Abstract: A method and apparatus is disclosed for introducing alterations or distortions in projected images. The distortions on the projected image are imperceptible to a human viewer. The distortions are recorded on recording devices and observable upon viewing the recorded images.

Abstract: Single crystal electro-optic material useful for recording phase holograms comprises lithium niobate or lithium tantalate doped with iron and silicon.

Abstract: A solid cylindrical recording medium is axially rotatable within a cavity in a transparent block, the medium and the cavity being complementary in shape. The medium and the block have substantially equal indexes of refraction whereby the medium does not refract light transmitted thereto through the block. Transmitted to the medium (through the block) is an object beam, representative of an image, and a reference beam, whereby the medium is exposed and a volume hologram of the image is recorded. A multiplicity of volume holograms of images may be recorded by rotating the medium with respect to the reference beam after each of a multiplicity of exposures.

Abstract: Photochromic lithium niobate crystals containing iron and either manganese or copper are useful for recording phase holograms in the photosensitive state of the crystals, which holograms can be permanently fixed and read out without loss in the quality of the image obtained therefrom over extended periods.