Holography

For other uses, see Holography (disambiguation).

Identigram as a security element in a German identity card.

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 it appears as if the object is in the same position relative to the recording medium as it was when recorded. 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 recorded image (hologram) appear three dimensional.

The technique of holography can also be used to optically store, retrieve, and process information. 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 that arbitrary scenes or videos can be shown on a holographic volumetric display.

Overview and history

File:Hologrammit.jpg

Hologram Artwork in MIT Museum.

Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (Hungarian name: Gábor Dénes), 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 which 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 and by Emmett Leith and Juris Upatnieks at University of Michigan, USA. Advances in photochemical processing techniques to produce high-quality display holograms were achieved by Nicholas J. Phillips.

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 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.

Theory

Holographic recording process

Though holography is often referred to as 3D photography, this is a misconception. A better analogy is sound recording where the sound field is encoded in such a way that it can later be reproduced. In holography, 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 that interference occurs between the two beams. The resulting light field generates a seemingly random pattern of varying intensity which is recorded in the hologram. It can be shown that if the hologram is illuminated by the original reference beam, the reference beam is diffracted by the hologram to produce a diffracted light field which is identical to the light field which was scattered by the object or objects. Thus, someone looking into the hologram "sees" the objects even though they are no longer present. There are a variety of recording materials which can be used, including photographic film.

The first cameras used something called a "pinhole lens". They consisted of a completely blacked-out box with a tiny pinhole on the side away from the film or screen. As a result, they only caught the scene before them from a single, tiny vantage point. The glass lenses that followed, were, in effect, simply giant pinholes, with all the light they collected being passed through a tiny point—a pinhole as it were—at the focal point of the glass lens before spreading out again before hitting the film or screen behind the lens.

The problem Dennis Gabor, the inventor of holography, set out to solve was how to take a picture of all the light passing through a large window, rather than just the light passing through one tiny pinhole. The person looking through this captured "window" would see the image in 3D by virtue of each of his or her eyes seeing the scene from a different viewpoint. Further, the person would be able to move his or her head around to the extent the window would allow to see the object from a variety of vantage points. (An early hologram from the 1960s featured an object with a glass magnifying lens mounted a few inches/centimeters in front of it. The viewer could, by ducking and bobbing his or her head, "look through" the image of the magnifier and, just as with a real magnifier, see different parts of the object behind it enlarged as they swept into view.)

Dennis Gabor, in effect, needed a fast shutter, one so fast that it could "freeze" all the light waves at their current phase just as they were passing through the window, a shutter that moved at the speed of light. His successful approach worked in a way analogous to the way a strobe light is used to "freeze" the motion of rapidly moving mechanical equipment, such as engines: If the light comes on at exactly the same moment during every rotation of a piece of equipment, the rotating part(s) will appear to be standing still.

The function analogous to a strobe light, in holography, is performed by the "reference beam." As you can see in the illustration above, a portion of the light from the laser, the "illumination beam," is aimed at the scene where it bounces off the objects in the scene directly onto the film—the window—with no pinhole or lens interposed. Another portion of the laser light, the "reference beam," instead of striking the object first, is split off from the original laser beam and directed straight onto the film.

To "play back" the scene (see the illustration, "Holographic reconstruction process," below), one resupplies the reference beam, shining it onto the developed film—the window. This reveals the originally-captured phase of the light waves as they passed through the window on their way from the objects in the scene. In effect, as the illustration shows, you can now "look through" the window and see the original object behind it.

Dennis Gabor's invention was not nearly as simple as a strobe light. To go deeper into the theory of holography, it is next necessary to understand Interference and diffraction. For those unfamiliar with these concepts, it is worthwhile to read their respective articles before reading further in this section.

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.

Plane wavefronts

A diffraction grating is a structure with a repeating pattern. A simple example is a metal plate with slits cut at regular intervals. Light rays travelling through it are bent at an angle determined by λ, the wavelength, and d, the distance between the slits, and is given by sinθ = λ/d.

A very simple hologram can be made by superimposing two plane waves from the same light source. One (the reference beam) hits the photographic plate normally and the other one (the object beam) hits the plate at an angle θ. The relative phase between the two beams varies across the photographic plate as 2π y sinθ/λ where y is the distance along the photographic plate. The two beams interfere with one another to form an interference pattern. The relative phase changes by 2π at intervals of d = λ/sinθ so the spacing of the interference fringes is given by d. Thus, the relative phase of object and reference beam is encoded as the maxima and minima of the fringe pattern.

