[edit] Overview and history
Holography was invented in 1947 by the Hungarian-British physicist Dennis Gabor (Hungarian name: Gábor Dénes),[1][2] work for which he received the Nobel Prize in Physics in 1971. Pioneering work in the field of physics by other scientists including Mieczysław Wolfke resolved technical issues that previously had prevented advancement. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, and the company filed a patent in December 1947 (patent GB685286). The technique as originally invented is still used in electron microscopy, where it is known as electron holography, but optical holography did not really advance until the development of the laser in 1960.The development of the laser enabled the first practical optical holograms that recorded 3D objects to be made in 1962 by Yuri Denisyuk in the Soviet Union[3] and by Emmett Leith and Juris Upatnieks at University of Michigan, USA.[4] Early holograms used silver halide photographic emulsions as the recording medium. They were not very efficient as the grating produced absorbed much of the incident light. Various methods of converting the variation in transmission to a variation in refractive index (known as "bleaching") were developed which enabled much more efficient holograms to be produced.[5][6][7]
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.[8] A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers.[9] Rainbow holograms are commonly seen today on credit cards as a security feature and on product packaging.
Another kind of common hologram, the reflection or Denisyuk hologram, can also be viewed using a white-light illumination source on the same side of the hologram as the viewer and is the type of hologram normally seen in holographic displays. They are also capable of multicolour-image reproduction.[10]
Specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface.[11] It works by reflectively or refractively manipulating bundles of light rays, whereas Gabor-style holography works by diffractively reconstructing wavefronts.
Most holograms produced are of static objects but systems for displaying changing scenes on a holographic volumetric display are now being developed.[12][13][14]
Holograms can also be used to store, retrieve, and process information optically.[15]
In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor lasers, such as those found in millions of DVD recorders and used in other common applications, can be used to make holograms and have made holography much more accessible to low-budget researchers, artists and dedicated hobbyists.
It was thought that it would 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.[16]
[edit] How holography works
Holograms are recorded using a flash of light that illuminates a scene and then imprints on a recording medium, much in the way a photograph is recorded. A hologram, however, requires a laser as the light source, since lasers can be precisely controlled and have a fixed wavelength, unlike white light, which contains many different wavelengths.
A shutter is required when taking a photograph to limit the time in which the film is exposed to light. Holography also requires a specific exposure time, and this can be done using a shutter, or by electronic timing of the laser.
This laser beam is generally aimed through a series of elements that change it in different ways - see Figure 2. The first element is a beam splitter, which divides the beam into two identical beams, each aimed in different directions:
- One beam, known as the illumination or object beam, is spread using lenses and directed onto the scene using mirrors, in order to illuminate it. Some of the light scattered (reflected) from this illumination falls onto the recording medium.
- The second beam, known as the reference beam, is also spread through the use of lenses, but is directed so that it doesn't come in contact with the scene, and instead travels directly onto the recording medium.
On the recording medium, the light waves of the two beams intersect and interfere with each other. It is this interference pattern that is imprinted on the holographic medium. The pattern itself is seemingly random, as this pattern represents the way in which the scene's light interfered with the original light source, but not the original light source itself. The interference pattern can be said to be an encoded version of the scene, requiring a particular key, that is, the original light source, in order to view its contents. This missing key is provided later by shining a laser, identical to the one used to record the hologram, onto the developed film which then recreates a range of the scene's original light.
When the original reference beam illuminates the hologram, it is diffracted by the recorded hologram to produce a light field which is identical to the light field which was originally scattered by the object or objects onto the hologram - see Figure 3. When the object is removed, an observer who looks into the hologram "sees" the same image on his retina as he would have seen when looking at the original scene. This image is known as a virtual image.
Figure 4 is a photograph of a hologram's surface. The object in the hologram is a toy van. It can be seen that it is no more possible to discern the subject of the hologram from this pattern than it is to identify what music has been recorded by looking at the hills and valleys on a vinyl record surface, or the pits on a CD. Also note that the holographic recording is described by the speckle pattern, rather than the "wavy" line pattern; the latter being an incidental result of interference between multiple reflections in the glass plate on which the film is mounted.
