Digital Speckle Pattern Interferometry (DSPI) Using A 100X100 Imaging Array

Digital speckle pattern interferometry (DSPI) is a variation of electronic speckle pattern interferometry (ESPI). Both methods use the same basic techniques for nondestructive testing, but DSPI processes speckle pattern data digitally in a computer instead of using analog electronics to enhance fringe contrast. Both testing methods are an outgrowth of holography and speckle interferometry. The system described here uses a Reticon 100x100 diode array camera with an integration time of 5 ms, instead of a television camera, coupled to an HP-9836C computer. The use of digital methods provides flexibility in measurement and processing. Results of measuring both static and dynamic object movement show high-contrast fringes with electronic noise at 1/500 of the dynamic range. A new technique for testing vibrating objects has been developed that significantly in-creases fringe visibility. It involves subtracting a reference frame containing only self-interference terms and no cross-interference term from a time-averaged data frame of the object vibration. This reference frame is created by vibrating a reference mirror at a high amplitude while the object is at rest. Using software written in assembly language, the maximum frame rate is 3 processed frames/sec. Trade-offs of using TV systems with analog processing vs diode arrays with digital processing are discussed.

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