We investigated the potentials, applicability and advantages of spectroscopic ellipsometry (SE) for the characterization of high-end photomasks. The SE measurements were done in the ultraviolet-near infrared (UVNIR) wavelength range from 300 nm to 980 nm, at angle of incidences (AOI) between 10 and 70° and with a microspot size of 45 x 10 μm² (AOI=70°). The measured spectra were modeled using the rigorous coupled wave analysis (RCWA) to determine the structural parameters of a periodic array, i.e. the pitch and critical dimension (CD). Two different types of industrial photomasks consisting of line/space structures were evaluated, the reflecting extreme ultraviolet (EUV) and the transmitting opaque MoSi on glass (OMOG) mask. The spectra of both masks show characteristic differences, which were related to the Rayleigh singularities and the missing transmission diffraction in the EUV mask. In the second part of the paper, a simulation based sensitivity analysis of the Fourier coefficients is presented, which is used to define the required measurement precision to detect a CD deviation of 1%. This study was done for both mask types to investigate the influence of the stack transmission. It was found that sensitivities to CD variations are comparable for OMOG and EUV masks. For both masks, the highest sensitivities appear close to the Rayleigh singularities and significantly increase at very low AOI. To detect a 1% CD deviation for pitches below 150 nm a measurement precision in the order of 0.01 is required. This measurement precision can be realized with advanced optical hardware. It is concluded that UV-NIR ellipsometry is qualified to characterize photomasks down to the 13 nm technology node in 2020.