View source for Holography

{{Short description|Recording to reproduce a three-dimensional light field}}
[[File:Holomouse2.jpg|thumb|Two photographs of a single hologram taken from different viewpoints]]

'''Holography''' is a technique that enables a [[Wavefront|wavefront]] to be recorded and later reconstructed. It is best known as a method of generating three-dimensional images, and has a wide range of other uses, including data storage, microscopy, and interferometry. In principle, it is possible to make a hologram for any type of [[Holography#Non-optical holography|wave]].

A '''hologram''' is a recording of an [[Physics:Wave interference|interference]] pattern that can reproduce a 3D [[Light field|light field]] using [[Physics:Diffraction|diffraction]]. In general usage, a hologram is a recording of any type of wavefront in the form of an interference pattern. It can be created by capturing [[Physics:Light|light]] from a real scene, or it can be generated by a computer, in which case it is known as a computer-generated hologram, which can show virtual objects or scenes. Optical holography needs a [[Physics:Laser|laser]] light to record the light field. The reproduced light field can generate an image that has the depth and [[Astronomy:Parallax|parallax]] of the original scene.<ref>{{Cite web |url=http://holocenter.org/what-is-holography |title=What is Holography? {{!}} holocenter |language=en-US |access-date=2019-09-02}}</ref> A hologram is usually unintelligible when viewed under [[Diffuse reflection|diffuse ambient light]]. When suitably lit, the interference pattern diffracts the light into an accurate reproduction of the original light field, and the objects that were in it exhibit visual [[Depth perception|depth cues]] such as [[Astronomy:Parallax|parallax]] and perspective that change realistically with the different angles of viewing. That is, the view of the image from different angles shows the subject viewed from similar angles.

A hologram is traditionally generated by overlaying a second wavefront, known as the reference beam, onto a wavefront of interest. This generates an interference pattern, which is then captured on a physical medium. When the recorded interference pattern is later illuminated by the second wavefront, it is diffracted to recreate the original wavefront.<ref name=Jesacher>{{Cite journal |last=Jesacher |first=Alexander |last2=Ritsch-Marte |first2=Monika |date=2016-01-02 |title=Synthetic holography in microscopy: opportunities arising from advanced wavefront shaping |url=http://www.tandfonline.com/doi/full/10.1080/00107514.2015.1120007 |journal=Contemporary Physics |language=en |volume=57 |issue=1 |pages=46–59 |doi=10.1080/00107514.2015.1120007 |issn=0010-7514}}</ref> The 3D image from a hologram can often be viewed with non-laser light. However, in common practice, major image quality compromises are made to remove the need for laser illumination to view the hologram.

A computer-generated hologram is created by digitally modeling and combining two wavefronts to generate an interference pattern image. This image can then be printed onto a mask or film and illuminated with an appropriate light source to reconstruct the desired wavefront.<ref name=Jesacher /> Alternatively, the interference pattern image can be directly displayed on a dynamic holographic display.<ref>{{Cite journal |last=Sahin |first=Erdem |last2=Stoykova |first2=Elena |last3=Mäkinen |first3=Jani |last4=Gotchev |first4=Atanas |date=2020-03-20 |title=Computer-Generated Holograms for 3D Imaging: A Survey |url=https://trepo.tuni.fi//bitstream/handle/10024/127486/ACM_CSUR_Sahin_revised_submitted.pdf |journal=ACM Computing Surveys |volume=53 |issue=2 |pages=32:1–32:35 |doi=10.1145/3378444 |issn=0360-0300}}</ref>

Holographic portraiture often resorts to a non-holographic intermediate imaging procedure, to avoid the dangerous high-powered [[Physics:Laser#Pulsed operation|pulsed lasers]] which would be needed to optically "freeze" moving subjects as perfectly as the extremely motion-intolerant holographic recording process requires. Early holography required high-power and expensive lasers. Currently, mass-produced low-cost [[Engineering:Laser diode|laser diode]]s, such as those found on [[Engineering:DVD recorder|DVD recorder]]s and used in other common applications, can be used to make holograms. They have made holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.

Most holograms produced are of static objects, but systems for displaying changing scenes on dynamic holographic displays are now being developed.<ref>{{cite journal |last1 = Blanche |first1 = P.-A. |year =2010 |title =  Holographic three-dimensional telepresence using large-area photorefractive polymer |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. |last10 = Kathaperumal |first10 = M. |last11 = Rachwal |first11 = B. |last12 = Siddiqui |first12 = O. |last13 = Thomas |first13 = J. |last14 = Norwood |first14 = R. A. |last15 = Yamamoto |first15 = M. |last16 = Peyghambarian |first16 = N. |pmid = 21048763 |bibcode = 2010Natur.468...80B |s2cid = 205222841 |display-authors = 8 }}</ref><ref>{{Cite journal |last1=Smalley |first1=D. E. |last2=Nygaard |first2=E. |last3=Squire |first3=K. |last4=Van Wagoner |first4=J. |last5=Rasmussen |first5=J. |last6=Gneiting |first6=S. |last7=Qaderi |first7=K. |last8=Goodsell |first8=J. |last9=Rogers |first9=W. |last10=Lindsey |first10=M. |last11=Costner |first11=K. |last12=Monk |first12=A. |last13=Pearson |first13=M. |last14=Haymore |first14=B. |last15=Peatross |first15=J. |date=2018-01-25 |title=A photophoretic-trap volumetric display |url=https://www.nature.com/articles/nature25176 |journal=Nature |language=en |volume=553 |issue=7689 |pages=486–490 |doi=10.1038/nature25176 |pmid=29368704 |bibcode=2018Natur.553..486S |s2cid=4451867 |issn=1476-4687|doi-access=free }}</ref>

The word ''holography'' comes from the [[Social:Greek language|Greek]] words {{lang|grc|ὅλος}} (''holos''; "whole") and {{lang|grc|γραφή}} (''graphē''; "[[Social:Writing|writing]]" or "drawing").

