Fluorescent Lamps

The fluorescent lamp is a low-pressure discharge lamp using mercury vapour. It has an elongated discharge tube with anelectrode at each end. The gas usedto fill the tube comprises inert gas, whichignites easily and controls the discharge,plus a small amount of mercury, thevapour of which produces ultraviolet radiationwhen excited. The inner surfaceof the discharge tube is coated with a fluorescentsubstance that transformsthe ultraviolet radiation produced by thelamp into visible light by means of fluorescence.

When leaving the electrode(1) the electrons(2) collide with mercury atoms (3). The mercury atoms (4) are thus excited and in turn produce UV radiation (5). The UV radiation is transformed into visible light (7) in thefluorescent coating (6).

To facilitate ignition of the fluorescent lamp the electrodes usually take the form of wire filaments and are coated with metallic oxide (emissive material) that promotes the flow of electrons. The electrodes are preheated at the ignition stage, the lamp ignites when the voltage is applied.

Different luminous colours can be achieved through the combination of appropriate fluorescent materials. To achieve this three different luminous substances are frequently combined, which, when mixed together, produce white light. Depending on the composition of theluminous substances, a warm white, neutral white or daylight white colour is produced.

In contrast to point sources (see incandescent lamps, above) the light from fluorescent sources is radiated from a larger surface area. The light is predominantly diffuse, making it more suitable for the uniform illumination of larger areas than for accent lighting.

The diffuse light of the fluorescent lamp gives rise to soft shadows. There are no sparkling effects on glossy surfaces. Spatial forms and material qualities are therefore not emphasized. Fluorescent lamps produce a spectrum, which is not continuous, which means that they have different colour rendering compared with incandescent lamps. It is possible to produce white light of any colour temperature by combining fewer fluorescent materials, but this light still has poorer colour rendering properties than light with a continuous spectrum due to the missing spectral components. To produce fluorescent lamps with very good colour rendering properties more luminous substances have to be combined in such a way that the spectral distribution corresponds as closely as possible to that of a continuous spectrum. Fluorescent lamps have a high luminous efficacy. They have a long lamp life, but this reduces considerably the higher the switching rate. Both ignitors and ballasts are required for the operation of fluorescent lamps. Fluorescent lamps ignite immediately and attain full power within a short period of time. Instant reignition is possible after an interruption of current. Fluorescent lamps can be dimmed. There are no restrictions with regard to burning position. Fluorescent lamps are usually tubular in shape, whereby the length of the lamp is dependent on the wattage. U-shaped or ring-shaped fluorescents are available for special applications. The diameter of the lamps is 26 mm (and 16 mm). Lamp types with a diameter of 38 mm are of little significance. Fluorescent lamps are available in a wide range of luminous colours, the main ones being warm white, neutral white and daylight white. There are also lamps available for special purposes (e.g. for lighting food displays, UV lamps) and coloured lamps. The colour rendering properties of fluorescents can be improved at the cost of the luminous efficacy; enhanced luminous efficacy therefore means a deterioration in the colour rendering quality. Fluorescent lamps are usually ignited via an external starting device and preheated electrodes. Some models have integrated ignition, which means that they can do without a starting device altogether. These are mainly used in enclosed luminaires, for environments where there is a risk of explosion.

Compact Fluorescent Lamp

Compact fluorescent lamps do not function any differently from conventional fluorescent lamps, but they do have a more compact shape and consist of either one curved discharge tube or the combination of several short ones. Some models have an outer glass envelope around the discharge tube, which changesthe appearance and the photometric properties of the lamp.

Unlike conventional fluorscent lamps, both ends of the discharge tube(s) are mounted on a single cap.

Compact fluorescent lamps basically have the same properties as conventional fluorescents, that is to say, above all, high luminous efficacy and a long lamp life. Their luminous efficiency is, however, limited due to the relatively small volume of the discharge tube. The compact form does offer a new set of qualities and fields of application. Fluorescent lamps in this form are not only confined to application in louvred luminaires, they can also be used in compact reflector luminaires (e.g. downlights). This means that concentrated light can be produced to accentuate the qualities of illuminated objects by creating shadows.

Compact fluorescent lamps with anintegrated starting device cannot bedimmed, but there are models availablewith external igniting devices and four-pinbases that can be run on electronic controlgear, which allows dimming.

Compact fluorescent lamps are mainlyavailable in the form of tubular lamps,in which each lamp has a combination oftwo or four discharge tubes. Startingdevice and ballast are required to operatethese lamps; in the case of lamps withtwo-pin plug-in caps the starting device isintegrated into the cap.

Alongside the standard forms equippedwith plug-in caps and designed to be runon ballasts, there is a range of compactfluorescent lamps with integrated startingdevice and ballast; they have a screwcap and can be used like incandescentlamps. Some of these lamps have anadditional cylindrical or spherical glass bulbor cover to make them look more likeincandescent lamps. If these lampsare used in luminaires designed to takeincandescent lamps it should be noted thatthe luminaire characteristics will becompromised by the greater volume of thelamp.

Compact fluorescent lamp with integral ballast

Compact fluorescent lamp with integral ballast and screw cap. This lamp is mainly used domestically as an economic alternative to the incandescent lamp.

High-voltage fluorescent tubes

  • High-voltage fluorescent tubes work on the principle of low-pressure gas discharge, the gas being either an inert or rare gas or a mixture of inert gas and mercury vapour. In contrast to fluorescent lamps, the electrodes contained in these lamps are not heated, which means they have to be ignited and run on high voltage. As there are special regulations concerning installations run at 1000 V and more, high-voltage tubular lamps are usually operated at less than 1000 V. There are, however, high-voltage discharge lamps available that run at over 1000 V.
  • High-voltage fluorescent tubes have a considerably lower luminous efficacy than conventional fluorescent lamps, but they have a long lamp life. Rare-gas discharge does not allow much scope when it comes to producing different colours; red can be produced using neon gas or blue using argon. To extend the spectrum of colours available it is possible to use coloured discharge tubes. However, mercury is usually added to the inert gas and the resulting ultraviolet radiation transformed into the desired luminous colours using fluorescent material.
  • High-voltage fluorescent tubes require a ballast; they are operated on leakage transformers, which manage the high voltages required for ignition and operation.
  • High-voltage fluorescent tubes ignite instantly and they can be restarted when hot. There are no restrictions with regard to burning position.
  • High-voltage fluorescent tubes come in various diameters and lengths. Different tubular shapes can be manufactured to meet the requirements of specific applications, e.g. for written signs and company logos. They are available in a variety of colours.

  • Induction Lamps

    Induction lamps, like fluorescent lamps, belong to the family of low-pressure mercury gas discharge lamps. Unlike other discharge lamps they have no electrodes, which is why they are also called “electrodeless lamps”. The consequence of having no electrodes is a very long economic life of around 60,000 to 75,000 hours. This long life is also the main feature of induction lamps. They find their application in situations where lamp replacement is near-impossible or very expensive. Philips was the first company to introduce induction lamps onto the market, under the name QL, in 1991.

    Working principle

    In an induction lamp, the free-running electrons needed for the gas discharge are obtained by winding an induction coil around a ferrite core placed in or around a discharge vessel. In the case of the QL lamp,the coil is placed inside a bulb-shaped vessel

    The induction coil is connected to a high-frequency power source and acts like a primary winding in a transformer. In a transformer, an alternating current in a primary winding around an iron core creates an alternating magnetic field that in turn initiates an alternating current in a secondary coil wound around the primary coil

    In the discharge vessel, the mercury gas surroundingthe ferrite core acts as the secondary coil (since mercury is a metal). The secondary current initiated in the mercury consists of free-running electrons around the ferrite core

    As in a normal fluorescent gas discharge, these free-running electrons ionise and excite other mercury atoms, which results in the emission of the same radiation as in a tubular fluorescent lamp.

    Since the vessel’s interior is coated with the same fluorescent powder as in normal fluorescent lamps, the same light is obtained. The main difference with normal fluorescent lamps is the extremely long lifetime of induction lamps, since they have no electrodes. The high frequency of 2.65 MHz is generated by an electronic circuit.

    Lamp construction

    The discharge vessel is made of the same type of glass as normal fluorescent tubes, and has a cylindrical glass cavity in which the antenna is positioned.

    Photograph of induction lamp vessel and antenna. The antenna is placed in the cylindrical cavity of the vessel. This lamp is only partly covered with fluorescent powder in order to show the cylindrical cavity.

    The mercury in the vessel is present in the form of an amalgam, which is necessary for a stable operation of the lamp at the high operating temperature. Special precautions are taken in the design of the lamp to limit excessive heat build up around the antenna. The high-frequency power generator is connected to the antenna with a coaxial cable. The cable forms part of the electronic circuit and is therefore an integral part of the system

    Pear-shaped induction lamp, QL, with internal antenna.

    Some manufacturers have the coil with iron core around the outside of the discharge vessel, which then has a different, flatter shape than the pear-shaped version

    Induction lamp with external antenna.

    The antenna is placed in the cylindrical cavity of the vessel. This lamp is only partly covered with fluorescent powder in order to show the cylindrical cavity.

    Performance characteristics.

    Energy balance-Around 17 per cent of the input power of an induction lamp is radiated as visible light. The remaining part is lost as heat in the power generator,in the antenna, and in the discharge.

    System luminous efficacy-The luminous efficacy of QL lamps is smaller than that of normal fluorescent lamps. It varies, depending on the wattage, between 65 lm/W and 75 lm/W.

    Lumen-package range-The QL induction lamps are available in a lumen package range from some 3500 to 12 000 lumen (corresponding wattage range 55 W to 165 W).

    Colour characteristics-The fluorescent coating is of the same compositionas that in normal fluorescent tubes, so the colour characteristics are also the same.

    Lamp life-The lamp life of most induction lamps is extremely long. Based on a mortality rate of 20 per cent, QL lamps have a lifetime of between 60 000 and 75000 hours. This is almost seven years of continuous operation, day and night.

    Lamp price-Induction lamps are expensive: some 70 to 100 times more so than GLS (general lamp shape) lamps. But this high lamp price has to be balanced against the extremely-long lifetime of these lamps. This means that where lamp replacement is difficult, very expensive, or even impossible, the balance will certainly be in favour of the longer life of these lamps.

    Lamp-lumen depreciation-Lumen depreciation in induction lamps is determined by a decrease in the activity of the fluorescent powder. At 60 000 hours the depreciation is around 25 per cent, which is why this lifetime figure is quoted. The actual life of QL lamps is usually much longer.

    Run-up and re-ignition

    A high-voltage ignition pulse produced by the HFgenerator ignites the lamp within five seconds, afterwhich it emits its full light output within one minute.Thanks to the high-voltage ignition pulse, hot re-ignitionof the lamp is immediate.


    Most standard versions of induction lamps are notdimmable. However, some special versions, which aredimmable to 50 per cent, are available for use in roadlighting.

    Ambient-temperature sensitivity

    Amalgam is used instead of pure mercury to keepthe sensitivity to ambient temperatures withinpractical limits. Lamp position sometimes plays a rolein this as well. It is therefore important to follow therecommendations from the lamp manufacturer withrespect to the luminaire design

    Product range

    Induction lamps are available in versions with internal(QL) and external antenna. The correspondingshapes, as we have seen, are pear shaped and plane. They are normally available incolour-rendering types 800 and 900, with colourtemperatures of 3000 K and 4000 K.

    High Pressure Sodium Lamps

    High-pressure sodium gas discharge lamps belongto the group of HID lamps. The Philips designationfor high-pressure sodium lamps is SON, where “SO”stands for sodium. High-pressure sodium lamps arealso referred to as HPS lamps.

    SON lamps, in common with all high-pressuredischarge lamps, are relatively compact. By increasingthe vapour pressure in a sodium lamp, the spectrumaround the typical yellow sodium line broadens.The result is that colour rendering improves andthe colour appearance changes from yellow toyellow-white (sometimes, in commercial literature,called golden white), albeit at the cost of a decreasein efficacy. However, the resulting efficacy is morethan double that of a high-pressure mercury lamp.At its introduction in the late 1960’s, a very efficientalternative was thus obtained for the many highpressuremercury lamps employed at that time inroad lighting. Today, road-lighting installations all overthe world very often use high-pressure sodium lamps,although for some installations LED solutions havebecome an alternative.

    By further increasing the sodium pressure, the colourquality of the light improves to such an extent that wereally can speak of white light. These so-called WhiteSON lamps have a lower efficacy but sometimes offeran acceptable alternative to halogen and compactmetal halide lamps for accent lighting. The Philipsdesignation for White SON lamps is SDW.

    Working principle

    Low-pressure sodium lamps, operating at a low working pressure of discharge, emit a single monochromatic line of light at a wavelength of 589 nm. With increasingpressure, the radiation in the core of the dischargeis absorbed by the cooler surrounding gas and re-emittedin the form of radiation not of the 589 nmline but with wavelengths slightly smaller and slightlylarger than 589 nm. So, the 589 nm line graduallydisappears (called self-absorption), while in thewavelength area to the left and right of that value,more and more light is emitted (broadening of thespectrum). The phenomenon of self-absorption andspectrum broadening is illustrated in Fig. 10.1, whichshows examples of sodium lamps with differentoperating vapour pressures. The phenomenon isaccompanied by a loss of efficacy. At the operatingpressure of a normal high-pressure sodium (SON)lamp, the lamp has a yellow-white colour appearance(2000 K) and a moderate colour-rendering index(Ra) of approximately 25 at an efficacy, for the higherwattages, of some 140 lm/W. With further increasein operating pressure the same process continues,viz. widening of the spectrum at the cost of efficacy.With lamps that operate at a four-times-higherpressure (Philips designation SON Comfort) thecolour rendering improves to “fairly good” (Ra=65)at an efficacy of around 90 lm/W. A version with anoperating pressure ten-times-higher than that ofthe standard SON lamp is also produced: Philipsdesignation SDW, popularly called White-SON. Thisversion has an Ra of 80 and radiates white light with acolour temperature of 2500 K at an efficacy of around45 lm/W.

    Effect of sodium vapour pressure on the spectral power distribution of different sodium gas discharge lamps.

    Lamp construction

    The main parts of a high-pressure sodium lamp are:

  • discharge tube
  • fill gas
  • outer bulb
  • electrodes
  • lamp cap
  • getter
  • Main parts of a high-pressure sodium lamp (example SON-T)

    Discharge tube

    At high temperatures, sodium has the tendencyto migrate slowly through quartz . Wehave also seen that ceramic discharge tubes withstandsodium at very high operating temperatures. This iswhy, right from their introduction in 1966 (long beforemetal-halide lamps started using ceramic material),high-pressure sodium lamps have translucent, tubularshaped,ceramic discharge tubes.

    Fill gas

    The sodium is introduced into the gas dischargetube as a sodium-mercury amalgam composition,which partially vaporises when the lamp reaches itsoperating temperature.

    The sodium vapour is responsible for excitation andsubsequent light radiation, while the mercury gasacts to regulate the voltage of the lamp and reducesthermal losses (buffer gas).

    The starter gas normally added is xenon (anexception to this is found in the case of low-wattageSON lamps, which have a built-in igniter in the formof a bi-metal switch and a neon-argon mixture ̶ thePenning mixture also used in fluorescent lamps ̶ asstarting gas).

    It is known that by increasing the pressure of thestarting gas xenon, the luminous efficacy of the lampincreases by about 20 per cent. However, to ensureproper ignition at this higher starting-gas pressure anauxiliary ignition wire has to be added very close tothe discharge tube. Philips does this in a special seriesof high-efficacy lamps by integrating this auxiliaryignition wire, or strip (also called antenna), in the wallof the discharge tube (SON PIA, where PIA stands forPhilips Internal Antenna).

    Discharge tube without and with aninternal ignition strip (or PIA).

    From an environmental point of view it can bedesirable to have high-pressure sodium lamps that donot contain mercury. Such lamps are, in fact, available.In these mercury-free high-pressure sodium lamps,xenon is used not only as a starting gas but also asthe buffer gas that regulates the voltage and reducesthermal losses. Mercury-free high-pressure sodiumlamps can be recognised by a green ring on the top ofthe outer bulb.


    The electrodes employed in high-pressure sodiumlamps are basically the same as those found in high-pressure mercury lamps

    Outer bulb

    To thermally insulate the gas discharge tube and toprotect its components from oxidation, an outer bulbis employed. The outer bulb of standard high-pressuresodium lamps is either tubular (SON-T) or ovoid inshape (SON). The internal wall of the ovoid bulb is usually coated with a diffusing powder.

    Coated ovoid (SON), Tubular (SON-T), and two White SON (SDW-T with different two-pin lamp caps).

    The coated versions were introduced so as to obtainthe same light-emitting area as in normal ovoid,fluorescent-powder-coated, high-pressure mercurylamps. In this way the coated ovoid high-pressuresodium lamps can be used with the same luminaireoptics as those developed for high-pressure mercurylamps.

    This was especially important at the originalintroduction of high-pressure sodium lamps,when they were replacing many existing high-pressuremercury lamps that were then beingused in many road-lighting installations. Notethat the coating in high-pressure sodiumlamps is of the diffusing, non-fluorescenttype. Since SON lamps produce practicallyno UV radiation, there is no point in usingfluorescent powder.

    White SON lamps (SDW) are usually only available inthe tubular form. The glass used for the outer bulb forwattages of more than 100 W is hard glass.

    Lamp caps

    The lamp caps employed for normal high-pressuresodium lamps are of the Edison screw type. The whiteSON lamps (SDW) have a special bi-pin cap to ensureexact positioning in a luminaire.


    During the process in which the lamp is evacuated,it is impossible to remove all traces of air and watervapour. During the operation of the lamp minusculeparticles evaporate from the glass and metals in thetube. All these traces would lead to an unacceptablyshort life. To remove them a getter is added thatabsorbs these traces. The getter is usually in the formof a small piece of solid material.

    Performance characteristics

    Energy balance

    Figure below shows the energy balance of a middle-rangetype of high-pressure sodium lamp. Some 30 per centof the input power is emitted in the form of visibleradiation. Compare this with the 40 per cent of low-pressuresodium lamps and the 17 per cent of high-pressuremercury lamps.

    Energy balance of a metal halide lamp (SON-T).

    System luminous efficacy

    The efficacy of the compact white SON variesbetween approximately 30 lm/W and 45 lm/W. Thehigh-pressure sodium lamp with colour renderingindex of 60 (SON Comfort) has an efficacy between75 lm/W and 90 lm/W, and the normal high-pressuresodium lamps (SON with Ra around 25) an efficacyof between some 80 lm/W and 140 lm/W. Again, asalways, the higher the wattage, the higher the efficacy.

    Lumen-package range

    Compact White SON lamps are available inlumen packages from some 1500 to 5 000 lumen(corresponding wattage range 35 W - 100 W).Normal high-pressure sodium lamps (SON) are beingproduced in the approximate range 4000 - 150 000lumen (corresponding wattage range 50 W - 1000 W).

    Colour characteristics

    As with all gas discharge lamps, the high-pressuresodium lamp spectrum is discontinuous. Figs.10.6 -10.8 show the spectra of a normal SON lamp, a SONComfort lamp and a White SON lamp (SDW).

    Spectral energy distribution of a highpressure sodium lamp (SON): Tk 2000 K and Ra 25.

    Spectral energy distribution of a highpressure sodium comfort lamp (SON Comfort): Tk 2150 K and Ra 65.

    Spectral energy distribution of a White high-pressure sodium lamp (SDW): Tk 2500 K and Ra 80.

    The colour temperature of these versions rangesfrom 2000 K to 2500 K and the colour-renderingindex from 25 to 80. Since the spectrum of allversions is relatively strong in the red wavelength area,the rendition of human faces is often experiencedas being somewhat flattering. Of course, for indoorlighting the colour rendering of the normal highpressuresodium lamp is far from adequate. For roadlighting it is quite acceptable.

    Lamp life

    The lamp voltage of a high-pressure sodium lampincreases gradually with life.16 The chief cause of lampfailure is that the lamp voltage rises higher than thevoltage output of the ballast, causing the lamp toextinguish. When this happens, the lamp cools downand the pressure in the lamp decreases so that theigniter can ignite the lamp again. After some minutes,the lamp voltage again increases too much and thelamp extinguishes again. So, the normal end of life of ahigh-pressure sodium lamp is accompanied by this socalledcycling effect. Normal high-pressure sodiumlamps have an economic life of up to some 20 000hours (20 per cent mortality).

    The lifetime of White SON lamps is not determinedby the moment of actual failure of the lamps but bythe onset of too large a colour shift of the light, whichis caused by the gradual increase of lamp voltage.To greatly increase their lifetime, white SON lampstherefore employ an electronic voltage stabiliserintegrated into their control gear. The economiclifetime of compact White SON lamps, dependingon type, lies between some 8000 and 12 000 hours(based on a 20 per cent too-large colour shift).

    Lamp price

    White SON lamps are a factor 25 to 40 times moreexpensive than GLS lamps. Normal high-pressuresodium lamps are some 8 to 60 times more expensive,depending on colour quality and wattage.

    Lamp-lumen depreciation

    For the compact White SON lamps, lumen depreciationvalues vary between some 20 and 25 percent (after 10 000 hours). The normal high-pressuresodium lamps have a much smaller lumen depreciationof between approximately 5 and 10 per cent (after 20000 hours).

    Run-up and re-ignition

    The high-pressure sodium lamp must be ignited bya high-voltage pulse, typically 1.8 kV to 5 kV. Afterignition, the colour of the light is initially white(discharge in the starting gas), changing to yellowafter some twenty seconds as the sodium amalgamgradually vaporises and the vapour pressure rises, untilafter some 3 to 5 minutes, the nominal pressure andfull light output is reached. Re-ignition of the hot lamprequires the lamp to cool down for about one minuteto allow the pressure to decrease to a point wherethe ignition pulse can again ionise the sodium atoms.


    All high-pressure sodium lamps can be dimmed to acertain extent, depending on the type of dimmingequipment used.

    Lower wattages (100 to 150 watt) can be dimmedwith special electronic gear, which allows for dimmingto 20 per cent. Higher lamp wattages can be dimmedby including an extra inductive coil (ballast) in theballast circuit. Lamp colour remains virtually constantand lifetime is not affected.

    Ambient-temperature sensitivity

    The behaviour of high-pressure sodium lamps duringtemperature variations differs from that of otherdischarge lamps because of the excess of amalgamused in the lamp. Although the outer bulb offers somedegree of thermal isolation, the lamp-manufacturer’sspecifications should be followed in the design ofluminaires as far as its effect on lamp temperature isconcerned.

    Mains-voltage variation

    A 5 per cent mains-voltage variation has a 15 per centeffect on the light output of high-pressure sodiumlamps. The same 5 per cent mains-voltage variation hasa 5 per cent effect on lamp voltage.

    Product range

    High-pressure sodium lamps areavailable in three basic types: standard high-pressuresodium (SON), an improved-colour version of highpressuresodium (SON Comfort), and a compact,white-light, high-pressure sodium lamp (SDW). The standard high-pressure sodiumlamp is available in two forms: the standard one and anextra-high-efficacy version (SON PIA).Most high-pressure sodium lamps contain a very smallquantity of mercury, but there is also a version that iscompletely-free of mercury.

    For ease of replacement of high-pressure mercurylamps in existing installations with more- efficienthigh-pressure sodium types, a special type of high-pressuresodium lamp has been developed. This type(Philips designation: SON-H) uses a neon-argon(Penning) mixture as starting gas, and is fitted withan ignition coil surrounding the discharge tube. Thesefeatures allow the lamp to be operated on a standardhigh-pressure mercury-lamp ballast.

    High-pressure Mercury Lamps

    High-pressure mercury lamps belong to the group of High Intensity Discharge (HID) lamps. High-pressure mercury lamps are available in versions where the discharge takes place in vaporised mercury only (Philips designation: HPL) and in versions in which metal halides are added so that the discharge takesplace in mercury vapour and in vaporised metals from the metal-halide components. These latter types are called metal halide (or HPI) lamps. Given the special operating principle and construction of metal halide lamps, they are detailed further in a separate section at the bottom of this tab.

    High-pressure mercury lamps, like all high-pressure discharge lamps, are compact compared to low pressure discharge lamps. HPL lamps have a moderate efficacy and moderate colour rendering. With their cool-white light they were extensively used in road lighting, especially in built-up areas. Since the introduction of the more efficient high-pressure sodium lamps in the late 1960’s, these lamps have in many cases replaced normal high-pressure mercury lamps.

    Working principle

    The gas discharge principle of high-pressure mercury lamps is similar to that of all other gas-discharge lamps. In high-pressure mercury lamps the discharge takes place in vaporised mercury at a pressure of around 106 Pa (10 atmospheres). The spectrum is a line spectrum with emissions in the long-wave UV region and in the visible region at the yellow, green, blue and violet wavelengths. The lamp without fluorescent powder lacks red in its spectrum and has a bluish-white colour appearance and very poor colour rendering. In most high-pressure mercury lamps, fluorescent powders are used to improve the colour quality by converting a large part of the (small) UV component into visible radiation, predominantly in the red end of the spectrum (HPL lamps). The result is cool-white light of moderate colour rendering and improved efficacy.In high-pressure mercury lamps, fluorescentpowder is often employed to improve thecolour of the light. In contrast with low pressure mercury tubular and compact fluorescent lamps, the conversion of UV into visible in high-pressure mercury lamps does little to increase their efficacy (only some 5 per cent).

    Lamp construction

    The main parts of a high-pressure mercury lamp are:

  • discharge tube
  • fill gas
  • electrodes
  • outer bulb with fluorescent coating
  • lamp cap
  • Discharge tube

    In view of the high operating temperature, quartz is used for the discharge tube.

    Fill gas

    The discharge tube contains a small quantity of mercury (which completely evaporates during operation) and an inert gas filling.


    The main electrodes consist of a core of tungsten rod with a tungsten coil (impregnated with emissive material) wound around it. To aid starting, a normal high-pressure mercury lamp has not only an inert gas but also an auxiliary electrode. Because of this,a normal mercury lamp does not need an external igniter. The auxiliary electrode simply consists of a tungsten wire positioned very close to one of the main electrodes

    Discharge tube: on the right an enlargement, showing the auxiliary electrode and one main electrode.

    Outer bulb

    An outer bulb (usually ovoid in shape) with an inert gas filling isolates the gas discharge tube so that changes in ambient temperature have no influence on its proper functioning. It also protects the lamp components from corrosion at the high operating temperatures involved. For the smaller lamps, with their lower operating temperatures, normal glass is used, while for the other types hard glass is used.

    Fluorescent powder

    High-pressure mercury lamps usually employ fluorescent powder to improve the colour quality of the light emitted. The powder is provided as a coating on the inner surface of the outer bulb. Different fluorescent coatings are used to obtain different lamp types with different colour qualities and lamp efficacies.

    Lamp cap

    Lamp caps are of the Edison-screw type, with the wattage of the lamp determining their size (E27 andE40).

    Performance characteristics

    Energy balance

    Figure above shows the energy balance of a typical high-pressure mercury lamp, showing that approximately 17 per cent of the input power is emitted in the form of visible radiation. Compare this with a tubular fluorescent lamp (28 percent), a compact fluorescent lamp (20 per cent) and a low-pressure sodium lamp (40 per cent).

    Luminous efficacy

    Luminous efficacy varies with lamp wattage and with the colour quality of the lamp from some 35 lm/W to 60 lm/W.

    Lumen-package range

    High-pressure mercury lamps are produced in lumen packages between some 2000 and 60 000 lumen(corresponding wattages between 50 W and 1000 W).

    Colour characteristics

    High-pressure mercury lamps have a line spectrum. The two lines in the red part of the spectrum are obtained by conversion of ultraviolet radiation by the fluorescent powder. The colour characteristics are dependent on the composition and quality of the fluorescent powders used. Different compositions and qualities are used to produce lamps with colour temperatures between some 3500 K and 4500 K, with colour rendering index (Ra ) values of around 60 for high quality versions and around 40 for ordinary versions.

    Special energy distribution of a high-pressure mercury lamp (HPL-N)

    Lamp life

    As with most gas discharge lamps, lamp life is determined by emitter exhaustion. Economic life varies according to type between 10 000 and 15 000 hours (20 per cent mortality).

    Lamp price

    High-pressure mercury lamps are, depending on their wattage, six to forty times more expensive than GLS lamps.

    Lamp-lumen depreciation

    Lamp-lumen depreciation is caused by evaporation and scattering of electrode material (lamp blackening)and by the gradual decrease in the activity of the fluorescent powder. The point at which 20 per cent lumen depreciation occurs lies at around 10 000 to 15 000 hours.

    Run-up and re-ignition

    The run-up time of a high-pressure mercury lamp to its full temperature and corresponding nominal mercury pressure is some four minutes. The hot lamp will not restart until it has cooled sufficiently to lower the vapour pressure to the point at which re-strike with the voltage available is possible. The re-ignition time is in the order of five minutes.


    High-pressure mercury lamps cannot be dimmed.

    Mains-voltage variations

    A five per cent variation in the mains voltage changes both lamp current and light output by ten per cent. Over-voltage decreases lamp life and increases lamp depreciation because of the correspondingly higher current.

