KEYWORDS: Sensors, Interferometry, Signal processing, Data modeling, Bayesian inference, Chemical species, Electroluminescent displays, Monte Carlo methods, Data processing, Interference (communication)
Recent advances in the field of quantum technology offer the exciting possibility of gravimeters and gravity gradiometers capable of performing rapid surveys with unprecedented precision and accuracy. Measurements with sub nano-g (a billionth of the acceleration due to gravity) precision should enable the resolution of underground structures on metre length scales. However, deducing the exact dimensions of the structure producing the measured gravity anomaly is known to be an ill-posed inversion problem. Furthermore, the measurement process will be affected by multiple sources of uncertainty that increase the range of plausible solutions that fit the measured data. Bayesian inference is the natural framework for accommodating these uncertainties and providing a fully probabilistic assessment of possible structures producing inhomogeneities in the gravitational field. Previous work introduced the probability of excavation map as a means to convert the high-dimensional space belonging to the posterior distribution to an easily interpretable map. We now report on the development of the inference model to account for spatial correlations in the gravitational field induced by variations in soil density.
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.
Adaptive coded aperture imaging systems can resolve objects that are smaller than the pixel-limited resolution of the
detector focal plane array. This is done by combining multiple frames of data, where different frames are taken with
different coding patterns on the coded-aperture mask. In the mid-wave infrared the required signal to noise ratio
necessitates some form of light concentration. Optical design software has been used to model candidate optical systems
with the aim of achieving up to four times resolution enhancement along each linear dimension. As in some other
computational imaging systems, the requirements on the optical system are found to be different to those that are
normally used in more classical optical designs. The basic needs are a point-spread function of suitable extent that
changes gradually with angle and does not vary significantly with the expected changes in input spectra or system
temperature. Novel metrics have been derived and used to inform the optical design. The modeling and design trade-offs
and resulting performance are discussed.
Adaptive coded aperture imaging (ACAI) has the potential to enhance greatly the performance of sensing systems by
allowing sub detector pixel image and tracking resolution. A small experimental system has been set up to allow the
practical demonstration of these benefits in the mid infrared, as well as investigating the calibration and stability of the
system. The system can also be used to test modeling of similar ACAI systems in the infrared. The demonstrator can use
either a set of fixed masks or a novel MOEMS adaptive transmissive spatial light modulator. This paper discusses the
design and testing of the system including the development of novel decoding algorithms and some initial imaging
results are presented.
Traditionally, coded aperture techniques have been applied to short-wavelength imaging: X-rays and γ-rays. For these
wavelengths, it is valid to neglect diffraction and describe the operation of the imager in purely geometric-optics terms.
We have investigated coded aperture imaging in the visible band. The much longer wavelengths in this region of the
spectrum mean that diffraction effects cannot be neglected. We describe the effects of diffraction and the implications for
image resolution. We present experimental results from a lens-free coded-aperture imager operating in the visible band
and describe the techniques used to obtain good quality images of complex greyscale scenes.
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.
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.
Coded aperture imaging (CAI) has been used extensively at gamma- and X-ray wavelengths, where conventional
refractive and reflective techniques are impractical. CAI works by coding optical wavefronts from a scene using a
patterned aperture, detecting the resulting intensity distribution, then using inverse digital signal processing to
reconstruct an image.
This paper will consider application of CAI to the visible and IR bands. Doing so has a number of potential advantages
over existing imaging approaches at these longer wavelengths, including low mass, low volume, zero aberrations and
distortions and graceful failure modes. Adaptive coded aperture (ACAI), facilitated by the use of a reconfigurable mask
in a CAI configuration, adds further merits, an example being the ability to implement agile imaging modes with no
macroscopic moving parts. However, diffraction effects must be considered and photon flux reductions can have
adverse consequences on the image quality achievable.
An analysis of these benefits and limitations is described, along with a description of a novel micro optical electro
mechanical (MOEMS) microshutter technology for use in thermal band infrared ACAI systems. Preliminary
experimental results are also presented.
Previous applications of coded aperture imaging (CAI) have been mainly in the energetic parts of the electro-magnetic
spectrum, such as gamma ray astronomy, where few viable imaging alternatives exist. In addition, resolution
requirements have typically been low (~ mrad).
This paper investigates the prospects for and advantages of using CAI at longer wavelengths (visible, infrared) and at
higher resolutions, and also considers the benefits of adaptive CAI techniques. The latter enable CAI to achieve
reconfigurable modes of imaging, as well as improving system performance in other ways, such as enhanced image
quality. It is shown that adaptive CAI has several potential advantages over more traditional optical systems for some
applications in these wavebands. The merits include low mass, volume and moments of inertia, potentially lower costs,
graceful failure modes, steerable fields of regard with no macroscopic moving parts and inherently encrypted data
streams.
Among the challenges associated with this new imaging approach are the effects of diffraction, interference, photon
absorption at the mask and the low scene contrasts in the infrared wavebands. The paper analyzes some of these and
presents the results of some of the tradeoffs in optical performance, using radiometric calculations to illustrate the
consequences in a mid-infrared application. A CAI system requires a decoding algorithm in order to form an image and
the paper discusses novel approaches, tailored to longer wavelength operation. The paper concludes by presenting initial
experimental results.
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