VDT design factors
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Resolution
One aspect of image quality is resolution, or image sharpness. People tend
to prefer a sharply focused image. That is true even if symbol size and contrast
ratios are such that sharp focus might not seem to be important. There may
be a physiological explanation for that preference, since there is a relationship
between image resolution and the electrical activity generated in the brain
(Gomer and Bish, 1978).
Stimulation of the visual system produces measurable electrical activity
in the brain. The voltage differences between two electrodes placed on the
scalp can be recorded. These voltage differences are called "evoked potentials." The
evoked potential produced by an image of higher resolution is, within limits,
stronger and more clearly defined than one produced by an image of lower resolution
but similar contrast and equal total light output.
One of the problems in recommending a specific level of resolution is that
visual performance over the range of spatial frequencies that are of interest
does not change with differences in resolution so long as modulation is held
constant. That means that a Gaussian pattern is just as easy to see as a square
wave pattern.
Thresholds for Gaussian grating and square wave bar patterns (DePalma
and Lowry, 1962).
Figure 39 shows the detectability thresholds for Gaussian grating and square
wave bar patterns while Figure 40 shows the same function for a single bar.
There is no difference in the ability to detect the two types of patterns for
spatial frequencies of about five cycles or more per degree. It is not known
why people prefer to view sharp images.
Thresholds for Gaussian grating and square wave (0.5 cycle width)
bars (Shapely, 1974).
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