Holographic subtraction of two images was first realized by Gabor et al-1 by using a successive recording of the two images with a 180 degree phase shift between them and subsequent readout of the composite hologram. Other techniques for image subtraction are proposed2-5 . In Ref. 2 the ,rr phase shift between the image complex fields is ensured by spatially modulation of the images by periodic waves that are themselves 180 deg out of phase. Phase conjugation and Stokes' relation are also used3'4 . In Ref. 5 subtraction is fulfilled by the help of two orthogonal lineary polarized waves.
"Image Subtraction by Holographic Storage in Photorefractive BTO Crytals", Proc. SPIE 1183, Holography '89, (12 April 1990); doi: 10.1117/12.963819; https://doi.org/10.1117/12.963819