[edit] Holography vs. photography
Each point in the holographic recording includes light scattered from every point in the scene, whereas each point in a photograph has light scattered only from a single point in the scene which has been focused by a lens onto the film or the digital capture medium.A hologram differs from a photograph in several ways:
- The hologram allows the recorded scene to be viewed from a wide range of angles whereas the photograph gives only a single view.
- The reproduced range of a hologram adds many of the same depth perception cues that were present in the original scene, which are again recognized by the human brain and translated into the same perception of a three-dimensional image as when the original scene might have been viewed. The photograph is a flat two-dimensional representation.
- The developed hologram surface itself consists of a very fine, seemingly random pattern, which appears to bear no relationship to the scene which it has recorded. A photograph clearly maps out the light field of the original scene.
- When a photograph is cut in pieces, each piece shows only part of the scene. When a hologram is cut in pieces, the whole scene can still be seen in each piece - see Figure 8 below. Think of viewing a street outside your house through a 4ft x 4ft window, and then through a 2ft x 2ft window; You can see the same things through the smaller window, but you can see more at once through the 4ft window and you may need to change your viewing position to see everything in the smaller window.
- A photograph can be viewed in a wide range of lighting conditions, whereas holograms can only be viewed with very specific forms of illumination.
[edit] Physics of holography
For a better understanding of the process, it is necessary to understand interference and diffraction. Interference occurs when one or more wavefronts are superimposed. Diffraction occurs whenever a wavefront encounters an object. The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction. It is somewhat simplified but is accurate enough to provide an understanding of how the holographic process works.For those unfamiliar with these concepts, it is worthwhile to read the respective articles before reading further in this article.
[edit] Plane wavefronts
A diffraction grating is a structure with a repeating pattern. A simple example is a metal plate with slits cut at regular intervals. A light wave incident on a grating is split into several waves; the direction of these diffracted waves is determined by the grating spacing and the wavelength of the light.A simple hologram can be made by superimposing two plane waves from the same light source on a holographic recording medium. The two waves interfere giving a straight line fringe pattern whose intensity varies sinusoidally across the medium. The spacing of the fringe pattern is determined by the angle between the two waves, and on the wavelength of the light.
The recorded light pattern is a diffraction grating. When it is illuminated by only one of the waves used to create it, it can be shown that one of the diffracted waves emerges at the same angle as that at which the second wave was originally incident so that the second wave has been 'reconstructed'. Thus, the recorded light pattern is a holographic recording as defined above.
[edit] Point sources
When a plane wavefront illuminates a negative lens, it is expanded into a wave which appears to diverge from the focal point of the lens. Thus, when the recorded pattern is illuminated with the original plane wave, some of the light is diffracted into a diverging beam equivalent to the original plane wave; a holographic recording of the point source has been created.
When the plane wave is incident at a non-normal angle, the pattern formed is more complex but still acts as a negative lens provided it is illuminated at the original angle.
[edit] Complex objects
To record a hologram of a complex object, a laser beam is first split into two separate beams of light. One beam illuminates the object, which then scatters light onto the recording medium. According to diffraction theory, each point in the object acts as a point source of light so the recording medium can be considered to be illuminated by a set of point sources located at varying distances from the medium.The second (reference) beam illuminates the recording medium directly. Each point source wave interferes with the reference beam, giving rise to its own sinusoidal zone plate in the recording medium. The resulting pattern is the sum of all these 'zone plates' which combine to produce a random (speckle) pattern as in the photograph above.
When the hologram is illuminated by the original reference beam, each of the individual zone plates reconstructs the object wave which produced it, and these individual wavefronts add together to reconstruct the whole of the object beam. The viewer perceives a wavefront that is identical to the wavefront scattered from the object onto the recording medium, so that it appears to him or her that the object is still in place even if it has been removed. This image is known as a "virtual" image, as it is generated even though the object is no longer there.
