Reflection and contrast
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Other contrast-enhancement devices
Hoods and mesh screens may also be used to increase contrast.
HOODS
General purpose VDTs can be fitted with hoods. Hoods block unwanted illumination
from the workplace. Effective hoods must surround the VDT image and extend
close to the viewer's face. In fact, commercial hoods have compliant forehead
rests. Homemade hoods are usually fashioned from cardboard by the user in an
attempt to cope with situations of poor room illumination. This is always almost
a poor solution. A typical result is a shadow across the screen. In the shadow,
the contrast is adequate; outside, unimproved. Truly effective hoods are massive,
physically unpleasant devices which severely limit the display user's freedom
of movement. It is better to find the glare source and correct it than to attach
cardboard hoods to the VDT. Reorienting the VDT, repainting a surface behind
the VDT with a less reflective paint, closing a window blind for a part of
the day are typical, highly effective user solutions to discomfort glare.
MESH
A fine mesh can be fitted to a display screen. The mesh acts like a roughened
surface, eliminating specular reflections. The mesh is characterized by the
percent of the area that is open, typically 40%. This acts like a screen with
reduced transmissivity to improve contrast. The thickness of the mesh, the
aperture pitch and the tranmissivity act to exclude off-angle light. This restricts
the viewing angle. This could be an advantage since it improves privacy, or
a disadvantage if multiple viewers wish to see the screen.
Care must be exercised in mesh design. If a mesh is added to a VDT by the
user, the mesh vendor should certify that the mesh is consistent with the VDT.
The mesh aperature must be smaller than the image pitch. Also, color CRT's
have an aperture pitch associated with the phosphor and have other structural
details which, if ignored, can produce interference patterns (moire) with the
mesh pattern. An inappropriate mesh may produce other unwanted side effects.
Mesh filters are recommended if the room illumination is high and cannot
be controlled another way. These filters are tolerant of handling, but once
badly soiled, difficult to clean.
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