The spectroscopic ellipsometry allows the determination of the dielectric function and the thickness of a thin-film. In particular, the knowledge about the wavelength-dependency of the dielectric function is a necessity for the optical simulation of thin-film systems. In the ellipsometry experiment, the sample is illuminated with monochromatic and polarized light under a fixed angle of incidence. The change of the polarization of the reflected light is determined. This quantity is used for the fit of an adapted optical model that provides the dielectric function of the layers involved in the sample.