When the photographic plate is developed, the fringe pattern acts as a diffraction grating and when the reference beam is incident upon the photographic plate, it is partly diffracted into the same angle θ at which the original object beam was incident. Thus, the object beam has been reconstructed. The diffraction grating created by the two waves interfering has reconstructed the "object beam" and it is therefore a hologram as defined above.

Point sources

Holographic reconstruction process

A slightly more complicated hologram can be made using a point source of light as object beam and a plane wave as reference beam to illuminate the photographic plate. An interference pattern is formed which in this case is in the form of curves of decreasing separation with increasing distance from the centre (basically a sinusoidal zone plate).

The photographic plate is developed giving a complicated pattern which can be considered to be made up of a diffraction pattern of varying spacing. When the plate is illuminated by the reference beam alone, it is diffracted by the grating into different angles which depend on the local spacing of the pattern on the plate. It can be shown that the net effect of this is to reconstruct the object beam, so that it appears that light is coming from a point source behind the plate, even when the source has been removed. The light emerging from the photographic plate is identical to the light that emerged from the point source that used to be there. An observer looking into the plate from the other side will "see" a point source of light whether the original source of light is there or not.

This sort of hologram is effectively a concave lens, since it "converts" a plane wavefront into a divergent wavefront. It will also increase the divergence of any wave which is incident on it in exactly the same way as a normal lens does. Its focal length is the distance between the point source and the plate.

Complex objects

To record a hologram of a complex object, a laser beam is first split into two separate beams of light using a beam splitter of half-silvered glass or a birefringent material. One beam illuminates the object, reflecting its image onto the recording medium as it scatters the beam. The second (reference) beam illuminates the recording medium directly.

According to diffraction theory, each point in the object acts as a point source of light. Each of these point sources interferes with the reference beam, giving rise to an interference pattern. The resulting pattern is the sum of all point source + reference beam interference patterns.

When the object is no longer present, the holographic plate is illuminated by the reference beam. Each point source diffraction grating will diffract part of the reference beam to reconstruct the wavefront from its point source. These individual wavefronts add together to reconstruct the whole of the object beam.

The viewer perceives a wavefront that is identical to the scattered wavefront of the object illuminated by the reference beam, so that it appears to him or her that the object is still in place. This image is known as a "virtual" image as it is generated even though the object is no longer there. The direction of the light source seen illuminating the virtual image is that of the original illuminating beam.

Mathematical model

A light wave can be modeled 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 the absolute value and angle of the complex number. The object and reference waves at any point in the holographic system are given by U_O and U_R. The combined beam is given be U_O + U_R. The energy of the combined beams is proportional to the square of magnitude of the electric wave:

$|U_{O}+U_{R}|^{2}=U_{O}U_{R}^{*}+|U_{R}|^{2}+|U_{O}|^{2}+U_{O}^{*}U_{R}$

If a photographic plate is exposed to the two beams, and then developed, its transmittance, T, is proportional to the light energy which was incident on the plate, and is given by

$T=k[U_{O}U_{R}^{*}+|U_{R}|^{2}+|U_{O}|^{2}+U_{O}^{*}U_{R}]$

where k is a constant. When the developed plate is illuminated by the reference beam, the light transmitted through the plate, U_H is

$U_{H}=TU_{R}=k[U_{O}U_{R}^{*}+|U_{R}|^{2}+|U_{O}|^{2}+U_{O}^{*}U_{R}]U_{R}=k[U_{O}|U_{R}|^{2}+|U_{R}|^{2}U_{R}+|U_{O}|^{2}U_{R}+U_{O}^{*}U_{R}^{2}]$

It can be seen that U_H has four terms. The first of these is proportional to U_O, and this is the re-constructed object beam. The second term represents the reference beam whose amplitude has been modified by U_R². The third also represents the reference beam which has had its amplitude modified by U_O²; this modification will cause the reference beam to be diffracted around its central direction. 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.

