From Wikipedia, the free encyclopedia: Holography (from the Greek ὅλος hólos, "whole" + γραφή grafē, "writing, drawing") is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system (a camera or an eye) is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the image appear three-dimensional. The holographic recording itself is not an image - it consists of an apparently random structure of either varying intensity, density or profile - an example can be seen below. The technique of holography can also be used to store, retrieve, and process information optically. While it has been possible to create a 3-D holographic picture of a static object since the 1960s, it is only in the last few years[1] that arbitrary scenes or videos can be shown on a holographic volumetric display.[2][3]
[edit]Overview and historyHolography was invented in 1947 by the Hungarian-British[4] physicist Dennis Gabor (Hungarian name: Gábor Dénes),[5] work for which he received the Nobel Prize in Physics in 1971. Pioneering work in the field of physics by other scientists including Mieczysław Wolfke resolved technical issues that previously had prevented advancement. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, and the company filed a patent in December 1947 (patent GB685286). The technique as originally invented is still used in electron microscopy, where it is known as electron holography, but holography as a light-optical technique did not really advance until the development of the laser in 1960. The first practical optical holograms that recorded 3D objects were made in 1962 by Yuri Denisyuk in the Soviet Union[6] and by Emmett Leith and Juris Upatnieks at University of Michigan, USA.[7] Advances in photochemical processing techniques to produce high-quality display holograms were achieved by Nicholas J. Phillips.[8] Several types of holograms can be made. Transmission holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser light through them and looking at the reconstructed image from the side of the hologram opposite the source. A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers. Rainbow holograms are commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission hologram are commonly formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating that provides the light from "behind" to reconstruct their imagery. Another kind of common hologram, the reflection or Denisyuk hologram, is capable of multicolour-image reproduction, using a white-light illumination source on the same side of the hologram as the viewer. Specular holography[9] is a related technique for making three-dimensional imagery by controlling the motion of specularities on a two-dimensional surface. It works by reflectively or refractively manipulating bundles of light rays, whereas Gabor-style holography works by diffractively reconstructing wavefronts. One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers, such as those found in millions of DVD recorders and used in other common applications, which are sometimes also useful for holography. These cheap, compact, solid-state lasers can, under some circumstances, compete well with the large, expensive gas lasers previously required to make holograms and are already helping to make holography much more accessible to low-budget researchers, artists and dedicated hobbyists. It was thought that it would to be possible to use X-rays to make holograms of molecules and view them using visible light. However, X-ray holograms have not been created to date.[10] [edit]How holography worksA detailed theoretical account of how holography works is provided by Hariharan.[11] [edit]The basicsHolography is a technique which enables a light field to be recorded, and reconstructed later when the original light field is no longer present. It is analogous to sound recording where the sound field is encoded in such a way that it can later be reproduced. Though holography is often referred to as 3D photography, this is a misconception. A photograph represents a single fixed image of a scene, whereas a hologram, when illuminated appropriately, re-creates the light which came from the original scene; this can be viewed from different distances and at different orientations just as if the original scene were present. The hologram itself consists of a very fine random pattern, which appears to bear no relationship to the scene which it has recorded. To record a hologram, some of the light scattered from an object or a set of objects falls on the recording medium. A second light beam, known as the reference beam, also illuminates the recording medium, so thatinterference occurs between the two beams. The resulting light field generates a seemingly random pattern of varying intensity, which is recorded in the hologram. The figure on the right is a photograph of part of a hologram - the object was a toy van. The photograph was taken by backlighting the hologram with diffuse light, and focusing on the surface of the plate. It is important to note that the holographic recording is contained in the random intensity structure (which is a speckle pattern), and not in the more regular structure, which is due to interference arising from multiple reflections in the glass plate on which the photographic emulsion is mounted. It is no more possible to discern the subject of the hologram from this random pattern than it is to identify what music has been recorded by looking at the hills and valleys on a gramophone record surface or the pits on a CD. When the original reference beam illuminates the hologram, it isdiffracted by the recorded hologram to produce a light field which is identical to the light field which was originally scattered by the object or objects onto the hologram. When the object is removed, an observer who looks into the hologram "sees" the