# Line Beam Scanning-Based Ultra-Fast THz Imaging Platform
## 3 Experiments  
After designing and constructing the line beam scanning-based ultra-fast THz imaging platform, its performance was tested. The scanning mirror was adjusted to keep the optical axis parallel with the ground coordinate, and a Brunei tube working as a THz single point detector was scanned along the vertical direction to determine the optical axis position. The Brunei tube was scanned along the optical axis, since the intensity at the working plane should have the highest value, indicating that the distance between the working plane and the scanning mirror was 3.02 m. After the determination of the working plane, the Brunei tube was scanned in the working plane to measure the intensity distribution. The intensity distributions along the x and y axes (marked in Figure 1D) are shown in Figure 3A,B, respectively. There is a little difference between the numerically computed and practically measured results, it is often because the numerically calculated intensity distributions were obtained in the ideal condition which did not consider the aberration in the THz system caused by errors in the system fabrication and integration; however, these errors inevitably occurred in the designed THz imaging system, thus, broadening the THz intensity distribution; moreover, the noise of the THz detector also introduced background errors in THz intensity measurements, which also broadened the THz intensity distribution along the y-axis. Though intensity distribution broadened in the measured results, both the numerically computed and practically measured results are still close, via Gaussian fitting, it indicates that the FWHM of the measured line beam was 12.04 mm, close to the numerically evaluated FWHM as ~12 mm, proving the proposed THz imaging platform can reach the resolution of ~12 mm. Besides, both the numerically computed and practically measured intensity distributions along the y-axis with 200 mm, 300 mm, and 400 mm away from the central axis were also listed in Figure 3C-E,  
Figure 2: The mechanical system of the line beam scanning-based ultra-fast THz imaging platform. (**A**) Framework of the mechanical system; (**B**) Scanning mirror and the honeycomb hole array at its back side; (**C**) The elliptical cylindrical reflector. respectively. The coincidence between the numerically calculated and the practically measured results in Figure 3A-E proved the high-quality construction of the THz imaging platform. Since the line beam scanning-based THz imaging platform requires high-speed scanning for two-dimensional imaging, both the detecting region and the scanning speed were then measured. Figure 3F shows the scanning trajectory, indicating that the scanning speed could reach 170 ms/frame, the scanning range of the mirror was within \(\pm\)10.5\({}^{\circ}\), and the scanning range at the working plane could reach 2 m \(\times\) 0.8 m, all satisfying the scanning requirements. After the certification on the line beam scanning-based THz imaging platform, it was finally adopted in the safe screening of the human body THz imaging as shown in Figure 4. Both the images captured by the direct optical device and the proposed THz imaging platform provide information from different perspectives. With the reconstructed THz image, the metal fabricated objects as the metallic gun can be easily distinguished according to the high intensity of the collected THz signals  
Figure 3: Certification on the line beam scanning-based THz imaging platform. (**A**,**B**) Practically measured and numerically computed sectional intensity distributions along the x and y; (**C–E**) Practically measured and numerically computed sectional intensity distributions along the y-axis with 100 mm, 200 mm, and 300 mm away from the central axis. (**F**) The scanning trajectory of the line beam scanning-based THz imaging platform.