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Fundus Camera project

punkt Fundus Camera project

2017-01-20

Fundus Camera is an instrument used to examine human ocular fundus and thereby prevent further defects and diseases of retina by taking photo in vivo. Due to position of retina there is no way to have separated illumination and imaging path - they have to coincide at least just before examined eye and illuminating and imaging is simultaneously. It implicates few technical difficulties such as splitting/joining beams and back reflections. Retina’s reflection characteristic is very low so it is important to provide system that minimize back reflections coming from optical elements. Even good AR coatings do not ensure lower back reflections than signal reflected from retina.

The aim of the project was to design optical system for fundus camera and optimize it in terms of back reflections. From two designs of illumination path - internal and external, the first one was chosen. The difference is in position of the objective - in chosen method objective is mutual for both paths - illumination and imaging. It is more difficult to optimize due to more optical surfaces and thus bigger back reflections. But in the same time it cause much bigger illumination efficiency and allows using smaller lens diameters with larger field of view. Usually to minimize a number of elements in optical system (that compensate aberrations) aspherical lens is used. Our solution assume only spherical surfaces.

 

Systems with both external and internal illumination have their merits and drawbacks, however in this work the latter has been selected due to its increased illumination efficiency and possibility to achieve large field of view using smaller elements than in the former case. In internal illumination systems the common objective is shared by both illumination and imaging path. Both of them are coupled by a beam splitter or a mirror with a hole. Due to the fact that only a small fraction of light reaching retina is reflected, back reflections from all the surfaces of the objective may have greater power and make it impossible to obtain useful image. Very high quality antireflective coatings in many cases do not suffice to properly eliminate ghost images. Several features had been incorporated in the past into the design of the fundus camera system with internal illumination to reduce the problem of back reflections:

  1. minimizing the number of surfaces common for illumination and imaging path, hence single-lens objectives with aspherical surfaces are widely used;
  2. an image of the light source should be created on the pupil of the eye. By placing a central obscuration in the imaging path in a plane conjugate to the eye pupil, created image is in form of annulus, which eliminates corneal back reflections;
  3. regardless of the design of the objective, if the whole area of lenses was illuminated, back reflections close to the optical axis would be inevitable. In order to eliminate them a plate with black dot is introduced conjugate to the front surface of the objective. The dot absorbs light that would otherwise back reflect from the central part of this surface.

As mentioned above, aspherical single-lens objectives are commonly used. Their manufacturing requires however specialized equipment and testing methods. Additionally, correction of chromatic aberration is difficult. Due to those facts an alternative approach using a multi-lens objective with all spherical surfaces.

 

Figure 1 shows an exemplary fundus camera system with internal illumination. Field lens (1) directs the light from the source onto the plate with black dot (2). The dot is then imaged onto the front surface of the objective. The objective, consisting of lenses (4a), (4b) and (4c) creates the image of the light source in the pupil of the eye (5, Figure 2) and consequently illuminates the retina (6, Figure 2). Another purpose of the objective is to create an intermediate image of the retina, which is afterwards reimaged by the zoom lens (7) onto the detector (8). Thanks to the use of zoom lens in the imaging part it is possible to compensate patient’s refractive errors.

System shown in Figure 1 consists of 5 overlaid configurations. In each of them different surface of the objective is defined as a mirror and the paths of the reflected rays are traced to the detector. As can be seen from Figure 1, even when the central part of illuminating beam is absorbed by the black dot, back reflections from multiple surfaces of the objective reach the detector destroying retina image. Overall, in the presented case about 0.05% of the illuminating light is back reflected of the surfaces of the objective, which at first look might seem negligible, however in reality, due to the low efficiency of retina illumination, intensity of the ghosts is at comparable level to the retina image intensity.

In order to reduce the influence of back reflections on the image quality it is crucial to reoptimize the system so that the objective’s surfaces are shaped differently while keeping other imaging properties at desired level. In optical design programs, such as Zemax, it is possible to analyze the system in both sequential and non-sequential mode. The former offers standard ghost control merit function operands, however they do not allow ray tracing back reflections in an alternative optical path, which is required in the case of fundus camera with internal illumination. Non-sequential mode, on the other hand, lacks certain dedicated optimization tools and is less suited for an early optimization process. Hence a new method has been developed that enables incorporating ghost analysis in the internal illumination fundus camera using efficient sequential mode. This method might be also utilized in a wider range of applications where ghost control is critical.

In the proposed method the imaging system, as shown in Figure 2, is built in Zemax. Standard merit function may be used to allow its optimization. Additional line in the merit function calls user defined macro, in which system from Figure 1, including separate configuration for every surface of the objective, is defined. Back reflections for every configuration are analyzed and total detector flux is returned to the merit function. Its value becomes one of the function’s components and is minimized during optimization process. Its weight may be adjusted to balance system properties.

Described method has been used during the design of fundus camera and has proved to be more efficient than optimization using non-sequential ray tracing model. Once a mature solution is found, however, non-sequential analysis should also be performed to make sure there is no unforeseen parasitic light reflected towards the detector.

Acknowledgment

The work presented here was partially founded from the Programme Innovative Economy via National Strategic Reference Framework (Poland) and European Regional Development Fund under the contact UDA-POIG.01.04.00-14-286/11-00.

References

[1]    DeHoog,E., Schwiegerling, J.,"Fundus camera systems: a comparative analysis", APPLIED OPTICS 2009 January 10; 48(2): 221–228 

[2]    ISO 10940:2009 Ophthalmic instruments -- Fundus cameras

[3]    N. Shibata and M. Torii, “Fundus camera,” U.S. Patent 6,654,553 (2003)

[4]    DeHoog,E., Schwiegerling, J.,"Optimal parameters for retinal illumination and imaging in fundus cameras", APPLIED OPTICS 2008 December 20, Vol. 47, No. 36

[5] Zemax® 13 Optical Design Program User\'s Manual 2013 April 4

 

 


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