Figure 8.

Scanning pattern.

c. Depth Perception. Periods of low illumination greatly reduce the

ability of the eve to determine distances. Therefore, various cues must be

used to estimate distances at night. These monocular cues are usually used

at the subconscious level. Awareness of these cues by crew members may

enable them to look for and use cues that they are not in the habit of

using.

(1) Geometric perspective: The size and shape of an object changes

depending on the distance and angle from which it is viewed. These apparent

changes give a geometric perspective that is evaluated in three different

ways.

(a) Linear perspective. Objects appear to converge over

distance. Therefore, you estimate the distance to an object by comparing

the apparent separation to the known separation. For example, if you know

the distance between the navigation lights of an aircraft, you can estimate

the distance to the aircraft by comparing their apparent separation distance

to the known separation distance.

(b) Apparent foreshortening. The shape of an object tends to

appear elliptical over distance. Therefore, you estimate the distance

17

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