Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 220

Measurement of the average luminance coefficient Q 0 is much more complicated.
According to its definition, Q 0 is directly proportional to the luminance at the point
of the road surface considered that is lighted from a luminous ceiling with a constant
intensity in the direction of that point. De Boer and Vermeulen ( 1967 ) described this
rule of proportionality and constructed a reflectometer based on it. Their reflectome-
ter employed a large number of small lamps uniformly distributed above a ceiling
with a filter of alternating black and white rings to provide the required luminance
distribution of that ceiling (Fig. 14.16 b). Measurement is done under an angle of
1 ⓦ with a mirror system. This instrument was bulky, heavy and difficult to align
correctly. A much more handy version working on the same principle has recently
been prototyped. Here the required luminance distribution of the ceiling is obtained
by means of several, separately-controlled arrays of LEDs arranged above a dome
of diffusing acrylic glass (Panhans et al. 2009 ).
Some reflectometers have been developed that measure only a limited number
of luminance coefficients for a limited number of angles of light incidence (see
Fig. 14.16 c)—(Debergh and Embrechts 2008 ; Fellin et al. 2009 , Korobko et al. 2013 ).
The results of the measured luminance coefficients are used for an extrapolation
process to derive all luminance coefficients of the complete reflection table from
which both Q 0 and S1 are calculated. The limited number of angles of light incidence
are realized by small LEDs. The calculation software needed, is part of the control
software of the reflectometer.
A method to overcome the problem of the small observation angle of 1 ⓦ makes use
of the physical principle that states: when the angle of light incidence and directions
of observation are interchanged, the light rays behave the same. A device based
upon this principle illuminates the road surface under an angle of 1 ⓦ and measures
luminances with luminance detectors at many fixed observer positions: these, in the
normal reversed situation, represent light-source positions for important directions
of light incidence (Muzet 2008 ). So the situation is as in Fig. 14.16 c, with small
luminance detectors at the positions of the light sources and a fibre-optics system
that provides for light incidence under an angle of 1 ⓦ . The measurement results are
used as input for a mathematical model that delivers the S1 and Q 0 value of the
road-surface area measured.
Some reflectometers do not measure under the small observation angle of 1 ⓦ
but under one or more larger and more convenient angles. For example, one such
reflectometer (Laurent 2008 ) employs a set of luminance detectors mounted on four
semi-arcs, that view the sample under -angles of 5 ⓦ ,10 ⓦ ,20 ⓦ ,30 ⓦ ,40 ⓦ ,50 ⓦ ,60 ⓦ ,

70 ⓦ and 80 ⓦ (Fig. 14.16 d). Only four fixed light sources (light incidence angles

ʱ

:
0 ⓦ ,30 ⓦ ,50 ⓦ and 70 ⓦ ) are used. Detector arcs are mounted at five offset angles so as
to measure all

ʳ

-angles viz. 0 ⓦ ,10 ⓦ ,20 ⓦ ,30 ⓦ , and 150 ⓦ .
The luminance coefficients of the reflection table valid for the observation angle of 1 ⓦ
are, again, derived from a mathematical model. Of course, here too, the calculation
software required is part of the control software of the instrument.
All reflectometers have a sturdy cover, which also serves to screen light from
the lighting installation reaching the measuring area. Normally, measurements are
made during the night. This because it is very difficult, given the unevenness of road

ʳ -angles under five different ʲ