Design of a Cell Phone Lens-Based Miniature Microscope with Configurable Magnification Ratio

applied sciences

Article Design of a Cell Phone Lens-Based Miniature Microscope with Conﬁgurable Magniﬁcation Ratio

Xinjun Wan and Xuechen Tao *

School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 310000, China; [email protected] * Correspondence: [email protected]

Abstract: Application of cell-phone-based microscopes has been hindered by limitations such as inferior image quality, fixed magnification and inconvenient operation. In this paper, we propose a reverse cell phone lens-based miniature microscope with a configurable magnification ratio. By switching the objectives of three camera lens and applying the digital zooming function of the cell phone, a cell phone microscope is built with the continuously configurable magnification ratio between 0.8×–11.5×. At the same time, the miniature microscope can capture high-quality microscopic images with a maximum resolution of up to 575 lp/mm and a maximum field of view (FOV) of up to 7213 × 5443 µm. Furthermore, by moving the tube lens module of the microscope out of the cell phone body, the built miniature microscope is as compact as a <20 mm side length cube, improving operational experience profoundly. The proposed scheme marks a big step forward in terms of the imaging performance and user operational convenience for cell phone microscopes.

Keywords: microscope; variable magniﬁcation; miniaturization

Citation: Wan, X.; Tao, X. Design of a Cell Phone Lens-Based Miniature Microscope with Configurable 1. Introduction Magnification Ratio. Appl. Sci. 2021, Human beings have been supported by the microscope, which supported them in 11, 3392. https://doi.org/10.3390/ observing and recognizing things on a micro-scale, particularly in the aspect of biology and app11083392 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 Academic Editor: Manuel Armada 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 Received: 12 March 2021 such as tele-medicine, teaching, and environmental monitoring [2]. Accepted: 7 April 2021 Deriving from the impending demand for portability, small-scale size and cost com- Published: 9 April 2021 pression, 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 Publisher’s Note: MDPI stays neutral worldwide, the operation of these kinds of cell phone microscopes is so uncomplicated with regard to jurisdictional claims in that average users are able to carry out testing and upload the microscope images to the published maps and institutional affil- iations. 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 [7–9]. Copyright: © 2021 by the authors. Among them, recently the cell phone microscope approach, by using a reversed cell phone Licensee MDPI, Basel, Switzerland. camera lens as an object lens, demonstrates tremendous potential of the application with This article is an open access article respect to imaging performances [10–12]. Such reversed camera lens-based miniature distributed under the terms and conditions of the Creative Commons microscopes can conduct microscopic imaging with a field of view (FOV) up to mm level, Attribution (CC BY) license (https:// with the resolution at the micrometer level having little distortion [13]. Moreover, the struc- creativecommons.org/licenses/by/ ture makes it easier to realize some super-resolution methods to further enhance imaging 4.0/). performance, such as fluorescence imaging [14,15] and computational imaging [12,16].

Appl. Sci. 2021, 11, 3392. https://doi.org/10.3390/app11083392 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3392 2 of 11

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. There are numerous high-quality cell phone camera lenses commercially available nowadays, offering us a wide selection pool. Secondly, we suggest moving cell phone camera lenses outside of the cell phone body so as to realize an actual compact size microscope module, which is still connected to the cell phone. Therefore, the proposed cell phone microphone in this paper is expected to carry forward the advantages of low costs and direct access to the internet through the cell phone, simultaneously offering users capabilities of adjustable magnification ratios, really easy deployment and even array application in-field, which is of great importance for real application of cell phone microscopes.

2. Materials and Methods 2.1. Principle of Miniature Cell Phone Microscope The function of the cell phone camera lens is to image a meter-scale scene onto a CMOS image sensor with a size of several millimeters (Figure1A). At present, a cell phone is normally equipped with multiple cameras with diverse values of FOV, and the focal length of camera lenses varies within the range of 1–8 mm. Such camera lenses are usually composed of 5–9 highly complex aspheric lenses. Therefore it is capable of high-quality imaging with minute aberration and field curvature and with a resolution power of up to 400–600 lp/mm. Benefiting from the mass production of the cell phone industry, the cost of these precise camera lenses is fairly low compared with similar quality industrial camera lenses, making them attractive in the microscope domain. The principle of the proposed miniature microscope is as follows (Figure1B). The microscopic imaging system is usually a combination of the object lens, tube lens and camera sensor. Here, the cell phone camera serves as an accurate miniature component of the tube lens and CMOS sensor. The object lens of a microscope is realized with another cell phone camera lens, the direction of which is reversed. Then, the sample to be imaged is placed on the original CMOS sensor position, and the low field curvature feature of the cell phone camera lens makes it possible to image with a large FOV and high resolution. By combining the two cell phone lenses in this way, a microscope can be constructed with the following magnification ratio,

M = ftube/ fobject (1)

where ftube and fobject are focal length of tube lens and objective lens, respectively. Appl. Sci. 2021, 11, 3392 3 of 12

M  ftube / fobject (1)

where ftube and fobject are focal length of tube lens and objective lens, respectively. On this basis, we propose that the object lens can be a switchable camera lens module with a variable focal length so as to adjust the magnification of the microscope, which is a very desirable feature of the microscope. We can find a large pool of commercial cell phone camera lenses with different focal lengths, making this approach quite feasible. In addition, we suggest moving the miniature camera module (including tube lens and CMOS sensor) out of the cell phone, as shown in Figure 1B. The size of a miniature camera module is typically 10 mm × 10 mm × 8 mm, while the cell phone size is typically about 120 mm × 80 mm × 10 mm by comparison. Therefore, compared with the previously re‐ ported cell phone microscope, the final size of the microscope module can be greatly re‐ duced. The imaging operation can be greatly simplified, because the users only need to Appl. Sci. 2021, 11, 3392 deal with a cube with a side length of 10 mm. 3 of 11 Other advantages of the cell phone microscopes still exist in the proposed structure, including low costs and direct access to the Internet.

