Display technologies
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CRT selection
Two factors should be considered in CRT selection:
1. Size. Obviously, the usable part of the screen should be large enough
to display the intended maximum number of symbols in a legible size and format.
2. Angle. The greater the angle through which the electron beam sweeps (see
Figure 34), the more likely the tube will be susceptible to spot and linearity
distortion.
The distance between the gun at the back of the tube, which shoots the stream
of electrons, and the phosphor surface being excited changes as the beam travels
across the surface.
The geometry of a 90- and 110-degree CRT.
The distance from the point of deflection to the corner of the inner surface
of the screen is about 40% longer than the distance from electron gun to the
center of the display for a 110-degree tube. The difference is only about 10%
for a 90-degree tube. Pin cushioning is another distortion that must be corrected.
A line drawn across the top of the display surface would, without corrective
circuitry, appear to bow. As the length of the tube is shortened, this type
of distortion is increased and becomes more difficult to control.
The use of a longer tube has the advantage of improved image quality. It
tends, however, to increase the depth of the display and, as a consequence,
takes up more room on the desk. This is one of the trade-offs that must be
made in the development process.
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