VDTs and vision
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Your eye
Excessive realignment (vergence) and movement of the eyes from
one fixed point to another (e.g., from screen to source document)
can lead to fatigue and visual difficulty. This effect could be
minimized by keeping the various objects that need to be viewed
(the display screen, the documents, etc.) at equal distances from
the eye. Normal vergence movements that occur with reading do not
ordinarily produce noticeable fatigue. The effort required for convergence
with lateral movements appears to produce less visual discomfort
than do up and down movements.
The lens and ciliary apparatus
The lens is responsible for refraction of light, and its refractive
power is varied under the influence of the muscles of the ciliary
apparatus in a process known as accommodation. Comfortable viewing
depends on appropriate accommodation such that the retinal image
is no more than 0.5 diopter out of focus and no more than two-thirds
of the potential accommodation capacity is expended. Under certain
circumstances, such as shifting to infinity following prolonged
near viewing, accommodating ability may be temporarily inadequate
leading to a transient but noticeable myopia. (However, under normal
static viewing conditions, the accommodated lens remains in constant
motion.) Under normal viewing conditions, visual fatigue would not
be expected to result from a frequent changing of accommodation,
and in fact, none has been demonstrated.
The pupil
Pupil size is determined by the opposing actions of constricting
and dilating muscles and is influenced by light levels. Pupil size
is also related to accommodation, and, with typical VDT usage, changes
in pupil size are more likely to be due to differences in viewing
distances than to variable luminances. Visual acuity is related
to pupil size because, with increasing light levels, the pupil becomes
smaller reducing the eye's refractive error. The point at which
visual acuity ceases to improve is reached rapidly, but the depth
of focus continues to increase requiring less accommodative effort.
The effect on visual acuity is only of importance for low-light
levels. The loss of depth of field with increased pupil size is
a problem at low-light levels. Even under constant illumination,
the pupil is in a state of continuous movement. Thus, in its resting
state, the average diameter of the pupil is constantly changing
within the range of plus or minus 10 percent, at a rate of approximately
twice a second. Under normal circumstances, changes in light levels
are presented to the eye at a rate in harmony with pupillary responsiveness,
and pupillary fatigue does not seem to occur. With faster changes
(more than three per second), the pupil responds inadequately, and
can become fatigued and result in discomfort.
The retina
The cells of the retina translate light signals into electrical
signals that are sent to the brain for further processing into the
perceived visual image. In general, there is no visual fatigue related
to low versus high levels of retinal illumination because the rate
and level of retinal receptor signal generating activity is not
subject to a fatigue phenomenon. There is actually much less metabolic
activity by a factor of two or three in bright light as compared
to dark. Proper lighting on the job will reduce visual fatigue.
General considerations
Extensive research has concluded that VDT use does not
harm the eyes and long-term studies have found no indication
of visual impairment. Reading from a display is essentially
similar to other reading tasks. With prolonged, intensive
use temporary visual discomfort can develop. However,
this is frequently related to improper workstation design,
lighting problems, or inadequately corrected vision
for the work to be performed. In many cases, visual
discomfort may be ameliorated by appropriate attention
to workstation and visual considerations, for example,
keeping the display screen clean.
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