same image on his retina as he would have seen when looking at the original scene. This image is often called a virtual image, as it can be seen even though the object is no longer present. The figure shown at the top of this article is an image produced by a camera which is located in front of the developed hologram which is being illuminated with the original reference beam. The camera is focused on the original scene, not on the hologram itself. [edit]Holography explained in terms of interference and diffractionFor 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 whenever a wavefront encounters an object. The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction. It is somewhat simplified but is accurate enough to provide an understanding of how the holographic process works. For those unfamiliar with these concepts, it is worthwhile to read the respective articles before reading further in this article. [edit]Plane wavefrontsA diffraction grating is a structure with a repeating pattern. A simple example is a metal plate with slits cut at regular intervals. A light wave incident on a grating is split into several waves; the direction of these diffracted waves is determined by the grating spacing and the wavelength of the light. 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 fringe pattern whose intensity varies sinusoidally across the medium. The spacing of the fringe pattern is determined by the angle between the two waves, and on the wavelength of the light. The recorded light pattern is a diffraction grating. When it is illuminated by only one of the waves used to create it, it can be shown that one of the diffracted waves emerges at the same angle as that at which the second wave was originally incident so that the second wave has been 'reconstructed'. Thus, the recorded light pattern is a holographic recording as defined above. [edit]Point sourcesIf the recording medium is illuminated with a point source and a normally incident plane wave, the resulting pattern is a sinusoidal zone platewhich 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 wavefront illuminates a negative lens, it is expanded into a wave which appears to diverge from the focal point of the lens. Thus, when the recorded pattern is illuminated with the original plane wave, some of the light is diffracted into a diverging beam equivalent to the original plane wave; a holographic recording of the point source has been created. When the plane wave is incident at a non-normal angle, the pattern formed is more complex but still acts as a negative lens provided it is illuminated at the original angle. [edit]Complex objectsTo record a hologram of a complex object, a laser beam is first split into two separate beams of light. One beam illuminates the object, which then scatters light onto the recording medium. According to diffraction theory, each point in the object acts as a point source of light so the recording medium can be considered to be illuminated by a set of point sources located at varying distances from the medium. 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 the hologram is illuminated by the original reference beam, each of the individual zone plates reconstructs the object wave which produced it, and these individual wavefronts add together to reconstruct the whole of the object beam. The viewer perceives a wavefront that is identical to the wavefront scattered from the object onto the recording medium, so that it appears to him or her that the object is still in place even if it has been removed. This image is known as a "virtual" image, as it is generated even though the object is no longer there. [edit]A simplified mathematical model of the recording and reconstruction processA light wave can be modelled by a complex number U, which represents the electric or magnetic field of the light wave. The amplitude and phase of the light are represented by theabsolute value and angle of the complex number. The object and reference waves at any point in the holographic system are given by UO and UR. The combined beam is given by UO +UR. The energy of the combined beams is proportional to the square of magnitude of the combined waves as:
If a photographic plate is exposed to the two beams and then developed, its transmittance, T, is proportional to the light energy that was incident on the plate and is given by
where k is a constant. When the developed plate is illuminated by the reference beam, the light transmitted through the plate, UH is equal to the transmittance T multiplied by the reference beam amplitude UR, giving
It can be seen that UH has four terms, each representing a light beam emerging from the hologram. The first of these is proportional to UO. This is the reconstructed object beam which enables a viewer to 'see' the original object even when it is no longer present in the field of view. The second and third beams are modified versions of the reference beam. The fourth term is known as the "conjugate object beam". It has the reverse curvature to the object beam itself and forms a real image of the object in the space beyond the holographic plate. When the reference and object beams are incident on the holographic recording medium at significantly different angles, the virtual, real and reference wavefronts all emerge at different angles, enabling the reconstructed object to be seen clearly. [edit]The efficiency of a hologramThe efficiency of a hologram is a measure of the fraction of the reference beam energy which is converted into reconstructed beam energy. There are several recording medium and set-up parameters which affect holographic efficiency:
