Noise reduction techniques in optical nondestructive evaluation

The purpose of this doctoral work is to develop noise reduction techniques applicable to speckle interferometric techniques, such as electronic speckle pattern interferometry (ESPI) and shearography. This effort has led to the development of a new general category of speckle methods called additive-subtractive phase-modulated speckle interferometry (ASPM-SI). ASPM-SI methods utilize a phase-modulation of the object beam and a continuous reference-updating technique to decrease the effects of optical noise. To perform ASPM-SI, additive speckle interference images containing information about the same two states of deformation of a test object undergoing acoustic or pressure stressing are rapidly acquired in each CCD video frame. Meanwhile, phase modulation is introduced in every other frame during the video sequence (using a translating mirror for shearography or an electro-optic modulator for ESPI) and the additive interference images are subtracted sequentially using a real-time image processor. As a consequence of this modulation and image processing, the self-interference component of a speckle interference pattern (associated with additive interferometry) is removed and real-time fringe visibility (usually associated with substractive interferometry) is provided. Hence, the desirable characteristics of both additive speckle interferometry (noise protection) and subtractive speckle interferometry (real-time fringe visibility) have been successfully combined. The susceptibility of ASPM-SI methods to environmental noise caused by induced thermal noise is demonstrated to be lower than that of conventional subtractive speckle interferometric methods.; In order to more fully understand the inherent robustness of the ASPM-SI methods in the presence of noise, an analytical model is developed to describe the effects of in-plane translation noise on fringe visibility. In-plane translation generates a loss of correlation between additive interference images which reduces fringe visibility. The progression of decorrelation from full-correlation to total-decorrelation is known as partial decorrelation. The results of this partial decorrelation model for the ASPM-SI techniques are compared with the corresponding results for the conventional subtractive techniques. Finally, a recommendation, based on the experimental and analytical results involving noise reduction, is made for the configuration of a suitable optical NDE system for detection of defects in adhesively-bonded structures.