Civil Engineering Reference
In-Depth Information
Section 18.2.3.1 on Presbyopia) in which it is complicated with the need for optical assistance at near
viewing distances.
Driving is a task for which there is a minimum visual acuity requirement. Most studies on the relation-
ships between driving and visual measurements have shown weak relationships at best (Bailey and
Sheedy, 1988). The reasons for these poor relationships are many, including the facts that licensure
screening eliminates people with poor vision from the driving population and individuals with
poorer vision self-limit their driving. Most (41 of 50) states require a visual acuity of 20
40 in order
to obtain a regular driver's license. Most states also will consider issuing a driver's license (usually
restricted) for reduced levels of visual acuity. Most states also have stricter acuity requirements for
commercial driver licenses. Another aspect of visual acuity and driving is the standard to which
highway signs are designed, which is 1 in. of letter height for every 50 ft of recognition distance
(Mace, 1988). Mathematically this transforms to a requirement for 20
/
23 visual acuity.
Many occupations have specific visual acuity requirements (Mahlman, 1982). The requirements are
usually for corrected visual acuity, that is, the worker can meet the requirement with spectacles or
contact lenses. For example, a corrected visual acuity standard of 20
/
20 for police officers was justified
on the basis of the need to identify city street signs at appropriate distances when driving in emergency
conditions (Sheedy, 1980). Corrected visual acuity standards apply to occupations such as pilots (20
/
/
20
each eye), FBI (20
/
20 in one eye, 20
/
40 in the other) U.S. Border Patrol (20
/
20 each eye), firefighters
(20
40 in each eye), among others.
Some occupations also have requirements for uncorrected visual acuity, that is, a minimum level of
visual acuity is required without optical correction. Uncorrected visual acuity standards typically
apply for occupations where performance is critical and especially if it is feasible that glasses or
contact lenses could be dislodged during work such as for police officers. Examples of uncorrected
acuity requirements include firefighters (20
/
30 binocular), and commercial truck drivers (20
/
100 binocular), police officers (usually between 20
40-
/
/
20
60 each eye). In many
occupations with an uncorrected visual acuity standard, job applicants are allowed to have refractive
surgery to meet the standard.
The effects of visual acuity upon performance of only a few tasks have been studied. One study (Good
and Augsburger, 1987) analyzed the level of visual acuity required for adequate performance with a
firearm. Fifty subjects performed a “friend or foe” task under different levels of visual acuity (20 ft
viewing distance, 10 cd
200 dependent on jurisdiction), FBI (20
200), and U.S. Secret Service (20
/
/
/
45 vision was required to perform at the
threshold performance level. Facial recognition is a complex visual task, but one that can be related to
visual acuity (Bullimore et al., 1991). With photopic (daytime) lighting (100 cd
M2), analysis determined that 20
/
/
M2) subjects could
/
effectively identify faces at 20 ft with 20
72 vision and identify facial expressions with 20
80 vision.
/
/
The equivalent acuities required at 20 ft viewing distance for lower light levels were 20
52.
Another means to assess visual capabilities associated with different levels of visual acuity is to utilize
accepted medical or insurance categorizations. For example, best-corrected vision of 20
44 and 20
/
/
200 or worse is
defined as legal blindness in nearly all states and allows income tax advantage. Another useful categori-
zation of abilities related to visual acuity is provided by the International Classification of Diseases (ICD-
9-CM, 2004) that is used for insurance reimbursements, shown in Table 18.2. Functionality of different
/
TABLE 18.2 Classification of Levels of Impairment by Visual Acuity
Best Corrected Acuity
Classification
20
/
10-20
/
25
Normal vision
20
/
30-20
/
60
Near normal vision
20
/
70-20
/
160
Moderate visual impairment or low vision
20
/
200-20
/
400
Severe visual impairment or low vision, legal blindness (U.S.)
20
/
500-20
/
1000
Profound visual impairment or low vision, moderate blindness
less than 20
1000
Near total visual impairment, severe blindness, near total blindness
/
Source: ICD-9-CM, American Medical Association, Ann Arbor, MI, 2004.
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