R. Hamilton Shepard

Sub-pixel Layout for Super-Resolution with Images in the Octic Group

This paper presents a novel super-resolution framework by exploring the properties of non-conventional pixel layouts and shapes. We show that recording multiple images, transformed in the octic group, with a sensor of asymmetric sub-pixel layout increases the spatial sampling compared to a conventional sensor with a rectilinear grid of pixels and hence increases the image resolution. We further prove a theoretical bound for achieving well-posed super-resolution with a designated magnification factor w.r.t. the number and distribution of sub-pixels. We also propose strategies for selecting good sub-pixel layouts and effective super-resolution algorithms for our setup. The experimental results validate the proposed theory and solution, which have the potential to guide the future CCD layout design with super-resolution functionality.

Optical design and characterization of an advanced computational imaging system

We describe an advanced computational imaging system with an optical architecture that enables simultaneous and dynamic pupil-plane and image-plane coding accommodating several task-specific applications. We assess the optical requirement trades associated with custom and commercial-off-the-shelf (COTS) optics and converge on the development of two low-cost and robust COTS testbeds. The first is a coded-aperture programmable pixel imager employing a digital micromirror device (DMD) for image plane per-pixel oversampling and spatial super-resolution experiments. The second is a simultaneous pupil-encoded and time-encoded imager employing a DMD for pupil apodization or a deformable mirror for wavefront coding experiments. These two testbeds are built to leverage two MIT Lincoln Laboratory focal plane arrays – an orthogonal transfer CCD with non-uniform pixel sampling and on-chip dithering and a digital readout integrated circuit (DROIC) with advanced on-chip per-pixel processing capabilities. This paper discusses the derivation of optical component requirements, optical design metrics, and performance analyses for the two testbeds built.

Design architectures for optically multiplexed imaging.

Optically multiplexed imaging is the process by which multiple images are overlaid on a single image surface. Uniquely encoding the discrete images allows scene reconstruction from multiplexed images via post processing. We describe a class of optical systems that can achieve high density image multiplexing through a novel division of aperture technique. Fundamental design considerations and performance attributes for this sensor architecture are discussed. A number of spatial and temporal encoding methods are presented including point spread function engineering, amplitude modulation, and image shifting. Results from a prototype five-channel sensor are presented using three different encoding methods in sparse-scene star tracking demonstration. A six-channel optically multiplexed prototype sensor is used to reconstruct imagery from information rich dense scenes through dynamic image shifting.

Seidel aberrations of the Gabor superlens

Equations are presented for the third-order Seidel aberrations of the Gabor superlens (GSL) as a function of microtelescope channel position within the aperture array. To reveal the origin and form of increasing aberration with channel height, Seidel coefficients are derived as a function of the accumulating pitch difference between the lens arrays and the aberrations present in the centered channel. Two- and three-element Gabor lenses are investigated and their aberrations are expressed as a function of first-order design parameters. The derived theory is then compared to a real ray trace simulation to demonstrate the accuracy of third-order aberration theory to predict GSL image quality.Equations are presented for the third-order Seidel aberrations of the Gabor superlens (GSL) as a function of microtelescope channel position within the aperture array. To reveal the origin and form of increasing aberration with channel height, Seidel coefficients are derived as a function of the accumulating pitch difference between the lens arrays and the aberrations present in the centered channel. Two- and three-element Gabor lenses are investigated and their aberrations are expressed as a function of first-order design parameters. The derived theory is then compared to a real ray trace simulation to demonstrate the accuracy of third-order aberration theory to predict GSL image quality.

The design of SWIR imaging lenses using plastic optics

Plastic lenses are widely used in visible imaging systems and provide a number of advantages including reduced weight. However, their use in the short-wave infrared (SWIR) has been limited due to the presence of strong material absorption bands occurring at wavelengths above 1 micron. This paper explores the viability of using plastic optics in broadband SWIR imaging applications and the efficacy of using plastic lenses as a method of weight reduction. A design study is presented to reveal combinations of plastic and glass lenses suitable for aberration correction. Weight savings is quantified via a comparison to glass lenses to investigate the trade-off between using lower density plastic materials and the faster F/#s (i.e. larger lenses) required to compensate for the signal loss caused by their absorption.

