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| {{For|the documentary authorship|Holograph}}
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| [[File:Holomouse2.jpg|thumb|Two photographs of a single hologram taken from different viewpoints]]
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| '''Holography''' is a technique which enables [[three-dimensional]] images to be made. It involves the use of a [[laser]], [[interference (wave propagation)|interference]], [[diffraction]], light [[intensity (physics)|intensity]] recording and suitable illumination of the recording. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the image appear [[Three-dimensional space|three-dimensional]].
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| The holographic recording itself is not an image; it consists of an apparently random structure of either varying intensity, density or profile.
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| ==Overview and history==
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| The [[Magyars|Hungarian]]-[[British people|British]] physicist [[Dennis Gabor]] (in Hungarian: ''Gábor Dénes''),<ref>[[Dennis Gabor|Gabor, Dennis]]. (1948), A new microscopic principle, ''Nature'', 161, p 777-8</ref><ref>{{Cite journal|doi = 10.1098/rspa.1949.0075|first = Dennis|last = Gabor|year = 1949|title = Microscopy by reconstructed wavefronts|journal = Proceedings of the Royal Society|location = London|volume = 197|pages = 454–487|issue = 1051|postscript = <!--None-->|bibcode = 1949RSPSA.197..454G }}</ref> was awarded the [[Nobel Prize in Physics]] in 1971 "for his invention and development of the holographic method".<ref>{{cite web|url=http://www.nobelprize.org/nobel_prizes/physics/laureates/1971/ |title=The Nobel Prize in Physics 1971 |publisher=Nobelprize.org |date= |accessdate=2012-04-21}}</ref>
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| His work, done in the late 1940s, built on pioneering work in the field of X-ray microscopy by other scientists including [[Mieczysław Wolfke]] in 1920 and [[William Lawrence Bragg|WL Bragg]] in 1939.<ref>Hariharan, (1996), Section 1.2, p4-5</ref> The discovery was an unexpected result of research into improving [[electron microscope]]s at the [[British Thomson-Houston]] (BTH) Company in [[Rugby, Warwickshire|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 language|Greek]] words ὅλος (''hólos''; "whole") and γραφή (''[[-graphy|grafē]]''; "[[writing]]" or "[[drawing]]").
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| [[File:III-BIBI BEI BOB.jpg|thumb|upright|Horizontal symmetric text, by [[Dieter Jung (artist)|Dieter Jung]]]]
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| 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<ref name="denisyuk">{{Cite journal | |
| | title = On the reflection of optical properties of an object in a wave field of light scattered by it
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| | last = Denisyuk
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| | first = Yuri N.
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| | authorlink = Yuri Denisyuk
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| | coauthors=
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| | journal = [[Doklady Akademii Nauk SSSR]]
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| | volume = 144
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| | pages = 1275–1278
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| | year = 1962
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| | month =
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| | url =
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| | doi =
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| | publisher =
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| | issue = 6
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| }}</ref> and by [[Emmett Leith]] and [[Juris Upatnieks]] at the [[University of Michigan]], USA.<ref name="leith">{{Cite journal
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| | title = Reconstructed wavefronts and communication theory
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| | author = Leith, E.N.
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| | coauthors= Upatnieks, J.
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| | journal = J. Opt. Soc. Am.
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| | volume = 52
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| | pages = 1123–1130
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| | year = 1962
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| | month =
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| | url =
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| | doi =10.1364/JOSA.52.001123| publisher =
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| | issue = 10
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| }}</ref> 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.<ref>Upatniek J & Leaonard C., (1969), "Diffraction efficiency of bleached photographically recorded intereference patterns", Applied Optics, 8, p85-89</ref><ref>Graube A, (1974), "Advances in bleaching methods for photographically recorded holograms", Applied Optics, 13, p2942-6</ref><ref>N. J. Phillips and D. Porter, (1976), "An advance in the processing of holograms," Journal of Physics E: Scientific Instruments p. 631</ref>
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| 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.<ref>Hariharan, (2002), Section 7.1, p 60</ref> A later refinement, the [[rainbow hologram|"rainbow transmission" hologram]], allows more convenient illumination by white light rather than by lasers.<ref name = Benton>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</ref> Rainbow holograms are commonly used for security and authentication, for example, on credit cards and product packaging.<ref>Toal Vincent (2012), "Introduction to Holography", CRC Press, ISBN 978-1-4398-1868-8</ref>
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| Another kind of common hologram, the [[#The efficiency of a hologram|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.<ref>Hariharan, (2002), Section 7.2, p61</ref>
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| [[Specular holography]] is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface.<ref>{{cite web|url=http://www.zintaglio.com/how.html |title=specular holography: how |publisher=Zintaglio.com |date= |accessdate=2012-04-21}}</ref> It works by reflectively or refractively manipulating bundles of light rays, whereas Gabor-style holography works by diffractively reconstructing wavefronts.
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| Most holograms produced are of static objects but systems for displaying changing scenes on a holographic [[volumetric display]] are now being developed.<ref>{{cite web|url=http://www.tgdaily.com/hardware-features/53703-mit-unveils-holographic-tv-system?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+tgdaily_all_sections+%28TG+Daily+-+All+News%29|title=MIT unveils holographic TV system|accessdate = 2011-09-14}}</ref><ref>See [[Zebra imaging]].</ref><ref>{{cite journal | last1 = Blanche | first1 = P.-A. | year =2010 | title = Holographic three-dimensional telepresence using large-area photorefractive polymer | url =http://www.nature.com/nature/journal/v468/n7320/full/nature09521.html | journal = Nature | volume = 468 | issue = 7320| pages = 80–83 | doi=10.1038/nature09521 | last2 = Bablumian | first2 = A. | last3 = Voorakaranam | first3 = R. | last4 = Christenson | first4 = C. | last5 = Lin | first5 = W. | last6 = Gu | first6 = T. | last7 = Flores | first7 = D. | last8 = Wang | first8 = P. | last9 = Hsieh | first9 = W.-Y. | pmid = 21048763|bibcode = 2010Natur.468...80B }}</ref>
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| Holograms can also be used to store, retrieve, and process information optically.<ref>Hariharan, (2002), 12.6, p107</ref>
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| In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor or [[diode laser]]s, such as those found in millions of [[DVD recorder]]s 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.
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| It was thought that it would be possible to use X-rays to make holograms of molecules and view them using visible light. However, X-ray holograms have not been created to date.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/Hbase/optmod/holog.html |title=Holography |publisher=Hyperphysics.phy-astr.gsu.edu |date= |accessdate=2012-04-21}}</ref>
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| ==How holography works==
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| [[File:Holograph-record.svg|thumb|400px|Recording a hologram]]
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| [[File:Holography-reconstruct.svg|thumb|300px|Reconstructing a hologram]]
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| [[File:Holographic recording.jpg|thumb|right|Close-up photograph of a hologram's surface. The object in the hologram is a toy van. It is no more possible to discern the subject of a hologram from this pattern than it is to identify what music has been recorded by looking at a [[compact disc|CD]] surface. Note that the hologram is described by the [[speckle pattern]], rather than the "wavy" line pattern.]]
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| Holography is a technique that enables a light field, which is generally the product of a light source scattered off objects, to be recorded and later reconstructed when the original light field is no longer present, due to the absence of the original objects.<ref>Hariharan, (2002), Section 1, p1</ref> Holography can be thought of as somewhat similar to [[sound recording]], whereby a sound field created by vibrating matter like [[musical instrument]]s or [[vocal cords]], is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.
