A luminescence excited in a body by some form of electromagnetic radiation incident on the body. The term photoluminescence is generally limited to cases in which the incident radiation is in the ultraviolet, visible, or infrared regions of the electromagnetic spectrum; luminescences excited by x-rays or gamma rays are generally characterized by special names. The graph of luminous efficiency per unit energy of the exciting light absorbed versus the frequency of the exciting light is called the excitation spectrum. The excitation spectrum is determined by the absorption spectrum of the luminescent body, which it often closely resembles, and by the efficiency with which the absorbed energy is transformed into luminescence.