Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 218

Fig. 14.15 Schematic
laboratory set-up for
obtaining the reflection table
of a sample of road surface

ʳ

1°

ʲ

meter about the vertical axis through the point of measurement. The sample should
be mounted with the driving direction parallel to the 0 ⓦ -180 ⓦ

ʳ
angle can be set by moving the light source along a straight line such that its vertical
distance above the sample remains constant. A long (dark) corridor is needed for this
purpose. The solid angle subtended by the source at the point of measurement will
increase with decrease in the angle of light incidence (smaller angle

ʲ direction. The

); as is the case
in an actual road-lighting installation. If the measuring area covers the sample more
or less completely at large

ʳ

angles, it means that at smaller angles the measuring field
covers only part of the sample. Alternatively, as also illustrated in Fig. 14.14 , the light
source can be rotated over an arc about the sample. Here the solid angle subtended
by the light source at the point of measurement is constant, as is the measured size
at the sample. It is possible to introduce a variable-aperture diaphragm in front of
the source to make the solid angle change (just as is the case with the method of
the light source moving along a straight line). Of course, as with the measurement
of the light distribution of luminaires, here too a mirror or set of mirrors can be
employed to make the measuring set-up more compact. As an alternative to using
one moveable source, a number of fixed sources located so as to give the necessary
range of

ʳ

ʳ

angles, and switched on in turn to give a short flash, can be used. Such
a set-up has the advantage that the time needed to complete the rather large series
of measurements can be kept to a minimum. This is especially important where wet
sample measurements are concerned to avoid inaccuracies due to premature drying.

14.2.4.2 Field Measurements

The use of reflectometers makes it possible to measure a larger number of locations on
the actual road surface in order to get an impression of the variation in reflection prop-
erties over entire surface, thus obtaining the most representative values of S1 and Q 0 .
Different types of portable reflectometers have been developed. These are based
on four different principles of operation: