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66-406: Holography is a technique that enables a 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 wave . A hologram is a recording of an interference pattern that can reproduce

132-423: A transmission electron microscope (TEM) in an off-axis scheme. Electron beam is split into two parts by very thin positively charged wire. Positive voltage deflects the electron waves so that they overlap and produce an interference pattern of equidistantly spaced fringes. Reconstruction of off-axis holograms is done numerically and it consists of two mathematical transformations. First, a Fourier transform of

198-475: A 120 mm disc that uses a holographic layer to store data to a potential 3.9  TB , a format called Holographic Versatile Disc . As of September 2014, no commercial product has been released. Another company, 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

264-399: A 3D light field using 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 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 laser light to record

330-453: 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. In static holography, recording, developing and reconstructing occur sequentially, and

396-464: A large telescope due to spatial variations in the index of refraction of the atmosphere. The deviation of a wavefront in an optical system from a desired perfect planar wavefront is called the wavefront aberration . Wavefront aberrations are usually described as either a sampled image or a collection of two-dimensional polynomial terms. Minimization of these aberrations is considered desirable for many applications in optical systems. A wavefront sensor

462-428: A mask or film and illuminated with an appropriate light source to reconstruct the desired wavefront. Alternatively, the interference pattern image can be directly displayed on a dynamic holographic display. Holographic portraiture often resorts to a non-holographic intermediate imaging procedure, to avoid the dangerous high-powered pulsed lasers which would be needed to optically "freeze" moving subjects as perfectly as

528-557: 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 (pattern recognition of time-varying images), and optical computing . The amount of processed information can be very high (terabits/s), since

594-420: 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 ( speckle ) pattern as in the photograph above. When

660-462: A single focal distance may not exist due to lens thickness or imperfections. For manufacturing reasons, a perfect lens has a spherical (or toroidal) surface shape though, theoretically, the ideal surface would be aspheric . Shortcomings such as these in an optical system cause what are called optical aberrations . The best-known aberrations include spherical aberration and coma . However, there may be more complex sources of aberrations such as in

726-539: A small 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 Holographic Studios in New York City . Since then, they have been involved in the production of many holographs for many artists as well as companies. Sapan has been described as

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792-552: A small (typically 5 mW) helium-neon laser and inexpensive home-made equipment. Holography had been supposed to require a very expensive metal 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

858-439: A spherical wavefront will remain spherical as the energy of the wave is carried away equally in all directions. Such directions of energy flow, which are always perpendicular to the wavefront, are called rays creating multiple wavefronts. The simplest form of a wavefront is the plane wave , where the rays are parallel to one another. The light from this type of wave is referred to as collimated light. The plane wavefront

924-408: 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. A hologram can be made by shining part of the light beam directly into the recording medium, and the other part onto

990-441: A way to create holograms that can be viewed with natural light instead of lasers. These are called rainbow holograms . Holography is a technique for recording and reconstructing light fields. 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 musical instruments or vocal cords ,

1056-481: A way to express themselves and to renew Concrete Poetry . A small but active group of artists still integrate holographic elements into their work. Some are associated with novel holographic techniques; for example, artist Matt Brand employed computational mirror design to eliminate image distortion from specular holography . The MIT Museum and Jonathan Ross both have extensive collections of holography and on-line catalogues of art holograms. Holographic data storage

1122-462: Is a device which measures the wavefront aberration in a coherent signal to describe the optical quality or lack thereof in an optical system. There are many applications that include adaptive optics , optical metrology and even the measurement of the aberrations in the eye itself. In this approach, a weak laser source is directed into the eye and the reflection off the retina is sampled and processed. Another application of software reconstruction of

1188-501: Is a good model for a surface-section of a very large spherical wavefront; for instance, sunlight strikes the earth with a spherical wavefront that has a radius of about 150 million kilometers (1 AU ). For many purposes, such a wavefront can be considered planar over distances of the diameter of Earth. In an isotropic medium wavefronts travel with the same speed in all directions. Methods using wavefront measurements or predictions can be considered an advanced approach to lens optics, where

