Compressive spectral imaging (CSI) senses a scene by using two-dimensional coded projections such that the number of measurements is far less than that used in spectral scanning-type instruments. An architecture that efficiently implements CSI is the coded aperture snapshot spectral imager (CASSI). A physical limitation of the CASSI is the system resolution, which is determined by the lowest resolution element used in the detector and the coded aperture. Although the final resolution of the system is usually given by the detector, in the CASSI, for instance, the use of a low resolution coded aperture implemented using a digital micromirror device (DMD), which induces the grouping of pixels in superpixels in the detector, is decisive to the final resolution. The mismatch occurs by the differences in the pitch size of the DMD mirrors and focal plane array (FPA) pixels. A traditional solution to this mismatch consists of grouping several pixels in square features, which subutilizes the DMD and the detector resolution and, therefore, reduces the spatial and spectral resolution of the reconstructed spectral images. This paper presents a model for CASSI which admits the mismatch and permits exploiting the maximum resolution of the coding element and the FPA sensor. A super-resolution algorithm and a synthetic coded aperture are developed in order to solve the mismatch. The mathematical models are verified using a real implementation of CASSI. The results of the experiments show a significant gain in spatial and spectral imaging quality over the traditional grouping pixel technique.

Coded aperture design in mismatched compressive spectral imaging

A system and related method for coded aperture sensing, comprising: passing at least one scene wavefront from a target scene through at least one original coded aperture mask onto a focal plane array, producing a diffracted projection of the target scene; and processing the diffracted projection into a representation of the target scene by correlating a function of the diffracted projection with a function of a known array pattern of the at least one original coded aperture mask and by using at least one reconstructing wavefront for holographic reconstructing.

Holography-based device, system and method for coded aperture imaging

We present an investigation of the performance of coded aperture optical systems where the elements of a set of binary
coded aperture masks are applied over a sequence of acquired images. In particular, we are interested in investigating
code sequences and image reconstruction algorithms that reduce the optical fidelity and hardware requirements for the
system. Performance is jointly tied to the mask design, the image estimation algorithm, and the inherent optical response
of the system. As such, we adopt a simplified reconstruction model and consider generalized optical system aberrations
in designing masks used for multi-frame reconstruction of the imagery. We also consider the case of non-Nyquist
sampled (aliased) imagery. These investigations have focused on using a regularized least-squares reconstruction model
and mean squared error as a performance metric. Masks are found by attempting to minimize a closed form objective
that predicts the mean squared error for the reconstruction algorithm. We find that even with suboptimal solutions that
binary masks can be used to improve imagery over the case of an uncoded aperture with the same aberration.

An analysis of coded aperture acquisition and reconstruction using multi-frame code sequences for relaxed optical design constraints

In a previous paper we presented initial results for sub-detector-pixel imaging in the mid-wave infra-red (MWIR) using
an imager equipped with a coded-aperture based on a re-configurable MOEMS micro-shutter. It was shown in laboratory
experiments that sub-pixel resolution is achievable via this route.
The purpose of the current paper is to provide detail on the reconstruction method and to discuss some challenges which
arise when imaging real-world scenes. The number of different mask patterns required to achieve a certain degree of
super-resolution is also discussed. New results are presented to support the theory.

Sub-pixel super-resolution by decoding frames from a reconfigurable coded-aperture camera: theory and experimental verification

The requirements for a persistent wide-area surveillance system are discussed in the context of evolving
military operations. Significant emphasis has been placed on the development of new sensing
technologies to meet the challenges posed by asymmetric threats. Within the UK, the Electro-Magnetic
Remote Sensing Defence Technology Centre (EMRS DTC) has supported the research and
development of new capabilities including radio-frequency (RF) and electro-optic (EO) systems, as well
as work on sensor exploitation, with a goal of developing solutions for enhancing situational awareness.
This activity has been supported by field trials to determine the efficacy of competing technologies in
relation to realistic threat scenarios.

Systems for persistent surveillance

An earlier paper [1] discussed the merits of adaptive coded apertures for use as lensless imaging systems in the thermal
infrared and visible It was shown how diffractive (rather than the more conventional geometric) coding could be used,
and that 2D intensity measurements from multiple mask patterns could be combined and decoded to yield enhanced
imagery Initial experimental results in the visible band were presented Unfortunately, radiosity calculations, also
presented in that paper, indicated that the signal to noise performance of systems using this approach was likely to be
compromised, especially in the infrared
This paper will discuss how such limitations can be overcome, and some of the tradeoffs involved Experimental results
showing tracking and imaging performance of these modified, diffractive, adaptive coded aperture systems in the visible
and infrared will be presented The subpixel imaging and tracking performance is compared to that of conventional
imaging systems and shown to be superior System size, weight and cost calculations indicate that the coded aperture
approach, employing novel photonic MOEMS micro-shutter architectures, has significant merits for a given level of
performance in the MWIR when compared to more conventional imaging approaches

Adaptive coded aperture imaging in the infrared: towards a practical implementation

We describe an adaptive coded-aperture imager operating in the midwave IR. This consists of a coded-aperture mask, a set of optics, and a 4k×4k focal plane array (FPA). This system can produce images with a resolution better than the detector pixel limit by combining multiple frames of data recorded with different coding. This superresolution capability has been demonstrated both in the laboratory and with targets placed outside, the highest resolution being one-half of the FPA pixel pitch.

Adaptive coded-aperture imaging with subpixel superresolution.

There is an increasingly important requirement for day and night, wide field of view imaging and tracking for both
imaging and sensing applications. Applications include military, security and remote sensing. We describe the
development of a proof of concept demonstrator of an adaptive coded-aperture imager operating in the mid-wave infrared
to address these requirements. This consists of a coded-aperture mask, a set of optics and a 4k x 4k focal plane array
(FPA). This system can produce images with a resolution better than that achieved by the detector pixel itself (i.e. superresolution)
by combining multiple frames of data recorded with different coded-aperture mask patterns. This superresolution
capability has been demonstrated both in the laboratory and in imaging of real-world scenes, the highest
resolution achieved being ½ the FPA pixel pitch. The resolution for this configuration is currently limited by vibration
and theoretically ¼ pixel pitch should be possible. Comparisons have been made between conventional and ACAI
solutions to these requirements and show significant advantages in size, weight and cost for the ACAI approach.

An adaptive coded aperture imager: building, testing and trialing a super-resolving terrestrial demonstrator

The 3rd Generation Goodrich DB-110 system provides users with a three (3) field-of-view high performance Airborne
Reconnaissance capability that incorporates a dual-band day and nighttime imaging sensor, a real time recording and a
real time data transmission capability to support long range, medium range, and short range standoff and over-flight
mission scenarios, all within a single pod. Goodrich developed their 3rd Generation Airborne Reconnaissance Pod for
operation on a range of aircraft types including F-16, F-15, F-18, Euro-fighter and older aircraft such as the F-4, F-111,
Mirage and Tornado. This system upgrades the existing, operationally proven, 2nd generation DB-110 design with
enhancements in sensor resolution, flight envelope and other performance improvements. Goodrich recently flight tested
their 3rd Generation Reconnaissance System on a Block 52 F-16 aircraft with first flight success and excellent results.
This paper presents key highlights of the system and presents imaging results from flight test.

Gavin Dyer

Papers

Adaptive coded aperture imaging in the infrared: towards a practical implementation

Adaptive coded-aperture imaging with subpixel superresolution.

An adaptive coded aperture imager: building, testing and trialing a super-resolving terrestrial demonstrator

The Goodrich 3rd generation DB-110 system: successful flight test on the F-16 aircraft