Digital speckle pattern interferometry techniques, including both DSPI (ESPI) and shearography offer many well-known advantages to the experimental mechanist. Applications range tromrelatively simple NDI through sophisticated 3-dimensional strain measurement. Much development of these techniques has taken place, and they have come of age. Why are not these methods becomign standard, or at least common? Reasons seem to center on perceived problems with dirty and noisy environments, reputed high system costs that have ranged to megadollars, and the "optical mystique" that implies the methods can be used only in the lab by a Ph.D.. None of these reasons seem particularly valid anymore. This paper demosntrates that speckle interferometries that work in noisy environments can be built and used by non-experts at low cost. Following a brief review of the important concepts of speckle interferometry and shearography, two successful designs that can be assembled for a few thousand dollars are described. The design for the DSPI unit uses a reference scattering plate, meaning it approximates a common-path interferometer,a nd so is stable enough to be used in a noisy environment. In its simplest form, this interferometer is adequate for NDI and deformation analysis. Phase shifting can be added to this device to improve resolution and range of measurement applications. The design for shearography uses a Fresnel biprism. It is possible to combine these designs into one interferometer that can operate in either modes.
展开▼
