Portable Fundus Camera Research Articles

Miniaturized fundus camera based on cross-polarization and in vivo verification

Abstract As an important part of clinical examination, fundus examination can reveal early symptoms of both eye and chronic physical diseases, thus helping doctors and automated screening to diagnose and treat in time. However, widely used portable fundus cameras are often disturbed by stray light, which seriously affects the quality of fundus imaging, extremely detrimental to the clinic. In this work, we have successfully built a miniaturized fundus camera based on the cross-polarization method, which simplifies the system design while guaranteeing high-quality imaging compared with other methods. The simulation results of the optical path and the imaging effect in vivo show that cross-polarization has a powerful effect on eliminating stray light. Our approach provides a new solution for the field of fundus photography and an application-level advance for the popularization of medical resources.

Retinal Changes in Patients With Covid-19 and Different Expressiveness of Metabolic Changes.

To study the relationship between the severity of COVID-induced metabolic changes and the structure and frequency of retinal changes, according to funduscopy data in patients with different clinical courses of COVID-19. 117 patients with COVID-19 were examined. While examining patients, severity of the course of COVID-19, the expressiveness of changes in the metabolic status were determined; fundus image registration was performed with portable fundus cameras Pictor Plus Fundus Camera and VistaView (Volk Optical). As a result of the research, retinal changes were found in 49 (41.9 %) patients with COVID-19. In 8 (16.3 %) cases, clinically significant (vitreous hemorrhage, prethrombosis of the central retinal vein or branches of the central retinal vein, thrombosis of the central retinal vein or branches of the central retinal vein) COVID-induced retinal and ophthalmological changes were observed, which caused a decrease in visual acuity. In 41 (83.7 %) cases, clinically insignificant changes (cotton wool spots, narrowed retinal vessels, intraretinal and petechial hemorrhages, tortuosity and dilatation of retinal venules) COVID-induced retinal changes were observed. Clinically significant retinal changes occur in patients with a statistically significantly higher level of D-dimer and a greater percentage of lung parenchyma lesion than in the group of patients with clinically insignificant retinal changes (p < 0.05). The structure of retinal changes in patients with COVID-19 correlates with the severity of the clinical course of the disease and changes in the metabolic status of patients. Metabolic changes are correlated with retinal changes and can be predictive for preventing general vascular complications in COVID-19.

A fast and fully automated system for glaucoma detection using color fundus photographs

This paper presents a low computationally intensive and memory efficient convolutional neural network (CNN)-based fully automated system for detection of glaucoma, a leading cause of irreversible blindness worldwide. Using color fundus photographs, the system detects glaucoma in two steps. In the first step, the optic disc region is determined relying upon You Only Look Once (YOLO) CNN architecture. In the second step classification of ‘glaucomatous’ and ‘non-glaucomatous’ is performed using MobileNet architecture. A simplified version of the original YOLO net, specific to the context, is also proposed. Extensive experiments are conducted using seven state-of-the-art CNNs with varying computational intensity, namely, MobileNetV2, MobileNetV3, Custom ResNet, InceptionV3, ResNet50, 18-Layer CNN and InceptionResNetV2. A total of 6671 fundus images collected from seven publicly available glaucoma datasets are used for the experiment. The system achieves an accuracy and F1 score of 97.4% and 97.3%, with sensitivity, specificity, and AUC of respectively 97.5%, 97.2%, 99.3%. These findings are comparable with the best reported methods in the literature. With comparable or better performance, the proposed system produces significantly faster decisions and drastically minimizes the resource requirement. For example, the proposed system requires 12 times less memory in comparison to ResNes50, and produces 2 times faster decisions. With significantly less memory efficient and faster processing, the proposed system has the capability to be directly embedded into resource limited devices such as portable fundus cameras.

Preserving polarization maintaining photons for enhanced contrast imaging of the retina.

