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High-index materials (i.e., index of refraction —1.60) are available in both glass
and plastic for those who require a large degree of refractive correction and/or desire
lighter, thinner lenses. High-index materials are not as widely available, require AR
coats to improve optical clarity, and a scratch-resistant coating for durability. In
addition, most high-index materials do not accept tints as easily and are less shatterresistant
than polycarbonate.
COATINGS. Special coatings can be applied to lens materials for reasons such as
those previously mentioned. Crown glass and most plastic lenses require a specific
coating to block residual ultraviolet radiation. Plastic and polycarbonate lenses require
a scratch-resistant coating to prolong their useful life. The scratch-resistant coating
applied to polycarbonate lenses absorb tints and dyes. High-index materials benefit
from AR coatings to improve transmissivity due to their high reflective properties.
While AR coats can improve optical clarity, they are extremely porous, attracting
water and oils, making the lenses difficult to clean. Lenses with AR coatings should
be “sealed” with a smudge- and water-repellant coat that extends the useful life of the
AR coat and makes the lenses easier to keep clean. Coatings must be applied correctly,
and lenses must be meticulously cleaned for the process to be successful. Coated lenses
should be handled with care and not subjected to excessive heat to avoid delamination
or crazing.
TINTS. The choice of tints for sunglasses is practically infinite. The three most
common tints are gray, gray-green, and brown, any of which would be an excellent
choice for the aviator. Gray (neutral density filter) is recommended because it distorts
color the least. Some pilots, however, report that gray-green and brown tints enhance
vividness and minimize scattered (blue and violet) light, thus enhancing contrast
in hazy conditions. Yellow, amber, and orange (i.e., “Blue Blockers”) tints eliminate
short-wavelength light from reaching the wearer’s eyes and reportedly sharpen vision,
although no scientific studies support this claim.3 In addition, these tints are known to
distort colors, making it difficult to distinguish the color of navigation lights, signals,
or color-coded maps and instrument displays. For flying, sunglass lenses should screen
out only 70 - 85% of visible light and not appreciably distort
color. Tints that block more than 85% of visible light are not
recommended for flying due to the possibility of reduced visual
acuity, resulting in difficulty seeing instruments and written
material inside the cockpit.
POLARIZATION. Polarized lenses are not recommended
for use in the aviation environment. While useful for blocking
reflected light from horizontal surfaces such as water or
snow, polarization can reduce or eliminate the visibility of
instruments that incorporate anti-glare filters. Polarized lenses
may also interfere with visibility through an aircraft windscreen
by enhancing striations in laminated materials and mask the
sparkle of light that reflects off shiny surfaces such as another
aircraft’s wing or windscreen, which can reduce the time a pilot
has to react in a “see-and-avoid” traffic situation.
MATERIAL PROPERTIES CROWN GLASS CR-39 PLASTIC POLYCARBONATE
INDEX OF REFRACTION
Higher number = thinner lens 1.523 1.498 1.586
SPECIFIC GRAVITY
Higher number =heavier lens 2.5 1.32 1.20
DISPERSION
(Abbe value) Higher number =
fewer aberrations
59 58 31
STRENGTH Temperable Strong, SRC
required
Strongest, SRC applied
to lens blank
CHARACTERISTICS
Coatable, easily
fabricated, readily
available
Tintable, coatable,
easily
fabricated,
readily available
Coatable, special fabrication
equipment required, recommended
for children and athletes
Table 1. Properties of the three most common lens materials.
Figure 2. Illustration of non-uniform tints with
glass lenses for high hyperopic (left) and myopic
(right) corrections.
PHOTOCHROMIC. Glass photochromic lenses (PhotoGray® and PhotoBrown®),
like their plastic counterparts (Transitions®), automatically darken when exposed
to ultraviolet and become lighter in dim light. Most of the darkening takes place
in the first 60 seconds, while lightening may take several minutes. Although most
photochromic lenses can get as dark as regular sunglasses, i.e., 20% light transmittance
in direct sunlight, warm temperatures (>70°F) can seriously limit their ability to darken
and reduced ultraviolet exposure in a cockpit can further limit their effectiveness. In
　
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