Civil Engineering Reference
In-Depth Information
visual acuity levels can also be assessed with the visual efficiencies accepted by the American Medical
Association (PDR, 1998) used to determine post-trauma compensation, shown in Table 18.3.
The acuity level at which individuals decide to wear glasses or contact lenses habitually is an indication
of the acuity level at which people feel compromised in the performance of everyday tasks. Numerous
variables can influence the acuity level at which ophthalmic correction is worn habitually: age,
demands of visual tasks, illumination levels (day or night), individual tolerance to blur, and cosmetic
concerns among others. In a survey (U.S., 1964) of 7710 persons, only 2% of those not wearing
glasses had a visual acuity of less than 20
100. Unfortunately, they did not provide any details
/
between 20
100. An analysis of 9468 screening records of West German factory workers,
reported the habitual binocular visual acuities (with usual eye correction if worn) at work (Schober,
4-6 April, 1968). Only 3.62% of the workers had distance visual acuity of 20
20 and 20
/
/
50 or worse, and only
/
2.29% had near acuity of 20
40 at distance were measured
in 13.8% of the workers and at near in 9.09%. These data indicate that very few factory workers have
habitual acuities of 20
50 or worse. Acuities between 20
29 and 20
/
/
/
50 or worse — an indication that for these acuity levels most workers choose
to habitually wear ophthalmic correction. It appears that most people with acuity in the range of
20
/
/
32 to 20
/
50 will choose to habitually wear optical correction.
18.2.2 Refractive Error
The most common reason for reduced visual acuity is an uncorrected refractive error. Refractive error is
also sometimes utilized in occupational vision standards. As an example, the U.S. Air Force Academy sets
refractive error limits of
þ
2.00 D
1.00 D in any meridian and 0.75 D astigmatism (pilot) and
/2
þ
2.25 D in any meridian and 2.00 D astigmatism (navigator).
Uncorrected myopia and uncorrected astigmatism will reduce visual acuity. Uncorrected astigmatism
in magnitudes as low as 0.50 D can create blur at all working distances and has been shown (Wiggins,
1991) to be significantly associated with visual discomfort at work. Some myopic individuals choose
to perform near work without correction. Uncorrected myopia is seldom a reason for eye-related symp-
toms such as eyestrain. In workplace visual screenings, uncorrected myopic individuals will have reduced
visual acuity at distance, but normal or much better visual acuity at near distances.
Persons with high amounts of myopia, generally
3.00 D
/2
8.00 D or greater, are at increased risk for retinal
detachment and other retinal disorders. It is best that these persons not be placed in jobs that place
them at higher risk for head trauma and some occupations specifically exclude persons with high
myopia from employment consideration.
Uncorrected hyperopia can be a significant source of visual discomfort — especially for workers with
visually demanding work. This is because the hyperopic eye must use excessive accommodative effort to
see clearly at near working distances resulting in visual fatigue and eyestrain. Hyperopic workers can
usually exert the needed accommodation to exhibit good visual acuity at visual acuity screenings. Hyper-
opia, however, can be identified by re-measuring distance visual acuity through a
2
þ
1.00 D lens. This lens
TABLE 18.3 Percent Visual Efficiency Corresponding to Visual Acuity
Level
% Visual
Efficiency
% Visual
Efficiency
Visual Acuity
Visual Acuity
20
20
100
20
100
50
/
/
20
25
95
20
125
40
/
/
20
32
90
20
160
30
/
/
20
40
85
20
200
20
/
/
20
50
75
20
300
15
/
/
20
63
65
20
400
10
/
/
20
80
60
20
800
5
/
/
Source: PDR; Medical Economics Company, Inc. Montvale, NJ: 1998.
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