    Product range

    High-pressure mercury lamps are available in an ordinary version with poor colour rendering (Raof around 40), and in so-called comfort versions with an improved colour rendering of around 60.The bulb is ovoid in shape and increases in size with increase in wattage.

    Reflector lamp versionsare produced with a cone-shaped outer bulb and aninternal reflective coating on the front

    There is one version of the high-pressure mercury lamp, the “blended light lamp”, that does not need an external ballast. The ballast has simply been built into the lamp itself in the form of a tungsten filament. The lamp can be connected direct to the mains. The lightfrom the mercury discharge and that from the heated filament blend together (hence the name blendedlight lamp). The colour characteristics of this lampare therefore better than those of a normal high pressuremercury lamp, but this comes at the cost of aconsiderably lower efficacy.

    Metal Halide Lamps

    Metal halide lamps are high-pressure mercury lamps that contain metal halides in addition to the mercury. In the heated discharge tube, the metals of the halides take part in the discharge process and radiate their own spectrum. Compared with high-pressure mercury lamps, both colour properties and efficacy are considerably improved. Thanks to the fact that no fluorescent powder is needed, the small gas discharge tube itself is the light-emitting surface. This small light-emitting surface makes the lamps extremely suitable for use in reflector and floodlight luminaires. Originally, they were solely produced in extremely high lumen packages with relatively large lamp dimensions (about the size of a 1 litre bottle). Theselamps are particularly suited for use in flood lights for the lighting of stadiums. Today, compact metal halide lamps are available in small lumen packages, which makes them very suitable for use in compact reflector luminaires for accent lighting both indoors and outdoors. These versions have become an energy efficient alternative for halogen lamps. Special compact metal halide versions are being produced for use in film studios and theatres. The smallest metal halide lamps are used for car headlamps (these are xenon lamps, whose gas discharge tube is no larger than a match head). Philips designations for the different types of metal halide lamps are HPI, MHN,CDM, CPO, Mastercolour and Cosmopolis. Metal halide lamps belong to the group of high-intensity discharge (HID) lamps.

    Working principle

    Suitable metals that vaporise in the hot discharge tubeso as to contribute to the discharge process cannotbe added directly to the mercury in the dischargetube. This is because metals suitable for this purposeattack the wall of the discharge tube at the hightemperatures that are needed for the gas discharge tooccur. The solution to this problem has been found toadd these metals in the form of their non-aggressivechemical compounds with a halogen (iodine, bromideor chloride), hence the name metal halide lamps. Thesolid metal halides start evaporating after switching onthe lamp, once the mercury discharge has increasedthe temperature in the discharge tube sufficiently.When this vapour enters the area of the mercurydischarge in the centre of the tube with its very hightemperature (around 3000ºC), it dissociates into itsseparate elements: metals and halogen. There themetals in their pure, vaporised state, take part in thedischarge process and determine the efficacy andcolour characteristics of the radiation. The aggressivevaporised metal cannot reach the tube wall because atthe lower wall.

    Lamp construction

    The main parts of a metal halide lamp are:

  • discharge tube
  • metal-halide additives
  • fill gas
  • outer bulb
  • electrodes
  • lamp cap
  • Discharge tube

    The discharge tube is made of either quartz or aceramic material. Some metals of the metal- halidecompounds (especially sodium) have the tendency,at high temperatures, to migrate slowly through thequartz wall of the tube, with the result that thereis a gradual change in the colour properties of thelamp during its lifetime. The use of ceramic dischargettubes solves the problem. Such tubes are imperviousto these metals, even at high operating temperatures.Later we will see that high-pressure sodium lampsmake use of the same ceramic material. Metalhalide lamps using ceramic material have the Philipsdesignation CDM lamps. Figure below shows examples of both quartz and ceramic gas discharge tubes.

    Gas discharge tube made of quartz (HPI-T lamp) and ceramic (CDM lamp) respectively. The yellow material seen through the quartz tube is condensed metal halide.

    The ceramic material (PolyCrystallineAlumina, or PCA) cannot be softened andworked as can glass or quartz. A ceramicdischarge tube is therefore built up from ahollow ceramic cylinder that is closed bygluing circular ceramic flat plates (discs) onits two ends. Ceramic dischargetubes can be constructed to better tolerancesthan can quartz tubes. Since the ceramictube is produced by sintering minusculealuminium-oxide particles one cannot lookstraight through the wall, although the lighttransmittance is more than 90% (comparethis effect with a broken and fragmented carwindow: light passes through it, although onegets no sharp image when looking throughit). We then speak of translucent instead oftransparent material.

    Schematic view of a ceramic discharge tube.

    Ceramic gas discharge tubes are operated at ahigher temperature than are quartz tubes. The higheroperating temperature influences the spectrum of theradiation. The result is a lower colour temperaturecompared to that of quartz metal halide lamps, and a10 per cent higher efficacy.

    Metal halides additives

    In theory, some fifty different metals can be used formetal halide compounds, and different manufacturershave introduced various combinations of thesemetals. Examples of some of the metals used in metalhalide compounds are: sodium, thulium, thallium,indium, scandium, dysprosium and tin. Some ofthese metals belong to the group of elements calledrare earth metals. Most lamps use a mixture of atleast three different metal halides. Each differentcombination results in a different spectrum, but alsoin different efficacy, lumen maintenance and lifetimecharacteristics.

    Fill gas

    Besides metal halides and mercury, an inert gas isalso added to the discharge tube, like in most otherdischarge lamps.


    The electrodes of metal halide lamps are of thetype used in normal high-pressure mercury lamps.They consist of a tungsten rod, with a tungsten coilimpregnated with emissive material, wound aroundit. They are of heavier construction because of thehigher operating temperature.

    Outer bulb

    Most metal halide lamps use a hard-glass or (for verycompact versions) quartz outer bulb for protectionand for heat insulation of the discharge tube as in the first figure below. They may be evacuated or gas filled. In the caseof quartz outer bulbs a UV-blocking quartz is used tolimit harmful UV radiation. For the same reason, somehard-glass versions use a cylindrically-shaped UV-blockshield around the discharge tube, the second figure. Both single endedand double-ended outer bulbs are used. Hard glass, single-ended (top) and quartz, double-ended outer bulbs. Hard glass, single-ended outer bulb with UVblock shield around the discharge tube.

    The inner wall of the outer bulb reflects a very smallpart of the light that consequently cannot be verywell controlled by a luminaire reflector. But since thereflected amount is so small, this is normally not aproblem, except where very-high-lumen-output lampsare employed. However, in floodlight installations, avery small amount of uncontrollable light could giverise to disturbing light pollution. For these applicationsspecial lamps are therefore available that have noouter bulb. Some of these have short-arc gasdischarge tubes to allow of optimised beam control45 Philips Lighting Hardwarein all directions. For reasons of thermal stability andsafety, those lamps without an outer bulb must beused in specially-designed luminaire housings.

    Double-ended quartz metal halide lamp(short arc) not making use of an outer bulb (MHNSA).

    Lamp caps

    Metal halide lamps come with a great variety of lampcaps. The single-ended lamps of higher wattage have ingeneral E40 Edison screw caps. Some caps have specialelectrical insulation because of the high ignition pulses(between 600 V and 5000 V). The lower-wattage singleendedlamps have two-pin electrical connectionsof various types, sometimes with ceramic electricalinsulation. In order to be able to exactlyposition the gas discharge arc in a luminaire, somelamps are provided with a so-called pre-focussed lampcap. The double-ended lamps use lampcaps of the type that are also used in double-endedhalogen lamps. The lamps without an outer bulb again have different lamp cap. Various lamp caps for compact single-endedmetal halide lamps.

    Performance characteristics

    Energy balance

    Figure below shows the energy balance of a metal halidelamp type that belongs to the middle range as far asefficacy is concerned. Almost 25 per cent of the inputpower is emitted in the form of visible radiation. Energy balance of a metal halide lamp(HPI-T).

    System luminous efficacy

    The compact versions of the metal halide lamp havelumen efficacies (depending on the metal halidemixture and gas discharge tube material used) ofbetween 70 lm/W and 95 lm/W. When comparing thiswith normal halogen incandescent lamps with theirmaximum efficacy of 25 lm/W, it is clear that compactmetal halide lamps are often very suitable to replacehalogen lamps. The larger versions of metal halidelamps have efficacies from some 75 lm/W to slightlymore than 105 lm/W.

    Lumen-package range

    Compact versions are available in lumen packagesfrom some 1500 to 25 000 lumen (correspondingwattage range 20 W - 250 W). Larger versions rangefrom some 20 000 lumen to more than 200 000lumen (corresponding wattage range 250 W - 2000W). Special (compact) types for use in film studios,theatres and professional photography are producedin lumen packages of up to more than 1 000 000lumen (12 000 W).

    9.3.4 Colour characteristics

    By employing different metal-halide mixtures, lamps with different spectra can be produced. Like all gas discharge lamps, the metal halide lamp has a discontinuous spectrum. Figures below show the spectra of some typical metal halide lamps with different colour temperatures CCT and colour rendering index Ra. The same colour designation system is used as with fluorescent lamps.

    Spectral energy distribution of a metalhalide lamp, colour type 642: Tk 4200 K and Ra 65(HPI). Spectral energy distribution of a metalhalide lamp, colour 830: Tk 3000 K and Ra 80 CDM Spectral energy distribution of a metalhalide lamp, colour 942: Tk 4200 K and Ra 90 (CDM)

    Compact metal halide lamps are normally produced in the colour temperature range of 3000 K to 4500 K, in two colour-rendering varieties: Ra ca. 80 and ca. 90. The larger metal halide lamps are available in the colour temperature range from approximately 4000 K to 6000 K, with colour rendering Ra in the range from 65 to more than 90. The special, daylight, metal halide lamp types for film studio and theatre lighting have high colour temperatures of between 6000 K and 8000 K with colour rendering values Ra between 65 and more than 90.

    Lamp life

    The lamp life of most metal halide types is somewhat shorter than that of other gas discharge lamps.This is because the electrodes are heated to ahigher temperature with a correspondingly higher evaporation rate, and are gradually degraded and destroyed by chemical reactions with the metalhalides. We have also seen that different metal halide lamp types employ widely-different construction methods. As a consequence, lamp life varies strongly with type. The economic life of compact versions lies between some 7000 and 14 000 hours (20 per cent mortality). The high-lumen-package versions vary from 4000 hours (single-envelope types for stadium flood lighting) to some 10 000 hours (again 20 percent mortality). Those ceramic metal halide lamps specifically developed for use in road lighting (Philips designation Cosmopolis), where lights may be used 4000 hours a year, have lifetimes up to 20 000 hours (20 per centmortality).

    Lamp price

    Compact metal halide lamps are, depending on their construction and complexity, a factor 10 to 40 more expensive than GLS lamps. With non-compact metal halides lamps this factor varies, according to wattage, by a factor of between 25 and 70, whereas for the special lamps for the lighting of sports arenas the factor can go up to approximately 150.

    Lamp-lumen depreciation

    Metal halide lamps have a higher lumen depreciation than most other discharge lamps. This is because ofthe higher degree of blackening from evaporated electrode material. Here too, the type of construction and sort of metal halides used play a role. Lumen depreciation values vary between some 20 and 30 percent (after 10 000 hours). Some special types, such as those employed in road lighting, depreciate less rapidly (some 10 per cent after 10 000 hours).

    Burning position

    With the larger gas discharge tubes in particular, the burning position may affect the actual location of the various metals in the tube. This means that different burning positions may result in different colour shifts. Also, with some types of lamp construction, the burning position can influence the lifetime of the lamp: for example, because of attack of the electrodes by some of the metals of the halides. Many metal halide lamps therefore have restrictions as to their permitted burning position (although these are, of course, specified in their accompanying documentation). Compact, single-ended types, with either quartz or ceramic tube, have universal burning positions.

    Run-up and re-ignition

    The metal halides in the discharge tube need time to heat up, evaporate, and dissociate into metal and halide. During this process, which takes about two to three minutes, the light output and colour gradually change until the final stable condition is reached. If there is an interruption in the power supply, medium and high-wattage lamps will take approximately 10 to 20 minutes for the pressure in the lamp to decrease enough for it to re-ignite. Compact ceramic lamps reignite much faster after some 3 to 5 minutes


    The dimming of metal halide lamps is difficult because with the resulting decrease in temperature some of the metal halides condense, changing the constitution of the metal halides actually participating in the discharge. This in turn changes the colour properties of the light. By employing a specially-shaped burner, electronically-driven lamp versions have been developed that can be dimmed to some 50 per cent without suffering from this problem.

    Ambient-temperature sensitivity

    The very high operating temperature of metal halide lamps is not influenced by variations in ambient temperature. Consequently, the lumen output and colour quality of these lamps are also not influenced by ambient temperature.

    Mains-voltage variations

    Mains-voltage variations affect lumen output, lifetime and light colour. A mains voltage that deviates 10 percent from its nominal value will result in perceivable colour shifts. Thanks to the higher operating temperature of ceramic lamps the effect on colour change is here considerably smaller. Both up and down variations in the mains shorten lamp life.

    Product range

    Metal halide lamps are available in a wide variety of types for many different applications, including flood lighting, road lighting, accent lighting (both interior and exterior), car headlamps and film-studio lighting.

    Examples of metal halide lamps used mainly in indoor lighting. Examples of metal halide lamps used mainly in outdoor lighting. First three: for road and industrial lighting. Last four: for sports floodlighting.

    Low-pressure Sodium Lamps

    Low-pressure sodium lamps belong to the groupof High-Intensity Discharge (HID) lamps, becausethey are available in high-light output (and thus high luminous-intensity) versions.

    All low-pressure gas discharge lamps have in commonthe fact that they are long, like the tubular-fluorescentlamp, which belongs to the family of low-pressuremercury lamps. They are highly-efficient lamps with a goodlifetime but no colour rendition at all. Their applicationis therefore restricted to those situations wherecolour rendering is of no importance, as for exampleon motorways, railroad yards and in somesecurity-lighting situations.

    The Philips designation for low-pressure sodium lampsis SOX, where “SO” stands for sodium. Low-pressuresodium lamps are sometimes also referred to as LPSlamps.

    Working principle

    The gas discharge principle of low-pressure sodiumlamps in which the discharge takes place in vaporized sodium, is similar to that of low-pressure mercurylamps. The low-pressure sodium discharge emitsmonochromatic radiation in the visible range.Therefore, unlike low-pressure mercury lamps, theydo not need fluorescent powders to convert thewavelength of the radiation. The monochromatic(single wavelength) radiation is the reason that colourrendering is non-existent. The wavelength of themonochromatic radiation is 589 nm (yellowish light),which is very close to the wavelength for which theeye has its maximum sensitivity.

    It is mainlyfor this reason that the lamp has such an extremely highluminous efficacy (up to 190 lm/W).Like all gas discharge lamps (with very fewexceptions), a low-pressure sodium lamp cannot beoperated without a ballast to limit the current flowingthrough it.

    Lamp construction

    The main parts of a low-pressure sodium lamp are:
  • discharge tube
  • fill gas
  • electrodes
  • outer bulb with inner coating
  • lamp cap

  • Discharge tube

    Just as with the fluorescent lamp, the power dissipatedin this lamp largely determines the length of thedischarge tube. Especially for these lamps with higher wattages, the unfolded length has to bevery long. To reduce the actual length of the lamp,the discharge tube of low-pressure sodium lamps istherefore always U-shaped. Nevertheless, the highestlumen packages still require a lamp length of about 1.2m.

    The U-shaped discharge tube is made of sodium resistantglass and contains a number of small dimples, or hollows, where the sodium is depositedas a liquid during manufacture. After ignition, thedischarge first takes place through the inert gasmixture. As the temperature in the tube gradually increases, some of the sodium in the dimplesvaporises and takes over the discharge, which thenemits the monochromatic radiation. At switch-off, thesodium condenses and again collects at the dimples,these being the coldest spots in the tube (just aswater vapour condenses on the cool windows of aroom). Without the dimples the sodium would, aftersome switch-on switch-off cycles, gradually condensealong the whole inner tube wall, decreasing lighttransmission considerably.

    Fill gas

    The inert gas mixture of neon and argon, called the“Penning mixture”, acts as a starting gas and buffergas (to protect the electrodes). During start-up, thedischarge only takes place in this gas, which is why alow-pressure sodium lamp radiates deep-red light forsome 10 minutes during start-up

    Immediately after switch-on, a low-pressuresodium lamp emits just a little reddish light, whichgradually changes to the familiar yellow sodium lightwhen the lamp has fully warmed up.


    Most modern low-pressure sodium lamps have coldstartelectrodes. These consist of a triple-coiledtungsten wire, so that they can hold a large quantity ofemitter material.

    Outer bulb

    The optimum sodium-vapour pressure is reachedwhen the temperature of the wall of the dischargetube is maintained at 260°C. To reach thistemperature efficiently, the U-shaped discharge tube iscontained in an evacuated outer-glass tube. To furtherincrease the thermal insulation, this outer tube iscoated on its inner surface with an interference layerthat reflects infrared radiation but transmits visibleradiation. In this way, most of the heat radiation isreflected back into the discharge tube, so maintainingthe tube at the desired temperature, while visibleradiation is transmitted through the layer.This principle of infrared reflective interference layers isalso used in a special type of halogen lampBefore infrared reflection coatingswere employed, low-pressure sodium lampsused an evacuated double-walled tube(vacuum-flask principle), which was detachableand could be re-used after lamp replacement.Apart from the dimples, the bend of the U-tube alsoforms a cold spot where sodium can condense andaccumulate. The bend is therefore insulated by a heatreflectingmetal cap.Even small quantities of oxygen or water vapour couldshorten the lifetime of the tungsten wire throughcorrosion. An absorbent “getter” is therefore added toremove this oxygen or water vapour.

    Lamp cap

    All SOX lamps are provided with a bayonet-typelamp cap. This allows the discharge tube tobe accurately positioned. This is critical, because thelight distribution of a low-pressure sodium luminaire isdependent on the position of the U-shaped dischargetube.

    Performance Characteristics

    Energy balance

    Figure below displays the energy balance of a typical low pressuresodium lamp. It shows that approximately 40per cent of the input power is emitted in the form ofvisible radiation. This is the highest percentage of allgas discharge lamps. The remaining part of the inputpower is lost in the form of heat.

    System luminous efficacy

    The luminous efficacy of the system is stronglydependent on the wattage of the lamp, and rangesfrom 70 lm/W to 190 lm/W, for low and highwattages, respectively.

    Lumen-package range

    Low-pressure sodium lamps are available in therange from approximately 2000 to 30 000 lumen(corresponding wattage range: 18 W to 180 W).

    Colour characteristics

    As mentioned before, low-pressure sodium lampsemit monochromatic light in the yellowish part of thespectrum. Colour rendering is therefore nonexistent(Ra=0). The correlated colour temperatureis around 1700 K.

    Spectral energy distribution of low-pressuresodium lamps.

    Lamp life

    Apart from the normal cause of failure in gasdischarge lamps (viz. electrode emitter exhaustion),low-pressure sodium lamps may also fail becauseof cracks or leaks in the long discharge tube orouter bulb. This may especially be the case inenvironments where there are strong vibrations ˗as for example may occur in poorly-designed roadlightingluminaires during strong winds. Leakage inthe outer bulb disrupts the thermal isolation, whichin turn means that not enough sodium will vaporise.As a consequence, the discharge will take placein the starting-gas mixture, so emitting only thecorresponding deep red light.

    Economic lamp life is around 12 000 hours (based ona 20 per cent mortality rate). Some versions make useof special "getter" material to maintain a high vacuum,resulting in fewer failures during the economic lifetimeof the lamp: the economic lamp lifetime is about 15000 hours (again: mortality rate of 20 per cent).

    Lamp price

    From low wattage (18 W) to high wattages (131 Wand 180 W), the price of low-pressure sodium lamps isa factor 10 to 30 higher than that of GLS lamps.

    Lamp-lumen depreciation

    Lumen depreciation occurs through blackeningof the discharge tube by scattering of the emittermaterial of the electrodes and by discolouration ofthe glass caused by the sodium. Depending on thetype of control gear used, these effects are partiallycounteracted by a slow and gradual increase in thepower dissipated in the lamp.

    Burning position

    Electrodes and lead-in wires coming into contactwith condensed sodium can eventually suffer damage.To prevent this from happening, low-pressure sodium lamps have restrictions as to their burning position.Base-down burning positions in particular have to beavoided under all circumstances. Figure below shows thepermissible burning positions for Philips low-pressuresodium lamps. As can be seen, the restrictions are lesscritical for lower-wattage (viz. smaller) lamps, simplybecause they contain less sodium.

    Permissible burning positions of low-pressuresodium lamps.

    Run-up and re-ignition

    The sodium needs timeto vaporise while the discharge takes place in thestarting-gas mixture. The warming-up process takesabout 10 minutes. Nearly all low-pressure sodiumlamps re-ignite immediately. The exceptions arethe high-wattage (131 W and 180 W) lamps, whichrestrike after 10 minutes.


    Low-pressure sodium lamps cannot be dimmed.Dimming would decrease the lamp temperature sothat not enough sodium would remain in the vapourstate to maintain the sodium discharge.

    Ambient-temperature sensitivity

    The good thermal insulation afforded by the outerbulb ensures that lamp performance is almostindependent of ambient temperature. Also, thanksto the starting-gas mixture, starting, as well, is almostindependent of ambient temperature.

    Mains-voltage variations

    The variations in lamp current and lamp voltage as aconsequence of a change in mains supply voltage tendto cancel each other out, the net result being that thelamp wattage, and to a certain extent the luminousflux, remain practically constant over a wide range.

    Product range

    Just like TL5 tubular-fluorescent lamps, low-pressuresodium lamps are available in two versions, eachwith a different balance between efficacy and lumenoutput. The Philips names for these two types of low-pressuresodium lamps are SOX1 (the high-lumen outputversion) and SOX-E (the high-efficacy version).Length-for-length, the luminous efficacy of the SOX-Eseries is some 10 to 15 per cent higher than that ofthe SOX series, while the light output is some 20per cent less. SOX-E lamps can be operated on HFelectronic control gear, which further increases theirefficacy by between 15 and 35 per cent. The length ofthe SOX lamp increases considerably with wattage:the 18 W version (which is also known as the mini-SOX) has a length of 22 cm, while the 131 W and 180W versions both have a length of 112 cm .Fig. 7.8 The range of low-pressure sodium lamps(SOX-E) from 18 W to 131 W.

    The range of low-pressure sodium lamps (SOX-E) from 18 W to 131 W.
  • Humphry Davy (1778-1829, English)
  • Heinrich Geißler (1814-1879, German
  • Heinrich Göbel (1818-1893, German)
  • Joseph Wilson Swan (1828–1914, English)
  • Thomas Alva Edison (1847-1931, American)
  • Pavel Jablotschkow (1847-1894, Russian)
  • Carl Auer von Welsbach (1858-1929, Austrian)
  • Peter Cooper Hewitt (1861-1921, American)
  • Joachim Teichmüller (1866-1938, German)
  • Nick Holonyak, Jr (1928- , American)
  • Richard Kelly (1910-1977, American)
  • William Lam (1924-2012, American)

  • History of Architectural Lighting

    History of the Lightbulb

    1809 - First Arc Lamp Created

    1835 - First Constant Electric Light Demo

    1859 - Geissler Tube Created by removing all the air from glass tube and passing electric current through it

    1880 - Edison patents incandescent bulb with carbon filament

    1901 - Peter Cooper Hewitt creates precursor to flourescent light by passing an electric current through mercury vapor

    1904 - Tungsten replaces carbon in incandescent bulbs

    1908 - 'Edison Screw' light bulb socket developed and continues to be major socket used today

    1913 - Irving Langmuir discovers that filling bulb with inert gas like nitrogen doubled the bulbs efficiency

    1926-34 - European researchers experiment with neon tubes coated with phosphors thus sparking fluorescent lamp research in the U.S.

    1939 - GE and Westinghouse introduced fluorescent lamps at both the New York World's Fair and the Golden Gate Exposition in San Francisco

    1951 - By 1951, more light in the U.S. was being produced by linear fluorescent lamps than incandescent -- a change that was led by the need for efficient lighting during World War II

    1962 - While working for General Electric, Nick Holonyak, Jr., invented the first visible-spectrum LED in the form of red diodes. Pale yellow and green diodes were invented next

    1976 - Fluorescent Bulbs Go Spiral, the creation of the first compact flourescent light (CFL)

    1978 - LEDs Appear in Consumer Products

    1985 - First CFL Hits the Market prices ranging from $25-35 per bulb

    2008 - First Residential LED Bulb Hits the Market

    Electrical light sources

    Arc Lamps

    Gas light was doomed to go the way of most lighting discoveries that were fated to be overtaken by new light sources just as they are nearing perfection. In contrast to the oil lamp and gas lighting, which both started life as weak light sources and were developed to become ever more efficient, the electric lamp embarked on its career in its brightest form.

    From the beginning of the 19th century it was a known fact that by creating voltage between two carbon electrodes, an extremely bright arc could be produced. However, continuous manual adjustment was required, making it difficult for this new light source to gain acceptance. In addition, the first arc lamps had to be operated on batteries making them quite costly to operate.

    About mid-century self-adjusting lamps were developed, thereby eliminating the problem of manual adjustment. Generators that could guarantee a continuous supply of electricity were now also available.

    It was, however, still only possible to operate one arc lamp per power source; series connection – “splitting the light”, as it was called – was not possible, as the different burning levels of the individual lamps meant that the entire series was quickly extinguished. This problem was only solved in the 1870s.

    The simple solution was provided by Jablotschkow’s version of the arc lamp, which involved two parallel carbon electrodes set in a plaster cylinder and allowed to burn simultaneously from the top downwards.

    Jablotschkow’s version of the arc lamp

    Incandescent Lamps

    Now that light could be “divided up” the arc lamp became an extremely practical light source, which not only found individual application, but was also used on a wide scale. It was in fact applied wherever its excellent luminous intensity could be put to good use – once again in lighthouses, for stage lighting; and, above all, for all forms of street and exterior lighting. The arc lamp was not entirely suitable for application in private homes, however, because it tended to produce far too much light. It would take other forms of electric lighting to replace gas lighting in private living spaces.

    It was discovered at a fairly early stage, that electrical conductors heat up to pro-duce a sufficiently great resistance, and even begin to glow; in 1802 – eight years before his spectacular presentation of the first arc lamp – Humphrey Davy demon-strated how he could make a platinum wire glow by means of electrolysis.

    The incandescent lamp failed to establish itself as a new light source for technical reasons, much the same as the arc lamp. There were only a few substances that had a melting point high enough to create incandescence before melting. Moreover, the high level of resistance required very thin filaments, which were difficult to produce, broke easily and burnt up quickly in the oxygen in the air.

    First experiments made with platinum wires or carbon filaments did not produce much more than minimum service life. The life time could only be extended when the filament – predominantly made of carbon or graphite at that time – was prevented from burning up by surrounding it with a glass bulb, which was either evacuated or filled with inert gas.

    Pioneers in this field were Joseph Wilson Swan, who preceded Edison by six months with his graphite lamp, but above all Heinrich Goebel, who in 1854 produced incandescent lamps with a service life of 220 hours with the aid of carbonized bamboo fibres and air-void eau-de-cologne bottles.

    The actual breakthrough, however, was indeed thanks to Thomas Alva Edison, who in 1879 succeeded in developing an industrial mass product out of the experimental constructions created by his predecessors. This product corresponded in many ways to the incandescent lamp as we know it today – right down to the construction of the screw cap.

    The filament was the only element that remained in need of improvement. Edison first used Goebel’s carbon filament comprising carbonized bamboo. Later synthetic carbon filaments extruded from cellulose nitrate were developed. The luminous efficacy, always the main weakness of incandescent lamps, could, however, only be substantially improved with the changeover to metallic filaments.

    This is where Auer von Welsbach, who had already made more efficient gas lighting possible through the development of the incandescent mantle, comes into his own once again. He used osmium filaments derived through a laborious sintering process. The filaments did not prove to be very stable, however, giving way to tantalum lamps, which were developed a little later and were considerably more robust. These were in turn replaced by lamps with filaments made of tungsten, a material still used for the filament wire in lamps today.

    Discharge Lamps

    Following the arc lamp and the incandescent lamp, discharge lamps took their place as the third form of electric lighting. Again physical findings were available long before the lamp was put to any practical use. As far back as the 17th century there were reports about luminous phenomena in mercury barometers.

    But it was Humphrey Davy once again who gave the first demonstration of how a discharge lamp worked. In fact, at the beginning of the 18th century Davy examined all three forms of electric lighting systematically. Almost eighty years passed, however, before the first truly functioning discharge lamps were actually constructed, and it was only after the incandescent lamp had established itself as a valid light source, that the first discharge lamps with the prime purpose of producing light were brought onto the market. This occured at around the turn of the century. One of these was the Moore lamp – a forerunner of the modern-day high voltage fluorescent tube. It consisted of long glass tubes of various shapes and sizes, high voltage and a pure gas discharge process. Another was the low-pressure mercury lamp, which is the equivalent of the fluorescent lamp as we know it today, except that it had no fluorescent coating.

