Theory Of Hologram
Holography is a technique employed to make three-dimensional images. The size of the object can range from large cars to small particles on the micrometer scale. Holography originates from the work of the British/Hungarian scientist Dr. D. Gabor. He tried to improve the resolution of his electron microscope in 1947, using a mercury arc lamp, the incoherent light source resulted in distortions in his images. These images he called holograms after the Greek words “holos” means “whole or entirely” and “gram” meaning “message or recording”. He developed a technique by which three~ dimensional image of an object can be recorded and reconstructed by wavefront construction device with the help of a highly coherent laser beam, which is known as Holography. The recorded transparent photographic plate is named Hologram.
Holography is the process of recording a wave on square law medium and later releasing the same. A wave can carry information in the form of the modulation of amplitude, phase, and polarization. This information can be stored using holography. Its revival cone with the advent of the laser. Since then it has found applications in almost all branches of science, technology, and engineering.
To record a hologram of a complex object, a laser beam is first split into two separate beams of light using a beam splitter of half-silvered glass or a birefringent material expanded by a lens, as we need two coherent light waves to make a hologram.
One beam illuminates the object, reflecting its image onto the recording medium as it scatters the beam, which carries information about the object. This wave is called the object beam. The second (reference) beam illuminates the recording medium directly which has no information called the reference beam.
According to diffraction theory, each point in the object acts as a point source of light. Each of these point sources interferes with the reference beam,g1vmg rise to an interference pattern. The resulting pattern is the sum of a large number (strictly speaking, an infinite number) of point source and reference beam interference patterns. When the object is no longer present, the reference beam directly illuminates the holographic plate.
The object beam and the reference beam, generate an interference pattern, which IS recorded m the term of a hologram on the film emulsion. In Figg. the set-up for the recording of transmission hologram is shown. This .type‘ of the hologram is called transmission hologram because the light passes through the holographic plate. Other characteristic 0ftransmission holograms are that the object beam and the reference beam come in from the same side of the holographic film plate during the exposure. Holography is a two-step process in which it first records and then reconstructs the image in the second step.
Step I: Construction of Recording of Hologram:
Since both amplitude and phase information is to be recorded on media that responds to energy, we resort to interferometry. A coherent wave, called the reference wave, is added to the object wave at the plane of recording. The reference wave is usually either a spherical or a plane wave. Both amplitude and phase variations have been converted into irradiance variations to which the recording medium responds. The processed photographic record is called the hologram. Hologram recording involves the phenomenon of interference and hence, all the conditions necessary for obtaining high contrast fringes is on interference pattern must be fulfilled. A typical hologram recording geometry is shown in Fig.2.
Step II: Reproduction or. Reconstruction of Hologram:
To release the object wave from the hologram it is illuminated by a reference wave. Different perspectives of the object are seen when it is viewed through different regions of the hologram. Reconstruction ‘ involves the phenomenon of diffraction. The fine structure on the hologram created by interference diffracts the incident wave. Therefore, the conditions of monochromaticity and coherence are considerably relaxed.
- In ordinary photography, destruction of a portion may lead to loss of information but in the case of the hologram, each part contains information about the entire object. Therefore, a hologram is a reliable source of data storage.
- Holography can be used for testing stress, strain and surface deformation (static and dynamic) in objects.
- By viewing the three-dimensional acoustical holograms, in visible light, the internal structure of the object can be observed.
- This technique Will be highly useful in the fields of medicine and materials technology.