[edit] Mathematical model
A light wave can be modelled by a complex number U, which represents the electric or magnetic field of the light wave. The amplitude and phase of the light are represented by the absolute value and angle of the complex number. The object and reference waves at any point in the holographic system are given by UO and UR. The combined beam is given by UO + UR. The energy of the combined beams is proportional to the square of magnitude of the combined waves as:
If a photographic plate is exposed to the two beams and then developed, its transmittance, T, is proportional to the light energy that was incident on the plate and is given by
where k is a constant.
When the developed plate is illuminated by the reference beam, the light transmitted through the plate, UH is equal to the transmittance T multiplied by the reference beam amplitude UR, giving
It can be seen that UH has four terms, each representing a light beam emerging from the hologram. The first of these is proportional to UO. This is the reconstructed object beam which enables a viewer to 'see' the original object even when it is no longer present in the field of view.
The second and third beams are modified versions of the reference beam. The fourth term is known as the "conjugate object beam". It has the reverse curvature to the object beam itself and forms a real image of the object in the space beyond the holographic plate.
When the reference and object beams are incident on the holographic recording medium at significantly different angles, the virtual, real and reference wavefronts all emerge at different angles, enabling the reconstructed object to be seen clearly.
[edit] Recording a hologram
[edit] Items required
- a suitable object or set of objects
- a suitable laser beam
- optical components which enable the laser beam to be split into two, with one beam (the object beam) directed onto the object, and the other beam (the reference beam) directed onto the recording medium, enabling an interference pattern between the object beam and the reference beam to be created
- a recording medium which converts this interference pattern into an optical element which modifies either the amplitude or the phase of an incident light beam according to the intensity of the interference pattern.
- an environment which provides sufficient mechanical and thermal stability that the interference pattern is stable during the time in which the interference pattern is recorded[18]
The exposure time required to record the hologram depends on the laser power available, on the particular medium used and on the size and nature of the object(s) to be recorded, just as in conventional photography. This determines the stability requirements. Exposure times of several minutes are typical when using quite powerful gas lasers and silver halide emulsions. All the elements within the optical system have to be stable to fractions of a μm over that period. It is possible to make holograms of much less stable objects by using a pulsed laser which produces a large amount of energy in a very short time (μs or less).[19] These systems have been used to produce holograms of live people. A holographic portrait of Dennis Gabor was produced in 1971 using a pulsed ruby laser.[20][21]
Thus, the laser power, recording medium sensitivity, recording time and mechanical and thermal stability requirements are all interlinked.
Another very important laser parameter is its coherence.[22] This be envisaged by considering a laser to produce a sine wave whose frequency drifts over time; the coherence length can then be considered to be the distance over which it maintains a single frequency. This is important because two waves of different frequencies do not produce a stable interference pattern. The coherence length of the laser determines the depth of field which can be recorded in the scene. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram.
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. Holograms of flat shiny objects have been made by locating it very close to the recording plate.[23]
[edit] Hologram classifications
There are three important properties of a hologram which are defined in this section. A given hologram will have one or other of each of these three properties, e.g. we can have an amplitude modulated thin transmission hologram, or a phase modulated, volume reflection hologram.[edit] Amplitude and phase modulation holograms
An amplitude modulation hologram is one where the amplitude of light diffracted by the hologram is proportional to the intensity of the recorded light. A straightforward example of this is photographic emulsion on a transparent substrate. The emulsion is exposed to the interference pattern, and is subsequently developed giving a transmittance which varies with the intensity of the pattern - the more light that fell on the plate at a given point, the darker the developed plate at that point.A phase hologram is made by changing either the thickness or the refractive index of the material in proportion to the intensity of the holographic interference pattern. This is a phase grating and it can be shown that when such a plate is illuminated by the original reference beam, it reconstructs the original object wavefront. The efficiency (i.e. the fraction of the illuminated beam which is converted to reconstructed object beam) is greater for phase than for amplitude modulated holograms.
