We investigated so-called cross polarization (or polarization conversion) effects to extend the capabilities of conventional spectral-ellipsometry based scatterometry. Therefore, we developed a procedure how to adapt an automated rotating polarizer ellipsometer in order to determine the ratio between diagonal and off-diagonal elements of the Jones matrices for suitable samples. This so called generalized ellipsometry leads to six spectra instead of two, which contain more extractable information about the geometry of the sample under investigation, i.e. the diffraction grating. We created spectral libraries by using the rigorous coupled wave analysis (RCWA) method, which contain elements of the Jones matrix for different grating geometries. These libraries allow an analysis of measured spectra, as well as sensitivity studies and optimization for different analyzer angles and grating orientations with respect to the plane of the incoming light beam (conical diffraction). Due to sidewall effects which might cause polarization conversion, we demonstrate the advantage when employing conical diffraction, especially for measurements of asymmetric and 2D periodic structures.