Purpose
Demonstrate a smart phone based non-mydriatic fundus imaging system without additional imaging optics. Micro waveguide technology delivers specular free imaging. The hardware complexity is replaced with sophisticated software methods for non expert control and image reconstruction. Traditionally cell phone based fundus imaging is performed with the addition of a 20 diopter lens placed between the camera and subject, or as an attachment to other ophthalmic devices to achieve optimal magnification and off-axis illumination delivery. However, this requires non trivial alignment and expert knowledge to operate.
Methods
The system comprises of a smart phone (for initial studies a Galaxy S3 was chosen) coupled with a novel programmable waveguide to steer near co-axial illumination through the pupillary plane. The system is held 25mm from the surface of the cornea co-axial to the foveal center. The wave guide is coupled with the rear facing camera in a small clip-on element. Successive images are captured under varying computationally driven illumination strategies using a user driven visual stimulus control. A standard 500 mW retinoscope LED is used in combination with a programmable mask and polarized beam splitter to steer a projected 2mm beam through the pupillary plane. The S3 sensor is an 8MP (SONY) enabling image capture at 30 HZ, while the rapid auto focusing is disabled and camera perimeters are controlled manually through software. The native aperture of 2.8 f-stop is used with a pixel pitch of 1.4 um.
Results
This system resolves 50 um structures within a 5 degree field of view of the fundus at a distance of 25mm from the surface of the cornea. A 7 degree field of view is achieved by placing the camera at 18mm to the surface of the cornea in any given angle. 98% of corneal reflections caused from surface scattering are successfully removed during capture. The alignment axis for capturing different views extends to 30 degrees of the foveal axis in both nasal and dorsal views without special alignment. Traditional limitations in smart phone based imaging, such as image quality, noise and fixed depth of field are discussed.
Conclusions
Non mydriatic fundus imaging on smart phone platforms may prove to be a paradigm shift in rapid screening. In contrast to standard imaging systems the over-all data acquisition necessary for comprehensive analysis can be gathered via single capture on a micro camera configuration.
Keywords: 627 optic disc •
556 infant vision •
688 retina