Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 17

Fig. 3.8 Light scatter in the
eye due to glare

ʸ

fovea

scene

E eye

the uniformity), the worse will be the visual performance for objects seen against the
low-luminance part of the road surface, which is where the contrast value is low and
the contrast threshold is high. As an extra negative effect, large luminance differences
in the field of view also result in a lowering of the contrast sensitivity of the eye,
and give rise to so-called transient adaptation problems. This effect can be explained
by considering the brighter parts as forming glare sources for an observer looking
towards the darker parts of the road surface (Adrian and Eberbach 1968/1969 ).

3.1.3 Contrast and Glare

3.1.3.1 Disability Glare

Glare can take either of two forms: disability glare and discomfort glare. Disability
glare is the form that is responsible for a negative influence of glare on visual perfor-
mance. The mechanism by which the loss of visual performance due to the presence
of glare takes place can best be understood by considering the light scatter taking
place in the eye of the observer, Fig. 3.8 ).
A sharp image of the scene in the direct field of view is focused on the retina of the
eye, the resulting visual sensation being determined by the luminance of the scene.
At the same time, however, light coming from a glare source that is too close to the
direct line of sight is partly scattered in the eye lens and eyeball. Part of the scattered
light is re-directed towards the fovea, where it interacts negatively with the image of
the road ahead: it reduces its contrast. It acts as a bright veil drawn across the field
of vision. This veil can be considered as having a luminance—the equivalent veiling
illuminance (L veil )—proportional to the quantity of light scattered in the direction
of the retina. The overall effect on visual performance is then determined by two
components: scene luminance and equivalent veiling luminance. Holladay ( 1927 )
found, on the basis of visibility tests, the latter component to be dependent upon the
illuminance (E eye ) on the eye and the angle (

) between the viewing direction and the
direction of light incidence from the glare source and on the age (A) of the observer.

ʸ