Emission of light from certain heated substances as a result of previous exposure to high-energy radiation .
The radiation causes displacement of crystal lattice of the substance. Upon heating, the trapped electrons return to their normal, lower-energy positions, releasing energy in the process. The longer the substance is exposed to radiation, the greater is the energy released. By measuring the amount of light given off, the duration of exposure to radiation can be determined; thus, thermoluminescence has been used to determine the age of various minerals and archaeological artifacts.