An analysis is given of Savart plates for arbitrary angles between the optic axis and the plate normal. Conoscopic interference patterns of thin Savart plates cut nearly parallel to the optic axis are shown and the use of such plates combined with diffraction gratings is discussed.
T. H. Peek, "Use of Savart Plates in Grating Interferometers," Appl. Opt. 10, 1092-1096 (1971)