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| ===Laser===
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| Holograms are recorded using a flash of light that illuminates a scene and then imprints on a recording medium, much in the way a photograph is recorded. In addition, however, part of the light beam must be shone directly onto the recording medium - this second light beam is known as the [[reference beam]]. A hologram requires a [[laser]] as the sole light source. Lasers can be precisely controlled and have a fixed [[wavelength]], unlike sunlight or light from conventional sources, which contain many different wavelengths. To prevent external light from interfering, holograms are usually taken in darkness, or in low level light of a different color from the laser light used in making the hologram. Holography requires a specific [[Exposure (photography)|exposure]] time (just like photography), which can be controlled using a [[Shutter (photography)|shutter]], or by electronically timing the laser.
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| ===Apparatus===
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| A hologram can be made by shining part of the light beam directly onto the recording medium, and the other part onto the object in such a way that some of the scattered light falls onto the recording medium.
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| A more flexible arrangement for recording a hologram requires the laser beam to be aimed through a series of elements that change it in different ways. The first element is a [[beam splitter]] that divides the beam into two identical beams, each aimed in different directions:
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| *One beam (known as the ''illumination'' or ''object beam'') is spread using [[Lens (optics)|lens]]es and directed onto the scene using [[mirror]]s. Some of the light scattered (reflected) from the scene then falls onto the recording medium.
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| *The second beam (known as the ''reference beam'') is also spread through the use of lenses, but is directed so that it doesn't come in contact with the scene, and instead travels directly onto the recording medium.
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| Several different materials can be used as the recording medium. One of the most common is a film very similar to [[photographic film]] ([[silver halide]] [[photographic emulsion]]), but with a much higher concentration of light-reactive grains, making it capable of the much higher [[Optical resolution|resolution]] that holograms require. A layer of this recording medium (e.g. silver halide) is attached to a transparent substrate, which is commonly glass, but may also be plastic.
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| ===Process===
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| When the two laser beams reach the recording medium, their light waves intersect and [[Interference (wave propagation)|interfere]] with each other. It is this interference pattern that is imprinted on the recording medium. The pattern itself is seemingly random, as it represents the way in which the scene's light ''interfered'' with the original light source — but not the original light source itself. The interference pattern can be considered an [[encoded]] version of the scene, requiring a particular key — the original light source — in order to view its contents.
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| This missing key is provided later by shining a laser, identical to the one used to record the hologram, onto the developed film. When this beam illuminates the hologram, it is [[diffraction|diffracted]] by the hologram's surface pattern. This produces a light field identical to the one originally produced by the scene and scattered onto the hologram. The image this effect produces in a person's [[retina]] is known as a [[virtual image]].
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| ===Holography vs. photography===
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| Holography may be better understood via an examination of its differences from ordinary photography:
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| * A hologram represents a recording of information regarding the light that came from the original scene as scattered in a range of directions rather than from only one direction, as in a photograph. This allows the scene to be viewed from a range of different angles, as if it were still present.
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| * A photograph can be recorded using normal light sources (sunlight or electric lighting) whereas a laser is required to record a hologram.
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| *A lens is required in photography to record the image, whereas in holography, the light from the object is scattered directly onto the recording medium.
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| *A holographic recording requires a second light beam (the reference beam) to be directed onto the recording medium.
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| * A photograph can be viewed in a wide range of lighting conditions, whereas holograms can only be viewed with very specific forms of illumination.
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| * When a photograph is cut in half, each piece shows half of the scene. When a hologram is cut in half, the whole scene can still be seen in each piece. This is because, whereas each point in a [[photograph]] only represents light scattered from a single point in the scene, ''each point'' on a holographic recording includes information about light scattered from ''every point'' in the scene. It can be thought of as viewing a street outside a house through a 4 ft x 4 ft window, then through a 2 ft x 2 ft window. One can see all of the same things through the smaller window (by moving the head to change the viewing angle), but the viewer can see more ''at once'' through the 4 ft window.
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| * A photograph is a two-dimensional representation that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range of a hologram adds many more [[Depth perception|depth perception cues]] that were present in the original scene. These cues are recognized by the [[human brain]] and translated into the same perception of a three-dimensional image as when the original scene might have been viewed.
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| * A photograph clearly maps out the light field of the original scene. The developed hologram's surface consists of a very fine, seemingly random pattern, which appears to bear no relationship to the scene it recorded.
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| ==Physics of holography==
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| For a better understanding of the process, it is necessary to understand [[interference (optics)|interference]] and [[diffraction]]. Interference occurs when one or more [[wavefronts]] are superimposed. [[Diffraction]] occurs whenever a wavefront encounters an object. The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction. It is somewhat simplified but is accurate enough to provide an understanding of how the holographic process works.
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| For those unfamiliar with these concepts, it is worthwhile to read the respective articles before reading further in this article.
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| ===Plane wavefronts===
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| A [[diffraction grating]] is a structure with a repeating pattern. A simple example is a metal plate with slits cut at regular intervals. A light wave incident on a grating is split into several waves; the direction of these diffracted waves is determined by the grating spacing and the wavelength of the light.
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| A simple hologram can be made by superimposing two [[plane wave]]s from the same light source on a holographic recording medium. The two waves interfere giving a [[Interference (optics)#Between two plane waves|straight line fringe pattern]] whose intensity varies sinusoidally across the medium. The spacing of the fringe pattern is determined by the angle between the two waves, and on the wavelength of the light.
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| The recorded light pattern is a diffraction grating. When it is illuminated by only one of the waves used to create it, it can be shown that one of the diffracted waves emerges at the same angle as that at which the second wave was originally incident so that the second wave has been 'reconstructed'. Thus, the recorded light pattern is a holographic recording as defined above.
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| ===Point sources===
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| [[File:Zonenplatte Cosinus.png|upright|thumb|Sinusoidal zone plate]]
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| If the recording medium is illuminated with a point source and a normally incident plane wave, the resulting pattern is a [[Zone plate|sinusoidal zone plate]] which acts as a negative [[Fresnel lens]] whose focal length is equal to the separation of the point source and the recording plane.
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| When a plane wavefront illuminates a negative lens, it is expanded into a wave which appears to diverge from the focal point of the lens. Thus, when the recorded pattern is illuminated with the original plane wave, some of the light is diffracted into a diverging beam equivalent to the original plane wave; a holographic recording of the point source has been created.
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| When the plane wave is incident at a non-normal angle, the pattern formed is more complex but still acts as a negative lens provided it is illuminated at the original angle.
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| ===Complex objects===
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| To record a hologram of a complex object, a laser beam is first split into two separate beams of light. One beam illuminates the object, which then scatters light onto the recording medium. According to [[diffraction]] theory, each point in the object acts as a point source of light so the recording medium can be considered to be illuminated by a set of point sources located at varying distances from the medium.
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| The second (reference) beam illuminates the recording medium directly. Each point source wave interferes with the reference beam, giving rise to its own sinusoidal zone plate in the recording medium. The resulting pattern is the sum of all these 'zone plates' which combine to produce a random ([[speckle pattern|speckle]]) pattern as in the photograph above.
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| When the hologram is illuminated by the original reference beam, each of the individual zone plates reconstructs the object wave which produced it, and these individual wavefronts add together to reconstruct the whole of the object beam. The viewer perceives a wavefront that is identical to the wavefront scattered from the object onto the recording medium, so that it appears to him or her that the object is still in place even if it has been removed. This image is known as a "virtual" image, as it is generated even though the object is no longer there.