A thin hologram is one where the thickness of the recording material is significantly smaller than the spacing of the interference pattern which makes up the hologram. In a volume, or thick, hologram, the depth of the recording material is equal to or significantly greater than the fringe spacing. An amplitude modulated hologram is one where the optical transmittance of the recording medium varies with the intensity of the fringe pattern. A phase hologram is one where the phaseof the re-constructing reference beam varies according to the intensity of the recorded fringe pattern. A transmission hologram is one where the object and reference beams are incident on the recording medium from the same side, whereas a reflection hologram has the object and reference beams incident from opposite sides; the reconstructing beam is then incident on the hologram from the same side as that where the viewer of the reconstruction is located. [edit]Thin hologramsThe discussion above of how holography works, relates to a thin amplitude transmission hologram. The transmittance of the recorded hologram varies with the intensity of the interference pattern produced by the combined object and reference beams. A straightforward example of this is the use of photographic emulsion on a transparent substrate. The emulsion is exposed to the interference pattern, and is subsequently developed giving a transmittance which varies with the intensity of the pattern. In a phase transmission hologram, the transmittance of the recording is proportional to the phase of the recorded fringe pattern. It can be shown that when such a plate is illuminated by the original reference beam, it is diffracted into several different beams, one of which is equivalent to the original object wavefront. A phase hologram is made by changing either the thickness or the refractive index of the material in proportion to the intensity of the holographic interference pattern. Many of the recording media listed below act as phase recording media. Photographic emulsion recordings, which give amplitude reocrdings when developed under normal conditions, can be converted to phase modulation recordings by a process known as bleaching [12] It should be noted that if a thin hologram is illuminated with a broad spectrum light beam (for example a white light source), each wavelength will reconstruct an object beam of slightly differing shape and size, and the net effect will be that the original object will not be discernible. [edit]Volume hologramsIt might appear at first sight that a hologram could not be made using a recording medium whose thickness is much greater than the wavelength of the light used to make the hologram, because the interference pattern recorded will now be three dimensional and its structure will vary significantly with hologram depth so that it cannot give rise to a single reconstructed object beam. This is not the case, however. Consider a simple hologram made from two plane waves which intersect in the recording medium, one being incident normally, and the other incident at an angle θ as above. An interference pattern is formed consisting of planes of constant phase, whose spacing is given by d = λ/sin θ. If the hologram is illuminated with one of the original plane waves, Bragg's law shows that diffracted waves occur at angles given by sin θ = nλ/d, where n is an integer. The first of these beams. which is also the most powerful, can be shown to correspond to the second of the original beams, and is therefore effectively a reconstructed object beam. The arguments used above to show how a hologram can be made using a point source, and then a complex object which can be considered to be a set of point sources, can be applied again here to show that a volume hologram can reconstruct the object beam when illuminated by the original reference beam. A significant advantage of a volume hologram compared with a thin hologram is that the reconstructed beam only occurs at the Bragg angle, which means that if it is illuminated with a light source which has a broad spectrum of wavelengths, reconstruction occurs only at the wavelength of the original laser used. This allows the holographic reconstruction to be done using a white light source, as is the case with most display and security holograms. Reflection holograms can only be made using volume holograms. The main advantage of a reflection hologram is that the reference beam is incident on the same side of the hologram as where the viewer is located, making viewing more convenient. Volume reflection holograms can have either amplitude or phase modulation. A volume reflection hologram is often referred to as a Denisyuk hologram.[13] [edit]Theoretical maximum efficienciesThe maximum theoretical efficiencies of the various types of holographic recordings, as quoted by Hariharan,[11] are given in the table below.
[edit]Practical aspects[edit]Making a hologramThe object and the reference beams must be able to produce an interference pattern that is stable during the time in which the holographic recording is made. To do this, they must have the same frequency and the same relative phase during this time, that is, they must be mutually coherent. Many laser beams satisfy this condition, and lasers have been used to make holograms since their invention, though the first holograms by Gabor used "quasi-chromatic" light sources. In principle, two separate light sources could be used if the coherencecondition could be satisfied, but in practice, a single laser is always used. In addition, the medium used to record the fringe pattern must be able to resolve it, and some of the more common media used are listed below. The spacing of the fringes depends on the angle between the object and reference beams. For example, if this angle is 45° and the wavelength of the light is 0.5 μm, the fringe spacing is about 0.7 μm or 1400 lines/mm. A working hologram can be obtained even if not all the fringes are resolved, but the resolution of the image is reduced as the resolution of the recording medium decreases. These are discussed in a section below. Mechanical stability is also very important when making a hologram. If the phase of one beam changes with respect to the other due to vibration or air movement, the fringe pattern moves across the field of view. If the fringe pattern moves by one or more fringe spacings, the light intensity is averaged out, and no holographic recording is obtained. A relative path change of half a wavelength shifts the interference pattern by one fringe. Thus, the stability requirement is very stringent. Generally, the coherence length of the light determines the maximum depth in the scene of interest that can be recorded holographically. