## 3 Results and Discussion
Compared to the circular hole structure, the path of light propagation is wider, and the charge distribution is sparser. The enhancement effect of the local field is not as strong as that exhibited by the circle hole structure. Therefore, as shown in Figure 4a, the peak width of the transmission spectrum of the cylindrical structure is smaller than that of the circular hole structure. Similarly, the cross-shaped and hollow cross-shaped structures are the same. In the same vein, the structures of the material and geometric parameters are optimized for the long infrared band. Unsurprisingly, the thickness of the metal thin film does not significantly impact the transmittance, as seen in Figure 5 in the medium infrared band. Meanwhile, increasing the radius does not shift the transmission peak but does result in a noticeable increase in the FWHM. Additionally, the period \(P\) changes in a linear fashion, as predicted in the literature [34]. Figure 3: The transmission spectrum in the 3–5 μm band under different metal materials (Au, Ag, and Al) and geometric parameters (\(h\), \(r\), and \(P\)). (**a–c**) is the different \(h\), \(r\), \(P\) in the case of Au material; (**d–f**) is the different \(h\), \(r\), \(P\) in the case of Ag material; (**g–i**) is the different \(h\), \(r\), \(P\) in the case of Al material. In summary, the medium infrared band filter can be optimized through the careful consideration of material types, geometric parameters, and practical application. The recommended parameters for this filter include using Au as the metal material, a thickness of 0.1 \(\upmu\)m, a circle radius of 1.5 \(\upmu\)m, and a period of 4 \(\upmu\)m. For the long infrared band filter, the recommended parameters also include using Au as the metal material, a thickness of 0.1 \(\upmu\)m, a circle hole radius of 3.5 \(\upmu\)m, and a period of 8 \(\upmu\)m.  
Figure 4: The transmission spectrum and electric field distribution diagram under different structures: (**a**) transmission spectrum of different structures when the metal material is Au—the illustrations are schematic diagrams of different structure diagram; (**b–e**) electric field distribution diagram of different structures (circle, cylinder, cross-shaped, and hollow cross-shaped). Figure 5: The transmission spectrum in the 6–14 \(\upmu\)m band under different metal materials (Au, Ag, and Al) and geometric parameters (\(h\), \(r\), and \(P\)). (**a–c**) is the different \(h\), \(r\), \(P\) in the case of Au material; (**d–f**) is the different \(h\), \(r\), \(P\) in the case of Ag material; (**g–i**) is the different \(h\), \(r\), \(P\) in the case of Al material. To facilitate the identification of device performance based on its geometric parameters, we utilized the structural color theory to invert the transmission spectrum of filters from two distinct working bands in visible light (as depicted in Figure 6a) into the CIE1931 color space. From there, we extracted the corresponding color modules for easy visual discernment. The color effects are displayed in Figure 6b,c. By using inversion measures, the performance changes of the designed filter can be easily identified with the naked eye. This means that under normal lighting conditions, filters with different performance capabilities can be distinguished by their color differences. In Figure 6d, the electromagnetic EMI SE analysis of the two filters in the radar band of 1-18 GHz shows that the results exceed 45 dB. This suggests that the continuous metal film structure can achieve ultra-high efficiency in electromagnetic shielding.