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| ===Mathematical model===
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| A single-frequency light wave can be modelled by a [[complex number]] '''U''', which represents the [[electric]] or [[magnetic field]] of the [[electromagnetic radiation#Properties|light wave]]. The [[amplitude]] and [[Phase (waves)|phase]] of the light are represented by the [[Polar form|absolute value]] and [[Polar form|angle]] of the complex number. The object and reference waves at any point in the holographic system are given by '''U'''<sub>O</sub> and '''U'''<sub>R</sub>. The combined beam is given by '''U'''<sub>O</sub> + '''U'''<sub>R</sub>. The energy of the combined beams is proportional to the square of magnitude of the combined waves as:
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| <math>|U_O + U_R|^2=U_O U_R^*+|U_R|^2+|U_O|^2+ U_O^*U_R</math>
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| If a photographic plate is exposed to the two beams and then developed, its transmittance, '''T''', is proportional to the light energy that was incident on the plate and is given by
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| <math>T=kU_O U_R^*+k|U_R|^2+k|U_O|^2+ kU_O^*U_R</math>
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| where ''k'' is a constant.
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| When the developed plate is illuminated by the reference beam, the light transmitted through the plate, '''U'''<sub>H</sub> is equal to the transmittance '''T''' multiplied by the reference beam amplitude '''U'''<sub>R</sub>, giving
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| <math>U_H=TU_R=kU_O|U_R|^2+k|U_R|^2U_R+k|U_O|^2U_R+ kU_O^*U_R^2</math>
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| It can be seen that '''U'''<sub>H</sub> has four terms, each representing a light beam emerging from the hologram. The first of these is proportional to '''U'''<sub>O</sub>. This is the reconstructed object beam which enables a viewer to 'see' the original object even when it is no longer present in the field of view.
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| The second and third beams are modified versions of the reference beam. The fourth term is known as the "conjugate object beam". It has the reverse curvature to the object beam itself and forms a [[real image]] of the object in the space beyond the holographic plate.
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| When the reference and object beams are incident on the holographic recording medium at significantly different angles, the virtual, real and reference wavefronts all emerge at different angles, enabling the reconstructed object to be seen clearly.
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| ==Recording a hologram==
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| ===Items required===
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| [[File:Holography setup.jpeg|thumb|An optical table being used to make a hologram]]
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| To make a hologram, the following are required:
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| * a suitable object or set of objects
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| * a suitable laser beam
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| * part of the laser beam to be directed so that it illuminates the object (the object beam) and another part so that it illuminates the recording medium directly (the reference beam), enabling the reference beam and the light which is scattered from the object onto the recording medium to form an interference pattern
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| * a recording medium which converts this interference pattern into an optical element which modifies either the amplitude or the phase of an incident light beam according to the intensity of the interference pattern.
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| * an environment which provides sufficient mechanical and thermal stability that the interference pattern is stable during the time in which the interference pattern is recorded<ref>Hariharan, (2002), Section 7,1. p60</ref>
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| These requirements are inter-related, and it is essential to understand the nature of optical interference to see this. [[Interference (optics)|Interference]] is the variation in [[intensity (physics)|intensity]] which can occur when two [[light waves]] are superimposed. The intensity of the maxima exceeds the sum of the individual intensities of the two beams, and the intensity at the minima is less than this and may be zero. The interference pattern maps the relative phase between the two waves, and any change in the relative phases causes the interference pattern to move across the field of view. If the relative phase of the two waves changes by one cycle, then the pattern drifts by one whole fringe. One phase cycle corresponds to a change in the relative distances travelled by the two beams of one wavelength. Since the wavelength of light is of the order of 0.5μm, it can be seen that very small changes in the optical paths travelled by either of the beams in the holographic recording system lead to movement of the interference pattern which is the holographic recording. Such changes can be caused by relative movements of any of the optical components or the object itself, and also by local changes in air-temperature. It is essential that any such changes are significantly less than the wavelength of light if a clear well-defined recording of the interference is to be created.
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| The exposure time required to record the hologram depends on the laser power available, on the particular medium used and on the size and nature of the object(s) to be recorded, just as in conventional photography. This determines the stability requirements. Exposure times of several minutes are typical when using quite powerful gas lasers and silver halide emulsions. All the elements within the optical system have to be stable to fractions of a μm over that period. It is possible to make holograms of much less stable objects by using a [[Laser pulse|pulsed laser]] which produces a large amount of energy in a very short time (μs or less).<ref>Martinez-Hurtado et al. {{doi|10.1021/la102693m}}</ref> These systems have been used to produce holograms of live people. A holographic portrait of Dennis Gabor was produced in 1971 using a pulsed ruby laser.<ref>Hariharan, (2002), Figure 4.5, p44</ref><ref>{{cite web|url=http://webmuseum.mit.edu/browser.php?m=objects&kv=67243&i=14558|title = Photograph of Dennis Gabor standing beside his holographic portrait |publisher=MIT|accessdate= 2011-09-16}}</ref>
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| Thus, the laser power, recording medium sensitivity, recording time and mechanical and thermal stability requirements are all interlinked. Generally, the smaller the object, the more compact the optical layout, so that the stability requirements are significantly less than when making holograms of large objects.
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| Another very important laser parameter is its [[Coherence (physics)#Temporal coherence|coherence]].<ref>Hariharan, (2002), Section 4.2, p40</ref> This can be envisaged by considering a laser producing a sine wave whose frequency drifts over time; the coherence length can then be considered to be the distance over which it maintains a single frequency. This is important because two waves of different frequencies do not produce a stable interference pattern. The coherence length of the laser determines the depth of field which can be recorded in the scene. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram.
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| The objects that form the scene must, in general, have optically rough surfaces so that they scatter light over a wide range of angles. A specularly reflecting (or shiny) surface reflects the light in only one direction at each point on its surface, so in general, most of the light will not be incident on the recording medium. A hologram of a shiny object can be made by locating it very close to the recording plate.<ref>Hariharan, (2002), Figure 7.2, p62</ref>
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| ===Hologram classifications===
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| There are three important properties of a hologram which are defined in this section. A given hologram will have one or other of each of these three properties, e.g. an amplitude modulated thin transmission hologram, or a phase modulated, volume reflection hologram.
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| ====Amplitude and phase modulation holograms====
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| An amplitude modulation hologram is one where the amplitude of light diffracted by the hologram is proportional to the intensity of the recorded light. A straightforward example of this is [[photographic emulsion]] on a transparent substrate. The emulsion is exposed to the interference pattern, and is subsequently developed giving a transmittance which varies with the intensity of the pattern - the more light that fell on the plate at a given point, the darker the developed plate at that point.
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| A phase hologram is made by changing either the thickness or the [[refractive index]] of the material in proportion to the intensity of the holographic interference pattern. This is a [[Grating equation|phase grating]] and it can be shown that when such a plate is illuminated by the original reference beam, it reconstructs the original object wavefront. The efficiency (i.e. the fraction of the illuminated beam which is converted to reconstructed object beam) is greater for phase than for amplitude modulated holograms.
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| ====Thin holograms and thick (volume) holograms====
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| A thin hologram is one where the thickness of the recording medium is much less than the spacing of the interference fringes which make up the holographic recording.
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| A thick or [[volume hologram]] is one where the thickness of the recording medium is greater than the spacing of the interference pattern. The recorded hologram is now a three dimensional structure, and it can be shown that incident light is diffracted by the grating only at a particular angle, known as the [[Bragg's law|Bragg angle]].<ref>Lipson, (2011), Seection12.5.4, p443</ref> If the hologram is illuminated with a light source incident at the original reference beam angle but a broad spectrum of wavelengths; reconstruction occurs only at the wavelength of the original laser used. If the angle of illumination is changed, reconstruction will occur at a different wavelength and the colour of the re-constructed scene changes. A volume hologram effectively acts as a colour filter.
