Sunglasses are important protective devices for pilots. Pilot sunglasses can reduce visual fatigue, thereby reducing the possibility of pilot error. A good pair of sunglasses can also protect the eyes and vision from ultraviolet radiation too. For a pilot, choosing sunglasses is a difficult task because of the variety of types available and because marketing often addresses fashion rather than performance.
Actually, choose the type of pilot sunglasses will depend on your aircraft, the environment in which you fly, and your personal preference. Moreover, the features which are important in choosing sunglasses for flying can be considered as filter factors and frame factors. When it comes to filters, the total light transmittance should be taken into consideration, because Light striking the front of the filter can be reflected or absorbed, or can pass through the filter. The amount of light passing through the filter is called the transmittance, and the smaller this number, the stronger/darker the filter. For pilots, the optimal transmittance is 16% to 20%. Below 15% there is reduced visual acuity and color discrimination, even with neutral density filters. A filter with no tint is described as a neutral density filter. This reduces the total light incident on the eye, while preserving the relative proportion of different wavelengths Neutral grey is the best general purpose tint for flying, as this produces a neutral density filter. A filter with a tint will appear to be colored when held up to light. This is because it is absorbing the other colors in the spectrum. A green filter appears green, for example, because it is absorbing red and other wavelengths of light. The second factor is UV protection. UV (ultraviolet) light is harmful to the eyes and to the surrounding skin. It is a risk factor in cataract, age-related macular degeneration, and skin cancers. The intensity of UV radiation increases by 4% per 1000 feet of altitude. Stratospheric ozone shields the earth’s surface from UV light. Low altitude flight in geographical regions where the ozone layer is depleted, and flight in or above the ozone layer anywhere in the world, both carry an increased risk of UV damage. Flying over snow, sand and water increases UV exposure, as UV light is reflected. It is recommended that all pilot sunglasses block 100% of UVA, UVB and UVC. The last factor should be taken into consideration is polarizing filters. These work by selectively blocking plane polarized light. Light becomes plane polarized when it is reflected towards the observer. These glasses are therefore excellent for dealing with situations where large amounts of light is reflected from surfaces such as snow, sand or water. However, aircraft windscreens are often either intentionally or unintentionally polarized in the manufacturing process. When a wearer looks through the windscreen, they see Moiré patterns. These are interference patterns caused by interaction of two sets of polarized filters. Moreover, Some LCD displays emit polarized light. These displays can dim or disappear when the pilot rotates their head to look at them. Polarized sunglasses are, however, useful for seeing submerged objects in water. They therefore have limited, specific applications in aviation, such as fixed wing open cockpit or rotary wing doors off, for a crewmember or pilot non-flying looking out of the aircraft.