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					                            DEVELOPMENT OF A DIGITAL FUNDUS CAMERA

                                  Rodrigo Modugno 1, Ivan Nunes da Silva 2, Daniel dos Santos Jr 1
                                    Opto Eletrônica S/A, São Carlos, SP, Brazil,
                                  Universidade de São Paulo, São Carlos, SP, Brazil,

Abstract: The aim of this work is to present the project of a            coupled devices, known as CCD. The eye fundus image is
digital fundus camera, which was completely developed,                   projected over the sensor, causing each capacitor to
manufactured and tested. It describes each part of the                   accumulate an electric charge proportional to the light
equipment development, such as the optical, mechanical,                  intensity of each region of the image. As the price of
electronic and software projects. This work shows the                    photographic films, and theirs development, rapidly
feasibility of the development of high technology                        increases year after year, the use of this type of equipment
equipments in Brazil.                                                    became costly. Digital systems have higher initial costs, but,
                                                                         though the initial investment, theirs operational costs are
Keywords: Fundus camera, retina, digital.                                minimum, once no development or film are necessary. Due
                                                                         to this trend, digital systems are becoming more and more
                                                                         attractive to physicians and practices. Another advantage is
1. INTRODUCTION                                                          that the images shot can be promptly seen on the computer
                                                                         screen, resulting in a significant reduction of the exam,
    Fundus cameras, or retinal cameras, as its name                      diagnosis and treatment chain period. With digital systems,
suggests, are equipments used to take pictures of the eye                diagnosis and treatment can be done at the same day.
fundus (retina) and of its structures, such as macula, fovea                 Unlike the great majority of the equipments currently
and optical nerve head. Those structures, when properly                  being manufactured, where adaptations are made to couple
analyzed, provide means of diagnosis to a series of                      off-the-shelf digital cameras to an existing analog fundus
pathologies, which some of them not necessarily being                    camera, the equipment proposed in this article was
related to the vision itself.                                            completely developed with the goal of being 100% digital.
    The fundus camera here described consists of a capture               Thus, its optics, mechanics and software projects were
system based on the reverse Newtonian telescope, coupled                 optimized for a strong integration between all of those parts.
to a coaxial illumination system [1]. In this same optical                   Generally, fundus cameras can be used to capture two
system, spectral barrier and excitation filters are placed on            different types of images: retinographies and angiograms.
the capture and illumination optical paths, respectively.                    Color retinographies are photos taken using the visible
Optically coupled to the reverse telescope, there is a capture           spectrum1. Those images are usually used for comparison of
device, which can be an analog camera or, as currently                   previous exams. Red free photos are taken with green
being used and proposed on this article, a digital camera or             colored filter, enhancing retina vessels.
digital sensor. Figure 1 shows the sketch of a digital fundus                Angiograms are used for examining the circulation of the
camera.                                                                  vascular structures of the eye using the dye tracing method.
                                                                         Angriograms can be done using sodium fluorescein (FA),
                                                                         widely used for retinal diseases, or indocyanine green
                                                                         (ICG), used for choroidal diseases. To perform those exams,
                                                                         band pass spectral filters are required. They can be of two
                                                                         types: excitation, used for the illumination, exciting the dye
                                                                         causing it to emit light; and barrier, which will block the
                                                                         wavelengths different from the dye emission which we want
                                                                         to capture. Figure 2, on the following page, shows samples
                                                                         of each type of the above mentioned procedures.

         Fig. 1. Sketch of a coaxial digital fundus camera.
                                                                         1. The visible spectrum is the portion of the electromagnetic spectrum that
   Digital capture systems use, replacing the regular                    is visible to the human eye, ranges from 380 to 750nm.
photographic film, photosensitive sensors, usually charge

                                                                                                       Development of a Digital Fundus Camera
                                                                                     Rodrigo Modugno, Ivan Nunes da Silva, Daniel dos Santos Jr

                                                                              2.   PROPOSED SYSTEM

                                                                                   2.1. Mechanical System
                                                                                  The equipment’s mechanical project has two main
                                                                              objectives: offer the patient and the user conditions to
                                                                              perform a safe, precise, fast and comfortable exam and
                                                                              provide means to support the elements of the optical system,
                                                                              including the image sensor. This last feature requires a high
                                                                              level of precision, as, according to [1], the image quality
                                                                              would decrease significantly if the distances between optical
                                                                              elements differ from what was designed. In order to get an
                                                                              idea of the equipment’s dimensions, Figure 4 shows its
                                                                              external dimensions (in millimeters).

