Lamps and lighting by Jean-Jacques Damel incourt

There is now a wide demand for the provision of high-quality visual environments and consequently lighting engineers need to examine closely the numerous interacting aspects o f human activities. This paper reviews the characteristics o f the human visual system and how these aflect visual peformance under dt@erent lighting conditions. The way in which the techniques that are used to convert electrical energy to visible radiation influence the quality Ofthe radiation for lighting purposes is discussed and the paper concludes by linking the properties o f severalfamilies of lamps to the main lighting applications.

ublic lighting was originally introduced as a means of reducing crime. T h s is s d l an important reason for providing public lighting P today, crime and vandalism remaining a major

problem in most public areas. The recognition of people, the detection of obstacles, the avoidance of glare, and visual orientation are factors to be considered in combating crime and preventing fear.

In European countries, it is generally possible to distinguish between an old town centre and residential or industrial areas on the outskirts. To facilitate traffic movement between these areas, arterial or link roads as well as motorways have been created. Reduction of traffic accidents is a major reason for instalhng street and road lighting. Important considerations for t h s type of lighting are road brightness, glare reduction and visual guidance.

Definitions Candela: the unit of luminous intensity, defined such that the luminance of a black body radiator at the temperature of solidification of platinum is 60 cd/cm2.

Luminance: a measure of the brightness of a surface, measured in candelas per square metre of the surface radiating normally. It represents the density of luminous flux per steradian per square metre of surface viewed.

Lumen: the unit of luminous flux, equal to the amount of light emitted per second in unit solid angle 6om a uniform source of one candela. It is the amount of light which falls on unit area of a surface when the surface is at unit distance fiom a source of one candela.

Lux: the unit ofdumination or illuminance, equal to one lumen per square metre.

Road lighting and lighting for security must be present in all parts of a city but the solutions can be quite different depending on the location. For example, in town centres and tourist areas, greater attention is generally paid to guidance and decorative aspects of urban lighting. These installations also need to have a pleasant appearance during the daytime.

Sports staha and cultural event venues also need to be lit appropriately taking into consideration the different demands of players, spectators and television. Attention needs to be given to vertical and horizontal illuminances, glare effects, safety of the installation and security of people.

Likewise, shopping and community centres, parlung areas and industrial areas, whch have generally been isolated &om residential zones, require specific treatment.

Practical aspects, such as adaptation ofluminaires and poles, painting, h n g , access to cable joints and fuses, must be carefully examined.

Tables l a and b summarise some of the requirements for public lighting. Three classes of elements interact in viewing a scene: the source of light, the object that reflects, hffuses or otherwise alters this light and, finally, the human visual system (including the brain), whch interprets the information carried by the altered light.

Good seeing conditions and a comfortable visual environment depend on the relationship between man and light in terms of visual performance, visual comfort, etc. However, they also depend on more coniplex factors of a psychological or social nature. It should therefore be kept in mind that lighting is not an exact science: it is as much an art as a science.

The visual system

The visual system, comprising the eye and the brain, is the key element in determining what we see. Fig. 1 shows a simplified cross-section through an eye and a stylised view of the neural network in the retina.

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Table 1: Requirements for public lighting. Importance is indicated by the number of crosses.

Dynamc acuity/ Static acuity/ Ambience Visual hygiene/ Sense of self- Stereoscopy Colour No glare security

Luminance/ Colour/CRI/ Contrast Spectral

distribution

Link roads Roads Town centres Residential areas Shopping centres Public areas Illumination Museums Sports stadia Parkina areas

xxx xx xx X

xx xx

x to xxx xx xxx xx

X xx xx X

xxx xx xx

xxx X

X

Colour temperature/

Spectral distribution

xx xx xx xx xx xxx xx xx

Spatial continuity

of flux

Cylindrical illumination

xxx xx xx X xx X

xxx xx xxx

X

xx xx xxx xx xx xx xx xx xxx

! Projectors ' I Area of application adjustment

Link roads Roads Town centres Residential areas Shopping centres Public areas Illumination Museums Sports stadia Parking areas

X X X X X X X xx X X

xxx xx xx xx X xx xx xx

x to xxx X

xxx xx xx xx xx xx xx xx xx xx

xx X X X X X

x to xxx xx X

X

xxx xx X xx X X

xxx X

xxx X

As can be seen from Fig. 1, the eye is basically a classical optical system with a lens of variable focal length (the crystalline lens), an iris daphragm of variable diameter (the pupil), and a special sensitive receptor (the retina).