The purpose of this study is to demonstrate the feasibility of using polarization maintaining photons for enhanced contrast imaging of the retina. Orthogonal-polarization control has been frequently used in conventional fundus imaging systems to minimize reflection artifacts. However, the orthogonal-polarization configuration also rejects the directly reflected photons, which preserve the polarization condition of incident light, from the superficial layer of the fundus, i.e., the retina, and thus reduce the contrast of retinal imaging. We report here a portable fundus camera which can simultaneously perform orthogonal-polarization control to reject back-reflected light from the ophthalmic lens and parallel-polarization control to preserve the backscattered light from the retina which partially maintains the polarization state of the incoming light. This portable device utilizes miniaturized indirect ophthalmoscopy illumination to achieve non-mydriatic imaging, with a snapshot field of view of 101° eye-angle (67° visual-angle). Comparative analysis of retinal images acquired with a traditional orthogonal-polarization fundus camera from both normal and diseased eyes was conducted to validate the usefulness of the proposed design. The parallel-polarization control for enhanced contrast in high dynamic range imaging has also been validated.

Prevalence and associated factors of diabetic retinopathy among people with diabetes screened using fundus photography at a community diabetic retinopathy screening program in Nepal

BackgroundThis study aimed to assess the prevalence and associated factors of diabetic retinopathy (DR) and vision threatening DR (VTDR) among people with diabetes screened using fundus photography in Nepal.MethodsThis is a retrospective study among people with diabetes presented for DR screening using fundus photography from 2013 to 2019. Detailed demographics, duration of diabetes, medical history, visual acuity, and grading of DR on fundus photography were analyzed. Fundus camera used in the study were;Topcon digital fundus camera 900 CXR and digital portable fundus cameras (Nidek-10 portable non-mydriatric fundus camera; Versacam & Trade & Alpha, France), and a Zeiss portable fundus camera (Zeiss Visucout 100). Macula centred and disc centred 45 degree two images were taken from each eye. Pupil were dilated in cases where there was media haze in un-dilated cases. DR was graded using early treatment diabetic retinopathy study criteria. The images were graded by fellowship trained retina specialist. DR prevalence included any DR changes in one or both eyes.ResultsTotal of 25,196 patients with diabetes were enrolled. Mean age was 54.2 years with Standard Deviation (S.D):12.9 years, ranging from 6 years to 97 years. Type 1 and type 2 diabetes comprised of 451 people (1.79%) and 24,747 (98.21%) respectively. Overall, 1.8% of the images were un-gradable. DR prevalence was 19.3% (95% Confidence Interval (CI): 18.8 − 19.7%). DR prevalence in type 1 and type 2 diabetes was 15.5% (95% CI: 12.5 − 18.6%) and 19.3% (CI: 18.8 − 19.8%) respectively. Clinically significant macular edema (CSME) was found in 5.9% (95% CI: 5.6-6.2%) and VTDR in 7.9% (95% CI: 7.7-8.3%). In multivariate analysis, our study revealed strong evidence to suggest that there is meaningful association between DR and VTDR with duration of diabetes, diabetic foot, diabetic neuropathy, agriculture occupation, those under oral hypoglycaemic agents or insulin or both as compared to those under diet only, and presenting visual acuity > 0.3LogMAR.ConclusionPrevalence and associated factors for DR and VTDR were similar to other DR screening programs in the region. Emphasis on wider coverage of DR screening could help for timely detection and treatment of STDR to avoid irreversible blindness.

Comparing optic nerve head parameters in two Spanish dog breeds using digital planimetry

The optic discs of dogs exhibit considerable size, shape, and colour variations, depending on the degree of myelination. This variability makes the interpretation of lesions difficult, owing to the lack of reference patterns. This study aimed to compare optic nerve head (ONH) parameters determined by digital planimetry (DP) in two pure-bred dogs, Spanish Greyhound (SG) and Spanish Water Dog (SWD). The vertical and horizontal diameters, area, circularity, and the proportion of the ONH at the tapetal or non-tapetal zone (top height and bottom height) were calculated using image treatment software and compared between breeds.Significant between-breed differences were detected for all parameters, with SWD exhibiting greater height, width, area, top height, and lower circularity of the ONH than SG. Linear regression revealed that age significantly influenced mean disc height (R2 = 0.310; p = 0.0001), mean disc width (R2 = 0.280; p = 0.0001), mean disc area (R2 = 0.281; p = 0.0001), and circularity (R2 = 0.243; p = 0.0001). The multiple regression model significantly predicted mean disc height, width, and area [(S) (R2 = 0.715; p = 0.001), (R2 = 0.742; p = 0.001), and (S) (R2 = 0.736; p = 0.001), respectively], based on age and breed. Excellent concordance was observed between the measurements of experienced and novice researchers, and there were no differences between the parameters measured by the researcher and those obtained by the software. Planimetric ONH measurements can be easily performed by novice operators using a portable fundus camera and digital computer software. These results may be of considerable clinical value, but further studies are required because of the great variability of the ONH in dogs.