==History==
[[File:IntroductionToHolography1972.ogv|thumb|thumbtime=40|''Introduction to Holography'' (1972 educational film)]]
Holography was invented in 1948 by the Hungarian-British physicist [[Biography:Dennis Gabor|Dennis Gabor]]. He invented holography during the process of attempting to improve [[Image resolution|image resolution]] in [[Engineering:Electron microscope|electron microscope]]s.<ref>{{cite journal |last1 = Gabor |first1 = Dennis |year = 1948 |title = A new microscopic principle |journal = Nature |volume = 161 |issue = 4098 |pages = 777–8 |doi=10.1038/161777a0 |bibcode = 1948Natur.161..777G |pmid=18860291 |s2cid = 4121017 |doi-access = free }}</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 |volume = 197 |pages = 454–487 |issue = 1051 |bibcode = 1949RSPSA.197..454G |s2cid = 123187722 |doi-access = free}}</ref>

Gabor's work 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.<ref name="Hariharan 1996">{{cite book |last1=Hariharan |first1=P. |title=Optical Holography |date=1996 |publisher=Cambridge University Press |location=Cambridge |isbn=9780521433488}}</ref> This discovery was an unexpected result of research into improving [[Engineering:Electron microscope|electron microscope]]s at the [[Company:British Thomson-Houston|British Thomson-Houston]] Company (BTH)  in Rugby, England, and the company filed a [[Social:Patent|patent]] in December 1947 (patent GB685286). The technique as originally invented is still used in [[Physics:Electron microscopy|electron microscopy]], where it is known as [[Physics:Electron holography|electron holography]], but optical holography did not really advance until the development of the [[Physics:Laser|laser]] in 1960.

Gabor was awarded the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method".<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1971/ |title=The Nobel Prize in Physics 1971 |publisher=Nobelprize.org |access-date=2012-04-21}}</ref>

[[File:III-BIBI BEI BOB.jpg|thumb|Horizontal symmetric text, by Dieter Jung]]
The development of the [[Physics:Laser|laser]] enabled the first practical optical holograms that recorded 3D objects to be made in 1962 by [[Biography:Yuri Denisyuk|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 
|last = Denisyuk 
|first = Yuri N. 
|journal = Doklady Akademii Nauk SSSR 
|volume = 144
|pages = 1275–1278
|year = 1962
|issue = 6
}}</ref> and by Emmett Leith and [[Biography:Juris Upatnieks|Juris Upatnieks]] at the [[Organization:University of Michigan|University of Michigan]], US.<ref name="leith">{{Cite journal 
|title = Reconstructed wavefronts and communication theory
|author = Leith, E.N.
|author2=Upatnieks, J.
|journal = J. Opt. Soc. Am. 
|volume = 52
|pages = 1123–1130
|year = 1962
|doi =10.1364/JOSA.52.001123| issue = 10
|bibcode = 1962JOSA...52.1123L
}}</ref> Early holograms used [[Physics:Silver halide|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>{{cite journal |last1 = Upatnieks |first1 = J |last2 = Leonard |first2 = C |year = 1969 |title = Diffraction efficiency of bleached, photographically recorded interference patterns |journal = Applied Optics |volume = 8 |issue = 1 |pages = 85–89 |doi = 10.1364/ao.8.000085 |pmid = 20072177 |bibcode = 1969ApOpt...8...85U }}</ref><ref>{{cite journal |last1 = Graube |first1 = A |year = 1974 |title = Advances in bleaching methods for photographically recorded holograms |journal = Applied Optics |volume = 13 |issue = 12 |pages = 2942–6 |doi = 10.1364/ao.13.002942 |pmid = 20134813 |bibcode = 1974ApOpt..13.2942G }}</ref><ref>{{cite journal |last1 = Phillips |first1 = N. J. |last2 = Porter |first2 = D. |year = 1976 |title = An advance in the processing of holograms |journal = Journal of Physics E: Scientific Instruments |volume = 9 |issue = 8 |page = 631 |doi = 10.1088/0022-3735/9/8/011 |bibcode = 1976JPhE....9..631P }}</ref>

==Basics of holography==
[[File:Holograph-record.svg|thumb|400px|Recording a hologram]]
[[File:Holography-reconstruct.svg|thumb|300px|Reconstructing a hologram]]
[[File:Structure of a holographic recording.jpg|thumb|This is a photograph of a small part of an unbleached transmission hologram viewed through a microscope. The hologram recorded an image of a toy van and car. It is no more possible to discern the subject of the hologram from this pattern than it is to identify what music has been recorded by looking at a [[Engineering:Compact disc|CD]] surface. The holographic information is recorded by the speckle pattern.]]

Holography is a technique for recording and reconstructing light fields.<ref name="Hariharan basics">{{cite book |last1=Hariharan |first1=P |title=Basics of Holography |date=2002 |publisher=Cambridge University Press |location=Cambridge|isbn =  9780511755569}}</ref>{{rp| Section 1}} 
A light field is generally the result of a light source scattered off objects. Holography can be thought of as somewhat similar to sound recording, whereby a sound field created by vibrating matter like [[Engineering:Musical instrument|musical instrument]]s or [[Medicine:Vocal cords|vocal cords]], is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.<ref>{{Cite book|last=Richards|first=Keith L.|url=https://www.worldcat.org/oclc/990152205|title=Design engineer's sourcebook|date=2018|isbn=978-1-315-35052-3|location=Boca Raton|oclc=990152205}}</ref> However, it is even more similar to [[Engineering:Ambisonics|Ambisonic]] sound recording in which any listening angle of a sound field can be reproduced in the reproduction.

===Laser===
In laser holography, the hologram is recorded using a source of [[Physics:Laser|laser]] light, which is very pure in its color and orderly in its composition. Various setups may be used, and several types of holograms can be made, but all involve the interaction of light coming from different directions and producing a microscopic interference pattern which a [[Physics:Photographic plate|plate]], film, or other medium [[Photography|photographically]] records.

In one common arrangement, the laser beam is split into two, one known as the [[Physics:Signal beam|object beam]]<!-- the traditional term, despite the redirect that results --> and the other as the [[Physics:Reference beam|reference beam]]. The object beam is expanded by passing it through a lens and used to illuminate the subject. The recording medium is located where this light, after being reflected or scattered by the subject, will strike it. The edges of the medium will ultimately serve as a window through which the subject is seen, so its location is chosen with that in mind. The reference beam is expanded and made to shine directly on the medium, where it interacts with the light coming from the subject to create the desired interference pattern.

