In recent years gamma ray imagers such as the GammaCamTM and Polaris have demonstrated good imaging performance in the field. Imager performance is often summarized as “resolution”, either angular, or spatial at some distance from the imager, however the definition of resolution is not always related to the ability to image an object. It is difficult to quantitatively compare imagers without a common definition of image quality. This paper examines three categories of definition: point source; line source; and area source. It discusses the details of those definitions and which ones are more relevant for different situations. Metrics such as Full Width Half Maximum (FWHM), variations on the Rayleigh criterion, and some analogous to National Imagery Interpretability Rating Scale (NIIRS) are discussed. The performance against these metrics is evaluated for a high resolution coded aperture imager modeled using Monte Carlo N-Particle (MCNP), and for a medium resolution imager measured in the lab.
A novel and memory efficient method for computing diffraction patterns produced on large-scale focal planes by largescale
Coded Apertures at wavelengths where diffraction effects are significant has been developed and tested. The
scheme, readily implementable on portable computers, overcomes the memory limitations of present state-of-the-art
simulation codes such as Zemax. The method consists of first calculating a set of reference complex field (amplitude
and phase) patterns on the focal plane produced by a single (reference) central hole, extending to twice the focal plane
array size, with one such pattern for each Line-of-Sight (LOS) direction and wavelength in the scene, and with the
pattern amplitude corresponding to the square-root of the spectral irradiance from each such LOS direction in the scene
at selected wavelengths. Next the set of reference patterns is transformed to generate pattern sets for other holes. The
transformation consists of a translational pattern shift corresponding to each hole’s position offset and an electrical phase
shift corresponding to each hole’s position offset and incoming radiance’s direction and wavelength. The set of
complex patterns for each direction and wavelength is then summed coherently and squared for each detector to yield a
set of power patterns unique for each direction and wavelength. Finally the set of power patterns is summed to produce
the full waveband diffraction pattern from the scene. With this tool researchers can now efficiently simulate diffraction
patterns produced from scenes by large-scale Coded Apertures onto large-scale focal plane arrays to support the
development and optimization of coded aperture masks and image reconstruction algorithms.
One obstacle to optimizing performance of large-scale coded aperture systems operating in the diffractive regime has
been the lack of a robust, rapid, and efficient method for generating diffraction patterns that are projected by the system
onto the focal plane. We report on the use of the 'Shrekenhamer Transform' for a systematic investigation of various
types of coded aperture designs operating in the diffractive mode. Each design is evaluated in terms of its
autocorrelation function for potential use in future imaging applications. The motivation of our study is to gain insight
into more efficient optimization methods of image reconstruction algorithms.
Interest in Adaptive Coded Aperture Imaging (ACAI) continues to grow as the optical and systems engineering
community becomes increasingly aware of ACAI's potential benefits in the design and performance of both imaging and
non-imaging systems , such as good angular resolution (IFOV), wide distortion-free field of view (FOV), excellent
image quality, and light weight construct. In this presentation we first review the accomplishments made over the past
five years, then expand on previously published work to show how replacement of conventional imaging optics with
coded apertures can lead to a reduction in system size and weight. We also present a trade space analysis of key design
parameters of coded apertures and review potential applications as replacement for traditional imaging optics. Results
will be presented, based on last year's work of our investigation into the trade space of IFOV, resolution, effective focal
length, and wavelength of incident radiation for coded aperture architectures. Finally we discuss the potential application
of coded apertures for replacing objective lenses of night vision goggles (NVGs).
KEYWORDS: Sensors, Coded apertures, Prototyping, Signal detection, Telescopes, Imaging systems, Gamma radiation, Data acquisition, Computing systems, Control systems
We have developed a prototype of a scalable high-resolution direction and energy sensitive gamma-ray detection system
that operates in both coded aperture (CA) and Compton scatter (CS) modes to obtain optimal efficiency and angular
resolution over a wide energy range. The design consists of an active coded aperture constructed from 52 individual CZT
planar detectors each measuring 3×3×6 mm3 arranged in a MURA pattern on a 10×10 grid, with a monolithic
20×20×5 mm3 pixelated (8×8) CZT array serving as the focal plane. The combined mode is achieved by using the
aperture plane array for both Compton scattering of high-energy photons and as a coded mask for low-energy radiation.
