luminescence In all these processes the atoms of a substance are excited to higher energy levels by absorbing energy from an external source, which de excite and release electromagnetic radiation.Fluorescence and phosphorescence are differerent forms of luminescence. In fluorescence the emission of light occurs within very short time after the excitation of atom (10^-8s) while in phosphorescence the emission may persist for a long time up to several hours. According to the type of energy supplied, we distinguish photoluminescence, where energy is supplied in the form of light, electroluminescence where electrical energy is supplied, chemoluminescence or bioluminescence where energy comes from chemical reactions, and triboluminescence where mechanical energy is converted.