Theory and experiment show that for a hologram object of two or more object points, the nonlinearity of the photographic process causes reconstructed images in addition to both the desired reconstructed image and the higher order reconstructed images. It is theoretically and experimentally shown that for a plane object parallel to the hologram plane, some of these undesired images may be focused in the plane of the desired reconstructed image.
J. C. Wyant and M. P. Givens, "Undesired Light in a Reconstructed Hologram Image Caused by the Nonlinearity of the Photographic Process," Appl. Opt. 9, 810-814 (1970)