FigureFigure 1. Principle of of a a miniature miniature cell cell phone phone microscope microscope.. (A) ( AUnder) Under normal normal circumstances, circumstances, the cell the cell phonephone lens can can image image a alarge large field field of ofview view on ona highly a highly flat CMOS flat CMOS image image sensor. sensor. This means This meansthat that accordingaccording to the the principle principle of of reversibility reversibility of oflight light paths, paths, high high-quality‐quality microscopic microscopic imaging imaging can be can be carried out when the cell phone lens is reversed. (B) Switchable reversed cell phone lenses are carried out when the cell phone lens is reversed. (B) Switchable reversed cell phone lenses are used used as switchable objectives. They couple light from the sample through the tube lens and form as switchable objectives. They couple light from the sample through the tube lens and form the image the image of the sample on the CMOS image sensor. At last, the cell phone will process and dis‐ ofplay the microscopic sample on the images. CMOS The image microscope sensor. module At last, is the outside cell phone of the will cell process phone. and display microscopic images. The microscope module is outside of the cell phone. 2.2. Down‐Selection of Different Effective Focal Length (EFL) Objectives On this basis, we propose that the object lens can be a switchable camera lens module According to the concept of the switchable object lens, we purchased a series of off‐ with a variable focal length so as to adjust the magnification of the microscope, which is the‐shelf cell phone camera modules and disassembled them to obtain a series of lenses a very desirable feature of the microscope. We can find a large pool of commercial cell with different focal lengths (Figure 2A). As accurate camera lens information is not easy phoneto obtain, camera we firstly lenses measured with different the focal focal lengths lengths, of lenses, making as shown this in approach Figure 2B. quite The fixed feasible. Intube addition, lens module we suggest is composed moving of a the camera miniature module camera (Figure module 2C) with (including a known focal tube length lens and CMOS sensor) out of the cell phone, as shown in Figure1B. The size of a miniature camera module is typically 10 mm × 10 mm × 8 mm, while the cell phone size is typically about 120 mm × 80 mm × 10 mm by comparison. Therefore, compared with the previously reported cell phone microscope, the final size of the microscope module can be greatly reduced. The imaging operation can be greatly simplified, because the users only need to deal with a cube with a side length of 10 mm. Other advantages of the cell phone microscopes still exist in the proposed structure, including low costs and direct access to the Internet.

2.2. Down-Selection of Different Effective Focal Length (EFL) Objectives According to the concept of the switchable object lens, we purchased a series of off- the-shelf cell phone camera modules and disassembled them to obtain a series of lenses with different focal lengths (Figure2A). As accurate camera lens information is not easy to obtain, we ﬁrstly measured the focal lengths of lenses, as shown in Figure2B. The ﬁxed tube lens module is composed of a camera module (Figure2C) with a known focal length (EFL = 6 mm) and a Sony IMX258 sensor (pixel size = 1.12 µm). The object lenses are connected with the module to image a 100 µm wide line target (Edmund Optics). In this process, we designed an experimental system for precise adjustment of the working distance, building it with a 3D printer (Figure2D). As for illumination, we utilized a simple Appl. Sci. 2021, 11, 3392 4 of 12

Appl. Sci. 2021, 11, 3392 4 of 12

Appl. Sci. 2021, 11, 3392 (EFL = 6 mm) and a Sony IMX258 sensor (pixel size = 1.12 um). The object lenses are con‐ 4 of 11 (EFLnected = 6 withmm) the and module a Sony toIMX258 image sensora 100 um (pixel wide size line = 1.12target um). (Edmund The object Optics). lenses In are this con pro‐ ‐ nectedcess, we with designed the module an experimental to image a 100 system um widefor precise line target adjustment (Edmund of the Optics). working In this distance, pro‐ building it with a 3D printer (Figure 2D). As for illumination, we utilized a simple LED cess,LED we diffuse designed backlight an experimental in the transmission system for precise approach. adjustment The results of the working of the imaging distance, test are buildingdiffuse backlightit with a 3Din theprinter transmission (Figure 2D). approach. As for illumination,The results of we the utilized imaging a simple test are LED pre ‐ presented in Figure3. diffusesented backlight in Figure in3. the transmission approach. The results of the imaging test are pre‐ sented in Figure 3.

FigureFigure 2. MeasurementMeasurement of of selected selected off‐ off-shelfshelf cell phone cell phone camera camera lens. (A lens.) The ( Asix) Thecell phone six cell lenses phone lenses with distinct focal lengths are all obtained from disassembled cell phones of Huawei P20pro, Figurewith distinct 2. Measurement focal lengths of selected are all off obtained‐shelf cell from phone disassembled camera lens. cell (A) phones The six ofcell Huawei phone lenses P20pro, Mate20, Mate20, and LG G5. (B) The focal length measurement device consists of a diffused light source, withand distinct LG G5. focal (B) The lengths focal are length all obtained measurement from disassembled device consists cell phones of a diffused of Huawei light P20pro, source, calibration calibration plate, the lens to be measured and a camera module with known parameters. (C) The Mate20, and LG G5. (B) The focal length measurement device consists of a diffused light source, plate,image the of the lens tube to belens measured camera module and a with camera known module focal withlength known and Sony parameters. IMX258 sensor. (C) The (D) image The of the calibration plate, the lens to be measured and a camera module with known parameters. (C) The tubeexperimental lens camera system module consists with of a known microscope focal module, length anddisplacement Sony IMX258 stage, sensor.calibration (D) and The light experimental image of the tube lens camera module with known focal length and Sony IMX258 sensor. (D) The source. The frame is produced by 3D printing. experimentalsystem consists system of a consists microscope of a microscope module, displacement module, displacement stage, calibration stage, calibration and light and source. light The frame source.is produced The frame by 3D is produced printing. by 3D printing.

