Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 20

where:
TI is in percent;
L veil for an observer looking straight ahead in a direction parallel to the road axis
and 1 ⓦ

down from the horizontal;

3.1.3.2 Spectrum and Disability Glare

Research into a possible wavelength effect on disability glare has shown that there is
no or only a very marginal effect (Jainski 1962 ; Wooten and Geri 1987 ; Davoudian
et al. 2013 ; Niedling et al. 2013 ). As will be discussed in Chap. 4, the same cannot
be said for discomfort glare.

3.1.3.3 LEDs and Disability Glare

LED road lighting luminaires consist of an array of numerous small bright LEDs (see
Chap. 10). The consequence is that these luminaires have a much more pronounced
non-uniform luminance over their light-emitting surface as compared to luminaires
using more conventional light sources. As has already been explained, the disability
effect of glare is caused by scattering of light within the eye that leads to a bright
veil, that in turn masks the field of view. Considering this process, one may not
expect a large difference in disability glare effects for LEDs and conventional light
sources. However, recent research indicates that the pronounced non-uniformity of
the light-emitting surface of LED luminaires has an influence on discomfort glare.
Where in the past it was often reasoned that by limiting disability glare discomfort
glare would also be restricted to a large enough extent, this may no longer be true in
the case of LED luminaires (see Chap. 4).

3.1.4 Contrast and Vertical Illuminance

As we have seen, the principle of silhouette seeing, on which road lighting is based,
requires sufficiently high negative contrasts. This in turn means that the vertical
illuminance on the side of objects facing oncoming motorists should be restricted.

3.1.4.1 Vertical Illuminance and Glare Restriction

For symmetrical luminaires (relative to the longitudinal axis of the road) good glare
restriction automatically limits the vertical illuminance on objects provided by light
from the luminaires. Provided the glare restriction specification is good, there is there-
fore usually no need to also specify the limitation of vertical illuminance provided
by the luminaires.