Civil Engineering Reference
In-Depth Information
TABLE 18.1 Myopia and Uncorrected Visual Acuity
Myopia (D)
Pincus (1946)
Crawford et al. (1945)
Peters (1961)
Plano
20
/
20
20
/
20
20
/
20
2
0.25
20
/
22
20
/
20-20
/
25
2
0.5
20
/
30
20
/
35
20
/
30
2
0.75
20
/
44
20
/
40-20
/
50
2
1.00
20
/
50
20
/
52
20
/
50-20
/
60
2
1.25
20
/
63
20
/
70-20
/
80
2
1.50
20
/
80
20
/
91
20
/
80-20
/
100
2
1.75
20
/
100-20
/
200
2
2.00
20
/
100
20
/
200 or worse
2
2.50
20
/
200
20
/
100-20
/
200
2
3.00
20
/
300
20
/
400
2
3.50
20
/
300
2
4.00
20
/
400
2
4.5
20
/
470
2
5.00
20
/
533
2
5.50
20
/
615
2
6.00
20
/
890
Myopia and astigmatism will certainly reduce visual acuity. Uncorrected visual acuity for different
levels of myopia is shown in Table 18.1 (Pincus, 1946; Crawford et al., 1945; Peters, 1961). Hyperopia
will reduce visual acuity less predictably because younger individuals can use their accommodative mech-
anism (eye focusing ability) to add power to the eye in order to eliminate or reduce the blur caused by the
hyperopia. Accommodative ability predictably declines with age (thus necessitating reading glasses or
bifocals), therefore the visual acuity losses due to myopia and hyperopia approach one another in
older age groups. Proper spectacle and
or contact lens correction will significantly improve visual
acuity in most eyes with a refractive error. It was reported (Zerbe and Hofstetter, 1958) that 98% of
the population between the ages of 14 and 40 can be corrected to 20
/
20 vision in at least one eye.
/
18.2 Visual Acuity, Refractive Error, and Job Performance
18.2.1 Visual Acuity
Many jobs have a formal requirement for a minimum level of visual acuity and nearly all jobs have a
de facto visual acuity requirement; however, it is often difficult to exactly determine what the
minimum required level of acuity should be. This is because job performance depends upon identifying
many complex visual stimuli that are difficult to compare to acuity letters.
Reading text from a computer display at a typical viewing distance of 60 cm is a task example. The
angular size of 12 pt font at this distance is approximately 17
0
of arc, or would require visual acuity of
approximately 20
70 to identify. However, a person cannot comfortably or optimally function by
viewing text at threshold size. As a rule-of-thumb, text should be at least three times (3
/
rule) the
threshold size (Sheedy and Shaw-McMinn, 2002), smaller sizes result in less than optimal performance
and can also result in eye-related symptoms such as eyestrain and fatigue. Therefore, if the acuity demand
of the text on the display is 20
23.3 to read it comfortably. This
requires near-perfect vision of computer users, which is one factor in the high number of computer users
experiencing symptoms and argues for optimal visual correction of computer users. The 3
70, then users should have acuity of 20
/
/
rule can
be tested for a given worker by establishing the typical reading distance, then having the person view
the material or computer display from 3
the typical distance — if the worker is still able to discern
the words then s
he passes the test. This test is not easily performed on a person with presbyopia (see
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