Figure 3. Photos of 100 um wide line target obtained by the tube lens (EFL = 6 mm) matched with different cell phone camera lenses. (A–F) The pictures correspond to different object lenses in turn, FigureFigure 3. 3. PhotosPhotos of of100 100 umµ widem wide line linetarget target obtained obtained by the by tube the lens tube (EFL lens = 6 (EFL mm) = matched 6 mm) matchedwith with which are LG G5 super wide‐angle lens, P20pro wide‐angle lens, P20pro tele‐photo lens, P20pro different cell phone camera lenses. (A–FA) TheF pictures correspond to different object lenses in turn, differentstandard celllens, phone Mate20 camera wide‐angle lenses. lens ( and– )Mate20 The pictures standard correspond lens. to different object lenses in turn, whichwhich are are LG LG G5 G5 super super wide wide-angle‐angle lens, lens,P20pro P20pro wide‐angle wide-angle lens, P20pro lens, tele P20pro‐photo tele-photo lens, P20pro lens, P20pro standard lens, Mate20 wide‐angle lens and Mate20 standard lens. standardThe lens, imaging Mate20 test wide-angle results of the lens six and dismantled Mate20 standard lenses are lens. shown in Figure 3. The mag‐ nificationThe imaging ratio can test be results determined of the sixaccording dismantled to the lenses line widthare shown (in pixels) in Figure in the 3. Theimage, mag the‐ pixelThe size imagingon the sensor test and results the nominal of the six line dismantled width of the lenses sample. are The shown focal length in Figure of the3. The nificationmagniﬁcation ratio can ratio be can determined be determined according according to the line to width the line (in widthpixels) (inin the pixels) image, in the image, pixelthe pixel size on size the on sensor the sensorand the and nominal the nominal line width line of the width sample. of the The sample. focal length The of focal the length of the dismantled lenses can be calculated based on Equation (1). As shown in Table1, by selecting different lens modules, we can realize the switching range of magniﬁcation of ~3.9×. In the following experiments, we selected three lenses as the objectives in the miniature microscope, namely LG G5 super wide-angle lens (FL = 1.5 mm), Huawei P20pro standard lens (FL = 6 mm) and P20pro wide-angle lens (FL = 4 mm). The larger focal length lens (Huawei P20pro tele-photo) was not selected for aperture mismatch reasons, which will be discussed in the following part. Appl. Sci. 2021, 11, 3392 5 of 12

dismantled lenses can be calculated based on Equation (1). As shown in Table 1, by select‐ ing different lens modules, we can realize the switching range of magnification of ~3.9×. In the following experiments, we selected three lenses as the objectives in the miniature microscope, namely LG G5 super wide‐angle lens (FL = 1.5 mm), Huawei P20pro standard Appl. Sci. 2021, 11, 3392lens (FL = 6 mm) and P20pro wide‐angle lens (FL = 4 mm). The larger focal length lens 5 of 11 (Huawei P20pro tele‐photo) was not selected for aperture mismatch reasons, which will be discussed in the following part. Table 1. Measurement result of magnification ratio and calculated focal length. Table 1. Measurement result of magnification ratio and calculated focal length. Focal Length Focal Length Camera Lens Source MagnificationFocal Ratio Length Camera Lens Source MagnificationFocal Length Ratio Camera Lens Source Magnification Ratio (mm)Camera Lens Source Magnification Ratio (mm) (mm) (mm) Mate20 P20pro Mate20 2.4 2.5 P20pro 1.5 4.0 (Wide-angle) 2.4 2.5 (Wide-angle) 1.5 4.0 (Wide‐angle) (Wide‐angle) Mate20 Mate20 P20pro P20pro 1.2 1.25.0 5.0 0.75 0.758.0 8.0 (Standard) (Standard) (Tele‐photo) (Tele-photo) G5 G5 P20pro P20pro 3.9 3.91.5 1.5 1.0 1.06.0 6.0 (Super wide‐angle)(Super wide-angle) (Standard) (Standard)

2.3. Assembly of2.3. Miniature Assembly Microscope of Miniature with Microscope Switchable with Magnification Switchable Ratios Magnification Ratios We also suggestWe removing also suggest the tube removing lens module the tube from lens the module cell phone from theto form cell phone a truly to form a truly compact cell phonecompact microscope. cell phone In microscope. the experiment, In the we experiment, dismantled we a second dismantled‐hand a Huawei second-hand Huawei Honor 7 cell phoneHonor and 7 cell took phone its main and back took cameras its main with back a cameras 20M pixel with out a 20Mof the pixel package, out of the package, and connectedand them connected with a flexible them with cable a (Figure flexible 4A). cable This (Figure camera,4A). Thiswith camera,a known with focal a known focal length (EFL = length4.62 mm), (EFL is = equipped 4.62 mm), with is equipped a Sony IMX230 with a Sonysensor IMX230 (pixel size sensor = 1.12 (pixel um). size = 1.12 µm). Then, a compactThen, fixture a compact is designed fixture to is accommodate designed to accommodate the main camera the module main camera (the tube module (the tube lens) and the selectedlens) and camera the selected lens module camera (the lens objective module module), (the objective as shown module), in Figure as shown 4B. in Figure4B. The fixture canThe be fixtureinstalled can on be a installed3D printed on threaded a 3D printed ring threadedand the distance ring and between the distance the between the objective and objectivethe sample and can the be sample adjusted can by be rotating adjusted the by ring rotating to find the the ring best to focusing find the best focusing position. The sideposition. length The of sidethe whole length fixture of the wholeis only fixture about is15 only mm. about A commercial 15 mm. A LED commercial LED diffuse reflectiondiffuse backlight reflection is used backlight as the transmission is used as the light transmission source (Figure light source 4C), which (Figure can4C), which can be replaced withbe replacedother available with other light available sources in light practical sources applications. in practical Users applications. can control Users can control focusing adjustmentfocusing and adjustment capture microscopic and capture images microscopic through images cell phone through camera cell software. phone camera software.

