Depth estimation of internal defects from a sensitive frequency using thermal wave radar

Thermal wave radar is one of the active infrared thermographic techniques that detects defects by applying a broadband frequency modulated excitation and cross-correlation. Although it outperforms conventional methods in the depth resolvability and defect detectability, there still exist difficulties in the estimation of defect depth due to the distortion effect of heat diffusion and noises. This work aims to estimate the depth of an internal defect using a new parameter instead of the widely used blind frequency, namely the sensitive frequency, at which a maximum phase contrast is reached. The thermal wave radar is implemented by using a linear frequency modulated laser excitation. The phase contrast in function of frequency is obtained from the dual orthogonal demodulation algorithm. A linear relationship with calibrated coefficients is established between the sensitive frequency and the reciprocal of the square of depth. The effect of defect size on the linear relationship is studied, and a correction of the linear relationship is proposed to improve the accuracy of depth estimation. The proposed method is numerically verified and experimentally validated, and the results illustrated that the depths of defects with various aspect ratios can be well estimated.