Speckle-Shearing Interferometric Camera -- A Tool For Measurement Of Derivatives Of Surface-Displacement

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Abstract
A camera having a lens with two apertures and equipped with a shearing mechanism is employed to image a specimen which is illuminated by coherent light. With this arrangement, the speckle from one point on the surface is allowed to interfere with the speckle from a neighboring point so that a speckle-grid pattern may be detected in the image plane. By double-exposure or real-time techniques, the speckle-grid pattern corresponding to the deformed state of the specimen is added to the speckle-grid pattern corresponding to the undeformed state of the specimen. The resulting mechanical interference produces a moire-fringe pattern which depicts derivatives of surface-displacements. The camera will be referred to as "Speckle-shearing Interferometric Camera". Three applications of the speckle-shearing interferometric camera are described: (1) mea-surement of slopes of structural deflections, (2) in-plane strain measurement, (3) vibration analy-sis. Experimental demonstrations for each case of the applications are presented. Although the speckle-shearing interferometric camera is based on interferometric principles, it overcomes several of the limitations associated with holographic and speckle interferometries, namely: (1) the setup is simple and does not re-quire laborous alignment of optical components, (2) it does not require stringent mechanical and ambient atmospheric stabilities, (3) coherent requirements of light are greatly relaxed, (4) the sensitivity is greatly reduced, and (5) it does not require differentiation to obtain strains.

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