Holography – Principle Of Recording And Reconstruction Of 3-d Images

Introduction of Holography:

Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same position relative to the recording medium as it was when recorded. The image changes as the position and orientation of the viewing system changes in exactly the same way as if the object was still present, thus making the recorded image (hologram) appear three dimensional. The technique of holography can also be used to optically

store, retrieve, and process information. Holography is commonly used to display static 3-D pictures. Holography was invented in 1947 by Hungarian physicist Dennis Gabor for which he received the Nobel Prize in Physics in 1971.

Recording of a hologram:

In holography, some of the light scattered from an object or a set of objects falls on the recording medium. A second light beam, known as the reference beam, also illuminates the recording medium, so that interference occurs between the two beams. The resulting light field is an apparently random pattern of varying intensity which is the hologram. There are a variety of recording materials which can be used, including photographic film.

A very simple hologram can be made by superimposing two plane waves from the same light source. One beam (the reference beam) hits the photographic plate normally and the other one (the object beam) hits the plate at an angle θ. The relative phase between the two beams varies across the photographic plate as 2πysinθ where y is the distance along the photographic plate. The two beams interfere with one another to form an interference pattern. The relative phase changes by 2π at intervals of d = λ/sinθ so the spacing of the interference fringes is given by d. Thus, the relative phase of object and reference beam is encoded as the maxima and minima of the fringe pattern.

Reconstruction of three dimensional image:

The process of producing a holographic reconstruction involves the phenomenon of diffraction of light. When the photographic plate is developed, the fringe pattern acts as a diffraction grating and when the reference beam is incident upon the photographic plate, it is partly diffracted into the same angle θ at which the original object beam was incident. Thus, diffraction grating created by the two waves interfering has reconstructed the object beam and when we look into the hologram, we sees the object even though it may no longer be present.