Photochromic lenses are eyeglass lenses that darken on exposure to light of sufficiently high frequency, most commonly ultraviolet radiation. They are also known as light-adaptive lenses, variable tint lenses, and light intelligent lenses. The lenses are clear indoors and darken automatically when exposed to the sun or other sources of UV light. They are made of polycarbonate or another plastic, while glass lenses use visible light to darken. The lenses are principally used in glasses that are dark in bright sunlight but clear or lightly tinted in low ambient light conditions.
Photochromic lenses were developed in the 1960s by William H. Armistead and Stanley Donald Stookey at the Corning Glass Works Inc. . The glass version of these lenses achieves their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride) in a glass substrate.
Photochromic lenses are ideal for people who need to wear glasses all the time. The two main purposes of photochromic lenses are to protect the eyes from UV light and to eliminate the need for a separate pair of prescription sunglasses. They provide the benefits of glasses and sunglasses in a single frame. The lenses darken significantly within about a minute of exposure to bright light and take somewhat longer to clear. A range of clear and dark transmittances is available.
