Holography

TECHNICAL

Photography & Holography

In photography, one is concerned only with the brightness or irradiance distribution (square of the amplitude) of the image. The optical path to different parts of the object is not recorded as the photographic emulsion is a square law detector and records only the amplitude.

In holography, the aim is to record complete wave field (both amplitude and phase) as it is intercepted by a recording medium. The recording plane may not be even an image plane. The scattered or reflected light by the object is intercepted by the recording medium and recorded completely in spite of the fact that the detector is insensitive to the phase differences among the various parts of the optical field.
In 1948 Denis Gabor gave an ingenious solution to the problem of recording phase information by means of a background wave, which converts phase differences into intensity differences. He introduced a two step lensless imaging process known as wavefront reconstruction technique or holography (Greek word holos means whole, complete), in which an interference between the object field and the background wave (known as reference wave) is formed and recorded on a photographic material. The record known as a hologram (whole record) captures the complete wave which can be viewed at a later time by illuminating the hologram with an appropriate light beam.

Thus, in holography an interference between the light reflected and scattered by the object, called the object (signal) beam and a reference beam is created and recorded on a photographic emulsion. If the amplitude of the signal beam remains constant and the angle between the beams increases, the fringes will become finer. On the other hand, if the phase relation between the two interfering beams remain constant but the amplitude of the object beam changes, the contrast of the fringes will change. By this process the complex object information gets coded in the form of complicated fringe pattern.

The object can be considered to be made up of a large number of point sources distributed in a three dimensional space. Each point of the object will interfere with the reference and produce fringes. The fringe patterns generated by different points will be varying in orientation, contrast and spacing.

Gabor showed the applicability of this new process of wavefront recording by using a mercury discharge lamp and taking collinear object and reference beams. The original in-line technique of Gabor produces both virtual and real images on the same axis, thus an observer focussing on one image, always sees it accompanied by the out-of-focus twin image.

Characteristics of a Hologram

Off-axis Holography : Leith-Upatnieks Holography

Leith and Upatnieks in 1962 demonstrated a technique which made it possible to separate the twin images. In this technique a separate coherent reference wave is allowed to fall on the hologram plate during the recording process, at an offset angle to the beam from the object. The exposed plate is developed by normal photographic procedures so that the amplitude transmittance of the plate after development is proportional to the exposure.
The output consists of the following four terms

(a) attenuated direct beam.
(b) a halo around the direct beam.
(c) a virtual image of the object at the same distance from the hologram. This image is deflected off the axis at an angle.
(d) a real image at the same distance from the hologram in front of it. This image is deflected off the axis at an angle the opposite direction.

Thus an off-axis hologram generates virtual and real images angularly separated from each other and from the direct beam also.

Hologram Aberrations

Holograms suffer from aberrations caused by a change in the wavelength from construction to reconstruction and also by a mismatch in the reference and reconstruction beams. Both the chromatic and nonchromatic aberrations are quite important even when only small deviation from the recording geometry are present in the reconstruction geometry. The condition that will eliminate all the aberrations simultaneously is to duplicate exactly one construction beam in the reconstruction process.