Holography

A photograph records the real image of an object formed by a lens. A hologram, however, records the field distribution that results from the light scattered by an object. Since there is a one-to-one correspondence between the object and its scattered field, it is possible to record information about the object by mapping the scattered field. Actually the recording of the scattered field provides much more information about the object than that of the real image recorded in a photograph. For instance, one hologram can readily generate different real images that correspond to different viewing angles.

It would seem possible to record the scattered field by just placing a sheet of film near the scattering object, but unfortunately, phase information about the scattered field cannot be recorded in this way. Nevertheless this approach is used in certain cases, such as the determination of crystal structure from the scattered field of x-rays. The missing information about the phase has to be supplemented by chemical analysis. In holography, in order to record the phase information, a reference wave coming directly from the source to the film is superimposed on the scattered field coming from the object to the film.