The word holography comes from the Greek words meaning whole record and is based on the reconstruction of light wavefronts. Therefore it is important to first understand the principle of superposition and interference of waves. If two light wavefronts are traveling and pass a given point, the total intensity is given by the algebraic sum of the intensities of the individual wavefronts. When the phases of the two wavefronts are the same, the intensity is the sum of the incident intensities, but when the phase of the two wavefronts is 180 degrees apart the intensity is zero.
This effect is known as interference which is the most important element or basis of optical measurement techniques. Thus when two plane wavefronts of Intensities I1 and I2 are superimposed the intensity varies periodically between the maximum value (I1 + I2 + 2 √I1I2 ) and the minimum value (I1 + I2 - 2 √I1I2 ). This intensity variation is known as a fringe pattern and is in the form of a series of planes of uniform intensity which are parallel to the plane that bisects the angle between the two wavefronts or beams of light.
In summary any pair of light wavefronts of the same single frequency (monochromatic light) which are added together will give rise to a fringe pattern. The shape and spacing of the fringe pattern will depend on the nature of the wavefronts.
Holography is a technique whereby one wavefront can be recorded and subsequently reconstructed without the presence of the original wavefront. When reconstructing the original wavefront from the recording, a three dimensional image is observed. It is a fairly recent technique which although originally suggested in 1949 by Gabor, has only became practical with the availability of lasers.
Holographic interferometry is an extension of holography which has its usage in the scientific and engineering fields. There are two distinct techniques of holographic interferometry which enable very accurate surface displacements to be accomplished.
The "frozen fringe" or "double exposure" holographic interferometry involves the recording of the image of an object (in the same manner as attempting to make a hologram) and subsequently making a second recording of the object after it has been stressed minutely. Thus the recording medium (usually an emulsion film) contains two almost identical images of the object. After reconstructing the images, an interference between the images is noted in the form of a fringe pattern superimposed on the object's image. This fringe pattern, in the form of zebra markings (contours) is the measure of the dimensional changes of the object between its stressed and unstressed condition. An alternate techniques allows "real time" or "live fringe" patterns to be observed. This technique is accomplished by recording a single image of the object (i.e. making a hologram of it), and after processing the film is relocated in the exact position where it was during the recording. I f one views the reconstructed image of the hologram it is superimposed on the real object and thus any perturbation on the object will create interference of the two images and as such a fringe pattern will appear. It should be noted that such a technique is very demanding on stability, in that requires exact relocation of the processed hologram in its original position and compensation for the emulsion shrinkage that takes place during the chemical processing. With care these conditions can be satisfied resulting in the highly desired "real time" examination of the objects behaviour under stress. Practical arrangements for holographic interferometry make use of the offset reference beam principle, suggested by Leith and Upatnieks as shown in the schematic below.