Thermal wave imaging using lockin-interferometric methods

Abstract

We report about a technique where we transferred the Lockin-principle from Lockin-thermography to interferometry to perform thermal wave lockin-interferometry. This technique is based on speckle-interferometric imaging of periodical height changes going along with the temperature modulation in a thermal wave. We used both electronic speckle pattern interferometry and shearography setups and operated them with low frequency periodical heat deposition while a stack of interferometric fringe patterns was recorded. After unwrapping, each pixel of the stack was Fourier-analysed at the Lockin-frequency, giving an amplitude image and phase image of low frequency thermal deformation. Though this is very much like Lockin-thermography, the image generating mechanism is substantially different: The thermal wave generates periodical thermal expansion correlated with an overall deformation where the depth integral of the thermal wave is involved. At such a low frequency (below 1 Hz), deformation occurs simultaneously everywhere except in areas where thermal wave propagation is modified e.g. by boundaries, which affect the phase of deformation. Depth range is adjusted via modulation frequency as in lockin thermography.