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Artificial tears which contain preservatives may be irritating when used with contact lenses and are best
avoided.
11.4.28 In all cases involving use of contact lenses, proper, regular monitoring by an appropriate vision
care specialist is necessary.
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-29
Sunglasses
11.4.29 Sunglasses are useful and often necessary to decrease glare and reduce discomfort in bright
environments, particularly above clouds. In addition to reducing glare by cutting down transmission of the
visible spectrum, sunglasses for general wear should provide protection from ultraviolet radiation (UVR).
11.4.30 UVR is somewhat arbitrarily subdivided into three bands according to wavelength: UVA,
400–320 nm; UVB, 320–290 nm; and UVC, 290–200 nm. Very little UVC is present in terrestrial sunlight except
at high altitudes. Significant amounts of UVA and UVB reach the surface of the earth. UVB is the most
important band as far as harmful effects on biological systems are concerned. The level of UVB is largely
controlled by ozone in the atmosphere. Ozone (O3) is a gas present throughout the atmosphere but most
concentrated in a layer (“the ozonosphere”) 15–50 km above the earth’s surface where its concentration
reaches approximately one molecule per two million or 0.5 ppm.
11.4.31 Concern has been expressed that flight crew may be exposed to dangerous levels of UVR due to
the chlorofluorocarbon (CFC)-induced depletion of stratospheric ozone at altitudes between 25 and 100 km
where, otherwise, most of the UVR is absorbed. Measurements in aircraft cockpits have shown that aircraft
windows provide excellent protection against UVR, even at high altitudes.
11.4.32 However, visible light at the blue end of the spectrum (400 to 500 nm) may cause some retinal
damage, particularly in older individuals. The amount of blue light increases with altitude and 50 to 60 per cent
of this light is transmitted through a 3-cm-thick flight deck window. Flight crew are thus exposed to more blue
light than individuals on the ground. It is not known if this blue light exposure is harmful, but it is prudent to
recommend that flight crew, especially when flying towards the sun at high altitude, wear sunglasses.
11.4.33 Harmful effects of UVR on the skin and the eyes are well recognized. Ocular damage from UVR
(especially the UVB band) include: photokeratitis (snowblindness), pterygium, climatic droplet keratopathy,
cataract and possibly intraocular melanoma. It should be stressed that the risks from UVR exposure are much
greater on the beach than in any cockpit at high altitude.
11.4.34 The colour of sunglasses and the darkness of the tint are usually matters of personal preference.
However, colour-tinted spectacles alter colour perception, and the only type of sunglasses acceptable in the
aviation environment are neutral grey lenses which reduce overall brightness without altering the colour of
viewed objects. Many different types of sunglasses are available including some with graded tint — dark in the
upper portion of the lenses and clear in the lower part. In addition to the tint, good quality sunglasses absorb
at least 95 per cent of UVB, while the highest quality sunglasses provide 99 per cent absorption of UVB and
almost all the UVA.
11.4.35 The UVR absorbing properties are separate from the coloured tint in sunglasses so that it is
possible to have very dark sunglasses with very little UVR protection and vice versa. In selecting sunglasses,
the very dark tints should be avoided because these make it difficult to see the cockpit instruments (absorption
of up to 85 per cent of visible light is suitable). Polarizing sunglasses are not acceptable for flight crew because
of the disturbing reflections from certain glass and plastic laminates. Photochromic lenses darken rapidly and
automatically depending on the brightness of the ambient light. The clearing process, however, is slow and
they are therefore not recommended for flight crew because they do not increase light transmission sufficiently
quickly when flying from bright to dull ambient lighting conditions.
11.4.36 Tinted spectacles, prescription or otherwise, are for daytime use only and result in severe
reduction of visual performance if used in twilight or darkness.
III-11-30 Manual of Civil Aviation Medicine
Aphakia
11.4.37 Aphakia is absence of the eye’s crystalline lens. This is generally the result of cataract surgery but
may rarely occur from non-surgical trauma. Removal of the lens reduces the refractive power of the eye by
approximately 20 dioptres leaving it more or less hyperopic, depending on the original refractive error. In eyes
with high degrees of myopia, removal of the lens reduces or abolishes the myopia and surgical removal of the
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Manual of Civil Aviation Medicine 2(49)
