An analysis is given of Françon’s modification of the Savart plate at arbitrary angles to the optic axis. The factors limiting the useful field of view are examined, and it is shown that with the Françon modification, apart from improved fringe straightness, a considerable suppression of unwanted secondary fringe patterns is obtained. The use of large angles between the plate normal and the optic axis is shown to result in thick plates with required small shearing.
Th. H. Peek, "Generalization of Françon's Modification of the Savart Plate," Appl. Opt. 10, 2235-2239 (1971)