The depth of fringes is obtained for a polychromatic point source at an arbitrary incidence angle. The path difference, calculated by ray tracing, at the plane where the interference fringes vanish indicates that the temporal coherence length is the same as that in other interferometers. It is also found that the equivalent path difference, introduced by relative phase modulation on two interfering rays by two gratings, is as important as the actual path-length difference but in the opposite sign. (C) 1997 Optical Society of America
