Mapping the depth and thickness of flat metallic inclusions using lock-in thermography

In this work we propose a method to map simultaneously the depth and thickness of a flat metallic inclusion embedded in an opaque thermally insulating matrix. It is based on lock-in infrared thermography with a modulated and uniform illumination of the whole sample surface. In a previous paper we found an analytical expression for the resulting surface temperature oscillation. Here we measure the surface temperature at several frequencies, and we fit the analytical formula to the frequency spectrum recorded at each pixel. In this way, we obtain a local value of depth and thickness of the metallic inclusion. Experiments performed on polymeric samples containing calibrated metallic inclusions betray depth variations, which were not expected by the manufacturer, indicating that the metallic inclusion is not parallel to the sample surface. This result was confirmed with an optical microscope.