Holography

This article is about the photographic technique. For other uses of "hologram" and related terms, see Holography (disambiguation).

Holography (from the Greek, όλος-hòlòs whole + γραφή-grafè writing, drawing) is the science of producing holograms; it is a form of photography that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store, retrieve, and process information. It is common to confuse volumetric displays with holograms, particularly in science fiction works such as Star Trek, Star Wars, Red Dwarf, and Quantum Leap.

Identigram as a security element in a German Identity card (Personalausweis)

Overview

File:Hologrammit.jpg

Hologram Artwork in MIT Museum

Holography was invented in 1947 by Hungarian physicist Dennis Gabor (1900–1979), work for which he received the Nobel Prize in physics in 1971. It was made possible by pioneering work in the field of physics by other scientists like Mieczysław Wolfke who resolved technical issues that previously made advancements impossible. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England. The British Thomson-Houston company filed a patent on 1947-12-17 (and received patent GB685286), but the field did not really advance until the development of the laser in 1960.

The first holograms that recorded 3D objects were made by Yuri Denisyuk in the Soviet Union in 1962; later by Emmett Leith and Juris Upatnieks in University of Michigan, USA in 1962. 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 or other monochromatic sources. 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 aluminium coating which 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.

One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers—typically used by the millions in DVD recorders and other applications, but 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.

Types of Security Holograms

Holograms are classified into different types with reference to the degree of level of optical security incorporated in them during the process of master origination. The different classifications are described below:

2D / 3D "hologram" images are not true holograms. Secondary meaning.

These are by far the most common type of hologram - and in fact they are not holograms in any true sense of the words. The term "hologram" has taken on a secondary meaning due to the widespread use of a multilayer image on credit cards and driver licenses. This type of "hologram" consists of two or more images stacked in such a way that each is alternately visible depending upon the angle of perspective of the viewer. The technology here is similar to the technology used for the past 50 years to make red safety night reflectors for bicycles, trucks, and cars. These holograms (and therefore the artwork of these holograms) may be of two layers (i.e. with a background and a foreground) or three layers (with a background, a middle ground and a foreground). The matter of the middle ground in the case of the two-layer holograms are usually superimposed over the matter of the background of the hologram. These holograms display a unique multilevel, multi-colour effect. These images have one or two levels of flat graphics “floating” above or at the surface of the hologram. The matter in the background appears to be “under” or “behind” the hologram, giving the illusion of depth.

Dot matrix

These holograms have a maximum resolution of 10 micrometres per optical element and are produced on specialized machines making forgery difficult and expensive. To design optical elements, several algorithms are used to shape scattered radiation patterns.

Electron-Beam lithography

These types of hologram are originated using highly sophisticated and very expensive electron-beam lithography system and this is the latest technology in the world at present. This kind of technology allows the creation of surface holograms with a resolution of up to 0.1 micrometres (12,000 dpi). This technique requires development of various algorithms for designing optical elements that shapes scattered radiation patterns. This type of hologram offers features like the viewing of four lasers at a single point, 2D/3D raster text, switch effects, 3D effects, concealed images, laser readable text and true colour images.

The various kinds of features possible in security holograms are mentioned below:

Concealed Images

These usually take the form of very thin lines and contours. Concealed images can be seen at large angle light diffraction, and at one particular angle only.

Guilloche Pattern (High Resolution Line Patterns)

These are sets of thin lines of a complicated geometry (Guilloche patterns) drawn with high resolution. The technology allows continuous visual changes of colour along each separated lines.

Kinetic images

They can be seen when the conditions of hologram observations are being changed. Turning or inclining the hologram allows the movements of certain features of the image to be studied.

Microtexts or nanotexts

Dot matrix holograms are capable of embedding microtext at various sizes. There are three types of microtexts in holograms: high contrast microtexts of size 50 – 150 micrometres; diffractive grating filled microtexts of size 50 – 150 micrometres low contrast microtexts. Microtexts of sizes smaller than 50 micrometres are referred to as nanotext. Nanotext with sizes of less than 50 micrometres can be observed with a microscope only.