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| ====Transmission and reflection holograms====
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| A transmission hologram is one where the object and reference beams are incident on the recording medium from the same side. In practice, several more mirrors may be used to direct the beams in the required directions.
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| Normally, transmission holograms can only be reconstructed using a laser or a quasi-monochromatic source, but a particular type of transmission hologram, known as a rainbow hologram, can be viewed with white light.
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| In a reflection hologram, the object and reference beams are incident on the plate from opposite sides of the plate. The reconstructed object is then viewed from the same side of the plate as that at which the re-constructing beam is incident.
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| Only volume holograms can be used to make reflection holograms, as only a very low intensity diffracted beam would be reflected by a thin hologram.
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| ===Holographic recording media===
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| The recording medium has to convert the original interference pattern into an optical element that modifies either the [[amplitude]] or the [[phase (waves)|phase]] of an incident light beam in proportion to the intensity of the original light field.
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| The recording medium should be able to resolve fully all the fringes arising from interference between object and reference beam. These fringe spacings can range from tens of [[micrometers]] to less than one micrometer, i.e. spatial frequencies ranging from a few hundred to several thousand cycles/mm, and ideally, the recording medium should have a response which is flat over this range. If the response of the medium to these spatial frequencies is low, the diffraction efficiency of the hologram will be poor, and a dim image will be obtained. Standard photographic film has a very low or even zero response at the frequencies involved and cannot be used to make a hologram - see, for example, Kodak's professional black and white film<ref>{{cite web|url=http://www.kodak.com/eknec/documents/59/0900688a80300559/EpubBW400CN4036.pdf|title=Kodak black and white professional film||accessdate = 2011-09-14}}</ref> whose resolution starts falling off at 20 lines/mm — it is unlikely that any reconstructed beam could be obtained using this film.
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| If the response is not flat over the range of spatial frequencies in the interference pattern, then the resolution of the reconstructed image may also be degraded.<ref>Hariharan, (1996), Section 6.4, p88</ref><ref>Kozma A & Zelenka JS, (1970), Effect of film resolution and size in holography, Journal of the Optical Society of America, 60, 34–43</ref>
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| The table below shows the principal materials used for holographic recording. Note that these do not include the materials used in the [[Holography#Mass replication of holograms|mass replication]] of an existing hologram, which are discussed in the next section. The resolution limit given in the table indicates the maximal number of interference lines/mm of the gratings. The required exposure, expressed as milli[[joule]]s (mJ) of photon energy impacting the surface area, is for a long exposure time. Short exposure times (less than 1/1000 of a second, such as with a pulsed laser) require much higher exposure energies, due to [[reciprocity failure]].
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| {| class="wikitable"
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| |+General properties of recording materials for holography. Source:<ref>Hariharan, (2002), Table 6.1, p50</ref>
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| |-
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| ! Material !! Reusable !! Processing !! Type of hologram !! Theoretical maximum efficiency !! Required exposure [mJ/cm<sup>2</sup>] !! Resolution limit [mm<sup>−1</sup>]
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| |-
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| | rowspan=2| [[Photographic paper|Photographic emulsions]]
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| | rowspan=2| No
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| | rowspan=2| Wet || Amplitude || 6%
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| | rowspan=2| 1.5
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| | rowspan=2| 5000
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| |-
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| | Phase (bleached) || 60%
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| |-
| |
| | Dichromated gelatin || No || Wet || Phase || 100% || 100 || 10,000
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| |-
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| | [[Photoresist]]s || No || Wet || Phase || 30% || 100 || 3,000
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| |-
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| | Photothermoplastics || Yes || Charge and heat || Phase || 33% || 0.1 || 500–1,200
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| |-
| |
| | [[Photopolymer]]s || No || Post exposure || Phase || 100% || 10000 || 5,000
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| |-|-
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| | [[Photorefractive effect|Photorefractives]] || Yes || None || Phase || 100% || 10 || 10,000
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| |}
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| | |
| ----
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| ===Copying and mass production===
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| An existing hologram can be copied by [[Embossing (manufacturing)|embossing]]<ref>Iwata F & Tsujiiuchi J (1974), "Characteristics oof a photoresist hologram and its replica", Applied Optics, 13, p1327-36</ref> or optically.<ref>Hariharan, (2002), Section 11.4.1, p191</ref>
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| Most holographic recordings (e.g. bleached silver halide, photoresist, and photopolymers) have surface relief patterns which conform with the original illumination intensity. Embossing, which is similar to the method used to stamp out plastic discs from a master in audio recording, involves copying this surface relief pattern by impressing it onto another material.
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| The first step in the embossing process is to make a stamper by [[Electrophoretic deposition|electrodeposition]] of [[nickel]] on the relief image recorded on the photoresist or photothermoplastic. When the nickel layer is thick enough, it is separated from the master hologram and mounted on a metal backing plate. The material used to make embossed copies consists of a [[polyester]] base film, a resin separation layer and a [[thermoplastic]] film constituting the holographic layer.
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| The embossing process can be carried out with a simple heated press. The bottom layer of the duplicating film (the thermoplastic layer) is heated above its softening point and pressed against the stamper, so that it takes up its shape. This shape is retained when the film is cooled and removed from the press. In order to permit the viewing of embossed holograms in reflection, an additional reflecting layer of aluminum is usually added on the hologram recording layer. This method is particularly suited to mass production.
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| The first book to feature a hologram on the front cover was ''The Skook'' (Warner Books, 1984) by [[JP Miller]], featuring an illustration by Miller. That same year, "Telstar" by [[Ad Infinitum (band)|Ad Infinitum]] became the first record with a hologram cover and ''[[National Geographic (magazine)|National Geographic]]'' published the first magazine with a hologram cover.<ref>{{cite web|last=Freitas |first=Frank De |url=http://holographica.blogspot.com/2008/07/national-geographic-hologram-1984.html |title=Antiquarian Holographica blog |publisher=Holographica.blogspot.com |date=2008-07-30 |accessdate=2012-04-21}}</ref> Embossed holograms are used widely on credit cards, banknotes, and high value products for authentication purposes.<ref>Toal Vincent, 2012, Introcution to Holography, CRC Press, ISBN 978-1-4398-1868-8</ref>
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| It is possible to print holograms directly into steel using a sheet explosive charge to create the required surface relief.<ref>{{cite web|url=http://www.physorg.com/news124039000.html |title=Holograms with explosive power |publisher=Physorg.com |date= |accessdate=2012-04-21}}</ref> The [[Royal Canadian Mint]] produces holographic gold and silver coinage through a complex stamping process.<ref>{{cite web|url=http://www.mint.ca/store/coin/150-lunar-hologram-coin-year-of-the-rabbit-2011-prod990012|title = Lunar Holographic Coins|accessdate = 2011-09-14}}</ref>
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| A hologram can be copied optically by illuminating it with a laser beam, and locating a second hologram plate so that it is illuminated both by the reconstructed object beam, and the illuminating beam. Stability and coherence requirements are significantly reduced if the two plates are located very close together.<ref>Harris JR, Sherman GC and Billings BH, 1966, Copying hologram, Applied Optics, 5, 665-6</ref> An [[refractive index|index]] matching fluid is often used between the plates to minimize spurious interference between the plates. Uniform illumination can be obtained by scanning point-by-point or with a beam shaped into a thin line.
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| ==Reconstructing and viewing the holographic image==
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| When the hologram plate is illuminated by a laser beam identical to the reference beam which was used to record the hologram, an exact reconstruction of the original object wavefront is obtained. An imaging system (an eye or a camera) located in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing the original. When the lens is moved, the image changes in the same way as it would have done when the object was in place. If several objects were present when the hologram was recorded, the reconstructed objects move relative to one another, i.e. exhibit [[parallax]], in the same way as the original objects would have done. It was very common in the early days of holography to use a chess board as the object and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess pieces appeared to change.
