Optical system
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The Retina
When light strikes the retina, the retinal receptors (nerve endings called
rods and cones) generate electrical signals which are sent to the brain, where
they are translated into visual perception. The mechanism that controls the
rate at which the retinal receptors produce electrical signals is very complex.
The level of retinal activity, however, is not associated with fatigue in the
general sense of the term. That is, it is no more visually fatiguing to be
outdoors on an overcast day, when the level of retinal illumination is low,
than on a sunny day, when the level is much higher.
The task of the visual system is to detect luminance or color differences
in the field of view. If the visual system could not change its sensitivity
level, or the level of adaptation, those differences in luminance or color
would have to be relatively large to be detected. Adaptation allows the visual
system to become maximally sensitive to luminance differences at other levels
(Werblin, 1973). That type of adaptation depends on a network of nerves that
run laterally over the retina, which provides the primary mechanism for adaptation
at photopic light levels, or levels at which colors can be perceived.
There are two other adaptation mechanisms, specifically retinal pigment bleaching
and pupillary response, but they are only important in night vision.
Symbol perceptibility is a function of edge sharpness and contrast. The sharper
the edges of a symbol, the greater its apparent difference from the background.
Edge sharpness alone, however, is not enough to provide maximum perceptibility.
The importance of contrast is obvious. With very little contrast, either in
brightness or color, an edge (no matter how sharp) is hard to see. As contrast
is increased, edge perception is easier until a level is reached where additional
contrast is no longer helpful.
There is a retinal mechanism in the invertebrate eye called lateral inhibition
that tends to increase the difference between two adjacent light levels. This
form of image enhancement is observable in human perception, but the location
of the mechanism is thought by some authorities (Ratliff, 1972) to be at both
the retinal and cortical levels. The lateral inhibition function of the invertebrate
(such as a limulus) eye, however, is worth describing to characterize this
aspect of vision.
The rate at which a receptor in the eye sends impulses to the brain depends
on the level of adaptation, the amount of light falling on the receptor and
on the activity of receptors near it. When one receptor is discharging, it
slows the neural discharges from adjacent receptors. In other words, the more
active a receptor, the more it inhibits the activity in nearby receptors.
A representation of contrast enhancement at the retinal
level.
In Figure 18, receptors A and B are being struck by relatively high light
levels. Receptors C and D are being stimulated by relatively low light levels.
Because receptor B is near an area of lower activity, it does not receive
as much inhibition as a receptor at location A. For that reason, receptor B
will fire at a higher rate than receptor A even though both may be receiving
the same amount of light. Similarly, a retinal receptor at location C is nearer
to receptors that are firing at a higher rate than is a receptor at location
D. The rate of firing for a receptor at location C will be slowed down more
than for a receptor at location D.
That mechanism enhances the difference between adjacent brightness levels,
for example, between the symbol and background. This effect makes image edges
more visible. VDT images make both the contrast (often expressed as the luminance
ratio) large, and the gradient (the ratio of change in luminance to the change
in distance) large to exploit the effect.
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