New Image Processing Algorithms For The Analysis Of Speckle Interference Patterns

Abstract

A sequence of algorithms for processing interference patterns generated by a phase -shifting speckle interferometer is discussed. The processing yields the computation of in -plane displacement and strain components on the surface of an object, using a phase-shifting algorithm to compute the phase. Accurate phase measurement on a 512 *512 grid is achieved by pixel -synchronous digitizing of four interference patterns and subsequent calculation of the phase modulo 2n radians, using a two -dimensional look -up table. A pipeline of Datacube image processing modules is configurated to perform this measurement. Digital image processing algorithms have been developed for phase unwrapping, phase restoration and smoothing. During these processing steps, invalid pixels due to low modulation or saturation are neglected. For phase unwrapping an algorithm has been developed that, starting at a chosen start pixel, propagates a "wavefront" of unwrapped phase data through the data set and that takes a set of neighbouring pixels into account to detect 2Et steps. Subsequently, phase restoration is done by averaging valid neighbouring pixels. Basic binary image processing techniques are used to solve the problem of irregularly shaped objects due to holes and shadows. The measured phase change is used to compute the inplane displacement and strain components of the deformed object. Results of a measurement of in -plane displacement and strain components on the surface of a simple aluminium object are shown.

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