Photochromic materials are transparent materials which exhibit increased light absorption when they are exposed to light. They can be optical glasses or plastic materials, or sometimes delivered as a powder. The photochromic properties usually result from the addition of a photochromic substance to a transparent material.
Typically, photochromic materials are used in sunglasses which automatically adapt to the light conditions: they transmit most light if it is not too bright, but get increasingly dark when exposed to sunlight. The photodarkening can occur within less than a minute; the recovery to the clear state may take several minutes, depending on the temperature.
There are also photochromic windows, which are useful for automatically regulating the illumination with incoming daylight. As an alternative, one may use electrochromic windows, which are controlled with an electric voltage and can therefore be better adjusted to the needs, e.g. also taking into account whether the heating effect of the sunlight is the desirable or not.
Note that there are also various photodarkening effects in optical materials, e.g. some rare-earth-doped fibers, which may be reversible or irreversible. Photocromism, however, is by definition reversible, although there is often some amount of fatigue after extended use.
An older term for photocromism is phototropy.
Generally, photochromic materials exhibit some kind of photoisomerization effect, meaning that the chemical structure of the material can be reversibly transformed by the influence of absorbed light.
Specifically, many photochromic glasses are made by embedding silver chloride (AgCl) or another silver halide in microcrystalline form into some glass. When the silver chloride is exposed to ultraviolet light in the UV-A spectral region, it forms tiny silver particles which absorb light. That process is reversed by thermally activated processes when the exposure to ultraviolet light ends.
Photochromism can also result from other physical mechanisms, e.g. involving the formation of color centers or some structural changes of larger organic molecules such as spirooxazines, diarylethenes and azobenzenes.
Plastic optics can be made photochromic by adding certain organic dyes such as oxazines or naphthopyrans to a surface layer, applied as a photochromic coating. They also react to ultraviolet light.
For some materials, the wavelength dependence of the light-induced absorption leads to a color. For example, there are pigments which are white in the dark but change to a blue, yellow or red color under intense illumination.
Over longer times, the recovery to the clear state may not be complete due to aging processes (fatigue).
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