Holograms

A hologram is a recording of an optical interference pattern between light waves. To generate a hologram, two coherent light beams — called the object and reference beams — are made to overlap in a photosensitive material such as a photopolymer or silver-halide emulsion. The object beam propagates from the object and thus carries information about it, while the reference beam is used to record the hologram (see “The basics of holography”).

Source: http://physicsworld.com/cws/article/print/32689/1/PWlow2_02-08