Piloting demands good vision. But the aviation environment contains numerous hazards to our eyes that can affect our ability to accomplish our tasks, as well as threaten the long-term health of our eyesight.
The immediate threats include eye strain, glare and windshield failures. Long-term exposure to glare and ultraviolet (UV) radiation doubles a pilot’s chances of developing cataracts and other age-related vision-robbing diseases.
Any factor that increases sunlight exposure to the eyes will increase the risk from solar radiation. That harm is cumulative and over an extended period increases the chances of developing an ocular disorder later in life. If you operate a high-performance aircraft that spends a considerable part of its flight profile near the stratosphere or during those times of the day when UV radiation is most intense, your dosage of UV radiation increases markedly.
On the ground, UV is partially filtered by the earth’s atmosphere, but the higher you go, the less the protection. It is estimated that there is a 4% increase in UV radiation with each 1,000 ft. of elevation. According to an FAA study, “Optical Radiation Transmittance of Aircraft Windscreens and Pilot Vision,” flying over a thick cloud layer increases UV radiation by up to 80%. Fresh snow is a particularly good reflector and almost doubles a person’s UV exposure. In such conditions, wearing a pair of sunglasses with a close-fitting wraparound frame design is the best protection since UV-blocking lenses fail to block radiation from entering the eyes from the sides of the frame.
Aircraft windscreens provide only limited protection against some forms of UV radiation that penetrate the transparency, damage the retinal photoreceptors and, with long-term exposure, can cause permanent eye damage. Research conducted by Dr. Adrian Chorley, the original optometrist principal with Britain’s Civil Aviation Authority, showed that between 2008 and 2015 long-wave ultraviolet A (UVA) proved to be the most harmful to pilots’ eyes because a higher percentage of it penetrates the cockpit. And he noted that “there is good evidence that long-term exposure to solar radiation, especially the ultraviolet and blue light components, is a risk factor for cataracts and, to a lesser extent, age-related degeneration of the retina.”
Blue light is a short wavelength that progressively destroys light-detecting cells in the eyes, contributing to age-related macular degeneration, which is a leading cause of blindness. The hazard of bright sunlight and blue light increases with altitude.
Meanwhile, glare can come directly from the light source or can take the form of veiling glare, reflected from crazing or dirt on the windscreen. Glare directly affects a pilot’s ability to accurately detect objects, especially when the object is in the same direction as the glare. This negatively affects the ability to maintain visual awareness and therefore decreases flight safety. Studies have shown that reduced night vision can result from continued exposure to intensive glare (See “A Glaring Problem,” BCA, November 2018).
Exposure to glare and concentrating on a visually intense task can cause an unconscious clenching of the muscles around the eyelids, face, temples and jaws, leading to discomfort, pain from overuse, eye strain, increased fatigue and headache. Soreness of the eyes and neck, mild tearing, dryness, blurring or doubling of vision, light sensitivity, difficulty focusing on images, tightness of the temples or forehead, or a combination of these, are common descriptions by persons experiencing eye strain. Headaches are the most common symptom. Eye strain can be unpleasant and disruptive to one’s ability to concentrate and work. Other dangerous symptoms of eye strain include increased physical fatigue, decreased productivity and increased number of errors.
The correct sunglasses can reduce the adverse effects of bright sunlight, decrease eye fatigue, and protect the soft tissue in and around the eye from harmful solar radiation. In the evening, sunglasses can aid in adapting the eyes to darkness, which can be delayed by prolonged exposure to bright sunlight. Conversely, the wrong sunglasses can produce negative effects including difficulties viewing instruments and enhancing glare on the windshield.
When using sunglasses, there should be a proper balance between visibility of objects inside and outside the cockpit. While it’s tempting to use dark sunglasses to cut out glare, pilots should be aware that certain lenses can reduce their visual performance in specific flight environments — and increase the potential for accidents. Reducing transmitted light reduces visual acuity, as anyone who has driven from a bright road into a dark tunnel while wearing sunglasses can verify. Even moderately dark sunglasses can, on a bright day, cut your vision down from 20/20 to 20/40. Color discrimination may also be reduced while wearing dark lenses. Target detection and resolution, especially for visual targets of medium to low contrast, can also be degraded if using sunglasses that are too dark.
