Editing Holography (section)

==Basics of holography==
[[File:Holograph-record.svg|thumb|400px|Recording a hologram]]
[[File:Holography-reconstruct.svg|thumb|300px|Reconstructing a hologram]]
[[File:Structure of a holographic recording.jpg|thumb|This is a photograph of a small part of an unbleached transmission hologram viewed through a microscope. The hologram recorded an image of a toy van and car. It is no more possible to discern the subject of the hologram from this pattern than it is to identify what music has been recorded by looking at a [[compact disc|CD]] surface. The holographic information is recorded by the [[speckle pattern]].]]

Holography is a technique for recording and reconstructing light fields.<ref name="Hariharan basics">{{cite book |last1=Hariharan |first1=P |title=Basics of Holography |date=2002 |publisher=Cambridge University Press |location=Cambridge|isbn =  9780511755569}}</ref>{{rp| Section 1}} 
A light field is generally the result of a light source scattered off objects. Holography can be thought of as somewhat similar to [[sound recording]], whereby a sound field created by vibrating matter like [[musical instrument]]s or [[vocal cords]], is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter.<ref>{{Cite book|last=Richards|first=Keith L.|url=https://www.worldcat.org/oclc/990152205|title=Design engineer's sourcebook|date=2018|isbn=978-1-315-35052-3|location=Boca Raton|oclc=990152205}}</ref> However, it is even more similar to [[Ambisonics|Ambisonic]] sound recording in which any listening angle of a sound field can be reproduced in the reproduction.

===Laser===
In laser holography, the hologram is recorded using a source of [[laser]] light, which is very pure in its color and orderly in its composition. Various setups may be used, and several types of holograms can be made, but all involve the interaction of light coming from different directions and producing a microscopic interference pattern which a [[photographic plate|plate]], film, or other medium [[photography|photographically]] records.

In one common arrangement, the laser beam is split into two, one known as the [[signal beam|object beam]]<!-- the traditional term, despite the redirect that results --> and the other as the [[reference beam]]. The object beam is expanded by passing it through a lens and used to illuminate the subject. The recording medium is located where this light, after being reflected or scattered by the subject, will strike it. The edges of the medium will ultimately serve as a window through which the subject is seen, so its location is chosen with that in mind. The reference beam is expanded and made to shine directly on the medium, where it interacts with the light coming from the subject to create the desired interference pattern.

Like conventional photography, holography requires an appropriate [[Exposure (photography)|exposure]] time to correctly affect the recording medium. Unlike conventional photography, during the exposure the light source, the optical elements, the recording medium, and the subject must all remain  motionless relative to each other, to within about a quarter of the wavelength of the light, or the interference pattern will be blurred and the hologram spoiled. With living subjects and some unstable materials, that is only possible if a very intense and extremely brief pulse of laser light is used, a hazardous procedure which is rarely done outside of scientific and industrial laboratory settings. Exposures lasting several seconds to several minutes, using a much lower-powered continuously operating laser, are typical.

===Apparatus===
A hologram can be made by shining part of the light beam directly into 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 [[Lens (optics)|lenses]] and directed onto the scene using [[mirror]]s. 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 does not 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]] ([[silver halide]] [[photographic emulsion]]), but with much smaller light-reactive grains (preferably with diameters less than 20&nbsp;nm), making it capable of the much higher [[Optical resolution|resolution]] that holograms require. A layer of this recording medium (e.g., silver halide) 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 [[Interference (wave propagation)|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 key – the 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.

===Comparison with 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 (sunlight or electric lighting) 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 (the reference 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 large window, then through a smaller window. One can see all of the same things through the smaller window (by moving the head to change the viewing angle), but the viewer can see more ''at once'' through the large window.
* A photographic [[Stereoscopy|stereogram]] is a two-dimensional representation that can produce a three-dimensional effect but only from one point of view, whereas the reproduced viewing range of a hologram adds many more [[Depth perception|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.