Optical Fringe Analysis.
DEPARTMENT OF THE AIR FORCE WASHINGTON DC
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This report describes an apparatus for measuring in-plane surface displacement utilizing speckle diffraction interferometry. A double exposure specklegram is incrementally illuminated with a narrow laser beam. The diffraction pattern created upon passing through the specklegram strikes a mask having a variable transparency pattern covering a range of spatial frequencies. The correlation of the diffraction pattern and mask pattern projects through the mask and is optically integrated along lines of constant spatial frequency. The intensity of the luminous energy line created thereby represents the degree of correlation. The peak intensity, as measured by a calibrated linear detector array, corresponds to the spatial frequency of that point on the specklegram and the surface displacement between speckle recordings. Orientation of the displacement at each specklegram increment is obtained by using a dove prism to rotate diffraction pattern until a maximum spatial frequency is observed. Author