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| A holographic image can also be obtained using a different laser beam configuration to the original recording object beam, but the reconstructed image will not match the original exactly.<ref>Hariharan, (2002), Section 2.3, p17</ref> When a laser is used to reconstruct the hologram, the image is [[speckle pattern|speckled]] just as the original image will have been. This can be a major drawback in viewing a hologram.
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| White light consists of light of a wide range of wavelengths. Normally, if a hologram is illuminated by a white light source, each wavelength can be considered to generate its own holographic reconstruction, and these will vary in size, angle, and distance. These will be superimposed, and the summed image will wipe out any information about the original scene, as if superimposing a set of photographs of the same object of different sizes and orientations. However, a holographic image can be obtained using [[white light]] in specific circumstances, e.g. with volume holograms and rainbow holograms. The white light source used to view these holograms should always approximate to a point source, i.e. a spot light or the sun. An extended source (e.g. a fluorescent lamp) will not reconstruct a hologram since its light is incident at each point at a wide range of angles, giving multiple reconstructions which will "wipe" one another out.
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| White light reconstructions do not contain speckles.
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| ===Volume holograms===
| |
| {{main|Volume hologram}}
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| A volume hologram can give a reconstructed beam using white light, as the hologram structure effectively filters out colours other than those equal to or very close to the colour of the laser used to make the hologram so that the reconstructed image will appear to be approximately the same colour as the laser light used to create the holographic recording.
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| ===Rainbow holograms===
| |
| {{main|Rainbow hologram}}
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| [[File:Rainbow hologram.jpeg|thumb|Rainbow hologram showing the change in colour in the vertical direction]]
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| In this method, parallax in the vertical plane is sacrificed to allow a bright well-defined single colour re-constructed image to be obtained using white light. The rainbow holography recording process uses a horizontal slit to eliminate vertical [[parallax]] in the output image. The viewer is then effectively viewing the holographic image through a narrow horizontal slit. Horizontal parallax information is preserved but movement in the vertical direction produces colour rather than different vertical perspectives.<ref>Hariharan, (2002), Section 7.4, p63</ref> [[Stereopsis]] and horizontal motion parallax, two relatively powerful cues to depth, are preserved.
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| The holograms found on [[credit cards]] are examples of rainbow holograms. These are technically transmission holograms mounted onto a reflective surface like a [[PET film (biaxially oriented)|metalized polyethylene terephthalate]] substrate commonly known as [[polyethylene terephthalate|PET]].
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| ===Fidelity of the reconstructed beam===
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| [[File:broken hologram.jpg|thumb|Reconstructions from two parts of a broken hologram. Note the different viewpoints required to see the whole object]]
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| To replicate the original object beam exactly, the reconstructing reference beam must be identical to the original reference beam and the recording medium must be able to fully resolve the interference pattern formed between the object and reference beams. Exact reconstruction is required in [[holographic interferometry]], where the holographically reconstructed wavefront [[Interference (wave propagation)|interferes]] with the wavefront coming from the actual object, giving a null fringe if there has been no movement of the object and mapping out the displacement if the object has moved. This requires very precise relocation of the developed holographic plate.
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| Any change in the shape, orientation or wavelength of the reference beam gives rise to aberrations in the reconstructed image. For instance, the reconstructed image is magnified if the laser used to reconstruct the hologram has a shorter wavelength than the original laser. Nonetheless, good reconstruction is obtained using a laser of a different wavelength, quasi-monochromatic light or white light, in the right circumstances.
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| Since each point in the object illuminates all of the hologram, the whole object can be reconstructed from a small part of the hologram. Thus, a hologram can be broken up into small pieces and each one will enable the whole of the original object to be imaged. One does, however, lose information and the [[optical resolution|spatial resolution]] gets worse as the size of the hologram is decreased — the image becomes "fuzzier". The field of view is also reduced, and the viewer will have to change position to see different parts of the scene.
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| ==Applications==
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| ===Art===
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| Early on, artists saw the potential of holography as a medium and gained access to science laboratories to create their work. Holographic art is often the result of collaborations between scientists and artists, although some holographers would regard themselves as both an artist and a scientist.
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| [[Salvador Dalí]] claimed to have been the first to employ holography artistically. He was certainly the first and best-known surrealist to do so, but the 1972 New York exhibit of Dalí holograms had been preceded by the holographic art exhibition that was held at the [[Cranbrook Academy of Art]] in Michigan in 1968 and by the one at the Finch College gallery in New York in 1970, which attracted national media attention.<ref>{{cite web|url=http://holophile.com/history.htm |title=The History and Development of Holography|publisher=Holophile.com |date= |accessdate=2012-04-21}}</ref>
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| During the 1970s, a number of art studios and schools were established, each with their particular approach to holography. Notably, there was the San Francisco School of Holography established by [[Lloyd Cross]], The Museum of Holography in New York founded by Rosemary (Possie) H. Jackson, the Royal College of Art in London and the Lake Forest College Symposiums organised by Tung Jeong (T.J.).<ref>{{cite web|author=Integraf |url=http://www.integraf.com/tung_jeong.htm |title=Dr. Tung H. Jeong Biography |publisher=Integraf.com |date= |accessdate=2012-04-21}}</ref> None of these studios still exist; however, there is the Center for the Holographic Arts in New York<ref>{{cite web|url=http://www.holocenter.org |title=holocenter |publisher=holocenter |date= |accessdate=2012-04-21}}</ref> and the HOLOcenter in Seoul,<ref>{{cite web|url=http://www.holocenter.or.kr/ |title=Holocenter |publisher=Holocenter |date= |accessdate=2012-04-21}}</ref> which offers artists a place to create and exhibit work.
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| During the 1980s, many artists who worked with holography helped the diffusion of this so-called "new medium" in the art world, such as Harriet Casdin-Silver of the [[USA]], [[Dieter Jung (artist)|Dieter Jung]] of [[Germany]], and [[Moysés Baumstein]] of [[Brazil]], each one searching for a proper "language" to use with the three-dimensional work, avoiding the simple holographic reproduction of a sculpture or object. For instance, in Brazil, many concrete poets (Augusto de Campos, Décio Pignatari, Julio Plaza and José Wagner Garcia, associated with [[Moysés Baumstein]]) found in holography a way to express themselves and to renew [[Concrete Poetry]].
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| A small but active group of artists still use holography as their main medium, and many more artists integrate holographic elements into their work.<ref>http://www.universal-hologram.com/</ref> Some are associated with novel holographic techniques; for example, artist Matt Brand<ref>Holographic metalwork http://www.zintaglio.com</ref> employed computational mirror design to eliminate image distortion from [[specular holography]].
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| The MIT Museum<ref>{{cite web|url=http://web.mit.edu/museum/collections/holography.html |title=MIT Museum: Collections - Holography |publisher=Web.mit.edu |date= |accessdate=2012-04-21}}</ref> and Jonathan Ross<ref>{{cite web|url=http://www.jrholocollection.com/ |title=The Jonathan Ross Hologram Collection |publisher=Jrholocollection.com |date= |accessdate=2012-04-21}}</ref> both have extensive collections of holography and on-line catalogues of art holograms.
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| ===Data storage===
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| {{Main|Holographic memory}}
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| Holography can be put to a variety of uses other than recording images. [[Holographic data storage]] is a technique that can store information at high density inside crystals or photopolymers. The ability to store large amounts of information in some kind of media is of great importance, as many electronic products incorporate storage devices. As current storage techniques such as [[Blu-ray Disc]] reach the limit of possible data density (due to the [[diffraction]]-limited size of the writing beams), holographic storage has the potential to become the next generation of popular storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.