Fig. 2. Clock-wise starting from top-left: color retinography; red free
retinography; indocyanine Green angiography; sodium fluorescein

    The aim of this work is to enumerate the difficulties, and
respective solutions, found during the development of
optoelectromechanic equipment based on the Brazilian
economical situation. It is important to emphasize that the
proposed equipment is the first, and unique, that was
completely developed and manufactured in Brazil. The
solutions presented resulted in a high grade product, which
can compete with products of its class that are currently                              Fig. 4. Opto ADS external dimension (millimeters).
being developed or manufactured by companies around the
world. Besides that, encourage the Brazilian industry and                         To capture the images, the equipment shall be positioned
awake its interest in developing high technological level                     by the operator in a way that the illumination ring, which
equipments, showing, through the following results, that it is                will be described on Subsection 2.3, is placed on the
possible to develop, manufacture and sell this type of                        patient’s pupil. For that reason, the equipment must have
product in Brazil.                                                            mechanisms that provide movements on the three axes (X,
    Also justify this work the attempt to reduce the cost of                  Y and Z) and rotation on its own axis (θ). To that task, a
the equipment, making this exam, up to this moment, an                        mechanism composed of axial and radial bearings and ball
exclusivity of hospitals and particular practices of the                      screws was developed. Those devices provide a smooth
country, accessible to everyone.                                              movement, avoiding any kind of effort by the user. The
    This project was accomplished by a multidisciplinary                      equipment is positioned through a joystick, where the photos
team, being here described, in general way, each one of the                   can also be triggered. Figure 5 shows the CAD model of the
different executed tasks. For a better understanding, the                     developed mechanism.
project was split in three main parts: electro-mechanic
project, software project and optical project.
    Figure 3 shows the first prototype of Opto ADS during
clinical trials.

                                                                                              Fig. 5. Opto ADS positioning base .

                                                                                   Unlike the currently equipments available, the spectral
                                                                              filters of the Opto ADS are automatically positioned through
                                                                              brushless DC servomotors. Those filters, which are set on a
                                                                              filter wheel, are placed before the illumination system, and
                                                                              after the capture system.
           Fig. 3. Opto ADS prototype during clinical trial.

     The filters are positioned through a hybrid system that                        diopters. The developed mechanism can be seen on Figure
uses a rotary potentiometer for the crude adjustment, that is,                      8.
just the expected region on which the filter shall be. For the
fine positioning, micro switches are used, ensuring that the
filters are properly positioned. Figure 6 shows the developed

                                                                                        Fig. 8. CAD model detailing the focus adjustment mechanism.

                                                                                        This mechanism shall be smooth, at the same time
                                                                                    precise, as the quality of the captured image is determined,
                                                                                    besides the optical performance, by the focus adjustment
                (a)                                     (b)                         precision.
 Fig. 6. Spectral filters motorized filter wheel: (a) excitation filters; (b)           The mechanical project was realized with the help of the
                              barrier filters.                                      CAD development software Solid Works. As for the
                                                                                    mechanical system, we’ll see on the other used development
    The mechanical devices used to support the optical                              tools, that their choice was due to previous experience of the
elements had to be manufactured under a rigid quality                               development team, technical aspects and, sometimes,
control. This was necessary due to the fact that the proposed                       because of its wide use on the academic area.
optical capture system has a high performance and the
distance between those optical elements are crucial for the                             2.2. Electronic System
resulting image. Simulations were realized and the tolerance                            The Opto ADS electronic project provides an interface
for each of the mechanical parts had different values                               between the equipment’s capture unit, where the optical
according to the optical requirements.                                              systems and spectral filters are placed, and its
                                                                                    microcomputer. Besides that, the electronic system controls
                                                                                    the spectral filters motorized wheel, illumination power
                                                                                    supply and monitors the user’s interventions through the
                                                                                    control panel located at the top of the equipment’s moving
                                                                                        The control board can be considered the main part of the
                                                                                    capture unit, executing all of the tasks mentioned above.
                                                                                    Figure 9 presents the board block diagram. Some of the
                                                                                    blocks below will not be described, as theirs functions are
                                                                                    simple or not relevant.

