Spectroscopic ellipsometry is a sensitive and reliable diagnostic tool for the optical properties of thin films and multi-layer coatings. In this paper we have derived new formulas permitting qualitative and quantitative analysis of the surface microroughness in the case where the refractive index of the film is close to that of the substrate. Theoretical results are applied to the developing of experimental data for lanthanum fluoride and magnesium fluoride thin films.