Lighting
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Other considerations
The visual system seems to compensate easily for different illuminant spectral
distributions. Noon daylight has a high blue content, incandescent lamps have
a high red component, fluorescent lamps tend to have distributions between
natural light and incandescent light. Although differences in light coming
from different sources may be large, they are typically not noticed unless
both lights can be seen at the same time.
Although there may be some aesthetic reasons, there are no physiological
reasons why different types of lamps should not be used in the same area.
If half a room were lit by cool white fluorescent lamps and the other half
by warm white lamps, one
side of the room would look bluish and the
other side reddish. That may not be pleasing.
While color preference for environmental surroundings is obviously subjective,
green and blue seem to be preferred over red, and all three
over yellow. The saturation of a color is an important point for lighting.
Although some people appear to prefer deeper,
saturated colors, lighter
colors are most often used on walls and large
surfaces because they reflect more
light. A reasonably saturated green may reflect
only about 15% of the light striking it. A light green may have a reflectance
factor of about 50%. That difference may be quite noticeable in the
overall room light level.
Natural lighting poses some problems in office environments. Direct sunlight
may be over 100,000 lux. If an office area is large and there is light from
outdoors, considerable interior lighting may be required to maintain reasonable
light balance throughout the room. With very high ambient lighting, displays
(such as CRTs) that generate their own light may be washed out, that is, they
will have less contrast between the symbol and background. Glare also may be
a problem with higher intensities of natural light.
Owing to the brightness range of natural illumination, it is usually best
to position the display so that the screen is perpendicular to the plane of
windows. This not only helps keep windows out of the field of view, but reduces
the probability of bright light from windows being reflected into the user's
eye. Figure 57 shows the range of natural lighting levels that might
be expected at various times of the day and
under various weather
conditions.
Range of sunlight illumination. Value A is at a minimum before sunrise,
at a maximum at the zenith. Curve A represents unobstructed sun, B is for
average cloud cover and C is for storm cloud cover.
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