The optical properties of a film-substrate system with a non-abrupt interface are studied solving Maxwell's equations. The resulting relations include the transverse and longitudinal response both local but spatially varying. They involve the differences between the dielectric functions of the film and the adjacent isotropic bulk medium as integrals. The equations are used for a discussion of the experimental methods: reflectivity, ellipsometry, absorptivity, reflectance difference spectroscopy and spectroscopic difference ellipsometry. As an actual example the limits for the thickness determination of ultrathin SiO2-films on Si from ellipsometric data are pointed out.