## 3 Results and Discussion
Figure 5a shows the C-scan image of sample B measured with the time-domain THz tomography system. In this image, even delaminations with a width of 1 mm are visible (though dimly), but only Teflon inclusions with a size of at least 4 mm can be seen. Figure 5c shows the amplitude of the THz signal versus the defect size. For both delamination and Teflon inclusion, the signal amplitude reduces as the size decreases. Figure 5e and Video S5e show the 3D image of sample B measured with the time-domain THz tomography system. Fewer small defects are visible in the 3D visualization than in the C-scan image. Figure 5b,d,f shows the C-scan image, the amplitude of the THz signal versus the defect size, and the 3D image of sample B, respectively, measured with the frequency-domain THz tomography system. In the C-scan image (Figure 5b), delaminations with a width of 2 mm or greater and Teflon inclusions with a size of 6 mm or more are visible. The results for the amplitude of the THz signal versus the defect size in Figure 5d are similar to those in Figure 5c. The 3D image in Figure 5f and Video S5f is consistent with the C-scan image in Figure 5b. Small defects are not visible in either the C-scan or 3D images. Figure 3d shows the design of sample C, which has dimensions of 130 \(\times\) 130 \(\times\) 5 mm\({}^{3}\) and contains internal defects at different depths. Sample C has 9 mm-wide rectangular delaminations and 8 mm square Teflon inclusions. All of the defects are 0.2 mm thick. The delaminations and Teflon inclusions lie at depths of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4 mm. Figure 6a shows the C-scan image of sample C measured with the time-domain THz tomography system. This image shows delaminations at a depth of 3 mm or less and Teflon inclusions at a depth of 1.5 mm or less. The measurable depth is limited to 7.8 mm by ECOPS [16]. Defects at a depth of 3.5 mm or more cannot be seen, considering GFRP's refractive index of 2.1. Figure 6c shows the amplitude of the THz signal versus the defect depth. For both delamination and Teflon inclusion, the amplitude of the THz signal decreases with increasing depth due to attenuation of the THz beam through GFRP. Figure 6e and Video S6e show the 3D image of sample C, which is consistent with the C-scan image. Figure 6b,d,f shows the C-scan image, the amplitude of the THz signal versus the depth of the defect, and the 3D image of sample C, respectively, measured with the frequency-domain THz tomography system. In the C-scan image (Figure 6b), delaminations at a depth of 2 mm or less and Teflon inclusions at a depth of 1.5 mm or less are visible. Due to the WSL, the coherence length of CW THz radiation is shorter in the frequency-domain THz tomography system than in the time-domain THz tomography system [19]. The reduction in coherence with increasing depth results in a greater decrease in the THz signal amplitude in the frequency-domain THz tomography system than the time-domain THz tomography system. Thus, the results for the amplitude of the THz signal versus the depth of the defect in Figure 6d show a greater decrease in the amplitude with increasing depth than those in Figure 6c. Figure 6f and Video S6f show the 3D image of sample C, which is consistent with the C-scan image. Figure 5: (**a**) C-scan image, (**c**) THz signal amplitude versus defect size, and (**e**) 3D image of sample B obtained using the time-domain THz tomography system. (**b**) C-scan image, (**d**) THz signal amplitude versus defect size, and (**f**) 3D image of sample B obtained using the frequency-domain THz tomography system.