Rathod, Vishwanath Manik and Jana, Soumya
(2018)
Fundus Imaging using Aplanat.
Masters thesis, Indian Institute of Technology Hyderabad.
Abstract
Retinal photography requires the use of a complex optical system called a fundus
camera, capable of illuminating and imaging simultaneously. Because of restriction
of aperture stop(pupil), available fundus imaging system suffer limited field of view.
Hence peripheral area of retina remains undetected in traditional way. Also, system
being prone to common aberrations always makes them to compromise with quality
of image.
In this thesis we propose a system that uses an aberration free reflector called
Aplanat instead of the conventional lens system for the fundus imaging. So Imaging
optics will be based on reflection unlike the convectional system which uses lens system
and hence refraction principle which results in very negligible aberrations. Also, small
reflector size makes it a hand-held system with minimum complexity and power loss
for illumination purpose. Working under thermodynamic limit and high numerical
aperture makes it possible to inject maximum light at wide angle inside eye which
abolish the necessity of mydriasis.
The optical system was designed in Zemax Optical Studio 15.5 in mixed sequential
mode abilities by inserting aplanat as non sequential CAD object in sequential system
comprised of eye and imaging sensor. CAD object was designed using Solid Edge ST8
using the Cartesian data points created in MATLAB. This solid object then imported
to Zemax through CAD import ability. Present system propose 3 phase to image
retina completely. A narrow throat aplanat for for some part close to optical axis
leaving a small hole at optical axis. A wide throat aplanat to image peripheral area.
a normal lens system that will cover the center hole. Exploiting overlapping part in
the images from all the systems, stitching can be used to get the final complete image.
Conjugate plane of retina found to be a curved surface and inside the aplanat
which restricts us from using Zemax tool for the imaging purpose as it can not have
of axis multiple sensor at desired location in align with conjugate of retina to sense
ray bundle. Also, we loose smoothness and accuracy of reflector surface while im-
porting the CAD object. So 3D image reconstruction of retina was performed in tool
developed by project partner. All three phases covers almost 2000 wide field of retina
which counts for 87% of the retina surface. Exploiting overlapping part in the images
from all the systems, stitching can be used to get the final complete image.
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