Atomic and electronic defects of various types in solids which produce optical absorption bands in otherwise transparent crystals such as the alkali halides, alkaline earth fluorides, or metal oxides. They are general phenomena found in a wide range of materials. Color centers are produced by gamma radiation or x-radiation, by addition of impurities or excess constituents, and sometimes through electrolysis. A well-known example is that of the F-center in alkali halides such as sodium chloride (NaCl). The designation F-center comes from the German word Farbe, which means color. F-centers in sodium chloride produce a band of optical absorption toward the blue end of the visible spectrum; thus the colored crystal appears yellow under transmitted light. On the other hand, potassium chloride (KCl) with F-centers appears magenta, and potassium bromide (KBr) appears blue.