Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 11

Fig. 3.1 Decrease in
threshold contrast, C th , with
increase in background
luminance, L b for objects
subtending an angle of 4, 7
and 9 min of arc. Positive
contrast, observation time 0.2
s, observer age 30 years.
(Adrian 1989 )

1.00

C th

0.75

4 min

0.50

7min

0.25

9min

0

0.1 1 10

L b (cd/m 2 )

seen in silhouette and its contrast is negative; if, on the other hand, it is brighter than
its background its luminance contrast is said to be positive. The luminance contrast
needed for an object to become visible depends, amongst other things, on the lumi-
nances surrounding the object, as it is these that determine the adaptation condition of
the observer's eye. This is illustrated in Fig. 3.1 , which gives the relation between the
just-perceivable contrast—termed the threshold contrast, C th —and the background
luminance for three different sizes of objects (seen under a visual angle of 4, 7 and
9 min, respectively). The figure shows that the threshold contrast C th decreases as
the background luminance and the size of the visual angle increase. Increasing the
background luminance, viz. the luminance of the road surface, thus increases the
chances of the object being detected, because the threshold contrast for objects is de-
creased. Another way of expressing this positive effect of background luminance on
contrast seeing is by saying that a larger adaptation luminance increases the contrast
sensitivity of the eye.
When the contrast threshold for a situation is known, it is possible to calculate
whether or not an object with given size and reflectance factor is visible. For this
purpose Adrian ( 1989 ) published a comprehensive set of formulae with which the
contrast threshold can be calculated depending on, amongst other things, the back-
ground luminance, the degree of glare, the object size, the observation time and the
age of the observer. This set of formulae, sometimes called “the Adrian model”, is
given in Appendix B. The effect of age can also be calculated from these formulae.
The actual effect on visual performance of these age factors will be demonstrated in
a further section of this chapter, while Chap. 7 will explain the reasons for the loss
of vision with age.