Civil Engineering Reference
In-Depth Information
TABLE 18.5 Initial and Final Resting Position of the Eyes When Fixating
Peripheral Stimuli at Different Angles (degress)
Lateral stimulus amount
10
20
30
40
50
Initial eye rotation
10
20
28
33
41
Final eye rotation
2
5
11
15
19
Because of the strong preference to use head rather than eye movements, workers primarily adjust
body posture to a peripheral target in order that the eyes can operate comfortably. Clearly, “the eyes
lead the body.” If the worker must fixate a peripheral target for extended periods, the awkward
posture can result in musculoskeletal symptoms. It has been shown that optimal performance and
comfort are attained with a downward gaze angle of 10-15
and straight ahead in the lateral meridian
(Sheedy and Shaw-McMinn, 2002). Ideally visual tasks should be located so the eyes are depressed by
approximately 10
8
with the worker in a neutral musculoskeletal posture. For example, computer displays
should be located straight in front of the worker laterally, and the height adjusted so that screen center is
approximately 10 cm lower than the eyes (60 cm viewing distance) — resulting in 10
8
8
downward gaze.
18.2.3.4 Peripheral Vision — Visual Fields
The clinical term for peripheral vision is “visual fields.” There are several methods of clinically measuring
visual fields. Each method requires the eye being tested to maintain fixation on a single spot while
peripheral test spots or objects are shown and the patient responds when they are seen. The normal
extent of visual fields as tested with a fairly large peripheral target are listed in Table 18.6 (PDR, 1998)
and displayed in Figure 18.6.
Figure 18.6 shows that the visual fields of the two eyes do not perfectly overlap, resulting in a binocular
visual field that is larger than the component monocular fields. It has been shown (Good and Fogt, 1998)
that the area of the monocular visual field varies from 53.5 to 78.2% of the binocular visual field depend-
ing upon the direction of fixation.
Workers missing an eye (monocular individuals) have significant loss of peripheral vision compared to
normal binocular individuals. In a study (Johnson and Keltner, 1983) of 10,000 drivers, those with
binocular visual field loss had significantly higher accident and conviction rates. Although monocular
individuals are able to obtain passenger car driver licenses, some states require additional rearview
mirrors and require two eyes for commercial licenses. One way that a monocular individual can help
to compensate for loss of visual field is to move the head more. However, there is no evidence that mon-
ocular individuals make more head movements nor is it observed clinically. In tasks where peripheral
awareness is a requirement (e.g., professional driving, police activities), the reduced visual field of the
monocular individual can greatly decrease visual efficiency and create dangerous situations. Several occu-
pations such as police officer, firefighter, pilot, and commercial truck driver among others require two
eyes, often with a minimal required visual field in each eye. Monocular workers can be at greater risk
TABLE 18.6 Normal Extent of the Visual Field in Eight
Principle Meridians
Direction from Fixation
Degrees of Visual Field
Temporal
85
Down and temporal
85
Down
65
Down and nasal
50
Nasal
60
Up and nasal
55
Up
45
Up and temporal
55
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