The prototype instrument was built using two RENA-3 test systems, one each for the aperture and the focal plane,
stacked on top of each other at a distance of 130 mm. The test systems were modified to coordinate (synchronize)
readout and provide coincidence information of events within a user-adjustable 40-1,280 ns window. The measured
angular resolution of the device is <1 deg (17 mrad) in CA mode and is predicted to be approximately 3 deg (54 mrad) in
CS mode. The energy resolution of the CZT detectors is approximately 5% FWHM at 120 keV. We will present details
of the system design and initial results for the calibration and performance of the prototype.
The design and implementation of adaptive coded apertures (diffraction) has advanced significantly since the first SPIE
conference on Adaptive Coded Aperture Imaging and Non-Imaging Sensors held in 2007. Core algorithmic concepts
relating to coding and decoding techniques remain steeply based in its non-diffractive design origins. We discuss
adaptive coded aperture imaging's current capabilities in light of recent advances as well as methods of improvement for
future systems design. The advantages of implementing reconfigurable mask patterns compared to fixed ones will also
be discussed, as well as potential improvement in angular resolution by means of reconfigurable masks.
Originally developed more than four decades ago for imaging non-refracting radiation such as x-rays, gamma-rays, and neutrons, today coded apertures are poised for new applications in other domains of the electro-magnetic spectrum. Boasting excellent angular resolution, wide field-of-view, negligible image distortion, and light weight construction, coded apertures are increasingly attractive for certain IR and optical applications. Successful exploitation of the coded aperture technique to these new venues will require the systems engineer or designer to understand the fundamental principles of imaging with coded apertures. This presentation explores the special properties of coded aperture masks, the algorithms used to design them, the signal processing algorithms used to decode recorded data, as well as important design considerations for a successful system. The targeted audience is the engineer with minimal prior working knowledge of coded apertures who wants to understand the technology sufficiently well to reliably assess applicability for present and future needs.
KEYWORDS: Missiles, Protactinium, Monte Carlo methods, Navigation systems, Mathematical modeling, Motion estimation, Motion models, Statistical analysis, Optimization (mathematics), Signal to noise ratio
This work deals with the following question: using passive (line-of-sight angle) observations of a multistage
surface to air missile from an aircraft, how can one infer that the missile is or is not aimed at the aircraft. The
observations are assumed to be made only on the initial portion of the missile's trajectory. The approach is to
model the trajectory of the missile with a number of kinematic and guidance parameters, estimate them and
use statistical tools to infer whether the missile is guided toward the aircraft or not. A mathematical model is
presented for a missile under pure proportional navigation with a changing velocity (direction change as well
as speed change), to intercept a nonmaneuvering aircraft. A maximum likelihood estimator (MLE) is used for
estimating the missile's motion parameters and a goodness-of-fit test is formulated to test if the aircraft is the
aim or not. Using measurement data from several realistic missiles - single stage as well as multistage - aimed
at an aircraft, it is shown that the proposed method can solve this problem successfully. The key to the solution,
in addition to the missile model parametrization, is the use of a reliable global optimization algorithm with a
hierarchical search technique for the MLE. The estimation/decision algorithm presented here can be used for an
aircraft to decide, in a timely manner, whether appropriate countermeasures are necessary.
KEYWORDS: Missiles, Protactinium, Motion estimation, Mathematical modeling, Motion models, Statistical analysis, Solids, Monte Carlo methods, 3D modeling, Signal to noise ratio
This work deals with the following question: using passive (line-of-sight angle) observations of a missile from an aircraft, how can one infer that the missile is or is not aimed at the aircraft. The observations are assumed to be made only on the initial portion (about 1/4) of the missile's trajectory. The approach is to model the trajectory of the missile with a number of kinematic and guidance parameters, estimate them and use statistical tools to infer whether the missile is guided toward the aircraft. A mathematical model is constructed for a missile under pure proportional navigation with a changing velocity (direction change and speed change), to intercept a nonmaneuvering aircraft. A maximum likelihood estimator is presented for estimating the missile's motion parameters and a goodness-of-fit test is formulated to test if the aircraft is the aim or not. Using measurement data from a realistic missile aimed at an aircraft shows that the proposed method can solve this problem successfully. The estimation/decision algorithm presented here can also be used for an aircraft to decide whether appropriate countermeasures are necessary.
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