1254-423: 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

1320-481: Is an active area of research. The most common materials are photorefractive crystals , but in semiconductors or semiconductor heterostructures (such as quantum wells ), atomic vapors and gases, 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

1386-420: Is applied. The resulting image in the object domain is complex-valued, and thus, the amplitude and phase distributions of the object function are reconstructed. The original holographic scheme by Dennis Gabor is inline scheme, which means that reference and object wave share the same optical axis . This scheme is also called point projection holography . An object is placed into divergent electron beam, part of

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1452-481: Is commonly glass, but may also be plastic. 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

1518-494: Is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets . The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength , as shown in the inserted image. This is due to the addition, or interference , of different points on

1584-494: Is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter. However, it is even more similar to Ambisonic sound recording in which any listening angle of a sound field can be reproduced in the reproduction. In laser holography, the hologram is recorded using a source of 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

1650-399: 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,

1716-459: 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. 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 that

1782-400: 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 plane waves 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

1848-512: 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 exposure time to correctly affect

1914-518: The Greek words ὅλος ( holos ; "whole") and γραφή ( graphē ; " writing " or " drawing "). The Hungarian - British physicist Dennis Gabor invented holography in 1948 while he was looking for a way to improve image resolution in electron microscopes . 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. The formulation of holography

1980-779: The Michelson interferometer could be called a wavefront sensor, the term is normally applied to instruments that do not require an unaberrated reference beam to interfere with. Electron holography Electron holography is holography with electron matter waves . It was invented by Dennis Gabor in 1948 when he tried to improve image resolution in electron microscope. The first attempts to perform holography with electron waves were made by Haine and Mulvey in 1952; they recorded holograms of zinc oxide crystals with 60 keV electrons, demonstrating reconstructions with approximately 1 nm resolution. In 1955, G. Möllenstedt and H. Düker invented an electron biprism , thus enabling

2046-439: The "last professional holographer of New York". Wavefront Wavefronts usually move with time. For waves propagating in a unidimensional medium, the wavefronts are usually single points; they are curves in a two dimensional medium, and surfaces in a three-dimensional one. For a sinusoidal plane wave , the wavefronts are planes perpendicular to the direction of propagation, that move in that direction together with

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2112-724: 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. In Great Britain, Margaret Benyon began using holography as an artistic medium in the late 1960s and had a solo exhibition at the University of Nottingham art gallery in 1969. This

2178-790: The Royal College of Art in London and the Lake Forest College Symposiums organised by Tung Jeong . None of these studios still exist; however, there is the Center for the Holographic Arts in New York 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

2244-493: The above simplifications, Huygens' principle provides a quick method to predict the propagation of a wavefront through, for example, free space . The construction is as follows: Let every point on the wavefront be considered a new point source . By calculating the total effect from every point source, the resulting field at new points can be computed. Computational algorithms are often based on this approach. Specific cases for simple wavefronts can be computed directly. For example,

2310-563: The art world, such as Harriet Casdin-Silver of the United States, 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

2376-412: 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

2442-581: The extremely motion-intolerant holographic recording process requires. Early holography required high-power and expensive lasers. Currently, mass-produced low-cost laser diodes , such as those found on DVD recorders 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. The word holography comes from

2508-460: 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. Early on, artists saw

2574-425: The hologram is performed. The resulting complex image consists of the autocorrelation (center band) and two mutually conjugated sidebands. Only one side band is selected by applying a low-pass filter (round mask) centered on the chosen side-band. The central band and the twin side-band are both set to zero. Next, the selected side-band is re-positioned to the center of the complex image and the backward Fourier-transform

2640-405: The hologram. Holography may be better understood via an examination of its differences from ordinary photography : 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

2706-481: The holographic method". Optical holography did not really advance until the development of the laser in 1960. The development of the laser enabled the first practical optical holograms that recorded 3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and Juris Upatnieks at the University of Michigan , US. Early optical holograms used silver halide photographic emulsions as

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2772-420: The interaction of light coming from different directions and producing a microscopic interference pattern which a plate , film, or other medium photographically records. In one common arrangement, the laser beam is split into two, one known as the object beam and the other as the reference beam . The object beam is expanded by passing it through a lens and used to illuminate the subject. The recording medium

