Visualization analysis of internal defects in coatings based on terahertz imaging technology

As an emerging non-destructive testing (NDT) technology, terahertz (THz) waves exhibit exceptional penetration into non-metallic coatings. When internal defects exist in coatings, their interaction with THz waves induces distinct spectral variations in THz time-domain signals. Leveraging this principle, this study takes a THz time-domain spectroscopy system as the core and innovatively constructs an integrated technical framework of "transmission-mode THz detection-time-domain transmission modeling-super-resolution (SR) image reconstruction" to characterize epoxy resin coating samples. On one hand, a time-domain transmission model was established to identify internal defects and evaluate the thickness detection limit of THz waves in defective regions. On the other hand, THz imaging was integrated with SR image reconstruction to enable high-precision visual analysis of coating defects. Experimental results show that when the defect thickness exceeds 0.3 mm, the measurement error decreases progressively with increasing thickness; once the thickness surpasses 0.6 mm, the thickness detection error drops below 8.5%. Building on these findings, after integrating SR reconstruction technology, the estimation errors of defect area and volume are significantly reduced when the defect thickness exceeds 0.3 mm. Specifically, for defects thicker than 0.5 mm, the relative error of area estimation using thickness-reconstructed images is less than 6%, with the minimum error reaching 0.99%; the volume estimation error is minimized to 13.77%, representing a substantial improvement over direct thickness-image analysis. SR-reconstructed images provide enhanced pixel-level details, markedly improving the quantification accuracy of defect regions. This study demonstrates that combining SR reconstruction with THz imaging enables comprehensive assessment of coating uniformity and achieves segmentation and localization of internal defects. This innovative approach expands the application of THz imaging in industrial coating inspection and promotes the development of more precise and intelligent NDT technologies for quality control.