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Holography is the science and practice of making holograms. Typically, a hologram is a photographic recording of a light field, rather than of an image formed by a lens, and it is used to display a fully three-dimensional image of the holographed subject, which is seen without the aid of special glasses or other intermediate optics. The hologram itself is not an image and it is usually unintelligible when viewed under diffuse ambient light. It is an encoding of the light field as an interference pattern of seemingly random variations in the opacity, density, or surface profile of the photographic medium. When suitably lit, the interference pattern diffracts the light into a reproduction of the original light field and the objects that were in it appear to still be there, exhibiting visual depth cues such as parallax and perspective that change realistically with any change in the relative position of the observer. In its pure form, holography requires the use of laser light for illuminating the subject and for viewing the finished hologram. In a side-by-side comparison under optimal conditions, a holographic image is visually indistinguishable from the actual subject, if the hologram and the subject are lit just as they were at the time of recording. A microscopic level of detail throughout the recorded volume of space can be reproduced. In common practice, however, major image quality compromises are made to eliminate the need for laser illumination when viewing the hologram, and sometimes, to the extent possible, also when making it. Holographic portraiture often resorts to a non-holographic intermediate imaging procedure, to avoid the hazardous high-powered pulsed lasers otherwise needed to optically "freeze" living subjects as perfectly as the extremely motion-intolerant holographic recording process requires. Holograms can now also be entirely computer-generated and show objects or scenes that never existed. Holography should not be confused with lenticular and other earlier autostereoscopic 3D display technologies, which can produce superficially similar results but are based on conventional lens imaging. Stage illusions such as Pepper's Ghost and other unusual, baffling, or seemingly magical images are also often incorrectly called holograms. ==Overview and history== The Hungarian-British physicist Dennis Gabor (in Hungarian: ''Gábor Dénes''), was awarded the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method".〔(【引用サイトリンク】title=The Nobel Prize in Physics 1971 )〕 His work, done in the late 1940s, was built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and William Lawrence Bragg in 1939.〔Hariharan, (1996), Section 1.2, p4-5〕 The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston (BTH) Company in Rugby, England, and the company filed a patent in December 1947 (patent GB685286). The technique as originally invented is still used in electron microscopy, where it is known as electron holography, but optical holography did not really advance until the development of the laser in 1960. The word ''holography'' comes from the Greek words (''holos''; "whole") and (''graphē''; "writing" or "drawing"). The development of the laser enabled the first practical optical holograms that recorded 3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and Juris Upatnieks at the University of Michigan, USA. Early holograms used silver halide photographic emulsions as the recording medium. They were not very efficient as the produced grating absorbed much of the incident light. Various methods of converting the variation in transmission to a variation in refractive index (known as "bleaching") were developed which enabled much more efficient holograms to be produced.〔Upatniek J & Leaonard C., (1969), "Diffraction efficiency of bleached photographically recorded intereference patterns", Applied Optics, 8, p85-89〕〔Graube A, (1974), "Advances in bleaching methods for photographically recorded holograms", Applied Optics, 13, p2942-6〕〔N. J. Phillips and D. Porter, (1976), "An advance in the processing of holograms," Journal of Physics E: Scientific Instruments p. 631〕 Several types of holograms can be made. Transmission holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser light through them and looking at the reconstructed image from the side of the hologram opposite the source.〔Hariharan, (2002), Section 7.1, p 60〕 A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers.〔Benton S.A, (1977), "White light transmission/reflection holography" in Applications of Holography and Optical Data Processing, ed. E. Marom et al, ps 401-9, Pregamon Press, Oxford〕 Rainbow holograms are commonly used for security and authentication, for example, on credit cards and product packaging.〔Toal Vincent (2012), "Introduction to Holography", CRC Press, ISBN 978-1-4398-1868-8〕 Another kind of common hologram, the reflection or Denisyuk hologram, can also be viewed using a white-light illumination source on the same side of the hologram as the viewer and is the type of hologram normally seen in holographic displays. They are also capable of multicolour-image reproduction.〔Hariharan, (2002), Section 7.2, p61〕 Specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface.〔(【引用サイトリンク】title=specular holography: how )〕 It works by reflectively or refractively manipulating bundles of light rays, whereas Gabor-style holography works by diffractively reconstructing wavefronts. Most holograms produced are of static objects but systems for displaying changing scenes on a holographic volumetric display are now being developed.〔(【引用サイトリンク】title=MIT unveils holographic TV system )〕〔See Zebra imaging.〕 Holograms can also be used to store, retrieve, and process information optically.〔Hariharan, (2002), 12.6, p107〕 In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor or diode lasers, such as those found in millions of DVD recorders and used in other common applications, can be used to make holograms and have made holography much more accessible to low-budget researchers, artists and dedicated hobbyists. It was thought that it would be possible to use X-rays to make holograms of very small objects and view them using visible light. Today, holograms with x-rays are generated by using synchrotrons or x-ray free-electron lasers as radiation sources and pixelated detectors such as CCDs as recording medium. The reconstruction is then retrieved via computation. Due to the shorter wavelength of x-rays compared to visible light, this approach allows to image objects with higher spatial resolution. As free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds which are intense and coherent, x-ray holography has been used to capture ultrafast dynamic processes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「holography」の詳細全文を読む スポンサード リンク
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