Like conventional photography, holography requires an appropriate [[Physics:Exposure (photography)|exposure]] time to correctly affect the recording medium. Unlike conventional photography, during the exposure the light source, the optical elements, the recording medium, and the subject must all remain  motionless relative to each other, to within about a quarter of the wavelength of the light, or the interference pattern will be blurred and the hologram spoiled. With living subjects and some unstable materials, that is only possible if a very intense and extremely brief pulse of laser light is used, a hazardous procedure which is rarely done outside of scientific and industrial laboratory settings. Exposures lasting several seconds to several minutes, using a much lower-powered continuously operating laser, are typical.

===Apparatus===
A hologram can be made by shining part of the light beam directly into 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. 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 [[Physics:Beam splitter|beam splitter]] that divides the beam into two identical beams, each aimed in different directions:
* One beam (known as the 'illumination' or 'object beam') is spread using [[Physics:Lens (optics)|lenses]] and directed onto the scene using [[Physics:Mirror|mirror]]s. Some of the light scattered (reflected) from the scene then falls onto the recording medium.
* The second beam (known as the 'reference beam') is also spread through the use of lenses, but is directed so that it does not come in contact with the scene, and instead travels directly onto the recording medium.

Several different materials can be used as the recording medium. One of the most common is a film very similar to [[Engineering:Photographic film|photographic film]] ([[Physics:Silver halide|silver halide]] [[Physics:Photographic emulsion|photographic emulsion]]), but with much smaller light-reactive grains (preferably with diameters less than 20&nbsp;nm), making it capable of the much higher [[Physics: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.

===Process===
When the two laser beams reach the recording medium, their light waves intersect and 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.

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 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.

===Comparison with photography===
Holography may be better understood via an examination of its differences from ordinary [[Photography|photography]]:
* 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.
* A photograph can be recorded using normal light sources (sunlight or electric lighting) whereas a laser is required to record a hologram.
* 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.
* A holographic recording requires a second light beam (the reference beam) to be directed onto the recording medium.
* A photograph can be viewed in a wide range of lighting conditions, whereas holograms can only be viewed with very specific forms of illumination.
* 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 {{convert|4|x|4|ft|cm|abbr=on|order=flip}} window, then through a {{convert|2|x|4|ft|cm|-1|abbr=on|order=flip}} 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 {{convert|4|ft|cm|abbr=on|order=flip}} window.
* A photographic [[Stereoscopy|stereogram]] is a two-dimensional representation that can produce a three-dimensional effect but only from one point of view, 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 [[Biology:Human brain|human brain]] and translated into the same perception of a three-dimensional image as when the original scene might have been viewed.
* 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.

==Physics of holography==

{{Main|Physics:Physics of optical holography}}
For a better understanding of the process, it is necessary to understand interference and diffraction. Interference occurs when one or more wavefronts are superimposed. Diffraction occurs when 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 give an understanding of how the holographic process works.

For those unfamiliar with these concepts, it is worthwhile to read those articles before reading further in this article.

===Plane wavefronts===
A [[Physics:Diffraction grating|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 that is 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.

A simple hologram can be made by superimposing two [[Physics:Plane wave|plane wave]]s from the same light source on a holographic recording medium. The two waves interfere, giving a 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 by the wavelength of the light.

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 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.

===Point sources===
[[File:Zonenplatte Cosinus.png|upright|thumb|Sinusoidal zone plate]]

If the recording medium is illuminated with a point source and a normally incident plane wave, the resulting pattern is a [[Physics:Zone plate|sinusoidal zone plate]], which acts as a negative [[Physics:Fresnel lens|Fresnel lens]] whose focal length is equal to the separation of the point source and the recording plane.

When a plane wave-front illuminates a negative lens, it is expanded into a wave that 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 spherical wave; a holographic recording of the point source has been created.

When the plane wave is incident at a non-normal angle at the time of recording, the pattern formed is more complex, but still acts as a negative lens if it is illuminated at the original angle.

===Complex objects===
To record a hologram of a complex object, a laser beam is first split into two 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.

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 ([[Physics:Speckle pattern|speckle]]) pattern as in the photograph above.

When the hologram is illuminated by the original reference beam, each of the individual zone plates reconstructs the object wave that produced it, and these individual wavefronts are combined to reconstruct the whole of the object beam. The viewer perceives a wavefront that is identical with the wavefront scattered from the object onto the recording medium, so that it appears that the object is still in place even if it has been removed.

==Applications==

===Art===
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.

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 |access-date=2012-04-21}}</ref> In Great Britain, Margaret Benyon began using holography as an artistic medium in the late 1960s and had a solo exhibition at the [[Organization:University of Nottingham|University of Nottingham]] art gallery in 1969.<ref name=Coyle>{{Cite book| publisher = John Libbey and Company| isbn = 978-0-86196-266-2| pages = 65–88|editor-first1= Philip |editor-last1= Hayward | last = Coyle| first = Rebecca| title = Culture, Technology & Creativity in the Late Twentieth Century| chapter = Holography – Art in the space of technology| location = London, England| date = 1990| chapter-url = https://books.google.com/books?id=yLq3rM2at3cC&pg=PA67}}</ref> This was followed in 1970 by a solo show at the Lisson Gallery in London, which was billed as the "first London expo of holograms and stereoscopic paintings".<ref name="poster">{{cite web|title=Margaret Benyon Holography|url=http://www.lissongallery.com/exhibitions/margaret-benyon|website=Lisson Gallery|access-date=4 February 2016}}</ref>

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 [[Biography:Lloyd Cross|Lloyd Cross]], The Museum of Holography in New York founded by Rosemary (Posy) H. Jackson, the Royal College of Art in London and the [[Organization:Lake Forest College|Lake Forest College]] Symposiums organised by [[Biography:Tung Jeong|Tung Jeong]].<ref>{{cite web|author=Integraf |url=http://www.integraf.com/tung_jeong.htm |title=Dr. Tung J. Jeong Biography |publisher=Integraf.com |access-date=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 |access-date=2012-04-21}}</ref> and the HOLOcenter in Seoul, which offers artists a place to create and exhibit work.

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 United States, Dieter Jung of {{wipe|Germany}}, and [[Biography:Moysés Baumstein|Moysés Baumstein]] of {{wipe|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 [[Biography:Moysés Baumstein|Moysés Baumstein]]) found in holography a way to express themselves and to renew Concrete Poetry.

A small but active group of artists still integrate holographic elements into their work.<ref>{{cite web |url=http://www.universal-hologram.com/ |title=The Universal Hologram |website=Cherry Optical Holography}}</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|specular holography]].

