## 4 Conclusions  
In conclusion, herein, we presented and compared the performance of our time- and frequency-domain THz tomography systems to detect internal defects in GFR composites. The imaging performance of the THz tomography systems for a variety of thicknesses, sizes, and depths of defects is summarized in Table 1. In the time-domain THz tomography system, the minimum detectable thickness and size and the maximum detectable depth for delaminations are 0.025, 1, and 3 mm, respectively. The minimum detectable thickness and size and the maximum detectable depth for Teflon inclusions are 0.025, 4, and 1.5 mm, respectively. In the frequency-domain THz tomography system, the minimum detectable thickness and size and the maximum detectable depth for delaminations are 0.025, 2, and 2 mm, respectively. The minimum detectable thickness and size and the maximum detectable depth for Teflon inclusions are 0.1, 6, and 1.5 mm, respectively. Figure 6: (**a**) C-scan image, (**c**) THz signal amplitude versus defect depth, and (**e**) 3D image of sample C obtained using the time-domain THz tomography system. (**b**) C-scan image, (**d**) THz signal amplitude versus defect depth, and (**f**) 3D image of sample C obtained using the frequency-domain THz tomography system. In the high-speed THz tomography systems, high-speed A scan and beam steering are core technologies, as they enable users to rapidly acquire image data without moving an imaging target. This time-domain THz tomography technology is suitable for high-speed measurements, but is not well-suited for equipment miniaturization, environmental non-dependence, and cost effectiveness due to the use of femtosecond pulsed lasers and phase-locked loops. Currently, the measurement speed of the frequency-domain THz tomography system is inferior to that of the time-domain THz tomography system due to the low signal-to-noise ratio of the THz CW transmitter-receiver pair. However, the frequency-domain THz tomography technology is advantageous to equipment miniaturization, environmental non-dependence, and cost effectiveness because of the CW laser features. Therefore, the frequency-domain THz tomography technology will be more promising as a nondestructive inspection technology for composite materials that is usable in the field, along with further advancements in THz CW transmitters and receivers. THz tomography is a promising imaging technology for rapid non-contact nondestructive inspection of composite materials, and further research and development is required for practical applications. The following are available online at [[https://www.mdpi.com/article/10.3390/app1114933/s1](https://www.mdpi.com/article/10.3390/app1114933/s1)]([https://www.mdpi.com/article/10.3390/app1114933/s1](https://www.mdpi.com/article/10.3390/app1114933/s1)), Figure S4a: A-scan data obtained in places with a defect and without any defect using the time-domain THz tomography system, Video S4e: 3D image of sample A-1 obtained using the time-domain THz tomography system, Video S4e: 3D image of sample A-2 obtained using the time-domain THz tomography system, Video S4f: 3D image of sample A-1 obtained using the frequency-domain THz tomography system, Video S4f: 3D image of sample A-2 obtained using the frequency-domain THz tomography system, Video S5e: 3D image of sample B obtained using the time-domain THz tomography system, Video S5f: 3D image of sample B obtained using the frequency-domain THz tomography system, Video S6e: 3D image of sample C obtained using the time-domain THz tomography system, Video S6f: 3D image of sample C obtained using the frequency-domain THz tomography system. Conceptualization, J.-S.Y. and D.-S.Y. ; data curation, J.-S.Y. ; formal analysis, J.-S.Y. and D.-S.Y. ; funding acquisition, D.-S.Y. ; investigation, J.-S.Y. and D.-S.Y. ; methodology, J.-S.Y. ; project administration, D.-S.Y. ; resources, J.-S.Y. and D.-S.Y. ; software, J.-S.Y. and D.-S.Y. ; supervision, D.-S.Y. ; validation, J.-S.Y. and D.-S.Y. ; visualization, J.-S.Y. ; writing--original draft, J.-S.Y.