Early holograms had both the object and reference beams illuminating the recording medium normally, which meant that all the four beams emerging from the hologram were superimposed on one another. The off-axis hologram was developed by Leith and Upatnieks to overcome this problem. The object and reference beams are incident at well-separated angles onto the holographic recording medium and the virtual, real and reference wavefronts all emerge at different angles, enabling the re-constructed object beam to be imaged clearly.

Viewing the hologram

Photograph of a hologram in front of a diffuse light background - 8x8 mm

The picture on the right is a photograph, taken against a diffuse light background, of a hologram recorded on photographic emulsion. The area shown is about 8 mm by 8 mm. The holographic recording is the random variation in intensity which is an objective speckle pattern, and not the regular lines which are likely to be 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 than it is to identify the music on an gramophone record by looking at the structure of the gramophone record surface. When this hologram is illuminated by a divergent laser beam, the viewer will see the object used to make it (in this case, a toy van) because the light is diffracted by the hologram to reconstruct the light which was scattered from the object.

When one looks at a scene, each eye captures a portion of the light scattered from the scene, and the lens of the eye forms an image of the scene on the retina, in which light from each angular position is focused to a specific angular position in the image plane. Since the hologram reconstructs the whole of the scattered light field that was incident on the hologram, the viewer sees the same image whether it is derived from the light field scattered from the object, or the reconstructed light field produced by the hologram, and is unable to tell whether he or she is looking at the real or the virtual object. If the viewer moves about, the object will appear to move in exactly the same way whether he or she is looking at the original light field or the reconstructed light field. If there are several objects in the scene, they will exhibit parallax. If the viewer is using both eyes (stereoscopic vision), he or she will get depth information when viewing the hologram in exactly the same way as when he or she is viewing the real scene.

A hologram is not a 3D photograph. A photograph records an image of the recorded scene from a single viewpoint, which is defined by the position of the camera lens. The hologram is not an image, but an encoding system which enables the scattered light field to be reconstructed. Images can then be formed from any point in the reconstructed beam either with a camera or by eye. 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.

Since each point in the hologram contains light from the whole of the original scene, the whole scene can, in principle, be reconstructed from an arbitrarily small part of the hologram. To demonstrate this concept, the hologram can be broken into small pieces and the entire object can still be seen from each small piece. If one envisions the hologram as a "window" on the object, then each small piece of hologram is just a part of the window from which it can still be viewed, even if the rest of the window is blocked off.

One does, however, lose resolution as the size of the hologram is decreased—the image becomes "fuzzier." This is a result of diffraction and arises in the same way as the resolution of an imaging system is ultimately limited by diffraction where the resolution becomes coarser as the lens or lens aperture diameter decreases.

Viewing and authoring

The 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 coherence condition 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 the fringe patterns and some of the more common media used are listed below. The spacing of the fringes depends on the angle between object and reference beam. For example, if this angle is 45°, and the wavelength of the light is 0.5μm, the fringe spacing is about 0.7μm or 1300 lines/mm. A working hologram can be obtained even if all the fringes are not resolved, but the resolution of the image is reduced as the resolution of the recording medium decreases.

Mechanical stability is also very important when making a hologram. Any relative phase change between the object and reference beams due to vibration or air movement will cause the fringes on the recording medium to move, and if the phase change is greater than π, the fringe pattern is averaged out, and no holographic recording is obtained. Recording time can be several seconds or more, and given that a phase change of π is equivalent to a movement of λ/2 this is quite a stringent stability requirement.

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.

To reconstruct the object exactly from a transmission hologram, the reference beam must have the same wavelength and curvature, and must illuminate the hologram at the same angle as the original reference beam (i.e. only the phase can be changed). Departure from any of these conditions will give a distorted reconstruction. While nearly all holograms are recorded using lasers, a narrow-band lamp or even sunlight is enough to recognize the reconstructed image.

The reconstructed hologram is enlarged if the light used to reconstruct the hologram has a higher wavelength. This initially generated some interest since it seemed 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. This effect can be demonstrated using a light source which emits several different frequencies.

Exact reconstruction is achieved in holographic interferometry where the holographically reconstructed wavefront interferes with the live wavefront, to map out any displacement of the live object, and gives a null fringe if the object has not moved.

Holographic recording media

The 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.