[edit] Thin holograms and thick (volume) holograms
A thin hologram is one where the thickness of the recording medium is much less than the spacing of the interference fringes which make up the holographic recording.A thick or volume hologram is one where the thickness of the recording medium is greater than the spacing of the interference pattern. The recorded hologram is now a three dimensional structure, and it can be shown that incident light is diffracted by the grating only at a particular angle, known as the Bragg angle.[24] If the hologram is illuminated with a light source incident at the original reference beam angle but a broad spectrum of wavelengths, reconstruction occurs only at the wavelength of the original laser used. If the angle of illumination is changed, reconstruction will occur at a different wavelength and the colour of the re-consturcted scene changes. A volume hologram effectively acts as a colour filter.
[edit] Transmission and reflection holograms
A transmission hologram is one where the object and reference beams are incident on the recording medium from the same side. An optical arrangement for making a transmission hologram is shown in Figure 2. In practice, several more mirrors may be used to direct the beams in the required directions.Normally, transmission holograms can only be reconstructed using a laser or a quasi-monochromatic source, but a particular type of transmission hologram, known as a rainbow hologram, can be viewed with white light.
In a reflection hologram, the object and reference beams are incident on the plate from opposite sides of the plate. The reconstructed object is then viewed from the same side of the plate as that at which the re-constructing beam is incident.
Only volume holograms can be used to make reflection holograms, as only a very low intensity diffracted beam would be reflected by a thin hologram.
[edit] Holographic recording media
The recording medium has to convert the original interference pattern into an optical element that modifies either the amplitude or the phase of an incident light beam in proportion to the intensity of the original light field.The recording medium should be able to resolve fully all the fringes arising from interference between object and reference beam. These fringe spacings can range from tens of microns to less than one micron, i.e. spatial frequencies ranging from a few hundred to several thousand cycles/mm, and ideally, the recording medium should have a response which is flat over this range. If the response of the medium to these spatial frequencies is low, the diffraction efficiency of the hologram will be poor,and a dim image will be obtained. It should be noted that standard photographic film has a very low, or even zero, response at the frequencies involved and cannot be used to make a hologram - see, for example, Kodak's professional black and white film[25] whose resolution starts falling off at 20 lines/mm and it is unlikely than any reconstructed beam would be obtained using this film.
If the response is not flat over the range of spatial frequencies in the interference pattern, then the resolution of the reconstructed image may also be degraded.[26][27]
The table below shows the principal materials used for holographic recording. Note that these do not include the materials used in the mass replication of an existing hologram which are discussed in the next section. The resolution limit given in the table indicates the maximal number of interference lines/mm of the gratings. The required exposure is for a long exposure. Short exposure times (less than 1/1000 of a second, such as with a pulsed laser) require a higher exposure due to reciprocity failure.
| Material | Reusable | Processing | Type of hologram | Theoretical maximum efficiency | Required exposure [mJ/cm2] | Resolution limit [mm−1] |
|---|---|---|---|---|---|---|
| Photographic emulsions | No | Wet | Amplitude | 6% | 1.5 | 5000 |
| Phase (bleached) | 60% | |||||
| Dichromated gelatin | No | Wet | Phase | 100% | 100 | 10,000 |
| Photoresists | No | Wet | Phase | 30% | 100 | 3,000 |
| Photothermoplastics | Yes | Charge and heat | Phase | 33% | 0.1 | 500–1,200 |
| Photopolymers | No | Post exposure | Phase | 100% | 10000 | 5,000 |
| Photorefractives | Yes | None | Phase | 100% | 10 | 10,000 |
[edit] Embossing and mass production
An existing hologram can be replicated, either optically, similar to holographic recording or in the case of surface relief holograms, by embossing.[29] 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.[30] 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.[31]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. Embossed holograms are used widely on credit cards, banknotes, and high value products.[32]
It is possible to print holograms directly into steel using a sheet explosive charge to create the required surface relief.[33]
[edit] Reconstructing and viewing the holographic image
When the hologram plate is illuminated by a laser beam identical to the reference beam which was used to record the hologram, an exact reconstruction of the the original object wavefront is obtained. An imaging system (an eye or a camera) located in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing the original. When the lens is moved, the image changes in the same way as it would have done when the object was in place, as illustrated in Figure 1. If several objects were present when the hologram was recorded, the reconstructed objects move relative to one another, i.e. exhibit parallax, in the same way as the original objects would have done. It was very common in the early days of holography to use a chess board as the object and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess pieces appeared to change.A holographic image can also be obtained using a different laser beam configuration to the original recording object beam, but the reconstructed image will not match the original exactly.[34]. When a laser is used to reconstruct the hologram, the image is speckled just as the original image will have been. This can be a major drawback in viewing a hologram.
