Aviation Safety Letter 3/2003

How to avoid glaring errors

Photo courtesy of Randolph Engineering Inc.

A pilot who can't see is an accident waiting to happen. Without good glare protection, flying on bright, sunny days can be tiring and hazardous - and it can effect night flying too. Exposure to bright sunlight for a whole day without protection interferes with proper night adaptation for 12 to 24 hours! The following brief summary will focus on how to choose your sunglasses and the advantages and disadvantages of the various types.

There are three problems caused by bright sunlight: glare, infra-red (IR) radiation and ultraviolet (UV) radiation. Glare, although the most obvious nuisance - causing tearing, distraction and fatigue - is responsible for less serious problems than IR or UV radiation. Cutting down glare by using very dark sunglasses, however, can cause problems because reducing transmitted light reduces visual acuity, as anyone who has driven from a bright road into a dark tunnel whilst wearing sunglasses can verify. Even moderately dark sunglasses can, on a bright day, cut your vision down from 20/20 to 20/40.

On the ground, UV is partially filtered by the earth's atmosphere, but the higher you go, the less the protection. UV light is not filtered equally by all types of sunglasses and can damage the eye, causing early cataracts (lens opacities). Cheap sunglasses should be avoided as they may only cut down glare. Good sunglasses reduce light transmission to 12-20 percent, but should cut down UV transmission by at least 90 percent. Looking directly into the sun should be avoided as IR can quickly injure the sensitive retina at the back of the eye. Prolonged and unprotected exposure in bright sunlight, particularly if combined with a wide snow cover, can seriously degrade vision. As you can imagine, mountain climbers are quite familiar with the need for top-quality eye protection.

Sunglasses may be constant-gradient, photochromic or polarized. Polarized lenses are great for fishing, but bad for flying. Due to manufacturing stresses, there may be small areas of polarization in an aircraft canopy or windscreen and, if the angles of polarization in the glasses and the windscreen differ, a blind spot can be produced. Polarization may also interfere with depth and distance perception, particularly during a bank. Just what you need turning on final!

Photochromic lenses that darken with increasing UV light are good for driving, but polycarbonate aircraft canopies shield out much of the ultraviolet rays and may interfere with their proper darkening. Additionally, going from bright sunlight into cloud the glasses may take several minutes to lighten. Constant-gradient glasses come in various colours and are the most commonly used. All are about equally effective for glare, but green or grey lenses have the least adverse effect on your vision. Yellow lenses are good in haze, but less effective in bright sunshine. Sports orange lenses should not be chosen because they interfere with blue-green discrimination and may make red warning lights more difficult to see. Pilots with colour deficiencies should not use coloured lenses and should stick to a quality grey lense.

What is best? Where vision is concerned, do not gamble your eyes by using cheap sunglasses; also keep in mind that price is not always a good gage of quality, as some trendy polarized models costing well over $150 are not what you need at all. You should budget anywhere between $75 and $150 for good aviation sunglasses. Constant-gradient lenses that reduce light transmission to 15-20 percent and block 90 percent of UV light are ideal. Neutral grey, green or brown lenses are the most popular. Blue, orange or polarizing lenses should not be worn while flying. If in doubt, ask your Civil Aviation Medical Examiner for advice. In the long run, it is wiser to save your eyes than to save your money!

This article was originally published in ASL 3/1994, and has been slightly updated by our Civil Aviation Medicine Branch. - Ed.

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