The recording medium has to convert the interference pattern into an optical element which modifies either the amplitude or the phase of a light beam which is incident upon it. These are known as amplitude and phase holograms respectively. In amplitude holograms the modulation is in the varying absorption of the light by the hologram, as in a developed photographic emulsion which is less or more absorptive depending on the intensity of the light which illuminated it. In phase holograms, the optical distance (i.e., the refractive index or in some cases the thickness) in the material is modulated.

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 of typically some μm thickness). 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 mm of the gratings. The required exposure is for a long exposure. Short exposure times (less than 1/1000th of second, such as with a pulsed laser) require a higher exposure due to reciprocity failure.

General properties of recording materials for holography. Source:

Material	Reusable	Processing	Type of hologram	Max. efficiency	Required exposure [mJ/cm²]	Resolution limit [mm−1]
Photographic emulsions	No	Wet	Amplitude	6%	0.001–0.1	1,000–10,000
			Phase (bleached)	60%
Dichromated gelatin	No	Wet	Phase	100%	10	10,000
Photoresists	No	Wet	Phase	33%	10	3,000
Photothermoplastics	Yes	Charge and heat	Phase	33%	0.01	500–1,200
Photopolymers	No	Post exposure	Phase	100%	1–1,000	2,000–5,000
Photochromics	Yes	None	Amplitude	2%	10–100	>5,000
Photorefractives	Yes	None	Phase	100%	0.1–50,000	2,000–10,000
Elastomers	No	None	Phase	--	300	--

Mass replication of holograms

A hologram sticker on a Nokia mobile phone battery intended to show the battery is genuine (manufactured by Nokia), and not counterfeit.

An 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. 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.

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.

Applications

Data storage

Main 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 available SLMs can produce about 1000 different images a second at 1024×1024-bit resolution. With the right type of media (probably polymers rather than something like LiNbO₃), this would result in about 1 gigabit per second writing speed. Read speeds can surpass this and experts believe 1-terabit per second readout is possible. In 2005, companies such as Optware and Maxell have produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 TB (terabyte), 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.

Security

Main article: Security hologram

UBS Kinebar gold bars use holograms as a security measure.

Security holograms are very difficult to forge because they are replicated from a master hologram which 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, Euro 5/10/20/50/100/200/500 notes, South Korean won 5000/10000/50000 notes, Japanese yen 5000/10000 notes, etc. They are also used in credit and bank cards as well as passports, ID cards, books, DVDs, and sports equipment.

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 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 which 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.

During the 1970s a number of arts 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.). None of these studios still exist, however there is the Center for the Holographic Arts in New York and the HOLOcenter in Seoul which offer artists a place to create and exhibit work.

A small but active group of artists use holography as their main medium and many more artists 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.

Hobbyist use

“Peace Within Reach” a Denisyuk DCG hologram by amateur Dave Battin.

Since the beginning of holography, experimenters have explored the uses of holography. 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 countless hobbyists. 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 green light. It had been assumed that the sensitivity would be non existent. Jeff Blyth responded with the G307 formulation of DCG to increase the speed and sensitivity to these new lasers.

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.

A small group of amateurs are even constructing their own pulsed lasers to make holograms of moving objects.

Holographic interferometry

Main article: holographic interferometry

Holographic interferometry (HI) is a technique which 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.

Interferometric microscopy

Main 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.

As sensors or biosensors

Main 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.

Dynamic holography

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

There also exist holographic materials which 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 (terabit/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 µs, 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. But remember that in optics, addition and Fourier transform are already easily performed in linear materials, the second simply by a lens. This enables some applications like a device that compares images in an optical way.

The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals, but also 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 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).

Non-optical applications

In 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. The principle of electron holography can also be applied to interference lithography.

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, although such holograms have not yet been commercialized.

Other applications

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.

Holography in fiction

See also: Category:Holography in fiction

Holograms are often used as plot devices in science fiction.

