Science of Holography
Introduction:
- An object illuminated by coherent light is made to produce interference fringes in photosensitive material.
- Reillumination of the developed interference pattern by light of the same wavelength produces a three-dimensional image of the original object.
Basic principle:
- The basic principle of holography is recording the phase and amplitude of the light scattered from different parts of the object.
- The phase recording helps to record the depth of the material.
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 when a wavefront encounters an object.
Plane wavefronts
A diffraction grating is a structure with a repeating pattern. A simple example is a metal plate.
With slits cut or ruling at regular intervals. The slit allows the passage of light. The ruling is opaque. A light wave that is incident on a grating undergoes diffraction, resulting in several waves. The direction of these diffracted waves depends on the grating spacing and the wavelength of the light.
- The construction of the hologram is possible by superimposing two plane waves from the same light source on a holographic recording medium (coherent waves).
- 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 by the wavelength of the light.
The recorded light pattern is a hologram.
- Reconstruction of the image is possible by illuminating this hologram with only one of the waves used to create it.
Point sources
If we apply a point source and a normal incident plane wave to illuminate the recording medium, the resulting pattern will be a sinusoidal zone plate. That acts as a negative Fresnel lens.
When a plane wave-front illuminates a negative lens, it expands to a wave that appears to diverge from the focal point of the lens. Thus, when we illuminate the recorded pattern with the original plane wave, some light is diffracted into a diverging beam equivalent to the original spherical wave.
When the plane wave is incident at a non-normal angle at the recording, the pattern formed is more complex but still acts as a negative lens if the illumination is at the original angle. [7]
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Recording a hologram:
Light from a single source is passed through a beam splitter to get the output into two components.
- One part was allowed to fall directly on photographic material and named a reference wave (reference beam).
- The other part illuminates the object all around with suitable reflecting mirrors.
- The scattered light from the object is directed towards the photo-sensitive material and named an object wave (the object beam).
- Object and reference waves are coherent and produce interference patterns.
This pattern is recorded and processed.
The processed film is named a hologram.
- The film appears the same as that of a negative photograph.
- The working is the same as that of a diffraction grating.
Hologram | Photograph |
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Functionality
Holography uses a reference wave and an object wave. The reference wave can save the phase information as patterns of light and dark on a film. The object wave and reference wave must have the same wavelength to save the phase information, and they usually come from the same source.[7]
Recreation of hologram:
It is a process of reconstructing the image from the hologram. For this, we can use a readout wave (readout beam) having coherence identical to the reference wave used in the recording process. The hologram works as a grating. Then, we can observe two images at symmetrical positions (one image is real and the other is virtual). The virtual image is the exact three-dimensional replica of the object recorded.
Significant features:
- Three-dimensional image
- It contains all the information carried in the light wave reflected from the object. This property justifies the meaning of the Greek words holos (complete) and graphos (writing).
- If we break a hologram into small pieces, each piece will be a hologram of the complete object.
Laser holography
In laser holography, the source for recording holograms is a laser. Construction of all holograms involves the interaction of light from different directions and producing a microscopic interference pattern that a plate, film, or other medium photographically records.
I discussed the construction of the hologram and reproducing the image in the earlier session.
Like conventional photography, holography requires an appropriate exposure time to affect the recording medium correctly. Unlike traditional photography, during the exposure, the light source, the optical elements, the recording medium, and the subject must all remain motionless relative to each other to within about a quarter of the wavelength of the light, or the interference pattern will be blurred and the hologram spoiled. With living subjects and some unstable materials, that is only possible if we use a very intense and brief pulsed laser. [7]
Art
Artists know the potential of holography as a medium and gained access to science laboratories to create their work. Holographic art is the result of collaborations between scientists and artists.
