Philips LightingQuantities and Units, Measurements

Lighting Design andApplication Center

Correspondence Course

Lighting Application

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ContentsIntroduction 3

1. Why separate lighting quantities ? 32. Photometric quantities and units 5

2.1 Luminous flux 52.2 Luminous intensity 52.3 Illuminance 82.4 Luminance 9

3. Practical relations between quantities 113.1 Relation between luminous flux and luminous intensity 113.2 Relation between luminous flux and avenge illuminance 113.3 Relation between luminous intensity and illuminance 123.4 Relation between luminous intensity and luminance 153.5 Relation between illuminance and luminance 15

4. Other lighting units 16

5. Radiometric quantities and units 17

6. Measuring Instruments18

6.1 Visual photometers 186.2 Photoelectric photometers 18

7. Measuring techniques 217.1 Measurement of illuminance 217.2 Measurement of luminance 217.3 Measurement of luminous intensity 247.4 Measurement of luminous flux 247.5 Measurements carried out in the field 25

Conclusion 26

Exercise material 26

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IntroductionSpecial lighting concepts and units For the quantitative measurement of light a special set of concepts and

units has been adopted that bear no direct relationship to those used in otherdomains of physical science. This is in contrast with measuring practice in theother wavelength regions of the spectrum of electromagnetic radiation, which isgenerally based upon the familiar concepts of energy and power, and thereforeuses the related SI units - Joules and watts.

One reason why lighting uses its own standards is that the oldest unit of light, the'candIe', is based upon the flame of a standard lamp (see Lesson 2). The principalreason, however, is that a lighting unit must not only take the energy content of theradiation produced within the visible spectrum into account, but also the spectraldistribution of the sensitivity of the human eye. As has been explained in Lesson 4,the latter varies greatly with wavelength.

Four basic lighting quantities These considerations, together with requirements from lighting engineeringpractice, have resulted in the following four basic concepts of practical lightmeasurement.quantity symbol unitluminous flux Φ lumen (lm)luminous intensity I candela (cd)illuminance E lux (lx)luminance L candela per square metre (cd/m2)

These four basic quantities, their interrelationship and application in practicallighting engineering will be the subject of the first part of this lesson. The secondpart will be devoted to the measurement of light, the equipment used, and thevarious measuring methods. Priority will here be given to measurements carriedout on practical installations under practical conditions, rather than laboratorytechniques

Much of the trouble students sometimes experience in understanding the basics oflighting engineering can be attributed to confusion created by unfamiliarity with theabove-mentioned concepts and their proper application. It Is essential, therefore,that these should be well understood before proceeding with this course.

1. Why separate lighting quantities?Light measurement uses two dissimilar parameters When considering light - which is 'visible'

electromagnetic radiation - we are concerned on the one hand with energy and onthe other with a sensation obtained through the eye two principally dissimilarthings, This makes it difficult to talk about light in quantitative terms. To solve thisproblem. a convention has been adopted that fits in perfectly well with illuminatingengineering practice: viz. the product of radiant energy and eye sensitivity. Thus,light is regarded as radiation measured in terms of human eye sensitivity.

The light-watt In Lesson 4 it was explained that the sensitivity of the eye varies with wavelengthUnder conditions of photopic vision, the maximum sensitivity lies at 555 nm. Nowone could choose to define one watt of power radiated at a wavelength of 555 nmas being equal to one ‘light-watt'. One watt of power radiated at a differentwavelength within the visible range would then have to be multiplied by the relativeeye-sensitivity factor as defined by the spectral eye-sensitivity curve for photopicvision, that is to say the V(λ) curve (Fig. 1). By so doing, one arrives at the light-wan value corresponding to that wavelength. For example. for radiation with awavelength of 490 nm the eye sensitivity is only 20 per cent of that for radiationwith a wavelength of 555 nm. One watt of power radiated at 490 nm thereforeequals 0,2 light-watt.

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Fig. 1 The spectral or V(λ),curve for photopic vision.

The candela By adopting the light-watt as the unit of visible radiation for the quantitativeexpression of visual perception, ambiguity would have been completelyavoided. However this concept was not adopted because, long before it waseven formulated, a light unit had already been derived from one of the earliestreproducible lighting standards. This standard has been described in Lesson2, and was to become the 'candle', or, after 1948, the 'candela'. It was in fact aunit of light intensity in a given direction, that of the observer. A light sourceradiating a light intensity of one candela in all directions produces a well-defined quantity of light per second, which has been given the name 'lumen'.This has become the principal lighting standard in use today.

Maximum spectral luminous efficacy Calculations hare shown chat one watt of radiant power witha wavelength of 555 nm equals 683 lumens, This figure is known as the‘maximum spectral luminous 'efficacy’. Thus, one watt of power radiated at490 nm equals 0,2 x 683 = 137 lumens .

The lumen can therefore be defined as a certain quantity of radiant energyemitted per second, weighted against the spectral sensitivity of the humaneye.

Relative spectral eye sensitivity for photopic vision (555 nm = 1,00000)

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2 Photometric quantities and units

2.1 Luminous flux - symbol ΦΦ, unit lumen (lm)Definition of luminous flux Luminous flux is the concept for the total quantity of light emitted per second by

a light source (Fig 2). It is designated by the symbol Φ. The unit is the lumen(lm).

