# Design of a Cell Phone Lens-Based Miniature Microscope with Configurable Magnification Ratio
## 1 Introduction  
Human beings have been supported by the microscope, which supported them in observing and recognizing things on a micro-scale, particularly in the aspect of biology and medicine, since it was invented [1]. To date, apart from making efforts to advance the performance of microscopes, researchers have also been focused on the miniaturization and cost reduction of microscopes in order to replace conventional bench-top instruments and meet the need for practical applications under the condition of resource-limited circumstances such as tele-medicine, teaching, and environmental monitoring [2]. Deriving from the impending demand for portability, small-scale size and cost compression, a great deal of study has been conducted on cell phones on the basis of the microscopic imaging system. Due to the popularization of cell phones and cellular networks worldwide, the operation of these kinds of cell phone microscopes is so uncomplicated that average users are able to carry out testing and upload the microscope images to the internet without the requirement of a complicated training process, which displays the remarkable advantage in terms of efficiency and accessibility in resource-limited situations. Some practicable approaches and techniques have been proposed, such as optofluidic microscopes [3], lens-free holographic microscopy [4] and cell phone microscopes using ordinary objectives [5, 6], liquid lens, gel lens or simply a ball lens as the objective [7, 8, 9]. Among them, recently the cell phone microscope approach, by using a reversed cell phone camera lens as an object lens, demonstrates tremendous potential of the application with respect to imaging performances [10, 11, 12]. Such reversed camera lens-based miniature microscopes can conduct microscopic imaging with a field of view (FOV) up to mm level, with the resolution at the micrometer level having little distortion [13]. Moreover, the structure makes it easier to realize some super-resolution methods to further enhance imaging performance, such as fluorescence imaging [14, 15] and computational imaging [12, 16]. However, the practical applications of cell phone based miniature microscopes are still quite limited despite diversified proposed research. Unfortunately, miniature microscopes still suffer from a number of limitations [17]. The imaging performance of miniature microscopes is usually still inferior to ordinary bench-top systems. Reversed camera lens-based miniature microscopes can provide high-resolution and low-distortion microscope images [18], however, such miniature microscopes are of a fixed magnification ratio as reported, which is inconvenient compared with ordinary microscopes with switchable object lenses. Another problem is that it is not easy to handle cell phone miniature microscopes. Microscopic imaging requires precise control of the working distance between the microscope and sample by means of mechanism or shell [19], but it is usually inconvenient to fix and precisely adjust the large cell phone body. If complex adjusting structures are demanded, the miniature microscopes will lose advantages of portability and convenient deployment. Furthermore, in some applications, more than one miniature microscope is required to implement synchronous scanning and mosaic imaging when it comes to measuring the surface of large samples from the actual industry. It is difficult to accomplish with the reported designs to achieve the goal, as multiple cell phone cameras cannot be placed side by side closely and physically. Given this, we propose a cell phone lens-based miniature microscope with a configurable magnification ratio, which still adopts the reversed cell phone camera lens concept, but puts forward two major methods of the melioration. Firstly, we propose to select different focal length object lenses from off-the-shelf cell phone camera lenses, thus achieving the goal of configurable values of FOV and resolution.