Phase unwrapping for flaw detection: is it necessary?

The phase-stepping technique is becoming a common enhancement to digital shearography and holography systems used in NDE. Capturing a series of phase-stepped images, rather than just a pair, allows the calculation of surface deformation at every pixel; data form an equispaced, spatially dense set rather than a sparse collection of nonuniformly spaced points. Phase maps are calculated modulo 2 pi. If surface deformations are of sufficiently high amplitude, the phase will appear "wrapped." The next logical step is to "unwrap" the phase and make it continuous again. For flaw detection purposes, however, this difficult step should be questioned; it is sometimes not only unnecessary, but deleterious to the flaw detection process. Further image processing steps must sometimes be applied to the unwrapped phase in order to expose flaws once readily visible in the wrapped phase map. These steps, only applicable to an unwrapped phase map, can also serve to uncover some flaws not previously visible in the wrapped phase map. Finally, effective data visualization plays an important role in conveying information embedded in the wrapped or unwrapped phase maps, This paper introduces these issues and gives several examples, wrapped and unwrapped, with varying excitation type, material (specifically composite), flaw type, and image processing.