Civil Engineering Reference
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test, more exact testing requires examination with an anomaloscope available only in large clinics. In
some cases such as airline pilots, boat captains, and train engineers, it is critical to identify the color
of small lights. Job-related color vision testing can be performed with the Farnsworth Lantern test.
18.2.3.7 Vision and Work
Nearly every job has minimum de facto vision requirements. If a worker does not have the appropriate
visual abilities to meet those requirements it can result in nonperformance, inefficiency, and
or eye-
related or musculoskeletal symptoms. Through workplace screening of vision, worker education, ergo-
nomic observation and intervention, and provision of appropriate eye care, most employees can attain
the necessary visual abilities to perform most jobs.
Some jobs, however, because of their critical nature and high cost of nonperformance, require visual
abilities that exclude some individuals from consideration. In these situations it is important to set visual
requirements that are clearly job-related and not unnecessarily exclusionary.
/
Appendix 18.1 Measurement of Visual Acuity
Because visual acuity is an angular measurement, the measure of visual acuity can be used to quite
reliably predict the visual resolution abilities in comparison to an individual with 20
20 acuity. An indi-
/
vidual with 20
40 acuity will need to be at half the distance to see the same level of detail in a given target
as a person with 20
/
20 vision. If 20
20 vision enables detection of certain visual details at 480 ft, 20
40
/
/
/
will be able to see it at 240 ft. If 20
20 can see it at 12 ft, then 20
40 can see it at 6 ft. For a visual acuity of
/
/
20
4, etc. This type of
analysis is accurate for any distances greater than 5 or 6 ft from the individual. At distances closer than
this, the visual acuity of an individual can change depending upon their refractive condition.
The standard methods for designing visual acuity charts have been specified (Bailey and Lovie, 1976)
and standardized (ISO, 1994; ANSI, 1992). Most visual acuity charts have five letters per row. Typical
acuity letters have a stroke width to vertical size ratio of 1:5. Although the step size between rows can
vary among charts, the standard size progression to the next largest row is 10
0.1
or 1.2589. This
results in a logarithmic size progression, typical steps are shown in the following table. Although
Snellen equations are used for most clinical and occupational designations of visual acuity, logMAR
units are typically used for scientific investigational work. The minimum angle of resolution (MAR) is
the stroke width of the smallest identified row of letters. For 20
30, the viewing distance multiplier is 1
1.5, for 20
60 it is 1
3, for 20
80 it is 1
/
/
/
/
/
/
20 vision, this is 1' of arc, hence
/
logMAR
0.0. Each incremental row is 0.1 logMAR units. For critical measurements, each letter on a
five-letter row gives 0.02 credit towards final logMAR measure. Typical acuity steps in Snellen and
logMAR units are shown in the following (Appendix Table 18.1).
ΒΌ
Appendix Table 18.1 Standard Incremental Steps in Visual Acuity
20 ft Acuity
Designation
6 m Acuity
Designation
Letter Height
(min arc)
Stroke Height
(min arc)
logMAR
20
10
6
3
2.5
0.5
0.3
/
/
2
20
12.5
6
3.8
3.2
0.63
0.2
/
/
2
20
16
6
4.8
4.0
0.8
0.1
/
/
2
20
20
6
6
5.0
1.0
0
/
/
20
25
6
7.5
6.3
1.25
0.1
/
/
20
32
6
9.5
8.0
1.6
0.2
/
/
20
40
6
12
10
2.0
0.3
/
/
20
/
50
6
/
15
12.5
2.5
0.4
20
/
63
6
/
19
16
3.2
0.5
20
/
80
6
/
24
20
4.0
0.6
20
/
100
6
/
30
25
5.0
0.7
20
/
125
6
/
38
32
6.3
0.8
20
/
160
6
/
48
40
8.0
0.9
20
/
200
6
/
60
50
10.0
1.0
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