# Design of a High-Performance Digital Slit-Lamp Microscope with Five-Switched Zoom
## 1 Introduction  
As one of the most advanced analysis instruments in biomedical science, the microscope has been rapidly developed in recent years [1; 2; 3]. Among all microscopes, the slit-lamp microscope (SLM), invented by [PERSON], is most useful in clinical ophthalmology [4; 5]. It can be used to observe the structures of the eye anterior end and diagnose various eye diseases, such as cataract, corneal injury, and retinal detachment [6; 7; 8; 9]. The traditional SLM relies only on visual observation and needs manual mapping to record the signs of disease, which is inconvenient and unreliable. Nowadays, with the advent of cameras and the progress of digital technology, the digital slit-lamp microscope (DSLM) is favored by researchers in the field of ophthalmology due to the unique advantages of high resolution, quick acquisition, and storage of image information [10].  
DSLMs contribute to transmitting images for remote diagnosis. In order to quickly and accurately determine the signs of disease, capturing high-quality and high-resolution images is essential in telemedicine consultation. The main design of a conventional SLM does not include an image acquisition module. The digital image is obtained from the SLM using a camera or smartphone by means of an adapter in the eyepiece [11; 12; 13; 14]. [PERSON] et al. reported a method of image acquisition froma smartphone with an SLM via a simple adapter in the eyepiece [13]. A common disadvantage of these adapters is that they need to be repeatedly mounted and dismounted, and this is inconvenient during serial observations. These images obtained by an external device lose their diagnostic quality due to the lack of correction of optical aberrations and noise introduced by the adapter system. Hence, a built-in photographic lens and CCD design can improve the image quality of photography without camera adapters. The designed DSLM can connect directly to a computer via universal serial bus (USB) and quickly obtain diagnostic-quality images from the SLM, thereby reducing the time and effort required for preparing devices, particularly when making multiple consecutive observations or when the targets disappear quickly.  
In this study, we present and analyze design ideas for a high-performance DSLM. Based on the parallel optical path principle in stereoscopic microscopes, a five-switched DSLM with built-in photographic lens and CCD was designed using Zemax optical design software (Zemax, LLC, Kirkland, WA, USA). The microscope constituted a front objective, Galilean telescope system, and photographic lens; each of these was systematically investigated and discussed in the design. It is worth noting that the front objective part is non-coaxial with the other parts. In order to reduce the influence of decentration (non-coaxial), a progressive optimization process of gradual decentration correction was adopted when the coaxial system achieved high performance. The designed DSLM can realize five magnifications of 6\(\times\), 10\(\times\), 16\(\times\), 25\(\times\), and 40\(\times\) and obtain high-performance images directly without an adapter. Loose manufacturing and assembly tolerances are also provided for the design, and the system with loose tolerances can be manufactured with higher yield and lower cost. Our design with spherical surfaces has significant advantages, including low cost, easy assembly, and high-quality imaging, all of which make it attractive to clinical medicine in optometry.