Color centers are point defects or point defect clusters associated with trapped electrons or holes in normally transparent materials. These centers cause the solid to become colored when the electronic ground state of the defect is excited to higher energy states by the absorption of visible light [1, 2, 3, 4, 5]. [Note that transition metal and lanthanoid ion dopants that engender color in an otherwise colorless matrix are frequently called color centers. These are dealt with elsewhere (see “Cross-References”).]

The concept of color arising from point defects was initially developed in the first half of the twentieth century, principally by Pohl, in Germany. It was discovered that clear alkali halide crystals could be made intensely colored by diverse methods, including irradiation by X-rays, heating crystals in the vapor of any alkali metal, and electrolysis. Crystals with induced color were found to have a lower density than the crystals before treatment...