The lens can be made more or less convex by means of the cdiary muscles.

The diameter of the pupil can be varied from approximately 1 mm to 8 nun. This variation allows a rapid, but limited (less than 1:100), adaptation of the

eye to entrant light levels. Another means of adaptation is provided by chemical changes in the retinal receptors. These adaptations occur quickly for increasing lumi- nosity levels, but are slow-acting for decreasing luminosity levels, as is well known by night drivers.

The retina is a strongly heterogeneous structure with two classes of photoreceptors: cones and rods. These receptors differ in shape and also in sensitivity and spectral response. Rods have a single spectral response with a maximum near 500 nm, but there are three wide

crystalline lens

pupil

light

0 retina

- choroid

Fig. 1 The eye

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)O 500 600 700 wavelength, nm

a b

00 500 600 700 wavelength, nm

Fig. 2 Cone and rod sensitivity curves and influence of the spectral radiation distribution of a lamp on the colour rendering index (CRI). In (a) the vertical lines roughly represent the narrow emission bands of phosphors specially designed to excite the RGB cones near their maximum sensitivities. In (b) a radiation continuum has been added to improve the colour rendition (from 85 to 95).

overlapping spectral responses for cones. The cone responses allow colour to be perceived but also lead to the important phenomenon of metamerism-a sensation of the same colour may be obtained &om lights of distinct spectral compositions (Fig. 2). In the main these two f d e s of receptors are arranged in two dlfferent areas.

The first area represents the major part of the retina and is covered essentially by rods and a low density of cones. This area is well designed for low-intensity and low-definition vision.

The second area, called the fovea centrals, is a small zone near the vision axis. In this region, the cones are individually served by optic-nerve fibres, so that this small area, covering roughly one angular degree of visual field and completely free of rods, is responsible for precise and coloured vision. Ths small detection area in association with the small pupil aperture defines a narrow beam of light, makmg the eye sensitive to luminance.

Interconnections of indlvidual signals &om the rods and cones is achieved through the bipolar, ganglion, horizontal and amacrine cells. This allows information to be compressed and enables the differential detection of special structures of shape, colour and direction.

Unfortunately, the cones have little response at luminance levels below a few candelas per square metre (12 lux on white paper corresponds roughly to 3 cd/m2). These values therefore correspond to the boundary between photopic viewing (viewing in daylight or high illumination levels) and mesopic viewing (for example at dawn or during full

moorhght), when vision is acheved by a combination of cones and rods. As the French say: ‘La nuit, tous les chats sont gris’ (at night, all cats are grey).

Precise perception of the whole visual field is of course not possible directly because of the small detec- tion field defined by the fovea and the pupil. Fortu- nately, in normal vision the eyes are always on the move and their movementssmall oscdations and saccades -allow the brain to reconstruct our whole visual environment. We always see the world in our brain!

Finally near visual photopic condltions and at a h g h level of integration, three important functions and neuronal systems can be identified. The first is sensitive to luminance contrasts, movement and depth of focus; t h s system does not react to persistent information or coloured fields and is poorly suited for the analysis of small static structures. The second system is sensitive to coloured fields and grey shades; it does not detect movement, shapes or depth of focus. The third system is sensitive to tiny coloured contrasts under quasi-static examination.

In contrast to the central cones, the rods are believed to be combined in groups of large numbers of cells, providing sensitivity for low light level viewing but at the expense of resolution. Rods also play a major role in the detection of unexpected events in the peripheral environment.

Lighting conditions and sources

Ths section introduces parameters that are more easily determinable and suitable for use in light engineering.

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In most practical situations the eye works under performance is considerably lowered: the cones can not conditions where only photopic or mesopic vision is work properly and de td detection, depth-of-focus relevant and consequently the following dxussion d appraisal and colour judgement are greatly altered. generally be limited to these conditions, Under both photopic and mesopic conditions the

