Three-dimensional holograms

The story of the origin of holograms with recording in 3-D media is presented. It is pointed out that the capability of a volume photographic picture of an interference pattern of two waves to reconstruct one of the waves when being illuminated by the other one presents a general phenomenon on which holography is based. This phenomenon extends also to the case of a volume picture of travelling waves of intensity resulted from the interference of waves with different frequencies. An alternative method of holographic recording in 3-D media presents so-called selectograms. According to this method, the light scattered by an abject is split into two components by a diffractive grating that is positioned just in front of a thick-layered light-sensitive material. The interference pattern of these components is recorded in a 3-D light-sensitive medium. The reconstruction of the selectogram is performed by an extended diffuse source of light. An experiment on the recording of the reference-free selectogram in a thick-layered light-sensitive material reoxan is described. The advantages of the reference-free selectogram are low requirements as to the coherence of the recording and reconstructing light and its insensitivity to the vibration of the set-up during the recording.