Lighting
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Disability glare
The optical surfaces and media of the eye are not perfectly clear. Additionally,
there are internal reflections within the eye. Some of the light entering the
eye is scattered and reduces the contrasts that would otherwise be imaged on
the retina. This is usually of little importance except when the glare source
is relatively intense and near the line of sight Under these conditions, the
scattered light is called "disability glare" because it can noticeably
interfere with the visual task. There are, moreover, individual differences,
particularly when some forms of eye pathologies are considered in the amount
of light scattered within the eye (Wolbarsht, 1977).
Relationship between equivalent veiling luminanceand angular separation
between line of sight and glare source.
Disability glare may be measured in terms of an equivalent veiling illumination,
a uniform light that shines on the retina and reduces the contrasts of the
image. This happens when driving at night and headlights of an oncoming car
reduce the ability to see the road in front even though the eyes are not looking
directly at those headlights.
Disability glare, however, is not a common problem in the office environment,
where light levels are generally uniform. Very bright sources, such as overhead
lights, are at such an angle from the line of sight that they do not cause
disability glare.
Figure 55 shows the relationship between the angular separation from the
line of sight of glare source and its effect on the visibility of the task.
The glare source must be very near the object being viewed before there is
any noticeable degradation of the visual task. Even if the glare source does
not interfere with the visual task, it may still be annoying.
Discomfort Glare
Light fixtures, windows and reflections from shiny surfaces are common sources
of "discomfort glare" that may be objectionable even if they do not
interfere with the task.
When a glare source may become annoying. Luminous surfaces in the field
of visionwhich exceed the luminance levels defined by the curve may produce
a feeling of visual discomfort. This figure assumes a 1,000-lux ambient.
The angular separation between the line of sight and a potential glare
source is important: the greater the angle, the less likelihood of annoyance.
Figure 54 shows when a glare source may become annoying. The figure is based
on a 1,000lux ambient. A lower ambient light
level would produce a proportionately lower dividing line between discomfort
glare and an acceptable luminance distribution.
Age and Discomfort Glare
It is not uncommon to find lighting recommendations with correction factors
to compensate for age. At first, this seems right, since the average pupil
size is smaller for older people, and a smaller pupil lets in less light. The
lens also becomes less transparent with age,
which further reduces the amount of light reaching the retina. On the
other hand, older people tend to be considerably more sensitive to
discomfort glare. Accordingly, care should be taken so as not to do
more harm than good by raising ambient light levels for older workers.
Age and sensitivity to brightness.
Figure 55 shows the relationship between discomfort glare level and age.
It
is based on a study (Bennett, 1977) that used a 1degree source with a
background luminance of 6 cd/m2. Accordingly, the relationships
are for
just those conditions. One would expect the
curve to shift if the background luminance or target size were significantly
changed. What is important, however, is the relative change with age.
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