As mentioned in the previous section, luminous flux can be defined as:

the energy radiated by a light source per second, weighted against the spectralsensitivity of the human eye:

Some examples of practical light sources:

Bicycle headlamp 3 W 30 lmIncandescent lamp Softone 75 W 900 lmCompact fluorescent lamp SL18 W 900 lmFluorescent lamp 'TL’D 58 W/83 5200 lmHigh-pressure sodium lamp SON-T Plus 100 W 10 500 lmLow-pressure sodium lamp SOX-E 131 W 26000 lmHigh-pressure mercury lamp HPL-N 1000 W 58500 lmMetal halide lamp HPI-T 2000 W 190000 lm

Luminous efficacy As becomes clear from the above table, there is no fixed relationship betweenthe electrical energy dissipated in a lamp and the luminous flux radiated Theratio between luminous flux and power dissipation i$ called 'luminous efficacy,and is expressed in lumens per watt (lm/W). Each lamp type has its specificluminous efficacy.

2.2 Luminous intensity - symbol I, unit candela (cd)Definition of luminous intensity Luminous intensity is the concept for the concentration of light in a

specific direction, radiated per second {Fig. 3). It is designated by the symbol LThe unit is the candela (cd).

The luminous intensity can be defined as:

the luminous flux in certain direction, radiated per unit of solid angle.

Fig. 2 The luminous flux rt defined the quantity if light emitted per second. It canbe compared with the quantity of water passing a given point per second.

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Fig. 3 The luminous intensity is defined as the concentration of light in a specificdirection. It can be compared with the intensity of a water jet in a given direction.

A solid angle can be regarded as a cone This brings us to the concept of the solid angle, and its unit the‘steradian'. A solid angle can best be described as a measure for that portion ofspace about a point bounded by the surface of a cone whose top (vertex) is at thepoint (Fig. 4).

In general a light source will not radiate its luminous flux uniformly in all directions.If, however, we imagine a sufficiently narrow cone, with its vertex at the lightsource (which is considered as a point), then the luminous flux contained in thiscone will approximate a uniform distribution. The concentration of luminous fluxwithin this narrow cone can now be defined as the luminous flux in this conedivided by the opening of the cone. expressed in terms of the solid angle of thecone The result is called the luminous intensity (I), measured in candelas (cd). inthe direction of the centre-line of the cone.

Fig. 4 A solid angle con best be describedus the opening angle of a cone.

Fig. 5 A prone angle can be expressed indegrees or radians. One degree encloses ofthe circumference of a circle. One radianencloses 1/2π of the circumference of acircle.

Fig. 6 A steradian is the measure for a solidangle, enclosing the part of the surface of asphere (with the top of the angle as itscentre), with an area equal to 1/4π of thetotal surface of the sphere.

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A steradian is a measure for a solid angle The size of a solid angle is expressed in a similar manner aswith a plane angle, either in degrees or radians (Fig. 5). Imagine a sphereof arbitrary radius (r). with the top of the cone as its centre (Fig. 6). The partof the surface of the sphere which is enclosed by the cone is thenproportional to the solid angle (ω) of the cone. If the area fl the surface of thesphere enclosed by the cone is equal to the square of the radius (r2). thenthe corresponding solid angle is termed a ‘steradian’. So, if the enclosedarea of the surface of the sphere is not equal to but is, say, equal to A, thenthe solid angle ω = A / r2 = steradians.

A whole sphere contains 4π steradians The maximum possible solid angle will enclose the wholesphere. As the surface of a sphere of radius r is 4πr2, this solid angle will beequal to 4πr2 / r2 = 4 π steradians.Hence, a half-sphere will contain 2πi steradians

The concept of luminous intensity is of tremendous importance in lightingtechnology. as virtually no piece of lighting equipment emits its luminous fluxequally in all directions. This is quite deliberately so, for some directions areof far more importance to the observer than others, and some even have tobe avoided (glare!). The luminous intensity distribution of a lamp or luminaire,therefore, gives a good indication of its practical luminous efficiency and areaof application. Lesson 15 wilt deal further with this subject.

Some examples of practical light sources:Bicycle headlamp without reflector, in any direction 2,5 cdBicycle headlamp with reflector, centre of beam 250 cdIncandescent reflector lamp PAR38E 120 W Spot, centre of beam 9500 cdLighthouse. centre of beam 2 000 000 cd

2.2.1The photometric standard

In physics, the metre is the basic unit of length in the SI (system of units)Systeme Internationale d’Unités). Until recently, the physical standard was ametal bar of that length, made of a platinum-iridium alloy, kept at the 'Bureaudes Standards’ in Paris, France.

The physical photometric standard In photometry, the basic unit in the SI system Is the candela. Allother photometric units are derived from it. Prior to 1979 the physicalstandard for the candela was a black-body radiator (in practice formed by anarrow tube which was partly filled with pure thorium oxide). which was keptat the temperature of solidification of platinum (2042 K. see Fig. 7).The candela was defined as one sixtieth of the luminous intensity emitted by1 cm2 of the surface of the black-body radiator.

Fig. 7 > Cross-sectionaldrawing of the primaryphotometric standard as useduntil 1979. 1. black-bodyradiator in the form of a tubepartly filled with pure thoriumoxide; 2. platinum; 3, crucibleof thorium oxide; 4. insulationmaterial; S. outer container.

