Holographic interferometry

Holographic interferometry (HI) is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision (i.e. to fractions of a wavelength of light). These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing. It can also be used to detect optical path length variations in transparent media, which enables, for example, fluid flow to be visualised and analyzed. It can also be used to generate contours representing the form of the surface.

Holography is the two-step process of recording a diffracted light field scattered from an object, and performing image rendering. This process can be achieved with traditional photographic plates or with a digital sensor array, in digital holography. If the recorded field is superimposed on the 'live field' scattered from the object, the two fields will be identical. If, however, a small deformation is applied to the object, the relative phases of the two light fields will alter, and it is possible to observe interference. This technique is known as live holographic interferometry.

It is also possible to obtain fringes by making two recordings of the light field scattered from the object on the same recording medium. The reconstructed light fields may then interfere to give fringes which map out the displacement of the surface. This is known as 'frozen fringe' holography.

The form of the fringe pattern is related to the changes in surface position or air compaction.

Many methods of analysing such patterns automatically have been developed in recent years.

Several research groups published papers in 1965 describing holographic interferometry. While the first observations of phenomena that could be ascribed to holographic interferometry were made by Juris Upatnieks in 1963 the essential feature of the process was not understood until the work of Powell and Stetson. Their experiments were conducted over the period of October to December 1964, and they began with an investigation of the periodic coherence length of the HeNe laser being used. The compact laser beam was used to illuminate a spot on a small object was placed between two mirrors such that its image could be observed looking over one mirror into the tunnel of multiple reflections between the mirrors. Each image was 10 cm greater in path length than the one before it. Because these lasers had about three longitudinal modes, their coherence length was periodic, as described by the manufacturer, Spectra Physics in cooperation with the Perkin Elmer Corporation. This was demonstrated by recording a hologram of the view over one of the mirrors.

In one of the holograms, however, a dark band was observed in the closest image to the hologram, and it was observed to shift position with perspective. This band was not observable in the original laser beam and had to be something created by the holographic process. The confocal laser cavity consisted of a spherical mirror at the output end with a flat mirror at the center of curvature at the other end. Adjustment of the longitudinal spacing controlled the number of off-axis modes of oscillation, and it was observed that the laser was oscillating in more than one axis mode. The multiple laser modes were incoherent and did not interfere in the observable laser beam, so why did they interfere in the hologram reconstruction? Stetson put forth the idea that each mode existed in both the object and in the reference beam, and each pair recorded a separate hologram in the photographic plate. When these were reconstructed, both recordings reconstructed simultaneously from the same laser beam and the fields were then mutually coherent. Powell objected to this idea, because it implied that the hologram had the power to coherently reconstruct fields that were incoherent during its recording.

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