White light consists of light of a wide range of wavelengths. Normally, if a hologram is illuminated by a white light source, each wavelength can be considered to generate its own holographic reconstruction, and these will vary in size, angle, and distance. These will be superimposed, and the summed image will wipe out any information about the original scene, just as if you superimposed a set of photographs of the same object of different sizes and orientations. However, a holographic image can be obtained using white light in specific circumstances, e.g. with volume holograms and rainbow holograms.The white light source used to view these holograms should always approximate to a point source, i.e. a spot light or the sun. An extended source (e.g. a flourescent lamp) will not reconstruct a hologram since it light is incident at each point at a wide range of angles, giving multiple reconstructions which will "wipe" one another out.
White light reconstructions do not contain speckles.
[edit] Volume holograms
A volume hologram can give a reconstructed beam using white light, as the hologram structure effectively filters out colours other than those equal to or very close to the colour of the laser used to make the hologram so that the reconstructed image will appear to be approximately the same colour as the laser light used to create the holographic recording.[edit] Rainbow holograms
Main article: Rainbow hologram
The holograms found on credit cards are examples of rainbow holograms. These are technically transmission holograms mounted onto a reflective surface like a metalized polyethylene terephthalate substrate commonly known as PET.
[edit] Fidelity of the reconstructed beam
Any change in the shape, orientation or wavelength of the reference beam gives rise to aberrations in the reconstructed image. For instance, the reconstructed image is magnified if the laser used to reconstruct the hologram has a shorter wavelength than the original laser. Nonetheless, good reconstruction is obtained using a laser of a different wavelength, quasi-monochromatic light or white light, in the right circumstances.
Since each point in the object illuminates all of the hologram, the whole object can be reconstructed from a small part of the hologram. Thus, a hologram can be broken up into small pieces and each one will enable the whole of the original object to be imaged. One does, however, lose information and the spatial resolution gets worse as the size of the hologram is decreased — the image becomes "fuzzier". The field of view is also reduced, and the viewer will have to change position to see different parts of the scene.
[edit] Applications
[edit] 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 a scientist.Salvador Dalí claimed to have been the first to employ holography artistically. He was certainly the first and best-known surrealist to do so, but the 1972 New York exhibit of Dalí holograms had been preceded by the holographic art exhibition that was held at the Cranbrook Academy of Art in Michigan in 1968 and by the one at the Finch College gallery in New York in 1970, which attracted national media attention.[36]
During the 1970s, a number of art studios and schools were established, each with their particular approach to holography. Notably, there was the San Francisco School of Holography established by Lloyd Cross, The Museum of Holography in New York founded by Rosemary (Possie) H. Jackson, the Royal College of Art in London and the Lake Forest College Symposiums organised by Tung Jeong (T.J.).[37] None of these studios still exist; however, there is the Center for the Holographic Arts in New York[38] and the HOLOcenter in Seoul,[39] which offers artists a place to create and exhibit work.
During the 1980s, many artists who worked with holography helped the diffusion of this so-called "new medium" in the art world, such as Harriet Casdin-Silver of the USA, Dieter_Jung_(artist) of Germany, and Moysés Baumstein of Brazil, each one searching for a proper "language" to use with the three-dimensional work, avoiding the simple holographic reproduction of a sculpture or object. For instance, in Brazil, many concrete poets (Augusto de Campos, Décio Pignatari, Julio Plaza and José Wagner Garcia, associated with Moysés Baumstein) found in holography a way to express themselves and to renew the Concrete Poetry (or Shape Poetry).
A small but active group of artists still use holography as their main medium, and many more artists integrate holographic elements into their work.[40] Some are associated with novel holographic techniques; for example, artist Matt Brand[41] employed computational mirror design to eliminate image distortion from specular holography.