A robust optical coupler for alignment of superconducting nanowire detector arrays

We present an athermalized design and performance analysis of a robust imaging system used to couple light from an input fiber to a superconducting nanowire single photon detector.

Dynamic Optically Multiplexed Imaging

Optically multiplexed imagers overcome the tradeoff between field of view and resolution by superimposing images from multiple fields of view onto a single focal plane. In this paper, we consider the implications of independently shifting each field of view at a rate exceeding the frame rate of the focal plane array and with a precision that can exceed the pixel pitch. A sequence of shifts enables the reconstruction of the underlying scene, with the number of frames required growing inversely with the number of multiplexed images. As a result, measurements from a sufficiently fast sampling sensor can be processed to yield a low distortion image with more pixels than the original focal plane array, a wider field of view than the original optical design, and an aspect ratio different than the original lens. This technique can also enable the collection of low-distortion, wide field of view videos. A sequence of sub-pixel spatial shifts extends this capability to allow the recovery of a wide field of view scene at sub-pixel resolution. To realize this sensor concept, a novel and compact divided aperture multiplexed sensor, capable of rapidly and precisely shifting its fields of view, was prototyped. Using this sensor, we recover twenty-four megapixel images from a four-megapixel focal plane and show the feasibility of simultaneous de-multiplexing and super-resolution.

Refractive optically multiplexed LWIR imaging system

Using a novel computational imaging architecture, we double the field of view of a long-wave infrared microbolometer camera while maintaining resolution. Due to the compact designs enabled by this architecture and the critical impact of resolution on classification performance, this approach is compelling for surveillance applications where low size, weight, power and cost (SWaP-C) systems are desired. We detail the optical design, characterization, and performance of a compact, refractive, optically multiplexed imaging system for use in the long-wave infrared (8-12 μm). A pair of prisms are used to divide the aperture and expose the uncooled microbolometer focal plane to two fields of view simultaneously, doubling the number of output pixels and the horizontal field of view. The image is reconstructed by rotating the prisms about the optical axis, inducing opposing vertical shifts in the two channels. Focal length, field of view, MTF, and NEDT are used to compare performance to a conventional camera. Shifting methods for proper demultiplexing are discussed, and reconstructed images are offered as a demonstration of system performance.

Design and calibration of a wide field of view MWIR optically multiplexed imaging system

We describe the optical design and characterization testing of an optically multiplexed imaging system operating in the 3.4 to 5 micron waveband. The optical design uses a division of aperture method to overlay six images on a single focal plane and produce a 90 by 15 degree 6-megapixel field of view. Image disambiguation is achieved through image shifting enabled by piezo-actuated mirrors in the multiplexing assembly. This paper provides an overview of the optical design including focal plane selection, image resolution and distortion, pupil imaging, and aperture division geometry. A method of applying one and two-point non-uniformity correction using radiometric test data is suggested. Sensor-level per-channel image quality and sensitivity tests including MTF, 3D-noise and NEDT are shown to validate the design assumptions.

Towards an Optimized Gabor Superlens

The Gabor Superlens (GSL) combines light from an array of micro-telescopes to form a single composite image. This is achieved through an initial selection of micro-telescope and array parameters that satisfy a set of first order imaging conditions. Designing a GSL presents two design challenges that are not encountered in conventional (single-aperture) lens design: the array parameters couple design characteristics such as the F/# and field of view to the paraxial design, and the composite image quality can be dominated by aberration of individual elements rather than a summation of aberration contributions throughout the design. This paper begins with an assessment of the highly parameterized design space of the Gabor Superlens to clearly identify relationships between the initial selection of first order design geometry and the consequences they have on system performance. An overview of a streamlined design method follows. Increasingly sophisticated GSL designs are then investigated to demonstrate the effectiveness of using individually corrected lens groups to improve composite image quality.