Measuring displacements using digital speckle pattern interferometry

Abstract

When coherent light is incident upon an optically rough surface, the scattered light forms a pattern of dark and bright spots that has a characteristic granular appearance called speckles.
Speckles are unique, random patterns in space resulting from the interference of spherical waves coming off a diffusely reflecting or transmitting surface illuminated with coherent light. As the surface is displaced or deformed, the speckle pattern changes accordingly although bound to the original by a phase difference corresponding to the amount of displacement or deformation.
Speckle Pattern Interferometry makes use of this behavior to compare the speckle patterns produced by an object in different stages of displacement or deformation. The scheme is to record and correlate the speckle patterns corresponding to two states of deformation of the same object.
Digital Speckle Pattern Interferometry (DSPI) is an interferometric measurement technique using a coherent light source. An offshoot of Electronic Speckle Pattern Interferometry (ESPI) also known as Electronic Holography, DSPI is similar to holography and other speckle methods as it uses the interference of a reference beam and an object beam.
DSPI is particularly favored because measurements are done simply on the changes in the correlation of two speckle patterns. The implementation is simple and easy and the system is stable and flexible. The technique requires less laser power. The data is obtained optically and processed digitally (DSPI). The displacement information can be generated by mathematical operations on the digitized images, thus quantitatively resolve the spatial distribution ofthe deformation and locate material defects or highly stressed locations. It can be sensitized to independently measure one of the three displacement vectors. The improvement of the CCD technology makes it capable for real-time viewing and instantaneous analysis.
In this paper we evaluate the performance of DSPI as a method for measuring micrometer deformations against the established methods of Michelson Interferometry and Holographic Interferometry.