The propagation of conversion fronts and their forms are experimentally revealed by 9,10-diphenyl anthracene photo-oxidation in butyl methacrylate/methacrylic acid copolymer films, as an example. In addition to the absolute bleaching regime the partially bleached one was analysed and investigated. The latter case was realized by the addition of a u.v. -absorber-5-chloro-2-(2’ -hydroxy-5’-methyl-3-tert-butylphenyl)benzotriazole. The method of allowing for light losses under routine photochemical experiments with non-rigorous parallel light beams was suggested. This method was used for a quantitative description of front propagation kinetics.