Improved hybrid phase unwrapping algorithm in speckle interferometry

In many practical applications of speckle interferometry, the commonly known phase unwrapping approaches cannot cope with the stability and performance requirements of industrial environments. The use of low-cost optical components even increases the high noise levels of non-laboratory surroundings. Therefore, two fast window based techniques have been developped, which are then combined by a hybrid algorithm to obtain optimum performance and stability. The first method is a polynomial approximation method on the complex phasor image. A fast update technique on overlapping data. windows is used for those areas in an interferogram with fringe densities less than one fringe per window. In areas with higher fringe densities, a second FFT-based approach is used to perform local phase unwrapping. For each data window, the FFT is computed and the gradient of the underlying phase function is given by the peak position. This method is slower than the polynomial approach, but it proves stable with all fringe densities. Any noise not exceeding the fringe information in the frequency domain will not alter the result of the gradient computation. The algorithm has been tested with different types of industrial interferograms. Compared to pixel-based methods it proves in general more stable and faster. Especially images with high fringe densities but low level of detail can be handled very effiently since the window size and step width can be adjusted accordingly. The hybrid method is therefore suitable in an industrial environment, where quick response and a wide measurement range is needed.