Hologram
Hologram
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.
Holography was invented over Easter, 1947 by Hungarian physicist Dennis Gabor (1900–1979), for which he received the Nobel Prize in physics in 1971. The discovery was an unexpected result (or serendipity as Gabor would say) 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.
There are several types of holograms that can be made. The very first holograms were “transmission holograms”, which were viewed by shining laser light through them. 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 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.
Holographic reconstruction process
Once the film is processed, if 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.
Because of the need for interference between the reference and object beams, holography typically uses a laser in production. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.
Dynamic holography
There exist also holographic materials which don’t need the developing process and can record a hologram in a very short time. This allows to use holography to perform some simple operations in an all-optical way. Examples of applications of such real-time holograms include phase-conjugate mirrors (“time-reversal” of light), optical cache memories, image processing (pattern recognition of time-varying images), and optical computing.
The amount of processed information can be very high (Terabit/s), since the operation is performed in parallel on a whole image. This compensates the fact that the recording time, which is in the order of a µs, is still very long compared to the processing time of an electronic computer. The optical processing performed by a dynamic hologram is also much less flexible than electronic processing. On one side one has to perform the operation always on the whole image, and on the other side the operation a hologram can perform is basically either a multiplication or a phase conjugation. But remember that in optics, addition and Fourier transform are already easily performed in linear materials, the second simply by a lens. This enables some applications like a device that compares images in an optical way.
The search for novel nonlinear optical materials for dynamic holography is an active area of research. The most common materials are photorefractive crystals, but also in semiconductors or semiconductor heterostructures (such as quantum wells), atomic vapors and gases, plasmas and even liquids it was possible to generate holograms.
Information Courtesy of Wikipedia
