Reduced-phase dual-illumination interferometer (RPDII) is an off axis, single shot and single wavelength phase imaging technique to measure large objects without using unwrapping algorithms. Two beams of this interferometer illuminate a sample at different incident angles, two phases of the different incident angles and their phase difference are recorded. The phase difference between two beams can be controlled by adjusting the incident angles. The angle accuracy that decrease the RPDII accuracy have been studied. We have shown, the groove spacing of the grating and magnification of the lens system before sample, determine the incident angle accuracy. The ability of RPDII to unwrap large phases is shown by reconstructing phase of a step object without using unwrapping algorithms. The reconstructed image shows that the total inaccuracy is much more than the inaccuracy caused by incident angles.