The maximum achievable diffraction efficiency of organic photorefractive materials is strongly influenced by the properties of the nonlinear optical chromophores. Here we report on a novel evaluation method of frequency dependent ellipsometric measurements. We fit our theory to the phase shift between the alternating electric field and the detected signal as a function of the applied frequency. The presented method is based on an improved theoretical description of electro-optic relaxation spectroscopy when compared to earlier publications. It allows for the determination of the birefringence contribution C-BR to the refractive index change in materials with low glass transition temperature T-g. We apply our method to data obtained on highly efficient photorefractive guest-host systems based on polysiloxane. The nonlinear contributions were also determined and the results are briefly discussed. An error estimate for the value of C-BR yields an accuracy of about +/-10%. A comparison of electro-optic relaxation spectroscopy with photorefractive methods for determining the ratio C-BR/C-EO is presented. We conclude that this method allows for an estimate of the orientational enhancement with a relatively simple setup which is independent of the photorefractive experiment.