"Sunglasses are
a topic dear to the heart of aviators. There are
a number of factors which determine the
suitability or otherwise of sunglasses for use
in the aviation role.
Narrow frames that carry large lenses are the
most desirable from a field of view standpoint.
The most critical problem with frames arises
from the presence of wide sidearms which can
significantly impair the peripheral visual
field.
Lenses should not be too dark, and should
transmit at least 15% of incident light. The
tint used should be "neutral density" (N.D.),
that is, a greyish tint that does not distort
colour perception. Much has been written about
the perceived benefits of various coloured
lenses, especially in regard to contrast and
perceived brightness. Coloured lenses have been
shown to reduce the colour information received,
and to have a detrimental effect on both
perception and reaction time.
Lenses of polycarbonate material are preferred
because of their impact-resistance and ability
to absorb ultra-violet and infra-red rays.
However, these lenses can scratch easily. Harder
materials may be suitable, however these may be
more prone to shattering in an impact.
Different jurisdictions and countries have
standards to which sunglasses are manufactured.
Sunglasses should be chosen which conform to the
appropriate standard. For example, the current
Australian Standard ensures that sunglasses meet
acceptable standards for lens quality, frame
strength and lens retention and also ensures
that sunglasses provide adequate protection from
solar radiation.
Polarising sunglasses should not be used when
flying. The polarising filter interacts with the
cockpit transparency to produce a distorted and
degraded visual image. This effect can also be
seen with laminated car windscreens. Better keep
the polarised lenses for fishing.
Pilots who wear prescription spectacles can
choose from a number of options for glare
protection. Prescription sunglasses with N.D.15
lenses can be obtained, or N.D.15 clip-on or
flip-up sunglasses may be worn over prescription
spectacles.
Back to the original issue of photochromatic
lenses. Photochromatic lenses have several
disadvantages that render them unsuitable for
use by pilots.
Firstly, their transition times are relatively
slow. Photochromatic lenses take about five
minutes to increase their density to the level
of sunglasses, but more importantly, the
bleaching time from maximum to minimum density
can be as long as 30 minutes. Although there is
a rapid lightening of the lens in the first five
minutes, this may be too long when there is a
sudden variation in light during a descent into
or under cloud, or because of a rapid change in
cloud cover.
Their second disadvantage is that, even when
fully bleached, photochromic lenses still absorb
slightly more light than untinted lenses. Since
vision is critically dependent on ambient light
levels, even this small decrease of light
reaching the eye through photochromatic lenses
is undesirable, especially at night or in low
light levels. The inherent degradation of these
lenses with time causes them to progressively
become darker, as they become unable to reach
the fully bleached state. This effectively
prohibits their use in flying or controlling air
traffic.
