Reflection and contrast
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Screen orientation
This method involves positioning the visual display surface to avoid reflection
as much as possible. As Figure 44 indicates, light rays are reflected from
a surface at the same angle that they strike it. Reflections of rays from
the brightest sources are, therefore, unlikely to be a problem because those
sources are usually overhead and the rays are reflected downward. Reflections
of rays from dimmer sources may reach the eye, but, because of their dimness,
are not much of a problem.
Screen orientation used to reduce the probability that the reflections
from bright, overhead sources will reach the eye.
Figure 44 shows how screen orientation is used to reduce the probability that
the reflections from bright, overhead sources will reach the eye. Reflected
rays leave the surface at the same angle at which they strike it. The surface
is positioned so that reflections from bright, overhead sources will not be
reflected back to the user's eye. Light from a character or point of the phosphor
is not diffused when it passes through the front surface.
ISO has specified that "a display should be legible from any angle of view
up to at least 40 degrees from the normal to the surface of the display, measured
in any plane". For many CRT type displays, the image quality is not appreciably
affected over a wide range of viewing angles. For many flat panel displays,
however, image quality does depend on the angle of view. Therefore, it is important
to be able to specify a range of viewing angles over which image quality requirements
will be maintained.
Anti-reflective coating
This method, typically used on commercial lenses, involves treating the glass
surface of the display screen. Light travels at different speeds through different
media. It travels through about 25% to 50% slower through various types of
glass than through air. The index of refraction of a particular piece of glass
is the speed of light through a vacuum divided by the speed of light through
that glass.
When light rays pass from a medium of one index of refraction to a medium
of another, the abrupt change in the speed of light that results creates an
optical surface, and typically 4.25% of the light is reflected from a glass
to air interface.
Special thin-film optical coatings may be deposited on glass to change the
physics of the air/glass interface so that more light is transmitted through
the glass and less than 0.3% reflected from the surface. This process is apparent
on older camera lenses, which have a magenta-colored appearance. That type
of lens typically had a single layer of thin-film coating matched for the center
of the visual spectrum. Reflections in the green-yellow range are minimized,
but reflections from the ends of the spectrum, red and blue, are about the
same as for untreated glass. In that case, the red and blue reflections are
seen as magenta.
Multiple layers of thin-film optical coating may be used to widen the range
of minimized reflections. These coating may also be used on the screen of a
VDT to reduce reflections by 90% or more without any distortion or blurring
of the image.
A problem with thin-film, anti-reflection coating is that dirt and fingerprints
are much more noticeable on the surface, so the screen may have to be cleaned
more often.
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