Interferometric hologram evaluation and real-time vibration analysis of diffuse objects

Laser speckle limits the resolution that can be achieved in holography. Time-varying speckle, caused by scattering from moving objects, is even more troublesome. It can destroy the correlation needed to obtain fringes in holographic interferometry and even, ifthe fluctuations are rapid compared with the exposure time needed, the coherence needed to record a hologram. These problems are discussed, with particular reference to biomedical holography, and suggestions are made for minimizing their effect. Some positive aspects of time-varying speckle are also discussed. The fluctuations obey the same statistics as the spatial variations of a stationary speckle pattern. These statistics can be used to measure the movement of the scattering particles, a technique of particular value in biomedical science. Potential applications include monitoring blood flow, motility, and intracellular activity.

Speckle fluctuations and biomedical optics: implications and applications

Application of moire techniques to holography

New methods are introduced that utilize interferometric comparison of images of diffusely reflecting objects from different hologram plates that are sandwiched together. To make the plate positions optically and mechanically identical during reconstruction and exposure, they are also exposed in sandwiches. If the two sandwiched hologram plates are separated by a small distance a new method of fringe evaluation can be used. Fringes caused by object tilt between two exposures can be eliminated by an analogous, but much larger, tilt of the sandwich hologram during reconstruction. Even the direction of tilt, forward or backward, is found this way.

Sandwich hologram interferometry : a new dimension in holographic comparison

In this paper the authors offer an explanation for the holographic observation of vibration patterns that do not agree with the classical mode theory of vibrating objects. The equations are derived for the formation of hologram vibration interference fringes under the assumption that an object may vibrate in any linear combination of its permissible modes, even while executing forced vibration at a single frequency. The modes appear to combine by phasor addition in these equations to give fringe loci that depend strongly on the relative phase of the combining modes. A set of experiments are reported which demonstrate this behaviour and allow quantitative comparison to the theory. Finally, the equations are derived for combinations of modes vibrating with independent frequencies, and again a set of experiments is reported which demonstrates this behaviour and permits quantitative comparison. Here the resultant fringes appear as the product of the fringe patterns of the component modes.

Measuring combination mode vibration patterns by hologram interferometry

Holographic interferometry is a very powerful technique for the measurement of small deformations and displacements of opaque solid objects. Unfortunately, the quantitative interpretation of the fringe patterns is somewhat involved, and the large number of papers published on this subject in the past 10 years has caused considerable confusion. This review paper is an attempt to rationalize the field by classifying the many different interpretation schemes into four main techniques, and by offering guidelines for the choice of technique for different applications.

J. D. Briers

Papers

The interpretation of holographic interferograms