Cross-camera Performance of Deep Learning Algorithms to Diagnose Common Ophthalmic Diseases: A Comparative Study Highlighting Feasibility to Portable Fundus Camera Use

Purpose To compare the inter-camera performance and consistency of various deep learning (DL) diagnostic algorithms applied to fundus images taken from desktop Topcon and portable Optain cameras. Methods Participants over 18 years of age were enrolled between November 2021 and April 2022. Pair-wise fundus photographs from each patient were collected in a single visit; once by Topcon (used as the reference camera) and once by a portable Optain camera (the new target camera). These were analyzed by three previously validated DL models for the detection of diabetic retinopathy (DR), age-related macular degeneration (AMD), and glaucomatous optic neuropathy (GON). Ophthalmologists manually analyzed all fundus photos for the presence of DR and these were referred to as the ground truth. Sensitivity, specificity, the area under the curve (AUC) and agreement between cameras (estimated by Cohen’s weighted kappa, K) were the primary outcomes of this study. Results A total of 504 patients were recruited. After excluding 12 photographs with matching errors and 59 photographs with low quality, 906 pairs of Topcon-Optain fundus photos were available for algorithm assessment. Topcon and Optain cameras had excellent consistency (Κ=0.80) when applied to the referable DR algorithm, while AMD had moderate consistency (Κ=0.41) and GON had poor consistency (Κ=0.32). For the DR model, Topcon and Optain achieved a sensitivity of 97.70% and 97.67% and a specificity of 97.92% and 97.93%, respectively. There was no significant difference between the two camera models (McNemar’s test: x2=0.08, p = .78). Conclusion Topcon and Optain cameras had excellent consistency for detecting referable DR, albeit performances for detecting AMD and GON models were unsatisfactory. This study highlights the methods of using pair-wise images to evaluate DL models between reference and new fundus cameras.

Fundus Imaging via Infrared Camera without Mydriatics in Holstein Calves

This study aimed to record the ocular fundus images of healthy Holstein calves under field conditions. The fundus of 34 eyes of healthy 17 Holstein calves was examined with a fundus camera, which does not require mydriatics, as it has been designed especially for animals and provides imaging with infrared light. The findings showed that the green-yellow tapetal zone was dominant in all calves, the optic disc was oval, and the number of primary arteries and veins originating from the center varied between 4 and 5. The vascular pattern was holangiotic. A remnant of the hyaloid artery (Bergmeister’s papilla) was detected as a gray dot in the middle of the disc. It was noted that the non-tapetal area was homogeneous, brown-black, and rich in choroidal vascular structure. Imaging the ocular fundus is essential in diagnosing some systemic and hereditary diseases in farm animals. However, herd-based ophthalmoscopic screening in farm animals is difficult under field conditions. By using this portable fundus camera, fundus examination can be performed easily under field conditions without taking the animals to the hospital. The standard ophthalmoscopic fundus images of healthy Holstein calves presented in this study will contribute to the literature.

Computer vision cataract screening equipment: technical specifications

Difficulties in obtaining a diagnosis and treatment and financial and logistical problems are identified as the leading causes of the high prevalence of blindness due to cataracts worldwide. With the increasing availability of portable fundus cameras and slit-lamp adapters for smartphones, cataract diagnosis can be enhanced in remote regions at a low cost using easy-to-handle devices. The aim of this article is to describe a patent application for a device created to capture lens images and classify cataracts automatically based on computer vision methods. The results are displayed after rapid calculations using numerous cataract measurement-based parameters, such as average color, standard color deviation, and comparison of color histograms. Future clinical validation studies will be performed based on this description.

TREND database: Retinal images of healthy young subjects visualized by a portable digital non-mydriatic fundus camera.

