We present a critical study of the known reflectance (R) and transmittance (T) method to extract the optical constants of thin film-substrate systems. In order to study the effect of the usual assumptions (homogeneous layers, semi-infinite substrate, normal incidence angle for reflectance measurement, etc.) in the loss of solution during the numerical inversion process, we have developed a theoretical model that overcomes these approximations. Simulated film-substrate systems as well as RT measurements from a ZnSe film onto a CaF2 substrate have been used to show how accurate Rim thickness and optical constants can be obtained using a more complete optical model.