    The Moore lamp – like the high-voltage fluorescent tube today – was primarily used for contour lighting in architectural spaces and for advertising purposes; its luminous intensity was too low to be seriously used for functional lighting. The mercury vapour lamp, on the other hand, had excellent luminous efficacy values, which immediately established it as a competitor to the relatively inefficient incandescent lamp. Its advantages were, however, outweighed by its inadequate colour rendering properties, which meant that it could only be used for simple lighting tasks.

    Theater foyer lit by Moore Lamps

    There were two completely different ways of solving this problem. One possibility was to compensate for the missing spectral components in the mercury vapour discharge process by adding luminous substances. The result was the fluorescent lamp, which did produce good colour rendering and offered enhanced luminous efficacy due to the exploitation of the considerable ultra-violet emission.

    The other idea was to increase the pressure by which the mercury vapour was discharged. The result was moderate colour rendering, but a considerable increase in luminous efficacy. Moreover, this meant that higher light intensities could be achieved, which made the high-pressure mercury lamp a competitor to the arc lamp.

    Quantitative lighting design

    A good hundred years after scientific researchinto new light sources beganall the standard lamps that we know todayhad been created, at least in their basicform. Up to this point in time, sufficientlight had only been available during daylight hours. From then on, artificial light changed dramatically. It was no longer a temporary expedient but a form of lighting to be taken seriously, ranking with natural light.

    Illuminance levels similar to those of daylight could technically now be produced in interior living and working spaces or in exterior spaces, e.g. for the lighting of streets and public spaces, or for the flood lighting of buildings. Especially in the case of street lighting, the temptation to turn night into day and to do away with darkness altogether was great. In the United States a number of projects were realised in which entire towns were lit by an array of light towers. Floodlighting on this scale soon proved to have more disadvantages than advantages due to glare problems and harsh shadows. The days of this extreme form of exterior lighting were therefore numbered.

    Both the attempt to provide comprehensive street lighting and the failure of these attempts was yet another phase in the application of artificial light. Whereas inadequate light sources had been the main problem to date, lighting specialists were then faced with the challenge of purposefully controlling excessive amounts of light. Specialist engineers started to think about how muchlight was to be required in which situations and what forms of lighting were to be applied.

    Task lighting in particular was examined in detail to establish how great an influence illuminance and the kind of lighting applied had on productivity. The result of these perceptual physiological investigations was a comprehensive work of reference that contained the illuminance levels required for certain visual tasks plus minimum colour rendering qualities and glare limitation requirements.

    Although this catalogue of standards was designed predominantly as an aid for the planning of lighting for workplaces,it soon became a guideline for lighting in general, and even today determines lighting design in practice. As a planning aid it is almost exclusively quantity oriented and should, therefore, not be regarded as a comprehensive planning aid for all possible lighting tasks. The aim of standards is to manage the amount of light available in an economic sense, based on the physiological research that had been done on human visual requirements.

    The fact that the perception of an object is more than a mere visual task and that, in addition to a physiological process, vision is also a psychological process, was disregarded. Quantitative lighting design is content with providing uniform ambient lighting that will meet the requirements of the most difficult visual task to be performed in the given space, while at the same time adhering to the standards with regard to glare limitation and colour distortion. How we see architecture, for instance, under a given light, whether its structure is clearly legible and its aesthetic quality has been enhanced by the lighting, goes beyond the realm of a set of rules.

    Beginnings of new lighting design

    It was, therefore, not surprising that alongside quantative lighting technology and planning a new approach to designing with light was developed, an approach that was related far more intensely to architectural lighting and its inherent requirements.

    This developed in part within the framework of lighting engineering as it was known. Joachim Teichmüller, founder of the Institute for Lighting Technology in Karlsruhe, is a name that should be mentioned here. Teichmüller defined the term “Lichtarchitektur” as architecture that conceives light as a building material and incorporates it purposefully into the overall architectural design. He also pointed out – and he was the first to do so – that, with regard to architectural lighting, artificial light can surpass daylight, if it is applied purposefully and in a differentiated way.

    Lighting engineers still tended to practise a quantative lighting philiosophy. It was the architects who were now beginning to develop new concepts for architectural lighting. From time immemorial, daylight had been the defining agent. The significance of light and shadow and the way light can structure a building is something every architect is familiar with. With the development of more efficient artificial light sources, the knowledge that has been gained of daylight technology was now joined by the scope offered by artificial light. Light no longer only had an effect coming from outside into the building. It could light interior spaces, and now even light from inside outwards. When Le Corbusier described architecture as the “correct and magnificent play of masses brought together in light”, this no longer only applied to sunlight, but also included the artificially lit interior space.

    This new understanding of light had special significance for extensively glazed facades, which were not only openings to let daylight into the building, but gave the architecture a new appearance at night through artificial light. A German style of architecture known as “GläserneKette”(Glass chain) in particular interpreted the building as a crystalline, self-luminous creation. Utopian ideas of glass architecture, luminous cities dotted with light towers and magnificent glazed structures, à la Paul Scheerbart, were reflected in a number of equally visionary designs of sparkling crystals and shining domes. A little later, in the 1920s, a number of glass architecture concepts were created; large buildings such as industrial plants or department stores took on the appearance of self-illuminating structures afterdark, their facades divided up via the interchange of dark wall sections and light glazed areas. In these cases, lighting design clearly went far beyond the mere creation of recommended illuminances. It addressed the structures of the litarchitecture. And yet even this approach did not go far enough, because it regarded the building as a single entity, to beviewed from outside at night, and disregarded users of the building and their visual needs.

    Buildings created up to the beginning of the second world war were therefore characterised by what is, in part, highly differentiated exterior lighting. All this, however, made little difference to the trend towards quantitative, unimaginative interior lighting, involving in the mainstandard louvered fittings.

    In order to develop more far-reaching architectural lighting concepts, man had to become the third factor alongside architecture and light. Perceptual psychology provided the key. In contrast to physiological research, it was not simply a question of the quantitative limiting values for the perception of abstract “visualtasks”. Man as a perceiving being was the focus of the research, the question of how reality perceived is reconstructed in the process of seeing. These investigations soon led to evidence that perception was not purely a process of reproducing images, not a photographing of our environment. Innumerable optical phenomenaproved that perception involves a complex interpretation of surrounding stimuli, that eye and brain constructed rather than reproduced an image of the world around us.

    In view of these findings lighting acquired a totally new meaning. Light was no longer just a physical quantity that provided sufficient illumination; it became a decisive factor in human perception. Lighting was not only there to render things and spaces around us visible, it determined the priority and the way individual objects in our visual environment were seen.

    The influence of stage lighting

    Lighting technology focussing on man as a perceptive being acquired a number of essential impulses from stage lighting. In the theatre, the question of illuminance levels and uniform lighting is of minor importance. The aim of stage lighting is not to render the stage or any of the technical equipment it comprises visible; what the audience has to perceive is changing scenes and moods – light alone can be applied on the same set to create the impression of different times of day, changes in the weather, frightening or romantic atmospheres. Stage lighting goes much further in its intentions than architectural lighting does – it strives to create illusions, whereas architectural lighting is concerned with rendering real structures visible. Nevertheless stage lighting serves as an example for architectural lighting. It identifies methods of producing differentiated lighting effects and the instruments required to create these particular effects – both areas from which architectural lighting can benefit. It is therefore not surprising that stage lighting began to play a significant role in the development of lighting design and that a large number of well-known lighting designers have their roots in theatre lighting.

    Qualitative lighting design

    A new lighting philosophy that no longer confined itself exclusively to quantitative aspects began to develop in the USA after the second world war. One of the pioneers in the field is without doubt Richard Kelly, who integrated existing ideas from the field of perceptual psychology and stage lighting to create one uniform concept.

    Kelly broke away from the idea of uniform illuminance as the paramount criterion of lighting design. He substituted the issue of quantity with the issue of different qualities of light, of a series of functions that lighting had to meet to serve the needs of the perceiver. Kelly differentiated between three basic functions: ambient light , focal glow and play of brilliance.

    Ambient light corresponded to what had up to then been termed quantitative lighting. General lighting was provided that was sufficient for the perception of the given visual tasks; these might include the perception of objects and building structures, orientation within an environment or orientation while in motion.

    Focal glow went beyond this general lighting and allowed for the needs of man as a perceptive being in the respective environment. Focal glow picked out relevant visual information against a background of ambient light; significant areas were accentuated and less relevant visual information took second place. In contrast to uniform lighting, the visual environment was structured and could be perceived quickly and easily. Moreover, the viewer’s attention could be drawn towards individual objects, with the result that focal glow not only contributed towards orientation, but could also be used for the presentation of goods and aesthetic objects.

    Play of brilliance took into account the fact that light does not only illuminate objects and express visual information, but that it could become an object of contemplation, a source of information, in itself. In this third function light could also enhance an environment in an aesthetic sense – play of brilliance from a simple candle flame to a chandelier could lend a prestigious space life and atmosphere.

    These three basic lighting categories provided a simple, but effective and clearly structured range of possibilities that allowed lighting to address the architecture and the objects within an environment as well as the perceptual needs of the users of the space. Starting in the USA, lighting design began to change gradually from a purely technical discipline to an equally important and indispensible discipline in the architectural design process – the competent lighting designer became a recognised partner in the design team, at least in the case of large-scale, prestigious projects.

    Lighting engineering and lighting

    The growing demand for quality lighting design was accompanied by the demand for quality lighting equipment. Differentiated lighting required specialised luminaires designed to cope with specific lighting tasks. You need completely different luminaires to achieve uniform washlight over a wall area, for example, than you do for accentuating one individual object, or different ones again for the permanent lighting in a theatre foyer than for the variable lighting required in a multi-purpose hall or exhibition space. The development of technical possibilities and lighting application led to a productive correlation: industry had to meet the designers’ demands for new luminaires, and further developments in the field of lamp technology and luminaire design were promoted to suit particular applications required by the lighting designers.

    New lighting developments served to allow spatial differentiation and more flexible lighting. Exposed incandescent and fluorescent lamps were replaced by a variety of specialised reflector luminaires, providing the first opportunity to direct light purpose fully into certain areas or onto objects – from the uniform lighting of extensive surfaces using wall or ceiling washers to the accentuation of a precisely defined area by means of reflector spotlights. The development of track lighting opened up further scope for lighting design, because it allowed enormous flexibility. Lighting installations could be adapted to meet the respective requirements of the space.

    Products that allowed spatial differentiation were followed by new developments that offered time-related differentiation: lighting control systems. With the use of compact control systems it has become possible to plan lighting installations that not only offer one fixed application, but are able to define a range of lightscenes. Each scene can be adjusted to suit the requirements of a particular situation. This might be the different lighting conditions required for a podium discussion or for a slide show, but it might also be a matter of adapting to changeswithin a specific environment: the changing intensity of daylight or the time of day. Lighting control systems are therefore a logical consequence of spatial differentiation, allowing a lighting installation to be utilised to the full – a seamless transition between individual scenes, which is simply not feasible via manual switching.

    There is currently considerable research and development being under taken in the field of compact light sources: among the incandescents the halogen lamp, whose sparkling, concentrated light provides new concepts for display lighting. Similar qualities are achieved in the field of discharge lamps with metal halide sources. Concentrated light can be applied effectively over larger distances. The third new development is the compact fluorescent lamp, which combines the advantages of the linear fluorescent with smaller volume, thereby achieving improved optical control, ideally suited to energy-efficient fluorescent downlights,for example.

    All this means that lighting designers have a further range of tools at their disposal for the creation of differentiated lighting to meet the requirements ofthe specific situation and the perceptual needs of the people using the space. It can be expected in future that progress in the field of lighting design will depend on the continuing further development of light sources and luminaires, but above all on the consistent application of this ‘hardware’ in the interest of qualitative lighting design. Exotic solutions – using equipment such as laser lighting or lighting using huge reflector systems – will remain isolated cases and will not become part of general lighting practice.

    Terms & Units

    Luminous flux

    Luminous flux describes the total amount of light emitted by a light source. Although this radiation could also be measured or expressed in watts, this does not describe the optical effect of a light source since the varying spectral sensitivity of the eye is not taken into account. To include the spectral sensitivity of the eye the luminous flux is measured in lumen.

    Luminous efficacy

    Luminous efficacy describes the luminous flux of a lamp in relation to its power consumption and is therefore expressed in lumen per watt (lm/W).

    The maximum value theoretically attainable when the total radiant power is transformed into visible light is 683 lm/W. Luminous efficacy varies from light source to light source, but always remains well below this optimum value.

    Quantity of light

    The quantity of light, or luminous energy (US), is a product of the luminous flux emitted multiplied by time; luminous energy is generally expressed in lm x h.

    Luminous intensity

    An ideal point-source lamp radiates luminous flux uniformly into the space in all directions; its luminous intensity is the same in all directions.

    In practice, however, luminous flux is not distributed uniformly. This results partly from the design of the light source, and partly on the way the light is intentionally directed. It makes sense, therefore, to have a way of presenting the spatial distribution of luminous flux, i.e. the luminous intensity distribution of the light source.

    The unit for measuring luminous intensity is candela (cd). The candela is the primary basic unit in lighting technology from which all others are derived. The candela was originally defined by the luminous intensity of a standardised candle.

    Later thorium powder at the temperature of the solidification of platinum was defined as the standard

    Since 1979 the candela has been defined by a source of radiation that radiates 1/683 W per steradian at a frequency of 540 x 1012 Hz.


    Illuminance describes the measurement of the amount of light falling onto (illuminating) and spreading over a given surface area.

  • Illuminance is the means of evaluating the density of luminous flux. It indicates the amount of luminous flux from a light source falling on a given area.
  • Illuminance need not necessarily be related to a real surface. It can be measured at any point within a space.
  • Illuminance can be determined from the luminous intensity of the light source.
  • Illuminance decreases with the square of the distance from the light source (inverse square law).
  • Illuminance E indicates the amt of luminous flux from a light source falling on surface A

    The illuminance at a point Ep is calculated from the luminous intensity l and the distance a between the light source and the given point


    Luminance describes the measurement of the amount of light emitting, passing through or reflected from a particular surface from a solid angle. Luminance is defined as the ratio of luminous intensity of a surface (cd) to the projected area of this surface (m2).

    The luminance of a luminous surface is the ratio of luminous intensity l and the projected surface area Ap

    Luminance is the basis for describing perceived brightness; the actual brightness is, however, still influenced by the state of adaptation of the eye, the surrounding contrast ratios and the information content of the perceived surface.

    Thermal Radiators

    It is not just by chance that the 380 to 780 nm range forms the basis for our vision, i.e. “visible light”. It is this very range that we have at our disposal as solar radiation on earth in relatively uniform amounts and can therefore serve as a reliable basis for our perception.

    The human eye utilizes this part of the spectrum of electro-magnetic waves to gather information about the world around us. It perceives the amount and distribution of the light that is radiated or reflected from objects to gain information about their existence or their quality; it also perceives the colour of this light to acquire additional information about these objects.

    The human eye is adjusted to the only lightsource that has been available for millions of years – the sun. The eye is therefore at its most sensitive in the area in which we experience maximum solar radiation. Our perception of colour is therefore also attuned to the continuous spectrum of sunlight.

    The first artificial light source was the flame of fire, in which glowing particles of carbon produce light that, like sunlight, has a continuous spectrum. For a long time the production of light was based on this principle, which exploited flaming torches and kindling, then the candle and the oil lamp and gas light to an increasingly effective degree.

    With the development of the incandescent mantle for gas lighting in the second half of the 19th century the principle of the self luminous flame became outdated; in its place we find a material that can be made to glow by heating –the flame was now only needed to produce the required temperature. Incandescent gas light was accompanied practically simultaneously by the development of electric arc and incandescent lamps, which were joined at the end of the 19th century by discharge lamps.

    In the 1930s, gas light had practically been completely replaced by a whole range of electric light sources, whose operation provides the bases for all modern light sources. Electric light sources can be divided into two main groups, which differ according to the processes applied to convert electrical energy into light. One group comprises the thermal radiators, they include incandescent lamps and halogen lamps. The second group comprises the discharge lamps; they include a wide range of lightsources, e.g. all forms of fluorescent lamps, mercury or sodium discharge lamps and metal halide lamps.

    Incandescent Lamps

    The incandescent lamp is a thermal radiator. The filament wire begins to glow when it is heated to a sufficiently high temperature by an electric current. As the temperature increases the spectrum of the radiated light shifts towards the shorter wavelength range – the red heat of the filament shifts to the warm white-light of the incandescent lamp.

    General service lamp(left) and pressed-glass lamp with integrated parabolic reflector(right).

    Depending on lamp type and wattage the temperature of the filament can reach up to 3000 K, in the case of halogen lamps over 3000 K. Maximum radiation at these temperatures still lies in the infrared range, with the result that in comparison to the visible spectrum there is a high degree of thermal radiation and very little UV radiation. Lack of a suitable material for the filament means that it is not possible to increase the temperature further, which would increase the luminous efficacy and produce a cool white luminous colour.

    As is the case with all heated solid bodies– or the highly compressed gas produced by the sun – the incandescent lamp radiates a continuous spectrum. The spectral distrbution curve is therefore continuous and does not consist of a set of individual lines.

    The heating of the filament wire results from its high electrical resistance –electrical energy is converted into radiant energy, of which one part is visible light. Although this is basically a simple principle, there are a substantial number of practical problems involved in the construction of an incandescent lamp. There are only a few conducting materials, for example,that have a sufficiently high melting point and at the same time a sufficiently low evaporation rate below melting point that render them suitable for use as filament wires.

    Nowadays practically only tungsten is used for the manufacture of filament wires, because it only melts at a temperature of 3653 K and has a low evaporation rate. The tungsten is made into fine wires and is wound to make single or double coiled filaments.

    In the case of the incandescent lamp the filament is located inside a soft glassbulb, which is relatively large in order to keep light loss, due to deposits of evaporated tungsten (blackening), to a minimum. To prevent the filament from oxidizing, the outer envelope is evacuated forlow wattages and filled with nitrogen or a nitrogen-based inert gas mixture for higher wattages. The thermal insulation properties of the gas used to fill the bulb increases the temperature of the wire filament, but at the same time reduces the evaporation rate of the tungsten, which in turn leads to increased luminous efficacy and a longer lamp life. The inert gases predominantly used are argon and krypton. The krypton permits a higher operating temperature – and greater luminous efficacy. Due to the fact that it is so expensive, krypton is only used in special applications.

    A characteristic feature of incandescent lamps is their low colour temperature -the light they produce is warm in comparison to daylight. The continuous colour spectrum of the incandescent lamp provides excellent colour rendition. As a point source with a high luminance, sparkling effects can be produced on shiny surfaces and the light easily controlled using optical equipment. Incandescent lamps can therefore be applied for both narrow-beam accent lighting and for wide-beam general lighting.


  • Incandescent lamps easily dimmed.
  • No additional control gear is required for their operation and
  • the lamps can be operated in any burning position.

  • Disadvantages:

  • low luminous efficacy, and
  • a relatively short lamp life,
  • .

    Special incandescent lamps are available with a dichroic coating inside the bulb that reflects the infrared component back to the wire filament, which increases the luminous efficacy by up to 40 %.

    General service lamps (A lamps) are available in a variety of shapes and sizes. The glass bulbs are clear, matt or opal. Special forms are available for critical applications(e.g. rooms subject to the danger of explosion, or lamps exposed to mechanical loads), as well as a wide range of special models available for decorative purposes.

    A second basic model is the reflector lamp (R lamp). The bulbs of these lamps are also blown from soft glass, although,in contrast with the A lamps, which radiate light in all directions, the R lamps control the light via their form and a partly silvered area inside the lamp. Another range of incandescents are the PAR(parabolic reflector) lamps. The PAR lamp is made of pressed glass to provide a higher resistance to changes in temperature and a more exact form; the parabolic reflector produces a well-defined beam spread.

    In the case of cool-beam lamps, a subgroup of the PAR lamps, a dichroic,i.e. selectively reflective coating, is applied. Dichroic reflectors reflect visible light, but allow a large part of the IR radiation to pass the reflector. The thermal load on illuminated objects can therefore be reduced by half.

    Halogen Lamps

    It is not so much the melting point of the tungsten (which, at 3653 K, is still a relatively long way from the approx. 2800 K of the operating temperature of incandescents) that hinders the construction of more efficient incandescent lamps, but rather the increasing rate of evaporation of the filament that accompanies the increase in temperature. This initially leads to lower performance due to the blackening of the surrounding glass bulb until finally the filament burns through. The price to be paid for an increase in luminous efficiency is therefore a shorter lamp life.

    Halogen lamp for mainsvoltage with screw capand outer envelope(left). The outer envelopemeans that the lampcan be operated withouta protective glasscovering. Low-voltage halogen lamp with pinbase and axial filament in a quartz glass bulb(right).

    One technical way of preventing the blackening of the glass is the adding of halogens to the gas mixture inside the lamp. The evaporated tungsten combines with the halogen to form a metal halide,which takes on the form of a gas atthe temperature in the outer section of thelamp and can therefore leave no depositson the glass bulb. The metal halide is split into tungsten and halogen once again t the considerably hotter filament and the tungsten is then returned to the coil. The temperature of the outer glass envelope has to be over 250° C to allow the development of the halogen cycle to take place. In order to achieve this a compactbulb of quartz glass is fitted tightlyover the filament. This compact formnot only means an increase in temperature,but also an increase in gas pressure,which in turn reduces the evaporation rateof the tungsten.

    Halogen cycle: combination of evaporated tungsten and halogen to produce tungsten halide in the peripheral area. Splitting of the tungsten halogens backto the filament.

    Compared with the conventional incandescent the halogen lamp gives a whiter light – a result of its higher operating temperature of 3000 to 3300 K; its luminouscolour is still in the warm white range. The continuous spectrum produce sexcellent colour rendering properties.

    The compact form of the halogen lamp makes it ideal as a point-source lamp; its light can be handled easily and it can create attractive sparkling effects. The luminous efficacy of halogen lamps is well above that of conventional incandescents –especially in the low-voltage range. Halogen lamps may have a dichroic, heat reflecting coating inside the bulbs, which increases the luminous efficacy of these lamps considerably.

    The lamp life of halogen lamps is longer than that of conventional incandescents. Halogen lamps are dimmable. Like conventional incandescent lamps, they require no additional control gear; low-voltage halogen lamps do have to be run on a transformer, however. In the case of double-ended lamps, projector lamps and special purpose lamps for studios the burning position is frequently restricted. Some tungsten halogen lamps have to be operated with a protective glass cover.


    LEDs are solid-state radiators where the light is created inside solid-state material. Light emission can be obtained when an electric current passes through specific types of semiconductor material. Common semiconductor diode chips, used today in so many electrical circuits, all use much the same technology. The light-radiating diode versions are called Light-Emitting Diodes, or LEDs. Because of their character they are sometimes also referred to as “opto-electronic” devices. Until the mid-nineties of the last century, LEDs had a low lumen output and low efficiency, making them only suitable as small indicator lamps. Today, the efficacy of LEDs is comparable to that of gas discharge lamps. The lumen output of a single LED can be more than that of a 75 watt incandescent lamp. To distinguish these LEDS from the indicator type of LEDs they are referred to as high-brightness or highpower LEDs.

    Further improvements in high-brightness LEDs are expected, ultimately leading to efficacies of probably slightly more than 200 lm/W (for white-light LEDs). This is approximately twice the efficacy of today’s most efficient white-light gas discharge lamps. With its light-emitting surface of some 0.5 mm2 to 5 mm2, an individual LED chip represents the smallest artificial light source currently available. LEDs have a long lifetime and are, given the solid-state material, extremely sturdy. They are available in white and in coloured-light versions. The coloured versions, in multi-LED format, are extensively used in traffic signs. Coloured versions were also the first ones to be used on a large scale for lighting: specifically, the exterior floodlighting of buildings and monuments. Both the efficacy and the colour quality of white LEDs have been improved so much that they are now used in many different lighting applications, including road lighting, indoor accent lighting, domestic lighting and automobile lighting. Examples of the use of LEDs for office lighting and outdoor sports lighting can also be found. Given the potential for a further increase in their efficacy, the number of LED applications is bound to increase. Their small size and their availability in many different colours, and the simplicity of lighting control, both in terms of dimming and colour changing, are properties that permit of completely new applications.

    Working Principle

    Principle of Solid State Radiation

    Like any diode, a LED consists of layers of p-type and n-type of semiconductor material.

    The n-type of material has an excess of negativelychargedelectrons whereas the p-type material has adeficiency of electrons, viz. positively-charged holes.Applying a voltage across the p-n semiconductor layerpushes the n and p-type atoms towards the junctionof the two materials. Here the n-type ofatoms “donate” their excess electrons to a p-typeof atom that is deficient in electrons. This processis called recombination. In doing so, the electronsmove from a high level of energy to a lower one,the energy difference being emitted as light. Thewavelength of the light is dependent on the energyleveldifference between the p and n materials, whichin turn is depends on the semiconductor materialused: different semiconductor materials emit differentwavelengths, and thus different colours, of light. Thespectrum is always a narrow-band spectrum (quasimonochromatic light).

    Semiconductors are made of materialthat is a poor conductor of electricity.By adding specific impurities to thematerial, a process called doping, theatoms of the material get either extraelectrons or a deficiency of electrons.This doping process makes the materialmore conductive, which is the reason forthe name semi-conductor. The materialwith extra electrons is an n-type ofsemiconductor (negatively charged) andthe material with a deficiency of electronsviz. positively-charged “holes”, is a p-typeof semiconductor. In the n-type of materialthe extra electron of an atom moves in anouter orbit with a correspondingly-higherenergy level. In the p-type of material themissing electron of an atom was movingin a lower orbit with lower energy level.After applying a voltage across the p-njunction, an n-atom can meet a p-atomat the junction and the electron of then-atom falls into the lower orbit of thep-atom with the correspondingly-lowerenergy level. The energy difference may beradiated as radiation (flow of photons) ormay heat up the material. Semiconductormaterial is chosen with a value of theenergy difference that results in radiationin the visible range (according to E=h . c/ λ; The processis very much like the process of excitedelectrons in a gas discharge falling back totheir original orbit with a lower energylevel while emitting light. In this sense it issurprising that the expression “solid statedischarge” and “solid state discharge lamp” is hardly ever used.

    Not all recombinations result in light emission. Some recombinations are non-radiative and just heat up the solid material. This limits the efficiency of light creation. A further limitation of the efficiency is caused by absorption of light in the solid material of the chip itself. Improvements in radiative recombination efficiency and light-extraction efficiency have been the most important reasons for the dramatic improvements of efficacy of LEDS during the past decade. Further improvements will be sure to greatly increase the efficacy of LEDs over the coming decade.

    Principle of White LEDs

    The spectrum of a single LED is always narrow. Consequently, its light is coloured. White LED light can nevertheless be obtained by combining three (or more) differently-coloured LED chips. A common method is to combine red, green and blue LED chips into a single module to produce white light.

    However, the colour rendering of such a “RGB white light” system is not good, since large areas of the full colour spectrum are not included in its light. Sometimes an amber colour chip is added to the RGB combination to improve the colour quality of the light (RGBA LED). Research is going on to produce single, multi-layer LED chips, each layer producing a specific colour of light. A single LED producing red, green and blue light would therefore result in white light. Good-quality white light, which is especially important when it comes to providing good colour rendering, is obtained by using a blue LED chip in combination with fluorescent material that converts much of the blue light into light of different wavelengths spread over almost the whole visible spectrum.

    TOP: colour temperature Tk=4000K, colourrendering Ra=70
    BOTTOM: colour temperature Tk=2750K, colour rendering Ra=85.

    The working principle of fluorescent powders converting UV radiation into visible radiation is the same for powders converting blue light into visible radiation of longer wavelengths. In LED technology it is customary to call such fluorescent materials, phosphors: hence white LEDs based on this principle are called “white-phosphor LEDs”. By mixing different phosphors in different proportions, white LEDs producing different tints of white light with different colour-rendering capabilities can be made.

    LED Construction

    The LED chip is embedded in a larger structure for mechanical protection, for the electrical connections, for thermal management, and for efficient light outcoupling. The main parts of a LED are:

  • semiconductor LED chip
  • reflector cup
  • supporting body
  • electrodes and bond wires
  • heat sink
  • primary and secondary optics
  • phosphors (for white LEDs)

  • Semiconductor chip material

    The p-n semiconductor sandwich forms the heart of the LED and is called the LED chip, or die

    The semiconductor material used in an LED determines the difference in energy level between the n and p junction which in turn determines the wavelength and thus the colour of the light emitted. For LEDs, compound semiconductor material is used, which is composed of different crystalline solids. These are doped with very small quantities of other elements (impurities) to give their typical n and p properties. Materials used in semiconductors for high-brightness LEDs must also be able to handle the necessary electrical currents, heat and humidity, and must have a high degree of translucency, as in the case with crystalline solids. The elements aluminium, indium, gallium and phosphide (ALInGaP) are used, in different compositions, to produce the colours amber, orange and red in high-brightness LEDs.

