Visual Comfort for Motorists - Road Lighting: Fundamentals, Technology and Application - page 53

Fig. 4.4 The photograph of a
lighted road which was
employed by De Boer when
making assessments of
discomfort glare. The holes at
the position of the luminaires
was filled completely and
homogeneously with the light
of various types of light
sources

4.3.2 Spectrum and Discomfort Glare

As has been mentioned in Chap. 3 there is no or only a very marginal effect of
the spectrum on disability glare. The discomfort glare sensation, however, is clearly
influenced by the spectrum of the light source used. De Boer ( 1955 , 1974 ) carried out
early investigations with observers appraising discomfort glare on a model with low-
pressure sodium, high-pressure sodium and high-pressure mercury light (Fig. 4.4 ).
On the basis of these investigations it was concluded that the 1976 CIE formula
of the discomfort glare control mark should incorporate a correction factor of

+ 0.4

to 0.6 for high-

pressure sodium lamps (Van Bommel and de Boer 1980 ). This is because these
lamps result in lower discomfort glare than lamps with a cooler light colour (higher
colour temperature).
With the introduction of LED light sources that are available in a great variety of
spectra, the effect of the spectrum on discomfort glare has received renewed interest.
Recent research shows that the spectral sensitivity for discomfort glare is not solely
determined by the photopic or by a combination of the photopic and scotopic spectral
eye sensitivity. Especially also the short-wavelength type of cone cells (so called “S”
or “blue sensitive” cones) play a role (Bullough 2009 ; Fekete et al. 2010 ; Bodrogi
et al. 2012 ; Akashi et al. 2013 ; Niedling et al. 2013 ). This explains why light sources
with a relatively large amount of short wavelengths (blue light) result in higher
discomfort glare effects than do long-wavelength spectra (yellowish light).
So far, much of this research has been carried out with narrow-wavelength light
sources (quasi monochromatic). Preliminary tests with conventional light sources
(high-pressure sodium, fluorescent and metal halide lamps) and broad-wavelength
white light LED sources indicate that in the colour temperature range of 2200-6000
K the difference in glare control mark varies from some 0.5-2 point in the nine-point
de Boer scale to the advantage of the lower-colour-temperature versions (Niedling
et al. 2013 ; Zhu et al. 2013a ; Lin et al. 2014 ). Ongoing research will hopefully result

+ 0.8 in the case of low-pressure sodium lamps and + 0.3 to +