The basics of holography
Holograms are created by making two coherent light beams — called the object and reference
beams — overlap in a photosensitive material. The object beam propagates from an object and
thus carries information about that object, while the reference beam is used to both record and
read-out the hologram. The optical interference pattern is physically stored as a change in
absorption, refractive index or thickness of the recording material — thus turning it into a series of
interference fringes (i.e. a diffraction grating) that contain information about the amplitude and
phase of the two original light beams. By illuminating the grating with a read-out beam, a weak
copy of the original object beam — and thus the object — reappears.