Figure 4. FabricatedFigure 4. miniature Fabricated cell miniature phone microscope. cell phone microscope. (A) After modification, (A) After modification, the cell phone the camera cell phone can cam be led‐ out of the cell phone byera a can flexible be led cable. out (ofB, Cthe) Image cell phone of the by miniature a flexible microscope cable. (B,C) configuration Image of the andminiature working microscope scene. configuration and working scene. The constructed miniature cell phone microscope has several merits compared with The constructedthe former miniature reported cell systems. phone microscope Firstly, the has whole several system merits is highly compared portable, with especially the the former reportedmicroscope systems. module. Firstly, Secondly, the whole the system operation is highly of the miniatureportable, especially microscope the is really simple, microscope module.even in Secondly, the field, the which operation is due toof the compactminiature size. microscope There is is no really need simple, for complicated focal even in the field,length which adjustment is due to rigs.the compact Thirdly, thesize. FOV There can is be no easily need switchedfor complicated by shifting focal three objectives length adjustmentwhose rigs. diameters Thirdly, werethe FOV made can equal be easily by 3D switched printed by adapters. shifting Switchablethree objec‐ magnification tives whose diametersenables thewere proposed made equal microscope by 3D printed to observe adapters. the sample, Switchable select magnification region of interest (ROI) to enables the proposedenlarge microscope the magnification to observe and reviewthe sample, with select a higher region resolution, of interest similar (ROI) to to the operation of enlarge the magnificationbench-top microscopes. and review with Finally, a higher the image resolution, quality similar of the to microscope the operation matches of 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 (Figure5) and advanced build-in image processing functions. The largest digital zooming ratio of Huawei Honor 7 is ~4×, 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 Appl. Sci. 2021, 11, 3392 6 of 12

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 Appl. Sci. 2021, 11, 3392 all equipped with digital zooming functions (Figure 5) and advanced build‐in image6 pro of 11‐ cessing functions. The largest digital zooming ratio of Huawei Honor 7 is ~4×, 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 image imageresolution. resolution. Therefore, Therefore, the digital the digital zooming zooming of cell of phones cell phones has brought has brought real realbenefits beneﬁts to mi to‐ microscopecroscope performance. performance.

Figure 5.5. Two mobile photos taken inin normalnormal environment.environment. The zoom function of thethe cellcell phonephone camera cancan be be operated operated manually manually in in the the interface. interface. (A ()A Photo) Photo before before zooming. zooming. (B )(B Photo) Photo after after zooming. zoom‐ ing. 3. Results 3.1.3. Results Microscopic Imaging Performance Tests 3.1. MicroscopicFirstly, we evaluatedImaging Performance the imaging Tests performance of the constructed miniature cell phone microscope by imaging a resolution test chart(USAF-1951 standard resolution target), Firstly, we evaluated the imaging performance of the constructed miniature cell through which the values of FOV and resolution at three magnification ratio settings could phone microscope by imaging a resolution test chart(USAF‐1951 standard resolution tar‐ be determined. The captured images exhibit high clarity and little distortion, which are get), through which the values of FOV and resolution at three magnification ratio settings comparable to the bench-top systems (Figure6). All the images are obtained directly could be determined. The captured images exhibit high clarity and little distortion, which from the cell phone without any post-treatment. When the zooming function of the cell phonesare comparable is not applied, to the bench the diagonal‐top systems values (Figure of the 6). FOV All at the three images magnification are obtained ratios directly are 2.7from mm, the 6.0 cell mm phone and without 9.0 mm, any corresponding post‐treatment. to the When magnification the zooming ratios function of 3×, of 1.2 the× and cell 0.8phones×, respectively. is not applied, When the the diagonal digital values zooming of the of the FOV cell at phones three magnification is applied and ratios the sample are 2.7 ismm, re-focused, 6.0 mm and the 9.0 FOV mm, is shrunken,corresponding yet theto the image magnification reveals more ratios detailed of 3×, 1.2× features. and 0.8×, We evaluatedrespectively. the When image the resolution digital zooming based on of the the intensity cell phones contrast is applied of the and line the profile sample between is re‐ thefocused, horizontal the FOV and is verticalshrunken, directions. yet the image The reveals microscope more candetailed distinguish features. the We features evaluated of 575the lp/mm,image resolution 322 lp/mm based and on 203 the lp/mm intensity when contrast the three of lenses the line are profile used, respectively.between the Ifhori we‐ takezontal the and FOV vertical into consideration, directions. The we microscope can find that can the distinguish constructed the miniature features of microscopes 575 lp/mm, outperform322 lp/mm and some 203 bench-top lp/mm when systems the inthree terms lenses of their are used, comprehensive respectively. imaging If we performance,take the FOV whichinto consideration, makes them we quite can attractive find that inthe the constructed applications miniature of large microscopes sample screening outperform and microscopicsome bench‐top reviewing. systems Thein terms low costof their and comprehensive compact size addimaging more performance, advantages towhich the proposedmakes them miniature quite attractive microscope. in the applications of large sample screening and microscopic reviewing. The low cost and compact size add more advantages to the proposed miniature 3.2.microscope. Real Sample Imaging Test We used the miniature microscope to conduct imaging tests of real samples, which included a plant terminal bud slice, a kidney vertical slice and a fiber connective tissue slice. Such samples are commercially available for educational purposes. With the help of the miniature microscope, we can take impressive microscopic images of the samples (Figure7). We can find that the switching of the magnification ratio proposed here has important value in the process of sample imaging. Take the imaging of the plant terminal bud slide as an example. The 1.2× mag ratio image gives users an overall view of the bud sample and finds the region of interest(ROI), and digital zooming offers a ~4× larger image of the central region with more details. However, users can not observe the cell nucleus clearly since the resolution using this lens module is only about 400 lp/mm. By shifting Appl. Sci. 2021, 11, 3392 7 of 11

