Holography records the wave interference between a holographic object wave and a known holographic reference wave, then uses that record to reconstruct a wavefront. The record is a hologram. A photographic plate or photodetector measures light intensity; the relative wave phase is encoded in the positions and contrast of interference fringes, rather than measured directly as an optical oscillation. Reillumination makes the recorded pattern act as a diffraction grating that produces an image-bearing field.
The useful change of viewpoint is to preserve a field that can propagate again, instead of only a focused brightness picture. Recovered complex amplitude permits refocusing with the Fresnel propagator; a spatially extended hologram can also supply different viewing directions. The recoverable information is limited by the collected aperture, recorded spatial frequencies, optical coherence and separation of unwanted diffraction orders.
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