Sunglasses can create an allure. They are a way of hiding and of getting attention at the same time. They are a fashion accessory with a purpose -- eye protection -- and that takes an edge off the embarrassment that sometimes comes with vanity. It feels good just putting them on.

For a few dollars, there are trendy styles sold on the street, but they may be little more than props for your face. Often made of cheap plastic and distorted colored glass, they do not adequately provide the bare essentials of eyewear -- comfortable vision in bright sunshine and protection from ultraviolet (UV) light.

In the best pair of sunglasses, nothing is more important than what meets the eye ... the lens. It's not the style or shape of the glasses that counts, only the UV-absorbing ability of the glass or plastic lens. For once in life, there is no conflict between being sensible and looking cool.

Damage to the eyes can occur slowly, by small amounts of sun over a long period of time, according to Donald Pitts, professor of environmental optometry and visual science at the University of Houston. Or, damage can be caused by brief exposure to high levels of UV radiation -- like an hour in a tanning booth, eyes open, reading a newspaper.

Although the research is not conclusive, the correlation between UV light and damage to the eye -- linked to cataracts, dry-eye syndrome, night blindness -- is stronger than the correlation between smoking and lung cancer, according to Pitts.

"UV should be kept completely from the eye," says Pitts, who designed sun visors for NASA to protect astronauts against solar and infrared radiation. He complains that the standards for sunglasses are not high enough.

Here are a few facts to help you decide if the shades you wear or those you buy are sunworthy:

While most sunglasses block some UV radiation, the amount varies. Glasses purchased off the street should not be expected to screen out UV, according to Pamela Crupi, certified optician and eyewear consultant to O'Goggle's, a sunglasses store in Georgetown. "Most people expect to lose them. They aren't for sun protection," says Crupi, who adds that glasses off the street rarely can be upgraded. Because the frames are not made of "zyl," a special grade of plastic, cheap sunglasses cannot be heated and the lenses cannot be replaced.

Darker is not necessarily better. Some of the darkest sunglasses could be the most harmful. Extremely dark lenses, without UV protection, can dilate the pupil and allow larger amounts of radiation to reach the eye. The American Optometric Association (AOA) suggests as a general rule that sunglasses be dark enough so that the eyes cannot be seen clearly through the lenses. At the same time, though, the lens should screen out 75 to 90 percent of available light.

Sunglass lenses come in rainbow colors these days. The AOA recommends medium to dark gray lenses because they do not alter color perception. Brown or green lenses are still popular, but do distort color. This could be a problem for men in particular, 10 percent of whom, warns Pitts, have abnormal color perception problems as it is.

Plastic or glass lenses? Plastic lenses are lighter weight and slightly more impact-resistant. The AOA recommends industrial-strength high-impact lenses for people who play eye-hazardous sports, such as tennis, baseball and softball.

The lenses on nonprescription sunglasses can be checked to make sure they are optically perfect. Hold the glasses out and look through the lens, fixing on an object across the room. Then move the glasses around a little. There should be no wavy lines or distortions, no waves or wiggles.

The fit of the sunglasses is important. "The glass should fit pretty close to your face, or close to the orbit of the eye and not extend too wide out to the temple, then more light comes in from the top," says Pitts. "Your sunwear should fit as well as prescription lenses." The AOA does not recommend sun lenses that clip on prescription glasses because they can scratch your regular glasses.

Maximum Protection Who needs maximum UV protection? Women taking oral contraceptives and people taking acne medications containing tetracyclines, which increase sensitivity to the sun, need maximum protection, according to the AOA. There are 97 kinds of drugs -- many tranquilizers and diuretics -- which cause photosensitivity, according to Pitts. "I don't think we know all of them yet," he says. "The prudent thing is to assume that the drugs you take might make you photosensitive."

People using sunlamps -- or sun beds -- need maximum UV protection, just as anyone who has had cataract surgery and has not had a "UV absorbing interocular lens" put in, needs to be particularly careful.

Contact lens wearers are more "photophobic," more bothered by the sun, than other people, although it is not known why. In a control study with animals, those wearing normal soft contact lenses showed more damage from UV light than the animals without contacts. Pitts recommends contact lens users wear sunglasses when outdoors. A UV-absorbing soft contact lens is waiting FDA approval.

Many sunglasses these days are sold with tags that tell how UV-absorbing they are. Those absorbing from 290 to 400 nm (nanometers) are considered to provide maximum protection.

Special Types of Lenses Polarizing lenses are good for people bothered by glare caused by reflected light. That is why polarizing lenses are also called "anti-glare" lenses.

Photosensitive lenses -- the ones that go from clear to dark on their own -- became popular in the '70s, but have a few problems. The lenses do not return to clear quickly enough, some complain. This also creates problems for older wearers, who typically need more light in order to see well.

Upgrading Your Glasses Many opticians offer protective coatings for existing glasses. If you are unsure of the UV-absorbing ability of your sunglass lenses, some opticians have a machine that can give an instant, although crude, estimate of a lens' UV absorption.

An ultraviolet coating, a dye that is absorbed into the lens, can reduce the transmission of UV light. There are some UV coatings that can be used on regular clear lenses and can be done while you wait. An anti-reflective coating can be put on the back of the glasses so there are no ghost images. There is a "ski coat" that helps skiers see through blinding, snow-reflected light and a scratch-resistant coating that is frequently added to plastic lenses, or mixed with other coatings to give them longer life.

Sunglasses, everybody loves them. They hide your age, they make you look good -- teen-agers can look older, the older can look younger, the dull can look interesting. But allure aside, one should know what to expect from a pair of sunglasses, what they should do for your eyes other than darken the view.

There now, stop squinting.