This paper show that both direct and reverse quantum yields, the UV/visible spectra of unstable open forms and the quantum yields of photodegradation can be extracted by dynamic modelling and inverse treatment of the plots of Abs vs time under continuous irradiation at various selected wavelengths. The basic concepts underlying this dynamic approach are described here using experimental examples in liquid solution. This method was used for a quantitative study of the photochromic spirooxazines. Contrary to fast methods (flash-photolysis), kinetic studies under continuous irradiation are performed under similar conditions to those encountered in natural or industrial environments.