to the G5 lens module objective, users can obtain the 3× image before zooming and the 11.5× image after zooming. Users are now able to discriminate the internal structures of the cells with micron-level resolution. The workﬂow is similar to that of bench-top systems. Therefore, users may feel it easier to learn and practice the miniature microscope. The Appl. Sci. 2021, 11, 3392 7 of 12 captured images are already colored images, which indicate that the proposed systems have great potential for histology sample imaging.

Figure 6. Evaluation of imaging performance of the built microscope. LG G5 super wide‐angle lens, P20proFigure wide 6. Evaluation‐angle lens ofand imaging P20pro performancestandard lens are of the taken built as targets microscope. to take LG photos G5 superof resolution wide-angle lens, chart.P20pro When wide-angle the zooming lens function and P20pro of the standardcell phones lens is not are applied, taken the as targets diagonal to values take photos of the field of resolution ofchart. view When (FOV) theat the zooming three magnification function of ratios the cell are phones 2.7 mm, is 6.0 not mm, applied, 9.0 mm, the which diagonal correspond values to of the field theof viewmagnification (FOV) at ratios the three of 3×, magnification1.2× and 0.8×, respectively. ratios are 2.7 The mm, FOV 6.0 is reduced mm, 9.0 when mm, using which the correspond digital to the zoom of the cell phones, but the microscope can distinguish the features of 575 lp/mm, 322 lp/mm magnification ratios of 3×, 1.2× and 0.8×, respectively. The FOV is reduced when using the digital and 203 lp/mm, respectively. Appl. Sci. 2021, 11, 3392 zoom of the cell phones, but the microscope can distinguish the features of 575 lp/mm,8 322of 12 lp/mm

3.2.and Real 203 Sample lp/mm, Imaging respectively. Test We used the miniature microscope to conduct imaging tests of real samples, which included a plant terminal bud slice, a kidney vertical slice and a fiber connective tissue slice. Such samples are commercially available for educational purposes. With the help of the miniature microscope, we can take impressive microscopic images of the samples (Figure 7). We can find that the switching of the magnification ratio proposed here has important value in the process of sample imaging. Take the imaging of the plant terminal bud slide as an example. The 1.2× mag ratio image gives users an overall view of the bud sample and finds the region of interest(ROI), and digital zooming offers a ~4× larger image of the central region with more details. However, users can not observe the cell nucleus clearly since the resolution using this lens module is only about 400 lp/mm. By shifting to the G5 lens module objective, users can obtain the 3× image before zooming and the 11.5× image after zooming. Users are now able to discriminate the internal structures of the cells with micron‐level res‐ olution. The workflow is similar to that of bench‐top systems. Therefore, users may feel it easier to learn and practice the miniature microscope. The captured images are already col‐ ored images, which indicate that the proposed systems have great potential for histology sample imaging.

FigureFigure 7. 7.ImagingImaging tests tests of real of real biological biological samples. samples. Sample Sample images images of a plant of a plantterminal terminal bud slice, bud a slice, a kidney slice and a fibrous connective tissue slice are obtained with the built microscope using the kidney slice and a ﬁbrous connective tissue slice are obtained with the built microscope using the P20pro wide‐angle lens and the LG G5 Super wide‐angle lens as objectives. We also used the zoom functionP20pro in wide-angle the imaging lens process. and the LG G5 Super wide-angle lens as objectives. We also used the zoom function in the imaging process. 3. Discussions The imaging tests exhibit that the proposed miniature microscope can provide mi‐ cron‐level resolution and large FOV microscopic imaging with minute cost and high port‐ ability. Next, the limiting factors of miniature cell phone microscopes are discussed. The first problem is whether the FOV of the miniature microscope can be enlarged. According to Equation (1), the longer the focal length of the object lens switched to, the larger the FOV for the proposed microscope is. However, for the cell phone tele‐photo lens, the increase of focal length brings with it a smaller angle of imaging, since they are usually designed for the fixed size CMOS sensor. If a long focal length tele‐photo cell phone lens (for example, P20pro tele‐photo, EFL = 8 mm) is used, we find the resulting image is effective only in the central circular region, which is decided by the maximum imaging angle of the tele‐photo lens (Figure 8). Compared with Figure 6, we can see that the magnification ratio decreases as expected, and the effective FOV also decreases. There‐ fore, we abandoned the tele‐photo lens as the objective. If users really demand an even larger FOV, a specially designed tele‐photo lens with larger imaging angle may be re‐ quired, which may increase the cost of the total system.

Figure 8. Picture of resolution test chart using P20pro tele‐photo lens (EFL = 8 mm). Theoretically, when the longer focal length lens is used as an objective lens, the magnification will be reduced and the FOV will become larger. In practice, the FOV may be limited to a smaller circle by the objective lens imaging angle limitation. Appl. Sci. 2021, 11, 3392 8 of 12

Figure 7. Imaging tests of real biological samples. Sample images of a plant terminal bud slice, a Appl. Sci. 2021, 11, 3392 kidney slice and a fibrous connective tissue slice are obtained with the built microscope using8 of 11the P20pro wide‐angle lens and the LG G5 Super wide‐angle lens as objectives. We also used the zoom function in the imaging process.

