Jackie Kennedy once said that only when she left hers off could she go
unrecognised. Jack Nicholson wore his to collect his Oscars, and Miles Davis
kept his on no matter how dark and smoky the jazz club.
If you are in any doubt about the power of sunglasses, just look at the
movies. From Lolita (1962), where the eponymous teenage nymphet
peeps over a heart-shaped pair, to The Big Chill (1983), in which a
laid-back William Hurt is defined by his Porsche and shades, via the Blues
Brothers and Men in Black right up to the current hit The
Matrix, dark glasses loom large.
They seem to appeal to something deep in human psychology. Supposedly, the
first dark glasses were worn by judges in imperial China to prevent the accused
from reading their eyes before guilt or innocence had been proclaimed. They
still have the power to hide thoughts and emotions, and even to transform
personalities. People revel in the cool remoteness that comes from covering the
eyes.
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But sunglasses are more than mere aids to vanity鈥攖hey are there to
reduce glare too. Until the 19th century, for example, the Inuit wore mask-like
contraptions of wood and walrus skin with a narrow slit to see through, to
protect against snow blindness. And nowadays we expect sunglasses to block the
Sun鈥檚 ultraviolet light, which causes cancer and cataracts of the eyes.
Behind the glamour of sunglasses lurks a competitive, high-tech business
worth nearly $3 billion a year in the US alone. But the technological
demands of moulding glass, plastic and metal into effective optical devices have
influenced style: the still-popular aviator style once worn by General
MacArthur, for instance, is a classic case of form following function. In the
1920s, when aircraft became powerful enough to climb above the clouds into
strong sunlight, pilots needed sunglasses darker than any available. The
American government asked the Bausch & Lomb optical company to help. B&L
developed a greenish glass that transmitted only 15 per cent of visible light.
They formed it into large, teardrop-shaped lenses that covered the whole visual
field and were mounted in thin wire frames to save weight. The result, the
Ray-Ban Aviator model, became a stylish civilian accessory鈥攎uch as leather
bomber jackets did鈥攁nd started a trend towards shades dark enough to hide
the eyes.
The traditional lens material, glass, now has to compete against a young
upstart, plastic. Plastic better withstands the rigours of popular sports such
as snowboarding and mountain biking. One maker of plastic sunglasses, the Oakley
company in California, likes to show how a high-speed steel pellet barely dents
a polycarbonate plastic lens, whereas it turns a glass lens into a shower of
jagged shards. But plastic is difficult to shape with precision and easy to
scratch, and it can turn cloudy after long exposure to sunlight, so there are
doubts about its durability. Some in the industry say 鈥減lastics come and go, but
glass is forever鈥�.
Both glass and plastic lenses can be polarised to remove the glare of
reflected light. Sunlight reflected off a horizontal surface, such as the road
in front of your car as you drive, is more or less horizontally polarised,
meaning that the light waves vibrate in a horizontal plane. Vertically
polarising sunglasses block that reflected light, just as a comb held vertically
blocks the passage of a second comb held horizontally.
But in an example of duelling technologies, polarised shades are a bad choice
if you use electronic devices with those familiar black-on-grey liquid-crystal
displays. These also polarise light, so they can suddenly turn black when seen
at certain angles through polarised glasses 鈥攚hich could be disconcerting,
to say the least, for an airline pilot on final landing approach.
Sharp shooters
Modern technology can even improve vision by eliminating the effects of
scattered light. As the British physicist John Tyndall discovered, and Lord
Rayleigh explained theoretically in 1871, light is scattered or deflected by the
molecules of the air. The shorter the light鈥檚 wavelength, the more it is
scattered. That鈥檚 why the sky is blue: blue light is scattered in all directions
as sunlight travels through the atmosphere.
Scattering also blurs the outlines of distant objects, bathing far-away
mountains in a blue haze. Eliminate the blue, diffused light rays and you get a
clearer image, as target shooters know. They wear glasses tinted yellow to block
blue light and define the target more sharply.
Most people do not want to live in a yellow world, but fortunately there are
subtler ways to remove these undesired wavelengths. Revo, a company based near
Silicon Valley in California, uses technology that NASA developed during the
1980s for coatings on cameras and telescopes to be used in space. Although the
details are proprietary, they admit that the method is based on destructive
interference: when two light waves of the same wavelength are superimposed so
that crest matches trough and trough matches crest, the waves add up to zero,
eliminating the light.
Revo deposits extremely thin layers of transparent material on its lenses.
Light is reflected back and forth within each layer, and these trapped waves
interfere, cancelling each other at a particular wavelength that depends on the
film thickness. No light of that wavelength flows through the back of the layer,
and so none of that colour reaches the eyes.
This technique gives extraordinary command over colour, like reaching inside
a rich musical chord and neatly excising a single note. As a result, Revo
sunglasses can be tuned to deliver something very close to natural colour, while
improving the clarity of distant objects and heightening contrast within both
sunny and shady scenes. But this combination of qualities does not come cheap:
Revo shades cost $150 or more.
Sunglasses with this level of technology are much like sports cars: their
sleek exterior hides a great deal of serious, high-performance machinery. But
you can get behind the lenses of a pair of gleaming high-tech sunglasses for
much less than it costs to get behind the wheel of a gleaming high-tech sports
car鈥攁nd, once there, you鈥檙e perfectly placed to project your dreams onto
the dark, dark lenses of your shades.