Ilya Agurok

Optimization of two-glass monocentric lenses for compact panoramic imagers: general aberration analysis and specific designs.

Monocentric lenses have recently changed from primarily a historic curiosity to a potential solution for panoramic high-resolution imagers, where the spherical image surface is directly detected by curved image sensors or optically transferred onto multiple conventional flat focal planes. We compare imaging and waveguide-based transfer of the spherical image surface formed by the monocentric lens onto planar image sensors, showing that both approaches can make the system input aperture and resolution substantially independent of the input angle. We present aberration analysis that demonstrates that wide-field monocentric lenses can be focused by purely axial translation and describe a systematic design process to identify the best designs for two-glass symmetric monocentric lenses. Finally, we use this approach to design an F/1.7, 12 mm focal length imager with an up to 160° field of view and show that it compares favorably in size and performance to conventional wide-angle imagers.

Fiber-coupled monocentric lens imaging

Monocentric lenses have proven exceptionally capable of high numerical aperture wide-field imaging - provided the overall system can accommodate a spherically curved image surface. We will present a summary of recent work on the design optimization and experimental demonstrations of monocentric wide-field imaging, including systems based on waveguide coupling of the image to conventional focal plane sensor(s).

Spectrally adaptive light filtering

A technology of spectrally adaptive light filtering has been demonstrated, which allows changing the transmission curve of an optical filter in an octave-wide spectral interval with the kilohertz rate under full computer control. This technology utilizes the unique capability of acousto-optic tunable filters to transmit light in several spectral intervals simultaneously, driven by radio signals with complex spectra. A computer algorithm to synthesize the driving signal for an arbitrary given spectral transmission curve was developed. This algorithm has been used for various applications of this developed technology in the visible, SWIR, and MWIR spectral intervals.

Image processing for cameras with fiber bundle image relay

Some high-performance imaging systems generate a curved focal surface and so are incompatible with focal plane arrays fabricated by conventional silicon processing. One example is a monocentric lens, which forms a wide field-of-view high-resolution spherical image with a radius equal to the focal length. Optical fiber bundles have been used to couple between this focal surface and planar image sensors. However, such fiber-coupled imaging systems suffer from artifacts due to image sampling and incoherent light transfer by the fiber bundle as well as resampling by the focal plane, resulting in a fixed obscuration pattern. Here, we describe digital image processing techniques to improve image quality in a compact 126° field-of-view, 30 megapixel panoramic imager, where a 12 mm focal length F/1.35 lens made of concentric glass surfaces forms a spherical image surface, which is fiber-coupled to six discrete CMOS focal planes. We characterize the locally space-variant system impulse response at various stages: monocentric lens image formation onto the 2.5 μm pitch fiber bundle, image transfer by the fiber bundle, and sensing by a 1.75 μm pitch backside illuminated color focal plane. We demonstrate methods to mitigate moiré artifacts and local obscuration, correct for sphere to plane mapping distortion and vignetting, and stitch together the image data from discrete sensors into a single panorama. We compare processed images from the prototype to those taken with a 10× larger commercial camera with comparable field-of-view and light collection.

Optimization of high-performance monocentric lenses

The recent application of monocentric lenses for panoramic high-resolution digital imagers raises the question of the achievable performance limits of this lens structure and of techniques for design optimization to approach these limits. This paper defines the important regions of the design space of moderate complexity monocentric lenses and describes systematic and global optimization algorithms for the design of monocentric objective lenses of various focal lengths, apertures, and spectral bandwidths. We demonstrate the trade-off between spectral band, F-number and lens complexity, and provide design examples of monocentric lenses for specific applications.

Real-time polarization-sensitive multispectral automatic imaging system for object contrast enhancement and clutter mitigation

Physical Optics Corporation has developed an automatic polarization sensitive multispectral imaging system for real-time object-to-background contrast enhancement. This system is built around an acousto-optic tunable filter and liquid crystal achromatic rotator of light polarization.

Spectrally adaptive imaging camera for automatic target contrast enhancement

This presentation describes the major performance characteristics of the developed camera and demonstrates some results of target contrast enhancement in laboratory and field experiments.

Digital image processing for wide-angle highly spatially variant imagers

High resolution, wide field-of-view and large depth-of-focus imaging systems are greatly desired and have received much attention from researchers who seek to extend the capabilities of cameras. Monocentric lenses are superior in performance over other wide field-of-view lenses with the drawback that they form a hemispheric image plane which is incompatible with current sensor technology. Fiber optic bundles can be used to relay the image the lens produces to the sensors planar surface. This requires image processing to correct for artifacts inherent to fiber bundle image transfer. Using a prototype fiber coupled monocentric lens imager we capture single exposure focal swept images from which we seek to produce extended depth-of-focus images. Point spread functions (PSF) were measured in lab and found to be both angle and depth dependent. This spatial variance enforces the requirement that the inverse problem be treated as such. This synthesis of information allowed us to establish a framework upon which to mitigate fiber bundle artifacts and extend the depth-of-focus of the imaging system.

Enhanced Field of View Fiber Coupled Image Sensing

Fiber-coupled sensors allow detection of spherical image surfaces. We investigate the field of view possible with a single straight fiber bundle, demonstrating how the field can be extended by annular microprisms near the image surface.

Curved fiber bundles for monocentric lens imaging

Monocentric lenses allow high resolution panoramic cameras, where imaging fiber bundles transport the hemispherical image surface to conventional focal planes. Refraction at the curved image surface limits the field of view coupled through a single bundle of straight fibers to less than ±34°, even for NA 1 fibers. Previously we have demonstrated a nearly continuous 128° field of view using a single lens and multiple adjacent straight fiber-coupled image sensors, but this imposes mechanical complexity of fiber bundle shaping and integration. However, a 3D waveguide structure with internally curved optical fiber pathways can couple the full continuous field of view onto a single focal plane. Here, we demonstrate wide-field imaging using a monocentric lens and a single curved fiber bundle, showing that the 3D bundle formed from a tapered fiber bundle can be used for relaying a 128° field of view from a curved input to the planar output face. We numerically show the coupling efficiency of light to the tapered bundle for different field of views depends on the taper ratio of the bundle as well as center of the curvature chosen for polishing of the fiber bundle facet. We characterize a tapered fiber bundle by measuring the angle dependent impulse response, transmission efficiency and the divergence angle of the light propagating from the output end of the fiber.