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| Currently available [[spatial light modulator|SLMs]] can produce about 1000 different images a second at 1024×1024-bit resolution. With the right type of media (probably polymers rather than something like [[lithium niobate|LiNbO<sub>3</sub>]]), this would result in about one-[[gigabit per second|gigabit-per-second]] writing speed. Read speeds can surpass this, and experts believe one-[[terabit per second|terabit-per-second]] readout is possible.
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| In 2005, companies such as [[Optware]] and [[Maxell]] produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 [[terabyte|TB]], which they plan to market under the name [[Holographic Versatile Disc]]. Another company, [[InPhase Technologies]], is developing a competing format. While many holographic data storage models have used "page-based" storage, where each recorded hologram holds a large amount of data, more recent research into using submicrometre-sized "microholograms" has resulted in several potential [[3D optical data storage]] solutions. While this approach to data storage can not attain the high data rates of page-based storage, the tolerances, technological hurdles, and cost of producing a commercial product are significantly lower.
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| ===Dynamic holography===
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| In static holography, recording, developing and reconstructing occur sequentially, and a permanent hologram is produced.
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| There also exist holographic materials that do not need the developing process and can record a hologram in a very short time. This allows one to use holography to perform some simple operations in an all-optical way. Examples of applications of such real-time holograms include [[phase-conjugate mirror]]s ("time-reversal" of light), optical cache memories, [[image processing]] (pattern recognition of time-varying images), and [[optical computing]].
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| The amount of processed information can be very high (terabits/s), since the operation is performed in parallel on a whole image. This compensates for the fact that the recording time, which is in the order of a [[microsecond]], is still very long compared to the processing time of an electronic computer. The optical processing performed by a dynamic hologram is also much less flexible than electronic processing. On one side, one has to perform the operation always on the whole image, and on the other side, the operation a hologram can perform is basically either a multiplication or a phase conjugation. In optics, addition and [[Fourier transform]] are already easily performed in linear materials, the latter simply by a lens. This enables some applications, such as a device that compares images in an optical way.<ref>R. Ryf et al. [http://ol.osa.org/abstract.cfm?id=65530 High-frame-rate joint Fourier-transform correlator based on Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> crystal], Optics Letters '''26''', 1666–1668 (2001)</ref>
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| The search for novel [[:Category:Nonlinear optical materials|nonlinear optical materials]] for dynamic holography is an active area of research. The most common materials are [[photorefraction|photorefractive crystals]], but in [[semiconductor]]s or [[Heterojunction|semiconductor heterostructures]] (such as [[quantum well]]s), atomic vapors and gases, [[plasma (physics)|plasmas]] and even liquids, it was possible to generate holograms.
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| A particularly promising application is [[optical phase conjugation]]. It allows the removal of the wavefront distortions a light beam receives when passing through an aberrating medium, by sending it back through the same aberrating medium with a conjugated phase. This is useful, for example, in free-space optical communications to compensate for atmospheric turbulence (the phenomenon that gives rise to the twinkling of starlight).
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| ===Hobbyist use===
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| [[File:Contest3.jpg|thumb|''Peace Within Reach'', a Denisyuk DCG hologram by amateur Dave Battin]]
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| Since the beginning of holography, experimenters have explored its uses. Starting in 1971, [[Lloyd Cross]] started the San Francisco School of Holography and started to teach amateurs the methods of making holograms with inexpensive equipment. This method relied on the use of a large table of deep sand to hold the [[optics]] rigid and damp [[vibration]]s that would destroy the image.
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| Many of these holographers would go on to produce art holograms. In 1983, Fred Unterseher published the ''Holography Handbook'', a remarkably easy-to-read description of making holograms at home. This brought in a new wave of holographers and gave simple methods to use the then-available AGFA [[silver halide]] recording materials.
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| In 2000, [[Frank DeFreitas]] published the ''Shoebox Holography Book'' and introduced the use of inexpensive [[laser pointer]]s to countless [[hobby]]ists. This was a very important development for amateurs, as the cost for a 5 mW laser dropped from $1200 to $5 as semiconductor laser diodes reached mass market. Now, there are hundreds to thousands of amateur holographers worldwide.
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| By late 2000, holography kits with the inexpensive laser pointer diodes entered the mainstream consumer market. These kits enabled students, teachers, and hobbyists to make many kinds of holograms without specialized equipment, and became popular gift items by 2005.<ref name="IEEE">Stephen Cass: ''[http://spectrum.ieee.org/consumer-electronics/gaming/holiday-gifts-2005 Holiday Gifts 2005 Gifts and gadgets for technophiles of all ages: Do-It Yourself-3-D]''. In ''IEEE Spectrum'', November 2005</ref> The introduction of holography kits with self-developing film plates in 2003 made it even possible for hobbyists to make holograms without using chemical developers.<ref name="physicsteacher">Chiaverina, Chris: ''[http://www.litiholo.com/Hologram%20Kit%20article%20Physics%20Teacher%20Nov%202010.pdf Litiholo holography - So easy even a caveman could have done it (apparatus review)]''. In ''The Physics Teacher'', vol. 48, November 2010, pp. 551-552.</ref>
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| In 2006, a large number of surplus Holography Quality Green Lasers (Coherent C315) became available and put Dichromated Gelatin (DCG) within the reach of the amateur holographer. The holography community was surprised at the amazing sensitivity of DCG to green [[light]]. It had been assumed that the sensitivity would be non-existent. Jeff Blyth responded with the G307 formulation of DCG to increase the speed and sensitivity to these new lasers.<ref>{{cite web|url=http://www.holowiki.com/index.php/G307_DCG_Formula |title=A Holography FAQ |publisher=HoloWiki |date=2011-02-15 |accessdate=2012-04-21}}</ref>
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| Many film suppliers have come and gone from the silver-halide market. While more film manufactures have filled in the voids, many amateurs are now making their own film. The favorite formulations are Dichromated Gelatin, Methylene Blue Sensitised Dichromated Gelatin and Diffusion Method Silver Halide preparations. Jeff Blyth has published very accurate methods for making film in a small lab or garage.<ref>{{cite web|url=http://www.holowiki.com/index.php/Special:Search?search=Blyth&go=Go |title=Many methods are here |publisher=Holowiki.com |date= |accessdate=2012-04-21}}</ref>
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| A small group of amateurs are even constructing their own pulsed lasers to make holograms of moving objects.<ref>{{cite web|url=http://cabd0.tripod.com/holograms/index.html |title=Jeff Blyth's Film Formulations |publisher=Cabd0.tripod.com |date= |accessdate=2012-04-21}}</ref>
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| ===Holographic interferometry===
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| {{Main|holographic interferometry}}
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| Holographic interferometry (HI) is a technique that enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light).<ref>Powell RL & Stetson KA, 1965, J. Opt. Soc. Am., 55, 1593–8</ref><ref>Jones R and Wykes C, Holographic and Speckle Interferometry, 1989, Cambridge University Press ISBN 0-521-34417-4</ref> It can also be used to detect optical-path-length variations in transparent media, which enables, for example, fluid flow to be visualized and analyzed. It can also be used to generate contours representing the form of the surface.
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| It has been widely used to measure stress, strain, and vibration in engineering structures.
