# Terahertz Spiral Spatial Filtering Imaging
## 4 Experimental Results
Figure 5: Experimental results for the letters objects. (**a**,**d**,**g**) The optical images of “T”, “H” and “Z”. (**b**,**e**,**h**) The THz bright-field (BF) imaging results. (**c**,**f**,**i**) The THz SSF imaging results. Further, we studied the capability of enhancing the imaging contrast of the tested object with low intensity and phase jumps of the THz SSF imaging system. As shown in Figure 6a, there is a hardly visible scratch on the HDPE piece, which was used as the testing object with low imaging contrast. The width of the scratch is about 0.6 mm, which is close to the size of the receiving aperture of the Golay cell. Therefore, the edges of the scratch were not extracted in the THz BF image (see Figure 6b). Notably, in comparison, it the much darker background in the THz SSF image is highlighted, as shown in Figure 6c. For a quantitative comparison, the intensity curves of the blue and red dashed lines at \(x=0\) mm and \(x=-2.5\) mm are plotted in Figure 6d,e, respectively. For the THz BF images, the edge contrasts are calculated as only 45.59 (edge 1) and 45.52% (edge 2) for Figure 6d, and 26.02 (edge 3) and 31.16% (edge 4) for Figure 6e, respectively. The edge contrasts can be enhanced significantly after SSF imaging; they increase to 94.83 (edge 1) and 85.00% (edge2) for Figure 6d, and 85.20 (edge 3) and 62.15% (edge 4) for Figure 6e, respectively. Since the incident THz beam is a quasi-elliptic Gaussian beam, the beam is not zero after passing through the THz SSF imaging system, resulting in the background in the THz SSF image. Thus, the edge contrast is greatly enhanced after THz SSF imaging, but there are still weak background effect in the THz SSF images, leading to the edge contrast not being close to 100%, especially for edge 4 in Figure 6c. Figure 6: Experimental results for the tested object with low imaging contrast. (**a**) The hardly visible scratch on the HDPE piece. (**b**) The THz BF imaging result. (**c**) The THz SSF imaging result. (**d,e**) The intensity curves corresponding to the blue and red dashed lines in (**b,c**), respectively. Considering that imaging contrast enhancement for natural targets is more meaningful and challenging, we imaged a leaf using the proposed THz SSF imaging system (see Figure 7a). As different parts of the leaf have different the THz transmittances, a complex THz BF image was formed under the irradiation of the THz wave. From the THz BF image of the leaf shown in Figure 7b, the main veins and major lateral veins of the leaf are clearly visible. Meanwhile, some small lateral veins are blurred and some are even submerged by noise. The main vein and lateral veins are black in the THz BF image, indicating that these regions are nearly not transparent to the THz wave. For the regions partly transparent to the THz wave, the maximum transmittance is less than 35% due to the low SNR in the image. After THz SSF imaging, the resultant image is displayed in Figure 7c. The background of the THz BF image is clearly suppressed. The edges of main vein are acquired and most of the lateral veins are highlighted in the image. The edge contrasts of the specific the lateral veins are enhanced for the red marked areas 1, 2, and 3 in Figure 7c, which are blurred in Figure 7b. Figure 7: Experimental result for a leaf. (**a**) The optical image of the leaf. (**b**) The THz BF imaging result. (**c**) The THz SSF imaging result.