  Fig. 7. CAD model detailing the optical elements of the capture and
                        illumination systems.

    Figure 7, above, shows the Opto ADS optical elements
and how they are fixed to the equipment structure.
    Due to singularities on each patient’s eyes, was made
necessary the development of a focus adjustment
mechanism in order to compensate the diopters2 that
different patients may have. This system modulates the
distance between the last lens of the capture system and the
                                                                                                     Fig. 9. Control board block diagram.
image sensor. The focus mechanism was designed to cover
the range of -20 to +20 diopters, which guarantee that the                          2. A measurement of refractive error. Hyperopia is measured in terms of
equipment can be used with the great majority of patients as                        positive diopters. Myopia is measured in terms of negative diopters.
the common refractive errors range between +6 to -6

                                                                                                   Development of a Digital Fundus Camera
                                                                                 Rodrigo Modugno, Ivan Nunes da Silva, Daniel dos Santos Jr

    The communication between the embedded electronic                        Among the established requirements for the optical
and the Opto ADS software, running on the microcomputer,                 system, the most important need was that it should be
is done through a CAN3 interface. This interface was chosen              coaxial, that is, the capture system is part of the illumination
because of its roughness, wide availability of specialized               system optical path. This choice was due to the fact that the
literature, previous knowledge by most of the developers                 homogeneity of the projected illumination on the retina is
and by the wide range of microcontrollers with integrated                far better when this type of architecture is used. This higher
CAN interfaces.                                                          homogeneity results in good quality images, which is the
    It’s through this interface that the filters are selected, the       aim of this optical project. It was also established that the
illumination power is controlled and messages regarding the              field of view (FOV) of the equipment should be of 45°, as
equipment’s conditions are received. This communication                  this would be an intermediate value of the available
interface has also some secondary functions like software                equipments, which range from 30º to 60º, according to its
version query. The used communication baud rate, 125kb/s,                application.
is far below CAN’s limits which could reach up to 1 Mb/s.                    Even possessing the technical know-how to develop and
    The illumination control is done through a closed loop               manufacture aspheric lenses4, it was decided that only
system, and its power level is set using one of the                      spherical lenses, whose surface profile is based on a sphere,
microcontroller’s PWM output ports. Its feedback, based on               would be used. This decision was based on the possibility of
the measurement of the halogen lamp current, is received                 reaching the necessary optical performance using only this
using one of the many analog inputs available. This control              type of lenses, which are less expensive, increasing the
system, even though simple, was developed in a closed loop               competitiveness of the equipment in the international and
to ensure that eventual faults on the illumination control               national markets.
would not expose the patient to unnecessary light exposure.                  To the development of the equipment’s optical project,
This control is optocoupled, electrically insulating the high            the Code V software was used. Developed by Optical
power circuitry, lamp control and power supply, from the                 Researches Associates (ORA), it holds all of the necessary
microcontrolled circuitry.                                               tools for the development of projects based on geometrical
    For the spectral filter positioning, a hybrid system, as             and physics optics.
described on the mechanical project, was developed. The                      To get deeper on the optical subject, wel’ll subdivide the
control of those filter wheels is made through a PWM,                    optical project in two parts: illumination optical system and
which provides an acceleration during the change of filters              capture optical system.
and a deceleration when the filter position is about to be
reached and the micro switch shall be constantly verified by                  2.3.1. Illumination optical system
the microcontroller to ensure a precise positioning. The                     Unlike the capture optical system, the illumination optics
value of the rotary potentiometer conditioned signal is read             must have low resolution, avoiding the lamp filament, and
using an analog input, while the micro switch is monitored               the illumination non-uniformity, to be projected over the
using a general purpose I/O port. This hybrid system                     retina. The illumination system must project a light ring in
provides a fast filter change, which is very important during            order to get the light inside through the patient’s pupil. Once
an exam, as during the infusion of the dye the operator may              inside, the light ring spreads creating an illumination spot all
have to change from one pair of filters to another.                      over the eye fundus.
    The Opto ADS electronic project was developed using                      In the Opto ADS, the light source used is a 21 Volts and
the Altium 2004 software, developed by the same name                     150 Watts halogen lamp, whose power is supplied by a
company. This software allows the development of printed                 variable switched power supply. This light was coupled to
circuit boards, known as PCBs and electronic schematics. It              an optical fiber bundle, where, in one of its side, the fibers
is also possible to simulate most of the circuitry. This tool            are arranged in the shape of a ring. The optical fiber based
reduced in a significant way the development period of the               illumination allowed the halogen lamp to be placed outside
equipment’s electronic system.                                           the equipments optical head, avoiding the great heat flow
    It is important to emphasize that the imaging module,                released by the lamp. This heat flow, once released inside
which includes the CCD sensor and its electronic                         the equipment, would decrease the capture optical system
conditioning and communication, was not approached on                    performance, as it is sensitive to temperature changes and
this chapter. This module is provided by a third part;                   gradients. Figure 10, on the following page, shows the
however, the development of an image module by Opto                      complete illumination system.
Eletrônica S/A is among the next steps of the project.