Topological characterization of the Retinal microvascular nEtwork visualized by portable fuNDus camera (TREND) is a database comprising of 72 color digital retinal images collected from the students of the Faculty of Medicine at the University of Montenegro, in the period from February 18th to March 11th 2020. The database also includes binarized images of manually segmented microvascular networks associated with each raw image. The participant demographic characteristics, health status, and social habits information such as age, sex, body mass index, smoking history, alcohol use, as well as previous medical history was collected. As proof of the concept, a smaller set of 10 color digital fundus images from healthy older participants is also included. Comparison of the microvascular parameters of these two sets of images demonstrate that digital fundus images recorded with a hand-held portable camera are able to capture the changes in patterns of microvascular network associated with aging. The raw images from the TREND database provide a standard that defines normal retinal anatomy and microvascular network geometry in young healthy people in Montenegro as it is seen with the digital hand-held portable non-mydriatic MiiS HORUS Scope DEC 200.This knowledge could facilitate the application of this technology at the primary level of health care for large scale telematic screening for complications of chronic diseases, such as hypertensive and diabetic retinopathy. In addition, it could aid in the development of new methods for early detection of age-related changes in the retina, systemic chronic diseases, as well as eye-specific diseases. The associated manually segmented images of the microvascular networks provide the standard that can be used for development of automatic software for image quality assessment, segmentation of microvascular network, and for computer-aided detection of pathological changes in retina. The TREND database is freely available at https://doi.org/10.5281/zenodo.4521043.

The Role of Telemedicine, In-Home Testing and Artificial Intelligence to Alleviate an Increasingly Burdened Healthcare System: Diabetic Retinopathy.

In the presence of the ever-increasing incidence of diabetes mellitus (DM), the prevalence of diabetic eye disease (DED) is also growing. Despite many improvements in diabetic care, DM remains a leading cause of visual impairment in working-age patients. So far, prevention has been the best way to protect vision. The sooner we diagnose DED, the more effective the treatment is. Thus, diabetic retinopathy (DR) screening, especially with imaging techniques, is a method of choice for vision protection. To alleviate the burden of diabetic patients who need ophthalmic care, telemedicine and in-home testing are used, supported by artificial intelligence (AI) algorithms. This is why we decided to evaluate current image teleophthalmology methods used for DR screening. We searched the PubMed platform for papers published over the last 5 years (2015–2020) using the following key words: telemedicine in diabetic retinopathy screening, diabetic retinopathy screening, automated diabetic retinopathy screening, artificial intelligence in diabetic retinopathy screening, smartphone diabetic retinopathy testing. We have included 118 original articles meeting the above criteria, discussing imaging diabetic retinopathy screening methods. We have found that fundus cameras, stable or mobile, are most commonly used for retinal photography, with portable fundus cameras also relatively common. Other possibilities involve the use of ultra-wide-field (UWF) imaging and even optical coherence tomography (OCT) devices for DR screening. Also, the role of smartphones is increasingly recognized in the field. Retinal fundus images are assessed by humans instantly or remotely, while AI algorithms seem to be useful tools facilitating retinal image assessment. The common use of smartphones and availability of relatively cheap, easy-to-use adapters for retinal photographs augmented by AI algorithms make it possible for eye fundus photographs to be taken by non-specialists and in non-medical setting. This opens the way for in-home testing conducted on a much larger scale in the future. In conclusion, based on current DR screening techniques, we can suggest that the future practice of eye care specialists will be widely supported by AI algorithms, and this way will be more effective.

Improvised Fundus Camera

Through the years, examination of fundus of the eye has come a long way. Technology has played a major part in the development of fundus imaging from direct and indirect ophthalmoscopy up to portable fundus cameras. However, the high cost of fundus cameras in the market creates an impediment in acquiring and utilizing such. The aim of this research was to improvise a cheaper yet fully functional fundus camera. A quantitative quasi-experimental type of research to rate the created prototype by a total of 23 respondents was applied. The device was rated according to functionality, design, ease of use, and cost, using numerical ratings from 4 to 1, with 4 being the highest and 1 the lowest. Results: Data revealed that the overall rating for the device was 4 or excellent. The “cost” achieved the highest, while the “design” had the lowest ratings respectively. Conclusion: The study shows that the created prototype can efficiently function as fundus camera with much cheaper cost of production than fundus cameras available in the market.

Wide field of view handheld smart fundus camera for telemedicine applications.

Purpose: We report that a large field-of-view (FOV) retinal image can be acquired by a smart fundus camera. Approach: This handheld system consists of a Raspberry Pi board, a touch screen display, a customized optical lens group, a ring light-emitting diode, and a Li-battery. Results: Wide FOV of is observed with proper lens configuration and can be expanded using image stitching algorithms. Conclusions: This customized handheld fundus camera provides better image quality than cellphone-based fundus imaging solutions and offers more operational features than traditional portable fundus cameras. It may benefit field-portable ophthalmic diagnostic applications.