Salvador Dalà claimed to have been the first to employ holography artistically. He was the first and best-known surrealist to do so. [2] The 1972 New York exhibit of Dalà 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. [8] In Great Britain, Margaret Benyon began using holography as an artistic medium in the late 1960s and had a solo exhibition at the University of Nottingham art gallery in 1969. [9] In 1970, by a solo show at the Lisson Gallery in London, which was the first London expo of holograms and stereoscopic paintings.[10]
During the 1970s, they established several art studios and schools with a particular approach to holography—notably, the San Francisco School of Holography by Lloyd Cross. The Museum of Holography in New York, founded by Rosemary (Posy) H. Jackson, the Royal College of Art in London, and the Lake Forest College Symposiums organized by Tung Jeong. [11] None of these studios still exist; however, there is the Center for the Holographic Arts in New York[12] and the Center in Seoul, 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 United States, Dieter Jung 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 Concrete Poetry.
A small but active group of artists still integrate holographic elements into their work. [13] Some are associated with novel holographic techniques. For example, artist Matt Brand[14] employed computational mirror design to eliminate image distortion from specular holography.
The MIT Museum[15] and Jonathan Ross[16] have extensive collections of holography and online catalogs of art holograms.
The possibility of recording a large number of images on a single hologram and the ability of a part of the hologram to reconstruct the complete information opens up the scope for storing much data. For example, a single hologram of 100 sq cm in size can accommodate the entire data of one volume of Encyclopedia Britannica.
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 medium has significance as many electronic products incorporate storage devices. Current storage techniques, such as Blu-ray Discs, 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 the usage of the volume of the recording media instead of just the surface. The available SLMs (Spatial Light Modulators) can produce about 1000 different images a second at a 1024×1024-bit resolution. It could result in one gigabit-per-second writing speed. [17]
A spatial light modulator is a device that can control the intensity, phase, or polarization of light in a spatially varying manner. A simple example is an overhead projector transparency. When we use the term SLM, it means computer-controlled transparency.
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, a format called Holographic Versatile Disc. As of September 2014, no commercial product came out.
Another company, InPhase Technologies, was developing a competing format. They went bankrupt in 2011 and sold to Akonia Holographics, LLC.
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 sub-micrometer-sized micro holograms has resulted in several potential 3D optical data storage solutions. [7]
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. [7]
Holographic interferometry
Holographic interferometry (HI) is a technique that enables static and dynamic displacements of objects with optically rough surfaces to optical interferometric precision (i.e., to fractions of a wavelength of light). [18][19]. To learn more about holographic interferometry, please read my other article (https://hubpages.com/education/holographic-interferometry).
Uses:
- To detect optical-path-length variations in transparent media.
- Enabling, visualization, and analysis of fluid flow is an example.
- To generate contours representing the form of the surface or the isodose regions in radiation dosimetry. [20]
- To measure stress, strain, and vibration in engineering structures.
Holography and data processing:
Pattern recognition: is of very great importance in cybernetics.
Consider the example of recognizing the letter 'P' from a text.
Prepare a hologram of the size of the letter P in the given text.
The reference wave is from a bright source. Object wave is the light reflected from P. A scanning device moves the hologram over the lines in the text. Each time the hologram meets the letter P in the text, the scanner responds with a bright flash. Here, the wave scattered by the letter P acts as a readout wave, reconstructing the image.
Dynamic holography
In static holography, recording, developing, and reconstructing occur sequentially, resulting in a permanent hologram.
There also exist holographic materials that do not need the developing process. It can record a hologram in a short time. This process allows one to use holography to perform some simple operations optically.
Examples:
- Phase-conjugate mirrors (time-reversal of light)
- Optical cache memories.
- Image processing (pattern recognition of time-varying images).
- Optical computing.
The amount of processed information can be very high (terabits/s) since the operation is parallel to a whole image. This method compensates for recording time, which is in the order of a microsecond, which 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. In optics, addition, and Fourier transform are already easily performed in linear materials, the latter simply by a lens. It enables some applications, such as a device that optically compares images. [21]
The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals. It was possible to generate holograms in semiconductors or semiconductor-heterostructures (such as quantum wells), atomic vapors and gases, plasmas, and even liquids.
