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  • Holography is a technique which enables three-dimensional images to be made. It involves the use of

  • a laser, interference, diffraction, light intensity recording and suitable illumination

  • of the recording. 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

  • image appear three-dimensional. The holographic recording itself is not an

  • image; it consists of an apparently random structure of either varying intensity, density

  • or profile.

  • Overview and history The Hungarian-British physicist Dennis Gabor,

  • was awarded the Nobel Prize in Physics in 1971 "for his invention and development of

  • the holographic method". 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. 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 optical holography did not

  • really advance until the development of the laser in 1960. The word holography comes from

  • the Greek words ὅλος and γραφή.

  • 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 and by Emmett Leith and

  • Juris Upatnieks at the University of Michigan, USA. 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 were developed which enabled much more efficient holograms

  • to be produced. 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 rather than by lasers. Rainbow holograms are commonly used

  • for security and authentication, for example, on credit cards and product packaging.

  • 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. Specular holography is a related technique

  • for making three-dimensional images by controlling the motion of specularities on a two-dimensional

  • surface. 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.

  • Holograms can also be used to store, retrieve, and process information optically.

  • 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 molecules

  • and view them using visible light. However, X-ray holograms have not been created to date.

  • How holography works

  • 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.

  • 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 Holograms are 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, which can be controlled

  • using a shutter, or by electronically timing the laser.

  • Apparatus 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 is spread using lenses and directed onto the scene using mirrors. Some of the

  • light scattered from the scene then falls onto the recording medium.

  • The second 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 of the most common is a film very similar to photographic film,

  • but with a much higher concentration of light-reactive grains, making it capable of the much higher

  • resolution that holograms require. A layer of this recording medium is attached to a

  • transparent substrate, which is commonly glass, but may also be plastic.

  • Process When the two laser beams reach the recording

  • medium, their light waves, intersect and interfere with each other. It is this interference pattern

  • that is imprinted on the recording medium. The pattern itself is seemingly random, as

  • it represents the way in which the scene's light interfered with the original light source

  • but not the original light source itself. The interference pattern can be considered

  • an encoded version of the scene, requiring a particular keythe original light source

  • in order to view its contents. This missing key is provided later by shining

  • a laser, identical to the one used to record the hologram, onto the developed film. When

  • this beam illuminates the hologram, it is diffracted by the hologram's surface pattern.

  • This produces a light field identical to the one originally produced by the scene and scattered

  • onto the hologram. The image this effect produces in a person's retina is known as a virtual

  • image. Holography vs. photography

  • Holography may be better understood via an examination of its differences from ordinary

  • photography: A hologram represents a recording of information

  • regarding the light that came from the original scene as scattered in a range of directions

  • rather than from only one direction, as in a photograph. This allows the scene to be

  • viewed from a range of different angles, as if it were still present.

  • A photograph can be recorded using normal light sources whereas a laser is required

  • to record a hologram. A lens is required in photography to record

  • the image, whereas in holography, the light from the object is scattered directly onto

  • the recording medium. A holographic recording requires a second

  • light beam to be directed onto the recording medium.

  • A photograph can be viewed in a wide range of lighting conditions, whereas holograms

  • can only be viewed with very specific forms of illumination.

  • When a photograph is cut in half, each piece shows half of the scene. When a hologram is

  • cut in half, the whole scene can still be seen in each piece. This is because, whereas

  • each point in a photograph only represents light scattered from a single point in the

  • scene, each point on a holographic recording includes information about light scattered

  • from every point in the scene. It can be thought of as viewing a street outside a house through

  • a 4 ft x 4 ft window, then through a 2 ft x 2 ft window. One can see all of the same

  • things through the smaller window, but the viewer can see more at once through theft

  • window. A photograph is a two-dimensional representation

  • that can only reproduce a rudimentary three-dimensional effect, whereas the reproduced viewing range

  • of a hologram adds many more depth perception cues that were present in the original scene.

  • These cues are recognized by the human brain and translated into the same perception of

  • a three-dimensional image as when the original scene might have been viewed.

  • A photograph clearly maps out the light field of the original scene. The developed hologram's

  • surface consists of a very fine, seemingly random pattern, which appears to bear no relationship

  • to the scene it recorded. Physics of holography

  • For a better understanding of the process, it is necessary to understand 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 simplified but is accurate enough to provide an understanding of how the holographic

  • process works. For those unfamiliar with these concepts,

  • it is worthwhile to read the respective articles before reading further in this article.

  • Plane wavefronts A diffraction grating is a structure with

  • a repeating pattern. A simple example is a metal plate with slits cut at regular intervals.

  • A light wave incident on a grating is split into several waves; the direction of these

  • diffracted waves is determined by the grating spacing and the wavelength of the light.

  • A simple hologram can be made by superimposing two plane waves from the same light source

  • on a holographic recording medium. The two waves interfere giving a straight line fringe

  • pattern whose intensity varies sinusoidally across the medium. The spacing of the fringe

  • pattern is determined by the angle between the two waves, and on the wavelength of the

  • light. The recorded light pattern is a diffraction

  • grating. When it is illuminated by only one of the waves used to create it, it can be

  • shown that one of the diffracted waves emerges at the same angle as that at which the second

  • wave was originally incident so that the second wave has been 'reconstructed'. Thus, the recorded

  • light pattern is a holographic recording as defined above.

  • Point sources

  • If the recording medium is illuminated with a point source and a normally incident plane

  • wave, the resulting pattern is a sinusoidal zone plate which acts as a negative Fresnel

  • lens whose focal length is equal to the separation of the point source and the recording plane.

  • When a plane wavefront illuminates a negative lens, it is expanded into a wave which appears

  • to diverge from the focal point of the lens. Thus, when the recorded pattern is illuminated

  • with the original plane wave, some of the light is diffracted into a diverging beam

  • equivalent to the original plane wave; a holographic recording of the point source has been created.

  • When the plane wave is incident at a non-normal angle, the pattern formed is more complex

  • but still acts as a negative lens provided it is illuminated at the original angle.

  • Complex objects To record a hologram of a complex object,

  • a laser beam is first split into two separate beams of light. One beam illuminates the object,

  • which then scatters light onto the recording medium. According to diffraction theory, each

  • point in the object acts as a point source of light so the recording medium can be considered

  • to be illuminated by a set of point sources located at varying distances from the medium.

  • The second beam illuminates the recording medium directly. Each point source wave interferes

  • with the reference beam, giving rise to its own sinusoidal zone plate in the recording

  • medium. The resulting pattern is the sum of all these 'zone plates' which combine to produce

  • a random pattern as in the photograph above. When the hologram is illuminated by the original

  • reference beam, each of the individual zone plates reconstructs the object wave which

  • produced it, and these individual wavefronts add together to reconstruct the whole of the

  • object beam. The viewer perceives a wavefront that is identical to the wavefront scattered

  • from the object onto the recording medium, so that it appears to him or her that the

  • object is still in place even if it has been removed. This image is known as a "virtual"

  • image, as it is generated even though the object is no longer there.