Real-time anti-node visualization of vibrating distributed systems in noisy environments using defocused laser speckle contrast analysis |
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Authors: | Lionel Keene Fu-Pen Chiang |
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Affiliation: | a Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049-1684, USA b Department of Mechanical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2300, USA |
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Abstract: | A simple optical technique is described that allows for the direct whole-field visualization of anti-nodal patterns in noisy environments. The technique involves the combination of defocused laser speckle imaging with the laser speckle contrast analysis (LASCA) algorithm to relate local contrast reduction of a laser speckle pattern to the magnitude of the gradient vectors that arise due to the tilt deflection of a vibrating surface. The method requires no reference beam and is tolerant of environmental noise. The sensitivity of the technique is a direct function of the degree of defocus of the speckle pattern and thus can be readily adjusted. By ratioing the instantaneous contrast to the reference image contrast, a simple method for thresholding the noise floor (noise at reference state) is attained, thus increasing the method's tolerance to ambient disturbances. Results from forced vibration via acoustic excitation of a square and rectangular membrane—with and without simultaneous white noise excitation—are shown. Results are compared to theoretical predictions for an ideal membrane, with good agreement. |
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