Holography is a technique that enables a wavefront to be recorded and later re-constructed. Holography is best known as a method of generating real three-dimensional imagesbut it also has a wide range of other applications. In principle, it is possible to make a hologram for any type of wave.
A hologram is made by superimposing a second wavefront normally called the reference beam on the wavefront of interest, thereby generating an interference pattern which is recorded on a physical medium. When only the second wavefront illuminates the interference pattern, it is diffracted to recreate the original wavefront.
Holograms can also be computer-generated by modelling the two wavefronts and adding them together digitally. The resulting digital image is then printed onto a suitable mask or film and illuminated by a suitable source to reconstruct the wavefront of interest.
The Hungarian - British physicist Dennis Gabor in Hungarian: Gábor Dénes [1] [2] was awarded the Nobel Prize in Physics in "for his invention and development of the holographic method". His work, done in the late s, was built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in and William Lawrence Bragg in The technique as originally invented is still used in electron microscopywhere it is known as electron holographybut optical holography did not really advance until the vad är hologram of the laser in The word holography comes from the Greek words ὅλος holos ; "whole" and γραφή graphē ; " writing " or " drawing ".
A hologram is a recording of an interference pattern which can reproduce a 3D light field vad är hologram diffraction. The reproduced light field can generate an image which still has the depth, parallaxand other properties of the original scene. The holographic medium, for example the object produced by a holographic process which may be referred to as a hologram is usually unintelligible when viewed under diffuse ambient light.
It is an encoding of the light field as an interference pattern of variations in the opacitydensityor surface profile of the photographic medium. When suitably lit, the interference pattern diffracts the light into an accurate reproduction of the original light field, and the objects that were in it exhibit visual depth cues such as parallax and perspective that change realistically with the different angles of viewing.
That is, the view of the image from different angles represents the subject viewed from similar angles. In this sense, holograms do not have just the illusion of depth but are truly three-dimensional images. The development of the laser enabled the first practical optical holograms that recorded 3D objects to be made in by Yuri Denisyuk in the Soviet Union [6] and by Emmett Leith and Juris Upatnieks at the University of MichiganUS.
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.
Optical holography needs a laser light to record the light field. In its early days, holography required high-power and expensive lasers, but currently, mass-produced low-cost laser diodessuch as those found on 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.
A microscopic level of detail throughout the recorded scene can be reproduced. The 3D image can, however, be viewed with non-laser light. In common practice, however, major image quality compromises are made to remove the need for laser illumination to view the hologram, and in some cases, to make it.
Holographic portraiture often resorts to a non-holographic intermediate imaging procedure, to avoid the dangerous high-powered pulsed lasers which would be needed to optically "freeze" moving subjects as perfectly as the extremely motion-intolerant holographic recording process requires.
Holograms can now also be entirely computer-generated to show objects or scenes that never existed.