Visual performance is a quantity measurable as a luminance distribution may also affect visual per- percentage error for a given population. There are formance through the glare effect, which is caused by several well known classical parameters that govern diffusion of light withm the eye. This phenomenon visual performance: angular size of the detd, lumin- increases when the luminance of the dsturbing source ance and colour contrasts, luminance increases or when the angular distance adaptation, luminance distribution, between the disturbing source and the nature of the lighting system, Visual vision axis decreases. The glare effect presentation time, temporary and also increases with age. permanent vision diseases. The choice performance is Consequently, a lighting system (i.e. - , - - ,

of determining parameter (lunllnance measurable power supply + source + luminaires) or colour contrast, detection of must be designed fi-om several different movement and position, etc.) depends as a percentage aspects-photometry, colour appear- on the nature of the scene, taking into ance and colour rendition, time depen- account the properties of the eye error for a dence, efficacy, simplicity-and must . . presented in the previous section. produce a visual environment suitable

for the activity for which it is designed. In photopic conditions, the influence given population of angular size, luminance contrasts and luminance adaptation can be summarised in terms of the variation of contrast sensitivity with spatial fiequency where retinal illuminance adaptation is taken as a parameter.

Note that the large spatial fiequency response seems to be obtained by parallel matching the outputs of separate channels. Therefore when information is to be displayed it is very important not to address two different sets of information to the same channel.

In scotopic (i.e. low light) conditions (e.g. in the total darkness outside a car’s headhghts) or mesopic con- ditions (e.g. in a car’s headlights), human visual

The power supply may be a Cradi- tional50 Hz supply or an electronic supply, ‘intelligent’ or otherwise. The light source is characterised by its luminous flux in lumens, its luminous efficacy in lumens per watt and its spectral distribution. The spectral distribution determines the colour quality of the source. In practice the latter is defined by its colour temperature, which gives the observer’s perception of the light (e.g. warm white, cold white), and its colour rendering index (CRI, in the range 0-loo), which expresses the extent to whch artificial light is able to render a ‘reference’ colour of an object.

The light source is always enclosed in a luminaire,

green thalium line

1000

400 500 600 700 800 900

wavelength h, nm

Fig. 3 Black body emissions at 2700 and 5200 K, mean sensitivity V(A) of the human eye and simplified emission (green 535 nm TI line) of a mercury-thallium discharge lamp

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Table 2: Parameters for high- and low-pressure discharges

Electron density, m-3 1 0” 1017 Atom density, m-3 io24 1 0’‘ Electrodatom

collision frequency, s-l 10” 109

Plasma phase LTE* no LTE Power density, W/m3 107 1 0’ Luminance, cdh2 1 0 6 103 Thermal losses hiah low

*LTE = local thermodynamic equilibrium

whch defines the luminous intensity distribution of the luminous flux. If rigorous control of the luminaire emission photometry is required, point or line sources are needed. Clearly the choice of lamps is of critical importance in the design of a lighting installation

Converting electrical power to visible radiating power

The first artificial source of light was fire. Until 1870 it was also practically the only artificial light source. Consequently our visual system has two basic reference sources: natural light (whch has a hgh colour temperature of 5000-10 000 K and high illumination levels of 5000-100000 lux) and fire (which has a low colour temperature of 1500-2500 K and low lllumination levels of 10-100 lux).

Fire is regarded as a thermal source of light. In thermal sources, the emitted light depends directly on the temperature of the source, as in black and grey bodies. Tungsten lamps are good examples of thermal sources.

Until now the maximum temperature practically

obtainable for a solid body has been about 3400 K. Fig. 3 shows black body emissions at 2700 K and 5200 K (the approximate colour temperatures of a standard tungsten lamp and the sun, respectively) and the mean sensitivity of the human eye. As can be seen, the resulting luminous efficacy of the 2700 K radiation, about 15 lm/W, is seriously limited by the temperature.

For the 5200 K black body emission, the luminous efficacy is about 100 lm/W, but that temperature is not attainable with a solid body. However, a temperature of 5200 K is easily obtained in a discharge lanip. Fig. 3 shows the emission of the green thalhum line in a mercury-thallium hgh-intensity discharge (HID) lamp. The efficacy of this green line radiation is about 625 h / W and the maximum of the line emission is close to the black body emission for the same wavelength. Furthermore, in a dscharge, it is easier to modi@ the nature and pressure of the emitting body, the temperature and so on.

Pressure is a determinant parameter. Table 2 gives an idea of some quantities for hgh- and low-pressure dscharges.

With a dlscharge source there is a trapping problem: for a given electrical power we must minimise the useless thermal losses and maximise the useh1 radiation. It is also necessary to trap some species, for example the electrons, which transfer power, through collisions, from the electrical field created by the external generator to atoms.