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram. Certain pen laser pointers have been used to make small holograms (see External links). The size of these holograms is not restricted by the coherence length of the laser pointers (which can exceed several meters), but by their low power of below 5 mW. The objects that form the scene must, in general, have optically rough surfaces so that they scatter light over a wide range of angles. A specularly reflecting (or shiny) surface reflects the light in only one direction at each point on its surface, so in general, most of the light will not be incident on the recording medium. The light scattered from objects with a rough surface forms an objective speckle pattern that has random amplitude and phase. The reference beam is not normally a plane wavefront; it is usually a divergent wavefront that is formed by placing a convex lens in the path of the laser beam. [edit]Reconstructing and viewing the hologramTo reconstruct the object beam, the hologram plate is illuminated with a reference beam which is similar to the reference beam used in the recording. The reconstructed object beam is diffracted from the hologram, and an image of the object is formed when an imaging lens (an eye or a camera) is placed into the reconstructed beam, even though the object is no longer present. An image can be formed from any point in the reconstructed beam. If the lens is moved, the image changes in the same way as it would have done when the object was in place. If several objects were present when the hologram was recorded, the reconstructed objects will exhibit parallax in the same way as the original objects would have done. It was very common in the early days of holography to use a chess board as the object and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess pieces appeared to change. [edit]Fidelity of the reconstructed beamTo replicate the original object beam exactly, the reconstructing reference beam must be identical to the original reference beam and the recording medium must be able to fully resolve the interference pattern formed between the object and reference beams. Any change in the shape, orientation or wavelength of the reference beam gives a distorted reconstruction. For instance, the reconstructed image is magnified if the laser used to reconstruct the hologram has a shorter wavelength than the original laser. Nonetheless, good reconstruction is obtained using a laser of a different wavelength and reflection hologramscan be reconstructed using white light or sunlight. Exact reconstruction is achieved in holographic interferometry, where the holographically reconstructed wavefront interferes with the live wavefront, giving a null fringe if there has been no movement of the object and mapping out the displacement if the object has moved. If the recording medium is unable to fully resolve the intereference pattern between object and reference beam, the spatial resolution of any images formed from the reconstructed beam is reduced,[dubious – discuss] i.e. the image becomes 'fuzzier'. Since each point in the object illuminates all of the hologram, the whole object can be reconstructed from a small part of the hologram. Thus, a hologram can be broken up into small pieces and each one will enable the whole of the original object to be imaged. One does, however, lose information and the spatial resolution gets worse as the size of the hologram is decreased — the image becomes "fuzzier". [edit]Holographic recording mediaThe recording medium must be able to resolve the interference fringes as discussed above. It must also be sufficiently sensitive to record the fringe pattern in a time period short enough for the system to remain optically stable, i.e., any relative movement of the two beams must be significantly less than λ/2. It is possible to record holograms in certain materials using a high-power pulsed laser technique that uses only a couple of nanoseconds to record the holographic pattern.[14] The recording medium has to convert the interference pattern into an optical element that modifies either the amplitude or the phase of the holographic interference pattern to produce either an amplitude or a phase hologram. Most materials used for phase holograms reach the theoretical diffraction efficiency for holograms, which is 100% for thick holograms (Bragg diffraction regime) and 33.9% for thin holograms (Raman-Nath diffraction regime, holographic films typically some micrometers thick). Amplitude holograms have a lower efficiency than phase holograms and are therefore used more rarely. The table below shows the principal materials for holographic recording. Note that these do not include the materials used in the mass replication of an existing hologram. The resolution limit given in the table indicates the maximal number of interference lines per millimeter of the gratings. The required exposure is for a long exposure. Short exposure times (less than 1/1000 of a second, such as with a pulsed laser) require a higher exposure due to reciprocity failure.