    For the colours blue, green and cyan the elements indium, gallium and nitride (InGaN) are used. It was the Japanese Nakamura who, in 1993, succeeded for the first time in producing a blue LED suitable for mass production. This was the “missing link” that enabled the production of white-phosphor LEDs and the production of white LED light on the basis of mixing the light of red, green and blue LEDS (RGB mixing). Since then, the development of highbrightness LEDs for lighting has taken off very rapidly. Today a very small area of the spectrum, in greenish-yellow, is still missing.


    Unfortunately, the LED chip is a “photon or light trap”: that is to say, much of the light emitted within the chip is internally reflected by its surfaces (borders between the material and air) and ultimately, after multi reflections, absorbed in the material (heating up the material). Only light that hits the outer surface more or less perpendicularly (± approximately 20 degrees) can leave the material.

    By giving the chip a specific shape and by keeping it thin, the so-called light-extraction efficiency can be improved. Fig. 11.9 gives an example of such a specifically-shaped chip.

    Reflector Cup

    In many cases, the chip is placed in a reflector cup which, because of its shape, helps to direct the lightin an upwards direction. Highly-reflective materialis used: for example, metal or ceramic material. Thereflector cup can be considered to be part of the primary optics of the primary optics of the LED

    Primary and secondary Optics

    The silicon lens on top of the LED chip serves as protection for the chip. More importantly, it helps in increasing the light extraction from the chip and as such is essential for a high lumen efficacy of the LED. The lens itself does not shape the beam. Thematerial of the lens and of the reflector cup shouldbe such that degradation (for example lens yellowing)is minimised in order to minimise lumen depreciation over life.

    Secondary optics to accurately shape the lightbeam for different applications can be separately incorporated in the LED luminaire or fitted direct to the LED housing. The figure below shows a number of different secondary optics fitted directly to the LED housing.

    Electrodes and bond wires

    In order to be able to apply power to the chip, thep and n parts of the chip have metal contacts calledelectrodes (the cathode is connected to the n-partof the chip and the anode to the p part). Bondwires connect the electrodes with the electricalconnections. They are usually gold wires. Fig. 11.9shows such an electrode (anode) with a bond wire.Since the electrodes intercept light leaving the chip,the dimensioning of the electrodes and bond wires,especially on the side of the main light-escape route, isone of the factors that determines the light efficiency of the LED.

    Heat sink

    LEDs do not radiate infrared radiation and consequently give a cool beam of light. However, this does not mean that they do not generate heat. Non-radiative recombinations of electrons and holes in the p-n sandwich, and light trapped in the chip, do heat up the chip. The larger the power of the chip and the lower its luminous efficacy, the higher is this heating effect. The higher the temperature of the p-n junction in the chip, the lower the light output of the chip. A too-high chip temperature also seriously shortens LED life, and it also slightly shifts the emitted wave length and thus the colour of the LED. Effective thermal management is therefore critical for a proper functioning of LEDs. All high-power high-brightness LEDs therefore have a heat sink of high-thermal conductivity material (like aluminium or copper) on their rear side to conduct the heat away from the chip and through the luminaire housing to the surrounding air. LED luminaires must therefore incorporate in their design thermal conduction and convection features (such as cooling fins) to dissipate the heat to the immediate surroundings.

    For retrofit LED lamps (LED bulbs), the size of theheat sink is limited by the size of the bulb. The heatsink therefore has a limited capacity, thus limitingthe power of the retrofit LED bulbs employed (withtoday’s LED efficacies, to something like 75 wattincandescent-lamp equivalent). If active cooling is employed, larger powers are permissible.


    The most important method for producing white light with LEDs is by applying a phosphor coating to a blue-light LED that converts part of the blue light into longer-wavelength green, yellow and red light. Different compositions of different phosphors are used to produce white light of different colour tints. Since the basis of the process is the blue light of the chip, the final efficacy will become higher as more blue is kept in the light. However, this implies a high colour temperature (cool-white light) and relatively-poor colour rendering. If, with a different phosphor composition, more blue lightis converted, the colour quality will improve at the expense of a somewhat-lower efficacy. We see here the same balancing effect between colour quality and lamp efficacy that we have seen with conventional lamps.

    The phosphor is often applied on or very near to the blue LED chip.

    The phosphor is often mixed with a solvent and then poured over the chip. This is called a nonconformal coating process. It results in a variation of the thickness of the phosphor layer. This thickness variation causes a variation of the colour temperature in the light beam as shown in Fig.11.13 left. PhilipsLumileds employ a unique process for coating thechip with a conformal phosphor layer. The resultant advantage of employing this process is that there is no colour variation in the light beam.

    Remote-phosphor LED modules

    In the case of multi-LED units, the phosphor issometimes applied at a greater distance from theLEDs. Such modules are called remote-phosphorLED modules. Here, a number of blueLEDs are placed inside a mixing chamber of highand diffuse reflective material. The phosphor layer,positioned remotely from the LEDs on the bottom ofthe chamber, converts the blue light of the chips intowhite light. In this way, thanks to the mixing process,small differences in light output and or colour ofindividual chips are not visible. The risk for disturbingglare is also reduced because the light intensity fromthe large-sized phosphor layer is much lower thanthe intensities of small, individual LEDs. Some typesof retrofit Philips LED bulbs apply a similar remote phosphor technology.

    The phosphors used for the blue-light conversion appear yellow when they are not activated; that is to say, when the LED is not switched on. This sometimes makes people think, erroneously, that the blue light is filtered through a yellow filter. It is really wavelength conversion and not light filtering that takes place.

    Multiple LED modules

    The luminous flux of one individual LED is quite low compared to that of conventional light sources. Multiple LEDs are therefore often packed on a printed-circuit board (PCB) to obtain an LED module emitting a high luminous flux. The PCB establishes the electrical connections between all components and the external electrical driver. The PCB must also conduct the heat from the heat sinks of the LEDs to the outside world. PCBs can be of glass-fibre reinforced epoxy material, ceramic material or metal core material (aluminium, with a thin layer of fibreglass for electrical insulation).

    Conventionally, the electrical connections are made by soldering and in that case we speak of Surface Mounted Devices, SMDs. Since high temperatures can damage the chips, SMD type of packaging requires an accurate process control. A more advanced way of packaging LEDs on PCB’s is the so-calledC hip On Board method, COB. Here the LED chips are directly, without a substrate layer, connected to the PCB with conductive glue. The electrical connections are directly made through the bond wires. No soldering is required, a higher packaging density is possible and thermal management can be optimized more easily. What packaging method is most suitable, also taking the economic aspect into account, is dependent on the type of application.

    Performance Characteristics

    Temperature of the chip junction

    With risingtemperature of the p-n chip junction, the performanceof LEDs decreases: particularly the light output andlifetime. The performance data are usually specified fora junction temperature (Tj) of 25ºC. However, undernormal operating conditions, a junction temperatureof 60ºC to 90ºC is easily obtained. Depending onLED type, the lumen output falls to 60 to 90 per centwhen the junction temperature increases from 25ºCto 80ºC. As the power dissipated by the LED remainsthe same, this affects both the luminous efficacy andthe lumen output. Specification of performance dataat a junction temperature of both 25ºC and say 80ºCwould give a much better insight into what we mayexpect under real-life conditions. Amber and redLEDs are the most sensitive to changes in junction temperature, and blue LEDs the least.


    The mass production of LEDs results in LEDs of the same type varying in coulour, light output and voltage. In order to ensure that LEDs nevertheless conform to specification, LED manufacturers use a process called binning in their production process. At the end of the manufacturing process, LED properties are measured and LEDs are subsequently sorted into subclasses, or “bins” of defined properties. As far as the colour quality is concerned, the tolerances in these definitions are such that visible differences in colour between LEDs from the same bin are minimised. As the definition of a bin does not change with time, the same quality is also assured from production run toproduction run. With the advancement of knowledge concerning LED materials and the mass-production process, we may expect that binning will ultimately no longer be required. In fact Philips Lumileds has already introduced LED products onto the market of the highest-possible quality without using the binning process (“binning-free LEDs”).

    Energy Balance

    The energy balance of a LED is much easier to specifythan that of conventional light sources. This is because no energy is radiated in the UV and infrared region of the spectrum, which means that the energy balance comprises only visible radiant energy and heat energy. Today, White LEDs transform 20 to 30 per cent of the input power into visible light and the remaining part (70 to 80 per cent) into heat. The light percentage comes close to that of fluorescent lamps, and will soon supersede.

    System luminous efficacy

    Sometimes, luminous efficacies are specified for the bare chip. The ancillary devices described in the previous sections, which are essential for a proper functioning of LEDs, absorb light. The only realistic thing to do, therefore, is to specify luminous efficacy (and light output as well) for the total LED package. As with most conventional lamps, the luminous efficacy of LEDs is dependent on the power of the LED and on the colour quality of the light it produces. Higher-power LEDs have higher efficacies, while those with better colour rendering have lower efficacies. At the end of 2011, coolwhite LEDs were commercially available in efficacies up to slightly more than 100 lm/W, and warm-white LEDs with colour-rendering indices larger than 80 in efficacies of around 80 lm/W (all lm/W values include driver losses). These figures do not take into account losses in secondary optics. Retrofit LED bulbs with warm-white light (around 2700 K) and colour-rendering index better than 80 are available in efficacies up to 70 lm/W. Here, too, cooler-white versions are slightly more efficient.

    Lumen-package range

    Today, single LEDs exist in lumen packages varyingfrom a few lumens (indicator lamps) to some 1000 lumens. In the latter case, severe screening is called for to restrict glare, because so much light comes from such a very small light-emitting surface. By mounting multi LEDs on a printed circuit board, LED modules can be achieved with much larger lumen packages.

    Colour characteristics

    Principally, LEDs have a quasi-monochromatic, narrow band spectrum.

    The figure above shows the spectra of blue, green and red LEDs. All the colours of the spectrum can be made with the exception of a small gap in the green-yellowish region. White light can be produced by mixing different colours: as is done in RGB lighting. With some LED modules that make use of RGB mixing, the different coloured LEDs can be controlled(dimmed) individually. With such LED modules the colour of the light can be dynamically changed from white to all colours of the spectrum. White light of moderate-to-excellent colour rendering can be obtained by phosphor LEDs and can have a near continuous spectrum. By applying different phosphors, white light within the colour temperature range of 2700 K to 10,000 K can be produced. Often, the higher-colour-temperature versions have only moderate colour rendering (Ra between 50 and 75). In the lower-colour-temperature versions, LEDs are available with good (Ra larger than 80) to excellent colour rendering (Ra larger than 90 or even 95). As with all lamps, better colour quality comes at the cost of lower efficacy. White LEDs, like some fluorescent lamps, have one or more narrow peaks in their spectrum. The general colour rendering index Ra does not always give a good enough representation of the colour rendering capability of these light sources. CIE is therefore investigating new methods for assessing the colour rendering properties of white light sources with the goal of recommending a new colour-rendering metric.

    Beam control

    For lighting designers, one of the most interesting properties of a LED is its small light-emittin gsurface. This allows the creation of very accuratel ydefined beams. As an illustration of this, Fig. 11.1 8shows a near-parallel light beam made with a LEDlineluminaire that is impossible to create withconventional light sources

    Conversely small ligh temittin gsurfaces often need properly designed an dengineered screening in order to limit excessive glare .Multi-LED luminaires have also mult- light beams tha tmay cause multiple shadows because a lighted objec tis illuminated from many slightly-different directions .With RGB colour mixing, this may lead to disturbing ,multi-coloured shadows. How well the individua lbeams overlap and mix is for some applications an important quality criterion.

    LED Life

    In the case of high-performance LEDs it takes a very long time before they actually fail ̶ usually considerably more than 50 000 hours. Before that time, however, their lumen depreciation is so great that the LED is no longer giving sufficient light for most applications. Therefore, for LED lifetime specifications, the length of time that it takes to reach a certain percentage of its initial light value is used. Based on a depreciation value of 70 per cent, lifetime values of between 35,000 and 50,000 hours are common for high-performance LEDs. LED-bulbs, with their limited space for handling heat, have a lifetime of some 25 000 to 35 000 hours (25 to 35 times longer than an incandescent lamp). The temperature of the chip’s junction has an influence on both the number of actual LED failures and the precise point in time when the 70 per cent lumen maintenance point is reached.

    LED-lumen depreciation

    The electric current passing through the chip’s junction, and the heat generated in it, degrade the chip material and is so responsible for light depreciation. Decolouration of the housing and yellowing of the primary lens may be further reasons for lumen depreciation. In white phosphor LEDs, chemical degradation of the phosphor material also causes lumen depreciation. For high-performance LEDs, 30 per cent lumen depreciation is reached at around 35 000 to 50 000 hours. Lumen-depreciation values are very much dependent on what temperature the chip’s junction reaches in its application. Run-up and re-ignition LEDs give their full light output immediately after switch-on and after re-ignition.


    LEDs can be dimmed by simple pulse-width modulation down to five per cent of full light output. Not all retrofit LED lamps can be dimmed on normal, commercially-available dimmers. Special retrofit LED lamps that are designed to be dimmed on such dimmers are so specified on their packaging.

    Ambient-temperature sensitivity

    Limitation of the junction temperature of the LED chip is essential for the proper functioning of LEDs in terms of lumen output, lumen efficacy, lamp life, and even colour properties. In high-temperature environments the products perform worse, while at low temperature they perform better. The actual influence of the junction temperature is different for the different types of LEDs. In extreme temperature environments, therefore, relevant information for a particular product has to be obtained from the manufacturer. LED drivers are designed to drive LEDS on constant current. In this way the influence of mains-voltage variations is not an issue.

    UV and IR component

    LEDs radiate only visible radiation. There is no ultraviolet or infrared radiation.

    Product range

    LED products are available as:

  • single LEDs

  • multiple LEDs on flat or three-dimensional PCBboards

  • LED modules (or LED engines) with secondary optics and with or without built-in driver thatcan be used in the same way as lamps. Interfacesare standardised for interchangeability

  • multiple LEDs on strings

  • retrofit LED lamps with built-in driver
  • For users it can be important that LED modules (also called LED engines), like most conventional lamps, are interchangeable with products from different manufacturers. Zhaga,a global industry-wide cooperation, produces standard specifications for the interfaces of LED engines. The specified interfaces are not dependent on the LED technology used in the LED engine, because the LED engine is treated as a “black box”. LED engine manufacturers can therefore develop and innovate their product completely independently. Luminaire manufacturers that make use of such engines can also develop and innovate independently, because the luminaire is also treated as a black box. Interchangeability is achieved by defining interfaces for the physical dimensions, and for the thermal, electrical and photometric (beam) properties of the LED engine.


    OLEDs are flat, solid-state light sources built up of organic semi-conductor layers. In contrast to the high-brightness, quasi-point-source LED, an OLED is a planar light source of low to medium brightness. The name OLED stands for Organic Light Emitting Diode. Serious development of OLEDs only started in the mid-nineties. OLEDs can be produced to give most colours of the spectrum, and white. The technology permits the production of OLED-windows that are transparent when not switched on.

    OLED lighting products are now gradually coming onto the market in sizes up to some 30 cm x 30 cm with moderate efficacy and lifetime. The expectation is that the size, the efficacy, and the lifetime will rapidly improve. Initially, the most interesting application for OLEDs is architectural and decorative lighting. For general lighting purposes the efficacies have to be improved much further. Ultimately, white-light, large sized OLEDs with efficacies up to 150 lm/W would seem to be possible.

    Working principle

    The process that is responsible for the emission of light in OLEDs is very similar to that with LEDs: positively-charged holes and negatively-charged electrons are pushed through semiconductor layers towards each other and recombine. Part of these recombinations results in the emission of light. The colour of the light is dependent on the composition of the semiconductor material. White light can be obtained by bringing phosphorescent material in the emissive layers.


    The organic layers are placed between electrodes, the one on the light-extraction side being transparent. The layers are supported by a glass substrate and, for protection of the organic materials against oxygen and water, are sealed in glass. Development is going on to substitute the glass seal by thin-film encapsulation. This development not only reduces the thickness of OLEDs but also opens up the possibility of bendable OLEDs. The first laboratory examples have already been produced.

    Controlling Light

    Luminaires have a number of functions.The first is to accommodate one or morelamps plus any necessary control gear.Mounting, electrical connection andservicing must be as easy and safe aspossible.

    The construction of the luminairesguarantees that the user is protected fromcontact with live components (electricshock) and that there is no dangerof surrounding surfaces and objects overheating(fire prevention). Luminaires thatare to be applied under specific operatingconditions – e.g. rooms where there is adanger of an explosion, or damp or humidspaces – must be designed to meet themore stringent requirements. Besides theseelectrical and safety aspects luminairesalso have an aesthetic function asan integral part of the architectural designof a building. It is equally importantthat the form and arrangement ofthe luminaires and the lighting effects areappropriate.

    The third and perhaps most essentialtask the luminaire has to fulfil is tocontrol the luminous flux. The aim is toproduce a light distribution in accordancewith the specific functions the luminaireis required to fulfil, utilizing energy aseffectively as possible.

    Even in the days of the first artificial lightsource, the flame, luminaires were developedto ensure that the light source couldbe mounted and transported safely. Withthe advent of considerably stronger lightsources – first gas lighting and laterelectric lamps – it became more importantto construct luminaires that could controlluminance and ensure that the light wasdistributed as required.

    Luminaire technology was first confinedto providing a shielding element forthe lamp and reducing the luminous intensityof the lamp by means of diffusinglampshades or canopies. This was one wayof limiting glare, but did not controlthe distribution of the light, whichwas absorbed or able to scatter in undefineddirections. You will still find thiscombination of lamp and lampshadetoday – especially in the decorativemarket – in spite of their being relativelyinefficient.

    The introduction of reflector and PARlamps, which were used widely in the USA,marked the first step towards controllinglight purposefully and efficiently. In theselight sources the light is concentratedby the reflectors that form an integral partof the lamp and efficiently directed asrequired in defined beam angles. Incontrast to luminaires with exposed lamps,the lighting effect was therefore nolonger confined to the vicinity of theluminaire. It became possible to accentuatespecific areas from almost any pointwithin the space. The reflector lamptook on the task of controlling the light;the luminaire only served as a device tohold the lamp and as a means for limitingglare.

    One disadvantage of reflector lampswas the fact that every time you replacedthe lamp you also replaced the reflector,which meant high operating costs. Apartfrom that, there were only a few standardisedreflector types available, eachwith different beam angles, so for specialtasks – e.g. asymmetrical light distributionin the case of a washlight – there was frequentlyno suitable reflector lamp available.The demand for more differentiatedlighting control, for enhanced luminaireefficiency and improved glare limitationled to the reflector being taken from thelamp and integrated into the luminaire.This means that it is possible to constructluminaires that are designed to meet thespecific requirements of the light sourceand the task which can now be applied asinstruments for differentiated lightingeffects.

    Principles of controlling light


    In the case of reflection, the light falling onto a surface is fully or partially reflected, depending on the reflecting coefficientof this surface. Besides reflectance the degree of diffusion of the reflected light is also significant. In the case of specular surfaces there is no diffusion. The greater the diffusing power of the reflecting surface, the smaller the specular component of the reflected light, up to the point where only diffuse light is produced.

    Specular reflection is a key factor in the construction of luminaires; the purposeful control of light can be achieved through specially designed reflectors and surfaces, which also define the light output ratio.

    reflection factor (reflecting coefficient) indicates the ratio of the reflected luminous flux to the luminous flux falling on a surface


    Transmission describes how the light fallingon a body is totally or partially transmitteddepending on the transmissionfactor of the given body. The degreeof diffusion of the transmitted light mustalso be taken into account. In the caseof completely transparent materials thereis no diffusion. The greater the diffusingpower, the smaller the direct componentof the transmitted light, up to the pointwhere only diffuse light is produced.Transmitting materials in luminairescan be transparent. This applies to simplefront glass panels, or filters that absorbcertain spectral regions but transmit others, thereby producing coloured light or a reductionin the UV or IR range. Occasionallydiffusing materials – e.g. opal glass oropal plastics – are used for front covers inorder to reduce lamp luminance and helpto control glare.


    Absorption describes how the light falling on a surface is totally or partially absorbed depending on the absorption factor of the given material.

    In the construction of luminaires absorption is primarily used for shielding light sources; in this regard it is essential for visual comfort. In principle, absorption is, however, not wanted, since it does not control, but rather wastes light, thereby reducing the light output ratio of the luminaire. Typical absorbing elements on a luminaire are black multi-grooved baffles, anti-dazzle cylinders, barn doors or louvers in various shapes and sizes.

    When transmitted from one medium with a refractive index of n1 into a denser medium with a refractive index of n2 rays of light are diffracted towards the axis of incidence. For the transition from air to glass the refractive index is approx. n2/n1=1.5.

    When transmitted through a medium of a different density, rays are displaced in parallel.

    There is an angular limit for the transmission of a ray of light from a medium with a refractive index of n2 into a medium of less density with a refractive index of n1. If this critical angle is exceeded the ray of light is reflected into the denser medium (total reflection). For the transition from glass to air the angular limit is approx. 42 degrees.

    Light guides function according to the principle of total internal reflection


    When beams of light enter a clear transmitting medium of differing density – from air into glass and vice versa from glass into the air, for example – it is refracted, i.e. the direction of its path is changed. In the case of objects with parallel surfaces there is only a parallel light shift, whereas prisms and lenses give rise to optical effects ranging from change of radiation angle to the concentration or diffusion of light to the creation of optical images. In the construction of luminaires refracting elements such as prisms or lenses are frequently used in combination with reflectors to control the light.

    Typical ray tracing through an asymmetrical prismatic panel (top left), symmetrical ribbed panel (top right), Fresnel lens (bottom left) and collecting lens (bottom right).


    Interference is described as the intensification or attenuation of light when waves are superimposed. From the lighting point of view interference effects are exploited when light falls on extremely thin layers that lead to specific frequency ranges being reflected and others being transmitted. By arranging the sequence of thin layers of metal vapour according to defined thicknesses and densities, selective reflectance can be produced for specific frequency ranges. The result can be that visible light is reflected and infrared radiation transmitted, for example – as is the case with cool-beam lamps. Reflectors and filters designed to produce coloured light can be manufactured using this technique. Interference filters, so-called dichroic filters, have a high transmission factor and produce particularly distinct separation of reflected and transmitted spectral ranges.


    Reflectors are probably the most important elements in the construction of luminaires for controlling light. Both reflectors with diffusely reflecting surfaces – mostly white or with a matt finish – and highly specular surfaces are used. These reflectors were originally made of glass with a mirrored rear surface, which led to the term which is still used today: mirror reflector technology. Anodized aluminium or chrome or aluminium-coated plastic are generally used as reflector material today. Plastic reflectors are reasonably low-priced, but can only take a limited thermal load and are therefore not so robust as aluminium reflectors, whose highly resistant anodized coating provides mechanical protection and can be subjected to high temperatures. Aluminium reflectors are available in a variety of qualities, ranging from high quality super-purity aluminium to reflector swith only a coating of pure aluminium. The thickness of the final anodized coating depends on the application; for interior applications it is around 3–5μm, for luminaires to be used in exterior spacesor chemically aggressive environments up to 10 μm. The anodizing process can be applied to the aluminium coil (coil anodizing) or on the finished reflectors(stationary anodizing), which is more expensive.

    The surfaces of the reflectors can have a specular or matt finish. The matt finish produces greater and more uniform reflector luminance. If the reflected lightbeam is to be slightly diffuse, be it to attain softer light or to balance out irregularities in the light distribution, the reflector surfaces may have a facetted or hammered finish. Metal reflectors may receive adichroic coating, which can control light luminous colour or the UV or IR component. Light distribution is determined to a large extent by the form of the reflector. Almost all reflector shapes can be attributed to the parabola, the circle or the ellipse.

    Ray tracing from a point light source when reflecting on circle(top-left), ellipse(top-right), parabola(bottom-left) and hyperbola(bottom-right).

    Parabolic reflectors

    The most widely used reflectors are parabolic reflectors. They allow light to be controlled in a variety of ways – narrow beam, wide-beam or a symmetrical distribution, and provide for specific glare limitation characteristics. In the case of parabolic reflectors the light emitted by a light source located at the focal point of the parabola is radiated parallel to the parabolic axis. The more the light source deviates from the ideal point source – in relation to the diameter of the parabola – the more the rays of light emitted will diverge. If the reflector contour is constructed by rotating a parabola or parabolic segment around its own axis, the result is a reflector with narrow-beam light distribution. In the case of linear light sources a similar effect is produced when rectangular reflectors with a parabolic cross section are used.

    If the reflector contour is constructed by rotating a parabolic segment around an axis, which is at an angle to the parabolic axis, the result is a reflector with wide-beam to batwing light distribution characteristics. Beam angles and cut-off angles can therefore basically be defined as required, which allows luminaires to be constructed to meet a wide range of light distribution and glare limitation requirements.

    Parabolic reflectors can also be applied with linear or flat light sources – e.g. PAR lamps or fluorescent lamps, although these lamps are not located at the focalpoint of the parabola. In this case the aim is not so much to produce parallel directional light but optimum glare limitation. In this type of construction the focal point of the parabola lies at the nadir of the opposite parabolic segments, with the result that no light from the light source located above the reflector can be emitted above the given cut-off angle. Such constructions are not only possible in luminaires, but can also be applied to light control systems; parabolic louvers – e.g. in skylights – direct the sunlight so that glare cannot arise above the cut-off angle.

    Parabolic reflector for glare limitation for flat and linear light sources. When the focal point is located at the nadir (1) of the opposite parabolic segment no light is radiated above the cut-off angle .

    Parabolic reflector with short distance between focal point and apex of the reflector; reflector acts as a cut-off element. (left) Parabolic reflector with greater distance between focal point and apex of the reflector; no shielding for direct rays.(center) Parabolic reflector with greater distance between focal point and apex of the reflector, spherical reflector as a shielding element.(right)

    Parabolic reflector with intense directional light (left). Wide-angle parabolic reflector with cut-off angle Π(right).

    Darklight reflectors

    In the case of the above-mentioned parabolicreflectors clearly defined light radiation– and effective glare limitation – isonly possible for exact, point light sources.When using larger radiating sources –e.g. frosted incandescent lamps – glarewill occur above the cut-off angle; glareis visible in the reflector, although thelamp itself is shielded. By using reflectorswith a variable parabolic focal point(so-called darklight reflectors) this effectcan be avoided; brightness will then onlyoccur in the reflector of larger radiatingsources below the cut-off angle, i.e. whenthe light source is visible.

    Darklight technology. By using reflectors with a variable parabolic focal point no light is radiated above cut-off angle, especially in the case of volume radiating sources.

    Spherical reflectors

    Through the calculation of specific reflector contours various cutoff angles and distribution characteristics can be obtained for the same ceiling opening and lamp geometry.

    In the case of spherical reflectors the light emitted by a lamp located at the focal point of the sphere is reflected to this focal point. Spherical reflectors are used predominantly as an aid in conjunction with parabolic reflectors or lens systems. They direct the luminous flux forward onto the parabolic reflector, so that it also functions in controlling the light, or to utilize the light radiated backward by means of retro reflection back toward the lamp.

    Involute reflectors

    Here the light that is emitted by the lamp is not reflected back to the light source, as is the case with spherical reflectors, but reflected past the lamp. Involute reflectors are mainly used with discharge lamps to avoid the lamps over-heating due to the retro-reflected light, which would result in a decrease in performance.

    Involute reflectors: light radiated by the lamp is reflected past the light source.

    Elliptical reflectors

    In the case of elliptical reflectors the light radiated by a lamp located at the first focal point of the ellipse is reflected to the second focal point. The second focal point of the ellipse can be used as an imaginary, secondary light source.

    Elliptical reflectors are used in recessed ceiling wash lights to produce a light effect from the ceiling downwards. Elliptical reflectors are also ideal when the smallest possible ceiling opening is required for downlights. The second focal point will bean imaginary light source positioned at ceiling level; it is, however, also possible to control the light distribution and glare limitation by using an additional parabolic reflector.

    Elliptical reflectors in double-focus downlights (left), wallwashers (middle) and spotlights (right).

    Lens systems

    In contrast to prismatic louvers, lenses are used practically exclusively for luminaires for point light sources. As a rule the optical system comprises a combination of one reflector with one or more lenses.

    Collecting lenses

    Collecting lenses direct the light emitted by a light source located in its focal point to a parallel beam of light. Collecting lenses are usually used in luminaire constructions together with a reflector. The reflector directs the overall luminous flux in beam direction, the lens is there to concentrate the light. The distance between the collecting lens and the light source is usually variable, so that the beam angles can be adjusted as required.

    Collecting lens (left) and Fresnel lens (right). The beam angle can be varied by changing the distance between lens and light source.

    Fresnel lenses

    Fresnel lenses consist of concentrically aligned ring-shaped lens segments. The optical effect of these lenses is comparable to the effect produced by conventional lenses of corresponding shape or curvature. Fresnel lenses are, however, considerably flatter, lighter and less expensive, which is why they are frequently used in luminaire construction in place of converging lenses.

    The optical performance of Fresnel lenses is confined by aberration in the regions between the segments; as a rule the rear side of the lenses is structured to mask visible irregularities in the light distribution and to ensure that the beam contours are soft. Luminaires equipped with Fresnel lenses were originally mainly used for stage lighting; in the meantime they are also used in architectural lighting schemes to allow individual adjustment of beam angles when the distance between luminaires and objects varies.

