Common disorders
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Disorder frequencies / comfort zones
Refractive errors, that is, optical errors of the lens system of the eye, are
very common. It has been estimated that 65% of the population requires eyeglasses
under some circumstances (Campbell and Durden, 1983). A distribution of refractive
errors is shown in Figure 29.
The incidence of ocular refraction based on 194 persons (Sorsby,
et al, 1957).
Those data are based on 194 persons and do not include presbyopes (Sorsby,
Benjamin, Darby, Sheridan and Tanner, 1957).
Refractive errors are fairly obvious to a person. There are other conditions
of the visual system, however, that a person might not be aware of that may
have a considerable influence on visual comfort. For example, there is a condition
known as "convergence insufficiency" that may affect 20% to 30%
of the working population (Grieve and Archibald, 1942). Convergence insufficiency
is frequently associated with asthenopic (eye strain) symptoms of close work,
such as blurring, diplopia (double vision) and headaches (Letourneau, Lapierre
and Lamont, 1979) and is a condition where there is no significant phoria
at far point but a significant amount of exophoria (tendency to underconvergence)
at near point. There is also a reduced near point of convergence with this
condition. People with convergence insufficiency have no difficulty in aligning
their eyes to view distance objects. They must, however, exert extra effort
to make their eyes converge for viewing near objects. That extra effort may
immediately, or over time, produce some visual discomfort. This condition
may be relieved by a prism correction in their eyeglasses.
Zones of Visual Comfort
Viewing distance is an important consideration for visual comfort, particularly
for prolonged near point tasks that are typical of office work. Such tasks
place demands on both the focusing mechanism and the alignment mechanism
of the eyes. If either of these mechanisms is overly stressed for a long
period of time, visual discomfort may result.
For any given viewing distance, there is some range over which the eyes
may converge or diverge and still maintain clear single binocular vision This
may be measured over a person's range of accommodation and an area may be
plotted to show the zone of clear single binocular vision.
An example of a zone of clear single binocular vision. The subject
measured had a maximum accommodation response of 4.5 diopters (Davson, 1976).
The area of comfort for vergence, as first proposed by Percival (1982),
would be the center third of the range of vergence for a given viewing distance.
This recommendation (Alpern, et al, 1961) has received a degree of clinical
validity over the years. Whycer (1978) has further limited that area of visual
comfort by recommending that no more than two-thirds of the person's available
accommodation be required for viewing a task. The area of visual comfort would
be the center third of the width of area shown in Figure 30 and the height
of that area reduced by one-third. Some people may require eyeglasses to reach
that level of visual comfort.
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