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| ===Interferometric microscopy===
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| {{Main|Interferometric microscopy}}
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| The hologram keeps the information on the amplitude and phase of the field. Several holograms may keep information about the same distribution of light, emitted to various directions. The numerical analysis of such holograms allows one to emulate large [[numerical aperture]], which, in turn, enables enhancement of the resolution of [[optical microscopy]]. The corresponding technique is called [[interferometric microscopy]]. Recent achievements of interferometric microscopy allow one to approach the quarter-wavelength limit of resolution.<ref name="U1">{{Cite journal
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| |url=http://www.opticsexpress.org/abstract.cfm?id=134719
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| | author=Y.Kuznetsova
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| | coauthors=A.Neumann, S.R.Brueck
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| | title=Imaging interferometric microscopy–approaching the linear systems limits of optical resolution
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| | journal=[[Optics Express]]
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| | volume=15
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| | pages=6651–6663
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| | year=2007
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| | doi=10.1364/OE.15.006651
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| |bibcode = 2007OExpr..15.6651K
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| | pmid=19546975
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| |issue=11}}</ref>
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| ===Sensors or biosensors===
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| {{Main|Holographic sensor}}
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| The hologram is made with a modified material that interacts with certain molecules generating a change in the fringe periodicity or refractive index, therefore, the color of the holographic reflection.<ref>{{cite journal |author= AK Yetisen, H Butt, F da Cruz Vasconcellos, Y Montelongo, CAB Davidson, J Blyth, JB Carmody, S Vignolini, U Steiner, JJ Baumberg, TD Wilkinson and CR Lowe |title=Light-Directed Writing of Chemically Tunable Narrow-Band Holographic Sensors. |journal= Advanced Optical Materials |year=2013 |doi= 10.1002/adom.201300375 }}</ref><ref>{{cite doi|10.1021/la102693m}}</ref>
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| ===Security===
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| {{Main|Security hologram}}
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| [[File:Hologramm.JPG|thumb|left|''Identigram'' as a security element in a German identity card]]
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| Security holograms are very difficult to forge, because they are [[holography#Mass replication of holograms|replicated]] from a master hologram that requires expensive, specialized and technologically advanced equipment. They are used widely in many [[currency|currencies]], such as the [[Brazilian real|Brazilian]] 20, 50, and 100-reais notes; [[Pound sterling|British]] 5, 10, and 20-pound notes; [[South Korean won|South Korean]] 5000, 10000, and 50000-won notes; [[Japanese yen|Japanese]] 5000 and 10000 yen notes; and all the currently-circulating banknotes of the [[Canadian dollar]], [[Danish krone]], and [[Euro]]. They can also be found in credit and bank cards as well as [[passport]]s, ID cards, [[book]]s, [[DVD]]s, and [[sports equipment]].
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| ===Other applications===
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| Holographic scanners are in use in post offices, larger shipping firms, and automated conveyor systems to determine the three-dimensional size of a package. They are often used in tandem with [[checkweigher]]s to allow automated pre-packing of given volumes, such as a truck or pallet for bulk shipment of goods.
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| Holograms produced in elastomers can be used as stress-strain reporters due to its elasticity and compressibility, the pressure and force applied are correlated to the reflected wavelength, therefore its color.<ref>'Elastic hologram' pages 113–117, Proc. of the IGC 2010, ISBN 978-0-9566139-1-2 here: http://www.dspace.cam.ac.uk/handle/1810/225960</ref>
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| ==Non-optical holography==
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| In principle, it is possible to make a hologram for any [[wave]].
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| [[Electron holography]] is the application of holography techniques to electron waves rather than light waves. Electron holography was invented by Dennis Gabor to improve the resolution and avoid the aberrations of the [[transmission electron microscope]]. Today it is commonly used to study electric and magnetic fields in thin films, as magnetic and electric fields can shift the phase of the interfering wave passing through the sample.<ref>R. E. Dunin-Borkowski et al., Micros. Res. and Tech. vol. 64, pp. 390–402 (2004)</ref> The principle of electron holography can also be applied to [[interference lithography]].<ref>K. Ogai et al., Jpn. J. Appl. Phys., vol. 32, pp.5988–5992 (1993)</ref>
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| [[Acoustic holography]] is a method used to estimate the sound field near a source by measuring acoustic parameters away from the source via an array of pressure and/or particle velocity transducers. Measuring techniques included within acoustic holography are becoming increasingly popular in various fields, most notably those of transportation, vehicle and aircraft design, and NVH. The general idea of acoustic holography has led to different versions such as near-field acoustic holography (NAH) and statistically optimal near-field acoustic holography (SONAH). For audio rendition, the wave field synthesis is the most related procedure.
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| ''Atomic holography'' has evolved out of the development of the basic elements of [[atom optics]]. With the Fresnel diffraction lens and [[atomic mirror (physics)|atomic mirrors]] atomic holography follows a natural step in the development of the physics (and applications) of atomic beams. Recent developments including [[atomic mirror (physics)|atomic mirrors]] and especially [[ridged mirror]]s have provided the tools necessary for the creation of atomic holograms,<ref name="holo">{{Cite journal| title = Reflection-Type Hologram for Atoms | author = F. Shimizu | coauthors=J.Fujita |date=March 2002 |journal=[[Physical Review Letters]] |volume=88 | issue = 12 |page=123201 | doi = 10.1103/PhysRevLett.88.123201 | pmid=11909457 | bibcode=2002PhRvL..88l3201S}}</ref> although such holograms have not yet been commercialized.
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| ==Things often confused with holograms==
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| Effects produced by [[lenticular printing]], the [[Pepper's Ghost]] illusion (or modern variants such as the [[Musion Eyeliner]]), [[tomography]] and [[volumetric displays]] are often confused with holograms.<ref>{{cite news|url=http://www.bbc.co.uk/news/business-12328160 |title=Holographic announcers at Luton airport |publisher=Bbc.co.uk |date=2011-01-31 |accessdate=2012-04-21}}</ref><ref>{{cite web|last=Farivar |first=Cyrus |url=http://arstechnica.com/science/news/2012/04/tupac-hologram-merely-pretty-cool-optical-illusion.ars |title=Tupac "hologram" merely pretty cool optical illusion |publisher=Arstechnica.com |date=2012-04-16 |accessdate=2012-04-21}}</ref>
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| The Pepper's ghost technique, being the easiest to implement of these methods, is most prevalent in 3D displays that claim to be (or are referred to as) "holographic". While the original illusion, used in theater, recurred to actual physical objects and persons, located offstage, modern variants replace the source object with a digital screen, which displays imagery generated with [[3D computer graphics]] to provide the necessary [[depth perception|depth cues]]. The reflection, which seems to float mid-air, is still flat, however, thus less realistic than if an actual 3D object was being reflected.
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| Examples of this digital version of Pepper's ghost illusion include the [[Gorillaz]] performances in the [[2005 MTV Europe Music Awards#Performances|2005 MTV Europe Music Awards]] and the [[48th Grammy Awards#Performances|48th Grammy Awards]]; and [[Tupac Shakur]]'s virtual performance at [[Coachella Valley Music and Arts Festival]] in 2012, rapping alongside [[Snoop Dogg]] during the latter's set with [[Dr. Dre]].<ref>{{cite news|url=http://marquee.blogs.cnn.com/2012/04/16/tupac-returns-as-a-hologram-at-coachella/ |title=Tupac returns as a hologram at Coachella |work=The Marquee Blog - CNN.com Blogs |publisher=CNN |date= 16 April 2012|accessdate=2012-04-21}}</ref>
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| During the [[United States presidential election, 2008|2008 American presidential election]], [[CNN]] debuted its tomograms to "beam in" correspondents including musician [[will.i.am]] as "holograms".
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| An even simpler illusion can be created by [[Video projector|rear-projecting]] realistic images into semi-transparent screens. The rear projection is necessary because otherwise the semi-transparency of the screen would allow the background to be illuminated by the projection, which would break the illusion.
