A variety of materials and compounds change color when exposed to light and revert to their original color when placed in the dark. This reversible color change phenomenon induced by light is called photochromism. Since many compounds exhibit this phenomenon over A spectral range wider than the visible region, a broader definition for photochromism has been established as a reversible change of a single chemical species between two energy states with distinct absorption spectra and such a reversible change is induced in at least one direction by the action of the electromagnetic radiation.
Nori Y. C. Chu,
"Photochromic plastics", Proc. SPIE 10304, Large-Area Chromogenics: Materials and Devices for Transmittance Control, 1030406 (28 March 1990); doi: 10.1117/12.2283609; https://doi.org/10.1117/12.2283609