# Design of a Cell Phone Lens-Based Miniature Microscope with Configurable Magnification Ratio
## 2 Materials and Methods
### Assembly of Miniature Microscope with Switchable Magnification Ratios  
We also suggest removing the tube lens module from the cell phone to form a truly compact cell phone microscope. In the experiment, we dismantled a second-hand Huawei Honor 7 cell phone and took its main back cameras with a 20M pixel out of the package, and connected them with a flexible cable (Figure 4A). This camera, with a known focal length (EFL = 4.62 mm), is equipped with a Sony IMX230 sensor (pixel size = 1.12 \(\upmu\)m). Then, a compact fixture is designed to accommodate the main camera module (the tube lens) and the selected camera lens module (the objective module), as shown in Figure 4B. The fixture can be installed on a 3D printed threaded ring and the distance between the objective and the sample can be adjusted by rotating the ring to find the best focusing position. The side length of the whole fixture is only about 15 mm. A commercial LED diffuse reflection backlight is used as the transmission light source (Figure 4C), which can be replaced with other available light sources in practical applications. Users can control focusing adjustment and capture microscopic images through cell phone camera software.  
The constructed miniature cell phone microscope has several merits compared with the former reported systems. Firstly, the whole system is highly portable, especially the microscope module. Secondly, the operation of the miniature microscope is really simple, even in the field, which is due to the compact size. There is no need for complicated focal length adjustment rigs. Thirdly, the FOV can be easily switched by shifting three objectives whose diameters were made equal by 3D printed adapters. Switchable magnification enables the proposed microscope to observe the sample, select region of interest (ROI) to enlarge the magnification and review with a higher resolution, similar to the operation of bench-top microscopes. Finally, the image quality of the microscope matches that of the bench-top systems, since all the lenses utilize the high industrial quality of the cell phones.  
We noticed another advantage of the cell phone microscope. Modern cell phones are all equipped with digital zooming functions (Figure 5) and advanced build-in image processing functions. The largest digital zooming ratio of Huawei Honor 7 is \(\sim\)4\(\times\), thus giving users more flexibility in continuous magnification adjustment. According to our tests, compared with the zero zoom condition, the digital zoom images provide higher  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Camera Lens Source**} & \multirow{2}{*}{**Magnification Ratio**} & **Focal Length** & \multirow{2}{*}{**Camera Lens Source**} & \multirow{2}{*}{**Magnification Ratio**} & **Focal Length** \\  & & **(mm)** & & & **(mm)** \\ \hline Mate20 & \multirow{2}{*}{2.4} & \multirow{2}{*}{2.5} & P20 pro & \multirow{2}{*}{1.5} & \multirow{2}{*}{4.0} \\ (Wide-angle) & & & & & \\ \hline Mate20 & \multirow{2}{*}{1.2} & \multirow{2}{*}{5.0} & P20 pro & \multirow{2}{*}{0.75} & \multirow{2}{*}{8.0} \\ (Standard) & & & & & \\ \hline G5 & \multirow{2}{*}{3.9} & \multirow{2}{*}{1.5} & P20 pro & \multirow{2}{*}{1.0} & \multirow{2}{*}{6.0} \\ (Super wide-angle) & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Measurement result of magnification ratio and calculated focal length.  
Figure 4: Fabricated miniature cell phone microscope. (**A**) After modification, the cell phone camera can be led out of the cell phone by a flexible cable. (**B,C**) Image of the miniature microscope configuration and working scene.  
image resolution. Therefore, the digital zooming of cell phones has brought real benefits to microscope performance.