Sunglasses that are inherently cool. But unlike other things considered cool–body piercing, tattoos and platform shoes, for instance–wearing sunglasses not only makes good fashion sense, it makes good scientific sense.

That’s because good sunglasses protect against long-term eye damage caused by the ultraviolet radiation contained in sunlight, the same ultraviolet radiation that causes sunburn and skin cancer.

Ultraviolet radiation is made up of the high-energy rays whose wavelengths are just beyond the violet end of the visible spectrum. Since we can’t see ultraviolet radiation, blocking those rays doesn’t affect our vision.

The ultraviolet radiation that reaches the ground is divided into two main types, UV-B and UV-A. UV-B is more damaging because it has a shorter wavelength and therefore more energy. That shorter wavelength means it is absorbed by the cornea and lens of the eyes. UV-A has lower energy, but because it has a longer wavelength can penetrate deeper into the eye. Sunlight contains much more UV-A than UV-B.

Studies indicate that ultraviolet radiation can contribute to cataracts, pterygium (a growth of tissue on the white of the eye), cancer of the skin around the eye, photokeratitis (“snowblindness,” essentially a very painful sunburn of the cornea), and long-term degeneration of the eye.

All of which makes wearing sunglasses a great idea–provided they block 97 to 100 percent of UV radiation (information which should be printed on the tag).

That has nothing to do with how dark the lenses are. In fact, dark, poor-quality sunglasses may actually be more harmful than wearing no sunglasses at all–because dark glasses cause your pupils to open wider, allowing more ultraviolet radiation to penetrate deeper into your eyes. On the other hand, even completely non-tinted lenses in ordinary glasses can be made UV-proof. The UV protection is provided either by chemicals added to the lens during manufacture or by a special coating.

Expensive glasses don’t necessarily block UV better than cheap ones, either. In one test, a $2 pair of plastic children’s glasses blocked UV just as well as expensive brand-name glasses do.

However, more expensive glasses also offer a number of other benefits, including optically clear lenses free of distortion. One of the latest technological advances showing up on high-end sunglasses is a synthetic version of the pigment melanin, the stuff that turns our skin dark when we tan–our built-in method of blocking UV. Applied to sunglasses, this blocks not only UV, but also the high-energy visible part of the spectrum, the blue-violet light that’s just below UV in wavelength.

This blue-violet portion of the spectrum may also contribute to long-term eye and skin damage, so blocking it is a good idea. As well, blocking the blue end of the spectrum improves vision, because the short blue-light wavelengths focus slightly in front of the retina–which means they don’t form a sharp image. This is called “blue blur,” and getting rid of it is the idea behind those “blue blocker” sunglasses you may have seen advertised.

On the other hand, blocking the blue wavelengths can affect color perception to the point that it’s hard to recognize traffic signals–which brings up another point about sunglasses, that different types of glasses are best suited to different activities.

Yellow tints amplify the view and clarify a hazy landscape by absorbing blue light. These are good for outdoor sports like skiing, especially on hazy or overcast days.

Rose lenses also cut the haze; pinkish tinting can also improve the visibility of some computer screens and reduce glare from indoor fluorescent lights.

Brown and amber sunglass tints intensify depth perception and contrast, making whites lighter and darks darker. They’re also soothing to the eyes, which makes them good for drivers and pilots.

Gray lenses have the least effect on colors, which makes them valuable for photographers and artists.

Green tints cut bright light without dimming your vision. Green lenses with a brownish hue are especially good for golfers because they help highlight a golf ball against the blue sky and green fairway.

Polarized lenses eliminate more than 95 percent of horizontal glare, enhance colors and accentuate contrast and sharpness. They’re especially good for water sports and high-glare driving.

In the end, the kind of sunglasses you buy will depend as much on your fashion sense as on anything else–but forearmed with a little scientific knowledge, you can do your best to ensure that those glasses that make you look cool are also protecting and enhancing your vision–and that’s really cool.

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