It is a misconception to define holography as 3D photography as it depends on hologram theory. Hologram is basically a resulting light field which has a varying intensity. It is a phenomenon which occurs when a part of the light scattered from an object or objects falls on a particular medium which records it. It results to an interference between the two beams which actually creates the light field. However, photographic film is one of the different recording materials available.
Construction of hologram and hologram theory
The construction of a simple hologram requires superimposing of two plane waves which has the same light source. It is due to interference which occurs due to the superimposition of one or more wavefronts. The phenomenon is used to construct simple holograms when the reference beam strikes the photographic plate normally along with the object beam, which hits the plate at a certain angle. As the two beams interfere to form a interference pattern it results to a change in the relative phase. The maxima and the minima of the fringe pattern is the relative phase of the object and the reference beam after it is encoded. Then comes the role of diffraction, which takes place whenever a wavefront comes in contact with an object. The fringe pattern acts as a diffraction grating and as the reference beam gets diffracted at a certain angle when it is incident upon the photographic plate along with the original beam, which is also incident. As the two waves interferes, it causes diffraction grating which reconstructs the object beam creating a hologram.
A complicated hologram can be made by using a point source of light when an object beam and a plane wave is used as a reference beam. It creates an interference pattern which is formed in curves of decreasing separation and increasing separation from the centre.
Hologram from complex object
An hologram of complex object can be made with the help of a beam splitter as it splits the laser beam. As a result the object is illuminated by one beam and the other illuminates the recording medium. Each point in the object acts a point source of light which interferes with the reference beam. The hologram gets illuminated even when the object is not there as diffraction grating diffracts the reference beam to reconstruct the wavefront. As each of the wavefronts add together to reconstruct the whole of the object beam, it creates a wavefront, same to the wavefront, which the object scattered, thus creating a virtual image. This is how transmission holograms work.