holography (hŏlŏgˈrəfē, hō–) [key], method of reproducing a three-dimensional image of an object by means of light wave patterns recorded on a photographic plate or film. Holography is sometimes called lensless photography because no lenses are used to form the image. The plate or film with the recorded wave patterns is called a hologram. The light used to make a hologram must be coherent, i.e. of a single wavelength or frequency and with all the waves in phase. (A coherent beam of light can be produced by a laser.) Before reaching the object, the beam is split into two parts; one (the reference beam) is recorded directly on the photographic plate and the other is reflected from the object to be photographed and is then recorded. Since the two parts of the beam arriving at the photographic plate have traveled by different paths and are no longer necessarily coherent, they create an interference pattern, exposing the plate at points where they arrive in phase and leaving the plate unexposed where they arrive out of phase (nullifying each other). The pattern on the plate is a record of the waves as they are reflected from the object, recorded with the aid of the reference beam. When this hologram is later illuminated with coherent light of the same frequency as that used to form it, a three-dimensional image of the object is produced; it can even be photographed from various angles. This technique of image formation is known as wave front reconstruction. Dennis Gabors, a British scientist who in 1948 developed the wave theory of light (itself first suggested by Christopher Huygens in the late 17th cent.) can be viewed as the father of theoretical holography. However, no adequate source of coherent light was available until the invention of the laser in 1960. Holography using laser light was developed during the early 1960s and has had several applications. In research, holography has been combined with microscopy to extend studies of very small objects; it has also been used to study the instantaneous properties of large collections of atmospheric particles. In industry, holography has been applied to stress and vibrational analysis. Color holograms have been developed, formed using three separate exposures with laser beams of each of the primary colors (see color). Another new technique is acoustical holography, in which the object is irradiated with a coherent beam of ultrasonic waves (see sound; ultrasonics); the resulting interference pattern is recorded by means of microphones to form a hologram, and the photographic plate thus produced is viewed by means of laser light to give a visible three-dimensional image.
See G. W. Stroke, An Introduction to Coherent Optics and Holography (2d ed. 1969); T. Okoshi, Three-Dimensional Imaging Techniques (1976); N. Abramson, The Making and Evaluation of Holograms (1981); J. E. Kasper and S. A. Feller, The Complete Book of Holograms (1987).
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