Surface breaking defects are detected using a scanning laser source to deposit heat into a sample surface. Any lateral flow of heat is disturbed by such a defect, with a change in thermal spot shape being detected by an infrared camera. An apparent increase in temperature as the laser passes over a defect is due to a localised change in emissivity, allowing defects to be distinguished from surface markings. Defects have been detected in both stainless steel and aluminium.
Burrows, S. E., Dixon, S., Pickering, S. G., Li, T., & Almond, D. P. (2011). Thermographic detection of surface breaking defects using a scanning laser source. NDT and E International, 44(7), 589-596. https://doi.org/10.1016/j.ndteint.2011.06.001