Evaluation of the continuous wavelet transform method for phase measurement in Digital Speckle Pattern Interferometry

The Fourier transform method has become a popular technique to retrieve the phase map encoded by Digital Speckle Pattern Interferometry (DSPI) fringes. When closed fringes need to be analyzed, carrier fringes must be introduced in the pattern to eliminate the sign ambiguity that appears in the phase distribution. Recently, a method based on the application of the continuous wavelet transform (CWT) has been reported to evaluate phase maps in SPI. The CWT method does not produce ambiguities in the phase sign even when carrier fringes are not introduced in the interferometer. In the present paper the performance of the CWT phase-extraction method is evaluated using computer-simulated fringes, approach that allows knowing precisely the phase map contained in the pattern. It is shown that only DSPI fringes that verify the stationary phase approximation and its analytic asymptotic limit can be processed with this method. The influence of the filtering process to smooth the DSPI fringes and of the method used to extend the fringe pattern edges is also analyzed. Additional drawbacks that emerge when this method is applied are finally discussed.