The effect of light scattering on standard optical measurements of thin films is presented. According to the Davies-Bennett theory, the scattering is introduced quantitatively in the calculation of the energy transmission and reflection coefficients. The relations of continuity are adapted to perform reflectivity simulation in the case of rough interfaces. A comparison with reflectivity spectra of PSi layer is made and shows that when the scattering is neglected, the absorption coefficient of the material is overestimated. Using the roughness amplitude dependence vs. the current density of formation, we perform a simulation of a double layer structure. It then becomes possible to characterize by optical analysis the quality of the interfaces in multilayers, which presently is under considerable investigations.