Lighting
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Overview
The conditions of the surroundings in which a tool or piece of equipment is
used to perform a task can have a significant impact on the employee's ability
to perform that task. We will consider two such factors --- noise and
lighting.
The proper amount of light must always take into account the human visual
requirements for a task. Proper lighting, however, is a relative term.
Lighting that is good for one task may be inappropriate for another. This
section covers some considerations when specifying
or evaluating lighting conditions.
The general attitude toward lighting has been "more is better." This
is not necessarily true. Figure 51 shows that recommended light levels over
the past seventy years have dramatically increased for the same tasks. The
eye, however, has not changed. In fact, average vision is probably better now
than before because of the greater use of eyeglasses.
The eye, as a whole, does not react very quickly to changes in light levels.
For example, the eye does not react nearly as fast to sudden large changes
in light as does the ear to sudden large changes in sound. Although the eye
is useful over a wide range of illumination, it takes time to adjust to a particular
level.
Common artificial lighting does not impose any extra demands on a
person's vision and, in many cases, it may be superior to natural lighting.
Artificial light is easy to control and to direct into a specific area. A properly
lighted environment may be maintained artificially
for long periods of time, regardless of outside, or natural, conditions.
Artificial light, however, may produce lighting imbalances that exceed those
commonly found in the natural environment. In night driving, for example, the
natural lighting imbalances are exceeded when the driver tries to see
the road but has oncoming headlights in the
field of view. The percentage of
light reflected by a surface is essentially
a constant. As the natural light level changes, the amount of light
reflected from the various surfaces in the environment changes proportionately.
On the other hand, artificial lighting may be unevenly distributed
and result in areas of light which in themselves might be usable, but
together with other areas of light may pose a problem for a person
who must visually alternate between them.
Variations in luminant color may be greater under artificial than under natural
lighting conditions and settings. In nature, direct sunlight is not the only
source that illuminates surfaces. They may be illuminated by rays that are
direct, filtered or reflected from other selectively absorbing surfaces. These
variations are minor and the visual system tends to compensate for them. Artificial
lighting and settings may considerably bias the effective luminant color. This
is not necessarily bad.
Lighting should match the requirements of the task. Most of the tasks associated
with visual displays do not require precise visual acuity. Most objects (for
example, the VDT display surface and source documents) to be viewed are two
dimensional, and indirect lighting is appropriate for visual tasks associated
with them. On the other hand, tasks such as visual inspection of parts for
surface defects, or tasks that require rapid perception of complex, threedimensional
surfaces, are made easier by some degree of specular lighting (illumination
coming mostly from one direction). Shadows are more apparent and the angle
of the surface relative to the light source may influence the apparent brightness
of the surface.
The advantages of diffuse lighting are twofold: there are fewer hot spots,
or glare surfaces, in the visual field and the contrasts created by the shape
of objects tend to be "softer." The result, in terms of luminous
intensities, is a more uniform field.
The 1976 Uniform Color Space.
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