    Projecting systems

    Projecting systems comprise an elliptical reflector or a combination of spherical reflector and condenser to direct light at a carrier, which can be fitted with optical accessories. The light is then projected on the surface to be illuminated by the main lens in the luminaire. Image size and beam angle can be defined at carrier plane. Simple aperture plates or iris diaphragms can produce variously sized light beams, and contour masks can be used to create different contours on the light beam. With the aid of templates (gobos) it is possible to project logos or images.

    In addition, different beam angles or image dimensions can be selected depending on the focal length of the lenses. In contrast to luminaires for Fresnel lenses it is possible to produce light beams with sharp contours; soft contours can be obtained by setting the projector out of focus.

    Projector with projecting system: a uniformly illuminated carrier (1) is focussed via a lens system (2). The ellipsoidal projector (left) with high light output, and the condenser projector (right) for high quality definition.

    Prismatic systems

    Another optical means of controlling light is deflection using prisms. It is known that the deflection of a ray of light when it penetrates a prism is dependent on the angle of the prism. The deflection angle of the light can therefore be determined by the shape of the prism.

    If the light falls onto the side of the prism above a specific angle, it is not longer refracted but reflected. This principle is also frequently applied in prismatic systems to deflect light in angles beyond the widest angle of refraction.

    Prismatic systems are primarily used in luminaires that take fluorescent lamps to control the beam angle and to ensure adequate glare limitation. This means that the prisms have to be calculated for the respective angle of incidence and combined to form a lengthwise oriented louvre or shield which in turn forms the outer cover of the luminaire.


    Many luminaires can be equipped with accessories to change or modify their photometric qualities. The most important are supplementary filters, which provide coloured light, or reduce the UV or IR component. Filters may be made of plastic foil, although glass filters are more durable. Apart from conventional absorption filters there are also interference filters(dichroic filters) available, which have high transmission and produce exact separation of transmitted and reflected spectral components.

    Wider and softer light distribution can be achieved using flood lenses, whereas sculpture lenses produce an elliptical lightcone. Additional glare shields or honeycomb anti-dazzle screens can be used to improve glare limitation. In the case of increased risk of mechanical damage, above all in sports facilities and in areas prone to vandalism, additional protective shields can be fitted.

    Luminaires I

    Stationary Luminaires

    -- an integral part of the architecture. Occasionally it is possible to vary light direction, but rigid mounting usually means that the light direction is also fixed. Stationary luminaires can be further subdivided according to luminaire characteristics and design.


    In their basic form, downlights radiate light vertically downwards. They are usually mounted on the ceiling and illuminate the floor or other horizontal surfaces. On vertical surfaces – e.g. walls – the light patterns they produce have a typical hyperbolic shape (scallops).

    Downlights are available with different light distributions. Narrow-beam downlights only light a small area, but give rise to fewer glare problems due to their steep cut-off angle. Some downlight forms have supplementary louvre attachments in the reflector aperture as an extra protection against glare.

    In the case of downlights with darklight reflectors the cut-off angle of the lamp is identical to the cut-off angle of the luminaire, thereby producing a luminaire with optimal wide-angle light distribution and light output ratio.

    Recessed downlight for incandescent lamps. Darklight Technology, where the cut-off angle of the lamp is identical to the cut-off angle of the luminaire.

    Double-focus downlight with ellipsoidal reflector and additional parabolic reflector with especially small ceiling aperture.

    Washlights with darklight reflectors and additional ellipsoidal segment for the wall lighting.

    Directional spotlight with adjustable reflector lamp and darklight reflector. Directional spotlights can generally be rotated 360° and inclined to 40°. This means that the directional spotlight can be directed at both horizontal and vertical surfaces.

    Air-handling downlight designed for an incandescent lamp. The convection heat produced by the lamp is removed with the air flow.


    In contrast to downlights, uplights emit light upwards. They can therefore be used for lighting ceilings, for indirect lighting by light reflected from the ceiling or for illuminating walls using grazing light. Uplights can be mpounted on or in the floor or wall.

    Up-downlights combine a downlight and an uplight in one fixture. These luminaires are applied for the simultaneous lighting of flor and ceiling or for grazing lighting over a wall surgace. They are available in wall and pendant versions.

    Sectional drawing of a recessed floor luminaire for halogen reflector lamps.

    Wall-mounted combined uplight and downlight for PAR lamps.

    Louvered Luminaires

    Louvered luminaires ae designed for linear light sources such as fluorescent lamps. Their name derives from their anti-dazzle attachments that may be anti-glare louvers, light controlling specular reflectors or prismatic diffusers.

    Being fitted wth linear light sources of low luminance louvered luminaires produce little or no modeling effects. They generally have wide-beam light distribution, with the result that louvered luminaires are predominantly used for olighting wide areas.

    Louvered luminaires are usually long and rectangular in shape (linear fluorescents); square and round versions are also availahble for compact fluorescent lamps. Similar to down lights, they are available for recessed or surface mounting or as pendant fixtures.

    Louvred luminaire for fluorescent lamps with darklight reflector and involute upper reflector.

    Mounting options for louvered luminaires: recessed ceiling, surface, mounting on tracks, walls, floor-standing or pendant mounting.

    2 versions of louvered luminaires:
    1) luminaire with transverse louvers (top)
    2) luminaire with parabolic louvers (bottom)

    2 versions of louvered luminaires:
    1) luminaire with parabolic louvers and prismatic lamp diffuser for improving contrast rendition (top)
    2) luminaire with prismatic louver (bottom).

    Lamp arrangement in louvered luminaires:
    1) standard arrangement above the transverse louvres (top left)
    2) Lamp position to increase the cut-off angle (centre left)
    3) Twin-lamp version with lamps arranged horizontally and vertically (below left and top right)
    4) Lateral position for asymmetrical light distribution (centre right)
    5) Twin-lamp version with twin-louvre (below right).

    Asymmetric & Direct-Indirect Louvered Luminaires

    Asymmetric Louvered Luminaires radiate light in one direction. Used for uniform lighting of walls or to avoid glare caused by light projected onto windows or doors.

    Direct-indirect louvered luminaires are suspended from the ceiling or wall-mounted. They produce a direct component on horizontal surfaces beneath the luminaire and at the same time light the ceiling and provide diffuse ambient lighting.

    Asymmetric louvered luminaires:
    1) The wall can be lit by tilting the symmetrical reflector

    2) Lighting using a wallwasher with an elliptical side reflector

    3) Lighting without a wall component (e.g. in the vicinity of a window) using a luminaire with a flat side reflector.

    Typical light distribution curves for louvered luminaires:
    1) direct-indirect luminaire with a predominantly direct component

    2) direct-indirect luminaire with a predominantly indirect component

    3) indirect luminaire.

    Wash Lights

    -- designed to provide uniform lighting over extensive surfaces, mainly walls, ceilings and floors, therefore. They are included in the group downlights and louvred luminaires, although washlights do have their own luminaire forms.

    Wall Washers

    Wallwashers illuminate walls and depending on how they are designed, also a part of the floor. Stationary wallwashers are available as recessed and surface-mounted luminaires.

    1) Wallwasher for compact fluorescent lamps.

    2) Wallwasher with ellipsoidal reflector for halogen lamps.

    3) Wallwasher with sculpture lens and reflector attachment for reflector lamps.

    Wallwasher for fluorescent lamps :

    1) The direct light component is cut off, the reflector contour produces especially uniform lighting over the wall surface.

    2) a supplementary prismatic diffuser below ceiling level provides light directly from the top of the wall.

    3) Cantilever-mounted wallwasher.

    Ceiling Washlights

    Ceiling washlights are designed for brightening or lighting ceilings and for indirect ambient lighting. They are installed above eye height on the wall or suspended from the ceiling. Ceiling washlights are generally equipped with tungsten halogen lamps for mains voltage or with high-pressure discharge lamps.

    Wall-mounted ceiling washlight. The reflector contours produce uniform ceiling lighting.

    Versions of ceiling washlights:

    1) wall-mounted

    2) freestanding luminaire mounted on a stand

    3) suspended version..

    Floor washlights

    Floor washlights are mainly used for lighting hallways and other circulation zones. Floor washlights are mounted in or on the wall at relatively low levels.

    Wall-mounted floor washlight. The direct light component is restricted, the reflector shape produces uniform lighting of the floor.

    Floor washer versions:

    1) Round and

    2) Square versions for incandescent lamps or compact fluorescent lamps

    3) Rectangular version for fluorescent

    Movable Luminaires

    In contrast to stationary luminaires movableluminaires can be used in a variety oflocations; they are generally used in tracksystems or in light structures. Movableluminaires usually also allow changesin light direction, they are not confinedto a fixed position, but can be adjusted andrepositioned as required.


    Spotlights are the most common form of movable luminaires. They illuminate a limited area, with the result that they are rarely used for ambient lighting but predominantly for accent lighting. In view of their flexibility with regard to mounting position and light direction, they can be adjusted to meet changing requirements. Spotlights are available in a variety of beam angles. Their narrow-beam light distribution provides for the lighting of small areas from considerable distances, whereas the wider light distribution inherent in wide-beam spotlights means that a larger area can be illuminated using a single spotlight. Spotlights are available for a wide range of light sources. Since the aim is generally to produce a clearly defined, narrow beam, designers tend to opt for compact light sources such as incandescent lamps, halogen lamps and high-pressure discharge lamps, occasionally also compact fluorescent lamps. Wide-beam spotlights are mainly designed for larger lamps, such as double-ended halogen lamps and high-pressure discharge lamps or compact fluorescent lamps, whereas point sources, such as low-voltage halogen lamps or metal halide lamps provide an especially concentrated beam of light. Spotlights can be equipped with reflectors or reflector lamps. Some models can be equipped with converging lenses or Fresnel lenses to vary the beam angle. Spotlights with projecting systems allow a variety of different beam contours by the use of projection of masks or templates (gobos). Another characteristic of spotlights is that they can be equipped with a wide range of accessories or attachments, such as flood or sculpture lenses, colour filters, UV or infrared filters and a range of antidazzle attachments, such as barn doors, anti-dazzle cylinders, multigroove baffles or honeycomb anti-dazzle screens.

    A distinction is made between narrow beam angles of approx. 10° (spot) and wide-beam angles of approx. 30° (flood).

    An especially wide beam angle of approx. 90° is characteristic for floodlights designed for the lighting of wall surfaces.

    Spotlights for low-voltage halogen lamps can be operated on low-voltage tracks; the transformer can be mounted on the ceiling or be in an exposed position on the track (above)

    When operating on mains voltage tracks the transformer is usually integrated into the adapter or mounted on the luminaire (below)


    Wallwashers are not only available as stationary luminaires, but also as movable luminaires. In this case it is not so much the light direction that is variable, but the luminaire itself. On track, for example, movable wallwashers can provide temporary or permanent lighting on vertical surfaces. Movable wallwashers are generally equipped with halogen lamps for mains voltage, metal halide lamps or with fluorescent lamps (linear and compact types).

    Different versions of movable wallwashers. They can be adjusted to different wall heights and distances.

    Wallwashers for halogen lamps and compact fluorescent lamps and linear fluorescent lamps.

    Secondary Reflector Luminaires

    The widespread use of personal computer workstations in modern-day office spaces has led to a greater demand for improved visual comfort, above all with regard to limiting direct glare and discomfort glare. Glare limitation can be provided through the use of VDT-approved luminaires, or through the application of indirect lighting installations.

    Exclusively indirect lighting that provides illumination of the ceiling will avoid creating glare, but is otherwise ineffective and difficult to control; it can produce completely uniform, diffuse lighting throughout the space. To create differentiated lighting and provide a component of directed light, it is possible to combine direct and indirect lighting components in a two-component lighting system. This may consist of combining task lighting with ceiling washlighting, or the use of direct-indirect trunking systems.

    The use of secondary reflectors, which is a relatively new development, makes for more comprehensive optical control. This means that the ceiling, which represents an area of uncontrolled reflectance, is replaced by a secondary reflector which is integrated into the luminaire and whose reflection properties and luminance can be predetermined. The combination of a primary and a secondary reflector system produces a particularly versatile luminaire, which is able to emit exclusively indirect light as well as direct and indirect light in a variety of ratios. This guarantees a high degree of visual comfort, even when extremely bright light sources such as halogen lamps or metal halide lamps are used, and while still being possible to produce differentiated lighting.

    Direct-indirect and indirect secondary reflector luminaire

    Direct-indirect and indirect secondary reflector luminaire

    Fiber Optic Systems

    Light guides, or optical fibres, allow light to be transported at various lengths and around bends and curves. The actual light source may be located at a considerable distance from the light head. Optical fibres made of glass are now so well developed that adequate amounts of luminous flux can now be transmitted along the fibres for lighting applications.

    Fibre optics are used above all in locations where conventional lamps cannot be installed due to size, for safety reasons or because maintenance costs would be exorbitant. The especially small-dimensioned fibre ends lend themselves perfectly to the application of miniaturised downlights or for decorative starry sky effects. In the case of showcase lighting, glass display cases can be illuminated from the plinth. Thermal load and the danger of damaging the exhibits are also considerably reduced due to the fact that the light source is installed outside the showcase.

    In the case of architectural models several light heads can be taken from one strong central light source, allowing luminaires to be applied to scale.

    Typical light guide fixtures:
    1) Downlight with reflector (top-left)
    2) Downlight with lens optics (bottom-left)
    3) Directional spotlight with reflector (top-right)
    4) Directional spotlight with lens optics (bottom-right).

    Fibre optic system comprising projector, flexible light guides and individual light heads for the ends of the fibres. Inside the projector the light emitted by a lowvoltage halogen lamp is focussed onto the Common End of the bundle of fibres by means of an elliptical reflector. The light is then conveyed along the individual light guides, and emitted at the ends of the fibres through the attached light heads.

    Luminaires II

    A luminaire is a device that controls the distribution of the light emitted by a lamp or lamps and that includes all the items necessary for fixing and protecting the lamps (and sometimes the gear) and for connecting them to the electricity-supply circuit. In American English, the term fixture is usually used instead of luminaire, while in Anglo-Saxon countries, the term fitting is also sometimes used instead of luminaire.

    The principal characteristics of luminaires can be listed under the following headings:

  • Optical
  • Mechanical
  • Electrical
  • Thermal
  • Aesthetics
  • Luminaire characteristics are widely different for the different areas of lighting application, namely indoor general lighting, indoor accent lighting, road lighting, and flood lighting. These differences also call for different types of photometric documentation.

    Optical characteristics

    The optical characteristics of a luminaire determine the shape of its light beam, or light-output distribution. The light distribution of a luminaire defines how the luminous flux radiated by the lamp or lamps is distributed in the various directions within the space around it. Different lighting applications require different light distributions and thus different luminaires.

    The desired light distribution of a luminaire is obtained through the application of one or more of the physical phenomena:

  • reflection
  • refraction
  • diffuse transmission
  • Many luminaires also make use of shielding in one form or another, principally to obtain the required degree of glare control and to limit light pollution. The shielding function may be performed by refractors or diffusers or by mirror reflectors, by white-painted surfaces or, where very stringent glare control is required, by black surfaces. Typical techniques employed to control light distribution are illustrated in figure below on the basis of road-lighting examples.

    Control of light by means of reflection and screening by the reflector and by the luminaire’s body(top), by refraction and screening by the luminaire’s reflector body(middle), and by diffusion through an opal enclosure and screening by reflector cap (bottom)


    Many conventional luminaires are provided with a reflector (sometimes in conjunction with another light-control element) in order to create the appropriate light distribution. The reflecting material that is used for reflectors can be specular, spread or diffuse. The three basic types of reflection:

  • Specular reflectors
  • Specular reflectors (also called high-gloss mirror reflectors) are used when a precise form of light distribution is required, as in floodlights, spotlights and road-lighting luminaires. The reflector creates multiple images of the light source. The most widely used material is sheet aluminium, which has the strength needed to produce a stable reflector. To obtain a highly-specular finish, the aluminium ispolished: mechanically, chemically, electrolytically, or by a combination of these processes. Reflectance values are around 0.70. Alternatively, commercial- grade aluminium can be clad with a thin layer of super purity aluminium or silver. With aluminium, reflectance values of up to 0.80 can be obtained, while with silver a reflectance of more than 0.90 is possible. Finally, there is vacuum metalising, in which a specular layer of aluminium is deposited on a suitably-smooth substrate(metal, glass or plastics). The resulting reflectance, which is somewhere between 0.80 and 0.90, is dependent on both the substrate material and the quality of the metalising process.

  • Spread reflectors
  • With spread reflectors (sometimes also called half matt reflectors) there is no sharp mirror image of the light source. They are employed where amoderate degree of optical control is required, with the emphasis on producing a beam with smooth transitions. Such reflectors also help to smoothout discontinuities in the light distribution caused by inaccuracies in the shape of the reflector. Spread reflection is produced by hammering very small dimples and bumps into a specular surface or by brushing or etching it.

  • Diffuse reflectors
  • At the other extreme from specular reflection is diffuse reflection, which is also called matt reflection. Here light incident on the reflector is scattered in all directions, so there is no mirror image of the light source. Matt reflectors cannot provide sharp beam control, but are employed where diffuse or nonfocused light distributions are required. Matt-finished metals and white glossy paints on metal or glossy white plastics provide near-diffuse reflection. The small specular component due to the gloss is of no practical optical significance; the gloss merely serves to facilitate cleaning. Reflectance values can be in the range 0.85 to 0.90. Ceramic materials or finishes have completely-diffuse reflection characteristics with extremely high reflectances of up to 0.98.

    Reflector forms

    There are three basic reflector forms: plane, curved, and facetted.

    Basic reflector forms.

    Plane reflectors - When using a simple plane, or straight-sided, reflector the light emitted by the light source is reflected according to the material of the reflecting surface, viz. specularly or diffusely. Plane reflectors are often used to screen off the direct light from the light source. Accurate beam shaping is not very well possible with plane reflectors, but by changing the symmetry of the reflectors, the direction in which the bulk of the light is emitted can be changed.

    Curved reflectors - The best optical performance is obtained when using a curved reflector. Depending on the curvature, many different types of beams can be created. A curved reflector may be cylindrical, parabolic, elliptical, hyperbolic, or some other contour to suit a particular application. The circular and parabolically-shaped reflectors are the ones most commonly used. In some cases, these shapes can be combined.

    Cylindrical, parabolic and combinations of circular and parabolic reflector shapes.

    The most important optical property of a parabolic reflector is that a point source of light placed in its focus will produce a parallel beam of reflected rays with the greatest intensity in its centre. If the lightsource is not at the focus but in front or behind it, the reflected rays are no longer parallel. Thus, by choosing the position of the lightsource relative to the focus point, the desired beamshape (narrow to wide) can be created. Since a lampis never a real point source, deviations from the theoretical beam shape for a point light source as sketched above, will always occur. The smaller the light source relative to the size of the reflector, the more accurately can the beam be shaped.

    Facetted reflectors - Smooth-curved reflectors have to be produced to a high degree of accuracy, because even small deviations from their intended shape will produce undesirable discontinuities in their light distribution (striations). This will not occur with a facetted reflector. A facetted reflector consists of a number of adjacent, plane or curved, facets that together approximate a curve, that is an approximation of a parabolic curve. The width of beam produced by the facetted reflector is somewhat greater than that of a smooth-curved refelctor.


    Refractors are used to create the desired luminaire light distribution by passing the light from the source through a refractor. The angle through which the light is bent is dependent on both the shape of the refractive material and its refractive index (Snell’s law).

    Bending of light by refraction (according to Snell’s law) from the incident αi to the refraction αr.

    Refracting devices are either lenses or prisms. The type of refractor most commonly employed in indoorlighting is the lens found in tubular fluorescent lamp luminaires intended for general lighting. It consists of a horizontal plastic panel which is mounted just below the lamp. The panel is flat on the top and has a special pyramidal (prism) or lens structure on the underside, which directs the light in certain directions and reduces the brightness under specific angles. Where in the past prismatic controllers were used with relatively large-sized prisms, we see today more advanced micro-prism or micro-lens-type refractors that give more accurate possibilities to shape the light distribution. These types of refractors are also used to produce LED luminaires for general indoor lighting and for road lighting. Refracting glass bowls were in the past sometimes used for high-pressure mercury and sodium road lighting luminaires. They have become obsolete because they are heavy, but more so because lighting control in the upwards direction, and therefore control of light pollution, is not easily attainable.

    Micro-lens type of refractor for a LED luminaire for general indoor lighting. (below) LED road-lighting luminaire with lenses (bottom: enlarged lens panel).


    Translucent diffusers enlarge the apparent size of the light source. They scatter the light of the lamp in all directions without defining its light distribution. They serve mainly to reduce the brightness of the luminaire and thus the glare created by it. Diffusers are made of opal glass or translucent plastic, commonly acrylic or polycarbonate. The material should besuch that it scatters the light whilst producing the minimum amount of absorption.


    Screening the lamp from direct view

    Screening is employed to hide the bright lamp or lamps from direct view. The degree to which a lamp is hidden from view is expressed by the shielding angle α: the larger the shielding angle, the better the degree of shielding. The higher the brightness (luminance) of the lamp, the more strict are the requirements for the shielding angle (viz. larger shielding angle required).

    Luminaire screening as defined byscreening angle α.

    The luminaire reflector housing itself, or a built-in baffle, can provide the screening function. When the sole purpose of the louvre is to shield the lamp from view, diffuse-reflecting material is used, such as a white-plastic louvre or, in the case of flood lights, matt-black metal rings.

    Lamp shielding by the reflector itself (left) and by an internal baffle (right). Simple louvre (left) to shield the lamp in a fluorescent-lamp luminaire, and (right) a flood light louvre.

    Shielding devices are often combined with the function of defining the light distribution, in which case highly-reflective material is used for the louvre. In luminaires for fluorescent lamps, relatively cheap flat profile reflector louvres are often used to shield thelamp from direct view when viewed lengthwise. Parabolic reflector louvres, in addition to screening the lamp from direct view, also redirect the light downward thus improving the efficiency of the luminaire because the light refected from the louvre effectively contributes to defining the light distribution.

    Flat-profile louvre elements (left) and parapolic louvre (right). Prevention of disturbing reflections in display screens With poorly-designed indoor-lighting luminaires it is not just a direct view of the lamps that can be disturbing, but also the fact that the lamps can produce disturbing reflections in visual-display (e.g.computer) screens. With poor-quality screens in particular, these reflections may hamper legibility. However, the louvres needed for shielding the lamp from direct view can be so designed that these disturbing reflections are minimized for all directions around the luminaire. (Philips term: Omni directional Luminance Control, OLC). Louvre design that provides omni directional control of disturbing reflections in display screens.

    Colour filters

    In certain lighting applications, in particular display lighting and decorative flood lighting, colour is sometimes used to help achieve the desired aesthetic effect. In the past, colour filters attached to luminaires containing white light sources were extensively used for this purpose. Both absorption and dichroic(interference) filters were used, although absorption filters in particular lower the efficacy of the total lighting system. Typical transmittance values are, for blue absorption filters 5 per cent, for green absorption filters 15 per cent, and for red absorption filters 20 per cent. The consequence of the light absorption is that these filters become warm, which with high-power flood lights may damage the filter. The solution where such flood lights are employed is to use dichroic filters, which are a more expensive alternative. Today, coloured LED light sources are normally employed where coloured lighting is required. Here the colour comes directly from the lamp itself, so the efficacy of the lighting system is much higher.

    Absorption-type colour filters.

    Light-distribution characteristics

    The light distribution of a luminaire defines how the luminous flux radiated by the luminaire is distributed in the various directions within the space around it. This is also called luminous intensity distribution, since it is specified in terms of luminous intensities in all the directions in which the luminaire radiates its light). The luminous intensity diagram can be thought of as ”the light fingerprint” of a luminaire, in digital form (I-Table), and is the basis of all lighting calculations.

    Light distribution of a luminaire given by its luminous intensity diagram. The arrows represent the luminous intensities (I) in the directions specified. Here the light distribution is given for all planes, although it is usually only given for one (e.g. the blue curve) or two, mutually perpendicular, planes.

    Basic photometric data that can be calculated from the light distribution are the beam spread and the luminaire light output ratio. For all types of luminaires and for all types of application these data provide an insight into the photometric quality of the luminaire.Closely related to this data are application-specificphotometric data, as contained in iso-illuminance and iso-luminance diagrams, work-space utilisation factors, road-surface luminance and illuminance yield factors,and glare-specific diagrams. The basic photometric data will be described in more detail in the following paragraphs.


    Coordinate system

    To specify the complete light distribution (luminous intensity distribution) of a luminaire in all directions,two different standardised systems of co-ordinates are employed. The actual system chosen depends on the type of luminaire, the type of lamp, the way the luminaire is mounted in normal use, and its application. The right choice of system facilitates the measurement process and simplifies subsequent lighting calculations. Usually, the system used for indoor and road- lighting luminaires is the socalled C-Gamma abbreviated to C-γ system, and for floodlights the B-Beta abbreviated to B-β system.

    C-γ system of coordinates The C-Gamma system of coordinates used for representing the light distribution of a luminaire.

    In the C-γ system of co-ordinates the axis of rotationof the C-planes is vertical and passes through thecentre of the luminaire. The position ofa particular C-plane is defined by the included angleC (00 to 3600) between it and the C=00 referenceplane.31 A direction in a particular C-plane is indicated by the angle Gamma (γ), which ranges from 00 (down)to 1800 (up).

    Luminous intensity (I) table

    The luminous intensity or I-Table of a luminaire is the digital form of the light distribution of a luminaire and is the basic input for all lighting-calculation software. It is specified in the C – Gamma co-ordinate system(Table 15.1).

    Table 15.1 Part of a table of luminous intensity values for a specific luminaire.B-β system of coordinates The B-β system of coordinates used for representing the light distribution of a floodlight luminaire.

    In the B-β system, which is the system used forfloodlights, the axis of intersection of the B-planescorresponds to the axis of rotation of the floodlight. The position of a particular B-plane isdefined by the angle B (00 to 1800) between it andthe B=00 reference plane. The reference plane(B=0°) is perpendicular to the front glass of the floodlight. Angle B can be positive or negative.

    There is a second reference plane, perpendicular to the floodlight’s axis of rotation and passing through thecentre of the unit. This is called the “main plane”.A direction in a particular B-plane is indicated bythe angle Beta (β), while a direction in the mainplane is indicated by the angle B. With this systemof coordinates it is possible to define the luminousintensity distribution of a given floodlight in the mainplane over a range of B angles, or in any B-plane overa range of Beta angles.


    Polar intensity diagram

    Fig. 15.20 The polar luminous intensity diagram. Left:indoor-lighting “TL” luminaire, right: road-lightingluminaire. Blue curve: C 0° - 180° plane, green curve:C 90° - 270° plane, red curve: C plane throughdirection of maximum intensity.

    The polar luminous intensity diagram provides a good indication of the shape of thelight distribution of a luminaire. In this diagram, theluminous intensity is presented in the form of curvesand given in cd /1000 lm of the nominal lamp flux ofthe lamps employed. Each curve represents one Cplane. Curves are given for only the more importantplanes. Where the light distribution of the luminaireis rotationally symmetrical, which is the case formany downlights, projectors and industrial highbayluminaires, only one curve (for one C plane) isindicated in the diagram. This is because all other Cplanes have the same luminous intensity distribution.When the light distribution is not symmetrical, morecurves are indicated.

    In top figure below is an example of a common “TL” luminaire intensity distribution. The continuous blue line is the intensity distribution in a plane across the luminaire. The dotted green line is the intensity distribution in a plane through the length of the luminaire.

    In bottom figure above is an example of a road-lighting luminaire intensity distribution. That the light distribution across the luminaire is asymmetrical is easily noticeable from the green curve: more light is radiated towards the road rather than along the curbside. For road-lightingluminaires, the light distribution curve through the Cplane where the maximum intensity is radiated is alsousually shown (red curve).

    Planes for which luminous intensitycurves are given for road-lighting luminaires (red isdirection of maximum intensity). 15.2.3

    Beam spread

    For well-defined beams as obtained from spotlights,downlights and floodlights, the term beam spread orbeam width is used to distinguish one type of beam from another. Beam spread is defined as the angle,in a plane through the beam axis, over which theluminous intensity drops to 50 per cent of its peakvalue. To provide extra information on thebeam characteristics, the beam spread for another percentage (for example 10 per cent) is sometimes stated as well. Care needs to be taken with axially symmetric luminaires that the beam spread figurerefers to the total angle. Some literature gives beamspread as the angle from the axis. As an example, the polar curve below would normally be described as a60 degree beam but sometimes as 30 degree or 2 x30 degree.

    Beam spread defined by angle β on basisof ½ Imax (50 per cent of peak intensity).