Fig. 8 >> Secondarystandards mostly take thefarm of carefully calibratedfilament lamps of special construction.

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The photometric standard defined in terms of radiant power In 1979 the reference standard for thecandela was redefined as fellows: 'the luminous intensity in a specific directionof a source emitting monochromatic radiation of a frequency of 540 x 1012 Hz(555 nm), and of which the radiant intensity in that direction as 1/683 watt persteradian'. In other worth: 1/683 'light-watts' per steradian.

For practical use in laboratories, secondary standards, or sub-standards, areused. These are carefully calibrated incandescent filament lamps of specialconstruction (Fig. 8).

2.3 Illuminance - symbol E, unit lux (Ix)Definition of illuminance Illurninance is the quantity of light, or luminous flux, falling on a unit area of a

surface (Fig. 9). It is designated by the symbol E. The unit is the Lux (lx). Onelux equals one lumen per square metre (lm/m2).

Illuminance can be defined as:

the ratio of the luminous flux incident on a surface to the area of that surface.Or E = Φ / A (1)

The illuminance is independent of the direction from which the luminous fluxreaches the surface.

Some practical example are:Summer, at noon. under a cloudless sky 100 000 luxDitto, but in the shade 10 000 luxIn the open, under a heavily-overcast sky 5000 luxArtificial light, in well-lit office 1 000 luxArtificial light, average living-room 100 luxStreet lighting 5 - 30 luxFull moon, on a clear night 0,25 lux

Fig. 9 Illuminance is defined as the quantity of light received on a unit area ofsurface per second. it can be compared with the amount of (rain)water fallingon an area of surface per second.

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Fig. 10 Surfaces of differentreflective properties, orviewed from differentangles, show differentluminances to the observer.

2.4 Luminance - symbol L, unit candela per squaremetre (cd/m2)

Definition of luminance Luminance is the concept for the luminous intensity emitted per Unit of areaof a surface in a specific direction (Fig. 10) The surface can itself be light-emitting. Or transmitting - like the surface of a lamp or the sun - but it canalso reflect light from another source (and thus act as a secondary lightsource). It is designated by the symbol L. The unit is the candela per squaremetre (cd/m2).

Luminance can be defined as:

the ratio of the luminous intensity from a surface in a given direction to theapparent area of that surface

Or: L = I / Aa (2)

Fig. 11 When viewed fromdifferent directions, theapparent area of thispyramid will change inshape and size.

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Fig 12. Two surfaces ofthe some luminance mayevoke different brightnessimpression. The graysquare in this illustrationlooks darker against thewhite back ground thanagainst the blackbackground, although theluminances are the same.

Apparent area By apparent area is meant (he projection of any area of the surface inquestion on a plane that is at right angles to the direction of view (Fig. 11).For a sphere. For example, the total apparent area in any direction is thearea of the cross-section of the sphere.

Surfaces with different reflecting properties will, with the same illuminance, radiate different luminousintensities, and therefore have different luminances.

As both the luminous intensity and the apparent area are independent of distance, the luminance isalso independent of distance. On the other hand, luminance is generallydependent on the direction of observation, unless the surface has perfectlydiffuse reflecting or emitting properties.

Brightness The luminous intensity radiated by a light source or an illuminated surface perunit of apparent area (i.e. the luminance) evokes a sensation of brightness.However, luminance is an objective measure, whereas brightness Is asubjective evaluation made by the observer. The latter is largely dependenton the luminance of the surface, but also on other factors, such as theoverall luminance distribution in the held of view or, in other words, theadaptation luminance of the eye (Fig. 12).

The most important lighting quantity What we really 'see' in life are luminances, or ratherluminance variations in the field of view It is therefore the most importantquantity in lighting engineering, although the other three - luminous flux,luminous intensity and illuminance - are generally easier to work with whenperforming calculations or measurements.

Some practical examples:Surface of the sun 1 650 000 000 cd/m2

Filament of a clear incandescent lamp 7 000 000 cd/m2

Bulb of a 'Softone' incandescent lamp 200 000 cd/m2

Fluorescent lamp 5 000 - 15 000 cd/m2

Surface of the full moon 2 500 cd/m2

Sun-lit beach 15 000 cd/m2

White paper (reflectance 0,8) under 400 lux 100 cd/m2

Grey paper (reflectance 0,4) under 400 lux 50 cd/m2

Black paper (reflectance 0,04) under 400 lux 5 cd/m2

Road surface under artificial lighting 0,5 - 2 cd/m2

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Fig. 13 This opal globeluminaire radiates (almost)uniformly in all directions.Therefore the relationbetween luminous flux andluminous intensity given informula (3) is applicable.

3. Practical relations between quantities

The definitions of the quantities described so far in fact also inform us aboutthe relationships between them- Nevertheless, the relationships thusexpressed are often not very suited to practical use. It is possible, however,to derive from the basic relationships a number of formulae that are importantand useful for practical lighting engineering

3.1 Relation between luminous flux (ΦΦ) and luminousintensity (I)

Relation between Φ and I for uniform radiators The luminous intensity in any direction of a lightsource whose light distribution is uniform in all directions, is equal to theluminous flux divided by 4π. Thus:

l = Φ / 4π (3)

For example, an incandescent lamp of 2000 lumens, fitted in a globeluminaire of opal glass with a transmittance of 0,9 will (approximately) have inany direction a luminous intensity of: 2000 x 0,9 / 4π = 143 cd (Fig. 1 3).