The MIT Museum<ref>{{cite web|url=http://web.mit.edu/museum/collections/holography.html |title=MIT Museum: Collections – Holography |publisher=Web.mit.edu |access-date=2012-04-21}}</ref> and Jonathan Ross<ref>{{cite web|url=http://www.jrholocollection.com/ |title=The Jonathan Ross Hologram Collection |publisher=Jrholocollection.com |access-date=2012-04-21}}</ref> both have extensive collections of holography and on-line catalogues of art holograms.

===Data storage===
{{Main|Holographic data storage}}
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 medium 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.
Currently available [[Physics:Spatial light modulator|SLMs]] can produce about 1000 different images a second at 1024×1024-bit resolution which would result in about one-gigabit-per-second writing speed.<ref>{{Cite journal |last1=Lang |first1=M. |last2=Eschler |first2=H. |date=1974-10-01 |title=Gigabyte capacities for holographic memories |url=https://dx.doi.org/10.1016/0030-3992%2874%2990061-9 |journal=Optics & Laser Technology |language=en |volume=6 |issue=5 |pages=219–224 |doi=10.1016/0030-3992(74)90061-9 |bibcode=1974OptLT...6..219L |issn=0030-3992}}</ref>

In 2005, companies such as Optware and Maxell produced a 120&nbsp;mm disc that uses a holographic layer to store data to a potential 3.9&nbsp;[[Terabyte|TB]], a format called [[Holographic Versatile Disc]]. As of September 2014, no commercial product has been released.

Another company, [[Engineering:InPhase Technologies|InPhase Technologies]], was developing a competing format, but went bankrupt in 2011 and all its assets were sold to Akonia Holographics, LLC.

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 [[Physics:3D optical data storage|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.

===Dynamic holography===
In static holography, recording, developing and reconstructing occur sequentially, and a permanent hologram is produced.

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 mirrors ("time-reversal" of light), optical cache memories, [[Image processing|image processing]] (pattern recognition of time-varying images), and [[Optical computing|optical computing]].

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|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>

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 photorefractive crystals, but in [[Physics:Semiconductor|semiconductor]]s or [[Physics:Heterojunction|semiconductor heterostructures]] (such as [[Physics:Quantum well|quantum well]]s), atomic vapors and gases, [[Physics:Plasma|plasmas]] and even liquids, it was possible to generate holograms.

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).

===Hobbyist use===
[[File:Contest3.jpg|thumb|''Peace Within Reach'', a Denisyuk DCG hologram by amateur Dave Battin]]

Since the beginning of holography, many holographers have explored its uses and displayed them to the public.

In 1971, [[Biography:Lloyd Cross|Lloyd Cross]] opened the San Francisco School of Holography and taught amateurs how to make holograms using only a small (typically 5&nbsp;mW) helium-neon laser and inexpensive home-made equipment. Holography had been supposed to require a very expensive metal [[Physics:Optical table|optical table]] set-up to lock all the involved elements down in place and damp any vibrations that could blur the interference fringes and ruin the hologram. Cross's home-brew alternative was a sandbox made of a cinder block retaining wall on a plywood base, supported on stacks of old tires to isolate it from ground vibrations, and filled with sand that had been washed to remove dust. The laser was securely mounted atop the cinder block wall. The mirrors and simple lenses needed for directing, splitting and expanding the laser beam were affixed to short lengths of PVC pipe, which were stuck into the sand at the desired locations. The subject and the [[Physics:Photographic plate|photographic plate]] holder were similarly supported within the sandbox. The holographer turned off the room light, blocked the laser beam near its source using a small [[Engineering:Relay|relay]]-controlled shutter, loaded a plate into the holder in the dark, left the room, waited a few minutes to let everything settle, then made the exposure by remotely operating the laser shutter.

In 1979, Jason Sapan opened the [[Physics:Holographic Studios|Holographic Studios]] in {{wipe|New York City}}. Since then, they have been involved in the production of many holographs for many artists as well as companies.<ref>{{cite news | url=https://www.thedailybeast.com/new-yorks-hologram-king-is-also-the-citys-last-pro-holographer | title=New York's Hologram King is Also the City's Last Pro Holographer | newspaper=The Daily Beast | date=27 May 2014 | last1=Strochlic | first1=Nina }}</ref> Sapan has been described as the "last professional holographer of New York".

Many of these holographers would go on to produce art holograms. In 1983, Fred Unterseher, a co-founder of the San Francisco School of Holography and a well-known holographic artist, published the ''Holography Handbook'', an easy-to-read guide to making holograms at home. This brought in a new wave of holographers and provided simple methods for using the then-available AGFA [[Physics:Silver halide|silver halide]] recording materials.

In 2000, Frank DeFreitas published the ''Shoebox Holography Book'' and introduced the use of inexpensive [[Physics:Laser pointer|laser pointer]]s to countless hobbyists. For many years, it had been assumed that certain characteristics of semiconductor laser diodes made them virtually useless for creating holograms, but when they were eventually put to the test of practical experiment, it was found that not only was this untrue, but that some actually provided a [[Coherence length|coherence length]] much greater than that of traditional helium-neon gas lasers. This was a very important development for amateurs, as the price of red laser diodes had dropped from hundreds of dollars in the early 1980s to about $5 after they entered the mass market as a component of [[Engineering:DVD|DVD]] players in the late 1990s. Now, there are thousands of amateur holographers worldwide.

By late 2000, holography kits with inexpensive laser pointer diodes entered the mainstream consumer market. These kits enabled students, teachers, and hobbyists to make several kinds of holograms without specialized equipment, and became popular gift items by 2005.<ref name="IEEE">Stephen Cass: ''[https://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 [[Physics:Photographic plate|plates]] in 2003 made it possible for hobbyists to create holograms without the bother of wet chemical processing.<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)] {{webarchive|url=https://web.archive.org/web/20120208030253/http://www.litiholo.com/Hologram%20Kit%20article%20Physics%20Teacher%20Nov%202010.pdf |date=8 February 2012 }}''. In ''The Physics Teacher'', vol. 48, November 2010, pp. 551–552.</ref>

In 2006, a large number of surplus holography-quality green lasers (Coherent C315) became available and put dichromated gelatin (DCG) holography 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 this sensitivity would be uselessly slight or 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 |access-date=2012-04-21 |url-status=dead |archive-url=https://web.archive.org/web/20101106135632/http://www.holowiki.com/index.php/G307_DCG_Formula |archive-date=6 November 2010}}</ref>

