A grid is projected onto an object and focused into a viewing camera which is offset from the angle of illumination. When the image is superimposed by double exposure or by filtering with the image of another object, contour-difference moiré fringes appear. By the use of two offset projectors, measurements of contour sums or contour doubling may be accomplished. The properties, limitations, sensitivity, and procedures are analyzed and discussed. The method lends itself to image enhancement and is extended to the study of vibration analysis in real time. In the latter application, time-average moiré fringes appear according to the zeroth-order Bessel function, which contains the modal amplitude function in its argument.
J. Der Hovanesian and Y. Y. Hung, "Moiré Contour-Sum Contour-Difference, and Vibration Analysis of Arbitrary Objects," Appl. Opt. 10, 2734-2738 (1971)