1968, No. 10 299

The design of road lighting.for given luminance and uniformity

J. Verrneulen and B. Knudsen

When a road-lighting installation is planned, a compromise has to be found beiween thequality of the installation and the expenditure required. This compromise can be arrived attoday from the recommendations which various national and international bodies havepublished, specifying values for minimum permissible average luminance and maximum per-missible non-uniformity. The design procedure described here makes use of a direct andpractical way of expressing the relation between these quantities and the multiplicity ofparameters for a road-lighting system - which include the complicated reflection charac-teristics of the road surface. The outcome is a new procedure giving a rapid determi-nation of the maximum permissible spacing of the lamps and of the most suitable type oflantern fitting.

The first essential for good observation on a lightedroad is an adequate level of luminance, i.e. the lumi-nous flux which the road reflects per unit surface andper unit solid angle in the direction of the observer.Another important factor is that there should be nogreat difference in luminance; it should be as uniformas possible. When producing a design for a roadlighting installation it is therefore necessary to deter-mine the luminance distribution of the design. Theluminance of a given part of the road surface dependson its illumination level and reflection characteristics.Generally speaking, a road surface is neither a perfect(specular) reflector nor a perfect diffuser, but some-thing in between [11. Owing to the complicated direc-tional dependence of the reflection characteristics,the luminance distribution of a lighting installationcannot be expressed in a simple mathematical ex-pression; it can only be derived from a large numberof measurements carried out point by point. Untilrecently this meant that it was not possible to produceseveral alternative designs at short notice - which iswhy international road-lighting recommendationshave only quoted illumination levels.Nowadays, however, computers can speed up the

processing of the data obtained from point-by-pointmeasurements, and measuring instruments have beendeveloped which can determine in a single measure-

Ir. J. Vermeulen is with, and B. Knudsen was formerly with,the Philips Lighting Laboratory, Eindhoven.

ment the average luminance ..of a given road surfacein given lighting conditions. The large quantity ofdata made available in this vyay has made it possibleto lassify road surfaces in xisting lighting installa-tions, and this has made it simpler to produce newdesigns. Another result is that international recom-mendations can now specify average luminance levelsand values of uniformity, expressed for example asthe ratio of the average and minimum luminance [21.With a given lantern at a given height above the

road surface, the spacing of the light sources is thevariable that determines the uniformity of the lumi-nance and its average level. The usual practice is forcomparative designs to be made for a number oflanterns and to choose the type which permits thegreatest spacing between light sources. This is becausethe cost of a lighting installation is largely determinedby the number of lamp posts required. Progressivestandardization of the posts' sets a limit to the choice

[I] Here it is very important whether the road surface is dry,damp or wet; see J. Bergmans, Lichtreflectie door wegdek-ken, Thesis, Delft 1938; J. B. de Boer and A. Oostrijck,Reflection properties of dry and wet road surfaces and asimple method for their measurement, Philips Res. Repts. 9,209-224, 1954; W. Kebschull, Die Reflexion trockner undfeuchter Strassenbelge, Thesis, Berlin 1968.

Since the reflection characteristics vary with the wetness ofthe road surface in a way which is difficult to predict, thedesign procedures described in this article relate only todry surfaces.

[2] International recommendations for the lighting of publicthoroughfares, Publication C.I.E. (Commission Internatio-nale de l'Eclairage) No. 12 (E 3.3.1), Paris 1965.

300 PHILlPS TECHNICAL REVIEW VOLUME 29

of the height of the light source, and in some casesthe mounting height is determined by local conditions.As a rule the procedure in drawing up a design is todeterrnine first of all the greatest spaci ng between light

These two stages of the design procedure will nowbe dealt with, in the same order. First we must describethe geometrical configuration of the light source, theroad surface and the observer.

Fig. 2. Arrangement for measuring luminance factors on a sample of road surface in thelaboratory. The sample is placed on a turntable to which a luminance meter is attached. Therotation of the table corresponds to the variation of the angle (seefig. I). The light source,which gives a narrow beam of light, travels along a rail 35 metres long at a height of 2.9metres. The beam of light remains automatically directed upon the sample of road surface.In this way the angle y is varied, while the luminous intensity I in this arrangement is inde-pendent of y and o. The measuring device records the product Cf C053 y.

sources which is still compatible with the uniformity Configuration of light source, road surface and observer

required. A calculation is then made ofthe light-source Fig. I illustrates a road lighting installation. Aspacing that gives the required average luminance light source S (which may be treated as a point source)level; the smaller of the two distances is the maximum is at a height h above the road surface. The point Pspacing that may be used in the design. of the road surface illuminated by S is seen by an

1968, No. 10 DESIGN OF ROAD LIGHTING 301

/ /" ......-r r--...

