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System Optimization of Compact Monocentric Lens Imagers

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Abstract

Conventional “fisheye” imagers map wide fields of view onto flat image sensors, but suppressing field curvature imposes harsh design tradeoffs, especially as the focal length grows. Monocentric lenses are the polar opposite. The symmetry in a lens made of concentric hemispherical surfaces cancels most of the geometrical aberrations, and enables high-resolution image formation on a hemispherical image surface [1-3]. Monocentric lenses open a previously unachievable domain, but the question remains how the image can be recorded, and the stray light blocked, for a spherical image surface. One way is to relay-image overlapping areas of the sphere to planar sensors, using aperture stops in each relay to control stray light, and image processing to fuse the overlapping image boundaries [4-6]. This enables multi-Gigapixel real-time imaging, but the relay optics (e.g., 221 sets for a 2.5Gpix imager) introduce complexity and cost. Here we describe an alternative, using high-resolution fiber bundles for image transfer to planar sensors. The use of a dense array of high-index optical fibers to couple light from one surface to another was explored in the early 1960s [7] and has since been extensively used for medical applications such as endoscopy. Using a curved fiber bundle to interface a monocentric lens was one of the first conceived for this new technology [8], but early bundles were unsuitable due to the low contrast between the core and cladding indexes. Modern fiber bundles from Schott fiber optics have a core and cladding index of 1.81 and 1.48, incorporate index-matched absorptive glass to suppress cladding modes, and can be drawn to an extremely fine pitch. Figure 1 shows the basic geometry and a spherically symmetric 2-glass design, a high-resolution 12 mm focal length F/1.7 lens with uniform light collection to within 2x over a 120° field of view.

© 2013 Optical Society of America

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