Optical Stepped Thermography of Defects in Photovoltaic Panels

Defects in photovoltaic panels can reduce the effective working area, and decrease the performance of the photovoltaic panels. Therefore, it is significant to identify such defects and to feed the fault information up to the production chain. Active infrared thermographic non-destructive testing has the advantages of non-contact, fast speed and large imaging area, and has been widely used in the detection of defects in various materials. In this article, a stepped thermography of the defects, which result in degradation of energy conversion efficiency of cells in photovoltaic panels, was proposed. The front surface of the photovoltaic panel was optically stimulated by halogen lamps in step heating way, while an infrared camera was employed to monitor the temperature evolution of the front surface and to capture infrared image sequences. The thermal image sequences were processed using data processing algorithms, focusing on enhancing the defect signatures. The techniques of background subtraction, discrete Fourier transform, polynomial fitting, first-order derivative, cross correlation coefficient and histogram equalization were mainly used. Compared to the raw thermal images, the reconstructed images using suitable algorithms are more helpful to evaluate the defects. The results show that the optical stepped thermography combined with suitable post data processing is a fast and effective technique to identify the defects in photovoltaic panels.