Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed. Therefore, when an imaging system, e.g. a camera, is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present.
Holography can be thought of somewhat similar to sound recording, where a sound field created by a vibrating tool, like musical instruments or vocal chords, is encoded in such a way that it can be reproduced (played) later without the presence of the original vibrating tool.
A basic requirement for holography is the coherence of the source because the recording process employs an interference phenomenon between the reference and scattered (diffracted) wave. For point-source holography, this coherence is classified by two properties:
i) The spatial coherence that is given by the size of the source,
ii) The temporal coherence that is the degree to which the wave can be approximated as monochromatic.