“Photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths.”
It's of particular significance to systems which use lasers to transmit information along optical fibres. The super-transparent fibres can gradually darken when exposed to light - particularly at short wavelengths. This can (eventually) compromise the efficiency of fibre-optic systems, and also high powered laser welding equipment.
The phenomenon has not yet been explained.
Further details International Society for Optics and Photonics