# Design of a High-Performance Digital Slit-Lamp Microscope with Five-Switched Zoom
## 2 Design of the DSLM Optical System  
### Design Principle  
The DSLM optical system is composed of two parallel optical paths which are identical and correspond to the left and right eyes. The two parallel optical paths have a common front objective, and a schematic illustration is shown in Figure 1. The received light from the object is collected by the front objective and sequentially delivered to the Galilean telescope. Next, the incident light is divided into two beams by a beam splitter. One light is delivered to a tube lens and eyepiece, and the other is sent to a built-in photographic lens and CCD for imaging. Since a Galilean telescope can provide two magnifications, direct and inverted, magnifications of 6\(\times\), 10\(\times\), 25\(\times\), and 40\(\times\) can be achieved with two Galilean telescopes. A 16\(\times\) magnification is provided by removing the Galilean telescopes (empty). Switching of telescopes is done by rotating the drum. In other words, five magnifications can be realized in the design.  
Figure 1: Optical principle of the digital slit-lamp microscope (DSLM).  
Notably, the DSLM is a non-coaxial (decentered) optical system. As shown in Figure 1, the optical axis of the front objective is assumed to be on the dash-dotted line denoted M, and the optical axis of the Galilean telescope, beam splitter, tube lens, and eyepiece are on the other dash-dotted line, denoted N. The optical axes M and N are parallel to each other and spaced with longitudinal distance \(e\). It is well known that the aberrations of a non-coaxial optical system are the sum of the individual surface contributions. However, the forms of the aberrations have changed and become more complex, which increases the difficulty of aberration correction [15; 16; 17; 18]. According to the calculation in [18], additional aberrations will be introduced due to the decentration (non-coaxial), which increase on the original basis with no new types of aberrations. This means that the additional aberrations in a non-coaxial system can be minimized when the aberrations are corrected to their minimum in the coaxial system. In addition, a small amount of decentration has little impact on the aberrations of the optical system on the premise that the primary aberration is small before decentration. In this case, the designed non-coaxial optical system can transition from the coaxial design, and the amount of decentration is increased gradually. An optimization process is employed in the designed non-coaxial system after the coaxial system achieves high performance. The optical system of the DSLM is divided into three subsystems: the front objective, Galilean telescope, and photographic lens. First of all, the subsystems are designed optimally based on the coaxial system. Then, the vertical position of the front objective has a tiny displacement offset from the optical axis (forming a non-coaxial axis). Optimization of the design is performed again in the non-coaxial system. The progressive optimization process should be repeatedly conducted until the performance of the design meets requirements [19; 20; 21].