Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 19

Fig. 3.9 Contrast threshold
curve and influence of the
veiling luminance on both the
threshold value and the
effective contrast with and
without glare

0.3

C th

C o

a

c

C th, glare
C eff, glare

b

0.03

L b L b + L veil

0.1 10

(cd/m 2 )

L back

In the figure this corresponds to the decrease from point a to point c.
The decrease in loss of effective contrast is larger than the decrease in threshold
contrast. In other words, an object that can just be seen when there is no glare
(threshold contrast) cannot be seen when glare is present unless the actual contrast
is increased. This “masking” effect forms the basis of the measure for loss of visual
performance due to glare: the so-called threshold increment, TI. This is defined as
the amount of extra contrast required to again just make the object visible under glare
conditions, relative to the effective contrast. This corresponds in Fig. 3.9 to:

c − b

TI =

b
CIE ( 1976 ) prescribes that the threshold increment should be determined on the basis
of the perception of an object that subtends an angle of 8 min of arc at the eye of the
observer.
In practice, of course, contrasts will not be increased, and the value of the thresh-
old increment then indicates the loss of visual performance due to glare. For each
different background luminance, that is to say for each part of the road surface viewed
by an observer, the effect of glare on visual performance will be different, and a dif-
ferent value for the threshold increment will be obtained. CIE ( 1976 ) recommends
employing as an overall measure of disability glare the relative threshold increment
(TI) based on the average road-surface luminance value as background luminance.
CIE states that for the luminance range 0.05 cd/m 2 < L av < 5 cd/m 2

the threshold

increment is approximated with the formula:

L veil
L 0,8
av

TI = 65 ∗