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Contrast Enhancement Of ESPI Vibration Patterns By Speckle Averaging In A Video Frame Store

The analysis of vibration modes of engineering components is an important aspect of industrial testing. Holographic interferometry is one technique for studying whole areas of an object at the same time. Electronic speckle pattern interferometry (ESPI) uses a TV video system to perform a similar function by means of the speckle effect from an object that is illuminated with coherent laser light. While useful for converting light intensities into phase information relating to movement of the surface, speckles represent unwanted video noise in the final interferogram on the monitor screen. This paper presents a technique for reducing the speckle noise by sequentially averaging a large number of interferograms with de-correlated speckle patterns in a video frame store. A silicon-target storage tube or a digital frame store is used to produce high contrast ESPI fringes. This simplifies analysis by eye from the screen or by computer from the digital frame store. The combination of real time frequency scanning and contrast enhancement at a chosen frequency in only a few seconds makes ESPI a useful industrial test instrument.