Kodak and Agfa, the former major suppliers of holography-quality silver halide plates and films, are no longer in the market. While other manufacturers have helped fill the void, many amateurs are now making their own materials. 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 these 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 |access-date=2012-04-21 |url-status=dead |archive-url=https://web.archive.org/web/20120307232834/http://www.holowiki.com/index.php/Special%3ASearch?search=Blyth&go=Go |archive-date=7 March 2012}}</ref>

A small group of amateurs are even constructing their own pulsed lasers to make holograms of living subjects and other unsteady or moving objects.<ref>{{cite web|url=http://cabd0.tripod.com/holograms/index.html |title=Jeff Blyth's Film Formulations |publisher=Cabd0.tripod.com |access-date=2012-04-21}}</ref>

===Holographic interferometry===
{{Main|Physics:Holographic interferometry}}

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>{{cite journal | last1 = Powell | first1 = RL | last2 = Stetson | first2 = KA | year = 1965 | title =  Interferometric Vibration Analysis by Wavefront Reconstruction| journal = J. Opt. Soc. Am. | volume = 55 | issue = 12 | pages = 1593–8 | doi = 10.1364/josa.55.001593 | bibcode = 1965JOSA...55.1593P }}</ref><ref>{{cite book |last1=Jones |first1=Robert |last2=Wykes |first2=Catherine |title=Holographic and Speckle Interferometry |date=1989 |publisher=Cambridge University Press |location=Cambridge |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 or the isodose regions in radiation dosimetry.<ref>{{cite journal|last1=Beigzadeh |first1=A.M. |last2=Vaziri |first2=M.R. Rashidian |last3=Ziaie |first3=F. |title=Modelling of a holographic interferometry based calorimeter for radiation dosimetry|journal=Nuclear Instruments and Methods in Physics Research A|date=2017|volume=864|pages=40–49|doi=10.1016/j.nima.2017.05.019|bibcode=2017NIMPA.864...40B}}</ref>

It has been widely used to measure stress, strain, and vibration in engineering structures.

===Interferometric microscopy===
{{Main|Physics:Interferometric microscopy}}

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|numerical aperture]], which, in turn, enables enhancement of the resolution of optical microscopy. The corresponding technique is called [[Physics:Interferometric microscopy|interferometric microscopy]]. Recent achievements of interferometric microscopy allow one to approach the quarter-wavelength limit of resolution.<ref name="U1">{{Cite journal
| author=Y.Kuznetsova
|author2=A.Neumann, S.R.Brueck
| title=Imaging interferometric microscopy–approaching the linear systems limits of optical resolution
| journal=[[Physics:Optics Express|Optics Express]]
| volume=15
| pages=6651–6663
| year=2007
| doi=10.1364/OE.15.006651
|bibcode = 2007OExpr..15.6651K
| pmid=19546975
|issue=11| doi-access=free
}}</ref>

===Sensors or biosensors===
{{Main|Physics:Holographic sensor}}

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 |first1=AK |last1=Yetisen |first2=H |last2=Butt |first3=F |last3=da Cruz Vasconcellos |first4=Y |last4=Montelongo |first5=CAB |last5=Davidson |first6=J |last6=Blyth |first7=JB |last7=Carmody |first8=S |last8=Vignolini |first9=U |last9=Steiner |first10=JJ |last10=Baumberg |first11=TD |last11=Wilkinson |first12=CR |last12=Lowe |title=Light-Directed Writing of Chemically Tunable Narrow-Band Holographic Sensors |journal= Advanced Optical Materials |volume=2 |issue=3 |pages=250–254 |year=2013 |doi= 10.1002/adom.201300375 |s2cid=96257175 |url=https://www.repository.cam.ac.uk/handle/1810/293246 }}</ref><ref>{{Cite journal | last1 = MartíNez-Hurtado | first1 = J. L. | last2 = Davidson | first2 = C. A. B. | last3 = Blyth | first3 = J. | last4 = Lowe | first4 = C. R. | title = Holographic Detection of Hydrocarbon Gases and Other Volatile Organic Compounds | doi = 10.1021/la102693m | journal = Langmuir | volume = 26 | issue = 19 | pages = 15694–15699 | year = 2010 | pmid =  20836549}}</ref>

===Security===
{{Main|Security hologram}}
[[File:Hologramm.JPG|thumb|left|''Identigram'' as a security element in a German identity card]]
[[File:Visa Dove Hologram Rainbow.jpg|thumb|Dove hologram used on some [[Finance:Credit card|credit card]]s]]
Holograms are commonly used for security, as they are replicated from a master hologram that requires expensive, specialized and technologically advanced equipment, and are thus difficult to forge. They are used widely in many [[Finance:Currency|currencies]], such as the [[Finance:Brazilian real|Brazilian]] 20, 50, and 100-reais notes; [[Finance:Pound sterling|British]] 5, 10, 20 and 50-pound notes; South Korean 5000, 10,000, and 50,000-won notes; [[Finance:Japanese yen|Japanese]] 5000 and 10,000 yen notes, Indian 50, 100, 500, and 2000 rupee notes; and all the currently-circulating banknotes of the [[Finance:Canadian dollar|Canadian dollar]], [[Finance:Croatian kuna|Croatian kuna]], [[Finance:Danish krone|Danish krone]], and [[Finance:Euro|Euro]]. They can also be found in [[Finance:Credit card|credit]] and [[Finance:Bank card|bank card]]s as well as passports, ID cards, books, food packaging, [[Engineering:DVD|DVD]]s, and sports equipment. Such holograms come in a variety of forms, from adhesive strips that are laminated on packaging for fast-moving consumer goods to holographic tags on [[Engineering:Consumer electronics|electronic products]]. They often contain textual or pictorial elements to protect identities and separate genuine articles from counterfeits.

