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Holography

In the case of ordinary photographs, it is possible only to obtain the view in a particular plane. The camera lens can be focused only in a particular plane and the details of the field nearer and farther than the focused plane will not be recorded. All the other planes are out of focus. The main reason for this is that the photograph records only the intensity distribution in a particular plane.

If on the other hand, it is possible to record the amplitude and the phase distribution in any plane between the object and the observer, it helps to obtain the complete field of view as originally observed.

This type of recording is done in holography and holographic studies make it possible to have complete study of the field of view at any other time.

Gabor (1948) has introduced for the first time the holographic method of recording and retrieving the image.

S is a point source of light and O is a small object. On XY, the secondary wavelets from O superimpose on the strong primary waves from S. As the primary wave is uniform and more intense than the secondary wave, the variation in intensity across XY is dependent on the variation in phase across it. It is not determined by the variation in intensity across the secondary wave. In other words, the presence of strong coherent background helps to record information about the phase of the diffracted light. This technique was first introduced by Zernike. The pattern obtained on XY is called a hologram and its photograph is taken, keeping the time of exposure extremely small.

For reconstructing the field of view, the photographic plate is developed by reversal and if this developed plate is inserted at the place XY and only a source S is used, on looking through this plate towards S, the object will appear at the point O. In this way the original field of view is observed.

Holography has been used in holographic interferometry. Holography is also useful in the microscopic examination of certain kinds of specimen. If one desires to make a prolonged a prolonged examination of a small specimen suspended in a medium, it is necessary to focus the microscope off and on due to the change of position of the specimen. This difficulty can be overcome by taking a short exposure holograph of the specimen. The reconstructed holographic image can be examined continuously by focusing the microscope.

Holography is also useful to provide a high capacity system for image storage and reexamination.