4.3. Discussions Discussions TheThe imaging imaging tests tests exhibit exhibit that that the the proposed proposed miniature miniature microscope microscope can providecan provide micron- mi‐ levelcron resolution‐level resolution and large and FOV large microscopic FOV microscopic imaging imaging with minute with minute cost and cost high and portability. high port‐ Next,ability. the Next, limiting the factorslimiting of factors miniature of miniature cell phone cell microscopes phone microscopes are discussed. are discussed. TheThe first first problem problem is is whether whether the the FOV FOV of of the the miniature miniature microscope microscope can can be be enlarged. enlarged. AccordingAccording to to Equation Equation (1), (1) the, the longer longer the the focal focal length length of of the the object object lens lens switched switched to, to, the the largerlarger the the FOV FOV for for the the proposed proposed microscope microscope is. is. However, However, for for the the cell cell phone phone tele-photo tele‐photo lens,lens, the the increase increase of of focal focal length length brings brings with with it it a a smaller smaller angle angle of of imaging, imaging, since since they they are are usuallyusually designed designed for for the the fixed fixed size size CMOS CMOS sensor. sensor. If If a a long long focal focal length length tele-photo tele‐photo cell cell phonephone lens lens (for (for example, example, P20pro P20pro tele-photo, tele‐photo, EFL EFL = = 8 8 mm) mm) is is used, used, we we find find the the resulting resulting imageimage is is effective effective only only in in the the central central circular circular region, region, which which is is decided decided by by the the maximum maximum imagingimaging angle angle of of thethe tele-phototele‐photo lens lens (Figure (Figure 88).). Compared Compared with with Figure Figure 6,6 ,we we can can see see that thatthe themagnification magnification ratio ratio decreases decreases as expected, as expected, and the and effective the effective FOV also FOV decreases. also decreases. There‐ Therefore,fore, we abandoned we abandoned the tele the‐ tele-photophoto lens lensas the as objective. the objective. If users If users really really demand demand an even an evenlarger larger FOV, FOV, a specially a specially designed designed tele tele-photo‐photo lens lens with with larger larger imaging imaging angle angle may may be be re‐ required, which may increase the cost of the total system. quired, which may increase the cost of the total system.

Figure 8. Picture of resolution test chart using P20pro tele‐photo lens (EFL = 8 mm). Theoretically, Figure 8. Picture of resolution test chart using P20pro tele-photo lens (EFL = 8 mm). Theoretically, when the longer focal length lens is used as an objective lens, the magnification will be reduced and when the longer focal length lens is used as an objective lens, the magniﬁcation will be reduced and the FOV will become larger. In practice, the FOV may be limited to a smaller circle by the objective thelens FOV imaging will become angle limitation. larger. In practice, the FOV may be limited to a smaller circle by the objective lens imaging angle limitation.

The proposed miniature microscope is formed by connecting two cell phone lenses. The mismatch between the position and the size of the two lens apertures would naturally have a negative impact on the imaging quality of the micro-imaging system. In Table2, the resolution is evaluated based on the central part of the image, where the influence of aperture mismatch is minimal. It can be expected that vignetting may occur in the off-axis field of view due to aperture. From previous results, we can find that the corner area of the image is obviously darkened at a low magnification ratio (Figure6). Resolution is also influenced by this factor. When we shifted the 8th and 9th group lines of the resolution test chart from middle to boundary of the field of view, the aberration aggregation can be observed and the resolution capability gets worse, as the resolvable 9-1 lines become contrast-less in the corner of the image (Figure9). For some applications, the aberration on the edge would further weaken the available field of view, even though the imaging amount in the center is acceptable, especially when scanning or stitching is needed. By shortening the distance between the two lenses in the fixture, the influence of aperture mismatch may be reduced. In addition, when the objective lens is selected, the cell phone lens with a larger aperture is preferred. Other limiting factors also include color non-uniformity of the microscope image, short working distance, etc. However, despite all these limitations, the proposed miniature cell phone microscope with switchable magnification marks the improvement of imaging performance and user operational acceptance. Appl. Sci. 2021, 11, 3392 9 of 12

The proposed miniature microscope is formed by connecting two cell phone lenses. The mismatch between the position and the size of the two lens apertures would naturally have a Appl. Sci. 2021, 11, 3392 9 of 11 negative impact on the imaging quality of the micro‐imaging system. In Table 2, the resolution is evaluated based on the central part of the image, where the influence of aperture mismatch is minimal. It can be expected that vignetting may occur in the off‐axis field of view due to Tableaperture. 2. Measurement From previous result of the results, magnification we can ratio,find FOVthat andthe resolution.corner area of the image is obviously darkened at a low magnification ratio (Figure 6). Resolution is also influenced by this factor. Before Zooming After Zooming When we shifted the 8th and 9th group lines of the resolution test chartHighest from Resolution middle to bound‐ Magnification Ratio FOV/um Magnification Ratio FOV/um (lp/mm) ary of the field of view, the aberration aggregation can be observed and the resolution capa‐ G5 bility3.0 gets worse, 2137 as× the1613 resolvable 9 11.5–1 lines become 505 contrast× 381‐less in the corner 574.7 of the image (Super wide-angle) (Figure 9). For some applications, the aberration on the edge would further weaken the avail‐ P20pro able1.2 field of view, 4809 even× 3629 though the imaging 4.9 amount 1178in the× center889 is acceptable, 406.4 especially when (Wide-angle) scanning or stitching is needed. By shortening the distance between the two lenses in the fix‐ P20pro ture,0.8 the influence 7213 of× 5443aperture mismatch 3.3 may be reduced. 1749 × 1320 In addition, when 228.1 the objective lens (Standard) is selected, the cell phone lens with a larger aperture is preferred.