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 checkweighers to allow automated pre-packing of given volumes, such as a truck or pallet for bulk shipment of goods.
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> Holography technique can also be effectively used for radiation dosimetry.<ref>{{cite journal |last1=Beigzadeh |first1=A.M. |title=Modelling of a holographic interferometry based calorimeter for radiation dosimetry |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=864 |pages=40–49 |doi=10.1016/j.nima.2017.05.019 |year=2017 |bibcode=2017NIMPA.864...40B }}</ref><ref>{{cite journal |last1=Beigzadeh |first1=A.M. |title=Double-exposure holographic interferometry for radiation dosimetry: A new developed model |journal=Radiation Measurements |date=2018 |volume=119 |pages=132–139 |doi=10.1016/j.radmeas.2018.10.010 |bibcode=2018RadM..119..132B |s2cid=105842469 }}</ref>

==== High security registration plates ====
High-security holograms can be used on license plates for vehicles such as cars and motorcycles. As of April 2019, holographic license plates are required on vehicles in parts of India to aid in identification and security, especially in cases of car theft. Such number plates hold electronic data of vehicles, and have a unique ID number and a sticker to indicate authenticity.<ref>{{cite web |title=Why has the government made high security registration plates mandatory |url=https://economictimes.indiatimes.com/industry/auto/auto-news/why-has-the-government-made-high-security-registration-plates-mandatory/what-is-hsrp/slideshow/79936271.cms |website=The Economic Times |publisher=ET Online |access-date=18 July 2021}}</ref>

==Holography using other types of waves==
In principle, it is possible to make a hologram for any [[Wave|wave]].

[[Physics:Electron holography|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>{{cite journal | last1 = Ogai | first1 = K. | display-authors = etal | year = 1993 | title =  An Approach for Nanolithography Using Electron Holography| journal = Jpn. J. Appl. Phys. | volume = 32 | issue = 12S | pages = 5988–5992 | doi = 10.1143/jjap.32.5988 | bibcode = 1993JaJAP..32.5988O | s2cid = 123606284 }}</ref>

[[Physics:Acoustic holography|Acoustic holography]] enables sound maps of an object to be generated.  Measurements of the acoustic field are made at many points close to the object.  These measurements are digitally processed to produce the “images” of the object.<ref>{{cite web |title=Acoustic Holography |url=https://www.bksv.com/en/knowledge/applications/noise-source-identification/acoustic-holography |website=Bruel and Kjaer |access-date=3 September 2022}}</ref>

Atomic holography has evolved out of the development of the basic elements of [[Physics:Atom optics|atom optics]]. With the Fresnel diffraction lens and atomic mirrors atomic holography follows a natural step in the development of the physics (and applications) of atomic beams. Recent developments including atomic mirrors and especially [[Physics:Ridged mirror|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 |author2=J.Fujita |date=March 2002 |journal=[[Physics:Physical Review Letters|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.

[[Physics:Neutron|Neutron]] beam holography has been used to see the inside of solid objects.<ref>{{Cite news|url=https://www.nist.gov/news-events/news/2016/10/move-over-lasers-scientists-can-now-create-holograms-neutrons-too|title=Move Over, Lasers: Scientists Can Now Create Holograms from Neutrons, Too|last=Swenson|first=Gayle|date=2016-10-20|work=NIST|access-date=2017-04-04|language=en}}</ref>

Holograms with x-rays are generated by using [[Physics:Synchrotron|synchrotron]]s or x-ray [[Physics:Free-electron laser|free-electron laser]]s as radiation sources and pixelated detectors such as [[Engineering:Charge-coupled device|CCDs]] as recording medium.<ref>{{cite journal | last1 = Eisebitt | first1 = S. | display-authors = etal   | year = 2004 | title = Lensless imaging of magnetic nanostructures by X-ray spectro-holography  | url = https://zenodo.org/record/1233277| journal = Nature | volume = 432 | issue = 7019| pages = 885–888 | doi = 10.1038/nature03139 |bibcode = 2004Natur.432..885E | pmid=15602557| s2cid = 4423853 }}</ref> The reconstruction is then retrieved via computation. Due to the shorter wavelength of [[Physics:X-ray|x-ray]]s compared to visible light, this approach allows imaging objects with higher spatial resolution.<ref>{{cite journal | last1 = Pfau | first1 = B. | display-authors = etal   | year = 2014 | title =Influence of stray fields on the switching-field distribution for bit-patterned media based on pre-patterned substrates  | url =https://hal.archives-ouvertes.fr/hal-01282859/file/Pfau_APL_2014.pdf | journal = Applied Physics Letters | volume = 105 | issue = 13| page = 132407 | doi = 10.1063/1.4896982 |bibcode = 2014ApPhL.105m2407P | s2cid = 121512138 }}</ref> As [[Physics:Free-electron laser|free-electron laser]]s can provide ultrashort and x-ray pulses in the range of [[Femtosecond|femtosecond]]s which are intense and coherent, x-ray holography has been used to capture ultrafast dynamic processes.<ref>{{cite journal | last1 = Chapman | first1 = H. N. | display-authors = etal   | year = 2007 | title = Femtosecond time-delay X-ray holography | url = http://bib-pubdb1.desy.de//record/83807/files/Nature-merged.pdf| journal = Nature | volume = 448 | issue = 7154| pages = 676–679 | doi = 10.1038/nature06049 |bibcode = 2007Natur.448..676C | pmid=17687320| s2cid = 4406541 }}</ref><ref>{{cite journal | last1 = Günther | first1 = C.M. | display-authors = etal   | year = 2011 | title = Sequential femtosecond X-ray imaging  | journal = Nature Photonics | volume = 5 | issue = 2| pages = 99–102 | doi = 10.1038/nphoton.2010.287 |bibcode = 2011NaPho...5...99G }}</ref><ref>{{cite journal | last1 = von Korff | first1 = Schmising | year = 2014 | title = Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation | url = http://bib-pubdb1.desy.de/record/169124/files/DESY-2014-02806.pdf| journal = Physical Review Letters | volume = 112 | issue = 21| page = 217203 | doi = 10.1103/PhysRevLett.112.217203 | bibcode=2014PhRvL.112u7203V}}</ref>

==False holograms==

[[File:Pyramid_holographic_3D_holographic_projection_phone_projector_3D_holographic_projection_3D_mobile_phone_naked_eye_3D_pyramid.jpg|thumb|A Pepper's ghost illusion made from a clear plastic [[Frustum|frustum]]]]

[[File:Hatsune Miku - Tell Your World (Live @ Anime Friends 2017).webm|thumb|Shows making using of projected images are erroneously marketed as "holographic"]]

