Photochromic sunglasses -- also known as Transitional or Light Adjusting -- is one of the fastest growing segments of eyewear on the market today. And since driving an automobile is one of the most common applications of sunglass wearing, it's a good idea to be aware of this fact.
Many windshields are manufactured with ultraviolet filters in them, and do a fairly decent job of blocking UV rays from a driver's eyes. Before investing in a pair of photochromic glasses for the purpose of driving, know your windshield. The eyewear's photochromic lenses only work with ultraviolet exposure. If most of the UV rays are blocked by your windshield, your glasses will not function like you expect.
Rumor has it that the Photochromic industry is working on or has found a solution to overcome the above problem. If or when we hear more about lenses that darken even when their UV trigger is blocked, we'll post the information.