The simplest light type from the viewpoint of a mathematical description of photoreactions is the monochromatic and isosbestic irradiation (at λisos, see Sect. 2.9 ). We start here from that relatively scarce irradiation condition since, if they occur, isosbestic points exist only in limited numbers for any given reaction. Isosbesticity also means that there should be more than one reactive species absorbing the incident light.