   2.3. Optical System
    The quality of captured eye fundus images using the
Opto ADS are intimately related to the quality of its optical
system. The system must have high resolution, displaying
with sharpness even the tiniest vascular structures of the
retina. Due to the different spectral bands used during the              ________________
exams, it is necessary that the system resolution is high for            3.   Controller Area Network. Was officially announced in 1986 by Robert
                                                                              Bosch GmbH and was initially developed to be used on automotive
the spectral range that includes the blue and the near                        applications.
infrared (400 to 850nm), being required, therefore, a                    4.   An aspheric lens is a lens whose surfaces have a profile that is neither
complex optical system with fairly high performance.                          a portion of a sphere nor of a circular cylinder.

                                                                         Those spectral filters guarantee the quality of the
                                                                     captured images, especially when referring to angiograms.
                                                                     Infrared and UV lights could decrease image contrast, thus,
                                                                     reducing image sharpness.
                                                                         Table 1 details the bandwidth of each spectral filter pair
                                                                     used and the respective exam where they are used.

                                                                          Table 1. Excitation and barrier filters and their bandwidths.

                                                                                                            Bandwidth (nm)
                                                                           Exam type
                                                                                                   Excitation               Barrier

         Fig. 10. Complete illumination system schematic.               Color retinography         400 to 700               not used
                                                                      Red Free Retinography         500 to 580              not used
   2.3.2. Capture optical system                                       Sodium Fluorescein
                                                                                                    400 to 480             500 to 580
    As previously described, the equipment optics was
developed for a field of view (FOV) of 45°. Generally,                  Indocyanine Green
                                                                                                    730 to 800             820 to 930
equipments used for the diagnosis of glaucoma have a                      Angiography
narrow field of view, like 30º, while equipments used for
pathologies like retinal detachment have a wider field of               We can see on the table above that the bandwidth of
view (60º). Based on that information, the developed                 excitation and barriers filters used on angiograms have a gap
equipment has an intermediate field of view, allowing it to          between their band pass. Crosses between those filters
be used for the diagnosis of broader types of pathologies.           would result in image glares and loss of contrast.
    As a coaxial optical system, where illumination and
capture share part of their optical path, the optimization had          2.4. High Level Application Software
to be done to the complete optical system. It was necessary              The Opto ADS software was defined as being the
as changes to capture system could decrease the illumination         interface between the user, usually a physician, and the
system performance. This optimization procedure was                  optical and electronic systems. Besides that, the software
described in [1].                                                    provides a friendly interface to data storage, exam preview
    The capture system consists of the following lenses              and printing and image processing. For supporting the
assemblies: main objective, afocal and auxiliary objective.          equipments assembly and maintenance, the software also
The total number of optical elements necessary to project            provides information regarding its operational condition and
the retina image over the image sensor is 17. Figure 11              alignment aid features.
shows the complete capture system.                                       In order to handle and display all of the captured images,
                                                                     the software has a clear and simple interface, allowing users
                                                                     that are not familiar with microcomputers to operate the
                                                                     equipment and its software without difficulty. This intuitive
                                                                     interface has the aim of making the migration of users used
                                                                     to analog fundus cameras an easy task, and almost not time
                                                                         It was also a software requisite the storage of the
                                                                     patients’ information on a database. Searching tools, by
                                                                     name or code, are also available. The images can also be
           Fig. 11. Complete Capture system schematic                stored on secure removable medias, like DVDs or CD, as
                                                                     eventual damages to the microcomputer could lead to a
   The optical system performance will be quantitatively             complete data loss.
evaluated on Subsection 3.1.                                             The Opto ADS software uses a series of Microsoft
                                                                     Windows and third part drivers. For that reason, during its
   2.3.3. Excitation and Barrier spectral filters                    initialization, all of the necessary drivers are initialized and
   When an exam is selected by the user, a pair of filters is        verified, the communication interface with the capture unit
automatically set by the equipment. From this pair, the              is tested and the files and folders necessary for the properly
excitation filter is placed between the optical fiber bundle         software execution have their integrity checked. If any
output and the illumination optical system entrance. The             failure occur during those initial tests the software returns an
barrier filter is placed between the afocal and the auxiliary        error flag and a message box is displayed with the
objective.                                                           description of the possible solution.
                                                                         The Opto ADS software is based in one main screen,
                                                                     where all of the software functions are accessible. As its