Validation of a portable, non-mydriatic fundus camera compared to gold standard dilated fundus examination using slit lamp biomicroscopy for assessing the optic disc for glaucoma.

PurposeTo evaluate the sensitivity and specificity of a portable non-mydriatic fundus camera to assess the optic disc for glaucoma.MethodsWe conducted a single-site, cross-sectional, observational, instrument validation study. Non-mydriatic fundus photographs centred at the optic disc were obtained from 276 eyes of 68 glaucoma and 70 normal patients, using a portable fundus camera (Smartscope, Optomed, Oulu, Finland). A senior Glaucoma consultant, masked to the patient’s study participation, performed a gold standard dilated fundus examination to make the diagnosis of glaucoma. Following this, a mydriatic photograph was taken by a standard table-top fundus camera. All the images were digitalized and de-identified by an independent investigator and presented to two remote graders, masked to the patients, their diagnoses, and photographic modality. Based on individual disc characteristics, a diagnosis of screening positive or negative for glaucoma was made. In the end, the independent investigator re-identified the images. Sensitivity and specificity to detect glaucoma with the undilated Smartscope camera was calculated compared to dilated fundus examination.ResultsGrading remote images taken with the portable non-mydriatic fundus camera showed a sensitivity of 96.3% (95% confidence interval (CI): 91.6–98.8%) and 94.8% (95% CI: 89.7–97.9%) and a specificity of 98.5% (95% CI: 94.9–99.8%) and 97.8% (95% CI: 93.9–99.6%) for the two graders respectively as compared to gold standard dilated fundus examination.ConclusionThe non-mydriatic Smartscope fundus images have high sensitivity and specificity for diagnosing glaucoma remotely and thus may be an effective tool for use in community outreach programs.

Comparison of Angiography Performance of Low-cost portable Fundus camera with standard FFA camera – Validation Study

Study Design: In an observational cross-sectional study, 91 eyes of 46 consecutive patients underwent simultaneous Fundus Fluorescein Angiography (FFA) for common retinal disorders. The FFA images captured by both cameras were graded by 3 Retinologists (DVS, GM, RB) in a structured format. 7 characteristics of FFA images that were used for grading were Foveal Avascular Zone (FAZ) visualization, identifying branch retinal vein, Identification of any abnormal hyper fluorescence at macula, Identification of Micro aneurysms, Identification of capillary non perfusion areas and identification of leakage from retinal neovascularization, Image quality and Impact on Clinical decision. Results were captured and statistically analyzed for interobserver and intercamera agreements.

A Unique Experience of Retinal Diseases Screening in Nepal.

BackgroundRecently, the number of retinal patients is increasing in Nepal. The retinal problems are the second common cause of blindness in Nepal. Retinal diseases are challenging to manage due to the lack of simple and cost-effective screening methods, limited human resources, poverty, and lack of awareness in Nepal. Hospital for Children, Eye, ENT & Rehabilitation Services (CHEERS), Nepal conducted retina screening camps and education in communities through Retina Eye Care of Nepal project (RECON) in the years 2017 and 2018.Materials and MethodsWe screened retinal patients with an indirect ophthalmoscope coupled with plus 20 Dioptre lens and portable fundus camera. We referred the patients needing interventions to the base hospital (CHEERS) using a screening algorithm, where they received retinal laser and surgery. Besides, we also provided on-site educational programs for retinal disease awareness to the community.ResultsRetinal problems found from two static outreach clinics (SORC) and a day screening and treatment service (DSTS) were 18.3%, 20%, and 20%, respectively, while from a remote rural health camp (RRHC) was 2.61%. Likewise, patients referred to CHEERS for retinal laser or other retinal interventions were 15%, 16.6%, 16%, and 1.96% from 2 SORCs, 1 DSTS, and 1 RRHC, respectively.ConclusionRetina camp is a cost-effective approach to early detection and referral for retinal-related vision impairment patients. It is also one opportunity to educate the community. The purpose of the project was to provide retina care to the community who otherwise could not afford it. We recommend a similar screening model on a large scale in a low-resource setting for the maximum benefits to the rural community.

