Maksutov-Cassegrain inspired high object-space resolution lens for smartphone iris imaging

We present a compact catadioptric lens for use in smartphone iris imaging. With an emphasis on object-space resolution, this lens has a large aperture. To fit within typical shallow phone casing, the lens is folded axially, with the folding arrangement inspired by the Maksutov telescope; the resulting z-height is less than 5.5 mm. Two mirrors give a double ray path, with the secondary mirror centrally obscuring the lens aperture. Using a single refracting element, axial chromatic aberration is limited by using low-dispersion glass. Aspheric coefficients are used on all surfaces, for aberration compensation. The refocussing lens is designed to focus on a nearest object at-300 mm, with refocus achieved by a small axial shift of the primary mirror. The polychromatic MTF is above 0.4 across a 16-degree full field-of-view (FoV) for iris distances as large as-500 mm, for the near-infrared wavelengths of 920 ±35 nm. While the FoV is limited by the catadioptric arrangement, it is sufficiently large for this specialized application. This can be justified by considering that the human interpupillary distance is such that the two eyes subtend a full FoV of 8-deg or less at the closest distance of-300 mm.