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The word "holography" goes from Greek words "holos" - whole, entire and "grapho" - write. Holography is a method of recording volumetric images of objects using laser light. Hologram records not the image itself but the structure of the light wave reflected from the object (its amplitude and phase). Hologram is formed on the photoplate by two coherent light beams: the reference beam, going directly from the laser, and the signal (or object) beam, reflected by the object. When these beams meet on the photoplate they interfere, creating the interference pattern - very small alternate dark and light fringes and points.

On the exposed this way and developed photoplate there is no any image itself but the recorded interference pattern contains the encoded information about the object. When laser light goes through the system of fringes and points of interference pattern, reconstructs the image of the object because of diffraction. Looking through the hologram the viewer can change his point of view and see the volumetric image of the object from different sides.

Holography was invented (and named) by English physicist Dennis Gabor in 1947, while exploring the laws of imaging in optics and working on enhancement of electronic microscope. He came to a conclusion that full image of an object can be captured without objective lens, using just a beam of coherent monochromatic light. First holograms were recorded by him using mercury-vapor lamp, of a spectrum of which a very narrow range  was "cut off". Diameter of a beam was 1-2 microns and exposure time - some hours. Very small or translucent object was placed between source of light and photoplate, so the radiation of source of light acted like reference and object beam at the same time. That is why two objects located on one line were seen on a hologram during replay. Practical use of holography was impossible then, so it was forgotten for a long period of time.

After invention of a powerfull source of coherent light - laser, interest in holography re-ignited. In 1962 American opticians and radiophysicists Emmet Leith and Juris Upatnieks improved the scheme of Gabor, dividing reference and object beams -  now they met only on a photoplate. This allowed, first, to make holograms of non-translucent objects of complex shape, and second, to split the replayed images from each other. Scheme of Leith-Upatnieks became basic for modern holographic setups.

In the same time holographic methods of recording information interested Soviet physicist Yury Denisyuk. He invented principally new method of recording holograms in thick layer of photographic emulsion. In his work Denisyuk based on the method of obtaining colour photographic images, invented by french physicist Gabriel Lippmann in 1891. Light beam from the objective lens of Lippmann's camera reached photoplate with mercury, applied to it from opposite side (layer of mercury acted like a mirror). Reflected light waves interfered with incoming ones, creating standing waves in the thick of photographic emulsion. In the places where they occurred antinodes of blackening - reflective surfaces appeared, each of which reflected light only of specific color. The image was colorful but not volumetric. In Denisyuk scheme reference and object beams come to the plate from different sides and interfere. In the thick of emulsion layer in the places of interference maximums appear microscopic spots of blackening. Light, falling on the developed hologram, is reflected by them, and interfere, forming the replayed image of an object. In this case, hologram reflects only light the wavelenght of which is equal to the wavelength of a laser used for recording, all other wavelengths are suppressed. Volumetric hologram is viewed under ordinary white light, receiving monochrome image. Most of artistic holograms are Denisyuk holograms.

One of very wide-spread and famous types of holograms is rainbow holograms of Benton. Rainbow hologram is a sort of transmission hologram, recorded using the optic slit. This hologram is also viewed under ordinary white light. When the viewer moves, the image changes it's light, shimmering with all colors of rainbow. This gave this type of holograms its name - rainbow. This type of holograms is very popular because of its cheapness. One can see such holograms on audio- or video- disks, blanks of documents, goods etc.

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