Screen reflections
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Screen reflections
The first principle to keep in mind is that the optimal way to fix screen reflections
is to remove their source. The need for anti-glare screens strongly suggests
that the workplace is not designed or lit properly. Just about anything that
can cause disturbing reflections on a properly positioned VDT screen is located
where it will also cause direct glare in somebody's normal visual field.
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 surfaces, screen orientation, anti-reflective
coating, and environmental lighting.
Reflections on screens can be broadly lumped into two categories:
1. Too much light falls on the screen. This "washes out" the
image, reducing contrast to the point that the image is no longer easy to
see or to use. In addition, the light makes the surroundings very bright.
If the screen image is dark, it may seem like a "black hole." This
type of glare is often referred to as "disability glare": the ability
to use the display is seriously reduced, but this need not cause any actual
discomfort to the user (apart from frustration in using the display).
2. Bright areas of the environment form reflections from the screen surface.
These reflections are (a) distracting, and (b) superimpose on the screen
image and make it hard to see. This type of glare is more likely to be characterized
as "discomfort glare": acutely disturbing, these reflections may
cause visual discomfort as well as interference with task performance. As
you might suppose, the ways to cope with these types of reflections are quite
different.
Too much light
This problem can be offset in the obvious way - by turning down or blocking
the source(s) of illumination. Often, rearranging the workstation furniture
so the screen doesn't receive direct illumination (for example, from a nearby
window) will fix the problem. Too little light from the screen can amount
to the same thing as too much room illumination. A really dim display should
be replaced, if screen intensity cannot be increased satisfactorily.
If the room illumination cannot be reduced, and if the display screen luminance
is adequate, then display attachments may be useful, such as directional
filters and neutral-density filters. Directional filters include micro-louver
(tm: 3M Co.) and micro-mesh, and hoods. Directional filters just keep unwanted
light from striking the screen. They provide a darker screen background.
They also absorb a certain amount of light themselves, so the screen as a
whole is not as bright. Hoods over the screen can be used to block unwanted
illumination from the workplace. In general hoods are not recommended, however.
To eliminate glare a hood must surround the VDT image completely and extend
close to the viewer's face - consequently a hood can severely limit the display
user's freedom of movement. The role such a hood may play in inducing a static
posture is immediately obvious, even to persons who may have little interest
in ergonomics. Sitting in a fixed position for long periods of time can be
fatiguing. It is important to note that an often unrecognized result of display
screen glare is that it may force the user to adopt an uncomfortable posture
in an effort to minimize glare.
Neutral density filters are dark glass or plastic. They reduce incoming
light by a certain factor, and reflected light by the same factor again.
Light leaving the display is reduced only once. For example, 50% transmission
glass reduces incoming light by 50% and reflections by another 50%. Diffuse
reflection of ambient illumination is thus reduced by 75%. However, display
luminance is reduced by 50%. The ratio of display luminance to reflected
light (contrast) is improved by 50%. However, the display is much less bright
than without the filter in place. If the surroundings are very bright and
the screen is very dim then the screen will have poor luminance balance with
its surroundings. In extreme cases glare from the surrounding visual field
may have the same effect as reduced screen contrast.
In general, the best candidate for a filter to control excessive ambient
illumination is a display with adequate brightness adjustability. A dim display
is likely to be just as unusable with a filter as without it, in conditions
of excessive ambient illumination.
Reflections
Reflections are formed at both the front surface of the display screen and
the rear of the screen at the phosphor-glass interface. Nearly all modem
monitors provide some surface roughening at both surfaces to diffuse reflections.
This treatment is effective against small, bright sources, but has little
benefit where large, bright surfaces can be reflected.
The simpliest way to reduce reflections from sources that cannot be removed
is to turn or tilt the monitor so the reflections are not in the user's line
of sight. Most modem monitors have tilt and swivel bases that allow such
adjustments. If the monitor is tilted downward, light sources from above
the level of the monitor will not be reflected. The swivel adjustment can
also help to eliminate reflections with the exception of those coming from
papers on the desk or the user's light-colored clothing.
In extreme cases, especially where the ambient illumination is high, papers
and clothing are bright enough to produce disturbing reflections. Unfortunately,
corrective filters may make reflections worse. Since they make the screen
background darker, reflections will have a higher contrast against the screen.
In addition, filters also reduce image luminance, which makes it easier for
reflections to obscure the image, as well.
Two corrective approaches
First, if the screen presents light characters against a dark background (negative
polarity), reverse this relationship to have dark characters against a light
background (positive polarity). Graphical user interfaces such as OS/2 or
Microsoft Windows use a white screen background and dark characters. This
polarity choice makes reflections far less noticeable, and generally less
disturbing.
Second, consider an anti-reflection screen or coating for the screen. Anti-reflection
filters that really work are quite expensive. Often by combining multi-layer
anti-reflection coatings and a circular polarizer filter, these filters remove
the reflection from the filter itself as well as the VDT screen. They also
transmit rather little light, so there is excellent contrast enhancement
in excessive illumination. The drawback is that good ones are not inexpensive,
and the multi-layer coatings must be kept very free of dirt and fingerprints
in order to work at all.
It is essential to keep in mind that the usability requirements for the
display system must be met with any glare reduction filter or treatment.
Display luminance must still be high enough, and display resolution must
remain high. There are interactions between the filter and the screen. It
is not possible just to buy a filter and add it on to any old screen and
expect the result to be acceptable. However, properly selected and installed
anti-glare filters or coatings may be a useful part of the overall strategy
for coping with screen reflections.
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