Method of reproducing a three-dimensional image of an object by means of Light-Wave patterns recorded on a photographic plate or film. The object is illuminated
with a coherent beam of light produced by a Laser. Before reaching the object, the beam is split into two parts: the reference
beam is recorded directly on the photographic plate, and the other is reflected
(see Reflection) from the object and then is recorded. On the photographic plate the two
beams create an Interference pattern, exposing the plate at points where they arrive in phase. When this
photographic recording, called a hologram, is later illuminated with coherent
light of the same Frequency as that used to form it, a three-dimensional image of the object becomes
visible, and the object can be photographed from various Angles. Color holograms are formed using three separate exposures with laser beams of each
of the primary colors. In acoustical holography, a coherent beam of Ultrasonics waves (see Sound), instead of light, is used. The resulting interference pattern is recorded
with Microphones to form a hologram, which when viewed with laser light produces a visible
three-dimensional image. Holography has been combined with microscopy to study
very small objects; it also is used in industry in stress and vibrational
analysis.