From Wikipedia, the free encycloped

From Wikipedia, the free encyclopedia
For other uses, see Holography (disambiguation).
For the documentary authorship, see Holograph.
"Hologram" redirects here. For other uses, see Hologram (disambiguation).

Two photographs of a single hologram taken from different viewpoints
Holography is the science and practice of making holograms, which are normally encodings of light fields rather than of images formed by a lens. Holograms are usually intended for displaying three-dimensional images. The holographic recording itself is not an image; it consists of an apparently random structure of varying intensity, density or surface profile. When it is suitably lit, the original light field is recreated and the view of the objects that used to be in it changes as the position and orientation of the viewer changes, as if the objects were still there.

In its pure form, holography requires the use of laser light for illuminating the subject and for displaying the finished hologram, relying solely on the optical phenomena of interference and diffraction. Under optimal conditions, in a side-by-side comparison a holographic image is visually indistinguishable from the actual subject, if both are lit just as they were at the time of recording.

In common practice, however, major image quality compromises are made to eliminate the need for laser illumination when viewing the hologram, and sometimes, to the extent possible, also when making it. Non-holographic intermediate imaging procedures are often employed as an alternative to the high-powered and hazardous pulsed lasers otherwise needed for the holography of people and other living things. Holograms can now also be entirely computer-generated and show objects or scenes that never existed.

Holography should not be confused with lenticular and other earlier autostereoscopic 3D display technologies, which can produce superficially similar results but are based on conventional lens imaging. Stage illusions such as Pepper's Ghost and other unusual, baffling, or seemingly magical images are also often carelessly called holograms.

Contents [hide]
1 Overview and history
2 How holography works
2.1 Laser
2.2 Apparatus
2.3 Process
2.4 Holography vs. photography
3 Physics of holography
3.1 Plane wavefronts
3.2 Point sources
3.3 Complex objects
3.4 Mathematical model
4 Recording a hologram
4.1 Items required
4.2 Hologram classifications
4.2.1 Amplitude and phase modulation holograms
4.2.2 Thin holograms and thick (volume) holograms
4.2.3 Transmission and reflection holograms
4.3 Holographic recording media
4.4 Copying and mass production
5 Reconstructing and viewing the holographic image
5.1 Volume holograms
5.2 Rainbow holograms
5.3 Fidelity of the reconstructed beam
6 Applications
6.1 Art
6.2 Data storage
6.3 Dynamic holography
6.4 Hobbyist use
6.5 Holographic interferometry
6.6 Interferometric microscopy
6.7 Sensors or biosensors
6.8 Security
6.9 Other applications
7 Non-optical holography
8 Things often confused with holograms
9 Holography in fiction
10 See also
11 References
12 Reference sources
13 Further reading
14 External links
Overview and history[edit]
The Hungarian-British physicist Dennis Gabor (in Hungarian: 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 (BTH) 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. The word holography comes from the Greek words ὅλος (hólos; "whole") and γραφή (graphḗ; "writing" or "drawing").


Horizontal symmetric text, by Dieter 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 the University of Michigan, USA.[6] Early holograms used silver halide photographic emulsions as the recording medium. They were not very efficient as the produced grating 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 diode 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 very small objects and view them using visible light[citation needed]. Today, holograms with x-rays are generated by using synchrotrons or x-ray free-electron lasers as radiation sources and pixelated detectors such as CCDs as recording medium.[19] The reconstruction is then retrieved via computation. Due to the shorter wavelength of x-rays compared to visible light, this approach allows to image objects with higher spatial resolution.[20] As free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds which are intense and coherent, x-ray holography has been used to capture ultrafast dynamic processes.[21][22][23]

How holography works[edit]

Recording a hologram

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 the speckle 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.[24] 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[edit]
Hologram is 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 color 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[edit]
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