A related problem is that photochromic lenses don't always work effectively in cars, because ordinary glass windscreens naturally screen out most of the ultraviolet light. That means drivers really need a second pair of tinted or polarized sunglasses just for driving in. One final difficulty is that photochromic lenses don't last forever. After three years or so of continuous darkening and lightening, they become noticeably less reactive. This is less of a problem than it sounds, since many people change their eyeglasses at least this often. (Generally, you should get your eyes tested at least every two years and more often if you're older.) If, like me, you have a stable eye prescription and don't wear your eyeglasses too often, you might find it more of a nuisance: my photochromic lenses seem to have stopped reacting now roughly five years after I bought them.
