Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 21

3.1.4.2 Vertical Illuminance and Reflection from the Road Surface

Light from the luminaires not directly aimed at the object but at the area of road sur-
face in front of an object, contributes to the vertical illuminance at the target by the
light reflected from that road-surface area. This indirect contribution decreases the
value of the negative contrast of that object. The contribution is, of course, larger for
flat objects than for three-dimensional ones, where the reflected light from the road
surface cannot reach their upper sides. Adrian and Gibbons ( 1993 , 1999 ) showed
that the (negative) impact of the reflected light from actual road lighting installations
is, for certain object positions on the road, for flat objects, not negligible. Although
the same authors introduced a provisional calculation system for the indirect contri-
bution, a generally-applicable system is not yet available. This means that visibility
calculations based on contrast seeing, give a slightly too optimistic result.

3.2 Threshold Visibility

3.2.1 Revealing Power

The number of objects out of a defined set that are just detectable at various points
on a road can be calculated from the lighting values obtained from a road-lighting
installation. “Just detectable” means that they are at the threshold of visibility. If
the set of objects is chosen so as to represent the probability of the occurrence of
reflectance factors of pedestrian clothing, the percentage of objects detectable at each
point on the road is called "the revealing power " of the lighting installation (Waldram
1938 ). The probability of occurrence of reflectance factors of pedestrian winter cloth
has been measured by Smith ( 1938 ) and later by Hansen and Larsen ( 1979 ). Hansen
and Larsen measured the reflectance factors of cloth of 1024 pedestrians randomly
chosen on the street. In Japan, the number of objects larger than 10 cm found on
actual express ways, have been measured together with their reflectance. Around
1300 objects were analysed (Hirakawa et al. 2007 ). Figure 3.10 shows the results for
both clothing and objects.
Figure 3.11 (left) shows a rough representation of the reflectance factor occurrence
of Hansen and Larsen's clothing. The same figure (right) shows the objects that may
be visible under a road-lighting installation of not-too-good quality: many of the
darker, but only some of the brighter objects are visible against the bright road
surface. This is typical for a poor road-lighting installation: objects in the range of
moderate-to-high reflectance factors are invisible.
Revealing power is eminently suitable to provide an insight into how the different
aspects of road lighting, such as average road surface luminance (L av ), uniformity
(expressed as overall uniformity U o ) and glare restriction (expressed as threshold
increment TI), influence visibility. In order to provide a fundamental insight into
how these basic parameters influence visual performance, revealing power values for