Digital speckle-pattern shearing interferometry

Abstract

An application of digital image processing techniques to speckle-shearing interferometry is described. A present system consists of an image-shearing camera using a Fresnel biprism of small angles and a digital TV-image processing facility. This interferometer makes it easy to measure in quasi-real-time spatial derivatives of surface displacement and modal vibration amplitude of objects. A statistical theory is applied to analyze the formations of these fringes due to 3-D displacement. Interpretation of the result leads to conditions of maximum fringe contrast and the limitation of this technique. Experimental results in the three cases of slope of normal displacement, surface strain, and slope of vibration amplitude measurements are presented.

Vibration Measurement by Speckle Interferometry between High Spatial and High Temporal Resolution

“…. The subtraction of the speckled image of an object from that of the object at rest, as in (Nakadate, 1980) …”

Identification of antinodes and zero-surface-strain contours of flexural vibration with time-averaged speckle pattern shearing interferometry

“…The further development of the basic technique has led to a more convenient setup, shearography, which measures the vibration amplitude gradient directly. 2,3 The electronic version of this is often referred to as electronic speckle shearing interferometry. The capability of ESPI or shearography has now been enhanced by the use of high-speed computers with modern image processors.…”

“…The capability of ESPI or shearography has now been enhanced by the use of high-speed computers with modern image processors. [3][4][5] The phase-shift method has permitted both techniques for analyzing vibration quantitatively. 6 ESPI-based methods of measuring vibration have a few different techniques, for example, the timeaveraged technique, the stroboscopic technique, and the pulsed laser technique.…”

Practical Estimates of the Errors Associated with the Governing Shearography Equation

“…Its utility for qualitative flaw characterization has been demonstrated, and while there is a need for using shearography in NDE for quantitative analysis, a large amount of the research [2][3][4][5][6][7] has concentrated on the qualitative evaluation of structures and materials. The purpose of this paper is to begin building upon a foundation for the newly emerging quantitative shearography [8].…”