Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 16

Fig. 3.6 A set of objects with
reflectance factors typical of
those found in a pedestrian's
clothing ( top ), seen against
three different road-surface
backgrounds, in order of
increasing luminance. With
good road lighting most of the
objects are seen in negative
contrast (silhouette vision)
Fig. 3.7 Photograph taken by
Waldram at the beginning of
the last century to illustrate
the silhouette effect of road
lighting. A white and a black
cat are seen as dark
silhouettes against the bright
road surface of high
luminance. (Waldram 1938 )
early researchers investigating the luminance concept of road lighting. Figure 3.7
shows one of Waldram's original photographs. The white and black cats are seen as
grey and black silhouettes against the bright road surface.
Of course, the road surface is barely visible as a background whilst stuck in a
traffic jam. But because of the slow traffic speeds and short distances over which
visibility is required, poor visibility is not really then a problem. Once the traffic
moves on and speeds increase and visibility over greater distances is again required,
the road surface again forms the background to most objects on the road (Fig. 3.7 ).
To ensure sufficiently-high contrasts and thus a sufficient visual performance at
all locations on the road, the difference between the average and the minimum road-
surface luminance should not be too great. This can be ensured by having a high
enough ratio of minimum-to-average road surface luminance. This luminance ratio
of L min to L av is called the overall uniformity, U o . The smaller this ratio (i.e. the poorer