Photochromic lenses are eyeglass lenses that darken automatically when exposed to sunlight, then fade back when you return indoors. In most cases, photochromic lenses are clear (or nearly clear) indoors and darken to a medium sun tint outdoors. But there are exceptions.
Because the most popular photochromic lenses sold in the United States are made by Transitions Optical, many people — including some eye care practitioners — mistakenly call all photochromic lenses "transitions lenses" or "transition lenses." But there are many brands of photochromic lenses offered by different lens manufacturers (see below).
Other generic terms sometimes used for photochromic lenses include "light-adaptive lenses" and "variable tint lenses."
The molecules responsible for causing photochromic lenses to darken are activated by the sun's ultraviolet radiation. Because UV rays penetrate clouds, photochromic lenses will darken on overcast days as well as sunny days. But because the windshield and window glass of cars and trucks blocks most UV rays, photochromic lenses typically won't get as dark inside a vehicle as they do outdoors.
Photochromic lenses are available in nearly all lens materials and designs, including high-index lenses, bifocals and progressive lenses. An added benefit of photochromic lenses is that they shield your eyes from 100 percent of the sun's harmful UVA and UVB rays.
Because a person's lifetime exposure to sunlight and UV radiation has been associated with cataracts later in life, it's a good idea to consider photochromic lenses for children's eyewear as well as for eyeglasses for adults. Polycarbonate is the safest lens material for kids, providing up to 10 times the impact resistance of other lens materials.
Though photochromic lenses cost more than clear eyeglass lenses, they offer the convenience of reducing the need to carry a pair of prescription sunglasses with you everywhere you go.