Equipment: Road Surfaces - Road Lighting: Fundamentals, Technology and Application - page 164

has,
can, for motorized traffic, be neglected since it varies between only 0.5 and 1.5 ⓦ
for the stretch of road important to a road user (60-160 m ahead) (De Boer et al.
1952 ). Luminance coefficients are therefore determined with

In practice, for motorized traffic, the influence the angle of observation ʱ

held constant at 1 ⓦ .
At the time of writing, research is being considered on finding a practical road surface
reflection system for different observation angles; as for example, those relevant for
pedestrians (Chain 2008 ; Chain et al. 2011 ). For the conditions of observation for
car drivers, the luminance coefficient of a road surface can be said to be dependent
upon only two angles:

ʱ

ʲ and ʳ :

q = q ( ʲ , ʳ )

The calculation of luminance is made somewhat easier if the so-called reduced
luminance coefficient, R, is used in place of q , where

cos 3 ʳ

R = q ( ʲ , ʳ ) ∗

The luminance, L, at an element on the road given by a single source can then be
written as

I
h 2 ∗

I
h 2

cos 3 ʳ L = q ∗ E = q ∗ = R ∗

where:

R Reduced luminance coefficient of an element on the road surface as defined by

ʲ

ʳ

(cd/m 2 /lux) and

I Luminous intensity in the direction of the point (cd)

h Mounting height of the luminaire (m)

The complete reflection characteristics of a road surface can be given in a table in
which R values are specified for a number of combinations. Such a table is called

a reflection or R-table for the road-surface in question. An example of an R table
is given in Table 12.1 . Using a table such as this, it is a simple matter to calculate
the luminance produced by a single luminaire at any point on the road surface with
the formula given above. Repeating this for adjacent luminaires and summing the
results will then give the total luminance at the point. All road-lighting-calculation
software makes use of this method.
The remaining question now is how to obtain a reflection table for a particular
road surface so as to be able to carry out luminance calculations when designing
a road-lighting installation for that particular surface. The many values needed in
order to draw up a reflection table can only be accurately determined by performing
lengthy and tedious laboratory measurements on a representative sample of the road
surface (see Chap. 13). Portable reflectometers suitable for the measurement of road
surfaces in the field are not generally available, as will be discussed in Sect. 12.7.
Moreover, the road-lighting design often has to be made before the road has even
been constructed. Fortunately, there is an alternative method for obtaining a proper
reflection table on which to base the lighting design. This method uses a classification

ʲ - ʳ