Visual Comfort for Motorists - Road Lighting: Fundamentals, Technology and Application - page 51

Fig. 4.3 Luminaire and
installation characteristics
influencing discomfort glare

I 88

I 80

h'

F 76

L av

4.3 Glare

4.3.1 Discomfort Glare

It has already been shown that glare can have a distinct negative, disturbing effect on
visual performance. But this is not all: it may also have a distinct discomfort effect.
To distinguish between these two effects we speak of disability glare (already dealt
with in Sect. 3.1.3.1) and discomfort glare, respectively. These two effects may be
caused by partly different mechanisms in the eyes, nerves and brain. It has been
hypothesized that discomfort glare effects coincide with and can be studied through
uncomfortable contractions of the iris (changing the pupil size), eye lens and muscles
surrounding the eyes (Berman et al. 1994 ; Murray et al. 2002 ; Stone 2009 ; Zhu et al.
2013a ). Discomfort glare may divert the attention of the motorist away from the
scene to be surveyed towards the bright glare source, thereby reducing safety. This
seems to be especially a problem for beginning drivers.
Many investigations have been carried out, again especially in the seventies and
eighties of last century, in which observers were asked to appraise the degree of
discomfort glare experienced under both static and dynamic driving conditions, us-
ing both model and actual installations. These are discussed in some detail in Van
Bommel and de Boer ( 1980 ). From these investigations it was found that the de-
gree of discomfort glare is influenced by the following luminaire and installation
characteristics (see also Fig. 4.3 ):

• Luminaire characteristics

absolute luminous intensity (cd) at an angle of 80 ⓦ to the downward vertical
in the vertical plane parallel to the road axis (the smaller this intensity the
better),

I 80 :

run back ratio of the luminous intensities at 80 ⓦ and 88 ⓦ to the downward
vertical in the vertical plane parallel to the road axis (the larger the run-back
ratio the better),

I 80 /I 88 :

apparent flashed (light-emitting) area of the luminaires (m 2 ) as seen at an
angle of 76 ⓦ to the downward vertical (the larger the area the better);

F