Fig. 4 illustrates t h s specific problem oflight sources: photons must be able to escape but electrons must be trapped. One solution, shown in the figure, is to separate duties between a working gas (wg) and a trapping (or control) gas (tg) that satisfj the following

electron working gas 1

I I

- photon 0 control gas 1

a b C

Fig. 4 A specific difficulty in light sources. In (a) the gas is at low density and both electrons and photons escape. In (c) the gas is at high pressure and both electrons and photons are trapped. In (b) the duties of the working (wg) and trapping (tg) gases are separated-&, > N,, Etg > Ewrso that electrons are trapped but photons escape.

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Tungsten Flu0 LP HP Hg HP Na HP MH

= power, 0 - 3 kW = efficacy, 0 - 200 lmNV

life, 0 - 15,000 h

colour temperature, 0 - 6000 K

= colour rendering index, 0 - 100

=

LP Na

Fig. 5 Characteristics of incandescent (tungsten), low-pressure (LP) and high-pressure (HP) lamps

conditions: Pw8 << Pg (or Ng > Nwx, where N is the denisty of atoms), E,,<EC,, where P is the partial pressure of the gas and E the excitation energy. The worktng gas is excited and ionised. The trapping gas supports neither excitation nor ionisation: it only controls particle movements and hence conductivity and other material properties (but not, of course, photon trapping).

Photon escape may also be acheved through another mechanism: line broadening. The basic idea is very simple: if a given number of absorbing atoms are linearly &splayed over a broader hequency range the absorption effect is lowered exponentially In reality the dstribution d not be linear and the situation is a little more complex! Besides, the effect depends on the properties of the atoms and is more or less efficient depending on which atoms are chosen.

Some lamps and their properties

Fig. 5 summarises the main properties of six families of lamps. The following paragraphs try to show how these properties are acheved.

Tungsten lamps It has already been explained that the efficacy of

the tungsten lamp is low and depends strongly on the temperature of the tungsten wire. However evapora- tion of the tungsten also depends on this temperature.

Finally the parameter pair ‘efficacy-life’ offers us a wide choice: 12 1ndW-1000 h for classical tungsten lamps, 20 lm/W-2000 h for tungsten-halogen lamps for general lighting, and 30 lni/W-150 h for other uses.

Fluorescent low-pressure lamps The fluorescent lamp family exploits the properties

of the low-pressure mercury/rare-gas discharge. This discharge provides a good example of the separation of duties, with mercury at a pressure of a few pascals acting as the working gas and electron trapping being controlled by the rare gas at a pressure of a few hundred pascals.

In such a dscharge, it is possible to optimise the electron temperature so as to produce a high level of resonant radiation at a wavelength of 254 nm. More than 50% of the positive column power can be converted into ultraviolet radiation. Thus, despite a poor ultraviolet-to-visible radiation conversion factor, a luminous efficacy of 100 lm/W with a CRI of 85 can be achieved using HF excitation. However, a low pressure implies a low emission density-about 7 Im/cm3 for a 32 W lamp. Because of thermahsation (i.e. the electron temperature approaching the atom temperature), it is difficult to increase this value without decreasing lamp efficacy An 11 W compact fluorescent lamp has been produced which provides an emission density of 20 lm/cm3 and an efficacy of 82 lm/W

This well established lamp family stdl attracts interest.

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Table 3: Matching light sources to applications. T = tungsten lamps for general lighting; CF = compact fluorescent lamps; F = fluorescent lamps; MHL= metal halide lamps; WNa =white sodium lamps; LPNa = low-pressure sodium lamps; HPNa =high-pressure sodium lamps; HPHg = high-pressure mercury lamps; FHF = high-frequency flourescent lamps

Output, Efficacy, Life, Colour CRI, Projector Frequent Instant Sources E, ‘ J I L, temp., 0-100 (point starting light

klm lmiW kh 1000K source)

Domestic 0.4-3 2-4 90-100 Yes Yes T, CF, F Off ices 1-10 high >5 2.5-5 85-100 no no no F, CF, MHL

F, CF, MHL, WNa Commercial 1-10 high >5 2.5-6 85-100 yes no no Storage 1-10 high >10 no no no LPNa, HPNa Roads 10-100 high >10 Yes no

MHL, FHF, WNa Citycentre 1-20 high >10 2-4 85-100 no no no Sites 10-1 00 yes generally generally MHL, HPNa, . . .

no no

no HPNa or HPHg, MHL

Ways are being investigated for eliminating mercury, altering the phosphors and making changes in the excitation mode, flux and colour through electrical excitation etc.