    The terms “narrow beam”, “medium beam” and “wide beam” are frequently used to describe the beam spread (in the plane of interest) of a luminaire. An often-used, but by no means generally accepted, definition of these terms is that based on 50 per cent of the peak-intensity beam-spread value:

  • narrow beam: LT 20°
  • medium beam: 20° to 40°
  • wide beam: GT 40°
  • Classification of different beam spreadsbased on ½ Imax. 15.2.4

    Light output ratio

    The optical devices that shape the light distributionof a luminaire absorb light. This is the reason whythe total lumen output of the luminaire is lower thanthe lumen output of the lamp (or lamps) inside theluminaire. The light output ratio (η) is the ratio ofthe total lumen output of the luminaire to the lumenoutput of the lamp(s):

    η=φ luminaire / φ lamp

    The light output ratio of a luminaire is also, somewhat misleadingly, called “luminaire efficiency”. A truly efficient luminaire is one that has a light distribution that brings the light of the lamp efficiently to the spot or area where it is needed. For example, a barelamp luminaire with just a lamp holder and no optics has a very high light output ratio and thus very high “luminaire efficiency”, but is very inefficient in bringing the light to a particular area because it radiates light in all directions. A luminaire with suitable optics and thus a lower light output ratio can easily bring more of its light to the area required.


    Mechanical characteristics

    The mechanical function of the luminaire housing is threefold: it accommodates the various component parts of the luminaire, such as the optical system and the various components of the electrical system; protects these against external influences; and provides the means of mounting the luminaire in the installation.


    Sheet steel

    Sheet steel is generally chosen for the manufacture of tubular fluorescent luminaire housings for use indoors. The pre-painted sheet steel from the roll is white with diffuse reflection properties. Thus, after having been shaped in the luminaire factory into the desired luminaire form, no finishing-off operations are required.

    Stainless Steel

    Stainless steel is widely used for many of the small luminaire components, such as clips, hinges, mounting brackets, nuts and bolts, that have to remain corrosion free.

    Aluminium alloys

    Aluminium alloys, in which other elements havebeen added to the pure aluminium to improvetheir mechanical, physical and chemical protective properties, are used to manufacture cast, extruded and sheet-metal luminaires. Cast aluminium refers to the process in which molten aluminium alloy is poured (cast) in a mould. Extrusion is the process in which softened aluminium alloy is pressed through the openings of a die. Cast and extruded aluminium alloys are much used in housings for floodlight, road, and tunnel-lighting luminaires because they can be employed in humid and damp atmospheres without having to add protective finishes.

    Sheet aluminium is chiefly employed in luminaires for reflectors. The reflectors are anodized toimprove their reflection properties and to protectthem from becoming matt.


    Plastics are used for complete luminaire housings, for transparent or translucent luminaire covers, and for many smaller component parts. All-plastic houses can of course only be employed for light sources that have a relatively low operating temperature.

    Plastic covers are of methacrylate or polycarbonate. Methacrylate maintains its high light transmission properties over a long period, but its impact resistance is relatively low. The impact resistance of polycarbonates is very high and thus offers a high degree of protection against vandalism. It can be chemically treated to protect it from yellowing under the influence of ultraviolet radiation.


    Although glass is heavy, glass covers are used where these have to be positioned close to a light source having a high operating temperature. This is the case, for example, with HID flat-cover road-lighting luminaires and with most floodlighting luminaires. Two sorts of glass are used:

  • normal glass, where no special demands are placed on heat resistance
  • hard glass, where heat resistance, chemical stability and resistance to shock are important. Should hard glass break, it will disintegrate into small pieces.
  • Luminaires made completely out of glass are extremely heavy, and nowadays are seldom employed.


    Ceramic material is used in compact housings that are exposed to very high temperatures.


    All luminaires should have housings of sufficient rigidity to withstand normal handling, installation and use. With indoor-lighting luminaires for fluorescent lamps, stiffness and rigidity of construction is particularly important, since these lamps are relatively large and awkward to handle. Perhaps the most critical part of a luminaire as far as strength is concerned are the mounting brackets. The strength required here is covered by a safety factor: the mounting bracket(s) must be able to support at least five times the weight of the luminaire itself. With road-lighting and outdoor flood lighting luminaires, the mounting brackets must also be strong enough to withstand the highest conceivable wind loading for the location. Here a good aerodynamic shape for the luminaire can be advantageous, as it also serves to reduce the strength required for the lighting mast. The term “windage” is used to refer to the projected area of the luminaire against the wind. The smaller the windage, the lower is the resistance to the wind.

    Under some circumstances, the impact resistance of the luminaire itself is also important, particularly where protection against vandalism is called for. Reference can be made in this respect to standards that specify the required impact resistance for different conditions of use. These conditions vary from being able to withstand the impact of a falling object of 200 gram from a height of 10 cm (low impact resistance), to withstanding the impact of a 5 kg object from a height of 40 cm (vandal-proof luminaires).

    Resistance to pollution and humidity

    The atmosphere can contain many potentially corrosive gases which, in the presence of moisture vapour, will form highly-corrosive compounds. In all areas where this danger exists (notably in outdoor applications, indoor swimming pools and certain industrial premises) luminaires made from corrosion resistant materials or having protective finishes should be used. In such areas, the luminaire should protect the optical and electrical components it houses. It should, of course, be fully enclosed. The degree of protection provided by the luminaire is classified according to the International Protection code (IPcode) as described in an international IEC standard. The IP code consists of two numerals: IP • •.

  • The first numeral classifies the degree of protection against the ingress of solid foreign bodies (ranging from fingers and tools to fine dust) and protection against access to hazardous parts.
  • The second numeral classifies the degree of protection against the ingress of moisture. The higher the IP values, the better the protection.
  • IEC classification of lumiaires according to protection against harmful ingress of water (second digit of IP code).

    Dust and watertight (or waterproof) luminaire covers must always be used in conjunction with a sealing strip in combination with a strip channel for maximum effect. Due to the variation in temperature between the air inside and that outside the luminaire after switching on or off, pressure differences across the luminaire’s cover-seal are bound to occur. This effect is referred to as “luminaire breathing”. The seal should prevent corrosive gases, moisture and dust from being sucked into the luminaire during cooling off. The effectiveness with which the front cover seals the luminaire against ingress of solids and liquids, and the durability of this sealing function, is determined by the type and quality of the sealing material employed.

    Ease of installation and maintenance

    Many luminaires are of such a shape, size and weight as to make mounting them a difficult and time consuming operation. Mounting, but also relamping and cleaning, must usually be carried out high above ground level. So the ergonomic design of the luminaire should be such as to make these operations as easy and as safe as possible to perform. For example, covers should be hinged so that the electrician has his hands free to work on the lamp and gear. A good, ergonomically designed luminaire is one that can be mounted in stages: first the empty housing or a simple mounting plate, which is light and easily handled, then the remaining parts.

    Electrical characteristics

    The electrical function of a luminaire is to provide the correct voltage and current for the proper functioning of the lamp in such a way as to ensure the electrical safety of the luminaire.

    Lamp holders

    The most usual types of holder are the Edison screw, the bayonet and the pin. Most Edison screw and bayonet holders are made of plastics or porcelain, with metal parts for carrying the current. Porcelain is resistant to high temperatures and has a high voltage-breakdown resistance, which is important considering the high ignition voltage of HID lamps. The pin lamp holders for tubular fluorescent and compact fluorescent lamps are nearly always made of plastic. The metal contacts are spring loaded to ensure a constant contact pressure.

    Electrical wiring

    The electrical wiring in a luminaire must be such as to ensure electrical safety. This necessitates great care in the choice of wire used and its installation. There are a great many different types of wire available, in both single-core (solid) and multi-core (stranded) versions, all with various cross-sectional areas and clad with various thicknesses and qualities of insulation.

    Single-core wire is much stiffer than stranded wire, which means that fewer cable fasteners are needed to hold it in position. It is also easier to strip and is more suitable than stranded wire for the internal wiring of a luminaire. However, single-core wire is not suitable for use in luminaires that are subject to vibration and shock. In such cases a stranded wire must be used. The cross-sectional area (thickness) of the wire must be matched to the strength of the current flowing through it. The insulation of the wire used must be resistant to the high air temperature in the luminaire and the temperatures of the luminaire materials with which it is in direct contact. This is true not only under normal conditions of operation, but also in the presence of afault condition.

    Mains connection

    The method used to connect a luminaire to the power supply must be both quick and safe. The practice generally adopted is to incorporate a connection block in the housing, although prewired luminaires in which the electrical connection to the mains is automatically made when the unit is placed in position are also available.

    Electrical safety classification

    The electrical safety classification drawn up by the IEC embraces four luminaire classes.

    Table 15.4 IEC electrical safety classes.

  • Class 0: Applicable to ordinary luminaires only. These are luminaires having functional insulation, but not double or reinforced insulation throughout and with no provision for earthing.
  • Class I: Luminaires in this class, besides being electrically insulated, are also provided with an earthing point connecting all those exposed metal parts that could become live in the presence of a fault condition.
  • Class II: This class embraces luminaires that areso designed and constructed that exposed metalparts cannot become live. This can be achievedby means of either reinforced or double insulation. They have no provision for earthing.
  • Class III: Luminaires in this class are thosedesigned for connection to extra-low-voltage circuits (smaller than 50 VRMS, referred to as Safety Extra-Low Voltage, SELV). They have no provision for earthing. Figure below shows examples of luminaires of these fourclasses.
  • Thermal characteristics

    Temperature control

    A considerable amount of the electrical energy supplied to the lamp is converted into heat. The ballast adds to this heating effect within the luminaire. To protect the ballast from overheating, it is sometimes, especially with high-power lamps, screened off from the heat radiation of the lamp and placed in a separate compartment of the luminaire. With very high-powered lamps, it should be placed outside the luminaire in a special ballast box. For a given lamp/ballast combination, the working temperature reached by the luminaire is dependent upon three factors:

  • The volume of the luminaire. The greater the volume, the lower will be the temperature rise inside the luminaire.
  • The ease with which the heat generated within the luminaire can be conducted through it to the surrounding air. One way of promoting air flow through the housing is to make use of heat conducting materials in its construction. Most metals are good in this respect, while plastics, on the other hand, are thermal insulators and cannot therefore be employed as housing materials where high-power lamps are involved.
  • The cooling effect of the surrounding air. Good heat dissipation calls for large surface areas to bein contact with the surrounding air. Luminairesfor high-power lamps, such as high-bayluminaires, floodlights, and some LED luminaires, that are very sensitive to high temperatures, are therefore provided with cooling fins. Some types of industrial luminaires are provided with air vents in the top of the housing to allow the warm air to escape.
  • Luminaires are designed to meet the conditions under which they are most likely to be used. Themaximum ambient temperature, Ta, at which aluminaire can be operated safely, is indicated onthe type label on the product. If no temperature indication is given, the product is intended for use ata maximum ambient temperature of 25°C, which isthe case for the majority of luminaires designed forindoor applications. The use of luminaires above theirspecified maximum ambient temperature may reducesafety margins and will generally lead to a reduction of the lifetime of the various luminaire components. Luminaires designed for industrial applications have higher ambient-temperature limits, as high as 40°Cto 45°C, and in special cases even higher. Manymanufacturers offer for example products suitable for50°C which is often a requirement in the Middle Eastmarkets. 15.5.2 Protection against flammability Luminaire flammability The flammability of a luminaire operating under fault conditions is an issue with luminaires made of plastics. As an example: acrylic is superb optically, but a realfire hazard in large sheets. The combustion behaviour of luminaires is not just material dependent, it alsodepends on the shape and thickness of the luminairehousing. The IEC has defined a so-called glow-wiretest that assesses the fire hazard at different glowwiretemperatures (Table 15.5).

    Table 15.5 IEC fire-hazard classes.

    Flammability of mounting surfaceLuminaires cannot simply be mounted on any typeof surface ̶ there is the degree of fire risk to be considered. This is determined by the combinationof surface flammability and the temperature of theluminaire mounting plate.Whilst it is always safe to mount luminaires onnon-flammable building materials such as concreteand stone, other surface materials impose certainlimitations as specified in the IEC protection classes(Table 15.6). Luminaires for discharge lamps bearingan F-sign are also suitable for mounting on buildingsurfaces that do not ignite below 200°C, whileluminaires for discharge lamps bearing an FF-signhave a lower surface temperature and so can even bemounted on easily-flammable surfaces (Table 15.6). Most Philips indoor luminaires for surface mounting are of the F class. In very dusty environments where dust collected on top of the luminaire might ignite in the event of a fault, FF classified luminaires are required.

    Table 15.6 IEC protection classes of luminaires with regard to the flammability of the mounting surface.


    No less important than the functional characteristics of a luminaire is what is termed its aesthetic or visual appeal, that is to say its appearance and styling. In interiors, all non-recessed luminaires are clearly visible, and so whether switched on or not their design should be in harmony with that of the interior. In outdoor lighting, it is usually only the daytime appearance of the luminaires, when these are clearly visible, that is important: particularly in built-up areas, their design can make a positive contribution to the attractiveness of the locality.


    Indoor luminaires

    General lighting

    The term general lighting is used to denote the substantially uniform, functional lighting of a space without provision for special local requirements. Often, but not always, the general lighting is supplemented with accent or task lighting. General lighting luminaires can be divided according to their light distribution into direct, indirect and direct-indirect types. The direct-indirect types are available with differing upward and downward components. If the downward component is clearly larger than the upward component, the term semi-direct is used, while the versions where the upward component is the larger are referred to as being semi-indirect luminaires.

    From left to right: direct, indirect and direct-indirect types of luminaires.

    Recessed luminaires

    Ceiling-recessed luminaires are mostly of the direct type, although a small upward component (semidirect)can be obtained if the light-emitting surface is not flush with the ceiling and specific optics are employed. There are luminaires for various types of modular and non-modular ceiling systems. Certain types of recessed luminaires may be employed as air-handling luminaires.

    Recessed luminaires with a direct (top) and semi-direct (bottom) light distribution.

    Surface-mounted luminaires

    Surface-mounted luminaires are also normally of the direct type, but semi-indirect versions can beproduced as well. Many general lighting luminairesof the same type are available in both recessed andsurface-mounting versions, sometimes also in aversion for suspended mounting.

    Surface-mounted luminaires: for LED (left)and for TL.

    Suspended luminaires

    Suspended luminaires are available in a wide range of direct, semi-direct and semi-indirect versions. Theuse of suspended luminaires facilitates localisationof the general lighting, whereby some of the generallighting is concentrated on actual work places. Whilstthe shape of a recessed luminaire is hidden in theceiling, that of a suspended luminaire is clearly visible.Its design appearance therefore becomes much more important.

    With suspended luminaires, both the light characteristics and the design appearance are important.

    Free-standing luminaires

    For localised general lighting or for additional workspacelighting, free-standing luminaires can offer a flexible solution. Efficient versions makeuse of tubular fluorescent, compact fluorescent and LED light sources.

    Accent lighting


    Examples of surface-mounted and railmounted projectors for LED (left) and compact CDM.

    Projectors or spotlights are directional lighting luminaires that are used mainly to provide accent lighting. The projector obtains its directional light control in one of three ways: from the lamp itself, with its built-in reflector; from an external reflector built-into the luminaire; or from a combination of these two. Depending on the system employed, the beam spread varies from wide and medium to narrow. Most projectors have a joint construction so that the direction of the beam can easily be adjusted. Multiple projectors mounted in a single frame permit of a multiplicity of beam directions from one location.

    Projectors make use of compact metal halide, whiteSON, halogen and, more and more, LED light sources.

    Multi-projector, frame-mounted lighting system.


    Examples of recessed downlights: left, LED and right, compact fluorescent.

    Downlights are, in effect, spotlights recessed into, mounted on, or suspended from the ceiling. They are used not so much for the lighting of objects but for providing additional, concentrated lighton certain areas of a space. They therefore usuallyhave a medium or wide beam spread.Wall washersWall washers are used where a relatively large areahas to be uniformly lighted. School blackboards anddisplay shelves in shops are examples of where wallwashers might be employed for functional reasons.But wall washing also has an aesthetic or visual appeal.The luminaires are ceiling or wall mounted (recessed,surface mounted or suspended) and have theappropriate asymmetrical light distributions.MarkersThe small size of LEDs makes them extremelysuitable for recessed or surface-mounted wall andfloor markers. Besides their functional use in markinglocations in space and guiding people through space,they can also create stunning ‘light art’ and lightaccents.

    Example of wall and floor markers (LED).

    Industrial lighting

    Batten luminaires

    Batten luminaires are basic linear fluorescent or LED luminaires specifically designed for ease of mounting. Mounting is mostly surface or cablesuspended. They come in a variety of IP classes (upto waterproof IP 66 or 67) to suit many different industrial conditions. Separate accessories that caneasy be mounted to the basic luminaire, includereflectors, diffusers and louvres for lamp screening.

    .jpg Batten luminaires: with tubular fluorescentlamps and simple reflector, and with LEDs. Light lines An example of a light-line system.

    Light-lines make use of a trunking (support) systemto which lines of batten-type of luminaires specifically designed for that trunking system can be quickly and easily mounted. The trunking system consists of a strong rail that has two functions: it carries the luminaires and gear and it houses the electrical wiring for the supply of power (and, where relevant, for controlling different switching or dimmingsteps). The trunking system itself can be either surface mounted or suspended. The trunking sections are electrically joined together by coupling connectors that do not call for the use of tools. After the trunking system is installed the luminaires are simply clicked onto the trunking system. Spotlights with a special adapter can also be clicked onto the trunking system.

    The components of a trunking system: the catenary adapter (left), the trunking section(middle), and the cable track that is incorporated inthe trunking section (right). Luminaires mounted on the trunkingsystem. High and low-bay luminaires

    Mounting heights above approximately five metrescall for the use of luminaires housing powerful HIDlamps. With these high-intensity lamps good glarecontrol calls for high-precision optics. Depending onthe mounting height, narrow-beam, medium-beamor wide-beam light distributions are needed. In manyrotationally-symmetrical low and high-bay luminairessuch distributions can be obtained by moving the lampup or down in the reflector Many of these luminairesare of the suspended type, with built-in electrical gear.

    High or low-bay HID luminaire.

    These luminaires can be rather heavy and awkward to handle, and yet must often be mounted high up in industrial halls. They are therefore often designedas two easily-assembled parts: the lamp housingwith its control gear including electrical wiring, andthe reflector. Once the former is mechanically fixedin position and connected to the supply, a suitable reflector is simply clipped into position. Especially in industrial areas, the luminaires are equipped with a hard-glass cover. Depending on the intended application, these luminaires have either a relativelylow IP class (non-industrial areas with high ceilings) ora high IP class of up to 65 (dusty and humid industrialareas).


    Outdoor luminaires

    Outdoor lighting installations are designed to providetraffic safety (road lighting, tunnel lighting and urbanlighting), personal security (urban lighting), a pleasantand inviting night-time environment (urban lightingand architectural outdoor lighting), and the possibilityto work and play outdoors after dark (sports and arealighting).


    Road lighting

    Luminaires for road lighting with the focus on traffic safety are either mast, wall or suspension (span-wire)mounted. Their light distribution is such that the main vertical plane of symmetry lies at right angles to the longitudinal axis of the road, thus throwing the main part of their light along the road. Where inthe past both reflectors and large refractors were used to create that light distribution, today most road-lighting luminaires make use of reflectors or,in the case of LED luminaires, also refractor lenses. Mast-mounted road-lighting luminaires make use of two different mounting systems: bottom entry (posttop) or side-entry sockets. The side-entryversions are also used for wall mounting. Span-wire,or catenary, luminaires are suspended from a cable by means of brackets on their topside. The strength of the mounting sockets or brackets of road-lighting luminaires is important because these luminaires can be buffeted by strong winds. The aerodynamic shapeof the luminaire plays an important role in this respect as well. Not only must the bracket be strong enough to withstand the highest wind loading, it must also be rigid enough to prevent vibration of the housing, as this can lead to premature lamp failure and fracture of the support bracket. Side-entry and post-top mounting. Notethe screens on the luminaire on the right.Often the luminaire has a separate compartment forthe electrical control gear, but sometimesthe gear is located in the bottom of the mast.

    Luminaires with separate compartments for the control gear (driver) and for the lamp/reflector. Left: HID luminaire, right: LED luminaire (note the cooling fins of the LED lamp unit) 15.8.2

    Urban lighting

    The lighting installation in an urban environmentshould enhance the safety and security of motorists,cyclists and pedestrians, but it must also be pleasingand inviting as well. In addition to the road-lightingluminaires described in the previous section, in urbanareas we therefore see an ever-increasing number ofluminaires with a design that helps to give the area itsown pleasing identity. The relatively large freedom ofluminaire design when using small LED units is muchappreciated in this respect. At the relatively-low mastheights used in urban areas (3 m to 8 m), the daytimeappearance of the mast-luminaire combination is veryimportant. Masts, mounting brackets and luminairesin many different styles are employed, includingcustomized designs. Sometimes masts and luminairesare completely integrated into one product: true light columns.

    Mast-luminaire combinations as a singleproduct.

    The light distribution of urban lighting luminaires on long stretches of road has to be similar to that of “normal” road-lighting luminaires (casting the main part of their light along the road). On shorter stretches, and in squares, plazas and courts, a more rotationally-symmetrical light distribution is used.

    Architectural floodlighting

    Architectural highlighting of urban landmarks is another powerful tool used to give identity to an urban environment. Relatively small spot lights and flood lights based on LEDs can, because of their small light-emitting surface, give very narrow, near-parallel light beams. This makes them particularly suitable for the lighting of landmarks in an unobtrusive way. The luminaires are also produced with wider beams.

    Near-parallel light beam with a LEDline luminaire. Examples of LED floodlights for architectural outdoor lighting. Left: narrow-beam LEDline; middle: colour-variable floodlight; right:surface-mounted and recessed floodlights. All withpossibility for dynamic colour changing.

    High-power floodlights making use of both compactand normal HID lamps are employed for architecturaloutdoor lighting as well.

    Examples of HID floodlights for architectural outdoor lighting. 15.8.4

    Tunnel lighting

    Tunnel lighting luminaires are either wall or ceiling mounted. Because of the limited space in tunnels, the height of the luminaire is especially critical. The tunnel interior is always lighted by luminaires having a symmetrical light distribution relative to the axis of the tunnel. The tunnel entrance, where very high daytime lighting levels are required, can be lit with luminaires having a transverse symmetrical (the main beam is directed across the tunnel), an axialsymmetrical, or an asymmetrical counter-beam (aimedtowards oncoming traffic) light distribution.

    Different types of light distribution for tunnel-lighting luminaires.

    Whatever type of light distribution is chosen, it has to be fine-tuned for the actual tunnel width and tunnel height in question. Modular luminaire designs that allow for flexibility in lamp and optics combinations in one and the same housing are therefore essential.

    Examples of a modular tunnel-lightingluminaire concept.

    Sports and area lighting

    Floodlights for sports and area lighting make use ofall the different types of high-intensity HID lamps upto a power of 2000 W. LED floodlights are availableas multi-LED units for the intermediate intensityversions. The mounting height of the floodlightsvaries from some 10 m for the somewhat lowerpoweredunits to more than 40 m for the highpowerunits. Ease of mounting is essential wherethe higher mounting heights are concerned. Threebasic types of light distribution are employed:rotationally symmetrical (circular or ellipsoidal lightpattern); symmetrical about two perpendicular planes(rectangular light pattern); and asymmetrical. It is oftenwrongly thought that all rotationally-symmetricalfloodlights have a rotationally-symmetrical lightdistribution, whereas very often they have in fact anasymmetrical distribution. This is either because of theasymmetrical reflector shape or because of a built-inreflective screen to prevent glare.

    Examples of floodlight light distributions.Top left: asymmetrical light distribution (floodlightwith support for removable aiming telescope); topright: plane symmetrical light distribution about twoplanes; bottom: LED floodlight with asymmetricallight distribution.

    Most floodlight types are available in wide, mediumbeamand narrow-beam versions. (The latter in caseof asymmetrical beams in one of the main planes). Since a floodlight must be aimed once it is secured in position, it is equipped with a sturdy mounting bracket that allows the unit to be rotated in the two vertical and horizontal planes. Many floodlights are equipped with a simple gun-sight aiming device or, where more accurate or complex aiming is required, a support for mounting a telescopic sight.


    Luminaires always have to comply with theappropriate safety rules. ENEC (European NormsElectrical Certification) is the European mark fordemonstrating compliance with all European SafetyStandards. UL (Underwriters’ Laboratories) is thesimilar USA mark. Both certificationinstitutes know both prototype testing and testing ofthe production process.

    ENEC and UL certification marks.

    The number in the ENEC mark indicates the country ofthe institute that has given the European approval.

    Lighting Design Concepts

    The scientific application of artificial lighting is a relatively young discipline. In contrast to daylighting, which looks back on a tradition that developed gradually over several thousand years, the need to develop concepts for artificial lighting only became a requirement in the last century or two. Just 200 years ago planning using artificial light sources was confined to deciding where best to position the few candles or oil lamps available. Not really what might be referred to as adequate lighting design. Only in the last one hundred years, with the rapid development of efficient light sources, has lighting design acquired the tools that allow artificial lighting to be produced with adequate illuminance levels. This development is accompanied, however, by the task of defining the objectives and methods behind this new discipline, of deciding on the criteria by which the artificial light that is now available is to be applied.

    Quantitative Lighting Design

    The first and to date most effective concept has given rise to a set of standards or criteria for the lighting of workplaces. While decisions with regard to lighting in the private sector can be limited to the choice of suitably attractive luminaires, there is a clear interest in the field of the lighting of workplaces to develop effective and efficient forms of lighting. The main concern is which illuminance levels and types of lighting will ensure optimum visual performance, high productivity and safety at operating costs which are affordable.

    Guide values for illuminance E for various areas of activity in accordance with CIE recommendations.

    Both aspects of this task were examined in detail, i.e. both the physiological question of the correlation of visual performance and lighting, and the technical question of establishing criteria by which the quality of a lighting installation can be measured. The concept of quantitative lighting design with illuminance as the central criterion, followed by uniformity, luminous colour, shadow quality and the degree of glare limitation, developed at a relatively early stage. Taking such criteria as a basis, standards were compiled containing minimum illuminance levels on the relevant working area for a wide variety of activities, plus the minimum requirements for the other quality criteria.

    In practice, this would appear to require uniform, mostly horizontally oriented lighting over the entire space, which could best be effected by a regular arrangement of luminaires, e.g. continuous rows of fluorescent linear luminaires or louvred downlights. The illuminance level in each case is designed – in accordance with the demand for uniform lighting – to meet the requirements of the most complicated visual tasks that can be expected in the given space. The inevitable result is that the overall lighting is far too bright for all the other activities which will occur in the area.

    There is no doubt that quantitative lighting design concepts of this kind are successful within the framework of the task that has been set, as explained above. There is a proven correlation between the quality of the light and visual performance and this corresponds to the definable effect which the quality of the lighting has on efficiency and safety at the workplace. It is therefore justified to maintain that the standard lighting conditions recommended for a technical office must be different from those for a warehouse.

    But when consideration is given to the lighting required for working areas with different or changing activities the limits of quantitative lighting design concepts soon become apparent. If the task is to light a drawing board and a CAD workstation, for example – nowadays a frequent constellation – it soon becomes clear that the high illuminance level required for the drawing board is disturbing to the person working at the computer, indeed that the vertical light component required for work at the drawing board may make it impossible to work at the computer.

    The limits of quantitative lighting design soon become apparent in the actual field of application it was intended for, the lighting of workplaces. Questionable though it is, this kind of lighting philosophy is widespread and is generally adopted as the standard for architectural lighting. On closer investigation it is apparent that quantitative lighting design is based on an extremely simplified perception model. Our entire environment is reduced to the term “visualtask”; all architectural considerations, the information content and aesthetic quality of the visual environment are disregarded. The same applies to the definition of the perceiving person, who is effectively considered to be a walking camera – the only aspect that is taken into account is the physiology of our vision; the psychology of perception is disregarded altogether.

    Design concepts based on these principles, which only serve to ensure safety and a sound working economy, and ignore any other requirements the perceiving person may have of his visual environment, can lead to problems at the workplace. Outside the working environment the user would inevitably find such lighting inadequate; the lighting solution will remain clearly inferior to the feasible possibilities.

    Luminance-based design

    A quantitative lighting design concept that is oriented primarily towards providing a recommended illuminance level, lacks the criteria to develop a concept that goes beyond the requirements that would ensure productivity and safety to meet the needs of the architecture and the persons using the architectural space.

    In response to this problem a newkind of lighting philosophy is developed, as represented in Waldram’s “designed appearance” and Bartenbach’s “stable perception” models, for example. The aim of this approach is a process that not only provides adequate lighting for visual tasks but is also able to describe and plan the optical effect of an entire space.

    To be able to plan the visual effect of an environment the central reference quantity has to be changed. Instead of illuminance, which describes only the technical performance of a lighting installation, it is luminance that becomes the basic criteria–-a dimension that arises from the correlation of light and lit environment, thereby forming the basis for human perception. Changing the central quantity to luminance means that brightness and contrast ratios can be established for the entire perceived space, be it between visual task and background, between individual objects or between objects and their surroundings.

    This change of criteria does not make much difference to the lighting of visual tasks at the workplace, since the effect of different contrast ratios on visual performance are known and have been taken into consideration according to the degree of difficulty defined for the specific visual tasks. This does not apply for the lighting effect in the entire space. Considering luminance levels in this way means that brightness ratios produced by a lighting installation in correlation with the architecture and the illuminated objects can be ascertained and lighting concepts developed based on the distribution of brightness.

