Fig. 1. Terahertz frequency modulated continuous wave detection imaging system. (a) Working principle diagram of system; (b) physical map of system; (c) structural diagram of system

Fig. 2. Intensity distribution of focal plane. (a) Before optimization; (b) after optimization

Fig. 3. Photo and terahertz image of sample. (a) Photos of ABS and metal specimen; (b) terahertz C-scan images of ABS and metal specimen

Fig. 4. One-dimensional images before and after correction at different positions. (a) Point S on metal specimen; (b) point A₇ on ABS specimen

Fig. 5. Terahertz image of metal specimen. (a) B-scan image at Y=69; (b) three-dimensional image at Y=69; (c) C-scan images at different Z

Fig. 6. Echo signals of S and S₁--S₉ points on metal specimen

Fig. 7. Echo signals of front and back surfaces at A₁--A₈ points

Fig. 8. B-scan images of ABS specimen at Y=26 under different reference points. (a) S; (b) S₇; (c) S₈; (d) 0.3S+0.3S₇+0.4S₈

Fig. 9. High voltage insulation terminal. (a) Photo of sample; (b) B-scan profile at position with rotation angle of 0°; (c) terahertz echo signals at positions 1, 2, and 3 in Fig. 9(b) when rotation angle is 0°

Fig. 10. Echo signals in range of 0°--360° at different positions. (a)(c) Position 2; (b)(d) position 3

Fig. 11. Reconstructed terahertz three-dimensional images of high voltage insulation terminal from different perspectives. (a) Oblique view; (b) front view