Lockin-interferometry: Principle and applications in NDE

Abstract

Interferometry is relevant for non-destructive evaluation (NDE) since dimensional changes much smaller than an optical wavelength result in detectable signals. Fringe images obtained with Electronic-Speckle-Pattern-Interferometry (ESPI) or shearography display changes of surface topography between two states of an object, usually using a static load. Usually, hidden defects are found by comparing the observed fringe pattern to the one obtained on an intact reference component and to attribute observed differences to a defect. Our approach is a periodical object illumination with light that is absorbed in the surface to generate heat and a corresponding modulation of thermal expansion. At the same time fringe images are recorded (either with ESPI or shearography) to give a stack. Subsequently, each image is unwrapped and thereafter the time-dependent content of each pixel is Fourier transformed at the excitation frequency, so the result is local amplitude and phase of the modulated response at this frequency. The phase image displays local delay between excitation and response. This phase change depends on the depth where the defect is located since thermal waves are involved. In this paper, NDE-examples obtained using this new technique are presented. It is also shown how the achieved improvement as compared to conventional interferometry is up to an order of magnitude.