Fig. 1. Architecture of the detection system and schematic diagram of FMCW measurement principle. (a) Physical diagram of the system; (b) schematic workflow of terahertz FMCW radar; (c) principle of FMCW measurement

Fig. 2. Sample 1 structure and detection scheme

Fig. 3. Sample 2 structure and detection scheme

Fig. 4. Sample 3 structure and detection scheme

Fig. 5. Detection results and typical signal representation of sample 1. (a) Schematic diagram of the scanning scheme; (b) schematic diagram of typical signal range information; (c) imaging result of spectral point No. 1682; (d) imaging result of spectral point No. 1712

Fig. 6. Detection results and typical signal representation of sample 2. (a) Schematic diagram of typical signal range information; (b) imaging result of spectral point No. 1444; (c) imaging result of spectral point No. 1573

Fig. 7. Detection results and typical signal representation of sample 3. (a) Schematic diagram of typical signal range information; (b) imaging result of spectral point No. 1444; (c) imaging result of spectral point No. 1573

Fig. 8. Schematic diagram of information overlay phenomenon

Fig. 9. Schematic diagram of terahertz image structure division

Fig. 10. Schematic diagram of the image restoration algorithm

Fig. 11. Tomographic imaging repair process and three-dimensional reconstruction result of sample 1. (a) Schematic diagram of the repair process; (b) three-dimensional reconstruction result

Fig. 12. Three-dimensional reconstruction result of sample 2

Fig. 13. Tomographic imaging repair process and three-dimensional reconstruction result of sample 3. (a) Schematic diagram of the repair process; (b) three-dimensional reconstruction result

Fig. 14. Results of two scans in occlusion scenarios and three-dimensional reconstruction ofsample 3. (a) Front upper layer detection result; (b) front lower layer detection result; (c) overall front side detection result; (d) back upper layer detection result; (e) back lower layer detection result; (f) overall back side detection result; (g) three-dimensional reconstruction result