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phase of flight.
11.4.10 Multifocal lenses are well tolerated by most who try them and are available in many different forms.
See Figure 11-12. Those most useful in the flying environment are the following:
a) Bifocals — the top segment has the distance correction and the bottom segment the near
correction. The size, shape and placement of the reading segment is best determined by a
vision care specialist who is familiar with the requirements for medical certification.
b) Trifocals — the top segment has the distance correction, the bottom segment the reading
correction and the middle segment has a correction for intermediate distances such as
instrument panels, which may be a metre or more from the pilot’s eyes.
c) Progressive addition multifocals — usually called progressive add multifocals — are also
referred to as “invisible bifocals”. These are increasingly used for correction of presbyopia and
are cosmetically popular because there is no visible line across the lens. The top part of the
lens has the distance prescription. From near the centre of the lens the power increases
progressively towards the lower part of the lens. The lowest part of the lens has the reading
power so that there is a gradual transition from the distance portion to the near portion without
a dividing line and without prismatic jump which is present in ordinary bifocals and trifocals.
Theoretically there is a part of the lens which provides optimum correction for any distance
between infinity and the distance required for reading. Unfortunately all progressive
multifocals have peripheral areas of distortion at both sides of each lens making the so-called
progression channel rather narrow, particularly in the higher reading add powers. When first
introduced over thirty years ago, there was concern that the peripheral distortion areas in
these multifocals would cause problems for pilots during take-off and landing. This has not
been the case, and progressive add multifocals can be safely used by pilots, although some
do not like the peripheral distortion and choose not to wear them.
11.4.11 In the early stages of presbyopia bifocals work well in the cockpit. The top part of the lens is used
for distance and for the instrument panel and the bottom part of the lens for reading and any other visual task
at near. As the presbyopia increases, the instrument panel is no longer clearly seen through the top of the
bifocal lens and a correction is needed for this intermediate distance. The solution for this is a trifocal or a
progressive add multifocal.
11.4.12 As a general rule, the strength of the intermediate portion of a trifocal is half the strength of the
bottom portion or reading add. For example, if a given bifocal has a reading add of +2.00 dioptres, the strength
of the intermediate add would be +1.00 dioptre. As the term “add”4 indicates, these powers are simply added to
whatever prescription is necessary in the top portion of the lens, i.e. the distance correction.
11.4.13 The standard bifocals and trifocals usually work well in the cockpit. If there are problems with the
required focal distances, these distances should be measured in the aircraft or a simulator and the vision care
specialist provided with the numbers so that the appropriate corrections can be prescribed. Correct fitting of the
multifocals is critical. If the reading segment is too high, it will interfere with distance vision. If too low, the
wearer will have to raise his chin uncomfortably high in order to read.
11.4.14 Although not ideal from a human engineering point of view, many modern aircraft are equipped
4 Add: abbreviation for L. addetur — let there be added, used in spectacle prescriptions.
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-27
with a great number of gauges and switches located on overhead panels. This may present a problem for the
presbyopic pilot. If necessary a special “occupational multifocal” can be used with a small segment in the upper
part of the lens with power appropriate for the distance of the overhead instrument panel.
Figure 11-12. Examples of multifocal lenses useful in aviation
11.4.15 In presbyopic flight crew who wear progressive add multifocals, the problem of seeing overhead
instruments can be solved with the new stick-on bifocal segments in the upper part of the multifocal lens.
Progressive add multifocals are not available with a progressive add in the upper portion of the lens.
11.4.16 It should be emphasized that almost all the visual requirements for the older flight crew can be met
using ordinary multifocals. With proper communication between the flight crew member and the vision care
specialist it is almost always possible to provide comfortable, functional spectacles.
　
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