Blindness
The term blindness implies total or
partial loss of vision involving both eyes. The exact
level of vision defined as blindness, however,
varies in different countries because of differing
legal or social requirements. In the United States,
blindness is defined as unimprovable vision of
20/200 (6/60) or worse. This means that an
individual is generally considered blind who, even
with the use of ordinary eyeglasses, can see no
better at 20 ft (6 m) than a person with normal
vision can see at 200 ft (60 m). On the other hand,
the World Health Organization (WHO) Program
Advisory Group on the Prevention of Blindness
lists the vision level suggested as blindness as a
visual acuity of less than 10/200 (3/60), which is
twice as low as the U.S. definition. The WHO
level of visual acuity is also described as the
inability to count fingers in daylight at a distance of
10 ft (3 m), because in many regions a great
number of people cannot receive formal eye
examinations but may be tested by unspecialized
personnel. From data available in the mid-1980s,
the number of persons worldwide who have a
visual acuity of less than 10/200 is estimated as 28
million. This level of handicap precludes an
individual from functioning effectively in the
community without special assistance and
rehabilitation (see BLIND, EDUCATION OF
THE). Were the definition of blindness instead
taken as 20/200, as is done in a number of
industrialized countries besides the United States,
the total number of blind persons in the world
would be about 42 million. The prevalence and
causes of...
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