Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 217

Fig. 14.14 Sample being cast
in concrete; arrow indicates
driving direction. (Sørensen
and Nielsen 1974 )

be measured in a laboratory on a road surface sample cut or drilled out of an actual
road surface, or in-situ with a portable reflectometer on the actual road.

14.2.4.1 Laboratory Measurements

The road surface sample being measured must, of course, be as representative as
possible of the road surface as a whole. The size of the sample is important: too small,
and the chances are that it will not be truly representative of the surface from which
it was removed. A typical size for laboratory measurements would be something
like 10 30 cm. If one works with drilled road surface cores of 15 cm diameter,

three cores of the same area of the actual road surface should be measured to get a
representative measurement. The cutting or drilling-out must be performed in such a
way that the sample is not distorted or its surface damaged. To guard against distortion
in the case of cutting-out a large sample, the sample should immediately be supported
in some way: for example, in a cement mould so that the surface cannot bend and the
top part remains flat (Fig. 14.14 ). An alternative, resulting in a less heavy sample, is
to use rigid steel plates and to fix the samples with glue. The measurements should
then be completed as soon as possible, before significant surface changes can take
place due to drying out. The driving direction on the road should be noted, because
often the reflection properties are influenced by the vehicles driving over the surface.
The size of the area being measured should cover a large part of the sample,
again to obtain representative results for the actual road surface. Since the luminance
coefficient is defined as the ratio of the luminance to the illuminance, the values of
this coefficient for each of the 396 combinations of

×

can be obtained from
a series of luminance measurements carried out under the standardized observation
angle of 1 ⓦ . With proper calibration, there is no need to also measure the illuminance
at the sample. Figure 14.15 illustrates the principle behind two measurement set-ups
commonly employed.
The range of angles from

ʲ and ʳ

180 ⓦ (meter
looking away from source) are set by rotating the road sample together with the

0 ⓦ (meter facing the light source) to

ʲ = ʲ =