Introduction to holography
Holography is the science and practice of making holograms. Typically, a hologram is a photographic recording of a light field, rather than of an image formed by a lens, and it is used to display a fully three-dimensional image of the holographed subject, which is seen without the aid of special glasses or other intermediate optics. The hologram itself is not an image and is usually unintelligible when viewed under diffuse ambient light. It is an encoding of the light field as an interference pattern of seemingly random variations in the opacity, density, or surface profile of the photographic medium. When suitably lit, the interference pattern diffracts the light into a reproduction of the original light field and the objects that were in it appear to still be there, exhibiting visual depth cues such as parallax and perspective that change realistically with any change in the relative position of the observer.
Holography is proposed by D.Gabor in 1948, it initially aimed at solving the resolution problem of electron microscope with coaxial holography (Inline Hologram) . Because the invention of coherent light source (laser) (1964), holography was able to be realized.
Holography is distinct from 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 incorrectly called holograms.
Categories of Holography
There are three types of holography:
Hologram is a three-dimensional image that allows to see the depth of an image; when
it is viewed from different angles, different image is observed, so that three-dimensional
perception is achieved.
In all conventional imaging techniques, such as photography, we merely record the intensity
distribution in the original scene. As a result, all information of relative optical
paths to different parts of the scene is lost. But in holography it is different. What is
recorded on the photographic film is the interference pattern produced by the two waves
(reference wave and the light wave scattered from the object). The intensity at any point
in this pattern depends on the phase as well as the amplitude of the original object wave
. Accordingly, the processed photographic film, which is called a hologram, contains
information on both the phase and the amplitude of the object wave. When the resulting
hologram is subsequently illuminated with the reference wave, it contains enough information
about the phase and amplitude to permit their reconstruction. [1, 4]
In general, holography is an interference method of recording the light waves diffracted
by an object, illuminated with coherent light. Many methods exist for the recording and
playback of a hologram.
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Digital holography refers to the acquisition and processing of holograms with a digital sensor array typically a CCD camera or a similar device. Image rendering, or reconstruction of object data is performed numerically from digitized interferograms. Digital holography offers a means of measuring optical phase data and typically delivers three-dimensional surface or optical thickness images. Several recording and processing schemes have been developed to assess optical wave characteristics such as amplitude, phase, and polarization state, which make digital holography a very powerful method for metrology applications.
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Computer Generated Holography
Computer-generated holography (CGH) is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g. by digitally computing a holographic interference pattern and printing it onto a mask or film for subsequent illumination by suitable coherent light source.
Alternatively, the holographic image can be brought to life by a holographic 3D display (a display which operates on the basis of interference of coherent light), bypassing the need of having to fabricate a “hardcopy” of the holographic interference pattern each time. Consequently, in recent times the term “computer-generated holography” is increasingly being used to denote the whole process chain of synthetically preparing holographic light wavefronts suitable for observation.
Computer-generated holograms have the advantage that the objects which one wants to show do not have to possess any physical reality at all (completely synthetic hologram generation). On the other hand, if holographic data of existing objects is generated optically, but digitally recorded and processed, and brought to display subsequently, this is termed CGH as well. Ultimately, computer-generated holography might serve all the roles of current computer-generated imagery: holographic computer displays for a wide range of applications from CAD to gaming, holographic video and TV programs, automotive and communication applications (cell phone displays) and many more.
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