Photochromic lenses are
optical lenses that darken on exposure to specific types of light of sufficient intensity, most commonly
ultraviolet (UV) radiation. In the absence of activating light the lenses return to their clear state. Photochromic lenses may be made of
glass,
polycarbonate, or another
plastic. They are principally used in
eyeglasses that are dark in bright sunlight, but clear in low ambient light conditions. They darken significantly within about a minute of exposure to bright light, and take somewhat longer to clear. A range of clear and dark transmittances are available; one manufacturer makes one glass with transmittance reducing from 87% to 20%, and another reducing from 45% to 9%.