This equation it only of limited practical importance, as it is only valid for lightsources that radiate equal luminous intensities in air directions.

3.2 Relation between luminous flux (ΦΦ) and averageilluminance (Eav)

The average illuminance on a surface is equal to the luminous flux (Φinc)incident on that surface, divided by the area (A) of the surface (Fig. 14).Thus:

Eav = Φinc / A (4)

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Fig. 14 The relation betweenincident luminous flux andaverage illuminance according toformula (4).

If a luminous flux of 10 000 Im falls on a surface with an area of 12 m2, theaverage illurninance (Eav) will be 10 000/12 = 833 lux.

3.3 Relation between luminous intensity (I) andilluminance (E)

The inverse square law 1. The illuminance on a point in a plane perpendicular to the direction oflight incidence is equal to the luminous intensity in die direction of the point,divided by the square of the distance between the (point) light source andthe point in question (Fig. 15). If we call this distance d, the followingformula applies:

Ep = I / d2 (5)

for example, if a point light source emits a luminous intensity of 1200 cd ina direction perpendicular to a surface at a distance of 3 metres, theilluminance (Ep) at the point where the light strikes the surface will be1200/32 = 133 lux. If the source is at a distance of 6 metres from the lightsource, the illuminance will be 1200/62 = 33 lux.

This relationship is called the ‘inverse square law’. Strictly speaking, it isonly valid for point sources. In practice, however, the law holds goodapproximately as long as the distance between the light source and thepoint of calculation or measurement is more than three times the longestdimension of the light source. For laboratory measurements on luminaires,on the other hand, the distance should be at least five to ten times thelongest dimension of the luminaire

Fig 15. The relationbetween luminous intensityand illuminance on a planeperpendicular to thedirection of light incidence- the inverse square law

according to formula (5).

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sin 0° = 0 cos 0o = 1 tg 0o = 0sin 30o = 0,5 cos 60o = 0,5 tg 45o = 1sin 90° = 1 cos 90° = 0 tg 90o = ∞

Fig. 16 Basic goniometric formulae.

The cosine law 2. The illuminance at a point in a plane riot perpendicular to the direction of theluminous intensity is equal to the luminous intensity in the direction of thepoint, divided by the square of the distance between the light source and thepoint in question, multiplied by the cosine (Fig. 16) of the angle gamma thatthe direction of light incidence makes with the normal (perpendicular) to theplane (Fig. 14) Thus:

Ep = I cos γ / d2 (6)

This is called the cosine law.

For example. if a point light source radiates a luminous intensity of 1200 cd inthe direction of a point on a surface at 3 metres distance, and the light strikesthe surface a an angle of 600 to the normal to the surface, the illuminance (Ep)at that point will be equal to: (1200/32) x cos 60o = (120O/32) x 0.5 = 67 lux.

3.3.1 Horizontal illuminance

Illuminance on a horizontal surface For horizontal surfaces it will often he more practical tomodify the above formula by replacing the distance (d) between the lightsource and the calculation point by the vertical height (h) of the light sourceabove the surface (Fig. 19). The result is called the horizontal illuminance axthe point and the formula becomes:

Ehor = I cos γ / h2 (7)

Fig. 17 The relation between luminous intensity andilluminance on a plane not perpendicular to thedirection of light incidence - the cosine law - Fig. 18 The cosine law, modified for horizontalaccording to formula (6). illuminance, according to formula (7).

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Fig. 19 Calculation of the vertical illuminance Fig. 20 Calculation of the vertical illuminanceaccording to formula (8) according to formula (9).

3.3.2 Vertical illuminance

Illurninance on a vertical surface By rotating the system for horizontal illuminance through 90o, one obtainsthe illuminance on a vertical surface (Fig. 19). Thus:

Evert= I cos γ / d2 (8)

This is called the vertical illuminance at a point.

For practical reasons, this formula is often rewritten so as to substitute for the anglegamma between the angle of light incidence and the normal to the vertical surface,the vertical angle α between the direction of light incidence and die normal to thehorizontal surface, and the horizontal angle β indicating the orientation of thevertical surface with respect to the plane of light incidence (Fig. 20). Thus:

Evert = I sin α cos2 α cos β / h2 (9)

3.3.3 Hemispherical and semicylindrical illuminance

Illuminance on a spherical surface The illuminance on the curved surface of an infinitely smallhemisphere can be expressed as (Fig. 21):

E = I cos2 γ (1 + cos γ) / 4h2 (10)

Fig. 21 calculation of the hemispherical illuminance, Fig. 22 Calculation of the semicylindricalaccording to formula (10). illuminance, according to formula (11).

