Encyclopedia … combined with a great Buyer's Guide!

VLib
Virtual
Library
Sponsorship opportunity: support this popular resource, which serves the whole photonics community, and get recognition!

Photochromic Materials

Definition: optical materials which exhibit increased light absorption when being exposed to light

Alternative term: self-darkening materials

German: photochrome Materialien, selbsttönende Gläser

Category: optical materials

How to cite the article; suggest additional literature

Author:

Photochromic materials are transparent materials which exhibit increased light absorption when they are exposed to light. They can be optical glasses or plastic materials. The photochromic properties usually result from the addition of a photochromic substance to a transparent material.

Typically, such 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.

Operation Principle

Photochromic glasses are often 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 UVA 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.

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.

See also: optical materials, optical glasses, optical attenuators, eye protection, photodarkening
and other articles in the category optical materials

How do you rate this article?

Click here to send us your feedback!

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent
Comments:

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you enter any personal data, this implies that you agree with storing it; we will use it only for the purpose of improving our website and possibly giving you a response; see also our declaration of data privacy.

If you like our website, you may also want to get our newsletters!

If you like this article, share it with your friends and colleagues, e.g. via social media:

arrow