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| [[Crypton Future Media]], a music software company that produced [[Hatsune Miku]],<ref>{{cite web|url=http://www.crypton.co.jp/mp/pages/prod/vocaloid/ |title=クリプトン | VOCALOID2 - キャラクター・ボーカル・シリーズ |publisher=Crypton.co.jp |date= |accessdate=2012-04-21}}</ref> one of many [[Vocaloid]] singing synthesizer applications, has produced concerts that have Miku, along with other Crypton Vocaloids, performing on stage as "holographic" characters. These concerts use rear projection onto a semi-transparent DILAD screen<ref>{{cite web |last = G. |first = Adrian |title = LA’s Anime Expo hosting Hatsune Miku’s first US live performance on July 2nd |url = http://www.kawaiikakkoiisugoi.com/2011/06/16/las-anime-expo-hosting-hatsune-mikus-first-us-live-performance-on-july-2nd/ |accessdate = 20 April 2012 }}</ref><ref>{{cite web|author= |url=https://www.youtube.com/watch?v=ZCYJu7KSqQM |title="We can invite Hatsune Miku in my room!", Part 2 (video) |publisher=Youtube.com |date=2011-09-07 |accessdate=2012-04-21}}</ref> to achieve its "holographic" effect.<ref>{{cite news |last = Firth |first = Niall |title = Japanese 3D singing hologram Hatsune Miku becomes nation's strangest pop star |url = http://www.dailymail.co.uk/sciencetech/article-1329040/Japanese-3D-singing-hologram-Hatsune-Miku-nations-biggest-pop-star.html |publisher = Daily mail online |accessdate = 29 April 2011 |location = London |date = 12 November 2010 }}</ref><ref>{{cite web |title = Techically incorrect: Tomorrow's Miley Cyrus? A hologram live in concert! |url = http://news.cnet.com/8301-17852_3-20022743-71.html |accessdate = 29 April 2011 }}</ref><ref>{{cite web |title = Hatsune Miku – World is Mine Live in HD |url = http://www.youtube.com/watch?v=DTXO7KGHtjI&feature=related |accessdate = 29 April 2011 }}</ref>
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| In 2011, in Beijing, apparel company [[Burberry]] produced the "Burberry Prorsum Autumn/Winter 2011 Hologram Runway Show", which included life size 2-D projections of models. The company's own video<ref>{{cite web|author=|url=https://www.youtube.com/watch?v=9t5dCIuz2wY |title=Burberry Beijing - Full Show |publisher=Youtube.com |date= |accessdate=2012-04-21}}</ref> shows several centered and off-center shots of the main 2-dimensional projection screen, the latter revealing the flatness of the virtual models. The claim that holography was used was reported as fact in the trade media.<ref>{{cite web |url = http://www.vogue.it/en/shows/fashion-events/2011/04/burberry-in-china |title = Burberry lands in China |accessdate = June 14, 2011 }}</ref>
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| ==Holography in fiction==
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| {{Main|Holography in fiction}}
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| Holography has been widely referred to in novels, TV and movies.
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| ==See also==
| |
| {{div col|colwidth=20em}}
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| * [[Australian Holographics]]
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| * [[Autostereoscopy]]
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| * [[Computer-generated holography]]
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| * [[Digital holography]]
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| * [[Digital planar holography]]
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| * [[Holographic principle]]
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| * [[Holonomic brain theory]]
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| * [[Integral imaging]]
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| * [[List of emerging technologies]]
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| * [[Phase-coherent holography]]
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| * [[Plasmon#Possible applications|Plasmon - Possible applications]] (Full Color Holography)
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| * [[Tomography]]
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| {{div col end}}
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| ==References==
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| {{Reflist|30em}}
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| ==Reference sources==
| |
| {{Refbegin}}
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| *Hariharan P, 1996, Optical Holography, Cambridge University Press, ISBN 0-521-43965-5
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| *Hariharan P, 2002, Basics of Holography, Cambridge University Press, ISBN 0-521-00200-1
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| *Lipson A., Lipson SG, Lipson H, Optical Physics, 2011, Cambridge University Press, ISBN 978-0-521-49345-1
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| {{Refend}}
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| ==Further reading==
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| {{Refbegin|30em}}
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| * ''Lasers and holography: an introduction to coherent optics'' W. E. Kock, Dover Publications (1981), ISBN 978-0-486-24041-1
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| * ''Principles of holography'' H. M. Smith, Wiley (1976), ISBN 978-0-471-80341-6
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| * G. Berger et al., ''Digital Data Storage in a phase-encoded holograhic memory system: data quality and security'', Proceedings of SPIE, Vol. 4988, p. 104–111 (2003)
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| * ''Holographic Visions: A History of New Science'' Sean F. Johnston, Oxford University Press (2006), ISBN 0-19-857122-4
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| * {{Cite book|title = Practical Holography, Third Edition|last = Saxby|first = Graham|year = 2003|publisher = Taylor and Francis|isbn = 978-0-7503-0912-7}}
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| * ''Three-Dimensional Imaging Techniques'' Takanori Okoshi, Atara Press (2011), ISBN 978-0-9822251-4-1
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| * ''Holographic Microscopy of Phase Microscopic Objects: Theory and Practice'' Tatyana Tishko, Tishko Dmitry, Titar Vladimir, World Scientific (2010), ISBN 13 978-981-4289-54-2
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| * [http://www.fractal.ae/hologram/ Hologram Technology] by Fractal Systems
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| {{Refend}}
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| ==External links==
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| {{Commons category|Holography}}
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| * [http://www.lasersec.in Leading Holographic Company in India]
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| * [http://www.leiadisplay.com Vendor of holographic screens]
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| * [http://www.ihma.org International Hologram Manufacturers Association]
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| * [http://nobelprize.org/physics/laureates/1971/gabor-autobio.html The Nobel prize lecture of Denis Gabor]
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| * [http://www.media.mit.edu/spi/ MIT's Spatial Imaging Group with papers about holographic theory and Holographic video]
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| * [http://www.integraf.com/a-holography_medical_applications.htm Medical Applications of Holograms]
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| * [http://science.howstuffworks.com/hologram.htm How Stuff Works – holograms]
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| * Walker, John. (1992) [http://www.artdesigncafe.com/holographic-art-1992 "Holographic Art"]. ''Glossary of Art, Architecture & Design since 1945'', 3rd. ed.
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| * [http://holocenter.org Center for the Holographic Arts, New York – a non-profit organisation promoting holography]
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| * [http://www.zintaglio.com Specular holography art site]
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| * [http://news.bbc.co.uk/2/hi/technology/7230258.stm Faster way to produce holographic tiles]
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| * [http://amasci.com/amateur/holo1.html Abrasion, hand-drawn holograms]
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| * [http://holoforum.org/forum Holoforum – A place to discuss holography]
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| * [http://qed.wikina.org/holography/ Animations demonstrating holography] by QED
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| * [http://www.smartfuturesolutions.net/hologram/ Smart Future Solutions Holographic display]
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| * {{US patent|3506327}} — "''Wavefront reconstruction using a coherent reference beam''" — E. N. Leith et al.
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| {{Display technology}}
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| {{photography subject}}
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| {{Emerging technologies}}
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| {{Stereoscopy}}
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| {{Use dmy dates|date=May 2011}}
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| [[Category:Holography| ]]
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| [[Category:British inventions]]
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| [[Category:Emerging technologies]]
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| [[Category:Hungarian inventions]]
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| [[Category:Laser image generation]]
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| [[Category:Photographic techniques]]
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| [[Category:3D imaging]]
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