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Illuminance on a cylindrical surface Similarly, the Illuminance on the curved surface of an infinitelysmall vertical semi-cylinder can be expressed as (Fig. 22):

E = I sin α cos2 α (1 + cos β) / π h2 (11)

The practical usefulness of the concepts of semi spherical and semi-cylindricalilluminance is found in street and residential area lighting, where the illuminanceeffects on curvatures are more important for recognition of human faces thanilluminance on a point in a plane and seldom more than two light sources areinstrumental for the illuminance result

3.4 Relation between luminous intensity (I) and luminance(L)

The surface luminance of a light source or of a light-reflecting surface (secondarylight source) is equal to the luminous intensity divided by the apparent area (Aa) ofthe surface. Thus:

L = I / Aa (12)

A clear tubular high-pressure sodium lamp with a light-emitting area (dischargetube) of 100 mm length and 8 mm diameter radiates perpendicularly to thecylindrical surface with a luminous intensity of 4000 cd (Fig. 23). The surfaceluminance of the discharge tube, when viewed from the same direction, thenequals: 4000/(100 x 8) = 5 cd/mm2 = 5 000 000 cd/m2.

3.5 Relation between illuminance (E) and luminance (L)

Relation between E and L for mat surfaces In the case of a light-reflecting surface, the luminous intensity ofthe surface is usually nor known, but very often the illuminance on the surface is.For perfectly diffusing surfaces a relationship exists between the illuminance (E)on the surface, the surface reflectance (ρ), and the luminance (L) of the surface(Fig. 24). Thus:

L = ρ E / π (13)

Fig. 23 The surface luminance of the discharge tube Fig. 24 The luminance of a perfectly mat surface ina clear SON lamp, as a function of the luminous one direction as a function of the luminance canintensity, can be calculated according to formula (12) be calculated according to formula (13).

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For example. a sheet of mat paper is illuminated so as to receive an illuminance of500 lux. The reflectance of the surface is 0,7 (70%). The luminance of the sheet ofpaper in all directions then equals: 500 x 0,7 / π = = cd/m2.

This equation is valid for perfectly diffuse (man) surfaces. These display an equalluminance in all directions, no matter what the direction of view, The formula is notvalid for specular surfaces and for surfaces exhibiting compound reflection - such asroad surfaces - when viewed in the direction of the specular component

4. Other lighting units

Units in use before the adoption of the international candela The units lumen. candela. lux, and candelaper square metre are those adopted in 1948 for the SI system of units (SystèmeInternationale d’Unités). They are the only recommended lighting units. However, thereader might come across ocher lighting units, like those in use before 1948, or unitsderived from the (British) Imperial System of measures. These are therefore brieflylisted here.

unit description conversion factorLuminous fluxdekalumen 10 lumens 10 Imlight-watt 1 watt of radiant energy of a wavelength of 555 nm 683 Im

Luminous intensitycandle (international) derived from incandescent standard lamp 1 cdHefner candle derived from flame standard lamp 0,9 cd

IlluminanceMetrecandle 1 lumen per square metre 1 luxfootcandle 1 lumen per square foot 10.764 luxphot 1 lumen per square centimetre 10 000 luxnox 0.001 lumen per square metre 0,001 lux

Luminance *)lambert 1/π candles per square centimetre 3183 cd/m2

footlambert 1/π candles per square loot 3,426 cd/m2

nit 1 candle per square metre 1 cd/m2

stilb 1 candle per square centimetre 10000 cd/m2

apostilb 1/10 000 π candles per square centimetre (0,0001 lambert)0.3183 cd/m2

blondel = 1 apostilb 0,3183 cd/m2

skot = 0,001 apostilb 0.00032 cd/m2

*) All deprecated luminance units mentioned here are derived from the international candela. In olderGerman literature, however, one might encounter the 'Hefner’ apostilb, equal to 0,9 Internationalapostilb or 0,2865 cd/m2.

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Fig. 25 Plant growth depends on light - normal or artificial - and all green plantsrespond in much the same way to different spectral wavelengths (right).

5. Radiometric quantities and unitsNon-visible radiation In Section 1 it was explained that the photometric quantities and units are based

upon the visual sensation perceived by the human eye. For electromagneticradiation outside the visible region of the spectrum the quantities and unitstherefore have no meaning and the same holds true for those effects produced bylight that have nothing to do with the visual process. e.g. stimulation of plantgrowth and the various chemical processes encountered In photography andreprography.

Like the rods and cones in the retina of the human eye the active elementsresponsible for most of these photochemical processes show a specific andreproducible spectral sensitivity distribution. The spectral sensitivity curve for plantgrowth (photosynthesis), for example, is well-known, and practically independentof the species of plant (Fig. 25). It would thus be possible to create an individual'lumen' for each photochemical process; and indeed, it has been suggested thatthe 'phytolumen' be introduced as a measure for the influence of light on plantgrowth. This would have to be defined as: 'a certain quantity of radiant energyemitted per second, weighted against the standard plant-sensitivity curie forphotosynthesis'.

Radiometry follows the concept of radiant power However, in view of the great variety in photochemicalprocesses. such an approach would be neither feasible, nor practical.Therefore, for the measurement of radiation - called 'radiometry’ - a system ofquantities and units has been chosen solely on the basis of the energy content ofthe radiation within the wavelength range of interest The radiometric quantitiesclosely follow the photometric quantities! the four basic ones being: radiant flux.radiant intensity, irradiance, and radiance. The units, however, are all derivedfrom the SI unit of power. the watt (‘W). The following relations between the twosystems of measurement can be plotted:

Radiometry Photometryquantity unit quantity unit

radiant lux watt (W) luminous flux lumen (lm)

radiant intensity watt per steradian (W/sr) luminous intensity candela (cd) (= lm/sr)

irradiance watt per square metre illuminance lux (lx) (= lm/m2)

radiance watt per steradian luminance candela per square metresquare metre (W/sr.m2)

The interrelationships between the various radio-metric quantities are the sameas between photometric quantities.

