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.