Terahertz Non-Destructive Testing Technology for Coating Thickness Measurement Based on Surface Geometric Compensation

Tablet coating thickness is a crucial parameter affecting drug release rate and bioavailability, and its accurate measurement is of vital significance for pharmaceutical quality control. Aiming at the oblique incidence error of the traditional Terahertz Pulse Imaging (TPI) method when applied to biconvex curved surface tablets, this paper proposes a coating thickness measurement method combined with refraction angle compensation. The method reconstructs the tablet’s surface morphology by automatically locating normal incidence reference points, calculates the local surface normal direction, and introduces an optical path correction factor based on Snell’s Law to compensate for the geometric effects of the curved surface. Using azithromycin enteric-coated tablets as test samples, experimental results show that the traditional vertical incidence model leads to significant overestimation of coating thickness in the edge areas. In contrast, after compensation, the coating thickness distribution becomes more uniform, and the “edge thickening” phenomenon is eliminated. Statistical analysis indicates that the average coating thickness after compensation is 35.96 μm, which is consistent with the microscopic measurement results; the minimum detectable thickness reaches approximately 31 μm, verifying the high sensitivity and precision of the method. The proposed method provides an effective technical approach for the non-destructive and quantitative characterization of tablets with complex curved surfaces, and has important application value for Pharmaceutical Process Analytical Technology (PAT) and online quality monitoring.