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Fig 26 Photometer bench set-up for visual photometry. 1. photometer, Fig. 27 Visual photometer with2. Lamp being measured; 3. standard lamp; 4. diaphragms. built-in reference lamp for field

measurements.

6. Measuring instruments

6.1 Visual photometersTwo different measuring techniques Two fundamentally different techniques for the measurement of light, orphotometry as a termed, have been developed: the visual technique and the photoelectric technique.

Visual photometry compares luminances Visual photometry, the older of thetwo techniques. was practiced as long ago as 1760 by Bouguer and Lambert. Inmeasuring the luminous intensity of a light source, light from the source undertest is projected art a screen with diffusely transmitting or reflectingcharacteristics, along with the light from a standard light source of knownluminous intensity (Fig. 26). By varying the illuminance on the screen from oneof the two light sources, under controlled conditions, the luminances created bythe two sources are equalized, as judged by the eye of the observer. Therefore,visual photometry essentially involves the comparison of luminances.

Visual photometers have been improved over the years to a high degree ofprecision, and they remained in use until quite recently (Fig. 27). However. todaythey have virtually been ousted by photoelectric - also called physical -photometers. The photoelectric photometer is therefore the type referred to in thefollowing sections.

6.2 Photoelectric photometersPhotoelectric photometry measures illuminances In the photoelectric photometer the light falling upon the

surface of 'receptor' serves to generate a small electric current which can bemeasured.

Fig. 28 Circuit diagram of aphotometer based upon thephoto-emissive principle.To the right, the physicallayout of a photo-emissivecell: C = cathode, A =anode, B = glass bulb.

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Fig. 29 To the left, circuit diagram of a photometer based upon the Fig. 20 Circuit diagram of a photo-multiplier principle: C = cathode, D = dynode, A = anode. photometer based upon theTo the right, photo-multiplier tubes. photovoltaic principle.

Since, in photometry. light falling on an area of surface is defined as 'illuminance'it is in fact the illuminance that is measured with this class of instruments, which iswhy they are often called luxmeters. In physical photometry. therefore, allphotometric quantities have to be converted into illuminance values

The photocell employed in the photoelectric photometer can be of various types:• photo-emissive• photovoltaic• photo-conductive (e.g. photo-resistors, photodiodes or phototransistors)

Photo-emissive cells are vacuum diodes Photo-emissive cells are based on die principle that certainmetals release electrons from their surface when exposed to light. Within certainlimits, the rate of electron emission is proportional to the amount of light receivedby the surface, in other words the illuminance.

If the photo-emissive material is connected to the negative terminal of a tic source(thus forming the cathode), another metal electrode (the anode) is connected tothe positive terminal and both are placed in an evacuated or gas-filled glass bulb,then the electrons released from the cathode will move toward the anode Theresult is a minute electric current flowing through the circuit, which can bemeasured with an ammeter (Fit 28) As the electron emission rate is proportional tothe illuminance, the intensity of the current will be a measure for the amount oflight falling on the cathode

The photo-multiplier The current flowing through the circuit is invariably very small, which is why it isgenerally first amplified before being measured. Another possibility is to use a cellbased upon the 'photo-multiplier’ principle. Here, the electrons released by thephoto-cathode are first attracted by an auxiliary anode (dynode) which is itself ofthe electron-emitting type. Each electron hitting the auxiliary anode will releasefive to ten electrons from it, which will move to a next dynode carrying a highervoltage (Fig. 29). If several dynodes are placed one behind the other, the finalcurrent will be quite considerable, so chat it can be measured without problems-Photo-multiplier tubes are therefore very useful at low lighting levels.

Nowadays, the photo-emissive cell has largely given way to the photovoltaic cellfor use in photometry. as the latter offers considerable advantages, especially inportable equipment.

Photovoltaic cells are semi-conductor diodes Photovoltaic cells convert the incident light directly intoelectric power, and therefore need no external voltage source. However, tomaintain accuracy at low illuminance levels, and facilitate calibration, the outputcurrent of the cell is often amplified before being measured. The power for theamplifier is usually supplied from dry batteries.

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Fig. 31 Measuring circuit with Fig. 32 Measuring circuit with Fig. 33 Measuring circuit withphotodiode. phototransistor, also including an

amplifying stage.

Basically, a photovoltaic cell is a bipolar semiconductor, or 'diode’. It consists oftwo layers having different electric properties. sometimes separated by a thirdneutral or barrier layer. Under the influence of light, electrons migrate throughthe junction between the two layers. So, if the layers are externally connectedvia an ammeter a current is measured of a magnitude proportional to theamount of light incident on the cell (Fig. 30).

Initially, selenium was the most popular semiconductor material, but now siliconis almost universally used for the purpose. In contrast with selenium cells,silicon cells have no barrier layer.

Photometers based upon the photovoltaic principle are the most widely usednowadays for field measurements. They are mostly filtered to match, as far aspossible, the spectral sensitivity V(λ) curve of the human eye.

