Continuous Terahertz Wave Imaging for Debonding Detection and Visualization Analysis in Layered Structures

Continuous terahertz detection imaging technology is widely used in polymer detection. However, it is very difficult to quantitatively analyze defects due to the characteristics of focusing lenses. On the one hand, a lens with a short focal length means the achievement of a better spatial resolution and a poor depth resolution. On the other hand, a lens with a long focal length results in a better depth resolution and a worse spatial resolution. In addition, in order to ensure the positioning accuracy, the upper surface of a sample is often used as the focal plane for detection, which causes the energy outside the range of the focal depth to drastically decrease and seriously affect the imaging quality. In this paper, a method is proposed for debonding detection and quantitative analysis in layered structures. Firstly, by using multi-directional structural elements for measuring the difference between the debonding region and local surroundings, we extract defective edges and suppress clutters by establishing a multi-directional morphological filter. Then, the improved Otsu method is used for threshold segmentation and the population mean entropy is used to calculate the optimal threshold. Finally, the superiority of the proposed method is confirmed on synthetic datasets with low SNR images, and it is successfully applied to terahertz image processing. Moreover, the results of the proposed algorithm show that the debonding region detection error of the terahertz image is controlled within 10.17%-11.27%. It provides a new method and idea for quantitative analysis of terahertz detection in layered structures.