Photochromic lenses are lenses that arken on exposure to UV radiation. Once the UV is removed (for example by walking indoors), the lenses will gradually 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 1960s. More recently, plastic versions of these lenses have been commercialized. The first of these was the Photolite lens sold in the early 1980s by American Optical Corporation. The first commercially successful plastic photochromic lens was introduced by Transitions Optical in 1991.