FigureFigure 9. 9. ThreeThree sets of images takentaken of of the the resolution resolution test test chart chart as as it movesit moves from from the the FOV FOV center center to to boundary.boundary. ( (AA)) Lines of 575575 lp/mmlp/mm cancan bebe clearlyclearly distinguished distinguished when when it it is is at at the the center center of theof the FOV. FOV. (B(B) )The The 575 575 lp/mm contrast contrast is reducedreduced when when the the resolution resolution test test chart chart is movedis moved outward outward from from the the center. (C) The 575 lp/mm features are blurred at the corner of the FOV due to vignetting and optical center. (C) The 575 lp/mm features are blurred at the corner of the FOV due to vignetting and optical aperture mismatch. aperture mismatch.

More importantly, the proposed concept can be used as a miniature platform for various interesting micro-imaging research applications. In the cell culture applications, the real-time microscopic wireless observation can be realized (Figure 10A). The compact size of the microscope module makes it possible to be placed inside the temperature-controlled cell culturing tank, and the captured images can be transferred via the cell phone in a real-time manner. In addition, the low cost of the miniature microscope makes it possible to deploy such a module array to monitor an array of cell culturing plates simultaneously. This research is already in progress and has aroused great interest. In this paper, we have demonstrated the maximum magniﬁcation of 11.5× and a ~600 lp/mm resolution power, which is already one of the best results for such kinds of cell phone microscopes. However, Appl. Sci. 2021, 11, 3392 10 of 12

Table 2. Measurement result of the magnification ratio, FOV and resolution.

Before Zooming After Zooming Highest Resolution (lp/mm) Magnification Ratio FOV/um Magnification Ratio FOV/um G5 (Super wide‐an‐ 3.0 2137 × 1613 11.5 505 × 381 574.7 gle) P20pro 1.2 4809 × 3629 4.9 1178 × 889 406.4 (Wide‐angle) P20pro 0.8 7213 × 5443 3.3 1749 × 1320 228.1 (Standard)

Other limiting factors also include color non‐uniformity of the microscope image, short working distance, etc. However, despite all these limitations, the proposed minia‐ ture cell phone microscope with switchable magnification marks the improvement of im‐ aging performance and user operational acceptance. More importantly, the proposed concept can be used as a miniature platform for var‐ ious interesting micro‐imaging research applications. In the cell culture applications, the real‐time microscopic wireless observation can be realized (Figure 10A). The compact size of the microscope module makes it possible to be placed inside the temperature‐controlled cell culturing tank, and the captured images can be transferred via the cell phone in a real‐ time manner. In addition, the low cost of the miniature microscope makes it possible to deploy such a module array to monitor an array of cell culturing plates simultaneously. This research is already in progress and has aroused great interest. In this paper, we have demonstrated the maximum magnification of 11.5× and a ~600 lp/mm resolution power, which is already one of the best results for such kinds of cell phone microscopes. However, Appl. Sci. 2021, 11, 3392 10 of 11 the higher resolution is still expected to gain more application possibilities. We have al‐ ready been investigating the schemes of combining the proposed miniature microscope with other principles,the such higher as resolution total internal is still expectedreflection to gainimaging more application (Figure 10B), possibilities. structured We have light imaging (Figurealready 10C), beenetc. The investigating proposed the schemesscheme of enjoys combining a relatively the proposed large miniature FOV microscopein the flat field, making it suitablewith other for principles, carrying such out as total such internal super reflection‐resolution imaging methods. (Figure 10 B),In structured the near light future, advanced fluorescenceimaging (Figure microscopy 10C), etc. with The proposed low costs scheme and enjoys high aresolution relatively large will FOV be com in the‐ flat field, making it suitable for carrying out such super-resolution methods. In the near future, pletely possible. Otheradvanced interesting fluorescence applications microscopy may with lowinclude costs andplant high leaf resolution or water will beparticle completely real‐time monitoring,possible. etc. In Othermany interesting applications, applications the ~1 may‐mm include long plant working leaf or water distance particle of real-time the proposed microscopemonitoring, is acceptable etc. In manyand applications,as long as the this ~1-mm is satisfied, long working many distance application of the proposed microscope is acceptable and as long as this is satisfied, many application schemes can be schemes can be imaginedimagined and and designed designed onon the the merit merit of its of compact its compact size, reasonable size, reasonable resolution power, reso large‐ lution power, large FOV,FOV, tale-communicationtale‐communication ability ability and low and cost. low cost.

FigureFigure 10. Potential applications applications based based on the on proposed the proposed miniature miniature microscope. microscope. (A) Real-time (A microscopic) Real‐time wireless mi‐ croscopicobservation wireless of the enclosed observation cell culturing of the process. enclosed (B) Scheme cell culturing of a low-cost process. total internal (B) reflectionScheme miniatureof a low microscope.‐cost total (C) Scheme of a low-cost structured illumination miniature microscope for enhanced resolution. internal reflection miniature microscope. (C) Scheme of a low‐cost structured illumination minia‐ ture microscope for enhanced5. Conclusions resolution. Cell phone microscopes have been widely researched, the applications of which are still limited by the following factors, including inferior image quality, fixed magnification, inconvenient operation, etc. In this paper, we proposed a cell phone lens-based miniature microscope with a configurable magnification ratio. Inheriting the advantages of low costs and direct access to the internet via cell phone, the proposed miniature microscope simultaneously provides users with the ability of adjustable magnification ratios, truly easy deployment and even an array of applications in the field. These improvements are significant for the field applications of cell phone microscopes. On the basis of the proposed scheme, by switching three camera lens objectives and applying the digital zooming function of the cell phones, a set of miniature microscopes with magnification ratios continuously configurable among 0.8×–11.5× was built. The imaging tests exhibit that the proposed miniature microscope can provide high-quality microscopic imaging, with a maximum resolution of up to 575 lp/mm and a maximum FOV of up to 7213 × 5443 µm. In addition, by moving the tube lens module of the cell phone microscope out of the cell phone body, the built miniature microscope is as compact as a cube with a side length of 15 mm, profoundly advancing the operational experience. We believe that the proposed scheme has addressed the mentioned limitations of the cell phone-based microscopes and users may find it easier to learn and practice with such miniature microscopes. With the launch of more powerful cell phone cameras these days, we can foresee that more powerful miniature cell phone microscopes will be produced and put into use.

