phosphorescence (fŏsˌfərĕsˈəns) [key], luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescence in that it continues even after the radiation causing it has ceased. Phosphorescence was first observed in the 17th cent. but was not studied scientifically until the 19th cent. According to the theory first advanced by Philipp Lenard, energy is absorbed by a phosphorescent substance, causing some of the electrons of the crystal to be displaced. These electrons become trapped in potential troughs from which they are eventually freed by temperature-related energy fluctuations within the crystal. As they fall back to their original energy levels, they release their excess energy in the form of light. Impurities in the crystal can play an important role, some serving as activators or coactivators, others as sensitizers, and still others as inhibitors, of phosphorescence. Organo-phosphors are organic dyes that fluoresce in liquid solution and phosphoresce in solid solution or when adsorbed on gels. Their phosphorescence, however, is not temperature-related, as ordinary phosphorescence is, and some consider it instead to be a type of fluorescence that dies slowy.