There are many optical effects that are falsely confused with holography, such as the effects produced by [[Lenticular printing|lenticular printing]], the [[Physics:Pepper's ghost|Pepper's ghost]] illusion (or modern variants such as the [[Physics:Musion Eyeliner|Musion Eyeliner]]), [[Physics:Tomography|tomography]] and volumetric displays.<ref>{{cite news|url=https://www.bbc.co.uk/news/business-12328160 |title=Holographic announcers at Luton airport |publisher=Bbc.co.uk |date=2011-01-31 |access-date=2012-04-21}}</ref><ref>{{cite web|last=Farivar |first=Cyrus |url=https://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 |access-date=2012-04-21}}</ref> Such illusions have been called "fauxlography".<ref>{{cite web |url=https://light2015blog.org/2015/09/28/holographic-3d-technology-from-sci-fi-fantasy-to-engineering-reality/ |title=Holographic 3D Technology: From Sci-fi Fantasy to Engineering Reality |date=2015-09-28 |website=International Year of Light 2015 - Blog |archive-url=https://web.archive.org/web/20171030003249/https://light2015blog.org/2015/09/28/holographic-3d-technology-from-sci-fi-fantasy-to-engineering-reality/ |archive-date=2017-10-30}}</ref><ref>{{cite thesis |type=MFA |last=Gordon |first=Marcus A. |date=2017 |title=Habitat 44º |publisher=OCAD University |doi=10.13140/RG.2.2.30421.88802}}</ref>

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, involved 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|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.

Examples of this digital version of Pepper's ghost illusion include the Gorillaz performances in the 2005 MTV Europe Music Awards and the 48th Grammy Awards; and Tupac Shakur's virtual performance at Coachella Valley Music and Arts Festival in 2012, rapping alongside Snoop Dogg during his 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|access-date=2012-04-21}}</ref> Digital avatars of the swedish supergroup ABBA were displayed on stage in May 2022.<ref>{{cite news |last1=Brause and Mills |title=Super Trouper: ABBA returns to stage as virtual avatars for London gigs |url=https://www.reuters.com/lifestyle/super-trouper-abba-returns-stage-virtual-avatars-london-gigs-2022-05-26/ |website=Reuters |date=27 May 2022 |agency=Reuters |access-date=4 June 2022}}</ref> The ABBA performance used technology that was an updated version of Pepper's Ghost created by Industrial Light & Magic.<ref>{{cite web |title=ABBA's mysterious "Abbatars" revealed |url=https://www.graphicnews.com/en/pages/38425/entertainment-abbas-mysterious-abbatars-revealed-1 |website=Graphic News |access-date=4 June 2022}}</ref> American rock group KISS unveiled similar digital avatars in December 2023 to tour in their place at the conclusion of the End of the Road World Tour using the same Pepper's Ghost technology as the ABBA avatars.<ref>{{cite news |last1=Amorosi |first1=A. D. |title=KISS Says Farewell at Madison Square Garden, Before Passing the Torch to Band's Avatar Successors: Concert Review |url=https://variety.com/2023/music/concert-reviews/kiss-final-concert-review-madison-square-garden-avatars-1235819744/ |access-date=3 December 2023 |work=Variety |date=3 December 2023}}</ref>

An even simpler illusion can be created by [[Engineering: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.

Crypton Future Media, a music software company that produced [[Software:Hatsune Miku|Hatsune Miku]],<ref>{{cite web|url=http://www.crypton.co.jp/mp/pages/prod/vocaloid/ |script-title=ja:クリプトン |title=Crypton |publisher=Crypton.co.jp |language=ja |access-date=2012-04-21}}</ref> one of many [[Software:Vocaloid|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 2 July |url = http://www.kawaiikakkoiisugoi.com/2011/06/16/las-anime-expo-hosting-hatsune-mikus-first-us-live-performance-on-july-2nd/ |access-date = 20 April 2012 }}</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=ZCYJu7KSqQM | archive-url=https://ghostarchive.org/varchive/youtube/20211030/ZCYJu7KSqQM| archive-date=2021-10-30|title="We can invite Hatsune Miku in my room!", Part 2 (video) |publisher=Youtube.com |date=2011-09-07 |access-date=2012-04-21}}{{cbignore}}</ref> to achieve its "holographic" effect.<ref>{{cite web |title = Technically incorrect: Tomorrow's Miley Cyrus? A hologram live in concert! |url = http://news.cnet.com/8301-17852_3-20022743-71.html |access-date = 29 April 2011 }}</ref><ref>{{cite web |title = Hatsune Miku – World is Mine Live in HD |website = [[Company:YouTube|YouTube]]|url = https://www.youtube.com/watch?v=DTXO7KGHtjI |access-date = 29 April 2011 }}</ref>

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|url=https://www.youtube.com/watch?v=9t5dCIuz2wY | archive-url=https://web.archive.org/web/20111004055249/http://www.youtube.com/watch?v=9t5dCIuz2wY&gl=US&hl=en&amp;has_verified=1| archive-date=2011-10-04|title=Burberry Beijing – Full Show |publisher=Youtube.com |access-date=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 |access-date = 14 June 2011 }}</ref>

In [[Place:Madrid|Madrid]], on 10 April 2015, a public visual presentation called "Hologramas por la Libertad" (Holograms for Liberty), featuring a ghostly virtual crowd of demonstrators, was used to protest a new Spanish law that prohibits citizens from demonstrating in public places. Although widely called a "hologram protest" in news reports,<ref>{{cite web |url=http://revolution-news.com/first-hologram-protest-in-history-held-against-spains-gag-law/ |title=First Hologram Protest in History Held Against Spain's Gag Law |publisher=revolution-news.com |access-date=2015-04-13 |url-status=dead |archive-url=https://web.archive.org/web/20150413044945/http://revolution-news.com/first-hologram-protest-in-history-held-against-spains-gag-law/ |archive-date=13 April 2015}}</ref> no actual holography was involved – it was yet another technologically updated variant of the Pepper's ghost illusion.

Holography is distinct from [[Specular holography|specular holography]] which is a 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 |access-date=2012-04-21}}</ref> It works by reflectively or refractively manipulating bundles of light rays, not by using interference and diffraction.

