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The huge power solid-state lasers require large optical materials with high quality. The inhomogeneity must be required to be measured. Inhomogeneity measurement is often done at normal incidence by interferometer, while the size of large blanks is limited to the interferometer aperture. A five-step method to measure refractive index inhomogeneity over the interferometer aperture is proposed in this paper. The variation of the refractive index inhomogeneity of the glass blank is directly calculated using five interferograms measured at oblique incidence. The high repeatability of the results is given. The reliability of the method is further verified by comparing the same part measured at normal incidence.