Holography


Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object were still present, thus making the recorded image (hologram) appear three dimensional.

The technique of holography can also be used to optically store, retrieve, and process information. While holography is commonly used to display static 3-D pictures, it is not yet possible to generate arbitrary scenes by a holographic volumetric display..




Overview

Holography was discovered in 1947 by Hungarian physicist Dennis Gabor (19001979), work for which he received the Nobel Prize in Physics in 1971. It was made possible by pioneering work in the field of physics by other scientists like Mieczyslaw Wolfke who resolved technical issues that previously made advancements impossible.

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. The technique as originally invented is still used in electron microscopy, where it is known as electron holography, but holography as a light-optical technique did not really advance until the development of the laser in 1960.

The first holograms that recorded 3D objects were made in 1962 by Yuri Denisyuk in the Soviet Union and by Emmett Leith and Juris Upatnieks at University of Michigan, USA. Advances in photochemical processing techniques to produce high-quality display holograms were achieved by Nicholas J. Phillips.

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. A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light or other monochromatic sources rather than by lasers.

Rainbow holograms are commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission hologram are commonly formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminum coating that provides the light from "behind" to reconstruct their imagery.

Another kind of common hologram, the reflection or Denisyuk hologram, is capable of multicolor image reproduction using a white light illumination source 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, such as found in millions of DVD recorders and used in other common applications, 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.




Theory

Though holography is often referred to as 3D photography, this is a misconception. A better analogy is sound recording where the sound field is encoded in such a way that it can later be reproduced. In holography, some of the light scattered from an object or a set of objects falls on the recording medium.

A second light beam, known as the reference beam, also illuminates the recording medium, so that interference occurs between the two beams. The resulting light field is an apparently random pattern of varying intensity which is the hologram. It can be shown that if the hologram is illuminated by the original reference beam, a light field is diffracted by the reference beam which is identical to the light field which was scattered by the object or objects. Thus, someone looking into the hologram 'sees' the objects even though it may no longer be present. There are a variety of recording materials which can be used, including photographic film.




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 simplistic, but is accurate enough to provide an understanding of how the holographic process works.


Hologram  Wikipedia




In the News ...


Hologram messaging coming of age   BBC - November 3, 2010
It has long been a staple of science fiction films - the idea that you could send a moving 3D representation of someone to any location, even the far side of the galaxy. Now, US researchers claim this fantasy is very close to reality.

  Touchable Hologram Becomes Reality   PhysOrg - August 14, 2009




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