CLR (Covert Laser Readable) image

Dot matrix holograms also support CLR imagery, where a simple laser device may be used to verify the hologram's authenticity. Computing CLR images is a complicated mathematical task that involves solving ill-posed problems. There are two types of CLR: Dynamic CLR and Multigrade CLR. Dynamic CLR is a set of CLR fragments that produce animated images on the screen as the control device moves along the hologram surface. Multigrade CLR images produce certain images on the screen of the controlling device, which differ in the first and minus first orders of laser light diffraction. As a variant, a hidden image which is both negative and positive, in plus one and minus one order respectively, may be created.

Computer Synthesized 2D/3D and 3D images

This technology allows 2D / 3D images to be combined with other security features (microtexts, concealed images, CLR etc.) - this combination effect cannot be achieved using any other traditional technologies of origination.

True colour images

True colour images are very effective decorative pictures. When synthesized by computer, they may include microtexts, hidden images, and other security features, yielding attractive, high-security holograms.

Technical description

The difference between holography and photography is best understood by considering what a black and white photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour. Although colour holograms are possible, in most cases the holograms are recorded monochromatically.

Holographic recording process

Holographic recording process

To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). If these two beams are coherent, optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram. The central goal of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is reconstructed, producing a 3D image.

These recorded fringes do not directly represent their respective corresponding points in the space of a scene (the way each point on a photograph represents a single point in the scene being photographed). Rather, a small portion of a hologram's surface contains enough information to reconstruct the entire original scene, but only what can be seen from that small portion as viewed from that point's perspective. This is possible because during holographic recording, each point on the hologram's surface is affected by light waves reflected from all points in the scene, rather than from just one point. It's as if, during recording, each point on the hologram's surface were an eye that could record everything it sees in any direction. After the hologram has been recorded, looking at a point in that hologram is like looking "through" one of those eyes.

To demonstrate this concept, you could cut out and look at a small section of a recorded hologram; from the same distance you see less than before, but you can still see the entire scene by shifting your viewpoint laterally or by going very near to the hologram, the same way you could look outside in any direction from a small window in your house. What you lose is the ability to see the objects from many directions, as you are forced to stay behind the small window.

Because of the need for coherent interference between the reference and object beams, nowadays laser light is used to record holograms. But the first holograms were already recorded prior to the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.

In simple holograms the coherence length of the beam determines the maximum depth the image can have. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram. Also 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.

In a very simple setup the object consists of a phase and amplitude mask placed in a hole of a larger screen and the reference beam passes through a second hole at some distance and whereby it is diffracted so that it overlaps with the light from the object in the far field, where the fringes are recorded (see Nature Physics Vol. 2, No. 12 cover).

Holographic reconstruction process

Holographic reconstruction process

When the processed holographic film is illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase (except for rainbow holograms where the depth information is encoded entirely in the zoneplate angle). Because many viewpoints are stored, each of the viewer's eyes sees the image from a slightly different angle, so the image appears three-dimensional. This is known as stereopsis. The viewer can move his or her viewpoint and see the image rotate exactly as the original object would.

If several objects have been combined in a multi-holographic image, then running just one reference beam can reconstruct all the objects (this is more or less called "redintegration")

Holograms as diffraction gratings

A diffraction grating receives incoming light from the left which is diffracted by the slits

A diffraction grating is a transparent or reflective sheet with thin slits, the distance between them and their diameter being on the order of the wavelength of the light. Light rays travelling towards it are bent at an angle determined by the distance between the slits and the wavelength of the light.

When holograms are constructed, the reference beam and the object beam interfere with one another (see sections above), and the dark and light fringes of the interference pattern are recorded. When this photograph is developed, the light parts become clear and the dark parts opaque. The clear, light parts become like the slits of a diffraction grating, and the angle at which they bend incoming light (the reconstruction beam) is determined by the spacing between them, which in turn was determined originally by the object beam and reference beam, when the hologram's interference pattern was made. Thus the slits bend the reconstruction beam to be the exact angles at each point that the object beam was going at.

