Optical system
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Optical system
There is no medical evidence that the human eye is damaged by the light levels
and visual tasks of a typical office environment. It is true that some visual
tasks can cause symptoms commonly called "fatigue," and some can result
in a temporary reduction of visual ability. For example, a person who focuses
on a near object for a long time may require several seconds or even minutes
to regain normal distant vision. Fatigue and inconvenience, however, are not
the same as harm. Nevertheless, the concern most often expressed about VDTs is
the possibility of ill effects on the user's eyes. This section presents some
basic information about the human eye that can assist in selecting and using
VDTs and minimizing user discomfort.
Two terms are fundamental to a discussion of the
human optical system. These terms are focal length
and diopter. Specifically, they refer to the strength,
or light-bending power, of lenses. Focal length is
the distance from a lens to the point where the light
rays striking that lens coverage. Diopter is the reciprocal
of focal length, expressed in meters. For example,
a lens with a focal length of one meter has a strength
of one diopter, one-half meter, two diopters; one-quarter
meter, four diopters, and so on. Diopters are a convenient
measurement index because their values are additive.
Incoming parallel light rays striking a +5
diopter lens and converging at a focal point 20cm
distant.
Figure 8 shows a schematic of a positive spherical
lens of the type used to correct hyperopia, or farsightedness.
Incoming parallel light rays striking a -5diopter
lens and diverging
Figure 9 show a schematic of a negative lens of the
type used to correct myopia, or nearsightedness. Both
conditions are discussed later under "Common Disorders." Note
that negative lenses do not have a true focal point
because they cause light rays to diverge. The diopter
strength of a negative lens is determined by measuring
the distance from that lens to the point where the
diverging rays would converge if their paths
were reversed.
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