## 3 Calculation and Results  
### Analysis of Stray Light  
During analysis of the stray light, the three-dimensional model of the camera optical system and the field of view were exported from the optical software Zemax in the IGES format. The exported file was imported into UG, and the baffle and vanes were designed and exported in STEP format. Finally, import the file into TracePro optical analysis software to assign material properties of each component and perform stray light analysis of the optical system. At present, the Monte Carlo algorithm is commonly used to analyze the stray light in Tracepro software. Based on mathematical statistics and probability analysis, the Monte Carlo algorithm can track the incident light to analyze the ability to suppress stray light in the optomechanical system. In the process of stray light analysis, the material properties of optical and mechanical structures are defined as reflection, transmission, absorption, and surface scattering. The surface scattering properties of the optomechanical parts need to be set before ray tracing. Figure 6: Schematic diagram of the design of the vanes. The scattering intensity measurement on the surface of the parts is usually defined by the bidirectional scattering distribution function (BSDF), which can be expressed as:  
\[BSDF(\theta_{i},\phi_{i},\theta_{s},\phi_{s})=\frac{dL_{s}(\theta_{s},\phi_{s})}{ dE_{s}(\theta_{i},\phi_{i})} \tag{7}\]  
where \(\phi_{S}\) and \(\theta_{S}\) represent the azimuth and incident angle of reflected light. \(\phi_{i}\) and \(\theta_{i}\) represent the azimuth and incident angle of incident light. In the BSDF model of the surface of the optical mechanism, a relatively stable quasi-power reciprocal ABg model is selected, as shown in Figure 7. The scattering distribution function is as shown in Equation (8):  
\[BSDF(\overrightarrow{x})=\frac{A}{\left(B+\left|\overrightarrow{x}\right| ^{\overrightarrow{s}}\right)} \tag{8}\]  
where \(\left|\overrightarrow{x}\right|=\left|\overrightarrow{\beta}-\overrightarrow{ \beta}_{0}\right|\) and \(\overrightarrow{\beta}\) are unit vectors in the scattering direction. \(\overrightarrow{r}\) is the projection on the surface, \(\overrightarrow{\beta}_{0}\) is the unit vector in the mirror direction, and \(\overrightarrow{r}_{0}\) is the projection on the surface. Because the lengths of \(\overrightarrow{\beta}\) and \(\overrightarrow{\beta}_{0}\) are less than or equal to 1, \(\left|\overrightarrow{x}\right|\leq 2\). \(A\), \(B\), and \(g\) are three undetermined parameters of the \(ABg\) model. The parameters of the \(ABg\) model are defined as \(B\geq 0\), \(g\
eq 0\). If \(g=0\), it will become a Lambertian BSDF with a value of \(A/(B+1)\) and a total scattering of \(A/(B+1)\). If \(g<0\), the BSDF will increase with \(\left|\overrightarrow{\beta}-\overrightarrow{\beta}_{0}\right|\). UG 3D software was used to model the camera as a whole and export it as a STEP file. This file was imported into TracePro software. The ray tracing model is shown in Figure 8.  
The surface properties and material properties of the light source and parts need to be set strictly according to the actual working conditions. The working band of the camera is in the range of 400-760 nm, so the center wavelength of the light source is set to \(\lambda=580\) nm. The environment temperature is 20 \({}^{\circ}\)C, the lens surface transmittance is 96%, the absorption rate is 0.1%, and the scattering \(ABg\) model parameter are set as \(A=0.01\), \(B=0.0001\), and \(g=1\). The window is made of germanium material; the surface transmittance is set to 1. The material of the lens barrel, pressure ring, baffle, and vanes is aluminum, and the material of the machined parts is black anodized material, so the surface absorption rate is set to 97%, with a transmittance of 0, and the scattering \(ABg\) mathematical model parameters are \(A=0.001\), \(e=0.01\), and \(g=3.5\). The focal plane surface property is set to completely absorb. Figure 7: Mathematical model of \(ABg\).