In this work, we used a high accuracy synchrotron-based reflectometer to experimentally determine the effects of angular bandwidth limitations on high-NA EUV mask performance. We collected the scatterometry data of both the mask blank and absorber field, as well as mask pattern diffraction performance as a function of illumination angle, scatter angle, and wavelength. Gratings down to 44 nm half pitch on mask, up to 16° angle of incidence (AOI), and wavelength ranging from 13.3 to 13.7 nm were considered. Rigorous Coupled-Wave Analysis (RCWA) was used to model scatterometry results which were compared with measurement. The experimental measurements on a mask with a multilayer reflectivity of over 60% at 13.5 nm wavelength that peaked about 10° showed that computing the large area reflectivity as a function of wavelength and incident angle from 0 to 14° resulted in root mean square errors in reflectivity unacceptably as high as 13% and 0.8% of the incident beam for the multilayer and absorber respectively. This dropped to 5% and 0.7% respectively when interdiffusion and wavelength-dependent refractive indices were taken into account. Calibration by fitting to the measured data reduced the errors to 0.8% and 0.08% respectively. Measurements of patterned gratings compared to a simple binary mask with 60% clear-field energy transmission for a 176 nm pitch grating showed less zero order than expected and an imbalance the two first orders where one exceeded the simple binary mask prediction. The levels of these orders degraded further for smaller pitches at 88 nm and especially at large incident angles of 14° where diffracted orders had large angles compared to the angular bandwidth of the multilayer. Fortunately, RCWA-based modeling of the patterned mask assuming the simplest case of vertical absorber walls predicted a similar trend in the diffraction to that observed from the measurement.