2838-453: The light field. The reproduced light field can generate an image that has the depth and parallax of the original scene. A hologram is usually unintelligible when viewed under 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 cues such as parallax and perspective that change realistically with

2904-434: 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,

2970-410: 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 beam splitter that divides the beam into two identical beams, each aimed in different directions: Several different materials can be used as

3036-421: 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

3102-401: 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. The search for novel nonlinear optical materials for dynamic holography

3168-449: The pattern formed is more complex, but still acts as a negative lens if it is illuminated at the original angle. 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

3234-422: The phase is the control of telescopes through the use of adaptive optics. Mathematical techniques like phase imaging or curvature sensing are also capable of providing wavefront estimations. These algorithms compute wavefront images from conventional brightfield images at different focal planes without the need for specialised wavefront optics. While Shack-Hartmann lenslet arrays are limited in lateral resolution to

3300-426: 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

3366-422: 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 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. In 2005, companies such as Optware and Maxell produced

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3432-409: The recorded light pattern is a holographic recording as defined above. If the recording medium is illuminated with a point source and a normally incident plane wave, the resulting pattern is a 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. When a plane wave-front illuminates a negative lens, it

3498-402: The recording medium. One of the most common is a film very similar to photographic film ( silver halide photographic emulsion ), but with much smaller light-reactive grains (preferably with diameters less than 20 nm), making it capable of the much higher resolution that holograms require. A layer of this recording medium (e.g., silver halide) is attached to a transparent substrate, which

3564-406: The recording medium. They were not very efficient as the produced diffraction 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. A major advance in the field of holography was made by Stephen Benton , who invented

3630-407: 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

3696-407: The recording of electron holograms in off-axis scheme. There are many different possible configurations for electron holography, with more than 20 documented in 1992 by Cowley. Usually, high spatial and temporal coherence (i.e. a low energy spread) of the electron beam are required to perform holographic measurements. Electron holography with high-energy electrons (80-200 keV) can be realized in

3762-521: 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). Since the beginning of holography, many holographers have explored its uses and displayed them to the public. In 1971, Lloyd Cross opened the San Francisco School of Holography and taught amateurs how to make holograms using only

3828-424: 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

3894-452: The second wavefront, it is diffracted to recreate the original wavefront. 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

3960-417: The size of the lenslet array, techniques such as these are only limited by the resolution of digital images used to compute the wavefront measurements. That said, those wavefront sensors suffer from linearity issues and so are much less robust than the original SHWFS, in term of phase measurement. There are several types of wavefront sensors, including: Although an amplitude splitting interferometer such as

4026-438: The wave is scattered by the object (object wave) and it interferes with the unscattered wave (reference wave) in detector plane. The spatial coherence in in-line scheme is defined by the size of the electron source. Holography with low-energy electrons (50-1000 eV) can be realized in in-line scheme. It is important to shield the interferometric system from electromagnetic fields, as they can induce unwanted phase-shifts due to

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4092-419: The wave. For a sinusoidal spherical wave , the wavefronts are spherical surfaces that expand with it. If the speed of propagation is different at different points of a wavefront, the shape and/or orientation of the wavefronts may change by refraction . In particular, lenses can change the shape of optical wavefronts from planar to spherical, or vice versa. In classical physics , the diffraction phenomenon

4158-417: The wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. If there are multiple, closely spaced openings (e.g., a diffraction grating ), a complex pattern of varying intensity can result. Optical systems can be described with Maxwell's equations , and linear propagating waves such as sound or electron beams have similar wave equations. However, given

4224-703: Was an unexpected result of Gabor's research into improving electron microscopes at the British Thomson-Houston Company (BTH) in Rugby , England, and the company filed a patent in December 1947 (patent GB685286). The technique as originally invented is still used in electron microscopy, where it is known as electron holography . Gabor was awarded the Nobel Prize in Physics in 1971 "for his invention and development of

4290-500: 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". 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 (Posy) H. Jackson,

4356-420: 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 photographic plate holder were similarly supported within the sandbox. The holographer turned off the room light, blocked the laser beam near its source using

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