Thermoluminescence is a phenomenon observed in some materials when they are exposed to ionizing radiation and subsequently heated, thereby releasing light energy, as a result of the transition of electrons trapped at or near defect centers from metastable states to the conduction band. Thereafter, the relaxed electrons recombine with hole vacancies in defect centers associated with an allowed energy gap between the conduction band and the defect center. The light emitted is of varying magnitudes at different temperatures as some deep trap metastable levels are sequentially filled, emptied and emptied again during the heating process. The sequence of detrapping and recombination topography is a function of trap depth and energy gap size. The presence of trap levels within the forbidden energy gap of insulators or semiconductors, are associated with defect centers which arise from impurity ions or vacancies, whether stoichiometric or non-stoichiometric. The TL response has been used to investigate the nature of defect centers and therefore defect center chemistry in a wide variety of materials including those used as insulators in semiconductor and electronic devices, luminescent materials, phosphors, dosimeters, catalysts, thermoelectric materials, pigments, high-temperature superconductors and phase change materials.

Chen, R., & McKeever, S. W. (1997). Theory of thermoluminescence and related phenomena. World Scientific.‏