The phenomenon that the Optical Power losses in a medium can grow when the medium is irradiated with Light at certain wavelengths.

Various Transparent optical media such as optical fibers, Laser crystals or Nonlinear Crystal Materials exhibit photodarkening when they are irradiated with light at certain wavelengths. This means that the Transmission losses resulting from Absorption or Scattering grow with time. The involved mechanisms and the characteristic parameters can vary greatly, depending on the material. In many cases, the photodarkening mechanism involves the formation of color centers or other microscopic structural transformations in the medium. Photodarkening can lead to serious performance degradations and Lifetime limitations of optical devices, and its minimization may require the optimization of materials and operation conditions.