Reflection and contrast
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Surface reflections
The kind and amount of light reflected from any display screen surface depend
on the nature of that surface and the angle at which the light strikes it.
The percent of light beam reflected back from a smooth, untreated
glass surface as a function of the angle of incidence. This is for an air-to-glass
surface (Sears, 1958).
Figure 42 shows that about 4% of the light that strikes a smooth, untreated
glass surface at an angle of 50 degrees or less is reflected, and that the
percentage increased as the angle increases.
It is usually impractical to try to eliminate reflected light completely.
It can be reduced, however, to an acceptable level. The methods most often
used for this are diffusing surface, screen orientation, anti-reflective coating
and environmental lighting.
Diffusing surface
This method involves roughening the display surface to make it optically
irregular. While this does not reduce and may in fact increase the total amount
of light reflected, it tends to break up the reflected image. The more the
surface is roughened, the more the reflection is broken up. The reflection
becomes "softer" because it is spread over a wider area.
This may create two problems, however. The first is a blurring of the display
image, caused by the fact that the light rays generated by the excited phosphor
are also diffused when they pass through the glass of the screen and strike
the irregular front surface. The other problem is a masking illumination (a
veil of diffused light over the entire glass surface) that can result when
some reflections from the roughened surface reach the user's eyes.
A diffusing surface used to reduce the specular reflections.
Figure 43 shows a diffusing surface used to reduce the specular reflections.
It does not, however, reduce the total amount of reflected light. When light
rays from outside sources strike the first surface, they are scattered and
the reflected image is diffused, out of focus. The same thing happens to the
light rays coming from a character of point generated by the display; when
they pass through the diffusing surface, they are also diffused.
This effect is similar to the disability glare effect, which is discussed
later.
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