[edit]Embossing and mass productionAn existing hologram can be replicated, either in an optical way similar to holographic recording or, in the case of surface relief holograms, by embossing. Surface relief holograms are recorded in photoresists or photothermoplastics and allow cheap mass reproduction. Such embossed holograms are now widely used, for instance, as security features on credit cards or quality merchandise. The Royal Canadian Mint even produces holographic gold and silver coinage through a complex stamping process.[17] The first book to feature a hologram on the front cover was The Skook (Warner Books, 1984) by JP Miller, featuring an illustration by Miller. That same year, "Telstar" by Ad Infinitum became the first record with a hologram cover and National Geographic published the first magazine with a hologram cover.[18] The first step in the embossing process is to make a stamper by electrodeposition of nickel on the relief image recorded on the photoresist or photothermoplastic. When the nickel layer is thick enough, it is separated from the master hologram and mounted on a metal backing plate. The material used to make embossed copies consists of a polyester base film, a resin separation layer and a thermoplastic film constituting the holographic layer. The embossing process can be carried out with a simple heated press. The bottom layer of the duplicating film (the thermoplastic layer) is heated above its softening point and pressed against the stamper, so that it takes up its shape. This shape is retained when the film is cooled and removed from the press. In order to permit the viewing of embossed holograms in reflection, an additional reflecting layer of aluminum is usually added on the hologram recording layer. It is possible to print holograms directly into steel using a sheet explosive charge to create the required surface relief.[19] [edit]Applications[edit]ArtEarly 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.[20] During the 1970s, a number of art studios and schools were established, each with their particular approach to holography. Notably, there was the San Francisco School of Holography established by Lloyd Cross, The Museum of Holography in New York founded by Rosemary (Possie) H. Jackson, the Royal College of Art in London and the Lake Forest College Symposiums organised by Tung Jeong (T.J.).[21] None of these studios still exist; however, there is the Center for the Holographic Arts in New York[22] and the HOLOcenter in Seoul,[23]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 USA, Dieter Jung ofGermany, and Moysés Baumstein of Brazil, each one searching for a proper "language" to use with the three-dimensional work, avoiding the simple holographic reproduction of a sculpture or object. For instance, in Brazil, many concrete poets (Augusto de Campos, Décio Pignatari, Julio Plaza and José Wagner Garcia, associated with Moysés Baumstein) found in holography a way to express themselves and to renew the Concrete Poetry (or Shape Poetry). A small but active group of artists still use holography as their main medium, and many more artists integrate holographic elements into their work.[24] Some are associated with novel holographic techniques; for example, artist Matt Brand[25] employed computational mirror design to eliminate image distortion from specular holography. The MIT Museum[26] and Jonathan Ross[27] both have extensive collections of holography and on-line catalogues of art holograms. [edit]Data storageMain article: Holographic memory Holography can be put to a variety of uses other than recording images. Holographic data storage is a technique that can store information at high density inside crystals or photopolymers. The ability to store large amounts of information in some kind of media is of great importance, as many electronic products incorporate storage devices. As current storage techniques such as Blu-ray Disc reach the limit of possible data density (due to the diffraction-limited size of the writing beams), holographic storage has the potential to become the next generation of popular storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. Currently availableSLMs can produce about 1000 different images a second at 1024×1024-bit resolution. With the right type of media (probably polymers rather than something like LiNbO3), this would result in about one-gigabit-per-second writing speed. Read speeds can surpass this, and experts believe one-terabit-per-second readout is possible. In 2005, companies such as Optwareand Maxell have produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 TB, which they plan to market under the name Holographic Versatile Disc. Another company, InPhase Technologies, is developing a competing format. While many holographic data storage models have used "page-based" storage, where each recorded hologram holds a large amount of data, more recent research into using submicrometre-sized "microholograms" has resulted in several potential 3D optical data storage solutions. While this approach to data storage can not attain the high data rates of page-based storage, the tolerances, technological hurdles, and cost of producing a commercial product are significantly lower. [edit]Dynamic holographyIn 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 (pattern recognition of time-varying images), and 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, 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.