Fundus camera is a complex optical system for retinal photography, involving illumination and imaging of the retina.
Stray light is one of the most significant problems of fundus camera because the retina is so minimally reflective that
back reflections from the cornea and any other optical surface are likely to be significantly greater than the light reflected
from the retina. To provide maximum illumination to the retina while eliminating back reflections, a novel design of
illumination system used in portable fundus camera is proposed. Internal illumination, in which eyepiece is shared by
both the illumination system and the imaging system but the condenser and the objective are separated by a beam splitter,
is adopted for its high efficiency. To eliminate the strong stray light caused by corneal center and make full use of light
energy, the annular stop in conventional illumination systems is replaced by a fiber-coupled, ring-shaped light source
that forms an annular beam. Parameters including size and divergence angle of the light source are specially designed.
To weaken the stray light, a polarized light source is used, and an analyzer plate is placed after beam splitter in the
imaging system. Simulation results show that the illumination uniformity at the fundus exceeds 90%, and the stray light
is within 1%. Finally, a proof-of-concept prototype is developed and retinal photos of an ophthalmophantom are captured.
The experimental results show that ghost images and stray light have been greatly reduced to a level that professional
diagnostic will not be interfered with.