Luminescence is the emission of light from nonconducting solids in addition to thermal radiation. It is caused by the stimulation of trapped electrons in crystal defects and/or impurities, and their subsequent recombination with emission of light. Ionizing radiation from natural radioactivity and cosmic rays produces free charge carriers, which are partly stored in the crystal lattice. Since the concentration of the charge carriers increases with time, their numbers and thus the intensity of the luminescence signal can be employed for dating.
The phenomenon of luminescence can be categorised according to the form of energy supply
during the stimulation process as:
Thermoluminescence (TL, stimulated by heat)
Optical stimulated luminescence (OSL,stimulated by visible light)
Infrared stimulated luminescence (IRSL,stimulated by infrared light)
Radioluminescence (RL, stimulated byionizing radiation)