11 August 2008 Lockin-speckle-interferometry for non-destructive testing
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Interferometrical methods like Shearography or Electronic-Speckle-Pattern-Interferometry (ESPI) are being used for remote deformation measurements. For non-destructive testing, usually not the deformation of the whole inspected object is of interest, but only the changes in the deformation field that are caused by hidden defects. By applying the lockin technique, small local discontinuities can be monitored even on a large background deformation. Dynamic excitation is performed by modulation of absorbed light intensity while object deformation is continuously recorded to give a stack of fringe images. Instead of using only the information contained in the image with the best contrast, our technique evaluates the whole image stack with respect to the local response to the coded input. The periodical component of the deformation is extracted by Fourier transformation for the time dependent signal at each pixel. This way the relevant information contained in the image stack is compressed to an amplitude- and a phase angle image. As only defects contribute to a signal change in the phase image, the method is defect selective. Furthermore, the phase change depends on depth where the defect is located since thermal waves are involved. One more advantage is the substantial improvement of the signal-to-noise ratio.
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Philipp Menner, Philipp Menner, Henry Gerhard, Henry Gerhard, Gerd Busse, Gerd Busse, } "Lockin-speckle-interferometry for non-destructive testing", Proc. SPIE 7063, Interferometry XIV: Techniques and Analysis, 70630C (11 August 2008); doi: 10.1117/12.794650; https://doi.org/10.1117/12.794650

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