# Design of a High-Performance Digital Slit-Lamp Microscope with Five-Switched Zoom
## 2 Design of the DSLM Optical System
### Optimization  
As described in the previous section, the initial structures of the three subsystems were determined. Then, optimization must be undertaken to correct aberration. We monitored and optimized the imaging quality of each magnification by using multiconfigurations in Zemax software. In consideration of the fact that the DSLM is mainly used for ophthalmic examination, and the medical light source wavelength range is 560-610 nm [24], we selected the default visible wavelength band in Zemax. In the optimization, the three wavelengths of 486 nm (blue hydrogen F line), 588 nm (yellow sodium D line), and 656 nm (red hydrogen C line) were weighted and optimized during the design process. In Zemax, the RAED operand (real ray angle of exitance) can be adopted to make sure the incident and emergent light of the Galilean telescope are parallel. The EFLY operand (effective focal length in lens \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Basic Specifications**} \\ \hline Magnification of eyepiece & 12.5\(\times\) & Field Number & 20 mm \\ Image sensor type & 1/1.8\({}^{\prime\prime}\) CCD & Design spectrum & Visible light \\ Total visual magnification & & 6\(\times\), 10\(\times\), 16\(\times\), 25\(\times\), 40\(\times\) & \\ \multicolumn{2}{c}{**Front objective**} & \multicolumn{2}{c}{**Photographic lens**} \\ Focal length (mm) & 110 & Focal length (mm) & 50.7 \\ F-number & 2.75 & F-number & 3.28 \\ FOV (\({}^{\circ}\)) & 20.13 & FOV (\({}^{\circ}\)) & 10.2 \\ \multicolumn{2}{c}{**Galilean telescope**} \\ \multicolumn{2}{c}{**Objective**} & \multicolumn{2}{c}{**Eyeepiece**} \\ Focal length & 25\(\times\) & 80 & Focal length (mm) & 25\(\times\) & \(-\)50 \\ (mm) & 40\(\times\) & 50 & 40\(\times\) & \(-\)20 \\  & 25\(\times\) & 4 & 25\(\times\) & 4 \\ F-number & 40\(\times\) & 2.5 & F-number & 40\(\times\) & 2.5 \\ FOV (\({}^{\circ}\)) & 25\(\times\) & 5.08 & FOV (\({}^{\circ}\)) & 25\(\times\) & 8.13 \\ \multicolumn{2}{c}{40\(\times\)} & 3.25 & FOV (\({}^{\circ}\)) & 40\(\times\) & 8.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Design specifications of the DSLM. Figure 2: Initial structure of the optical system. (**a**) Structure of the doublet–singlet lens for the front objective. (**b**) Three-component objective as the initial structure of the photographic lens. (**c,d**) The initial structures of Galilean telescopes with 25\(\times\) and 40\(\times\) magnification, respectively. units) was applied to control the focal length of each subsystem. In addition, center thickness and edge thickness were also constrained to avoid overlap between lenses. The refractive index and Abbe number of glass were constrained to ensure reasonable material. The CDGM (CDGM GLASS CO., Ltd., Chengdu, Sichuan, China) glass catalogs were selected. Because the DSLM is a non-coaxial system, a progressive optimization strategy was used. In the first step of the optimization, a good optimization result was achieved in the original coaxial system. In the second step, the decentration of the front objective was enlarged gradually during the optimization. The maximum value of decentration is 10.4. It should be noted that, along the \(y\)-axis, the object point is no longer symmetrically distributed with regard to the optical axis, M. In order to accurate evaluate the imaging quality of the optical system, the actual light needs to be tracked from different field points. As shown in Figure 1, multiple field points in both the \(\pm Y\) and \(+X\) directions are required to correct the aberrations. The field points were placed at \((0,0)\) mm, \((0,2.25)\) mm, \((0,-2.25)\) mm, \((0,-3.15)\) mm, \((2.25,0)\) mm, and \((0,4.5)\) mm to describe the imaging quality of the entire optical system. Moreover, coma and astigmatism must be corrected firstly; these affect the image clarity directly. The complex operands, including TRAY (transverse aberration Y direction measured in the image plane with respect to the chief ray), SUMM (sum of two operands), and so on, are taken to control coma and astigmatism.