In this paper the two- grating interferometer is analysized by means of nonapproximation. It is shown that when the number and the contrast of fringes from the interferometer are not limited by the coherence of its illuminating light, it is also not influenced by the position in space of the light source and is still capable of yielding equal spacing straight fringe pattern. The general conditions are derived under which the temporal or/and spatial coherence of illumination has no influence on the fringe patterns. Further, the effect on the interferometer due to the mismatch of these conditions is discussed.