This should shed light on the matter.
Sunglasses for Pilots
There are many types of sunglasses available, but no single type is ideal for every pilot. Needs change based on age, light sensitivity, ambient lighting conditions and type of flying. Some sunglasses are not right for any pilot at any time. Valid reasons for wearing sunglasses in the aviation environment include improved night vision adaptation, enhanced contrast in the visual field, reduced glare, decreased UV exposure and avoidance of eye fatigue. Though style and appearance may be a consideration, the safety conscious pilot should focus on the proper selection of lens features rather than frame styles with cheap lenses.
Visual acuity varies with the light available and the sensitivity of an individual to various degrees of brightness. The pupil controls the amount of light reaching the retina. Older individual's eyes do not transmit as much light through the eye as younger people do. Therefore, many older individuals need more light for optimum acuity. They may want to use sunglasses that transmit more light. On high glare days, such as over snow or sand, the pupils contract to protect the eye from the glare. Sunglasses will reduce glare and allow the pupil to let more light on to the retina, thus enhancing vision.
Sunglasses for Pilots - Glare
Glare can also be caused by indirect blue wavelength light and UV light. UV light increases by 4% for every 1,000 feet of altitude and contributes to the blue colour of the sky. Some researchers feel this can cause a haziness on the retina decreasing visual acuity even when indirectly viewed. Fortunately, most windscreens eliminate much of this wavelength. Near sunrise and sunset, the atmosphere filters out this wavelength giving the sky its characteristic red-orange colour. Yellow lenses, often called "blue blockers" will block out this wavelength also and may improve vision on a hazy day. They may present decreased perception of some cockpit displays, however.
Sunglasses for Pilots - Tinted lenses
Tinted lenses distort colours to some extent. The yellow shaded "blue blockers" will alter colour perception if tinted enough to block out 30% of the light. Thus, these lenses should be used only by aviators on bright, hazy days and avoided in low light situations. Green and grey lenses have the least distorting effect on colour vision. Brown distorts colours slightly more, but can block some of the blue light blurring in haze.
Sunglasses for Pilots - Darkness
Darkness or degree of light reduction is calculated in percentage and listed by numbers. A #1 lens blocks only 20% of the incoming light and has little value for aviators. The exception may be the #1 Yellow lenses for hazy or smog filled days. The #2 lens blocks 70% of light and are useful for most aviation situations. It provides a balance of glare protection, luminescence reduction and UV protection without significantly reducing visual acuity. The light reduction of #3 (85%) may be useful for those pilots particularly sensitive to bright light while others may find the #3 lens reduces visual acuity. The #4 lens blocks out 95% of incoming light and significantly reduces visual acuity because the macula, where sharpest vision is found on the retina, requires light to activate the cones of the retina. Aviators wearing these lenses in flight will not meet FAA minimum distant visual acuity standards.
Sunglasses for Pilots - Mirrored glasses
Mirrored glasses use metal particles to reflect images. They scratch easily and can cause distortion or blind spots. While popular for Hollywood movie pilots, professional pilots should leave them to actors, policemen and other imitators.
Sunglasses for Pilots - Photochromatic lenses
Photochromatic lenses increase darkness when exposed to UV light. Because aircraft windscreens block most UV light, the lenses will not darken substantially inside an airplane or car. The military prohibits its pilots from using these sunglasses for good reason. Pilots flying open cockpit airplanes are the only ones who may benefit from this feature.
Sunglasses for Pilots - Gradient lenses
Gradient lenses usually have a darker tint on the upper portion of the lens and a gradually lightening color near the bottom. This may be useful when trying to view instruments on a very bright day. The lighter tinting below allows more light from the relatively dark instrument panel to reach the retina and improve visual acuity while blocking out the glare from the outside view.
Sunglasses for Pilots - UV protection
UV protection is desirable for lenses worn outdoors, although it is not as important for glasses worn inside the aircraft since this wavelength is already blocked by the windscreen. Glass and polycarbonate block nearly all of the UV-B light. Soft plastic lenses may block visible light but not block any UV wavelengths. The hazard for cataract formation in the individual using soft plastic lenses is increased because the pupil dilates in response to decreased visible light. The dilated pupil allows more UV light to enter and penetrate the lens increasing the risk of cataracts.
Sunglasses for Pilots - Scratch resistant coating
Scratch resistant coating may increase the life of polycarbonate lenses and plastic lenses. Ironically, polycarbonate will withstand direct hammer strikes without breaking, but scratches relatively easily. Glass will shatter, but is more resistant to scratching. Polycarbonate lenses are thinner and lighter than glass lenses.
Sunglasses for Pilots - Polarized lenses
Polarized lenses should not be worn by pilots in the cockpit. Glare from flat surfaces is blocked by polarized lenses which are oriented in parallel lines like closely spaced prison bars. Light parallel to the lines is transmitted while non-parallel light (glare) is blocked. Unfortunately, if the windscreen is polarized and the lenses are not precisely oriented the same as the windscreen, all light may be blocked. Changing bank angle and head position could create blind spots. For those who are boating and need glare protection from light reflected off the water, polarized lenses are excellent choices.
Sunglasses for Pilots - What we recommend
What we recommend: On bright days, consider using a neutral tint (green or grey) glass or polycarbonate lens that blocks 70-90% of the incoming light, possibly with a gradient that lightens on the lower portion of the lens. On a hazy or smog filled day, consider a yellow or brown lens that blocks 20% of the light, but avoid using it if colour perception (IFR flight), as opposed to visual acquisition (VFR flight), is important.
The military has found that some fighter pilots prefer the "high contrast" yellow visors for their helmets to enhance target acquisition, while others prefer not to wear yellow visors. At dusk or in lighting that is comfortable without sunglasses, remove them to increase visual acuity. Don't use polarized or photochoromatic lenses in the cockpit. Don't waste your money on soft plastic lenses or mirrored lenses. Scratch resistant coating may increase the life of polycarbonate lenses.
For additional information, please see the July-August 2002 Flight Safety Foundation article on "For Pilots, Sunglasses Are Essential in Vision Protection" and the Fall 2003 article from the Federal Air Surgeon's Medical Bulletin on "Sunglasses in Aviation: A Primer for Pilots" by Mr. Montgomery and Dr. Nakagawara of the FAA Vision Research Institute.
Optimum Vision and Eye Protection And Visual Acuity