Author Contributions: Investigation, X.W. and X.T.; methodology, X.W. and X.T.; resources, X.W.; supervision, X.W.; validation, X.W. and X.T.; writing—original draft, X.T.; writing—review and editing, X.W. and X.T. All authors have read and agreed to the published version of the manuscript. Appl. Sci. 2021, 11, 3392 11 of 11

Funding: This research was funded by the National Natural Science Foundation of China, grant number 61505107 and by the Shanghai Science and Technology Innovation Action Plan, grant number 19511104600. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data used to support the findings of this study are included within the article. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Schiffer, M.B.; Miller, A.R. The Material Life of Human Beings: Artifacts, Behavior, and Communication; Psychology Press: Hove, UK, 1999. 2. Petti, C.A.; Polage, C.R.; Quinn, T.C.; Ronald, A.R.; Sande, M.A. Laboratory medicine in Africa: A barrier to effective health care. Clin. Infect. Dis. 2006, 42, 377–382. [CrossRef][PubMed] 3. Zheng, G.; Lee, S.A.; Yang, S.; Yang, C. Sub-pixel resolving optofluidic microscope for on-chip cell imaging. Lab. Chip 2010, 10, 3125–3129. [CrossRef][PubMed] 4. Bishara, W.; Su, T.W.; Coskun, A.F.; Ozcan, A. Lensfree on-chip microscopy over a wide field-of-view using pixel super-resolution. Opt. Express 2010, 18, 11181–11191. [CrossRef][PubMed] 5. Skandarajah, A.; Reber, C.D.; Switz, N.A.; Fletcher, D.A. Quantitative imaging with a mobile phone microscope. PLoS ONE 2017, 9, e96906. [CrossRef][PubMed] 6. Cai, F.; Wang, T.; Lu, W.; Zhang, X. High-resolution mobile bio-microscope with smartphone telephoto camera lens. Optik 2020, 207.[CrossRef] 7. Breslauer, D.N.; Maamari, R.N.; Switz, N.A.; Lam, W.A.; Fletcher, D.A. Mobile phone based clinical microscopy for global health applications. PLoS ONE 2009, 4, e6320. [CrossRef][PubMed] 8. Smith, Z.J.; Chu, K.; Espenson, A.R.; Rahimzadeh, M.; Gryshuk, A.; Molinaro, M.; Dwyre, D.M.; Lane, S.; Matthews, D.; Wachsmann-Hogiu, S. Cell-phone-based platform for biomedical device development and education applications. PLoS ONE 2011, 6, e17150. [CrossRef][PubMed] 9. Switz, N.A.; D’Ambrosio, M.V.; Fletcher, D.A. Low-cost mobile phone microscopy with a reversed mobile phone camera lens. PLoS ONE 2014, 9, e95330. [CrossRef][PubMed] 10. McKay, G.N.; Mohan, N.; Butterworth, I.; Bourquard, A.; Sánchez-Ferro, Á.; Castro-González, C.; Durr, N.J. Visualization of blood cell contrast in nailfold capillaries with high-speed reverse lens mobile phone microscopy. Biomed. Opt. Express 2020, 11, 2268–2276. [CrossRef][PubMed] 11. Dong, S.; Guo, K.; Nanda, P.; Shiradkar, R.; Zheng, G. FPscope: A field-portable high-resolution microscope using a cellphone lens. Biomed. Opt. Express 2014, 5, 3305–3310. [CrossRef][PubMed] 12. Kim, J.-H.; Joo, H.-G.; Kim, T.-H.; Ju, Y.-G. A smartphone-based fluorescence microscope utilizing an external phone camera lens module. BioChip J. 2015, 9, 285–292. [CrossRef] 13. Kim, K.M.; Choe, S.-H.; Ryu, J.-M.; Choi, H. Computation of Analytical Zoom Locus Using Padé Approximation. Mathematics 2020, 8, 581. [CrossRef] 14. Chen, X.; Yu, L.; Kang, Q.; Sun, Y.; Huang, Y.; Shen, D. A smartphone-based absorbance device extended to ultraviolet (365 nm) and near infrared (780 nm) regions using ratiometric fluorescence measurement. Microchem. J. 2021, 164, 105978. [CrossRef] 15. Goenka, C.; Lewis, W.; Chevres-Fernández, L.R.; Ortega-Martínez, A.; Ibarra-Silva, E.; Williams, M.; Franco, W. Mobile phone- based UV fluorescence microscopy for the identification of fungal pathogens. Lasers Surg. Med. 2019, 51, 201–207. [CrossRef] [PubMed] 16. Diederich, B.; Wartmann, R.; Schadwinkel, H.; Heintzmann, R. Using machine-learning to optimize phase contrast in a low-cost cellphone microscope. PLoS ONE 2018, 13, e0192937. [CrossRef][PubMed] 17. Helmchen, F.; Fee, M.S.; Tank, D.W.; Denk, W. A miniature head-mounted two-photon microscope: High-resolution brain imaging in freely moving animals. Neuron 2001, 31, 903–912. [CrossRef] 18. Rolland, J.P.; Fuchs, H. Optical versus video see-through head-mounted displays in medical visualization. Presence Teleoper. Virtual Environ. 2000, 9, 287–309. [CrossRef] 19. Di Febo, R.; Casas, L.; Antonini, A. A smartphone-based petrographic microscope. Microsc. Res. Tech. 2021.[CrossRef][PubMed]