A hologram is a recording of the optical interference
pattern that forms at the intersection of two coherent optical beams. Typically,
light from a single laser is split into two paths, the signal path and the reference
path.. The beam that propagates along the signal path carries information, whereas
the reference is designed to be simple to reproduce. A common reference beam is
a plane wave: a light beam that propagates without converging or diverging. The
two paths are overlapped on the holographic medium and the interference pattern
between the two beams is recorded. A key property of this interferometric recording
is that when it is illuminated by a readout beam, the signal beam is reproduced.
In effect, some of the light is diffracted from the readout beam to "reconstruct"
a weak copy of the signal beam. If the signal beam was created by reflecting light
off a 3D object, then the reconstructed hologram makes the 3D object appear behind
the holographic medium. When the hologram is recorded in a thin material, the
readout beam can differ from the reference beam used for recording and the scene
will still appear.
To make the hologram, the
reference and object beams are overlapped in a photosensitive medium, such as
a photopolymer or inorganic crystal. The resulting optical interference pattern
creates chemical and/or physical changes in the absorption, refractive index or
thickness of the storage media, preserving a replica of the illuminating interference
pattern. Since this pattern contains information about both the amplitude and
the phase of the two light beams, when the recording is illuminated by the readout
beam, some of the light is diffracted to "reconstruct" a weak copy of
the object beam .If the object beam originally came from a 3-D object, then the
reconstructed hologram makes the 3-D object reappear.