Stray light mitigation in a novel endoscope for fallopian tubes

Stray light in an endoscope largely contributes to whether a signal can be detected or not. This FRED analysis used a novel endoscope designed for the fallopian tubes to show how common endoscope elements cause stray light contamination, and to offer suggestions on how to mitigate it. Standard and advanced optical raytracing was performed. Raytrace reports determined which ray paths caused the highest power and irradiance distributions after reflecting one or more times from an element in the system. The analysis revealed that the cover plate introduced significantly more stray light into the system than other endoscope components. The imaging lenses and variable stop reflectivity had a negligible impact on the signal. To obtain acceptable signal-to-noise ratio, the source numerical aperture (NA) was lowered to 0.35 and 0.25 to keep the stray light within the same order of magnitude and an order of magnitude lower, respectively than the desired signal. There was a single specular reflection off of the cover plate distal surface. This illumination reflected back into the imaging fiber without first scattering off the tissue, which resulted in high stray power at the back of the imaging lenses. The specular light appeared brighter at higher source NAs and saturated the desired signal at the edge of the imaging fiber. An NA between 0.25 and 0.35 provides maximum illumination to image the tissue, with minimal stray light degrading the desired signal.