Lockin-Speckle-Interferometry for non-destructive testing

摘要

Interferometrical methods like Shearography or Electronic-Speckle-Pattern-Interferometry (ESPI) are being used forremote deformation measurements. For non-destructive testing, usually not the deformation of the whole inspectedobject is of interest, but only the changes in the deformation field that are caused by hidden defects. By applying thelockin technique, small local discontinuities can be monitored even on a large background deformation. Dynamicexcitation is performed by modulation of absorbed light intensity while object deformation is continuously recorded togive a stack of fringe images. Instead of using only the information contained in the image with the best contrast, ourtechnique evaluates the whole image stack with respect to the local response to the coded input. The periodicalcomponent of the deformation is extracted by Fourier transformation for the time dependent signal at each pixel. Thisway the relevant information contained in the image stack is compressed to an amplitude- and a phase angle image. Asonly defects contribute to a signal change in the phase image, the method is defect selective. Furthermore, the phasechange depends on depth where the defect is located since thermal waves are involved. One more advantage is thesubstantial improvement of the signal-to-noise ratio.