Nondestructive thermographic detection of internal defects using pixel-pattern based laser excitation and photothermal super resolution reconstruction

Thermographic testing, as a non-destructive testing method, is a non-contact imaging technique for examining specimens by characterizing their thermal conduction behavior. One of the main limitations of thermography is the inherent limit to the resolution of the internal structures of any specimen due to the diffusive nature of heat conduction. Overcoming this limit is the goal of thermographic super resolution methods. In this work, we present a method using two-dimensional pixel pattern based active photothermal laser heating in conjunction with subsequent numerical reconstruction to achieve a high-resolution reconstruction of internal defect structures. Furthermore, a forward solution to the underlying inverse problem is proposed along with an appropriate heuristic to find the regularization parameters necessary for the inversion in a laboratory setting. This allows the generation of synthetic measurement data, opening the door for the application of machine learning based methods for future improvements towards full automation of the method. The proposed experimental approach is validated using the latest laser coupled DLP projector technology as a heat source. Finally, the proposed method is shown to outperform several established conventional thermographic testing techniques while improving the required measurement times by a factor of 8 compared to currently available photothermal super resolution techniques.