Holographic interferometry is certainly one of the most useful practical applications of holography. This subject was initiated principally by Stetson and Powell with a double exposure technique. In this paper a new technique is exposed which needs only one exposure of the photographic emulsion. This uses two reference beams instead of one as is customary in holography. Depending on the location of the phase object, two configurations are possible, each having some distinct properties. When the object is placed in the central beam, we get, upon reconstruction, an interferogram having twice the sensitivity of one coming from a single-pass interferometer. On the other hand if the object is located in one of the outer beams, the reconstruction step would simultaneously yield an interferogram of the phase object in a first order of diffraction, and the original object wave in a second order. Experimental confirmations are given.
Manoranjan De and Léandre Sevigny, "Three-Beam Holographic Interferometry," Appl. Opt. 6, 1665-1671 (1967)