    Designing concepts based on luminance levels is therefore not so much a question of simply changing reference criteria, but more of expanding the design analysis to the entire space. Whereas up to now an overall illuminance level was planned for the whole space based on the visual requirements for the most complicated visual task, the approach now is to plan luminance levels for all areas of a visual environment.

    This means that it is now possible to differentiate between the various visual tasks performed in a space, to define room zones where the lighting is adjusted to the specific activities carried out in these areas. At this stage it is possible to refer to the standards and recommendations laid down for quantitative lighting design when planning the lighting for the individual visual tasks.

    Design concepts based on luminance levels are, however, not confined to ascertaining spatial zones, which in turn would only lead to splitting a space up into a series of conventionally lit areas. The mainfeature of this kind of planning is that it is not directed towards the lighting for visual tasks, but towards the brightness ratio between the visual tasks and their respective surroundings, the balance of all luminances within one zone. This presumes that the lighting for one zone only allows optimum “stable” perception when the luminance contrasts do not exceed, or fall below, certain values. The aim is to create a constellation within which the visual task forms the brightest area in the environment, thereby holding the attention of the viewer. The luminance level of the surroundings should therefore be lower so as not to distract the person’s view, but remain within a defined range of contrast. The permissible scale of contrasts is a result of the state of adaptation of the eye while perceiving the visual task – in a “stable” environment the eye retains a constant state of adaptation, whereas an “instable” environment leads to a continuous, tiring adaptation through too low or too high background luminances.

    When planning a “stable” spatial situation with controlled luminance distribution it is no longer possible to base concepts on a standardised lighting installation. Just as luminance is a result of the correlation of the illuminance and reflectance of the surfaces, the lighting installation and the material qualities must be planned together when designing concepts based on luminance levels. The required luminance contrasts cannot only be achieved by varying the lighting, but also by determining surrounding colours. The most prominent example is the lighting for the Museum of Art in Berne, where the especially intense and bright appearance of the paintings on display is not achieved by increased illuminance levels, but through the grey colour of the walls. In this situation the paintings have a higher luminance than the relatively dark surrounding area, which makes their colours appear particularly intense-–similar to projected slides or lighting using framing projectors.

    At first glance this appears to be a promising concept, which avoids the weak points of quantitative lighting design and provides criteria for a perception-oriented design theory. Considerable doubts have arisen from the perceptual psychology sector, however, as to whether luminance and luminance distribution are suitable criteria for a lighting design theory based on human perception.

    Luminance is indeed superior to illuminance in as far as it forms the basis for perception – light itself is invisible. It can only be perceived when it is reflected by objects and surfaces. Luminance, however,is not identical to the brightness we actually perceive; the luminance pattern onthe retina only provides the basis for our perception, which is completed through complex processes in the brain. This also applies to luminance scales that are adjusted to the state of adaptation of the eye or the conversion into equidistant grades of brightness – there is no direct correlation between the image actually perceived and the luminance pattern on the retina.

    If luminance were the only factor that determined our perception, we would be helplessly exposed to an array of confusing patterns of brightness produced by the world around us. We would never be in a position to distinguish the colour and reflectance of an object from different lighting levels, or perceive three-dimensional forms. It is nevertheless exactly these factors of constancy, the forms and material qualities around us, that our perception is aimed at; changing luminance patterns only serve as an aid and a starting point, not as the ultimate objective of vision.

    Only with knowledge of the lighting conditions and with the aid of constancy phenomena can interpretations be made of the luminance pattern on the retina, and a familiar three-dimensional image arise from the mass of confusing parts. The brightness ratios that we actually perceive may deviate considerably from the underlying luminance pattern. In spite of its higher luminance a grey, overcast sky seen above a field of snow will appear to be darker than the snow. The decline in luminance over a wall surface lit from anangle is likewise ignored, whereas it has an increased effect on the sides of a cube. Colour ratios and grey values are thus corrected in differently lit areas, with theresult that we perceive a consistent scale.

    In every case, the registering ofluminances is deferred in favour of theconstant qualities of objects, which isinherent to our perception: the acquisitonof information about a given environmentclearly has higher priority than mereoptical images. This central aspect ofthe way we process information cannot betaken into consideraion by a theory ofperception based on luminance, however.Similar to quantitative lighting design,luminance technology adheres to a purelyphysiological concept, which reducesthe perceptual process to the creation ofoptical images in the eye, ignoring allother processes that take place beyondthe retina. The information content of ourperceived environment and the interestthis environment awakens in the perceivingbeing cannot be explained by this model -but it is this very interplay of informationand interests that allows the perceivedimage to be processed, the relativity ofluminances to be apprehended and theluminance patterns in the eye to be reinforcedor ignored.

    If the aim of perception is to process information,and if it takes place dependingon the information provided, it cannotunder any circumstances be examined irrespectiveof the information contentprovided by or inherent in a specific environment.In the light of this fact, anyattempt to define a set of general rulesfor lighting that are not based on aconcrete situation is of doubtful validity.This also applies to the attempt to makean abstract definition of “stable” lighting situations, which is what luminance based design strives to do.

    A general definition of the conditions required for the development of psychological glare - the most extreme form of an “unstable”, disturbing lighting situation- will fail due to the fact that the information content pertaining to the relevant glare sources is not available. It becomes apparent that glare does not only depend on stark luminance contrasts, but also on lack of information content with regard to the surface producing the glare. It is not the window with a view over the sunlit countryside that gives rise to glare, but– in spite of its lower luminance factor –the pane of opal glass that prevents this very view; a blue summer sky with a few clouds is not a source of glare, but the uniform grey-white sky of a dull day in November.

    If it is not possible to find an abstract definition for an “unstable” milieu, the attempt to describe ideal luminance patterns out of context is unsound. Maximum luminance contrast values of 1:3 or 1:10 between the object of attention and the proximate or broader ambient field have been laid down that confine the lighting designer’s range of expression to a dull average. Phenomena such as brilliance and accentuated modelling, which play a considerable role in imparting information about materials in our environment, are practically excluded; luminance situations such as we experience on any sunny day or on a walk in the snow, are considered to be unreasonable. But you can only decide whether a lighting situation is pleasing or unreasonable when you experience a specific situation; luminance contrasts on the beach are not too stark for someone taking a stroll, but they will bother someone who is trying to read a book.

    Just as the brightness we actually perceive cannot be derived from luminance, it is impossible to conclude the exact lighting conditions which are necessary to ensure good perception simply by examining the contrast range of a lit environment; the lighting designer is obliged to examine each specific situation, the information it provides and the perceptual requirements of the users of the space.

    The difficult aspect of evaluating the quality of lighting concepts is the exceptionally vast adaptability of the human eye: a perceptual apparatus that is able to provide usable results at 0.1 lux on a clear night or 100 000 lux on a sunny day, is not substantially disturbed in its performance by luminance contrasts of 1:100, and is entirely capable of balancing the effects of inadequate lighting design. It is therefore not surprising that lighting installations that do not take into account the essential requirements of the perceiving person generally meet with acceptance. Dissatisfaction with the lighting at a workplace, for example, is frequently not recognised by the person concerned as a result of poor lighting design –criticism is usually aimed in the direction of the innocent “neon lamp”.

    Progress made in the field of lighting design can therefore not be evaluated by simply differentiating between inadequate and optimum, or clearly correct or incorrect lighting solutions. In the case of the lighting of workplaces a quantitative design concept may prove to be clearly successful, even when the lighting is exclusively adjusted to optimising visual performance. Luminance technology can also be regarded as a step in the right direction; the expansion of the design analysis from the visual task to the entire space and the development of zone oriented lighting design all make for progress, and in turn have a positive effect on the quality of the lighting.

    Even if lighting solutions can be achieved using quantitative processes that are acceptable within the broad spectrum of visual adaptability, it does not mean to say that lighting has been designed that will comply with all essential perceptual requirements. Both quantitative lighting design and luminance technology remain at the level of purely physiologically oriented design, which does not provide any reliable criteria apart from the isolated consideration of visual tasks. Luminance technology is equally not able to keep both promises – the designer’s prediction of what the visual effect will be and the creation of (perceptually speaking)optimum, “stable” lighting situations; it is therefore not realistic to lay down a set of abstract criteria for brightness distribution that do not relate to a specific situation.

    The principles of perception oriented lighting design

    The main reason for our dissatisfaction with lighting concepts based on quantitative lighting design or luminance technology is the fact that they adhere strictly to a physiologically oriented view of human perception. Man is only seen as a mobile being for processing images; his visual environment is reduced to the mere “visual task”, at the best to an overall perceptual understanding of “table” and “wall”, “window” and “ceiling”. Seen from this angle,it is only possible to analyse a minor portion of the complex perceptual process, which comprises the eye and an abstract comprehension of the world around us; no attention is paid to the person behind theeye and the significance of the perceived objects.

    Only when we begin to go beyond the physiology of the eye and take a closer look at the psychology of perception can the conditions required for the processing of visual information be fully understood and all the factors involved in the correlation between the perceiving being, the perceived objects and light as a medium for allowing perception to take place be taken into account. In a concept that understands perception as more than a process for handling information, the visual environment is more than just a configuration of optically effective surfaces. In this way both information content and the structures and aesthetic qualities of a piece of architecture can be analysed adequately. Man is no longer seen as merely recording his visual environment, but as an active factor in the perceptual process– an acting subject, who can constructan image of a visual environment based on a wide variety of expectations and needs.

    Only when two main factors are correlated– the structural information provided by a visual environment and the needs of a human being in the given situation– does the so-called pattern of significance of a space develop. Only then is it possible to analyse the ranking of importance which relate to individual areas and functions. On the basis of this pattern of significance it is possible to plan the lighting as the third variable factor in the visual process and to design it accordingly. The need for orientation aids varies radically depending on the type of environment– light applied for guiding people through spaces can be of prime importancein a congress centre with constantly changing groups of visitors, whereas this task is considerably less important in familiar environments. The decision whether to graze the surface structure of a wall with light depends on whether this structure presents essential information, e.g. about its character as a medieval stone wall or whether such lighting will only reveal the poor quality of the plaster work.

    Perception-oriented lighting design, which is directed at the human being and his needs, can no longer be directed in primarily quantitative terms relating to illuminance and the distribution of luminance. To achieve lighting that is suitable for a given situation it is necessary to developa set of qualitative criteria, an entire vocabulary of terms, which can describe the requirements a lighting installation has to meet and comprise the functions of the light with which these requirements can be fulfilled.

    Richard Kelly

    A basic description of the various functions of light as a medium for imparting information – was developed in the fifties by Richard Kelly, a pioneer of qualitative lighting design.

    Kelly describes the first and basic form of light as ambient light. This is the light that provides general illumination of our environment. It guarantees that the surrounding space, plus the objects and persons in it, are visible. This form of overall, uniform lighting ensures that we can orient ourselves and carry out general tasks. It is covered to a large extent by the ideas underlying quantitative lighting design, except that ambient light in the Kelly sense is not the aim of a lighting concept, but only a basis for further planning. The aim is not to produce overall lighting of supposedly optimum illuminance, but differentiated lighting that can be developed taking ambient light as the basic level of lighting.

    To achieve differentiation, a second form of lighting is required that Kelly refers to as focal glow. This is the first instance where light becomes an active participant in conveying information. One important aspect that is taken into account here is the fact that our attention is automatically drawn towards brightly lit areas. It is therefore possible to arrange the mass of information contained in an environment via the appropriate distribution of brightness – areas containing essential information can be emphasized by accent lighting, information of secondary importance or disturbing information toned down by applying lower lighting levels. This facilitates the fast and accurate flow of information, the visual environment, with its inherent structures and the significance of the objects it contains, is easily recognised. This also applies to orientation within the space – e.g. the ability to distinguish quickly between a main entrance and a side entrance – and for the accentuation of objects, as we find in product displays or the emphasizing of the most valuable sculpture in a collection.

    The third form of light, play of brilliance is a result of the realization that light not only draws our attention to information, but that it can represent information in itself. This applies above all to specular effects, such as those produced by point light sources on reflective or refractive materials; the light source itself can also be considered to be brilliant. This “play of brilliance” can lend prestigious spaces in particular life and atmosphere. The effect produced traditionally by chandeliers and candlelight can be achieved in modern-day lighting design through the purposeful application of light sculptures or the creation of brilliance from illuminated materials.

    Richard Kelly, one of the pioneers of modern lighting design. In projects designed by leading architects, e.g. Mies van der Rohe, Louis Kahn or Philip Johnson, he developed the basic principles of differentiated lighting design, influenced by stage lighting.

    William Lam

    With his differentiation between the basic functions of light Kelly made a substantial contribution towards the theory behind qualitative lighting design. He provides a systematic presentation of the means available. The question that still remains open is: according to what criteria are these means to be applied? The lighting designer is obliged to continue to depend on his own instinct, experience and the inadequate support provided by the quantitative criteria laid down in the standards when it comes to analysing the particular lighting context – determining the special features of the space, how it is utilized and the requirements of the users.

    A basic description of the various functions of light as a medium for imparting information – was developed in the fifties by Richard Kelly, a pioneer of qualitative lighting design.

    Two decades pass, however, before William M. C. Lam compiles the missing catalogue of criteria: systematic, contextrelated vocabulary for describing the requirements a lighting installation has to meet. Lam, one of the most dedicated advocates of qualitative lighting design, distinguishes between two main groups of criteria.

    William Lam, lighting designer and dedicated theoretician of qualitative lighting design.

    He first describes the group of activity needs: the needs for information related to specific conscious activities. To understand these needs it is essential to know the characteristics of the various visual tasks to be performed; analysing activity needs is therefore in line with the criteria laid down for quantitative lighting. As far as the aims of lighting design are concerned, there is general agreement on this point; the aim is to design functional lighting that will provide optimum visual conditions for the specific task – be it work, movement through a space, or leisure activities.

    In contrast to the advocates of quantitative lighting design Lam objects strongly to uniform lighting aligned to the respective most difficult visual task; he proposes a far more differentiated analysis of all the activities that will take place according to location, type and frequency.

    Even more important than this new evaluation of a group of criteria that already existed, is what Lam calls his second complex, which comprises biological needs. In contrast to activity needs, which are derived from man’s occupations with specific tasks, biological needs covers the psychological need for information, the more fundamental aspects of the human relation to the visual environment. Whereas activity needs arise from specific conscious activities and are aimed at the functional aspects of a visual environment, biological needs comprise mainly unconscious needs, which allow us to evaluate a situation from an emotional point of view. They are concerned with our feeling of wellbeing in a visual environment.

    In his definition of biological needs he presumes that our attention is only dedicated to one visual task in moments of greatest concentration. Man’s visual attention is almost always extended to observe his entire surroundings. Any changes are perceived immediately, behaviour can be adjusted without delay to adapt to the changed situation.

    The emotional evaluation of a visual environment does not only depend on whether it provides the required information in a clear fashion or whether it withholds it from the observer – the feeling of unease that arises in confusing situations. We have all experienced the feeling of being disoriented in the mass of visual information at an airport or when looking for a specific office in a local authority building.

    For Lam the first of the basic psychological needs for environmental information is the need for orientation. Orientation can be understood in this case first in a spatial sense. It refers to how well destinations and routes can be identified: the spatial location of entrances, exits and what the environment specifically offers. This may be a reception, a special office or the individual departments of a department store. But orientation also comprises information about further aspects of the environment, e.g. the time of day, the weather or what is happening around us. If this information is missing, as may be the case in closed spaces in department stores or in the corridors of large buildings, for example, we feel the environment to be unnatural and even threatening; only when we have left the building can we suddenly make up for the information deficit – we establish that it has become dark and started to rain, for example.

    A second group of psychological needs is targeted at how well we can comprehend surrounding structures. It is important that all areas of the spaces are sufficiently visible. This is a decisive factor in our feeling of security in a visual environment. If there are niches and corridors we cannot see into or parts of a space are poorly lit, we feel uncomfortable and unsafe. Dark corners, e.g. in subways or dark corridors in hotels at night, may contain danger, in the same way as overlit areas.

    Comprehension of our surroundings does not mean that absolutely everything has to be visible, it comprises an element of structuring, the need for a clearly structured enviroment. We feel that a situation is positive when the form and structure of the surrounding architecture is clearly recognizable, and when important areas are designed to stand out against the given background. Instead of a confusing and possibly inconsistent flow of information the space thus presents itself as a clearly structured whole.

    When accentuating specific areas it is not only visual tasks that traditionally receive attention that should be underlined. A view outside or the presence of other points of interest, e.g. a work of art, can be equally effective.

    A third area consists of the balance between man’s need for communication and his right to clearly defined private spaces. Both extremes, complete isolation and complete public exposure, are felt to be negative; a space should promote contact to other persons, while at the same time allowing private spaces to be defined. A private space can be created by defining an area with light: a seated area or a conference table within a large room, for example, and making it stand out from its surroundings.

    Architecture & Atmosphere

    Both main groups of William Lam’s criteria describe man’s needs, his needs for a functional and perceptually sound environment. Besides this analysis, which is based on the needs of man as a perceiving being, it must not be forgotten that light and luminaires also make a substantial contribution towards the aesthetic effect of architectural design. When Le Corbusier describes architecture as “the correct and magnificent play of masses brought together in light”, he underlines the significance of lighting on the design of buildings.

    Lam’s demand for a clearly structured visual environment comes very close to fulfilling this task, but does not cover all aspects. It is certainly possible to structurea space according to the psychological needs of the users by applying different forms of lighting. Any decision to go for one of these approaches implies a decision to create a different aesthetic effect, a different atmosphere in the space. Apart from merely considering the needs of the perceiving being it is also necessary to plan the interplay of light and architecture.

    As with user-oriented lighting design, lightalso has a supporting function in architecture.It is a tool for rendering the givenarchitectural structures visible, and contributestowards their planned effect.Lighting can go beyond this subordinaterole and itself become an active componentin the design of the space. Thisapplies in the first place for light that isnot only able to render architecturevisible, but also to enhance the intended appearance. This applies primarily to luminaires and their arrangement. Luminaires can be discreetly integrated into the architecture – e.g. via recessed mounting in the ceiling. The fixture itself is not visible, it is only the light that has effect. But luminaires can also be added to architecture: in the form of a light structure, an alignment of spotlights or a light sculpture, the lighting installation itselfcan become an architectural element that can purposefully change the appearance of the space.




    Acronym for alternating current, which describes a source where the voltage changes polarity multiple times per second, with a frequency of 50 Hz or 60 Hz depending on the country.

    Accent Lighting

    Lighting that focuses its output in a narrow beam, drawing attention to specific decorative features or objects, making them stand out from their surroundings. Accent lighting is also useful in retail applications, where it can be used to draw attention to specific products and make them seem more appealing.


    American Lighting Association, a trade association that encompasses the USA, Canada and the Caribbean, focusing on residential lighting. ALA members include manufacturers, event organizers and designers.

    Ambient Lighting

    General lighting used to provide visibility in a built environment. Ambient lighting includes both artificial and natural lighting, and does not include task lighting and accent lighting.


    Measurement unit for electric current. In lighting installations, wiring and protections are calculated based on the amperes drawn by the lighting circuits, as well as their rated voltage.


    See Ampere.

    Amp Calculations

    Procedure of determining the current drawn by a lighting fixture or a circuit consisting of multiple fixtures. Once the current drawn in amperes is known, it is possible to size the wire for the lighting circuit, as well as electrical protection devices.

    Angle of Light

    Angle between the orientation of a light source and the viewing direction. For example, the angle of light is 0° when looking at a downlight directly from below, and increases progressively as the viewer steps away from it.

    The term is commonly used in theatrical lighting, to describe the angle between the stage lighting direction and the viewer’s line of sight.


    American National Standards Institute, a publisher of standards for US products and services. ANSI also coordinates national and international standards, allowing US products and services to be used elsewhere.

    ANSI Ballast

    A ballast that complies with ANSI standards.

    See Ballast.


    The intended use of a lighting product. Residential, retail, hospitality, healthcare and high-bay industrial are all examples of lighting applications.


    The intended use of a lighting product. Residential, retail, hospitality, healthcare and high-bay industrial are all examples of lighting applications.

    Arc Lamp

    Any lamp that establishes an electric arc between two electrodes. Arc lamps generally stimulate a gas, making it glow and generating a lighting output.

    Architectural Lighting

    Decorative lighting that is part of a building’s design and construction. It also provides ambient lighting as a secondary function.


    A chemical element, and one of the noble gases. It is used inside the bulbs of incandescent lamps to prevent oxidation of their filaments. Argon can also be used in gas-discharge lamps to emit violet light, or blue if mixed with mercury.


    A chemical element, and one of the noble gases. It is used inside the bulbs of incandescent lamps to prevent oxidation of their filaments. Argon can also be used in gas-discharge lamps to emit violet light, or blue if mixed with mercury.



    The lighting output of outdoor fixtures that is emitted opposite to the intended direction, generally an undesirable effect. For example, if the pole lamps in a parking lot emit backlight towards adjacent homes or apartments, it can be bothersome for the property owners.

    Backlight should not to be confused with backlighting, an accent lighting technique.


    Lighting designed to illuminate an object from behind, which causes an appealing glow effect around its edges. Backlighting is a type of accent lighting, and is commonly used to draw attention to works of art.

    Backlighting should not to be confused with backlight, an undesirable lighting effect that can be produced by outdoor fixtures.


    An opaque or translucent component that blocks direct sight of a lighting source.


    A component required by fluorescent and HID lighting fixtures. It controls the voltage and electric current provided to the lamp during ignition and operation, preventing overheating or premature failure. Depending on their internal construction, ballasts can be either magnetic or electronic.

    Ballast Factor (BF)

    Value that describes how a ballast affects the rated lighting output of a lamp. For example, a 3000-lumen lamp connected to a ballast with a BF of 0.90 or 90% will produce 2700 lumens (3000 lm x 90% = 2700 lm).


    The part of a lamp that connects to the lighting fixture, providing both physical support and electric power for the lamp. A lamp and fixture can only be used together if the lamp base matches the fixture socket. The bases of most residential lamps use either a screw or multiple pins.

    A lamp can have more than one base: for example, fluorescent tubes have one on each end.

    Base Temperature

    Maximum allowable temperature of a lamp base, which must be considered when designing the lighting fixture.


    A type of lamp base that connects with the socket using keyways instead of threads.

    Beam Angle

    Also known as beam spread, the beam angle is a value that describes the downward light cone emitted by a lighting fixture with a reflector. The beam angle is measured between the downward direction, where the lamp provides maximum lighting intensity, and the direction in which intensity drops to 50%. In other words, a lamp with a large beam angle spread its lighting into a wider cone.

    Bi-Pin Base

    A lamp base that uses two pins, as implied by its name.

    See Ballast.


    A type of lighting fixture commonly used outdoors, consisting of a short and broad post with a lamp on top. Bollards generally accomplish a double function, providing both outdoor lighting and decoration.

    BUG Rating

    BUG is an acronym for backlight, uplight and glare, and the term was developed by the Illuminating Engineering Society (IES) and International Dark-Sky Association (IDA) to describe the amount of light emitted by a fixture in unwanted directions.

    • Backlight is directed behind the luminaire.
    • Uplight is directed upwards.
    • Glare causes a visual impairment (See Glare).

    In the BUG rating, these effects are indicated in a scale from 0 to 5, where 0 indicates the effect is minimized and 5 indicates it is not controlled at all. Therefore, the best possible BUG rating is B0 U0 G0, and the worst possible rating is B5 U5 G5.


    A transparent casing that contains a light source, generally made from glass.

    Burn Position

    The operating position for which a lamp is designed. For example, some lamps can only operate in the base-up position. Lamps operating in positions for which they are not designed generally suffer reduced performance or short-term failure.



    Common term for the housing of a recessed downlight.

    Candela (cd)

    Measurement unit for luminous intensity, which is the amount of light emitted in a particular direction. Not to be confused with the lumen (lm), measurement unit for the total lighting output of a lamp or fixture, without describing a particular direction.

    Cave Effect

    An effect that occurs when lighting fixtures direct all of their lighting downward and little or no light is reflected back up towards the ceiling or upper wall portions. The cave effect is generally unwanted because it makes indoor spaces feel ominous, like the interior of a cavern.


    Part of a lighting fixture that covers the outlet box and wiring connections. Canopies often have decorative features.


    Certified Ballast Manufacturers Association, an organization that certifies ballasts as required by ANSI standards. Ballasts with the CBM seal have a guaranteed minimum ballast factor of 0.85 (See Ballast Factor).

    Ceiling Cavity

    The portion of a room that is above the lighting fixtures.


    International Lighting Commission (French: Commission internationale de l’éclairage), an authoritative organization in the lighting industry across the globe.


    Derived from "chandler" (candle maker), a chandelier is a designer/decorative light that hangs from the ceiling and consists of several branch-like parts that host lights or candles.

    Circline Lamp

    A subtype of fluorescent lamp where a fluorescent tube is bent into a circular shape, and where the ballast is typically located in the middle.

    Circuit Breaker

    Electrical protection device that is normally located within a distribution board. Each lighting circuit is connected to a circuit breaker, and it interrupts current automatically if an overload or fault is detected.

    Coefficient of Utilization (CU)

    The fraction of a lamp’s luminous output that reaches the work plane. The CU is influenced by the luminous efficiency of the fixture, as well as room geometry and colors.

    Cold Cathode Fluorescent Lamp (CCFL)

    A fluorescent lamp that emits electrons without heating its electrodes, applying only a high voltage (conventional fluorescent lamps use voltage and heating). CCFLs tend to be less efficient than conventional fluorescent lamps, but offer a service life of over 60,000 hours, comparable to that of LED lighting.

    Color Rendering Index (CRI)

    A metric used to describe how faithfully a light source can render the true colors of objects and spaces, where natural light sources like the sun have a perfect index of 100. Using lamps with a high CRI value is very important in high-end interior design, as they enhance the visibility of décor and fine details.

    Continuous Dimming

    Dimming method where the lamp output can be adjusted at any level between OFF and 100% output, without incremental steps.

    See Dimmer.

    Compact Fluorescent Lamp (CFL)

    A type of fluorescent lamp where the tube is bent into a compact shape to reduce space requirements, hence its name. CFLs often have a built-in ballast and screw bases, allowing them to replace incandescent and halogen bulbs directly.

    Cool White

    A light source with a correlated color temperature of around 4100K. The term comes from the fact that light sources at this color temperature value do not have the characteristic yellow hue of incandescent bulbs or warm white LED lamps.

    Cornice Lighting

    Fluorescent light built into a soffit at the top corner of a wall (i.e., where the wall meets the ceiling).

    Correlated Color Temperature (CCT)

    Unlike the CRI, which describes how faithfully a light source represents other objects, the correlated color temperature (CCT) describes the color output of the lamp itself. Some common CCT values include:

    • 2700K, with a warm tinge of yellow that creates appealing and relaxing environments
    • 4000K, a neutral white tone that strikes just the right balance between relaxation and concentration
    • 6500K, with a slight tinge of blue, which has an energizing effect

    Although the correct technical term is correlated color temperature, it is often shortened to only color temperature. It is also important to note that the CCT is not the real operating temperature of a lamp - it is the temperature to which you would have to heat a black body to make it glow with the same color. For example, an LED bulb with a CCT of 5000K glows in the same color as a black body heated to a real temperature of 5000K, but the LED bulb itself does not reach that temperature.

    Cove Lighting

    A type of lighting that generally directs it output towards the ceiling, and where individual fixtures are hidden in ledges. Cove lighting is often used for decorative purposes because it can emphasize the borders of walls, as well as ceiling features.

    Crest Factor

    In a ballast, the ratio of maximum lamp current to average operating current. The service life of lamps tends to be extended as the crest factor becomes lower.

    Cutoff Angle

    Viewing angle beyond with it is no longer possible to see a light source directly, measured from the direction exactly below the lamp (nadir).



    Acronym for Digitally Addressable Lighting Interface, a communication protocol for lighting automation.

    Daylight Harvesting

    A lighting design strategy that maximizes the use of natural light to reduce energy costs and create indoor spaces that feel natural and appealing.

    Daylight Lamp

    A lamp with a CCT value comparable to that of daylight, generally between 5500K and 6500K. It is important to note that the term does not refer to actual daylight, but rather artificial lighting that replicates its color.


    Acronym for Direct Current. Used to describe a power supply where the flow of electricity always takes place in the same direction, such as that provided to LED arrays by their drivers.

    Desk Lamp

    A compact fixture used for task lighting on a desk, and which is generally portable.

    Diffused Light

    Light produced by an extended surface, either directly or through reflection. Diffused light provides a uniform and soft distribution that minimizes shadows.


    A piece of glass or acrylic that has the purpose of scattering the light from a bulb, which results makes lighting more uniform and eliminates glare.


    Adjective used to describe a lamp or fixture whose lighting output can be modulated with a dimmer.

    See Dimmer.


    A device that regulates the lighting output of a lamp by controlling the power supplied. Dimmers can be used to make indoor environments more customizable and personal, and are also useful to save energy. It is important to note that not all lamps are compatible with dimmers, and incompatible types may be damaged.

