Optimization of fringe pattern calculation with direct correlations in speckle interferometry

A direct correlation technique is used to calculate correlation fringe patterns from consecutive speckle patterns acquired with a dual-beam electronic speckle interferometer. Although more calculations are required than in standard image differencing routines, an advantage of the method is that the illumination over the surface of the object need not be uniform. In the method, Pearson's coefficient of correlation between the intensities within a set of adjacent pixels is calculated. This has the added advantage of being directly related to the theoretical phase-dependent correlation. A mapping of this measure of correlation results in the correlation fringe pattern. Laboratory tests were carried out with in-plane translations ranging from 5 to 45 mum. The correlation calculations were carried out by using cells (square sets of pixels) in the raw speckle images with dimensions ranging from 2 pixels x 2 pixels to 19 pixels x 19 pixels. Both cell dimension and translation magnitude dependent decorrelation effects influence the quality of the correlation fringe patterns.