Contrast and color
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Flashing light sensitivity
When light is flashing, contrast sensitivity can be increased. The effect is
dependent on the frequency of flashing. Between 1 and 5 flashes per second,
a "just noticeable difference" is 1.005:1 rather than 1.05:1. The
best target size is symbol size, about 3 cycles per degree. The effect diminishes
rapidly for increasing temporal frequency and for increasing spatial frequency
(decreasing size). This property can be used to draw attention to a new area
of the screen, for example, when data are outside limits.
Peripheral Contrast Sensitivity
The parafovea and peripheral areas of the retina are less sensitive to contrast
than is the fovea. Nonfoveal perception, however, is important in reading
and visual search tasks.
Contrast sensitivity for the fovea and near periphery (Blackwell and Moldauer,
1980).
Figure 22 shows, the loss of contrast sensitivity for various distances away
from the fovea. The study referenced in the figure by Blackwell and Moldauer
(1960) used a one minute of arc disc presented for 0.01 second.
VDTs are not usually capable of these laboratory conditions. A one minute
disc is about the size of the period a the end of this sentence. Few VDTs can
present images for 1/100 second. Under ordinary office illumination, most VDTs
achieve less than eight shades of gray. The importance of these data to the
user of a VDT is that applications that must draw your attention to a target
in the periphery should use targets that are large enough, sufficiently different
from the background and presented for an adequate duration of time.
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