• The Jetsons (1962-3 television series) uses holograms as entertainment devices, replacing the television in many episodes
• Star Trek: The Animated Series (1974 television series) episode "The Practical Joker", the holodeck is introduced
• Star Wars (1977 film), use of the hologram in the movies and video games of the series to display people remotely communicating with each another
• Hello America (1981 book by J.G. Ballard), holographic technology is used by president Charles Manson to scare nomad peoples along the United States of America, showing images of American pop culture icons such as Gary Cooper, Mickey Mouse, or the Enterprise space ship.
• Star Trek: The Next Generation (1987 television series), uses the holodeck extensively; beginning with this series, various episodes and films throughout the Star Trek series feature holographic characters and ships
• Red Dwarf (1988 television series), after a catastrophic radiation leak inside the Jupiter mining vessel called 'Red Dwarf', crew member Second Technician Arnold Rimmer is resurrected as a hologram and walks about the ship and planets they encounter. Because he is a "soft-light hologram," he cannot touch anything as objects just pass right through him. However, later in the series he meets 'Legion,' a gestalt entity with advanced technology, who upgrades Rimmer's light bee - the object that projects his hologram by hovering around - changing his projection to what is called in the show "hard-light," thus allowing him to once again touch objects other than computer-generated.
• Back to the Future Part II (1989 film), a giant projection hologram is used as an advertisement for the (fictional) 2015 film Jaws 19
• Total Recall (1990 film), the main character uses a device, similar to a wristwatch, to produce a hologram of himself and deceive his foes
• Star Trek: Voyager (1995-2001 television series) introduced the Emergency Medical Hologram (EMH) doctor
• In the Yu-Gi-Oh! manga, a significant part of the story revolves around a game Duel Monsters played on a holographic image system.
• Stargate: SG-1 (1997-2007 television series), various characters appear as holograms in various episodes: The Asgard masquerade themselves holographically as Norse gods to the primitive peoples under their protection, Morgan le Fay in "The Pegasus Project" and Myrddin as a Merlin in "Avalon" and "Camelot" as a holographic sentry; Heliopolis "Book"; the puddle jumper starship has a holographic HUD
• Lost in Space (1998 film), June Lockhart (Maureen Robinson) appeared as Will's school principal "Cartwright" in a hologram
• Halo (series) (2001 video game) uses "holotanks" to display the avatar of an artificial intelligence construct.
• Stargate: Atlantis (2004-2009 television series), the Atlantis city-starship features a hologram room that allows access to the Ancient database in the form of holograms; an Ancient Control Chair contains holographic projectors; in the episode "Rising", Melia (a member of the Atlantean High Council during the first siege of Atlantis some ten millennia ago) is first seen as a hologram describing the history of the Ancients in the Pegasus Galaxy; Aurora-class battleship can project holograms remotely for communication purposes
• The Island (2005 film), a holographic projector surrounded the military compound where clones were kept to give the illusion of a tropical environment; holographic displays are present on various terminals, including the MSN information terminal in Los Angeles
• Dead Space (2008 video game), to replace the player's HUD, a holographic display shows up in front of the player's character
• Avatar (2009 film), holographic displays are used extensively on terminals and HUDs
• Iron Man and Iron Man 2, the 2008 and 2010 films.
• Enthiran (2010 film), Chitti, the robot, can be telecommunicated with using a "virtual calling" where each caller can be seen as a holographic projection in front of them during the call

See also

• Specular holography
• Volume hologram
• Digital holography
• Digital planar holography
• Holographic Sensor
• Holonomic brain theory
• Integral imaging
• Holographic principle
• Tomography
• List of emerging technologies
• Phase-coherent holography
• Australian Holographics
• Holographic data storage
• Computer generated holography

Further reading

• Hariharan, P (1996). Optical Holography: principles, techniques, and applications. Cambridge University Press. ISBN 978-0521439657.
• 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 holograhic memory system: data quality and security, Proceedings of SPIE, Vol. 4988, p. 104-111 (2003)
• Holographic Visions: A History of New Science Sean F. Johnston, Oxford University Press (2006), ISBN 0-19-857122-4

External links

• International Hologram Manufacturers Association
• U.S. patent 3,506,327 — "Wavefront reconstruction using a coherent reference beam" — E. N. Leith et al.
• The nobel prize lecture of Denis Gabor
• MIT's Spatial Imaging Group with papers about holographic theory and Holographic video
• Medical Applications of Holograms
• How Stuff Works - holograms
• Center for the Holographic Arts, New York - a non-profit organisation promoting holography
• Specular holography art site
• Faster way to produce holographic tiles
• Abrasion, hand-drawn holograms