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Photochromic lens
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 sunglasses 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%.
Molecules of silver chloride or another silver halide are embedded in photochromatic lenses. They are transparent to visible light without significant ultraviolet component, which is normal for artificial lighting. When exposed to ultraviolet (UV) rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape and absorb a significant percentage of the visible light, i.e., they darken. This process is reversible; once the lens is removed from strong sources of UV rays the silver compounds return to their transparent state.
==Invention==
Photochromic lenses were developed by Roger Araujo at the Corning Glass Works Inc. in the 1960s, and the process was used in the first mass-produced variable tint lenses.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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