                                                                                             Development of a Digital Fundus Camera
                                                                           Rodrigo Modugno, Ivan Nunes da Silva, Daniel dos Santos Jr

name suggests, it’s in the main screen where the user can           made at least once a week, no large amount of data will be
create a new patient, add a new physician, start a new exam         lost, even in a hard disk fatal error.
or review a previous one, print results and execute many                Figure 13 shows, through the entity-relationship model
other functions. When an exam is open, diagnosis tools are          (ERM), the relational database used on the Opto ADS
available, such as image’s overlay, area calculation and            software.
image processing functions. It is also through the main
screen that the user can access backup tools, in order to
preserve data integrity and free computer’s hard disk space.
    Figure 11 shows the software main screen displaying an
open exam. The thumbnails are the captured images or
images of a previous exam.

                                                                                Fig. 13. Opto ADS Entity-Relationship model

                                                                    3. RESULTS

                                                                         3.1. Optical System Performance
                                                                        The optical system analysis was only performed in the
                                                                    capture optical system. As described on Subsection 2.3.1,
                                                                    the illumination system was designed for a low resolution;
                                                                    therefore, this type of analysis was not necessary.
                                                                        After being manufactured and its optical system aligned,
                                                                    the performance analysis was divided in two phases: capture
                                                                    optical system analysis, which includes only optical
             Fig. 12. Opto ADS software main screen                 elements and complete capture optical system analysis,
                                                                    which also includes the sensors and electronic devices. The
    The Opto ADS control software was developed using               optical system performance analysis was quantitatively
C++ programming language. For the software coding, the              measured using an interferometer. This article, however,
Borland C++ Builder visual development environment was              addresses only to the analysis of the complete optical
chosen as it creates excellent visual aspect applications for       system, including CCD, performance.
the Microsoft Windows operational system.                               The complete optical system performance, including the
    The C++ programming language was chosen for being               CCD image sensor, was performed through the analysis of
widespread in the academic area, reducing the training              the modulation transfer function, or MTF. The MTF
period of the newly hired engineers.                                measurement can be defined as the magnitude response of
    At the present time, the fast development of new digital        the optical system to sinusoids of different spatial
fundus cameras required the development of the Opto ADS             frequencies.
to be done, necessarily, in the shortest possible period of             Among the various techniques available for the
time. Consequently, the same had to be done with the                measurement of the optical system’s MTF, the step
software project and coding period. This fact, associated to        response, also known as knife-edge or edge spread function
the small group of developers available at the company, led         (ESF), is actually the most used method [3]. Many reasons
to the use of agile methodologies. Those agile                      can be listed for the wide use of the step response for MTF
methodologies allow the development of high quality                 measurements. The first reason for its utilization is the low
software, comparable to traditional methodologies, in short         complexity level required to manufacture the target, as its
term. Similar methodology used during the development of            dimensions are not critical, like the reduced diameter used
the Opto ADS was applied to the software development of             by the point spread function (PSF) technique, or the slit
another equipment at Opto Eletrônica, the MUX-GSE, and              width used by the line spread function (LSF). Besides that,
was described in [2].                                               compared to the previous mentioned techniques, the edge
                                                                    spread function presents a much superior flux of energy,
   2.4.1. Database and data storage
                                                                    making its detection through CCD sensor more convenient.
    Data relative to the exams, like patient name, exam code,       One of the ESF disadvantage is that we have derivatives
physician name, diagnosis, images and video files shall be          during its calculus, therefore, enhancing the existing image
stored in a secure way. To store those information safely, a        noise [4]. For the Opto ADS, this problem was solved using
relational database was created and access tools, based on          the mean value of a series of images and the use of a
SQL5, created. To ensure that the capture images and videos         window on the spatial differentiation result, that is, just the
would not be deleted or lost due to software or hardware            transition area is used.
crashes, those files can be stored in removable medias (CDs             The system resolution was designed to resolve vessels
and DVDs). Every time a backup is performed, a complete             down to 0.01mm width, that is, 50 line pairs per
copy of the database is stored on the removable media. This         ________________
way, if properly used, which means that backups are being           5.   Structured Query Language. A standard command language that
                                                                         interacts and performs tasks with relational databases

