Enhanced resolution methods in shearography and holography for time-average vibration measurement

Abstract

Vibration measurement by time-average methods of different full-field techniques like speckle interferometry, digital holography or analog, optical holography is a powerful technique, able to provide vibration amplitude maps of high spatial resolution. The essential characteristic of full-field techniques is the simultaneous acquisition of data for all object points. Quantitative data processing aiming to obtain the full-field amplitude map is affected by several difficulties. The most important are the weak contrast of Bessel-type fringes and the speckle noise. The greatest obstacle in achieving complete amplitude field estimation comes from the orthogonal components of time-averaged interferograms, where the multiplicative, high-frequency phase noise covers the deterministic, vibration-related phase. Several researchers studied these problems in relation with the double-exposure method. In the present paper, the author presents in a single, unifying approach, these methods, common to all full-field interferometric techniques. An important reduction of multiplicative high-frequency phase noise allows obtaining fringe-averaged patterns whose intensity noise is much lower than in classical methods. The analysis leads to lower noise of the fringe patterns and extended measurement range, and also to a method of vibration-related phase estimation based on the mathematical inversion of the Bessel function, which may include in some stages subpixel precision.