I just read this on a web page. This seems to give pretty accurate info about this topic.
Here it goes:
Relay-Version: B 2.11 6/12/87; site scorn
Path: uunet!clyde.concordia.ca!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!julius.cs.ui uc.edu!apple!voder!pyramid!leadsv!claude
From:
[email protected] (Claude Goldsmith)
Newsgroups: rec.aviation
Subject: How Should Pilots Select Sunglasses?
Message-ID: <
[email protected]>
Date: Thu, 08 Nov 90 08:32:21 PST
Reply-To:
[email protected] (Claude Goldsmith)
Distribution: na
Organization: LMSC-LEADS, Sunnyvale, Ca.
Lines: 110
The newsletter published by USAIG in August has an interesting
article dealing with sunglasses for aviators. Since I don't have a
scanner handy and I'm a 2 finger typist, I'm not going to copy the
whole thing here. However some of the data and conclusions are
interesting and useful, so I'll extract a few things.
>From Aviation Insurance News; volume 21; number 4.
"There are several excellent 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 percent of the population reacts adversely to
glare."
"The older eye compares unfavorably with the younger one. Since
visual performance is directly related to image luminance, it follows
that the older eye, which is less responsive to changes in light
levels, is at a disadvantage. There are several reasons for this.
First, there is less increase in pupilary size with decreased
luminance. Second, changes in the lens and vitreous humor make the
older eye more sensitive to glare. Third, there is an overall
reduction in the transmission of light. This translates directly to
a need for more light to be able to see, and to age-related problems
with visual acuity in diminished light... Older pilots, therefore,
should wear sunglasses that allow the passage of more light to the
retina. It has been reported that to obtain the same contrast
detection performance as a 20-year-old, a 40-year-old needs 40
percent more light, and a 60-year-old needs 100 percent more light"
"A high-level light source, such as glare, causes pupilary
constriction. The smaller the aperature through which the eye must
see, the darker is the immage created on the retina... "
"The amount of light blocked by sunglasses is the key to how much
decrement takes place in visual acuity while the glasses are actually
in place. This adverse effect exists only while the glasses are
actually in place, and does not persist after the glasses have been
removed. Generally, the darker the lens, the larger the loss. This
is the reason why baseball players delay flipping down their
sunglasses from beneath their cap visors until they have located the
arcing ball."
"...A lens that darkens or lightens according to the intensity of
ambient light, or truly photosensitive lens, would appear to be the
most desireable, especially since the pilot needs metered light
abatement that is based on the severity of the exposure. However
those lenses that require UV-B to make them darken will not darken
when worn inside the airplane since UV-B does not penetrate the
canopy or windows. Thus, what appears to be the best answer to the
changeable needs of the cockpit, photochromatic lenses, turns out not
to be an answer at all."
"The amount of light that passes through a lens is the most critical
factor in selecting sunglasses to effect a compromise between visual
decrement, color distortion and glare or high ambient light
protection. Lenses are catagorized as being between a one and a four
according to percent of light transmission and it's basic color.
Thus, you can have a brown 3, gray 4, yellow 1, green 2 or any
combination of a color and a number. A number 1 lens cuts 20 percent
of available light, barely enough to be noticed, and except for
yellow, useful only in the world of fashion. A number 2 lens blocks
70 percent. A number 3 lens blocks 85 percent and a number 4 lens
blocks 95 percent. There is no place in aviation for a number 4 lens
of any color because of the severe decrement in visual acuity,
(though such lenses are available). A pilot with 20/20 vision
wearing number 4 glasses has a visual acuity between 20/40 and 20/60
while airborne in the cockpit, even though he could be a comfortable
20/20 on the sunbathed ski slopes using the same glasses."
"A Number 3 lens has utility only in unusually high light situations
such as flying into the sun, or flying VFR just on top in bright sun.
Interestingly, it is Number 3 lenses that are in common usage without
regard for whether acuity suffers. Visual acuity while wearing
Number 3 lenses can be degraded to an average of 20/30."
"The Number 2 lens should be the aviators friend, and then only when
judiciously worn. A 30 percent light transmission presents the world
with the same amount of light as that found in a 70 percent eclipse
of the sun. That is what these glasses do. Subdued light is the
result. Visual acuity is reduced minimally. Wearing Brown 2
glasses, the 20/20 pilot remains almost 20/20. Looking for traffic,
the pilot should remove them. As soon as meteorological conditions
permit, they should be returned to the case or pocket. Wear Yellow 1
glasses in haze or in the soup, and then only if they improve
vision."
"The effect of even a small difference in acuity on visual performance
is commonly underestimated." So say the investigators at the U.S.
Naval Aerospace Medical Research Laboratory in their December 1986
report on the use of sunglasses and visors by U.S. Navy fighter
pilots. Visor wearers were at a 1.8 nautical mile disadvantage in
sighting a target compared to those not so encumbered."
"Sunglasses should not be worn merely because they are available.
Ambient light translates directly into visual acuity. Losses in the
former impact the latter."
"Aviators sunglasses should be glass or polycarbonate; should transmit
not less than 25 percent of available light; should not distort
colors, distances, or shapes; should nullify the bluring of shortwave
reflected blue; and should increase contrast without
misrepresentation. Their adverse effect of visual acuity must be
known and they should not be worn during conditions of diminished
light. There can be special times when an adjunct pair of Yellow 1
glasses (that are not sunglasses) can improve vision..."
--
note: photochromatic lenses would enter a number 2 type lense in their lighter tone stage but it seems that cockpit glass will affect the photochromatic ability of the lenses to get darker, to some point.