File:Hologram diagram 4.jpg

An incoming plane wave interferes with another, travelling at a certain angle to the other.

To demonstrate this concept at its simplest, consider two plane waves interfering through a photographic plate. One hits the plane by the perpendicular direction (the reconstruction beam) and the other one by an angle θ (the object beam). The distance d between the slits is determined by the wavelength of the waves (they are in phase both time-wise and space-wise, so this is the same for both) and the angle between them. If we record the interference pattern through a particular one dimensional slice of the overall two dimensional pattern, we get the plate with the yellow stripes which represent where destructive interference occurs. If we develop this photographic plate so that the destructive interference stripes become slits, and take away the wave that was travelling at an angle (the object beam) and leave the one traveling perpendicular to the recorded pattern (the reference beam, which now becomes the reconstruction beam). The reconstruction beam will get bent by the slits left by the pattern.

The destructive interference leaves behind slits that become a diffraction grating. The incoming light is bent by them.

The diffraction grating created by the two waves interfering has recreated the "object beam": the plane wave that was originally travelling at an angle. To further demonstrate the concept, consider a point source and a plane wave interfering:

File:Hologram diagram 5.jpg

A point source of light and a plane wave interfere.

The spacing d between the destructive interference fringes gets smaller and smaller the further from the plate the light from the point source is. With a smaller d, the angle the reconstruction wave will get bent through will become sharper. If the photographic plate is developed, and the plane wave shone back through, the light will be bent at differing angles depending on the distance d between the slits.

File:Hologram diagram 6.jpg

The reference beam is shone through the developed photographic plate.

The diffraction grating reconstructs the point source. The light emerging from the photographic plate is identical to the light emerging when the point source used to be there. If you were standing on the other side of this simple hologram, your eyes would see the curved light rays (these are lines perpendicular to the wavefronts) and follow them perpendicularly back to where they meet, and tell your brain that there is a point there.

This is what our eyes do every day to see images. This is why we can see things that don't correspond directly to reality, like bent spoons in glasses, mirages, and our reflections in mirrors because our eyes faithfully follow back the light to where it came from whether the light actually started there or not: every time there is a discrepancy between reality and what we see, it is because light waves have been deviated or bent from their original course.

All objects that we see, we see as a collection of point sources. Each point on the object radiates out light as a point source and the collection of points our eyes see becomes a whole object. It is the same with holograms: every single point on the object records its own private interference pattern, which gets individually reconstructed, and our eyes see all these points reconstructed together to see the whole picture of the hologram all at once.

This explains why your view of the object in the hologram changes with your position; each time you move you are seeing a different ray emitted from each point source. (Like moving around in front of a window, you see the ray from different sides of objects depending where you're standing.) With normal photography, the camera records just one view, so when you move you are in effect seeing the same ray again and your view doesn't change. (You are seeing different rays from each droplet of ink, but each droplet of ink is one ray of the picture.) The hologram, in comparison, records every possible view there is to see, all at once.

Materials

It is possible to store the diffraction gratings that make up a hologram as phase gratings or amplitude gratings. In the former type the optical distance (i.e. the refractive index or in some cases the thickness) in the material is modulated. In amplitude gratings the modulation is in the absorption. Amplitude holograms have a lower efficiency than phase holograms and are therefore used more rarely. 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).

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, which are described in the following section. 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

These are by far the most common type of holograms preferred by customers. These holograms may be of two layers i.e. with a background and a foreground or three layers - with a background, a middle ground and a foreground. The matter of the middle ground in the case of the two layer holograms are usually superimposed over the matter of the background of the hologram.

These holograms display a unique multilevel, multi-color effect. These images have one or two levels of flat graphics “floating” above or at the surface of the hologram. The matter in the background appears to be “under” or “behind” the hologram, giving the illusion of depth. Apart from being the most cost-effective type of hologram the 2D / 3D Hologram, when viewed under a variety of sources of light, are extremely eye – catching.

