Main Page/PHYS 3220/Holography

Introduction

Holograms are three dimensional images of scenes or objects recorded on two dimensional film or glass plates coated with photographic emulsion. The hologram actually records the interaction of two beams of coherent light, and these are mutually correlated in the form of a microscopic pattern of interference fringes. The first beam of coherent light is called the reference beam and it directly illuminates the film. The second, or object beam travels from the light source to the object, and then is reflected onto the film. The interference pattern at the film is used to store the information of the depth of the recorded object. When properly illuminated, the film or photographic plate produces a three dimensional image which is the hologram.

In this experiment, you will produce different holographic plates using different optical beam configurations to gain a broad understanding for the physics principles behind holography, and also the various techniques employed in producing such holograms.

Method

You've learned throughout your physics education that light can be thought of as a wave with an amplitude and phase.In conventional photography, a lens is used to focus light from a point on a source onto a corresponding point on a photographic plate. This plate then forms an irradiance map of the original object. The amplitude is recorded in terms of the intensity, but the phase information of the wavefront is lost, and hence there is no perception of depth or parallax in the final photograph.

The basic concept of a hologram is shown in Fig. 1 using a simple example. On the top left, light incident on a point object is diffracted into a pattern of concentric diffraction rings of maxima and minima which can be recorded on a transparency. In a second step, light incident on this transparency will be focussed onto a point. To understand how a plate of concentric rings, which presents a multi-slit aperture to the incoming parallel beam, can act as a lens, there are many good books (e.g. ref 11) which describe this and you should read about Fresnel lenses and the zone plate. The same argument extended to two points for the object (middle diagram) shows two concentric rings. If this transparency is then exposed to light, each set of concentric rings will focus the light onto two separate points. Finally in the bottom example, you can see that a cluster of points (which is shown in this example as the letters A B C) produces a superimposed array of concentric rings for each point on the object. Each set of rings will refocus incident light onto a corresponding point and the net result is a reproduction of the object which are the letters A B C.