Sometimes changing from clear glasses to sunglasses can be a pain, if only there was a lens that changed from light to dark when you were in the sun. WAIT, THERE IS JUST SUCH A LENS. Photochromic lenses are the perfect answer for not having to change back and forth when walking in and out of buildings and offices. DR. Gary Heiting at All About Vision provides some great incite on how these lenses function.Photochromic lenses are eyeglass lenses that are clear (or nearly clear) indoors and darken automatically when exposed to sunlight. Other terms sometimes used for photochromic lenses include “light-adaptive lenses” and “variable tint lenses.”The most popular brand of photochromic lenses sold in the United States is Transitions Lenses (Transitions Optical). Because of this, some people — including some eye care practitioners— refer to photochromic lenses as “transitions lenses” or “transition lenses.” But there are other brands of photochromic lenses available as well.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.Photochromic lenses typically will not darken inside a vehicle because the windshield glass blocks most UV rays. Recent advancements in technology allow some photochromic lenses to activate with both UV and visible light, providing some darkening behind the windshield. Ask your optician for details.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.Adding anti-reflective coating to photochromic lenses enhances their performance even further. AR coating allows more light to pass through photochromic lenses for sharper vision in low-light conditions (such as driving at night), and eliminates bothersome reflections of sunlight and other light from the backside of the lenses in bright conditions.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.