VDTs and vision
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Eye care
One factor influencing comfort when using display terminals is eye lubrication,
which is especially important for wearers of contact lenses. VDT users tend
not to blink as frequently as people performing other reading tasks. Eye discomfort
from this staring effect is exacerbated by low humidity. In addition, many
display users maintain the same body position for long periods of time. Therefore,
all display users should be encouraged to focus on distant objects, to look
away from the terminal, and to move head and body periodically for comfort.
After-images
Occasionally, after-images are seen with VDT use. Terminals have the
ability to add moving and fixed patterns and color combinations that may
produce these after-images. Normal (negative) after-images, waterfall illusions,
the McCollough Effect ( complementary colored lines), and motion after-effects
may be produced. Of course none are specific to VDTs, most are short-lived,
and all are felt to be benign.
Glare
As noted above, glare can be minimized by appropriate attention to workstation
considerations such as proper lighting. Glare may be exacerbated by electrostatic
accumulation of dust particles, finger marks, or other sources of dirt on
the screen surface. Daily cleaning with an anti-static cloth can be helpful
in this situation. Unfiltered screens reflect approximately four percent
of the light falling upon them. Appropriate anti-reflecting filters reduce
this considerable and can enhance the effective contrast ratio.
Optical problems and refraction
Much visual discomfort in VDT use may be related to uncorrected refractive
errors of which the user is normally unaware but whose effects are exacerbated
by prolonged, exacting work. These errors include hyperopia, myopia, astigmatism
and phoria. This exacting work is especially pertinent for young uncorrected
hypermetropes and small-value astigmats. Uncorrected myopics improve their
focus by getting closer to their work; however, this close position can produce
discomfort, since it generates more head and body movements for task performance.
Extra-ocular muscle imbalance and resultant disturbance in convergence
may also contribute to visual discomfort. Convergence insufficiency and excess
require additional effort to maintain convergence for near objects. These
effects may be accentuated with viewing of the display screen when the line
of sight is directed above the horizontal. Viewing above the horizontal normally
promotes divergence. Exertion is necessary to maintain convergence in this
position, and this would be burdensome to the person with exophoria. This
may generate visual discomfort with time. Appropriate refractive corrections
can obviate these sources of visual discomfort.
For proper refraction, several factors should be considered. Refractive
measurements of several distances are needed: from the eye to the top of
the display screen, from the eye to the middle of the screen or even lower
on the screen, depending on the portion of the display most frequently used.
In addition, it will be helpful to know how frequently the user looks away
from the screen or other work surfaces. In some cases, the workstation can
be arranged for individuals to place documents at the same distance and height
as the VDT screen.
Depending upon length of use of the visual display and other work conditions,
the decision may be made to prescribe special monofocal lenses refracted
for the viewing distance of the particular user from the display screen rather
than from the usual 50 centimeters.
The most challenging refraction problem is with presbyopic patients. These
patients suffer discomfort from having to shift their gaze between parts
of bifocal lenses or between pairs of spectacles. Consideration should be
given to frequency and type of terminal use in prescribing for these patients.
Those who use their terminals intermittently or change focus between near
and distant objects frequently may feel more comfortable with bifocal lenses.
Those who use their terminals constantly may be more comfortable with special
spectacles refracted for the actual eye-to-screen distance.
Bifocal wearers almost always require the division between the two lens
parts to be higher than usual to allow for proper VDT use while minimizing
neck motion and muscle strain. In some cases, trifocals, or special single-purpose
monofocal lenses designed to optimize focusing at the actual working distance
may be useful in resolving these problems. Such lenses must be properly and
individually designed by knowledgeable professionals.
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