A photochromic glass lens automatically darkens in bright light and becomes lighter in
low light. Most of the darkening takes place in about half a minute, while the lightening
takes about five minutes. They come in a uniform or gradient tint. Although photochromic
lenses may be good UV-absorbent sunglasses it takes time for them to adjust to different
light conditions.
"Ground and Polished"
Some non-prescription glasses are ground and polished when they are manufactured to
improve the quality of the lenses. Non-prescription lenses that are not ground and
polished will not hurt your eyes. You do want to make sure that the lenses you buy are
made properly. To judge the quality of non-prescription sunglasses, look at something
with a rectangular pattern such as floor tile. Hold the glasses at a comfortable distance and
cover one eye. Move the glasses slowly from side to side, then up and down. If the lines
stay straight, the lenses are fine. If the lines wiggle, especially in the center of the lens, try
another pair.
"Impact Resistant"
All sunglasses must meet impact standards set by the Federal Food and Drug
Administration. No lens is truly unbreakable, but plastic lenses are less likely than glass
lenses to shatter when hit by a ball or stone. Most non-prescription sunglass lenses are
plastic. Polycarbonate lenses are a must for your sunglasses if you participate in
potentially eye hazardous work or sports. These lenses provide the most impact
resistance. Polycarbonate plastic, though tough and used in many sports sunglasses,
scratches easily. If you buy polycarbonate lenses, look for ones with scratch-resistant
coatings.