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. It is used in free-space optical communications to compensate for atmospheric turbulence (the phenomenon that gives rise to the twinkling of starlight).
Hobbyist use
Since the beginning of holography, many photographers have explored its uses and displayed them to the public.
In 1971, Lloyd Cross opened the San Francisco School of Holography. He taught amateurs how to make holograms using only a small (typically five mW) helium-neon laser and inexpensive homemade equipment. Holography had been supposed to require an expensive metal optical table set-up to lock all the involved elements down in place and dampen any vibrations that could blur the interference fringes and ruin the hologram. Cross's home-brew alternative was a sandbox made of a cinder block retaining wall on a plywood base, supported on stacks of old tires to isolate it from ground vibrations, and filled with washed sand to remove dust. [7] Securely mounted the laser atop the cinder block wall. Then, affixed the mirrors and simple lenses needed for directing, splitting, and expanding the laser beam to a PVC pipe. There was support for the subject and the photographic plate holder within the sandbox. The photographer turned off the room light, blocked the laser beam near its source using a small relay-controlled shutter, loaded a plate into the holder in the dark, left the room, waited a few minutes to let everything settle, and then made the exposure by remotely operating the laser shutter.
In 1979, Jason Sapan opened the Holographic Studios in New York City. Since then, they produced holographs for many artists as well as companies. [22]
Many of these holograms would go on to produce art holograms. In 1983, Fred Unterseher, a co-founder of the San Francisco School of Holography and a well-known holographic artist, published the Holography Handbook, an easy-to-read guide to making holograms at home. It was a new wave of photographers and provided simple methods for using the then-available AGFA silver halide recording materials.
Frank DeFreitas published the Shoebox Holography Book and introduced inexpensive laser pointers to countless hobbyists in 2000.
When they tested a practical experiment using a semiconductor diode, they found that the coherence length was greater than that of traditional helium-neon gas lasers. It was a significant development for amateurs, as the price of red laser diodes had dropped from hundreds of dollars in the early 1980s to about $5 after they entered the mass market as a component of DVD players in the late 1990s. Now, there are thousands of amateur photographers worldwide.
By late 2000, holography kits with inexpensive laser pointer diodes entered the consumer market. These kits enabled students, teachers, and hobbyists to make several holograms without specialized equipment and became popular gift items by 2005. [23] The introduction of holography kits with self-developing plates in 2003 made it possible for hobbyists to create holograms without the bother of wet chemical processing. [24]
In 2006, surplus holography-quality green lasers (Coherent C315) became available and put dichromate gelatin (DCG) holography within the reach of the amateur photographer. The holography community was surprised at the sensitivity of DCG to green light. This sensitivity would be uselessly slight or non-existent. Jeff Blyth responded with the G307 formulation of DCG to increase the speed and sensitivity of these new lasers. [25]
Kodak and Agfa, the former suppliers of holography-quality silver halide plates and films, are no longer in the market. While other manufacturers have helped fill the void, many amateurs are now making their materials. The favorite formulations are dichromate gelatin, Methylene-Blue-sensitised dichromate gelatin, and the diffusion method of silver halide preparations. Jeff Blyth has published very accurate methods for making these in a small lab or garage. [26]
Sensors or biosensors
Constructing a hologram with a modified material that interacts with certain molecules generates a difference in the fringe periodicity or refractive index and, therefore, the color of the holographic reflection. [28][29]
Security
Holograms have uses for security, the replicated version from a master hologram. It requires expensive, specialized, and technologically advanced equipment and is thus difficult to forge. They are used widely in many currencies, such as the Brazilian 20, 50, and 100-reais notes; British 5, 10, 20, and 50-pound notes; South Korean 5000, 10,000, and 50,000-won notes; Japanese 5000 and 10,000 yens; Indian 50, 100, 500, and 2000 rupee notes; and all the currently-circulating banknotes of the Canadian dollar, Croatian kuna, Danish krone, and Euro. We can find it in credit and bank cards, passports, ID cards, books, food packaging, DVDs, and sports equipment. Such holograms come in several forms, from adhesive strips laminated on packaging for fast-moving consumer goods to holographic tags on electronic products. They often contain textual or pictorial elements to protect identities and separate genuine articles from counterfeits.
