Photochromic lenses are lenses that darken on exposure to UV radiation. Once the UV is removed (for example by walking indoors), the lenses will return to their clear state. Photochromic lenses may be made of either glass or plastic. The glass version of this type of lenses was first developed by Corning in the 1960's. Plastic versions of these lenses have been available since the early 1990’s. 

The glass version of these lenses achieves their photochromic properties through the embedding of microcrystalline silver halide molecules in a glass substrate. Plastic photochromic lenses rely on organic photochromic molecules to achieve the reversible darkening effect. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight.

Automobile windows also block UV so the majority of these lenses (exception – DriveWear) will not activate very well in a car.