a. Night Blind Spot. The night blind spot can be experienced by
looking at a distant object on a dark night. If you stare directly at the
object it will disappear; however, if you look to the side of the object it
will reappear. The blind spot is from 5 to 10 degrees in width and is
centered in the field of vision (Figure 6). This means that as the distance
to an object increases, the size of the blind spot will also increase. For
example, at a distance of 3 feet an object the size of a screw can be lost
in the blind spot, but at a distance of 3,000 feet an object the size of a
large aircraft may be lost (Figure 7). To compensate for this blind spot a
person must use a good scanning technique. Although the effectiveness of a
technique varies with the individual, there is a suggested technique for an
effective scan (Figure 8). This technique uses a pattern that goes from
either the left to the right or from the right to the left, starting as far
out as objects can be recognized and working inward toward yourself.
Figure 7.
Effects of night blind spots.
b. Motion of the Eye. The light sensitive elements of the retina are
unable to perceive images while the eye is in motion, therefore, a stop-turn
method should be used. Each time the eye stop. you should concentrate on
an area about 30 degrees in width. The length of the stop will depend on
the clarity desired, but should be no longer than 2 to 3 seconds. When
moving from one viewing point to the next, you should overlap the previous
field of view by at least 10 degrees. Once an object has been detected, you
can continue to track the object by using off-center vision by focusing 10
degrees right, left, above or below the object.
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