The study of the light-induced behavior of various metal compounds. The physical and chemical properties of substances are generally altered by the absorption of light. Typical metal compounds have a characteristic number (coordination number) of molecules or ions (ligands) directly bonded to the metal center. Many of these compounds are colored, and it is possible that some metal compounds could mediate the transformation of solar radiation into useful chemical or electrical energy, or that they might scatter light in such a way that its frequency is increased.