## 5 Experimental Results and Discussion  
### Effect of Incident Power on Stray Radiation  
It can be seen from Equation (10) that the stray radiation intensity received by the detector image plane was related to the laser incident power. The actual situation would be more complicated, because the stray radiation intensity reaching the detector image plane is affected by many factors, such as the stray light incident angle and internal geometry of opto-mechanical system. The ceramic coating also had a saturation absorption effect. In order to verify whether the high-absorption ceramic coating proposed in this paper can suppress the stray radiation and the variation of the suppression effect with the laser power and the laser incident angle, the effects of different incident conditions on the intensity of stray radiation in the infrared opto-mechanical system with a ceramic coating on the aluminum alloy substrate were investigated experimentally. In the experiment, the optical axis of the infrared optical imaging system was adjusted to the angle of 50\({}^{\circ}\) with the laser propagation direction. Then, the laser incident power was gradually increased from 0.1 W to 0.8 W. Lastly, the stray radiation distribution image on the image plane of the infrared  
Figure 6: The design drawing and the three-dimensional schematic diagram of infrared imaging opto-mechanical system: (**a**) design drawing 1: first lens pressing ring; 2: second lens spacer; 3: lens barrel; 4: third lens spacer; 5: fourth lens pressing ring; 6: first lens; 7: second lens; 8: third lens; 9: fourth lens; (**b**) three-dimensional schematic diagram. Figure 7: Photographs of the infrared imaging opto-mechanical system: (**a**) without ceramic coating; (**b**) with ceramic coating. imaging detector was collected. The experimental results are shown in Figure 8. In order to compare and analyze the stray radiation images induced by lasers with different incident power, all images were normalized. As can be seen from Figure 8, the stray radiation distribution image mostly presented the shape of circular distribution. This is because the section of the inner wall of the lens was round and, therefore, the stray radiation into the opto-mechanical system was consistent with the geometry of the inner wall of the lens barrel. This experimental phenomenon could prove that the experimental results were indeed stray radiation images formed by multiple scattering from the inner wall in the opto-mechanical system. However, the two-dimensional spatial distribution of stray radiation intensity showed some randomness under different laser incident power, angle, and initial boundary conditions. The reason is that the spatial distribution of the spot energy for the initial incident laser in the experiment was different, and the surface roughness on the inner wall of the opto-mechanical system with and without coating was inconsistent, resulting in the different transmission paths of the stray radiation in the system. Therefore, the two-dimensional spatial distribution of the stray radiation intensity finally reaching the image plane of the detector was also different. Lastly, the stray light distribution on the image plane of the detector was randomly distributed. In order to describe the variation of stray light intensity with laser incident power more clearly, we counted the number of stray radiation points in each measurement result.