## 4 Experimental Setup  
The stray radiation experimental measurement setup of the infrared imaging system based on the high-absorption ceramic coating is shown in Figure 5. The system mainly consisted of a CO\({}_{2}\) laser at 10.6 \(\upmu\)m, laser beam expanding collimator, beam expander, infrared imaging opto-mechanical system and detector, DC power supply, laser power monitoring system, and constant-temperature heat source. In the experiment, the 10.6 \(\upmu\)m laser emitted by the CO\({}_{2}\) laser was output through the beam expanding and collimating optics. The collimated laser first split 1/10 of the laser energy by the beam splitter, so as to monitor the laser power in real time; the remaining laser was reflected into the infrared beam expanding optics by the reflector. Then, the expanded laser entered the infrared imaging system. Because the laser was incident at a certain angle outside the FOV, the laser entering the infrared opto-mechanical system scattered several times in the inner wall of the system and then reached the image plane of the detector. The stray radiation image collected was transmitted to the data processing center. The constant-temperature  
Figure 4: The atomic force microscope (AFM) measurement image of the ceramic coating surface. Figure 3: Measurement results of surface absorptivity for the ceramic coating and the aluminum alloy without coating in the range of visible to infrared light: (**a**) ceramic coating at \(\lambda\) = 0.2–2.5 \(\upmu\)m; (**b**) ceramic coating at \(\lambda\) = 2.0–16 \(\upmu\)m; (**c**) aluminum alloy without coating at \(\lambda\) = 0.2–2.5 \(\upmu\)m; (**d**) aluminum alloy without coating at \(\lambda\) = 2.0–16 \(\upmu\)m.  
heat source was placed in the FOV of the infrared imaging opto-mechanical system, which was used as a reference target of the experiment. In this experimental system, \(\alpha\) is the incident angle when the expanding beam irradiates the infrared imaging system, and \(\theta\) is the angle of FOV for the infrared imaging system. The main parameters of the CO\({}_{2}\) laser, infrared imaging system, and uncooled detector used in the experimental system are shown in Table 1.  
The design drawing and the three-dimensional schematic diagram of the infrared imaging opto-mechanical system are shown in Figure 6. The opto-mechanical system was made of aluminum alloy. In order to verify the absorption efficiency of stray radiation for the high-absorption ceramic coating, the LPPEO technology, combined with the forced convection technology for the coating uniformity control, was used to coat the ceramic coating on the inner wall of the infrared opto-mechanical system. The infrared opto-mechanical systems with and without ceramic coating were processed. The photographs of the system structure are shown in Figure 7. \begin{table}
\begin{tabular}{c c c} \hline \hline
**System Component** & **Parameter** & **Specifics** \\ \hline \multirow{3}{*}{CO\({}_{2}\) laser} & Maximum output power & 1 W \\  & Wavelength & 10.6 μm \\  & Power stability & \(\pm\)5\% \\ \hline \multirow{5}{*}{Infrared imaging Optical system} & Working wavelength & 8–12 μm \\  & Exit pupil diameter & 44 mm \\  & Entrance pupil diameter & 57 mm \\  & Minimum resolvable temperature difference (MRTD) & 450 mK (2.5 cycle/mrad) \\ \hline \multirow{5}{*}{Infrared detector} & Resolution & 640 \(\times\) 512 \\  & Pixel Size & 17 μm \\ \cline{1-1}  & \(f\)-Number & 1 \\ \cline{1-1}  & Field-of-view (FOV) angle & 10\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of stray radiation experimental measurement system. Figure 5: The stray radiation experimental setup of the infrared imaging system. Because of the noncomplete avoided background radiation, 10 background noise images were collected and averaged as the background noise images before the experiment. During the experiment, another 10 images were collected and averaged as stray radiation images with background noise. After the experiment, the collected data were processed.