## 3 Analysis and Discussion  
### Imaging System Analysis  
The image quality analysis evaluation index of Zemax is mainly Spot Diagram, Field Curvature Distortion, and MTF Curves [24]. Figure 8: A ray optics model to predict the light path. Figure 10: Lighting system design results. Figure 9: Corneal margin incident illumination. (**a**) The incident light of the cornea in the critical state is (**b**) higher than the incident light in the critical state and (**c**) lower than the incident light in the critical state. The object-image relationship of the optical system was as follows:  
\[i\big{(}u^{\prime},v^{\prime}\big{)}=\iint\limits_{-\infty}^{\infty}o(u,v)h \big{(}u^{\prime}-M_{u}u,v^{\prime}-M_{v}v\big{)}dudv \tag{9}\]  
In Equations (2)-(4), \(o(u,v)\) is the pattern of the object, \(i(u^{\prime},v^{\prime})\) is the image square pattern, \((u,v)\) and \((u^{\prime},v^{\prime})\) are the Cartesian coordinates of the corresponding object plane and image plane, \(M_{u}\) and \(M_{v}\) are the lateral magnifications of the object, and \(h(u^{\prime}-M_{u}u,v^{\prime}-M_{v}v)\) is the response of the system to the unit pulse \((u,v)\) at \(\delta(u,v)\). Generally, only the similarity of the distribution of objects and images is considered, and the scale of objects and images is not considered, so it was assumed that \(M_{u}=M_{v}=1\). Therefore, Equation (9) could be simplified as follows:  
\[i(u,v)=o(u,v)\times h(u,v) \tag{10}\]  
The integral value of the point image distribution function \(h(u,v)\) was normalized to obtain the point spread function of the fundus imaging system:  
\[PSF(u,v)=h(u,v)/\iint\limits_{-\infty}^{\infty}h(u,v)dudv \tag{11}\]  
\(PSF(u,v)\) denotes the radiant power per unit area [12], for which Fourier transform was the optical transfer function of the fundus imaging system:  
\[OTF(r,s)=\iint\limits_{-\infty}^{\infty}PSF(u,v)e^{[-i2\pi(ru+sv)]}dudv \tag{12}\]  
\(OTF(r,s)\) is a complex function. Therefore, it can be written as follows:  
\[OTF(r,s)=MTF(r,s)e^{[-iPTF(r,s)]} \tag{13}\]  
Among them, the modulo MTF(r,s) of the optical transfer function was the amplitude transfer function, which represented the modulation attenuation of the transfer harmonic component of the fundus imaging system, which could fully reflect all the effects of aberration except distortion, and was an important evaluation index. Figure 11 shows the MTF diagram of the imaging system; the vertical axis is the modulus value of the optical transfer function, which represents the contrast ratio, and the maximum value is 1 [24]. The horizontal axis represents the spatial frequency of the system in lp/mm. The larger the area enclosed by the MTF curve and the coordinate axis, the better the quality of the optical system and the clearer the image. The different colors in Figure 11 represent different field-of-view curves, and the dashed and solid lines represent the sagittal and meridian directions. Figure 11: MTF curve of the imaging system. According to pharmaceutical industry standard YY0634-2008 \"Ophthalmic Instruments Fundus Camera\" [25], in order to obtain better imaging quality, the general cut-off frequency MTF of the system should be greater than 0.2 [26]. As shown in the figure, the MTF modulus values of the whole field of view at 100 lp/mm were all above 0.2, which shows that the imaging system in this study could resolve the structural unit with a diameter of 5 um on the fundus. Figure 12 is the Spot Diagram and Field Curvature Distortion of the imaging system; the Spot Diagram gives the root mean square (RMS) radius value and geometric (GEO) radius of each field of view. The smaller the value, the better the imaging quality. It is generally believed that less than 6 um can be used to image fundus lesions clearly. It can be seen that the RMS radius of the diffuse spot in this imaging system was less than 3 um and the maximum RMS radius was 2.148 um, which meant that fundus lesions could be clearly imaged.