==In fiction==
{{Main|Physics:Holography in fiction}}

Holography has been widely referred to in movies, novels, and TV, usually in science fiction, starting in the late 1970s.<ref name=":0">{{Cite book|title=Holographic Visions: a History of New Science.|url=https://archive.org/details/holographicvisio00john_090|url-access=limited|last=Johnston|first=Sean|date=2006|publisher=Oxford University Press, UK|isbn=978-0191513886|location=Oxford|pages=[https://archive.org/details/holographicvisio00john_090/page/n427 405]–408|chapter=The Hologram and Popular Culture|oclc=437109030}}</ref> Science fiction writers absorbed the [[Social:Urban legend|urban legend]]s surrounding holography that had been spread by overly-enthusiastic scientists and entrepreneurs trying to market the idea.<ref name=":0" /> This had the effect of giving the public overly high expectations of the capability of holography, due to the unrealistic depictions of it in most fiction, where they are fully [[Volumetric display|three-dimensional computer projections]] that are sometimes tactile through the use of force fields.<ref name=":0" /> Examples of this type of depiction include the hologram of Princess Leia in ''Star Wars'', Arnold Rimmer from ''Red Dwarf'', who was later converted to "hard light" to make him solid, and the Holodeck and Emergency Medical Hologram from ''Star Trek''.<ref name=":0" />

Holography served as an inspiration for many video games with science fiction elements. In many titles, fictional holographic technology has been used to reflect real life misrepresentations of potential military use of holograms, such as the "mirage tanks" in ''Command & Conquer: Red Alert 2'' that can disguise themselves as trees.<ref name=":12">{{Cite book|title=Holograms: A Cultural History|last=Johnston|first=Sean F.|publisher=Oxford University Press|year=2015|isbn=978-0191021381|chapter=11 - Channeling Dreams}}</ref> [[Software:Player character|Player character]]s are able to use holographic decoys in games such as ''[[Software:Halo: Reach|Halo]]'' and ''[[Software:Crysis 2|Crysis 2]]'' to confuse and distract the enemy.<ref name=":12" /> ''Starcraft'' ghost agent Nova has access to "holo decoy" as one of her three primary abilities in ''[[Software:Heroes of the Storm|Heroes of the Storm]].<ref>{{Cite web|url=http://us.battle.net/heroes/en/heroes/nova/|title=Nova - Heroes of the Storm|website=us.battle.net|language=en-us|access-date=2019-10-20}}</ref>''

Fictional depictions of holograms have, however, inspired technological advances in other fields, such as [[Augmented reality|augmented reality]], that promise to fulfill the fictional depictions of holograms by other means.<ref>{{Cite book|title=The Hologram: Principles and Techniques|last=Richardson|first=Martin|others=Wiltshire, John D.|isbn=978-1119088905|location=Hoboken, NJ|oclc=1000385946|date = 2017-11-13}}</ref>

==See also==
{{cols|colwidth=21em}}
* [[List of file formats#3D graphics|3D file formats]]
* [[Physics:Computer-generated holography|Computer-generated holography]]
* [[Holographic display]]
* [[Augmented reality]]
* Australian Holographics
* [[Autostereoscopy]]
* [[Physics:Digital holography|Digital holography]]
* [[Biology:Digital holographic microscopy|Digital holographic microscopy]]
* [[Physics:Digital planar holography|Digital planar holography]]
* [[Engineering:Fog display|Fog display]]
* [[Physics:Holographic principle|Holographic principle]]
* [[Philosophy:Holonomic brain theory|Holonomic brain theory]]
* Hogel Processing Unit
* [[Integral imaging]]
* [[Social:List of emerging technologies|List of emerging technologies]]
* [[Physics:Phase-coherent holography|Phase-coherent holography]]
* [[Physics:Plasmon#Possible applications|Plasmon – possible applications]] (full color holography)
* [[Physics:Tomography|Tomography]]
* [[Volumetric display]]
* [[Volumetric printing]]
{{colend}}

==References==
{{Reflist|30em}}

==Bibliography==
{{Refbegin}}
* Hariharan P, 1996, Optical Holography, Cambridge University Press, {{ISBN|0-521-43965-5}}
* Hariharan P, 2002, Basics of Holography, Cambridge University Press, {{ISBN|0-521-00200-1}}
* Lipson A., Lipson SG, Lipson H, Optical Physics, 2011, Cambridge University Press, {{ISBN|978-0-521-49345-1}}
{{Refend}}

==Further reading==
{{Refbegin|30em}}
* ''Lasers and holography: an introduction to coherent optics'' W. E. Kock, Dover Publications (1981), {{ISBN|978-0-486-24041-1}}
* ''Principles of holography'' H. M. Smith, Wiley (1976), {{ISBN|978-0-471-80341-6}}
* G. Berger et al., ''Digital Data Storage in a phase-encoded holographic memory system: data quality and security'', Proceedings of SPIE, Vol. 4988, pp.&nbsp;104–111 (2003)
* ''Holographic Visions: A History of New Science'' Sean F. Johnston, Oxford University Press (2006), {{ISBN|0-19-857122-4}}
* {{Cite book|title = Practical Holography, Third Edition|last = Saxby|first = Graham|year = 2003|publisher = Taylor and Francis|isbn = 978-0-7503-0912-7}}
* ''Three-Dimensional Imaging Techniques'' Takanori Okoshi, Atara Press (2011), {{ISBN|978-0-9822251-4-1}}
* ''Holographic Microscopy of Phase Microscopic Objects: Theory and Practice'' Tatyana Tishko, Tishko Dmitry, Titar Vladimir, World Scientific (2010), {{ISBN|978-981-4289-54-2}}
* {{cite book|title=The Hologram: Principles and Techniques|editor-first1=Martin J. |editor-last1=Richardson |editor-first2=John D.|editor-last2=Wiltshire|publisher=Wiley|year=2017|isbn=9781119088905|oclc=1000385946|doi=10.1002/9781119088929|last1=Richardson|first1=Martin J.|last2=Wiltshire|first2=John D.}}

{{Refend}}

==External links==
* [https://web.archive.org/web/20041204092005/http://nobelprize.org/physics/laureates/1971/gabor-autobio.html The Nobel prize lecture of Dennis Gabor] [https://www.nobelprize.org/uploads/2018/06/gabor-lecture.pdf (pdf)]
* [http://science.howstuffworks.com/hologram.htm How Stuff Works – holograms]
* [http://qed.wikina.org/holography/ Animations demonstrating holography] by QED
* [https://youtube.com/watch?v=aTB2ryoWIFU&t=459 Making a single-beam hologram] at [[Company:YouTube|YouTube]]

{{Display technology}}
{{photography subject}}
{{emerging technologies|displays=yes}}
{{Stereoscopy}}


[[Category:Holography| ]]
[[Category:Photographic techniques]]
[[Category:3D imaging]]

{{Sourceattribution|Holography}}