302 PHTLIPS TECHNlCAL REVIEW

the isoluminanee diagram (.fig. 4a, b). This diagram isapplicable to the combination of a given type of roadsurface with a given type of lantern, and to thespecified position of the observer.

The height ofthe light source h (the mounting height)appears in these diagrams as the unit of length. Theluminance distribution changes uniformly with changesin the mounting height h.

VOLUME 29

which will give errors never greater than 17% forturning angles likely to be encountered on practicalroads. In such a situation the part of the diagrambehind the lantern is not rotated, and the part betweenthe lantern and the observer is rotated towards theobserver. In fact, the error of 17% is only found forbends with a small radius of curvature, as found insmaller roads.

Fig. 3b. The road surface to which the luminance-factor diagram of fig. 3a applies, lighted by alamp without a lantern (! = constant). The luminance distribution thus obtained is propor-tional to the luminance factor distribution (shown again on the right); the photograph givesa kind of perspective picture of the distribution.

Both the luminance-factor diagram of a road surface and thelight distribution of a lantern, given by fry,,)), can be measuredin the laboratory. The point-by-point multiplication of the twodistributions has been simplified by the introduction of computers;the isoluminanee diagram resulting from this multiplication cantherefore be produced entirely in the laboratory, making itpossible to dispense with time-consuming measurements outof doors, with their dependence on weather conditions.

If the observer is not situated on the line throughthe light source and parallel to the direction of theroad, or if there is a bend in the road, the luminance-factor diagram is then rotated with respect to the lightdistribution on the road surface. The product of thetwo will then yield a different isoluminanee diagram.However, it has been shown in our laboratory [3] [4]that there is a simple way of using the isoluminaneediagram for an observer under the row of lanterns

The design of road lighting of sufficient uniformity

On a road lighted by a row of lanterns there is apatch of minimum luminance between each pair oflanterns. The ratio of this minimum luminance Lm.into the average luminance L of the lighted road surfaceis a measure of the uniforrnity of the luminance. Thepresent internationally recommended value for theratio Lmin/L is at least 0.4 [2]. Since L is not so easyto determine, investigations have been made to findout whether the ratio between minimum and maximumluminance Lmin/Lmax might also be a useful measureof the uniformity. Both ratios were calculated for2160 lighting installations; the results are presentedin .fig. 5 [4]. The figure shows that there is a highdegree of correlation between the two ratios: thetwo dashed lines enclosing 95 % of the cases lie close

1968, No. 10 DESIGN OF ROAD LIGHTING

l~j / \. \/

.........20 V V 1\ \ j'--.

304 PHILlPS TECHNICAL REVIEW VOLUME 29

from each lantern is 0.125Lmax and the minimumluminance occurs at the point of intersection of theisoluminanee curves for 0.125 Lmax of the two lanterns.The design proeed ure therefore consists in moving theisoluminanee diagrams of two successive lanterns overeach other on the plan of the road to be lighted until asituation is reached where no point of the road surfacebetween two lanterns falls outside the 0.125Lmaxisol uminance curves of both lanterns (jig. 6a). Toachieve this, the 0.125 Lmax curves are moved apart

The spacing of the light sources, again expressed withh as the unit of length, is now found from the locationof the isoluminanee diagrams on the plan.If the observer is not under the row of lanter ns

- a case which applies to the drivers of oncomingvehicles on the offside of 0 in fig. 6a - then allthe isoluminanee diagrams are rotated (fig. 6b), andthis is also done if there is a bend in the road(fig. 6c). If the cases in fig. 6a and b indicate differentlight-source spacings, the smaller value must be chosen.

s

f.

Fig. 6. lsoluminanee curves O. I2S LIll'x for two successive lanterns whose spacing s is suchthat no point of the road falls all/side both curves.a) Straight road, observer 0 (driver of a car) directly under the row of lanterns.b) Straight road, observer 0 not directly under the row of lanterns (driver of oncoming

vehicle with respect to a).c) A bend in the road.

until their point of intersection coincides with the Average luminanceedge of the road. This is where the point of minimum Once the maximum light-source spacing compatibleluminance lies. In the immediate vicinity of this point with the required uniformity has been established, thethe luminance may perhaps fall to below the per- next problem is to find out whether the average Iurni-missible minimum of 0.25 Lmax if the contribution nance is high enough at this spacing. Since the lumi-from one lantern decreases faster than that from the nance distribution over the road surface is nowother increases. In a final design this should be in- known, the average luminance can be calculated as thevestigated by calculating the luminance at a number average of a large number of points. This is timeof poi nts around the point of intersection of the curves. consuming, however, and a faster method has there-

