## 3 Results
The iris opening diameter was optimized at 0.67 THz for the best resolution and image quality, using a QR code pattern engraved on the metal surface of a painted steel plate. As shown in Figure 3b-d, the resolution was improved when the iris opening diameter was decreased. The enhanced resolution is due to the diffraction by the iris aperture which increases the beam divergence angle to be larger than the acceptance angle of L2 (see Figure A1 of Appendix A). Thus, the size of the beam after passing L2 becomes 5 cm due to the clipping of the beam. The collimated beam shape would be similar to the Gaussian shape at high frequencies while becoming more similar to the flat-top hat shape at lower frequencies. Thus, the focused beam profile on the sample surface is expected to deviate from an ideal Gaussian shape and would be close to the Airy disk shape at low frequencies. For simplicity, however, we assumed a Gaussian beam profile for the resolution estimation. The resolution of the system was estimated by a positive-pattern USAF 1951 glass slide resolution reticle (Edmund Optics Inc.) at three different frequencies of 0.25 THz, 0.37 THz, and 0.67 THz, as shown in Figure 4a-c, respectively. The reticle is shown in Figure 4d. At 0.25 THz and 0.37 THz, up to the pattern 2 and 4 in the group -2 were resolved, respectively. These correspond to 0.280 and 0.353 line pairs/mm. In this pattern, the distances between the lines are 1.786 mm and 1.416 mm. At 0.67 THz, up to the pattern 4 of the group -1 was resolved, which corresponds to 0.707 line pairs/mm (0.707 mm line spacing). From those values, we estimate the system resolutions to be 1.786 mm, 1.416 mm, and 0.707 mm for 0.25 THz, 0.37 THz, and 0.67 THz, respectively. The experimental resolutions were compared to the theoretical spot sizes calculated by the Gaussian approximations as shown in Figure 4e. In the calculation, L3 was replaced by an ideal thin lens of 7 cm focal length. The focal diameters were calculated to be 2.13 mm, 1.44 mm, and 0.798 mm, for 0.25 THz, 0.37 THz, and 0.67 THz, respectively, as shown in Figure 4e. The experimental results and the theoretical calculations agreed well, which correspond to 1.48, 1.75, and 1.58 times of the wavelength for 0.25, 0.37, and 0.67 THz, respectively. Another interesting observation to be noted from Figure 4 is that the visibility of the metal mount, behind the glass reticle, strongly depends on the imaging frequency. The m  
Figure 4: Images of a USAF 1951 resolution target at (**a**) 0.25 THz, (**b**) 0.37 THz, and (**c**) 0.67 THz. (**d**) USAF 1951 target pattern mounted on a metal plated with several tapped holes. (**e**) Calculation results for the focusing of 5 cm beam diameter, collimated beam at three measured frequencies. In this simulation, L3 is replaced by an ideal thin lens of a 7 cm focal length. The images were obtained in mode 1. Figure 3: Effect of iris opening diameter on the image (at 0.67 THz). (**a**) QR code engraved on a painted metal plate. (**b**) THz image with fully opened iris. (**c**) THz image with 1.5 mm iris opening diameter. (**d**) THz image with 0.7 mm iris opening diameter. holes, one of them is indicated by the white arrow in Figure 4b,d. The holes are clearly identified at 0.37 THz, but not at other frequencies due to poor resolution (0.25 THz) and strong absorption due to the glass substrate (0.67 THz) [17; 18]. These results show the importance of frequency selection in the THz see-through imaging and the need for THz imaging systems to support wideband frequencies. As a more practical example of see-through imaging, a commercial laser pointer was imaged at 0.67 THz, as shown in Figure 5. Images were also obtained at 0.25 THz and 0.37 THz for comparison (see Figure A2 of Appendix A). The pointer is composed of two parts that can be mechanically combined, and a printed circuit board (PCB) is inside of the upper part and a USB connector and some electronics are inside of the lower part, as shown in Figure 5a.