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Sunglasses
Pilots: for
Sunglasses help safeguard a pilot’s most important sensory asset — vision. A quality pair
of sunglasses is essential in the cockpit environment to optimize visual performance.
Sunglasses reduce the effects of harsh sunlight, decrease eye fatigue, and protect ocular
tissues from exposure to harmful solar radiation. Additionally, they protect the pilot’s
eyes from impact with objects (i.e., flying debris from a bird strike, sudden decompression,
or aerobatic maneuvers). Sunglasses can also aid the dark adaptation process, which is
delayed by prolonged exposure to bright sunlight.
RADIATION. Radiation from the sun can damage skin and eyes when exposure is
excessive or too intense. Fortunately, the Earth’s atmosphere shelters us from the more
hazardous solar radiation (i.e., gamma and X-ray); however, both infrared and ultraviolet
radiation are present in our environment in varying amounts. This is dependant upon
factors such as the time of day and year, latitude, altitude, weather conditions, and the
reflectivity of surrounding surfaces. For example, exposure to ultraviolet radiation increases
by approximately 5 percent for every 1,000 feet of altitude.
Atmospheric infrared energy consists
of long-wavelength radiation (780
to 1400 nanometers [nm], see
Figure 1). The warmth felt from the
sun is provided by infrared radiation
and is thought to be harmless to the
skin and eyes at normal atmospheric
exposure levels. More hazardous to
human tissues is short-wavelength
ultraviolet radiation. Ultraviolet
is divided into three bandwidths:
UVA (400 – 315 nm), UVB (315
– 280 nm), and UVC (< 280 nm).1
Excessive or chronic exposure to
UVA and, to a greater extent, UVB,
can cause sunburn, skin cancers, and
is implicated in the formation of
cataracts, macular degeneration, and
other eye maladies.
The American Optometric Association recommends wearing sunglasses that incorporate
99 – 100% UVA and UVB protection. Fortunately, UVC, the most harmful form of
ultraviolet radiation, is absorbed by the atmosphere’s ozone layer before it reaches the
Earth’s surface. Some scientists believe, however, that depletion of the ozone layer may
allow more ultraviolet to pass through the atmosphere,2 making 100% ultraviolet protection
a wise choice when selecting eyewear.
LENS MATERIAL. The three most common lens materials in use today are
optical quality “crown” glass, monomer plastic (CR-39®), and polycarbonate plastic
(see Table 1). Lenses made from crown glass provide excellent optical properties (as
indicated by the high Abbe value). Crown glass is more scratch-resistant but heavier
and less impact-resistant than plastic. Glass absorbs some ultraviolet light; however,
absorption is improved by adding certain chemicals during the manufacturing process
Figure 1. Electromagnetic radiation spectrum
including visible, infrared, UVA, UVB, and UVC
wavelengths.
The American Optometric Association recommends wearing sunglasses that
incorporate 99 – 100% UVA and UVB protection. Fortunately, UVC, the most harmful
form of ultraviolet radiation, is absorbed by the atmosphere’s ozone layer before it
reaches the Earth’s surface. Some scientists believe, however, that depletion of the
ozone layer may allow more ultraviolet to pass through the atmosphere,2 making 100%
ultraviolet protection a wise choice when selecting eyewear.
LENS MATERIAL. The three most common lens materials in use today are
optical quality “crown” glass, monomer plastic (CR-39®), and polycarbonate plastic
(see Table 1). Lenses made from crown glass provide excellent optical properties (as
indicated by the high Abbe value).
Crown glass is more scratchresistant
but heavier and less
impact-resistant than plastic. Glass
absorbs some ultraviolet light;
however, absorption is improved
by adding certain chemicals during
the manufacturing process or by
applying a special coating. Glass
retains tints best over time; however,
for higher refractive correction, the
color may be less uniform, as parts
of the lens will be thicker than
others (see Figure 2).
CR-39® plastic lenses possess excellent optical qualities, are lighter in weight, and more
impact-resistant than glass lenses, but are more easily scratched, even when scratchresistant
coatings are applied. CR-39® lenses tint easily and uniformly, even for those
requiring a great deal of refractive correction, but do not hold tints as well as glass. CR-
39® plastic can be bleached and re-tinted if fading becomes excessive at some point.
　
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