UV light is not filtered equally by all types of sunglasses and can damage the eye, causing early cataracts. Cheap sunglasses should be avoided as they may only cut down glare. If one is unsure of the parameters of a particular tinted lens, ask an optical dispenser or eye doctor.
There’s a long-standing misperception that polarization protects eyes from UV light. It does not. It simply blocks a specific angle of visible light. While this is great for use on the water or snow since it blocks light reflection off the surface, it offers no real value in the cockpit. In fact, with modern LCDs, such sunglasses are actually a serious hinderance. This is significant since LCDs have their own polarization filter built-in and placing two polarization filters over each other can completely block vision.
And while polarized lenses eliminate reflected glare from a flat surface, looking through a laminated aircraft windscreen while wearing them can result in reduced and distorted images outside the aircraft. 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 result. Polarization may also interfere with depth and distance perception, particularly during a bank.
Photochromic lenses that darken with increasing UV light are good for driving, but polycarbonate aircraft canopies may interfere with their proper darkening. Additionally, when going from bright sunlight into clouds, the glasses may take several minutes to lighten, thus aviators may experience visual performance loss then or mountain shadow effects.
Constant-gradient glasses come in various colors and are the most commonly used. All are about equally effective for glare, but green or gray 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 worn because they interfere with blue-green discrimination and may make red warning lights more difficult to see. Pilots with color deficiencies should not use colored lenses and should stick to a quality gray lens. Blue-blocking lenses have been found to produce mild color discrimination losses in normal color vision subjects and severely reduce color discrimination for dichromatic (severe color-deficient) people.
So, what’s the best eye protection for pilots? Dr. Van B. Nakagawara of the Aeromedical Research Division’s Vision Research Team at the FAA’s Civil Aeromedical Institute recommends lenses that are free from distortions and imperfections, have adequate light transmissivity (approximately 12-20% overall light transmission), cut down UV transmission by at least 90%, and do not alter color perception. Since color vision is important to pilots, dark tints (i.e., <8% transmittance) and tints that distort color vision (such as blue-blocking lenses) should be avoided. The use of larger lens sizes and, as noted, wraparound frame styles can prevent sunlight from entering peripherally and affecting vision.
Sunglass lenses are usually composed of glass, polycarbonate or plastic. Glass lenses are more scratch resistant than cheap plastic ones and retain their tint best over time. Scratched lenses can greatly reduce their effectiveness to block UV light. Glass does have its issues, however. Such eyewear is generally heavier than other options and may have manufacturing defects leading to minute variations in thickness, which can distort color perception.
There are also plastic lens options. These lenses are much lighter and more impact resistant than glass ones but can easily be scratched. There are coatings that try to prevent scratches, but these are generally ineffective. Polycarbonate is the lightest and most scratch-resistant option but can absorb tints due to the coatings used.
Sunglass frames are another important design consideration for the aviator. A thick frame may break the seal of an oxygen mask, create an undesirable pressure-point along the temples (potentially leading to headaches), as well as admit unwanted noise under the muffs of headsets.
The windshields on turbine aircraft are designed to withstand the stresses of aerodynamic forces, pressurization, extreme thermal changes and bird strikes. There have been occasions in which the interior layers of an FAR Part 25 aircraft’s windshield have delaminated from the rest of the structure and violently sprayed glass debris on the pilots. Events including windshield failures, bird strikes or loose items encountered during turbulence or aerobatic flight have caused dangerous debris that could cause ocular trauma.
For instance, the flight crew of an Airbus 319 on August 2016 experienced a sudden failure of the windscreen. “Preflight and takeoff were normal. I was flying the aircraft. We were on autopilot as we climbed through FL 200 when the FO’s inner windshield shattered. It covered both pilots with small pieces of glass and continued to shatter and rain glass on the crew. We [advised] ATC and requested to return to ZZZ and [descend]. While descending, the FO got out the QRH and we went over the appropriate items. We were both wearing sunglasses at the time, which I think helped save our eyes.” (NASA accession number 1377500)
All pilots have an important self-interest to protect their vision. And one of the most important protective items to ensure that is an effective set of sunglasses designed to mitigate the many threats to seeing clearly in the aviation environment.
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