Fig. 1. Photograph and schematic of terahertz time-domain spectroscopy system. (a) Photograph; (b) working principle

Fig. 2. Schematic of tomographic short-time integral imaging

Fig. 3. Photograph of phenolic plastic plate and comparison of defect detection images of two tomographic methods. (a) Photograph of phenolic plastic plate; (b) defect size design; (c) detection result of tomographic imaging method; (d) detection result of tomographic short-time integral imaging method

Fig. 4. Implementation process of wavelet threshold denoising

Fig. 5. Schematic of interval setting of δ-σ evaluation rule

Fig. 6. Photograph and defect size design of phenolic plastic samples. (a) Photograph; (b) defect size design

Fig. 7. Pseudo-color maps of power spectral imaging. (a) Before preprocessing; (b) sym7, three layers, soft-threshold preprocessing; (c) sym7, five layers, hard-threshold preprocessing; (d) sym7, five layers, soft-threshold preprocessing

Fig. 8. Nondestructive detection signals of samples before and after preprocessing. (a) Before preprocessing; (b) sym7, three layers, soft-threshold preprocessing; (c) sym7, five layers, hard-threshold preprocessing (d) sym7, five layers, soft-threshold preprocessing

Table 1. Evaluation results of nondestructive detection images of different prefabricated defects in phenolic plastic samples