H&h -press u re mercury lamps Higher emission densities are wanted for

applications where a high illumination level and a limited number of source points are required. These can be achieved by increasing the gas density in the lamp. For example, in a 400 W high-pressure mercury lamp, an emission density of 1300 lm/cm3 can be obtained in a discharge running at 1.5 x IO5 Pa. However, because of ultraviolet emission, t h s lamp has a poor efficacy (55 lm/W) and a limited CRI (55 with fluorescent correction).

High-pressure sodium lamps At pressures higher than 10“ Pa local thermal

equilibrium is realised, thermal losses increase, and spectral lines are absorbed and broadened. In such condltions, in a high-pressure sodium lamp, the centre of the line is strongly absorbed but, due to the properties of sodium atom emission, the line is also strongly broadened and a lot of light can escape in the wings of the line profile. Thus, the high-pressure sodium dmharge shows an efficacy of greater than 110 lm/W for a 400 W lamp. For this lamp the emission density reaches 13 200 lm/cm3 and the lamp is an effective line emission source. The high efficacy of the hgh-pressure sodlum lamp is linked to the fact that the dominant emission, from the resonant line of sodmm, is in the visible part of the spectrum; however this property also limits the colour temperature and the CRI.

High-pressure metal halide lamps Better colour rendition (CRI 80-90) and high

efficacy (10&80 h/W) can be obtained with high- pressure metal hahde lamps. These results are obtained through complex chenical equilibrium and transport phenomena, and so, due to colour dispersion, the practical life of these lamps is generally more limited (400M000 h) than that of other dlscharge lamps (8000-20000 h). There is potential for much development in t h s area and t h s f a d y of lamps holds much promise.

Table 3 attempts to link the main lighting applications to light source properties. Of course other possible combinations exist depending on the imagination of the lighting engineer.

Short bibliography

1 OVERINGTON, I.: ‘Vision and acquisition’ (Pentech Press, London, 1976)

2 ‘Colorimetry’. CIE Publication no. 15.2-1 986 (CIE, Vienna, Austria, 1986, 2nd edn.)

3 ‘Method of measuring and specifying colour rendering of light sources’. CIE Publication no. 13.3-1995 (CIE, Vienna, Austria, 1995, new edn.)

4 ‘Spectroradiometric measurement of light sources’. CIE Publication no. 63-1984 (CIE, Vienna, Austria, 1984)

5 MAHNKE, E, and MAHNKE, R . H.: ‘Color and light in man-made environments’ (Van Nostrand Reinhold, New York, 1987)

6 BOYNTON, R. M.: ‘Human color vision’ (Holt, Rmehart & Winston, New York, 1979)

7 ELENBAAS, W: ‘The high pressure mercury vapour discharge’ (North Holland Publishing, Amsterdam, 1951)

8 WAYMOUTH, J.: ‘Electric discharge lamps’ (MIT Press, Cambridge, MA, USA, 1971)

9 DE GKOOT, J., and VAN VLIET, J.: ‘The high-pressure sodium lanip’. Philips Technical Library (Kluwer Technische Boeken BV, Deventer-Antwerpen, 1986)

10 MEYER, C., and NIENHUIS, H.: ‘Discharge lamps’. Philips Technical Library (Kluwer Technische Boeken BV, Deventer-Antwerpen, 1988)

11 Proceedings of the series of International Symposia on Incoherent Light Sources (2nd, Enschede, Netherlands, 9th-12th April 1979; 3rd, Toulouse, France, 18th-21st April 1983; 4th, Karlsruhe, Germany, 7th-10th April 1986; 5th, York, England, 10th-14th September 1989; 6th, Budapest, Hungary, 30th August-3rd September 1992; 7th, Kyoto, Japan, 27th-31st August 1995; 8th, Greifiwald, Germany, 30th August-3rd September 1998) (Available from Dr. G. Zissis, CPAT, Universitt Paul Sabatier, 118 rte de Narbonne, 31062 Toulouse cedex 04, France; E-mail zissis@cpa22.ups-tlse.fr)

0 IEE: 2000

Professor Jean-Jacques Damelincourt is Head of the ‘Intense Photon Sources’ team in the Centre de Physique des Plasnias et Applications de Toulouse (CPAT-UPS), Universitt Paul Sabatier, 118 rte de Narbonne, 31062 Toulouse cedex 4, France. E-mail damelincourt@cpat.ups-tlse.fr

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