Measurement of Vibrational Amplitude by Modulation of Projected Fringes

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Abstract
A full-field, variable sensitivity technique for mapping the vibrational amplitude of a diffuse surface is demonstrated. The surface under study is placed in an optical field consisting of two mutually coherent plane waves which propagate in different angular directions. At any instant of time the interference of these waves causes a fringe pattern to be formed on the surface under study. The time average, over one or more vibrational periods, of the intensity pattern on the surface can be obtained in real time by utilizing visual persistance or by making a time-average recording of an image of the surface. The visibility of the fringe pattern thus obtained is found to be modulated by a function of the local amplitude of vibration. This modulation can be observed directly or a photograph of the pattern can be optically processed to yield fringes that are loci of constant vibrational amplitude.