The nmnerieal aperurc (NA3 of the double beam thterferometers (DBD can affect the acotrate measurement of surface profiles. Based on the double beam interference microscope imaging theory, the fringe spacing will vary wide nonlinearly with the increasing of numerical aperture. The double beam interferometers require oblique incidence illmnination, including Miehelson, Mirth and Linnik lypes. The intensity distribution of the ilumination across its slop is constant so the correction factor depends mainly on the NA in the Michelson and Linnik interferometers. The correction factor expression is derived in the Miehelson interferometer. The reference surface is a central obscnration in the beam, which will reduec the effective mnnerical aperture in the Miran interferometer. At the same mumerical aperatre, larger central obscuration has a much effect on the correction factor. The correction factor expression is determined by considering the influence of central obseuration and oblique incidence in the Mirau interferometer.