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A photochromic lens is an optical lens that darkens on exposure to light of sufficiently high frequency, most commonly ultraviolet (UV) radiation. In the absence of activating light, the lenses return to their clear state. Photochromic lenses may be made of polycarbonate, or another plastic. Glass lenses use visible light to darken. They are principally used in glasses that are dark in bright sunlight, but clear, or more rarely, lightly tinted 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 is available. Two kinds of photochromic lenses were popularized, the first being glass containing silver halides. These silver-based lenses became largely obsolete with the introduction of photochromic organic compounds. The other type are plastic, usually polycarbonate combined with photochromic organic compounds. These processes are reversible; once the lens is removed from strong sources of UV rays the photochromic compounds return to their transparent state.