01-11-2012, 05:11 PM
Holography
holo tv.docx (Size: 304.87 KB / Downloads: 39)
Overview and history
The Hungarian-British physicist Dennis Gabor (Hungarian name: Gábor Dénes),[1][2] was awarded the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method".[3] His work, done in the late 1940s, built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and WL Bragg in 1939.[4] 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.
Portrait of Yuri Denisyuk, byDieter Jung
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[5]and by Emmett Leith and Juris Upatnieks at University of Michigan, USA.[6] 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.[7][8][9]
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.[10] A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers.[11] Rainbow holograms are commonly used for security and authentication, for example, on credit cards and product packaging.[12].
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.[13]
Specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface.[14] 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.[15][16][17]
Holograms can also be used to store, retrieve, and process information optically.[18]
In its early days, holography required high-power expensive lasers, but nowadays, mass-produced low-cost semi-conductor or LED 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.[19]
How holography works
Reconstructing a hologram
Close-up photograph of a hologram's surface. The object in the hologram is a toy van. It is no more possible to discern the subject of a hologram from this pattern than it is to identify what music has been recorded by looking at a CD surface. Note that the hologram is described by thespeckle pattern, rather than the "wavy" line pattern.
Holography is a technique that enables a light field, which is generally the product of a light source scattered off objects, to be recorded and later reconstructed when the original light field is no longer present, due to the absence of the original objects.[20] Holography can be thought of as somewhat similar to sound recording, whereby a sound field created by vibrating matter like musical instruments or vocal cords, is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.
Laser
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. In addition, however, part of the light beam must be shone directly onto the recording medium - this second light beam is known as the reference beam. A hologram requires a laser as the sole light source. Lasers can be precisely controlled and have a fixed wavelength, unlike sunlight or light from conventional sources, which contain many different wavelengths. To prevent external light from interfering, holograms are usually taken in darkness, or in low level light of a different colour from the laser light used in making the hologram.
Holography requires a specific exposure time (just like photography), which can be controlled using a shutter, or by electronically timing the laser.
Apparatus
A hologram can be made by shining part of the light beam directly onto the recording medium, and the other part onto the object in such a way that some of the scattered light falls onto the recording medium.
A more flexible arrangement for recording a hologram requires the laser beam to be aimed through a series of elements that change it in different ways. The first element is a beam splitter that 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. Some of the light scattered (reflected) from the scene then 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.
Several different materials can be used as the recording medium. One of the most common is a film very similar to photographic film(silver halide photographic emulsion), but with a much higher concentration of light-reactive grains, making it capable of the much higherresolution that holograms require. A layer of this recording medium (e.g. silver halide) is attached to a transparent substrate, which is commonly glass, but may also be plastic.
Process
When the two laser beams reach the recording medium, their light waves intersect and interfere with each other. It is this interference pattern that is imprinted on the recording medium. The pattern itself is seemingly random, as it 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 anencoded 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. When this beam illuminates the hologram, it is diffracted by the hologram's surface pattern. This produces a light field that is identical to the one originally produced by the scene and scattered onto the hologram. The image this effect produces in a person's retina is known as a virtual image
Holography vs. photography
Holography may be better understood via an examination of its differences from ordinary photography:
A hologram represents a recording of information regarding the light that came from the original scene as scattered in a range of directions rather than from only one direction, as in a photograph. This allows the scene to be viewed from a range of different angles, as if it were still present.
A photograph can be recorded using normal light sources (sunlight or electric lighting) whereas a laser is required to record a hologram.
A lens is required in photography to record the image, whereas in holography, the light from the object is scattered directly onto the recording medium.
A holographic recording requires a second light beam (the reference beam) to be directed onto the recording medium.
A photograph can be viewed in a wide range of lighting conditions, whereas holograms can only be viewed with very specific forms of illumination.
When a photograph is cut in half, each piece shows half of the scene. When a hologram is cut in half, the whole scene can still be seen in each piece. This is because, whereas each point in a photograph only represents light scattered from a single point in the scene, each point on a holographic recording includes information about light scattered from every point in the scene. Think of viewing a street outside your house through a 4 ft x 4 ft window, and then through a 2 ft x 2 ft window. You can see all of the same things through the smaller window (by moving your head to change your viewing angle), but you can see more at once through the 4 ft window.
A photograph is a two-dimensional representation that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range of a hologram adds many more depth perception cues that were present in the original scene. These cues are 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.
A photograph clearly maps out the light field of the original scene. The developed hologram's surface consists of a very fine, seemingly random pattern, which appears to bear no relationship to the scene it recorded.
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.
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 patternwhose 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.