1968, No. 10 DESIGN OF ROAD LIGHTING 305

fore been developed. To describe this method we mustintroduce two new parameters; we shall also describean instrument that can be used for measuring themagnitude of these parameters for an existing roadsurface. These parameters are the average luminancefactor qo -- which is small for a dark road surfaceand large for a light road surface -- and the quantityK, which is a measure of the spread of the luminancefactor q over the road surface [5]. The quantity K isdefined as follows:

qoK = 10glO --,

qmin

where qmln is the smallest value of q encountered. Ifthe road surface is a perfect diffuser, then q is every-where equal to qe; in this hypothetical case, therefore,qmln = qo and K = O. The more the road surfacediffers from a perfect diffuser, that is to say the morespecular reflection it gives, the higher becomes thevalue of K. Since the ratio qo/qmin can reach very highvalues for wet road surfaces (e.g. 104, compared withvalues between 1 and 4 for dry road surfaces), it isbetter to use the logarithm of this ratio as the charac-teristic quantity.

The quantity K characterizes the reflection charac-teristics of the road surface so completely that theluminance-factor diagrams of all road surfaces withthe same K may be assumed to be similar (to the ac-curacy required in calculating the average luminance).The consequence is that a difference in the averageluminance factor q of different road surfaces withthe same K just means that the road surface is lighteror darker. This amounts to saying that qo characterizesthe amount of the reflected light and K its distribution,and this approach is what is used in the calculationof the average luminance.

In practice it is necessary to use the quantity

qo

306 PHILlPS TECHNICAL REVIEW VOLUME 29

tional to the fraction of the surface that transmitslight.

The ceiling is situated in the instrument at a heightof 30 mm above the road surface under investigation;the point P to be measured is perpendicularly belowthe centre-point of the concentric circles. A smallarea around P is observed by a luminance meter froman angle of IO with the road surface [7). The size of

ment is calibrated by means of a standard surface ofknown average luminance factor.

To determine

1968, No. 10 DESIGN OF ROAD LIGHTING 307

Fig. 10. Standard configuration of a road with lanterns. Any conceivable configuration forwhich the average luminance is to be calculated can be built up froin the strips into which theroad is divided. If there are more rows of lanterns the calculation is done in steps and theresults added. '.

to the output of the lamp in the lantern, expressed byits luminous flux CP, and inversely proportional tothe surface area to be illuminated per lantern. If theroad is w metres wide and the spacing between thelamp posts is s metres, this area is ws m2. There isalso a more complicated dependence of the averageluminance on the light distribution of the lantern,the Kp value of the road surface, the configuration ofthe installation (single-sided, staggered, central, etc.)and the position of the observer. All these effects areincluded in a single proportionality factor 1:, and wecan therefore write:

L = qo 1: CPjws.

The value of 1: now has to be calculated for all possible

cases from the data relating to the road surface andlanterns.To summarize the many situations which can occur,

a road surface classification by

308 PHILlPS TECHNICAL REVIEW VOLUME 29

It simplifies the calculation if 7:j is not quoted separ-ately for each strip but straight away as the sum

11

~ 7:j,1=1

as a function of n. This is done by giving ~7:j curves.An example is shown in fig. lJ, for a lantern withthe light distribution of fig. 4a and a road surfacebelonging to class Il. The figure shows three curves,

~!

h !i!I

103 1;02 f;t, _ .EJ..1-'-

0.2_. - 02+IT) ~ _--- ----03t 0.1 ./-~.."V"0

1.~* ...!!!. OiL 0 0.5 1.0 1.5-~h~ !0.1.-.- .-" ,"

--------- --- -In~0.2

Fig. IJ. Graph of :ETj plotted on the transverse cross-sectionof the standard installation (fig. 10). These curves apply to thelantern whose light distribution is given in fig. 4 and a roadsurface of class 11.Above the graph there is a diagram of thedesign used for the calculated example in the text, in the correctsituation with respect to the graph and to the correct scale.There are three curves for the three observation positions 01,02 and 03.

corresponding to the three different observation po-sitions Ol, 02 and Oa .shown in the little drawingabove the graph in fig. 1l. The ~7:1 value is readfrom the graph by drawing the transverse cross-sectionof the. design on the horizontal axis, to scale. The b7:jvalue for the right-hand kerb, minus that for the left-hand kerb, gives the ~7:1 value for the width of the

road. The road illustrated in the little drawing abovethe graph extends in width from -O.lh to +l.Oh.The ~7:j value for the observation position 02 in thiscase is ~7:j = 0.20 - (-0.03) = 0.23.

Once the value of ~7:j has been determined in thisway, the required spacing s between the lamp postsat the desired average luminance L can be found fromequation (3).

In some lanterns lamps of different light outputcan be used without causing much change in the lightdistribution. If the spacing s required for the specifiedaverage luminance is appreciably less than that requiredfor adequate uniformity, it may be cheaper to use amore powerful lamp.If the manufacturer of a road-lighting lantern sup-

plies the isoluminanee diagram and the set of ~7:jcurves for the lantern with different positions of theobserver, for each of the five