A simple method for generating real time equal height fringes from two stereo transparencies is explained and demonstrated. The two transparencies are bleached converting them to phase transparencies. An image plane hologram is made of one of the transparencies, using a plane wave as the object wave. The second transparency is used to reconstruct the hologram. On those regions where the two transparencies are phase correlated the reconstructed wave is plane. A suitable filter is placed behind the hologram that allows only those regions of the reconstructed wave that are plane to pass. The light that is allowed to pass is used to illuminate an image plane giving a fringe of equal height.
V. P. Bennett,
"Detection of Image Coincidence in Stereo Pairs Using Phase Correlation," Optical Engineering 13(2), 132147 (1 April 1974). https://doi.org/10.1117/12.7971689