The eyes are especially susceptible to the sun because of the transparency of the outer tissues. Long-term exposure can speed up ageing of the macula, the most sensitive part of the retina and can also lead to cataracts.
Look
for sunglasses that meet the European safety standards. This means they will be
of good optical quality with break-resistant lenses, providing high levels of
protection against ultraviolet light, while not distorting colours. It's worth
buying a pair with a dark tint and with plastic rather than glass lenses, for
safety reasons. One of the key functions of sunglasses is to reduce the amount
of sunlight entering the eye. Look for a pair offering a light reduction of up
to 80%, that
is they will allow in only 20% of the sunlight. The size of the lens - the area
shielded - affects light admitted.
Expensive sometimes means better, but not necessarily. What really counts is
the degree to which the lenses filter out harmful UV rays. Look for the CE
mark, which proves they conform to the European Community Standard. They should
also satisfy British Standard BSEN1836, meaning they will provide high levels
of protection against damaging ultraviolet light.
Don't confuse the shade of the lenses with their ability to filter UV rays.
Dark sunglasses may still allow UV rays to enter the eye.
Polarising lenses will reduce reflective glare from water and land surfaces,
making them particularly good at improving vision in bright or hazy driving
conditions.
What about if you wear glasses either for reading or everyday?
Photochromic lenses were invented in the USA back in the 1960s but were very expensive when they first came to Britain and could be very slow to react.
The original lenses that darkened when exposed to UV rays were made from glass and although the technology has improved especially the speeding up of reaction time the science is still basically the same.
Glass photochrome lenses contain silver chloride crystals that when exposed to UV-A light ( 320 - 400nm) actually make dark elemental silver & when in the shade the silver reverts to an ionic state and the lenses go clear.
Plastic lenses are different because they use photochromic dyes which rearrange in UV to give darker colours. The dyes need to be in specialist plastics and each company has its own proprietary polymer. Some even sandwich the dye between the plastic. It is an area of lucrative research with a huge market.
The next generation of transitional lenses hope to deal with the fact that drivers can't really use them because many car windscreens actually have a UV blocking coating effectively stopping the sunglasses working.
And in the future?.........
The very latest technology announced recently uses an electrochromic polymer changing colour with electricity
Chemical engineer student Chunye Xu and colleagues from Seattle have been working on "smart" sunglasses that can lighten or darken on command.
Using power from a watch battery the polymer can change shade in one second with milliwatts of power when the button on the side arm is pressed.
Electrochromic polymers are not new but this application uses such a small amount of power and is very quick to respond.
As a prototype they are rather heavy but have real possibilities. The team are also working on glasses to correct sight defects.
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