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Thermal Waves for Imaging of Defects with Lockin‐Speckle Interferometry

ABSTRACT:  A temporal modulated deformation field was monitored by a phase shifted out‐of‐plane interferometer. The periodical deformation was generated by exposure to intensity modulated light which produces a thermal wave by absorption at the surface of the object. From the recorded sequence of speckle‐interferograms time‐varying absolute displacement maps are derived by an unwrapping process. The transformation in the frequency domain and the evaluation at the modulation frequency (lockin‐technique) allows to improve the signal‐to‐noise ratio and to investigate the depth where the defects are located. The measuring time for depth localisation can be reduced when the object is simultaneously excited and evaluated at several frequencies. The potential of this new method is shown on different model samples.