The MIT Museum[42] and Jonathan Ross[43] both have extensive collections of holography and on-line catalogues of art holograms.
[edit] 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 one-gigabit-per-second writing speed. Read speeds can surpass this, and experts believe one-terabit-per-second readout is possible. In 2005, companies such as Optware and Maxell produced a 120 mm disc that uses a holographic layer to store data to a potential 3.9 TB, which they plan to market under the name Holographic Versatile Disc. Another company, InPhase Technologies, is developing a competing format. While many holographic data storage models have used "page-based" storage, where each recorded hologram holds a large amount of data, more recent research into using submicrometre-sized "microholograms" has resulted in several potential 3D optical data storage solutions. While this approach to data storage can not attain the high data rates of page-based storage, the tolerances, technological hurdles, and cost of producing a commercial product are significantly lower.[edit] Dynamic holography
In static holography, recording, developing and reconstructing occur sequentially, and a permanent hologram is produced.There also exist holographic materials that do not need the developing process and can record a hologram in a very short time. This allows one to use holography to perform some simple operations in an all-optical way. Examples of applications of such real-time holograms include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image processing (pattern recognition of time-varying images), and optical computing.
The amount of processed information can be very high (terabits/s), since the operation is performed in parallel on a whole image. This compensates for the fact that the recording time, which is in the order of a microsecond, is still very long compared to the processing time of an electronic computer. The optical processing performed by a dynamic hologram is also much less flexible than electronic processing. On one side, one has to perform the operation always on the whole image, and on the other side, the operation a hologram can perform is basically either a multiplication or a phase conjugation. In optics, addition and Fourier transform are already easily performed in linear materials, the latter simply by a lens. This enables some applications, such as a device that compares images in an optical way.[44]
The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals, but in semiconductors 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).
[edit] Hobbyist use
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.[45]
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.[46]
A small group of amateurs are even constructing their own pulsed lasers to make holograms of moving objects.[47]
[edit] Holographic interferometry
Main article: holographic interferometry
Holographic interferometry (HI) is a technique that enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light).[48][49] It can also be used to detect optical-path-length variations in transparent media, which enables, for example, fluid flow to be visualized and analyzed. It can also be used to generate contours representing the form of the surface.It has been widely used to measure stress, strain, and vibration in engineering structures.
[edit] 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.[50][edit] 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.[51][edit] Security
Main article: Security hologram
Security holograms are very difficult to forge, because they are replicated from a master hologram that requires expensive, specialized and technologically advanced equipment. They are used widely in many currencies, such as the Brazilian real 20 note, British pound 5/10/20 notes, Estonian kroon 25/50/100/500 notes, Canadian dollar 5/10/20/50/100 notes, Euro 5/10/20/50/100/200/500 notes, South Korean won 5000/10000/50000 notes, and Japanese yen 5000/10000 notes. They are also used in credit and bank cards as well as passports, ID cards, books, DVDs, and sports equipment.
[edit] 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. Holograms produced in elastomers can be used as stress-strain reporters due to its elasticity and compressibility, the pressure and force applied are correlated to the reflected wavelength, therefore its color.[52][edit] Non-optical holography
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.[53] The principle of electron holography can also be applied to interference lithography.[54]
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,[55] although such holograms have not yet been commercialized.
[edit] Things often confused with holograms
Effects produced by Lenticular printing and Video projection, Pepper's Ghost, and Volumetric Projections are often confused with holograms.[56]In 2010, there was a series of concerts organized by Crypton Future Media which included Hatsune Miku, a singing synthesizer application with a female persona, performing on stage as a "holographic" character.[57][58][59] This effect was actually achieved through a special method of rear projection against a semi-transparent screen.
In 2011, in Beijing, apparel company Burberry produced the "Burberry Prorsum Autumn/Winter 2011 Hologram Runway Show", which included life size 2-D projections of models. The company's own video shows several centered and off-center shots of the main 2-dimensional projection screen, and all the 3-D effects stem from 2-D perspective.[60] The claim that holography was used was reported as fact in the trade media.[61]
Source Wikipedia
No comments:
Post a Comment