“…In the original work, the Gottingen group measured the absorption spectra of various alkali halides activated by small amounts of thallium (25,36,40), copper (23,135), silver (40,135), and lead (36). They studied the variation of the absorption with concentration of impurity (23,67,68) and found that, at least for low concentrations, the intensity of absorption is directly proportional to the concentration of the impurity.…”