millimeter with an response above 0.2. For the                        capture system. It is also on the company plans the
manufactured prototypes, the system response for the 50               development of a LED based illumination system.
lp/mm frequency was of approximately 0.25, therefore,
complying with the optical project specifications.                    ACKNOWLEDGMENTS
    The system MTF result was obtained through software
developed by Opto’s team and was based on the algorithm               The authors would like to thank Opto Eletrônica S/A, and its
proposed in [5]. The software validation was done using               managers, for making this project possible and allowing to
lenses with known MTF values. The measured values using               turn most of it into public domain. The author would also
those lenses were compared to the software’s results. The             like to thank every member of the development team that,
measurement error was lower than 5%. Figure 14 shows a                through hard work, made this project become a product.
typical Opto ADS MTF curve.

                                                                      [1] E. G. Carvalho. “Desenvolvimento de um Sistema Óptico para
                                                                          Retinografia e Angiografia Digital”. 83 pp. Dissertação
                                                                          (Mestrado em Física) – Instituto de Física de São
                                                                          Carlos, São Carlos, 2006.
                                                                      [2] D. Santos Júnior, “Implementação de Processo de Software
                                                                          Para Teste de Equipamentos Aeroespaciais”. 180 pp.
                                                                          Dissertação (Mestrado em Engenharia Elétrica) –
                                                                          Departamento de Engenharia Elétrica, Escola de Engenharia
                                                                          de São Carlos, Universidade de São Paulo, São Carlos, 2007.
                                                                      [3] E. Samei et al. “Comparison of edge analysis techniques for
                                                                          the determination of the MTF of digital radiographic system”.
                                                                          Phys. Med. Biol. Vol. 50, pp3613-3625, 2005.
     Fig. 14. Modulation transfer function evaluation software.       [4] G. D. Boreman, “Modulation Transfer Function in Optical and
                                                                          Electro-Optical Systems”, 110 pp, SPIE Press, 2001.
   3.1. Clinical try-out                                              [5] Optikos Comporation, “How to Measure MTF and other
                                                                          Properties of Lenses”. 64 pp, Cambridge, MA, 1999.
    Once the development was finished, three prototypes
were manufactured and allocated on practices for clinical
try-out. During this period, information regarding to its ease
of use, software features, roughness and, most important, its
image quality were collected. Retina specialists, users,
patients and maintenance personal were heard. Those tests
led to changes that resulted in the project described in this
           After the project validation, the equipment has
become the most complex medical device manufactured
within the company’s portfolio.

    Through the results presented in Section 3, we can
conclude that the project has achieved its performance
requirements. Moreover, a total of hundred and fifty
equipments were sold in Brazil and in other countries, which
confirm the good acceptance by users, practices and
    Due to the vast competitiveness in the medical
equipment market, added to its high level of demand, this
equipment will have a continuous development policy,
keeping it up to date with the market trends. Among the next
steps, there are the development of a high-resolution system,
using sensors over 10 mega pixels, and a non-mydriatic


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