Glossary
Photochromic lenses
Lenses that darken in sunlight and clear again indoors, reacting to ultraviolet light rather than being switched.
Photochromic lenses contain compounds that change structure when they absorb ultraviolet light, so the lens darkens outdoors and returns to clear indoors. The reaction is chemical and automatic; there is nothing to switch and no power involved. They are often referred to by the brand name Transitions.
Two things to know
The change is not instant in both directions: darkening takes seconds, clearing usually takes minutes. And because the trigger is ultraviolet light rather than brightness, most photochromic lenses barely darken behind a car windscreen, which filters UV out.