[28] The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals, but in semiconductors orsemiconductor 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 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). [edit]Hobbyist useSince the beginning of holography, experimenters have explored its uses. Starting in 1971, Lloyd Cross started the San Francisco School of Holography and started to teach amateurs the methods of making holograms with inexpensive equipment. This method relied on the use of a large table of deep sand to hold the optics rigid and damp vibrations that would destroy the image. Many of these holographers would go on to produce art holograms. In 1983, Fred Unterseher published the Holography Handbook, a remarkably easy-to-read description of making holograms at home. This brought in a new wave of holographers and gave simple methods to use the then-available AGFA silver halide recording materials. In 2000, Frank DeFreitas published the Shoebox Holography Book and introduced using inexpensive laser pointers to countlesshobbyists. This was a very important development for amateurs, as the cost for a 5 mW laser dropped from $1200 to $5 as semiconductor laser diodes reached mass market. Now, there are hundreds to thousands of amateur holographers worldwide. In 2006, a large number of surplus Holography Quality Green Lasers (Coherent C315) became available and put Dichromated Gelatin (DCG) within the reach of the amateur holographer. The holography community was surprised at the amazing sensitivity of DCG to greenlight. It had been assumed that the sensitivity would be non-existent. Jeff Blyth responded with the G307 formulation of DCG to increase the speed and sensitivity to these new lasers.[29] Many film suppliers have come and gone from the silver-halide market. While more film manufactures have filled in the voids, many amateurs are now making their own film. The favorite formulations are Dichromated Gelatin, Methylene Blue Sensitised Dichromated Gelatin and Diffusion Method Silver Halide preparations. Jeff Blyth has published very accurate methods for making film in a small lab or garage.[30] A small group of amateurs are even constructing their own pulsed lasers to make holograms of moving objects.[31] [edit]Holographic interferometryMain article: holographic interferometry Holographic interferometry (HI)[32][33] 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). 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. It has been widely used to measure stress, strain, and vibration in engineering structures. [edit]Holoprinters
A holoprinter is a holographic printing device that can print out full-colour digital holograms from a rendered 3D model or a video series. The machine can cost up to half a million dollars and is about the size of a small room. It uses red, green and blue lasers to write a series of dots, or holopixels, across a holographic medium. The holopixel contains information about the whole image from its own unique perspective. The information for each holopixel is computed from a series of rendered images generated via computer graphics. The holographic medium is typically a polymer film. The film may require development after exposure. It is then laminated on to a hard plastic backing. Printing a digital hologram can take several hours, as each holopixel dot has to be written individually in three colours, where the colours overlap within the medium. The size of a holopixel is typically around a square millimeter. There are only a few digital holoprinter manufacturers in the world, including Geola (Lithuania), View Holographics (UK) and Zebra Imaging (US). [edit]Interferometric microscopyMain article: 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, which, in turn, enables enhancement of the resolution of optical microscopy. The corresponding technique is called interferometric microscopy. Recent achievements of interferometric microscopy allow one to approach the quarter-wavelength limit of resolution.[34] [edit]Sensors or biosensorsMain article: 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.[35] [edit]SecurityMain article: Security hologram Security holograms are very difficult to forge, because they are replicated from a master hologram that requires expensive, specialized and technologically advanced equipment. They are used widely in many currencies, such as the Brazilian real 20 note, British pound 5/10/20 notes, Estonian kroon 25/50/100/500 notes, Canadian dollar 5/10/20/50/100 notes, Euro5/10/20/50/100/200/500 notes, South Korean won 5000/10000/50000 notes, and Japanese yen5000/10000 notes. They are also used in credit and bank cards as well as passports, ID cards,books, DVDs, and sports equipment. Holography allows for different levels of security, depending on budget and intensity of security. The highest level of security in fully custom holography, this involves the design and creation of unique images in three dimensions, cost can range from $5,000 to $15,000. For a tightly budgeted project, there are two choices of hologram: overprint holographic diffraction foil or custom etched diffraction material, which are not dimensional but diffract light into patterns of bright rainbow light. [edit]Other applicationsHolographic 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.[36] [edit]Non-optical holographyIn principle, it is possible to make a hologram for any wave. 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.[37] The principle of electron holography can also be applied to interference lithography.[38] Acoustic holography is a method used to estimate the sound field near a source by measuring acoustic parameters away from the source via an array of pressure and/or particle velocity transducers. Measuring techniques included within acoustic holography are becoming increasingly popular in various fields, most notably those of transportation, vehicle and aircraft design, and NVH. The general idea of acoustic holography has led to different versions such as near-field acoustic holography (NAH) and statistically optimal near-field acoustic holography (SONAH). For audio rendition, the wave field synthesis is the most related procedure. Atomic holography has evolved out of the development of the basic elements of 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 ridged mirrors have provided the tools necessary for the creation of atomic holograms,[39] although such holograms have not yet been commercialized. [edit]Things often confused with hologramsEffects produced by Lenticular printing and Video projection, and Pepper's Ghost are often confused with holograms.[40] In 2010, there was a series of concerts organized by Crypton Future Media which included Hatsune Miku, a singing synthesizer application and its female character, performing on stage as a "holographic" character.[41][42][43] This effect was actually achieved through a special method of rear projection against a semi-transparent screen. [edit]Holography in fictionSee also: Category:Holography in fiction Holograms are often used as plot devices in science fiction.
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