Holography
Traditionally, a holograph is a document written entirely in the handwriting of the person whose signature it bears. That is not what this article is about; this is about the more modern concept, not introduced until the 20th century.
Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, 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 and retrieve information.
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2 Technical Description 3 Holography in Art 4 Holographic Data Storage 5 External links |
Holography was invented in 1947 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971. The discovery was a chance result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, but the field did not really advance until the invention of the laser in 1960.
Various different types of hologram can be made. One of the more common types is the white-light hologram, which does not require a laser to reconstruct the image and can be viewed in normal daylight. These types of holograms are often used on credit cards as security features.
One of the most dramatic advances in the short history of the technology has been the mass production of laser diodes. These compact, solid state lasers are beginning to replace the large gas lasers previously required to make holograms. Best of all they are much cheaper than their counterpart gas lasers. Due to the decrease in costs, more people are making holograms in their homes as a hobby.
The difference between holography and photography is best understood by considering what a photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.
However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image (albeit, in most cases, a monochromatic one, though colour holograms are possible).
Hologram recording process
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.
Hologram reconstuction 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. Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move their 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 to produce them. 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.
Other applications of holograms include metrology and optical computing.
Holography can be applied to a variety of uses that other than recording images. Holographic data storage is a technique that can store information at high density inside crystals (ÃÂÃÂ la HAL 9000) or photopolymers. As current storage techniques such as DVD reach the upper 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 storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. This 3D aspect allows for a phenomenon known as Bragg volume selectivity to be utilised, whereby many information laden holograms can be superimposed/multiplexed in the same volume of medium. It is necessary to 'Bragg detune each hologram recorded with respect it's neighbours. This can be achieved by a number of methods, e.g. rotation of the media with respect the recording object and reference beams or changing the wavelength or phase of the recording laser beams for each hologram.
Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversable change), and rewritable media (where the change is reversable). Rewritable holographic storage can be achieved via the photorefractive effect in crystals:
Holograms can theoretically store equal to one bit per cubic block the size of the wavelength of light in writing. For example, light from a helium-neon laser is red, 632nm wavelength light. Using light of this wavelength, one square inch of holographic storage would be able to hold 1.63×1022 bits which is about 2,048,383,000 terabytes.
See also: optics.Overview
Technical Description
Holography in Art
Salvador DalÃÂÃÂ claims to have been the first to employ holography artistically.Holographic Data Storage
When the information is to be retrieved or read out from the hologram, only the reference beam is necessary. The beam is sent into the material in exactly the same way as when the hologram was written. As a result of the index changes in the material that were created during writing, the beam splits into two parts. One of these parts recreates the signal beam where the information is stored. Something like a CCD camera can be used to convert this information a more usable form.