Evaluation of an AI system for the detection of diabetic retinopathy from images captured with a handheld portable fundus camera: the MAILOR AI study.

To evaluate the performance of an artificial intelligence (AI) system (Pegasus, Visulytix Ltd., UK*) at the detection of diabetic retinopathy (DR) from images captured by a handheld portable fundus camera. A cohort of 6404 patients (~80% with diabetes mellitus) was screened for retinal diseases using a handheld portable fundus camera (Pictor Plus, Volk Optical Inc., USA) at the Mexican Advanced Imaging Laboratory for Ocular Research. The images were graded for DR by specialists according to the Scottish DR grading scheme. The performance of the AI system was evaluated, retrospectively, in assessing referable DR (RDR) and proliferative DR (PDR) and compared with the performance on a publicly available desktop camera benchmark dataset. For RDR detection, Pegasus performed with an 89.4% (95% CI: 88.0-90.7) area under the receiver operating characteristic (AUROC) curve for the MAILOR cohort, compared with an AUROC of 98.5% (95% CI: 97.8-99.2) on the benchmark dataset. This difference was statistically significant. Moreover, no statistically significant difference was found in performance for PDR detection with Pegasus achieving an AUROC of 94.3% (95% CI: 91.0-96.9) on the MAILOR cohort and 92.2% (95% CI: 89.4-94.8) on the benchmark dataset. Pegasus showed good transferability for the detection of PDR from a curated desktop fundus camera dataset to real-world clinical practice with a handheld portable fundus camera. However, there was a substantial, and statistically significant, decrease in the diagnostic performance for RDR when using the handheld device.

Prevalence of Posterior Segment Disease in Subjects Screened for Cataract- A Retrospective Study

Introduction: Glaucoma, retinal vein occlusion and diabetic retinopathy are the common posterior segment diseases affecting the eye and need to be specifically looked for in the community by Intra Ocular Pressure (IOP) measurement and a dilated fundus examination. Cataract screening alone in eye camps might not be adequate to prevent and treat these causes of blindness. Aim: To establish the need for routine screening of posterior segment diseases at primary and secondary eye care level by estimating the prevalence of this diseases in patients presenting with cataract in order to prevent avoidable blindness. Materials and Methods: This was a retrospective observational study conducted at Tertiary Care Hospital comprising of 1109 subjects who were screened for cataract and underwent cataract surgery, at screening camps from semi urban and rural localities in and around Hyderabad city between October 2018-March 2019. All subjects underwent detailed ophthalmic evaluation including visual acuity assessment, applanation tonometry, slit lamp examination and dilated fundus examination. Prevalence of blindness and moderate to severe visual impairment caused by posterior segment disease among these patients was analysed. Percentage distribution of the data was calculated mathematically. Results: Of the 1109 subjects, posterior segment diseases causing blindness was seen in 0.63%, severe visual impairment in 0.99% and moderate visual impairment in 3.78%. Glaucomatous optic neuropathy was the most common posterior segment disease followed by retinal vein occlusion and diabetic retinopathy in that order. Diabetes Mellitus (DM) and Hypertension (HTN) were associated with increased risk of glaucoma. Conclusion: Routine screening for posterior segment disease is recommended with the use of portable fundus cameras in the elderly population along with IOP measurement using hand held portable tonometer in screening camps in order to reduce the burden of avoidable blindness. Monitoring of life style diseases like HTN and DM at the time of cataract screening is needed for primary prevention of most of these diseases of the eye.

Near-infrared light-guided miniaturized indirect ophthalmoscopy for nonmydriatic wide-field fundus photography.

A portable fundus imager is essential for emerging telemedicine screening and point-of-care examination of eye diseases. However, existing portable fundus cameras have limited field of view (FOV) and frequently require pupillary dilation. We report here a miniaturized indirect ophthalmoscopy-based nonmydriatic fundus camera with a snapshot FOV up to 67° external angle, which corresponds to a 101° eye angle. The wide-field fundus camera consists of a near-infrared light source (LS) for retinal guidance and a white LS for color retinal imaging. By incorporating digital image registration and glare elimination methods, a dual-image acquisition approach was used to achieve reflection artifact-free fundus photography.

便携式免散瞳眼底相机光学系统设计

便携式免散瞳眼底相机光学系统设计