Light scattered from interface imperfections carries valuable information about its origins. For single surfaces, light-scattering techniques have become a powerful tool for the characterization of surface roughness. For thin-film coatings, however, solving the inverse scattering problem seemed to be impossible because of the large number of parameters involved. A simplified model is presented that introduces two parameters: Parameter δ describes optical thickness deviations from the perfect design, and param eter β describes the roughness evolution inside the coating according to a power law. The new method is used to investigate structural and alteration effects of HR coatings for 193nm , as well as laser-induced degradation effects in Rugate filters for 355nm .