Holography

1. Definition

Holography is a method of recording and displaying a three-dimensional image of an object, usually using coherent radiation from a laser and photographic plates (see diagrams below). The light from a laser is divided so that some of it (the reference beam) falls directly on a photographic plate. The other part illuminates the object, which reflects it back onto the photographic plate. The two beams form interference patterns on the plate, which when developed is called the hologram.

To reproduce the image of the object, the hologram is illuminated by coherent light, ideally the original reference beam. The hologram produces two sets of diffracted waves. One set forms a virtual image coinciding with the original object position and the other forms a real image on the other side of the plate. Both are three-dimensional. More recent techniques can produce holograms visible in white light.

2. How it works

Recording a Hologram: To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram or the interference pattern.

• Recording a hologram:
  

It is also important to note that these recorded fringes do not only directly represent their respective corresponding points in the space of a scene (the way each point on a photograph will only represent a single point in the scene being photographed). Rather, an individual section of even very small size on a hologram's surface contains enough information to reconstruct the entire original scene (within limits) as viewed through that point's perspective. This is possible because during holographic recording, each point on the hologram's surface is affected by light waves reflected from all points in the scene, rather than from just one point. It's as if, during recording, each point on the hologram's surface were an eye that could record everything it sees in any direction. After the hologram has been recorded, looking at a point in that hologram is like looking "through" one of those eyes.

To demonstrate this concept, you could cut out a small section of a recorded hologram; If you then view that cut-out section, you could still see most of the entire scene simply by shifting your viewpoint, the same way you would look outside from a small window in your house, for example.

Reconstructing a Hologram: 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. 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. Early holograms were made before the invention of the laser, however, and used other coherent light sources such as mercury-arc lamps.

• Reconstructing a hologram:
  

3. Some uses for holography

• In display holography, the advent of pulsed lasers has made it possible to capture images of plants, animals, and humans, as well as inanimate objects.

• Holographic packaging materials have become popular and cheap.

• The use of holograms for security on credit cards and bank-notes has reduced the opportunity for forgery.

• If a "double exposure" hologram is made of an object before and after the object is subjected to stress, called interferometry, the change in the shape of the object due to that stress is recorded as a series of light and dark bands. In industrial situations this can be used in component testing and quality control.

• Medical, dental and other records can be made both for teaching and for documentation.

• Particle physicists make holographic records of bubble-chambers from which accurate measurements can be made. (RossWaite^? - 04 Mar 2007)

• Many museums have made holograms of valuable articles in their collections, both for insurance purposes and to check for deterioration

• If a hologram is made of a lens, mirror, or other optical element, it is called a holographic optical element (HOE), and it acts in the same way as that element. HOEs can be used, for example, in bar-code readers.

4. History

• 1947: The idea of holography was first proposed by Dennis Gabor, who was working to improve the resolution of the electron microscope. The lack of a good coherent light source limited Gabor’s attempts at producing holograms with depth. Gabor used film transparencies to capture the holographic images and a mercury arc lamp, which was the most coherent light source that was available at the time. As a result, his holograms contained distortions and double images.

• 1960: The invention of the laser (pulsed-ruby), a coherent light source, by Dr. T.H. Maimam led to the rapid development of holography techniques and applications.

• 1962: The first laser transmission hologram of three-dimensional objects was created by Emmitt Leith and Juris Upatnieks. The image, a toy train and a bird, was very clear and showed depth but required a laser light to illuminate the hologram. Leith and Upatnieks used their “off-axis” technique that they had previously developed to create their hologram. This technique is currently used as the standard method from which most holograms are created.

• 1967: The first mass-distributed hologram, consisting of chess pieces on a board, was contained in the World Book Encyclopedia Science Yearbook.

• 1968: White-light transmission holography was invented by Dr. Stephen A. Benton. Ordinary white light can be used to view this type of hologram, resulting in a “rainbow” image from the colors of the spectrum that make up white light.

• 1971: Dr. Dennis Gabor was awarded the Nobel Prize in Physics for his work on holography.

• 1976: The first projected holographic movie was created. A pulsed holographic camera was used to record images at a rate of 20 frames per second and the developed film was projected onto a holographic screen. This screen focused the three-dimensional image out to several points in the audience. Only two or three people were able to see the movie in full depth without the use of 3D glasses.

• 1983: Mastercard International, Inc. was the first to use holography on its credit cards as a security measure. The holographic images are “printed” by stamping the holograph’s interference pattern onto the plastic cards. These embossed holograms (or white-light transmission holograms) can be copied millions of times at a cost of only a few cents apiece.

• 1984: National Geographic became the first major publication to put a hologram, consisting of an eagle, on its cover.

• 1988: The most ambitious holographic magazine cover was published by National Geographic. The entire cover was holographic. The front cover consisted of a double image of the Earth. From one point of view, the Earth appears whole, but when rotated, the planet appears to be exploding. This hologram represents the fragile nature of our planet. Along the spine of the magazine, the lettering was three-dimensional. The back cover consisted of a holographic advertisement.

5. Hologram examples

• Holography exhibit:
  

• Medical facial reconstruction:
  

6. Additional sources

• http://www.holophile.com/

• http://www.holography.ru/maineng.htm

• http://holographyforum.org/

7. Related subjects

Denis Gabor:

• http://nobelprize.org/nobel_prizes/physics/laureates/1971/gabor-autobio.html

• http://www.britannica.com/nobel/micro/223_92.html

Holographic Discs:

• http://en.wikipedia.org/wiki/Holographic_Versatile_Disc

• http://www.physorg.com/news967.html

Holographic Memory:

• http://computer.howstuffworks.com/holographic-memory.htm

• http://www.technologyreview.com/Infotech/14742/

-- RossWaite - 12 Feb 2007
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