Photo-conductive cells are variable resistors Photo-resistors are made of a material the resistanceof which decreases under the influence of the light As the resistor is a passiveelement it always needs an external source of electric power. i.e. a battery (fig.31). The materials used most often in photo-resistors are cadmium sulfide(CdS) and cadmium selenide (CdSe).

CdS-cells are extremely popular as light-meters in photographic cameras, hutthey show too much instability (or them to be suitable for use in precisioninstruments.

Photodiodes are in fact no different from photovoltaic cells, but rely on anexternal power supply. They are mostly of the silicon type. The conductivity of aphotodiode in the reverse direction steeply increases with the amount of lightfailing upon it. as does the current through the external circuit (Fig. 32)Photodiodes are sometimes used in measuring instruments but their mainapplication lies in light-dependent switching of electrical circuits.

Phototransistors display characteristics analogous to those of photodiodes butwith the additional advantage that they also work as amplifiers (Fig. 33) Theirnon-linearity, however, makes them unsuitable for measuring purposesalthough they are widely used in switching and lighting control systems.

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Fig. 34 Various luxmeters:Left older type, withseparate filters.Right: Modem luxmeter,with digital read-out andbuilt-in (left) w separate(right) photocell with bothcolour and cosinecorrection.

7. Measuring techniques

7.1 Measurement of illuminance

Limitations of field measurement accuracy In the foregoing section, it has been explained thatphotoelectric lightmeters measure the illuminance incident on the light-sensitive cell. Although the meters intended for field work are simple aridconvenient to use, most of them - at least the less expensive types -are notdesigned as precision instruments. Careful handling and frequent calibrationwill help to maintain their reliability, but measurements in the field should notbe expected to have an accuracy of better than five to ten per cent, andeven this can only be reached under favorable conditions with a goodinstrument, In addition, all photocells exhibit certain inherent characteristics,which the user must understand to obtain the best possible results (Fig. 34).

7.1.1 Spectral sensitivity

Adaptation to the eye- sensitivity curve The response of most photoelectric cells to the variouswavelengths of the visible spectrum is quite different From that of the humaneye Meters without colour correction are therefore only accurate under thekind of illumination tar which they are calibrated (usually light from a filamentlamp of a colour temperature of 2700 K). Many meters, however, aresupplied with a colour-correcting filter, which changes their spectralresponse to a reasonably dose approximation to the spectral sensitivitycurve of the human eye.

Colour correction Even with these meters, however, to obtain accurate results with lightsources whose spectral compositions show a considerable departure fromthat of a normal thermal radiator, a ‘colour correction factor' will have to beemployed. These colour correction factors are normally supplied by themanufacturer of the photometer. Meters provided with filters. so accuratethat for exacting field measurements no colour-correction factors need to beused, do exist, but are necessarily rather expensive.

7.1.2 ‘Cosine’ correction (correction for obliqueincidence)

Errors as result of a low angle of incidence Light striking the surface of the photo-electric cell at anangle, will produce an illuminance proportional to the cosine of the angle ofincidence (see Section 3,3), but this may not always result in an equivalentresponse from the cell There are two reasons for this: Firstly, light at obliqueangles will be more reflected by the protective cover-plate of the cell, andthus not reach the light—sensitive surface; and, secondly, the protective rimoften fired round the cell partly shades the light incident n low angles. Boththese effects become more manifest with increasing angle of incidence.

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Fig. 35 Cosine correction isespecially important whencarrying out measurementsin road-lighting installations,as very oblique angles oflight incidence then oftenoccur.

Since photometers are usually calibrated for light perpendicular to me surfaceof the cell. light entering at an angle. and also diffuse light, will give readingslower than the values following from calculation, unless some correctiveprocedure is applied. For illuminance measurements in the horizontal plane.the error may vary from a few per cent - for indoor areas with direct or semi-direct lighting from above, where only a small proportion of the light isreceived through reflection from the walls or ceiling - to 10 - 15 per cent forpredominantly indirect lighting, and even as much as 25 per cent if the lightcomes from the side-windows only. With floodlighting or roadlightinginstallations, where light incidence at very low angles is often the rule, theerror can be even larger (Fig. 35).

Correction systems Good photometers should incorporate some device - for example a diffusingshield that can be placed over the cell - designed to correct the receivingsurface in such a way that light arriving from all directions is properlyevaluated - so-called cosine correction. Modern photometers are oftenprovided with a photocell of a design chat ensures good cosine correctionunder all circumstances, without the need of a special adapter.

A well-corrected photometer follows the cosine law for all angles of incidence.Uncorrected meters should be used only if one is fully aware of theirlimitations.

7.1.3Fatigue

Adaptation to the prevailing illuminance level All photoelectric cells exhibit a certain degree offatigue - that is to say a tendency for the meter indication to drop off slowlyover a period of minutes until a constant reading is obtained. This effect ismost noticeable at high illuminance values, and therefore so if thephotoelectric cell has been covered, or the meter has been kept in the darkfor some time previously - or used at much lower illuminance levels.Therefore, before starting measuring, the meter should be allowed time toadapt (for as long as may be necessary) to the illuminance level at whichmeasurements are to be taken.

7.1.4Ambient temperature

Temperature dependence With some types of photoelectric cells, the sensitivity changes with theambient temperature. If measurements are taken at extremely high or lowtemperatures, the corresponding corrections should be made. Temperature-correction tables are usually supplied by the manufacturer of the meter.