    Direct Lighting

    Lighting where more than 90% of the light goes directly from the source to the area you wish to illuminate.

    Directed Light

    Light produced by point surfaces, which results in a concentrated output that accentuates edges and shadows. Directed light normally causes glare when the sources is viewed directly.


    A compact lighting fixture that directs its output downward, hence its name. Downlights can be recessed, surface-mounted or pendant.


    Piece of electronic equipment that transforms the main supply voltage into a lower DC voltage that is appropriate for LED lighting. Some LED lamps have a built-in driver, while others require one to be connected externally, just like the ballasts used by fluorescent and HID lamps.


    Edge Lit LED

    A type of LED fixture that uses a transparent acrylic pattern with etched dots, lines or patterns. The panel is encircled by an array of LEDs, and the etched pattern refracts the light uniformly in all directions.


    Describes how effectively a lighting fixture can convert electric power into lighting, measured in lumens per watt. This is like the gas mileage of a sports car, where the lighting output can be compared to miles traveled, and the electric power input is like fuel consumption.


    Conversion ratio between lighting power output and electric power input, measuring both quantities in watts. Not to be confused with efficacy, which describes the ratio between lumen output and watts consumed.

    Since lumens describe lighting output better than watts, efficacy tends to be a much more useful concept in lighting design.

    Electromagnetic Ballast

    See Magnetic Ballast.

    Electronic Ballast

    A subtype of ballast that uses power electronics to provide a high-frequency voltage and controlled current for fluorescent lamps. Electronic ballasts are lighter and more efficient than magnetic ballasts, and they eliminate humming and flickering issues.

    Electronic Transformer

    Smaller and lighter than magnetic transformers, electronic versions use electronic switching and special dimmers to step the line voltage down to the lamp voltage.

    Ellipsoidal Reflector Lamp (ER Lamp)

    This soft glass lamp collects and reflects light through a barrel about 2" in front of the fixture.

    Emergency Lighting

    Lighting designed to provide visibility when the normal lighting system fails, for example during blackouts. Emergency lighting is equipped with batteries, allowing it to operate long enough for a building to be evacuated.

    Energy Star

    An energy savings and sustainability program by the US Department of Energy and Environmental Protection Agency. Lighting products with the ENERGY STAR have been tested for superior energy efficiency.


    Energy Policy Act, a 1992 law that established minimum efficacy requirements for incandescent and fluorescent lamps, a well as labeling requirements.



    The wire coil that is heated to produce lighting in incandescent and halogen lamps, normally made from tungsten.

    Flat Panel Top LED

    A low-profile LED fixture that uses an edge-lit panel, designed to replace linear and parabolic fluorescent fixtures. This fixture type is available in recessed, surface-mounted and pendant versions.


    A phenomenon where a lamp blinks repeatedly, often caused by power supply issues, or a faulty ballast or driver.


    High-power lighting fixtures that typically use HID bulbs or their LED equivalents. They are generally used outdoors to emphasize specific objects or areas.

    Fluorescent Lamp

    One of the main types of lighting, far more efficient than incandescent and halogen bulbs, but outclassed by LED lighting. A fluorescent lamp uses electrodes to stimulate mercury vapor and produce ultraviolet (UV) radiation, which in turn stimulates the phosphor coating of the lamp to produce visible light.

    Fluorescent Tube

    A specific type of fluorescent lamp that has a tubular shape and comes equipped with pins at its ends, to be connected to the voltage output of a magnetic or electronic ballast. Fluorescent tubes are designated by the letter “T” followed by a number indicating its diameter in 1/8ths of an inch:

    • T12 = 12/8” or 1.5”
    • T8 = 8/8” or 1.0”
    • T5 = 5/8" or 0.625”

    Fluorescent tubes come in standard lengths, where the some of the most common are 24” (2’), 48” (4’) and 96” (8’).

    Flush Mount Lighting / Flush Mount LED Lights

    This kind of lighting is mounted to the ceiling with little or no gap between the ceiling and the light.

    Focused Light

    See Direct Light.

    Foot-candle (fc)

    Measurement unit for illuminance, or lumens per unit of area. One foot-candle is equivalent to one lumen per square foot (See Illuminance).

    Frosted Lens

    A white lens that is translucent but not transparent, which diffuses the output of a lamp.



    Visual impairment caused by a bright source of light, directly visible or reflected by a surface. There are two types of glare:

    • Discomfort glare causes an instinctive reaction to close the eyes and look away. This is the type of glare felt when exposed to a potent HID light or when the sun is directly visible through a window.
    • Disability glare impairs vision, but does not cause the same reaction as discomfort glare. If a light source gets reflected on your laptop screen, for example, it does not bother your eyes but distinguishing objects on the screen may be impossible.


    Hard Light

    A light source that creates shadows with a very sharp edge when cast on objects. Direct lighting from a concentrated source is generally hard light, and some examples are:

    • The sun in a day with clear skies.
    • A camera flash.
    • Highly directional lighting fixtures such as floodlights and spotlights.

    See Soft Light.

    Halogen Lamp

    An improved version of incandescent lamps, where the glowing filament is contained in halogen gas, hence its name. Halogen lamps are around 25% more efficient than their incandescent counterparts.

    Heat sink

    A lamp or luminaire component that is used to dissipate heat effectively. Heat sinks normally use materials with a high thermal conductivity and have a fin-like geometry so that their surface area in contact with the air is maximized.


    Acronym for high-intensity discharge, a type of lighting often used for industrial and outdoor settings due to its powerful output. Some examples of HID lighting are mercury-vapor, metal-halide, xenon, high-pressure sodium and low-pressure sodium lamps.

    All types of HID lamps produce lighting by stimulating an enclosed gas with an electric arc, and therefore they operate at high temperatures.

    High-Bay Lighting

    Lighting systems designed for ceiling heights of 25’ or more, commonly found in sports complexes, warehouses or industrial locations.

    See low-bay lighting.

    High-Efficiency Plasma (HEP)

    An emerging lighting technology that uses radiofrequency to stimulate a contained gas and create a small but very bright ball of plasma. HEP lighting offers a very high efficacy (over 90 lumens per watt) and perfect color rendition (CRI = 100).

    The technology became commercial very recently, however, and has not achieved a market share comparable to that of LED.

    High-Output (HO) Lamp

    Fluorescent tubes with a higher lumen output and rated power than conventional fluorescent tubes. For example, a normal 48” T5 tube may consume 28 watts and provide 2400 lumens, while an HO version may consume 54 watts and provide 5000 lumens.

    High Voltage Lamps

    Lamps that function at a high voltage and offer naturally diffused or scattered light.


    Acronym for high-pressure sodium, a subtype of HID lighting where excited sodium vapor is the source of light. The lighting output of HPS lamps is characterized by its warm yellow hue, and they are commonly used in cobra-head street lights.



    International Association of Lighting Designers, a global association that promotes best practices in lighting design, while providing training and scholarships.


    Institute of Electrical and Electronics Engineers, a global professional association with over 400,000 members. The IEEE is a technical authority that has published many standards and recommended practices for the electrical and electronic fields.


    Illuminating Engineering Society of North America, a technical authority in the lighting industry, with dozens of publications to its credit. IESNA has members and recognition throughout the world.


    The luminous flux on a surface, per unit of area. The illuminance requirements of built environments are determined by their intended purpose, and there are two common units of measurement:

    • Lux - Equivalent to one lumen per square meter.
    • Foot-candle - Equivalent to one lumen per square foot.

    Higher illuminance levels make surfaces appear brighter to the human eye and improve visibility.


    The use of lighting for practical or artistic purposes.


    Light produced by heating a material; Examples include the glow of a candle, an incandescent filament lamp, a shooting star, etc.

    Incandescent Lamp

    A type of lamp with a tungsten filament that glows when it carries current. Incandescent lighting has a perfect color-rendering index of 100, comparable to that of the sun, but is among the least efficient types of lighting.

    Indirect Lighting

    Lighting technique based on reflecting the output of a lamp on surfaces. An example is the lighting provided by torchiere fixtures, which emit their beam towards the ceiling to be reflected back down.

    Induction Lamp

    A type of gas-discharge lighting where the gas is not stimulated directly by electrodes, like in fluorescent or HID lighting, and where microwaves or radiofrequency are used instead. Induction lighting has a much longer service life than HID or fluorescent lighting because there are no electrodes being subject to wear each time the lamp is activated.

    Initial Lumens

    Lighting output of a new lamp, which diminishes with use.

    See Mean Lumens.

    Instant Start Ballast

    A type of fluorescent lighting ballast that applies a high-voltage pulse to the lamp, making it start instantly without preheating the electrodes. The drawback of instant-start ballasts is that they reduce the service life of lamps: electrode material is expelled each time the lamp is started, blackening its edges and causing eventual failure.

    Insulated Ceiling Fixture

    These lighting fixtures are installed into the ceiling and are in direct contact with insulation. They have a lower wattage and emit less heat.

    Integrated Lighting Fixture

    A type of lighting fixture that offers superior energy efficiency, by using a specially-designed LED array and internal geometry. Integrated lighting fixtures are generally more efficient than lamp-based LED fixtures, but they make retrofits more expensive because the entire fixture must be replaced, not only the lamps and ballasts.

    Interior Soffit Lighting / Soffit Light Fixtures

    Lights for portions of the ceiling that are lower than the primary surface; for example, the fixtures installed above the kitchen sink or on a beam or the underside of a balcony.

    International Dark-Sky Association (IDA)

    An international authority on light pollution and environmentally responsible outdoor lighting. Their main goal is to preserve night sky visibility throughout the world.

    IP Rating

    Ingress Protection rating, a two-digit code that indicates the resistance of a lighting fixture to solid particles and liquids, where higher digits indicate enhanced protection. The first digit indicates protection against solids, and the second indicates the protection degree against liquids.

    For example, an IP67 rating indicates a higher degree of protection than an IP54 rating.


    Kelvin (K)

    Measurement unit for temperature, although in the lighting industry it is more commonly used to indicate the correlated color temperature (CCT) of light sources.

    Kilowatt (kW)

    Measurement unit for electric power, equivalent to 1000 watts. This term should not be confused with kilowatt-hour.

    See Watt.

    Kilowatt-hour (kWh)

    Measurement unit for energy consumption. As implied by its name, it is equivalent to the amount of energy consumed by a one-kilowatt appliance running for one hour. Electric utility bills are often calculated based on kilowatt-hour consumption per month. This term should not be confused with kilowatt.



    The process of fusing together thin sheets of material so that the composite material is stronger and more stable.


    The specific component of a lighting fixture that emits light. They generally come with standard bases that fit into the sockets found in compatible fixtures. Some lamps have built-in ballasts or drivers, while others are connected to an external one contained in the fixture.

    Lamp Lumen Depreciation (LLD)

    A progressive reduction in the luminous output of a lamp throughout it service life.


    The part of a lighting fixture that provides support and power for a lamp with a matching base.

    Layering Light

    An interior design approach where several types of lighting are combined to achieve a specific ambience or mood.


    Acronym for light-emitting diode, a solid-state component that emits light when exposed to electric current. LED lighting represents the state-of-the-art in the industry, outclassing most other types of lighting in terms of energy efficiency, design flexibility and colors of light available.

    LED Array

    A group of LEDs mounted on a printed circuit board, capable of producing a lighting output.


    Leadership in Energy and Environmental Design, a resource efficiency certification framework for buildings, developed by the US Green Building Council.


    Lamp or luminaire component that has the goal of dispersing the lighting output so that the desired distribution pattern is achieved.

    Lensed Troffer

    A troffer-type luminaire that is covered by a lens, making the lighting distribution more uniform and eliminating glare.

    Light Loss Factor (LLF)

    A factor used in lighting design to account of the degradation in luminous output over time. The LLF accounts for many aspects such as reflector or lens degradation, dust accumulation, lamp degradation due to voltage surges or heat, etc.

    For example, if the LLF is 0.80 and a room needs 40,000 lumens, the lighting system will be designed to provide 50,000 initial lumens (50,000 lm x 0.80 = 40,000 lm).

    Light Pollution

    Any lighting output emitted in unwanted directions, which may have negative consequences. An example of light pollution is when parking lot fixtures emit bright light sideways, exposing drivers to glare.

    Light Scallops

    An effect of a downlighting plan. The downlights that are closest to the wall create light scallops and add aesthetic value to a room.

    Light Transformer

    Also referred to as dimmer switches, light transformers are electrical devices that are used to change the voltage of a circuit to make the light dimmer or brighter.

    Lighting Fixture

    See Luminaire.

    Linear Lighting

    Multiple LEDs (light-emitting diodes) aligned in a single strip and used for creating uninterrupted lines of directional lighting.


    IESNA standard that establishes the procedure to test the photometric and electrical properties of LED lighting.


    IESNA standard that establishes the procedure to test the lifespan of LED lighting.


    A translucent or opaque screen that blocks direct visibility of a light source and eliminates glare.

    Low-Bay Lighting

    Lighting systems designed for ceiling heights below 25’.

    See high-bay lighting.

    Low Voltage Fixtures / Low Voltage Lamps

    Lights that operate at a lower voltage than the standard household voltage. These incandescent lamps need a step-down transformer to bring the voltage down from 120 to 6, 12 or 24 volts.

    Low Voltage Track

    As the name suggests, this is a track that functions at a low voltage using a step-down transformer.


    Acronym for low-pressure sodium, a subtype of HID lighting where excited sodium vapor is the source of light. LPS lights have a very high efficacy, but their color rendering performance is very poor. This limits their use to some outdoor applications where the CRI is not important.


    Measurement unit for the lighting output of lamps or fixtures. The total lumens emitted and their spatial distribution are of paramount importance when creating appealing and luxurious indoor spaces. In lighting, lumens can be compared to miles traveled and watts can be compared to fuel consumption.

    Lumen Maintenance Life

    A metric used to describe the time in which the output of a lighting product diminishes to a specific percentage of its initial value. The lumen maintenance life is measured in hours and displayed by the letter L plus two digits. For example, the following lumen maintenance life would describe a product whose output decreases to 70% after 60,000 hours of use:

    • L70 = 60,000 hours

    Lumens per Watt

    Lumens Per Watt (LPW) is the light output per unit of energy consumption and it reflects the lighting system's efficacy.

    Lumens to Watts

    The power (P) in watts (W) can be calculated by dividing the luminous flux (ΦV) in lumens (lm) by the luminous efficacy (η) in lumens per watt (lm/W).


    A complete and functional lighting fixture. A luminaire includes the lamp, the ballast or driver, internal wiring, reflectors, lens and any additional components required to deliver light.

    Luminaire Dirt Depreciation (LDD)

    Progressive reduction in the output of a luminaire due to dust accumulation.

    Luminaire Efficiency

    The ratio between the lumens emitted by a complete luminaire to those emitted by the lamps within. A portion of lighting is always lost due to internal geometric features and reflection.

    Not to be confused with luminous efficacy.


    The brightness of an object or surface, as perceived by human eyesight from a specific direction. Luminance is measured in candelas per square meter (cd/m2). It is important to note than luminance changes depending on the viewing angle, and high luminance values are the direct cause of glare.

    Luminous Ceiling

    An uninterrupted plane of translucent ceiling panels with diffusing, fluorescent strip lights.

    Luminous Efficacy

    See Efficacy.

    Luminous Flux

    Total output emitted by a light source, measured in lumens. The luminous flux describes the total lighting output of a lighting fixture without considering direction. Not to be confused with luminous intensity.

    Luminous Intensity

    Lighting emission in a specific direction, measured in candelas. Luminous intensity changes depending on the viewing angle. Not to be confused with luminous flux.


    Measurement unit for illuminance, or lumens per unit of area. One lux is equivalent to one lumen per square meter.

    A key component of lighting designed is achieving a suitable illuminance level depending on the application at hand.


    Magnetic Ballast

    A type of ballast that uses a ferromagnetic core, similar to that of a transformer, to regulate the power supply provided to a fluorescent lamp. Magnetic ballast are heavier and less efficient than their electronic counterparts, and issues with flickering or humming are common.

    Mean Lumens

    Lighting output of a lamp or luminaire at 40 percent of its service life.

    See Initial Lumens.

    Medium Base

    Also known as E26 or standard base, it is the screw-shaped base used by most residential light bulbs.

    Mercury Lamp

    A subtype of HID lamp that produces its lighting output by stimulating mercury vapor, hence its name. Mercury lamps may use a phosphor coating to enhance lighting performance, and are commonly used in outdoor and industrial lighting applications.

    Metal Halide (MH)

    A subtype of HID lamp that produces its lighting output by stimulating vaporized metal-halide compounds, hence its name. Like mercury lamps, MH lamps are commonly used in outdoor and industrial setting.

    Ceramic metal halide is a subtype of MH lamp, where the arc tube is made from a ceramic material instead of quartz glass. This improves the color rendering index of the lamp.

    Mogul Base

    Also known as E39 base, it is larger than the medium E26 base and commonly used by HID lamps. Some LED replacement lamps for HID fixtures are compatible with the same types of ballasts and include a mogul base to use the same socket.

    Monochromatic Light

    A source of light whose output only has one wavelength. The dull yellow lighting of low-pressure sodium lamps is an example of monochromatic light.

    Mounting Height

    Depending on the application, mounting height can have two possible definitions:

    • Distance between the bottom of the fixture and the work plane.
    • Distance between the bottom of the fixture and the ground.

    MR Lamp

    MR is an acronym for multifaceted reflector, a component used to shape the output of a light bulb into a directional beam. MR lamps typically use incandescent, halogen or HID bulbs, and there are also LED replacements available. MR lamps are available with both screw bases and pin bases.

    The MR designation is followed by a numerical value indicating the lamp diameter in 1/8ths of an inch, where two of the most common types are MR11 and MR16.



    National Association of Innovative Lighting Distributors, a lighting industry association that provides ongoing training, a discussion forum, networking, business intelligence and information services.


    National Electric Code,a publication by the National Fire Protection Association, which establishes the requirements for fireproof electrical installations.


    National Electrical Manufacturers Association, a US-based electrical industry association that develops technical standards to ensure product quality and uniformity.

    NEMA Enclosure Type

    A numeric code that describes the degree of protection offered by an enclosure, according to the NEMA 250-214 standard. For example:

    • NEMA 2 = Indoor use, protection against falling dirt and light splashing of water.
    • NEMA 3R = Indoor or outdoor use, dirt protection, resistant to rain and snow.
    • NEMA 4X = Indoor or outdoor use, dust-tight, water-proof (including hosedown) and corrosion-proof.
    • NEMA 6P = All NEMA 4X benefits, and also submersible.


    National Institute of Standards and Technology, a US-based physical science laboratory that is a technical authority on standards, measurements and technology.


    Occupancy Sensor

    A device that uses infrared or ultrasonic radiation, or sound, to detect the presence of humans and switch the lights accordingly. Occupancy sensors are an effective energy-saving measure.


    Acronym for organic light-emitting diodes, flexible polymers based on organic carbon molecules, where the light source is spread across a surface as opposed to a point source.

    Opal Glass

    Semi-translucent white glass that owes its milky finish to the ingredients added to clear glass. It is used for diffusing light.

    Opaque Material

    A material that completely blocks visible light.


    PAR Lamp

    PAR is an acronym for parabolic aluminized reflector, and it is used to shape the output of a light bulb into a directional beam. PAR lamps typically use incandescent, halogen or HID bulbs, and there are also LED replacements available. PAR lamps are available with both screw bases and pin bases.

    The PAR designation is followed by a numerical value indicating the lamp diameter in 1/8ths of an inch. Some of the most common types are PAR20, PAR30 and PAR38.


    Term used to describe a louver with a parabolic shape.

    See Louver.

    Pendant Light / Pendant Fixture / Pendant Lamp

    A lighting fixture that is designed to hang from the ceiling, and which often uses a shade to prevent glare. Pendant lights can be used for both general and task lighting.


    The measurement of light and its properties.

    Photopic Lumens

    The portion of luminous output that gets a response from the cones in human eyes, which are responsible for daytime vision.

    Post Light

    Lighting fixture used to provide outdoor area lighting, where the lamp is found on top of a pole, often with a reflective housing that achieves a characteristic lighting pattern.

    Power Factor (PF)

    Ratio of real power to apparent power drawn by lighting fixtures and other electrical devices. The real power is represented by the actual watts consumed, while the apparent power is the multiplication product of voltage and current, measured in volt-amperes. Electric utility companies normally apply additional charges if the power factor of a building drops below a specified level.

    Programmed Start Ballast

    A ballast that preheats the electrodes of a fluorescent lamp before igniting it, which reduces their wear over time and extends the service life of the lamp. Preheating is accomplished by applying a very low voltage that is high enough to raise the temperature of electrodes, but without causing the lamp to ignite.

    Pull Down Light Fixture Parts

    Utilizing retractable functionality, this recessed downlight pulls down from the ceiling to aim at various angles.

    Pulse Start Ballast

    A type of ballast used with HID lighting, which uses a series of controlled voltage pulses to ignite the lamp, minimizing damage to the electrodes each time the lamp is turned on.



    Energy transmission in the form of waves. Light is a form of radiation, including infrared and ultraviolet light, which are invisible for humans.

    Rapid Start Ballast

    A type of ballast for fluorescent lamps, which preheats the electrodes and applies voltage simultaneously. This type of ballast is faster than an programmed start ballast, but slower than an instant start ballast. The resulting electrode damage when the lamp is started is intermediate between that of instant start ballast (high damage) and programmed start ballast (low damage), and the resulting service life is also intermediate as a result.

    Rated Lamp Life

    The time it takes for 50 percent of the lamps in a batch to reach the end of their service life.


    A cash incentive for a purchase. In the USA, many states have rebate programs for energy-efficient technologies (e.g. LED lighting) or renewable energy systems (e.g. solar panels)

    Recessed Can

    A specific type of lighting fixture with a cylindrical shape that is embedded in the ceiling, hence its name.

    Recessed Lighting / Recessed Fixture / Recessed Luminaire

    Also known as pot light, canister light or downlight, recessed lighting fixtures or luminaires are installed into the openings in a ceiling, appearing as if the light is shining from an elevated hole.


    A physical property of surfaces, equivalent to the ratio of reflected light to incident light.

    Reflective Lamp Shade / Reflective Lighting Fixtures

    A lamp or lighting fixture that produces directional beams. Reflective fixtures direct all the lighting downward (so that there is little or no reflection of light back in the opposite direction).


    An internal component of many lamps and luminaires. It has a reflective surface and its geometry is specially designed to provide a specific lighting distribution. Reflectors are often used with lamps that emit light rays in every direction (HID, fluorescent, etc.) to concentrate their output in a specific direction.

    Restrike Time

    Time required by an HID lamp to achieve full brightness after it has been turned off.


    A lighting system upgrade, generally with the goal of improving energy efficiency and site safety.

    Room Utilization Factor

    Ratio between the light that reaches the work plane and that emitted by the luminaires in the room.



    The resulting “colorfulness” when objects are exposed to a light source, compared to that resulting from natural lighting. If the colors appear more intense, the light source saturates them; on the other hand, if colors are dulled, the light source desaturates them.


    A wall-mounted lighting fixture, which generally has a decorative purpose.

    Scotopic Lumens

    The portion of luminous output that gets a response from the rods in human eyes, which are responsible for nighttime vision.

    Scotopic/Photopic (S/P) Ratio

    Ratio of scotopic to photopic lumens for a specific light source. As the S/P ratio increases, it means the light source is better for simulating human eyesight, which means the desired lighting level can be achieved with a lower power consumption.

    Self-Ballasted Lamp

    A lamp that has an integrated ballast, allowing direction connection to the supply voltage. CFL bulbs with a screw base are one of the best-known types.

    Semi Flush

    A ceiling lighting fixture that has a stem that separates (or creates a visible gap between) the light from the ceiling.


    A screen that prevents a light source from being viewed directly. Shades generally use opaque or translucent materials.

    Shadow Casting Light Fixtures

    Lamps that are designed to cast light and shadows, creating intricate geometrical or abstract patterns on surrounding surfaces.


    See lampholder.

    Soft Light

    A light source that creates gradual shadows, without a noticeable edge between lighted and dark areas. Soft light is generally created with diffuse lighting sources, such as:

    • The sun, when covered with clouds that diffuse its light.
    • Lighting fixtures with lens or diffusers.

    See Hard Light.

    Specular Reflection

    Reflection from a surface that is smooth and shiny, such as metallic kitchenware.


    Lighting fixture that produces a narrow downward beam, generally used for accent lighting or task lighting applications.


    Acronym for solid state lighting, any type of lighting that uses LEDs to produce light, instead of incandescent filaments, ignited gas or plasma. SSL includes OLEDs.

    Step Dimming

    Dimming method that uses incremental and fixed lighting levels, as opposed to gradual dimming from to OFF to 100% output.

    See Dimmer.

    Strip Light

    Flexible, dynamic and customizable, strip lights can be cut and stuck anywhere, according to project requirements.

    Suspension Light / Suspension Fixture / Suspension Lamp

    See Pendant Light / Pendant Fixture / Pendant Lamp

    Swing Arm Lamp

    Adjustable folding arm lamp that can be used to provide task lighting (lighting up a specific area). Also referred to as a floating arm lamp or balanced arm lamp.


    Task Lighting

    Lighting fixtures with the goal to improve visibility in an area where specific tasks will be carried out, hence their name. The use of under cabinet lights for food preparation areas in kitchens is an example of task lighting.

    Total Cost of Ownership (TCO)

    Cost of owning a lighting product over its entire lifetime. It includes the sales price, installation cost, energy consumption, maintenance, component replacements and decommissioning cost.


    A floor lamp that uses a reflector on top a pole to direct its entire output upward, which is then reflected from the ceiling and walls.

    Track Lighting

    Lighting configuration where several fixtures are mounted on a common track, which provides them with power and allows each of them to be oriented in a different direction.

    Translucent Material

    A material that allows a partial transmission of light, generally diffusing it and eliminating glare. Frosted glass is an example of a translucent material.

    Transparent Material

    A material that allows most or all of the light incident on it to pass through. Clear glass is a translucent material.


    A recessed lighting fixture, designed to be installed in an opening in the ceiling. Troffers typically have predetermined dimensions, such as 2’x2’ or 2’x4’.

    Twin Tube

    A type of CFL lamp where two parallel fluorescent tubes share the same base.


    U-Bend Lamp

    A type of fluorescent lamp where the tube is bent in a U shape, hence its name. U-bend lamps generally have two bases, one on each end, which attach to different lampholders.

    UL Label

    A label placed by Underwriters Laboratories, which means a product has been tested for fire safety and electrical safety.

    Ultrasonic Sensor

    An acoustic sensor that measures distances by sending and receiving ultrasonic signals.

    Ultraviolet Light

    A type of electromagnetic radiation, invisible to the human eye (i.e., beyond the visible spectrum) with a wavelength of 10 to 380 nanometers.

    Under cabinet lighting

    Installed under a cabinet or shelf, this lighting is used to produce localized lighting and can also serve as a night light. It can come in LED, fluorescent, incandescent versions.

    Universal Product Code

    A 12-digit code found in lighting products, which can be scanned at the point of sale.


    Lighting method where an object or surface is lit from below, with a luminaire that directs its output upward. The applications of uplighting are generally decorative.


    Valance Lighting

    Lighting that is installed above the upper edge of windows, where an opaque panel blocks direct vision of the fixture and the light is directed upward and downward as a result.

    Vandal-Resistant Fixture

    A lighting fixture that is resistant to breaking or tampering, generally intended for outdoor public spaces.

    Vanity Light

    Lighting located above, below or to the sides of a bathroom mirror.

    Vapor-Tight Fixture

    A lighting fixture that is enclosed and gasketed to prevent the entrance of vapors or gases.


    The electric potential difference between two contacts. Voltage drives electric current through lighting fixtures and other appliances, just like pressure drives the flow of water in plumbing installations.

    Volumetric Troffer

    A troffer that is specially designed for maximum optical performance, with a uniform lighting distribution that eliminates both glare and the cave effect.


    Wall Grazing

    Lighting effect where a wall with an irregular surface is illuminated so that there are both highlighted and shaded areas. This effect is only possible on walls with granular surfaces, such as those built from stone or exposed brick. The opposite effect is wall washing.

    Wall Sconce

    See Sconce.

    Wall Washing

    Lighting effect where a wall is illuminated so that surface irregularities are minimized, it seem smoother. The opposite effect is wall grazing.


    Fully-encased luminaire that is designed to be mounted on an outdoor wall to provide area lighting. Wallpacks are available in HID, CFL and LED versions.

    Warm White

    White light that is characterized by a yellow tinge. The term is generally used for lighting with a correlated color temperature (CCT) of around 3000K.


    Measurement unit for the electric power consumption of lighting fixtures, or any other appliance that runs with electricity. In lighting, lumens can be compared to miles traveled and watts can be compared to fuel consumption.

    Watts to Lumens

    To convert watts to lumens, multiply the power (P) in watts (W) with the luminous efficacy (η) in lumens per watt (lm/W).

    Work Plane

    The horizontal plane where activities are carried out, typically 30 inches above the floor. Lighting designs are generally based on providing a specified illumination level at the work plane.