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Cont. A fundamentally new method of recording of opticalimages known as holography is now available. The word holography originates from the Greek wordholo meaning the Whole and graphy meaning the writing i.e. holography means complete recording. Theprinciple of holography was first put forward by DenisGabor, of the Imperial College of Science andTechnology, University of London, in 1947.

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Cont.

In holography, the image of the object to be photographedis not recorded but the light waves reflected from theobject are recorded. The photographic record is called ahologram. Holography is a two step processes:

(1) Transformation of the object into hologram i.e. anobject illuminated by coherent light is made to produceinterference fringes in a photographic emulsion(2) Retransformation or reconstruction of hologram intothe image of the object i.e. re-illumination of the developedinterference pattern by light of same wavelength toproduce a three dimensional image of the original object.

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Whats holography?

Holography is the process or technique of makingthree-dimensional image of the object. A hologram

is produced by the interaction of two beams of laserlight , one is the object beam coming through theobject and another is the reference beam comingdirectly from the source.

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Principle of Holography During the recording process, object wave (wave illuminatingthe object) and the reference wave (coming directly fromsource) interfere in the plane of the recording medium andproduce interference fringes. This photographic plate carryingthe interference pattern is called Hologram .

The interference fringes contain all the information aboutthe intensity and the phase of the scattered beam from object.

In the reconstruction process, the hologram acts as diffractiongrating . This is illuminated by a wave called thereconstruction wave (in most cases this similar to thereference wave used for recording the hologram) and the

image of the object is reconstructed from the hologram.

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Construction of HologramsNeed a laser (highly coherent and monochromatic light),

lenses, mirror, photographic film, and an objectBeam splitter separates the laser light into two beams,reference beam and object beamReference beam aimed at a piece of holographic film by

mirrors.Object beam directed at object to be recorded toilluminate object and then meets reference beam at film.Produces interference pattern of object beam andreference beam which is recorded.Film is developed. This film is called a hologram.This hologram contains all the information about the

intensity and phases of the scattered waves from theobject.

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Construction of Hologram

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Reconstruction of an image

from Hologram To view the image, the hologram is again illuminated with another laser beam called the reconstructionbeam which is identical to the reference beam.

Hologram acts as a diffraction grating and diffracts thelight. This form a real image in front of the hologram and a

virtual image behind the hologram. The virtual image has all the characteristic of the

object. The real image can be photographed directly without using a lens just by placing a light sensitivemedium at the position of real image.

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Reconstruction of an image from Hologram

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Hololography vs. PhotographyPhotography is 2D record of a 3D object whereas holographygives a three dimensional form of original object. If any objectis hidden just behind another object then the observer can seethe hidden object in viewing the hologram.Hologram is the positive pattern whereas in conventionalphotography negative pattern is produced.In ordinary photography each region contains separate andindividual part of the original object. Destruction of a portionof a negative leads to an irrepairable loss of information

corresponding to the destroyed part. On the other hand, eachpart of a hologram contains information about the entireobject. Destruction of a part of hologram does not cause a lossof information about the object, each seaparate fragment is

capable of producing image with a reduced clarity.

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Hololography vs. Photography (contd)

The information holding capacity of a hologram isextremely high by recording several images of theobject whereas in ordinary photography a photofilmcannot be used to record several images. Without the knowledge of the reference wavefront thehologram cannot be deciphered.

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This diagram shows how a hologram is made.There is an important difference between aphotograph and a hologram: a photographconsists of pixels which, combined, show theoriginal image - a hologram consists of waveinterference patterns which describe or"encode" the original object. It requires anadditional step of creating the projection tomake the object visible again. As a consequence, if you cut a photograph inhalf, then half of its pixels will be missing.But if you cut a hologram in half, you can stilluse each of the pieces to project the whole 3-dimensional object that was recorded! Although there are no actual dots in ahologram, it could be said that each of ahologram's "dots" contains the whole image.

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Applications of HolographyDesign of containers tohold nuclear materialsCredit cards carrymonetary valueSupermarket scannersOptical Computers

Improve design ofaircraft wings andturbine bladesUsed in aircraft heads -up display Art Archival Recording offragile museum artifacts