Mass replication

A hologram on a Nokia mobile phone battery. This is intended to show the battery is 'original Nokia' and not a cheaper imitation.

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.

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 aluminium is usually added on the hologram recording layer.

Coherent combining of holograms

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.

Dynamic holography

The discussion above describes static holography, in which recording, developing and reconstructing occur sequentially and a permanent hologram is produced.

There exist also holographic materials which don't need the developing process and can record a hologram in a very short time. This allows 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 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).

Holographic data storage

Main article: Holographic memory

Holography can be applied 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 reach the denser 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.

Digital holography

Main article: digital holography

An alternate method to record holograms is to use a digital device like a CCD camera instead of a conventional photographic film. This approach is often called digital holography. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen or TV set.

Use of holography in banknotes

Holograms are used widely as a security device in many currencies such as the Brazilian real 20 note, British pound 5/10/20 notes, Canadian dollar 5/10/20/50/100 notes, Euro 5/10/20/50/100/200/500 notes, South Korean won 5000/10000 notes, Japanese yen 5000/10000 notes, etc.

Holography in 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 most notorious 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 1970's 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 Llyod Cross, The Museum of Holography in New York founded by Rosemary (Possie) H. Jackson, the Royal College of Art in London and the Lake Forrest 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 [1] and the HOLOcenter in Seoul [2] which offer artists a place to create and exhibit work.

A small but active group of artist use holography as their main medium and many more artists integrate holographic elements into their work.

The MIT Museum [3] and Jonathan Ross [4] both have extensive collections of holography and on-line catalogues of art holograms.

Holography as a hobby

“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 (invented by Jerry Pethic) to hold the optics rigid and dampen 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 hobbiests. This was a very important development for amateurs as it took the cost for a 5mw laser from $1200 to $5. 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 Blythe 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, Methelene Blue Sensitised Dichromated Gelatin and Diffusion Method Silver Halide preparations. Jeff Blythe 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.

Jeff Blythe's Film Formulations

Non-optical holography

The principles of holography refer to general properties of waves. In principle, holography can be applied to any beam, as far as the beam can be interpreted in terms of waves.

Electron holography

Main article: Electron holography

Electron holography is the application of holography techniques to electron waves rather than light waves.

Electron holography was invented by Dennis Gabor to improve the resolution and avoid the aberrations of the transmission electron microscope. Today it is commonly used to study electric and magnetic fields in thin films, as magnetic and electric fields can shift the phase of the interfering wave passing through the sample.

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

Acoustic holography

Main article: Acoustic holography

Acoustic Holography is the method for registering sound waves quite analogous to the optical holography.

Atom holography

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.

Holographic theories of brain function

An analogy between the distributed information in holograms and the distributed information in brains gave rise to a speculative idea termed holonomic brain theory.

References

1. Optical holography: principles, techniques, and applications P. Hariharan, Cambridge University Press; 2 edition (1996), ISBN 978-0521439657
2. Lasers and holography: an introduction to coherent optics W. E. Kock, Dover Publications (1981), ISBN 978-0486240411
3. Principles of holography H. M. Smith, Wiley (1976), ISBN 978-0471803416
4. 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)
5. Holographic Visions: A History of New Science Sean F. Johnston, Oxford University Press (2006), ISBN 0-19-857122-4

External links

• 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
• Explora Museum in Frankfurt/Main — Germany
• 3D Museum in Dinkelsbühl — Germany
• wikiHow - How to Make a Hologram
• MIT's Spatial Imaging Group with papers about holographic theory and Holographic video
• Medical Applications of Holograms
• How Stuff Works - holograms
• A wiki for making holograms
• Center for the Holographic Arts, New York - a non-profit organisation promoting holograpy
• Faster way to produce holographic tiles

See also

• Holographic Discs
• Phase-coherent holography
• Integral imaging
• Security printing
• Dennis Gabor
• Mieczysław Wolfke
• Nick Phillips