In the post office, they use holographic scanners, larger shipping firms, and automated conveyor systems to determine the three-dimensional size of a package. They are often used with check weighers 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 stress-strain reporters due to their elasticity and compressibility. Correlating the pressure and force applied to the reflected wavelength and, therefore, the color. [30] The holography technique is used for radiation dosimetry also. [31][32]
High-security registration plates
You can find high-security holograms on license plates for vehicles such as cars and motorcycles. As of April 2019, holographic license plates are required on vehicles in parts of India to aid in identification and security, especially in car theft cases. Such number plates hold electronic data of vehicles and have a unique ID number and a sticker to indicate authenticity. [33]
Holography using other types of waves
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. Dennis Gabor invented electron holography to improve the resolution and avoid the aberrations of the transmission electron microscope. Today, it is used for studying electric and magnetic fields in thin films, as magnetic and electric fields can shift the phase of the interfering wave passing through the sample. [34] The principle of electron holography can also be applied to interference lithography. [35]
Acoustic holography can generate sound maps of an object. We can measure the acoustic field at many points close to the object. These measurements are digitally processed to produce the images of the object. [36]
Atomic holography has evolved out of the development of the elements of atom optics. With the Fresnel diffraction lens and tiny mirrors, atomic holography follows a natural step in developing the physics of nuclear beams. Recent developments, including atomic mirrors and ridged mirrors, have provided the tools necessary for forging miniature holograms,[37] although such holograms have not yet come out commercially.
We can use neutron beam holography to see the inner part of solid objects. [38]
We can generate holograms with x-rays using synchrotrons or x-ray free-electron lasers as radiation sources and pixelated detectors such as CCDs as recording medium. [39] The reconstruction is possible via computation. Due to the shorter wavelength of x-rays compared to visible light, this approach allows imaging objects with higher spatial resolution. [40] As free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds, which are intense and coherent, x-ray holography is useful to capture ultrafast dynamic processes. [41][42][43]
Acoustic holography is a technique that allows three-dimensional distributions of sound waves called sound fields to be stored and reconstructed. To do this, sound passing through a surface is recorded as a two-dimensional pattern called a hologram.
- The hologram contains information about the phase and amplitude of the sound waves passing through.
- Using this pattern, we can reconstruct the entire three-dimensional sound field.
- Acoustic holography is similar in principle to optical holography. [44]
There are two forms of acoustic holography. Far-field acoustical holography (FAH) and near-field acoustical holography (NAH). [45][46] The distinction lies in the distance of the sound source to the hologram, which impacts the resolution of the reconstructed sound field. [44]
Method
- The hologram measures acoustic pressure away from the source using an array of transducers (microphones) or a single scanning transducer.
The next stage is data processing with a computer.
- Conversion of information from the time domain to the frequency domain using Fourier transforms.
- The result is a set of intermediate holograms, one for each frequency bin used in the transform.
- Reconstruction of hologram into individual waves with known propagation characteristics.
- Backpropagation of these waves to the source surface and the recomposition of the entire sound field by adding all the waves. [44]
Applications of acoustic holography
- Acoustic holography is becoming increasingly popular in various fields, notably transportation, vehicle and aircraft design, noise, vibration, and harshness (NVH).
The general idea of acoustic holography has led to advanced processing methods such as statistically optimal near-field acoustic holography (SONAH). [47]
For audio rendering and production- wave field synthesis and higher-order Ambisonics are related technologies for modeling the acoustic pressure field on a plane, in spherical volume, etc.,
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