Holography
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Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms; it is an advanced 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.
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[edit] Overview
Holography was invented, at least conceptually, in 1947 by Hungarian physicist Dennis Gabor (1900–1979), work for which he received the Nobel Prize in physics in 1971. 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 which recorded 3D objects were made by Emmett Leith and Juris Upatnieks in Michigan, USA in 1963 and by Yuri Denisyuk in the Soviet Union.
Several types of holograms can be made. The first holograms were "transmission holograms", which were viewed by shining laser light through them and looking at the reconstructed image off to one side. A later refinement, the "rainbow transmission" hologram allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission holograms are now commonly formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating which provides the light from "behind" to reconstruct their imagery. Another kind of common hologram (a Denisyuk hologram) is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light 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.
[edit] 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 color filters), which allows a limited reconstruction of the wavelength of the light, and thus its color.
However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost, and with it the three-dimensional effect. In a hologram, information from both the intensity and the phase is recorded. When illuminating the hologram with the appropriate light, it diffracts part of it into exactly the same wave (up to a constant phase shift invisible to our eyes) which emanated from the original scene, thus retaining the three-dimensional appearance. Although colour holograms are possible, in most cases the holograms are recorded monochromatically.
[edit] 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 miracle 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 recorded already 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.
[edit] 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 both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move his or her viewpoint and see the image rotate exactly as the original object would.
[edit] 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.
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 |
[edit] Mass replication
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 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.
[edit] 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 [2].
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 to remove 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 the atmospheric turbulence (the phenomenon that gives rise to the twinkling of starlight).
[edit] Holographic data storage
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. 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.
[edit] 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.
[edit] Holography in art
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.[3]
The Dalí Holograms were mastered in St. Louis, at the McDonnell Douglas Company who had just invested in a Ruby Pulse Laser and decided to, aside from meteorological purposes, make industrially oriented projection Holograms for presentations and trade shows. In London, Dalí assembled his models by hanging objects with wires inside of wooden frames. This technique allowed for overlapping and differences in depth.
Since then the quality of the holograms has increased dramatically, mainly due to better holographic emulsions. As of 2005 there are many artists who use holograms in their creations.
[edit] Holographic theories of brain function
Holonomic brain theory, originated by Karl Pribram and initially developed in collaboration with David Bohm, models cognitive function as being guided by a matrix of neurological wave interference patterns situated temporally between holographic Gestalt perception and discrete, affective, quantum vectors derived from reward anticipation potentials.
Pribram was originally struck by the similarity of the hologram idea and Bohm's idea of the implicate order in physics, and contacted him for collaboration. In particular, the fact that information about an image point is distributed throughout the hologram, such that each piece of the hologram contains some information about the entire image, seemed suggestive to Pribram about how the brain could encode memories. (Pribram, 1987). Pribram was encouraged in this line of speculation by the fact that DeValois and DeValois (1980) had found that "the spatial frequency encoding displayed by cells of the visual cortex was best described as a Fourier transform of the input pattern." (Pribram, 1987) This holographic idea lead to the coining of the term "holonomic" to describe the idea in wider contexts than just holograms.
[edit] Footnotes
- ^ Lecture Holography and optical phase conjugation held at ETH Zürich by Prof. G. Montemezzani in 2002
- ^ R. Ryf et al. High-frame-rate joint Fourier-transform correlator based on Sn2P2S6 crystal, Optics Letters 26, 1666-1668 (2001)
- ^ Source: http://www.holophile.com/history.htm, retrieved December 2005
[edit] See also
[edit] External links
- U.S. Patent 3506327 — "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
- Articles describing how to make holograms with a laser diode: [1], retrieved December 2005
- How Holographic Versatile Discs Work
- How Holographic Memory Will Work
- How Holographic Environments Will Work
- Very good simple and clear explanations
- HoloWiki - A Wiki for Holographers
- InPhase Technologies - "What is holographic storage?"
- Learn how to make your own holograms with no more than a board of acrylic glass, a compass, and the sun
- Tutorials for holographers
- Simple Tutorials for Making Holograms in Home and School
- Holography podcasts with worldwide guests
- MIT's Spatial Imaging Group with papers about holographic theory and Holographic video