Vision and Work - Fundamentals and Assessment Tools for Occupational Ergonomics - page 434

Right eye Left eye

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40

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− 100 − 80 − 60 − 40 − 00 0 0 0 0 0

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Horizontal (degrees)

FIGURE 18.6 Peripheral vision (visual fields) with eyes fixating straight ahead. Monocular fields overlap to create a
larger binocular visual field.

for injury and caution should be used in placing monocular workers. Monocular workers should always
wear eye protection — they don't have a spare.

18.2.3.5 Binocular Vision
One aspect of binocular vision, that is, enhanced visual fields compared to one eye alone, had been
discussed earlier. Other aspects of binocular vision will be discussed below.

18.2.3.5.1 Depth Perception
The cues we use to interpret depth in our environment are generally divided into two categories —
monocular cues and binocular cues. The monocular cues are obtained with only one eye and are the
same cues that exist in photographs, paintings, and motion pictures. These include cues such as size
(distant objects are visually smaller), motion parallax, overlay (nearer objects visually overlay distant
objects), geometric perspective, and relative height (distant objects are usually higher in the field of
view). A monocular individual has full use of these cues for judging depth.
A binocular individual has an additional sensation to judge depth called “stereopsis.” This binocular
sensation of depth arises from the fact that the two eyes are displaced laterally in our head. Each eye views
the scene from a slightly different angle and neural processing of these differences creates the sensation of
depth. The mechanism is not like a range finder that relies upon calibration of the rotational positions of
the two viewing points (in this case, the eyes), but rather it depends upon slight differences in the angles
between objects as viewed by the two eyes. A rangefinder gives absolute information about the distance
from observer to object, steropsis gives relative information about depth — that is, it allows accurate
determination whether one object is closer or farther than another. The effectiveness of stereopsis
decreases with viewing distance because the angular difference between the two eyes decreases with
viewing distance. Stereopsis is the sensation of depth that is obtained in 3D movies that require
special glasses.
Stereopsis is important for dentists, jewelers, assemblers, and crafts where the worker is manipulating
relatively small objects at near viewing distances. Normal individuals use stereopsis in the performance of
occupational-type tasks, performing depth-dependent tasks upto 30% slower with one eye occluded