I know some of this has been posted before, under other names, but as I wrot e the original, I thought I would copy the full text here.
Sunnies again! This has been on a few times over the board in various forums, here is a comment I have posted before, most recently in the "Questions" forum, under Serengeti sunglasses in the UK? (
http://www.pprune.org/ubb/NonCGI/For...ML/001437.html)
There are several reasons to wear sunglasses. It is generally accepted that glare is harmful to the eye and that protection from glare is therefore therapeutic, even though studies show that only 22% of the population reacts adversely to glare. For these people the pupils are nearly pinpoints under such exposure. Diminished retinal luminescence cause measurable visual decrements and sunglasses will improve this visual acuity problem by counteracting the amount of available light. Conversely another 11% are at the opposite end of the sensitivity spectrum, these people actually have improved acuity under high glare conditions and seem to have no need for sunglasses.
It is not necessary to wear lenses inside the aircraft to protect a pilot from the injurious part of the UV spectrum, because canopy or cockpit windows will also block UV-B. (Acrylics, however, will not block UV effects and Ultra light or open-cockpit flyers will need sunnies.) UV-C is absorbed by the atmospheric ozone and is not supposed to reach the earth. Infrared rays (as in a heated cockpit) are not known to be a eye hazard.
Sunglasses are therefore essential for most pilots in order to cope with glare. The most important consideration for sunglasses is that they reduce glare without affecting vision. Lenses should not be too dark and the standards given for pilots is that they transmit at least 15 per cent of incident light. The tint must be "Neutral density" (ND) which means that it doesn't affect colour perception. The recommended tint for aviation sunglasses is therefore ND15. (This is the US military specification standard for aviators sunglasses and tinted visors, and may have its scientific origins in a 'best guess scenario' more than fifty years ago.) Only sunglasses that conform to the Australian standards should be worn . Those marked "specific purpose sunglasses" are recommended.
Pilots who wear prescription sunglasses may either wear clip-on lenses or they can get their prescription sunglasses made with ND15 lenses. Clip-ons have the advantage of being easy to remove when there is a quick change from light to dark conditions.
If you chose to have your prescription lens made with a tint then it is essential to have another pair of untinted lenses for night and low glare flying. Pilots who wear look-overs are advised to use bifocals instead and then wear clip-ons.
There are two main materials, crown glass and a plastic known as CR39. A very tough poly carbonate lens material is also available. All three lens materials are acceptable. CR39 and polycarbonate have these advantages : highly impact resistant; light weight; and low thermal conductivity and thus less liable to fogging.
CR39 is more vulnerable to surface damage than glass and CR39 lenses must be carefully protected from scratching. CR39 can be obtained with an abrasion-resistant coating to reduce its susceptibility to surface damage.
Polaroid lenses should be left in your boat. (Everybody has one right ) Their chief attribute is that quality Polaroid lenses will completely eliminate glare coming from a flat surface that is of an angle of approximately 53%. A pilot wearing Polaroid lenses sees the world as constantly changing according to his (or her) angle of bank, as the angle of the glare is altered.
Liquid crystal displays work by electrically controlling filtered polarised light, the crystal is orientated by electric current, so as to polarised light to either pass through or be blocked by a polarised filter. All light emitted from LCD screens is therefore polarised, and cannot penetrate a polarised lens if that lens is orientated 90 degrees to the light - so any LCD may appear blank with polarised lenses on, and looking through a polarised window can result in no view at all!
Lens colors are seen in different ways: Green or grey are said to give the least color distortion, and are available in combination. Yellow has the capability of filtering reflected short-wave blue which is found in air contaminants such as fog, haze, smoke or smog. In certain conditions therefore yellow "blue blockers" can improve visual acuity, but not because they protect from glare. Yellow lenses that cut out more than 30% of ambient light can affect color perception and military pilots complain that depth perception is altered. Brown, if it is not too dark, will enhance contrast as well as doing a modicum of blue-blocking. Rose also increases contrast and blue-blocking offering a niche in car use.
In summary, aviators' sunglasses should: be glass or polycarbonate; transmit not less than 25% of available light; not distort colors, distances or shapes; nullify the blurring effect of short-wave reflected blue; have their adverse effect on visual acuity well understood; and not be worn under conditions of diminished light.
As an aside, I bought a pair of glass Serengettis with a brown tint for nearly $AUD300. I didn't like the weight of the glass lens at all, and the brown tint, while it was good for glare, made the green information on EFIS a little hard to see when I transistioned to EFIS. In addition the inside of the lens was highly relfective, so as I was sitting in shadow (in the cockpit) looking out at a bright field (like a cloud deck from above) they gave me a perfect image of my eyes looking back at me (reflected from the inside of the lens). That was difficult to ignore. I now fly with a pair of $AUD15 plastic sunnies I bought from a Chemist and I am much happier with them.
In Australia pilots may claim one pair of sunnies per year at tax time.
sources:
-CAA (Australia) Aviation Safety Digest (ASD) 150 (1991)
-ASD 136 (Autumn 1998)
-ASD 133
-CASA's Flight Safety Australia (Sept. 1997)
Copies of all of these articles are available for free if you contact (Australia'a CASA or BASI)