7.1.5Age effects

Regular calibration is essential Both the absolute sensitivity and the spectral sensitivity distributionof photoelectric cells may change with time. Yearly calibration of thephotometer on different types of light sources is therefore necessary.

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Fig. 36 Two types of luminance meters, the 'LMT’ meter (left) and the older'Morass' meter (right). The latter is especially suited few road-surfaceluminance measurements.

7.2 Measurement of luminance

Illuminance received from an area If an image of the apparent area of a surface whose luminance is tobe measured is projected on the photosensitive area of photo-emissive orphotovoltaic cell, the illuminance reading of this cell becomes proportional tothe luminance of the area in the direction of measurement. A luminancemeter thus consists of a photoelectric cell, and an optical system thatprojects an image of the area to be measured on the surface of the cell (Fig.36). The measuring circuit is calibrated to give luminance values in cd/m2.

Luminance meters can be distinguished into spot and area luminancemeters. Some are suitable for both types of measurement.

Fig. 37 Gonio-photometer,few measuring theluminous intensitydistribution of luminaires.

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7.3 Measurement of luminous intensity

Measuring the luminous intensity distribution Most luminous intensity measurements are made inthe laboratories of luminaire manufacturers in order to obtain the light (orluminous intensity) distribution characteristics of a particular lamp-luminairecombination. This involves measuring the illuminance on the photocell of themeter at various directions around the luminaire. Given the illuminance valueand the distance between the photocell and the luminaire, the luminousintensity can be calculated (see Section 33).

For this work gonio-photometers are used, in which either the luminaire, or asystem of mirrors (or both) is rotated with respect to a stationary photocell(Fig. 37).

7.4 Measurement of luminous flux

The Ulbricht sphere The luminous flux of a lamp can either be calculated from the luminousintensities measured in a great many directions, or measured direct in a so-called integrating photometer. or ‘Ulbricht sphere’. after the scientist who. in1900. described its principle for the first time. The lamp being measured issuspended at the centre of a large hollow sphere, painted matt-white to makeit perfectly diffusing (Fig. 38). Owing to the internal reflection, the illurninanceon any part of the sphere’s iris ide surface is proportional to the lamp’s totallight output A small window in the wall of the sphere allows this ilIum inanceto be measured, The set-up is calibrated against a standard lamp of knownluminous flux.

Whilst the Ulbricht sphere photometer is used by photometric laboratories todetermine the luminous flux of lamps, the luminous flux of a lamp-luminairecombination us more easily obtained by calculation, after measuring theluminous intensity distribution in all important directions (see the previousSection). The calculation procedure is nowadays generally carried our withthe aid of a computer.

Fig. 38 Two integrating photometers. Left. a two-metre sphere; right thefour-metre sphere an the laboratory of the Quality Department Lighting inEindhoven, which is suitable for measurements on the largest lamp types.

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Fig. 39 Measurement of the horizontal illuminance on Fig. 40 Car-mounted 'Morass' luminance meter forthe working plane. The lighting engineer has to take carrying out road-surface luminance measurementscare that hrs body does not throw shadows orreflections on the photocell.

7.5 Measurements carried out in the field

The measuring conditions must be known The outcome of any Lighting measurement can only beevaluated properly if the conditions prevailing at the time of measurementare known, For this reason, it is important to put on record all pertinentInformation. including the type of meter used, the ambient temperature, theoperating voltage of the lighting installation, the state of cleanliness of thelighting equipment and of the walls and ceilings that reflect the light, and,finally, the operating age of the lamps. For example, new discharge lampsshould burn for 100 hours before any measurements are made. The lightoutput of discharge lamps. and of fluorescent lamps Especially! also variessignificantly with ambient temperature. Therefore, these should always begiven sufficient warm-up time to attain a constant temperature in theluminaire.

Preventing shadows or stray-light When carrying out illuminance measurements the operator musttake every precaution to avoid shadows or stray-light - reflected from hisclothing - falling on the cell (Fig. 39). Also, in the case of luminancemeasurements, the test surface must be free from shadows or reflections.

Open-air measurements should not be taken during hazy weather, as lightabsorption in the air can be quite considerable under such circumstances(Fig. 40). Humid conditions can, moreover affect the transmittance of theprotective layer on the photoelectric cells.

It will be obvious that a single illuminance measurement only applies to thepoint where it is taken. Therefore, to obtain the average value over an area,a number of readings will have to be made.

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Conclusion

The measurement of light is based on a system of quantitative concepts and units that takesinto account the energy content of visible radiation, as well as the response of the humaneye to the various spectral wavelengths. The basic quantity is the luminous intensity, and theassociated unit is the candela - one of the seven basic units in the SI system ofmeasurement.

The other lighting quantities: luminous flux, illurninance and luminance, together with theirrelated units, have been derived from the luminous intensity. Their Mutual relationship Is laiddown In a number of formulae, which are of immense practical value in lighting engineering.

Lighting measurements originally involved visual comparisons with standard light sources,but today all measuring equipment works on the photoelectric principle. in essence.illuminance is the only quantity measured. The other photometric quantities must be obtainedby conversion from illuminance values by making use of the appropriate formulae.

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