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%.
In
one sort of technology, 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. In another sort of technology, organic photochromic molecules, when
exposed to ultraviolet (UV) rays as in direct sunlight, undergo a chemical
process that causes them to change shape and absorb a significant percentage of
the visible light, i.e., they darken. These processes are reversible; once the
lens is removed from strong sources of UV rays the photochromic compounds
return to their transparent state.
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