3,017 research outputs found
Geometric reconstruction methods for electron tomography
Electron tomography is becoming an increasingly important tool in materials
science for studying the three-dimensional morphologies and chemical
compositions of nanostructures. The image quality obtained by many current
algorithms is seriously affected by the problems of missing wedge artefacts and
nonlinear projection intensities due to diffraction effects. The former refers
to the fact that data cannot be acquired over the full tilt range;
the latter implies that for some orientations, crystalline structures can show
strong contrast changes. To overcome these problems we introduce and discuss
several algorithms from the mathematical fields of geometric and discrete
tomography. The algorithms incorporate geometric prior knowledge (mainly
convexity and homogeneity), which also in principle considerably reduces the
number of tilt angles required. Results are discussed for the reconstruction of
an InAs nanowire
Diffraction Analysis with UWB Validation for ToA Ranging in the Proximity of Human Body and Metallic Objects
The time-of-arrival (ToA)-based localization technique performs superior in line-of-sight (LoS) conditions, and its accuracy degrades drastically in proximity of micro-metals and human body, when LoS conditions are not met. This calls for modeling and formulation of Direct Path (DP) to help with mitigation of ranging error. However, the current propagation tools and models are mainly designed for telecommunication applications via focus on delay spread of wireless channel profile, whereas ToA-based localization strive for modeling of DP component. This thesis provides a mitigation to the limitation of existing propagation tools and models to computationally capture the effects of micro-metals and human body on ToA-based indoor localization. Solutions for each computational technique are validated by empirical measurements using Ultra-Wide-Band (UWB) signals. Finite- Difference-Time-Domain (FDTD) numerical method is used to estimate the ranging errors, and a combination of Uniform-Theory-of-Diffraction (UTD) ray theory and geometrical ray optics properties are utilized to model the path-loss and the ToA of the DP obstructed by micro- metals. Analytical UTD ray theory and geometrical ray optics properties are exploited to model the path-loss and the ToA of the first path obstructed by the human body for the scattering scenarios. The proposed scattering solution expanded to analytically model the path-loss and ToA of the DP obstructed by human body in angular motion for the radiation scenarios
Methods for Photoacoustic Image Reconstruction Exploiting Properties of Curvelet Frame
Curvelet frame is of special significance for photoacoustic tomography (PAT) due to its sparsifying and microlocalisation properties. In this PhD project, we explore the methods for image reconstruction in PAT with flat sensor geometry using Curvelet properties. This thesis makes five distinct contributions: (i) We investigate formulation of the forward, adjoint and inverse operators for PAT in Fourier domain. We derive a one-to-one map between wavefront directions in image and data spaces in PAT. Combining the Fourier operators with the wavefront map allows us to create the appropriate PAT operators for solving limited-view problems due to limited
angular sensor sensitivity. (ii) We devise a concept of wedge restricted Curvelet transform, a modification of standard Curvelet transform, which allows us to formulate a tight frame of wedge restricted Curvelets on the range of the PAT forward operator for PAT data representation. We consider details specific to PAT data such as symmetries, time oversampling and their consequences. We further adapt the wedge restricted Curvelet to decompose the wavefronts into visible and invisible parts in the data domain as well as in the image domain. (iii) We formulate a two step approach based on the recovery of the complete volume of the photoacoustic data from the sub-sampled data followed by the acoustic inversion, and a one step approach where the photoacoustic image is directly recovered from the subsampled data. The wedge restricted Curvelet is used as the sparse representation of the photoacoustic data in the two step approach. (iv) We discuss a joint variational approach that incorporates Curvelet sparsity in photoacoustic image domain and spatio-temporal regularization via optical flow constraint to achieve improved results for dynamic PAT reconstruction. (v) We consider the limited-view problem due to limited angular sensitivity of the sensor (see (i) for the formulation of the corresponding fast operators in Fourier domain). We propose complementary information learning approach based on splitting the problem into visible and invisible singularities. We perform a sparse reconstruction of the visible Curvelet coefficients using compressed sensing techniques and propose a tailored deep neural network architecture to recover the invisible coefficients
Engineering evaluations and studies. Volume 2: Exhibit B, part 1
Ku-band communication system analysis, S-band system investigations, payload communication investigations, shuttle/TDRSS and GSTDN compatibility analysis are discussed
Contour extraction from HVEM image of microvessel using active contour models
This thesis reports the research results on automatic contour extraction from high voltage electron microscope (HVEM) image of thick cross section montages of small blood vessels. The previous work on this subject, which was based on the conventional edge detection operations combined with edge linking, has proven inadequate to describe the inner structural compartments of microvessels. In this thesis, an active contour model (commonly referred to as Snakes ) has been applied to advance the previous work. Active contour models have proven themselves to be a powerful and flexible paradigm for many problems in image understanding, especially in contour extraction from medical images. With the developed energy functions, the active contour is attracted towards the edges under the action of internal forces (describing some elasticity properties of the contour), image forces and external forces by means of minimization of the energy functions. Based on this active model, an effective algorithm is implemented as a powerful tool for 2-D contour extraction in our problem for the first time. The results thus obtained turn out to be encouraging
Proceedings of the second "international Traveling Workshop on Interactions between Sparse models and Technology" (iTWIST'14)
The implicit objective of the biennial "international - Traveling Workshop on
Interactions between Sparse models and Technology" (iTWIST) is to foster
collaboration between international scientific teams by disseminating ideas
through both specific oral/poster presentations and free discussions. For its
second edition, the iTWIST workshop took place in the medieval and picturesque
town of Namur in Belgium, from Wednesday August 27th till Friday August 29th,
2014. The workshop was conveniently located in "The Arsenal" building within
walking distance of both hotels and town center. iTWIST'14 has gathered about
70 international participants and has featured 9 invited talks, 10 oral
presentations, and 14 posters on the following themes, all related to the
theory, application and generalization of the "sparsity paradigm":
Sparsity-driven data sensing and processing; Union of low dimensional
subspaces; Beyond linear and convex inverse problem; Matrix/manifold/graph
sensing/processing; Blind inverse problems and dictionary learning; Sparsity
and computational neuroscience; Information theory, geometry and randomness;
Complexity/accuracy tradeoffs in numerical methods; Sparsity? What's next?;
Sparse machine learning and inference.Comment: 69 pages, 24 extended abstracts, iTWIST'14 website:
http://sites.google.com/site/itwist1
Gap and channelled plasmons in tapered grooves: a review
Tapered metallic grooves have been shown to support plasmons --
electromagnetically coupled oscillations of free electrons at metal-dielectric
interfaces -- across a variety of configurations and V-like profiles. Such
plasmons may be divided into two categories: gap-surface plasmons (GSPs) that
are confined laterally between the tapered groove sidewalls and propagate
either along the groove axis or normal to the planar surface, and channelled
plasmon polaritons (CPPs) that occupy the tapered groove profile and propagate
exclusively along the groove axis. Both GSPs and CPPs exhibit an assortment of
unique properties that are highly suited to a broad range of cutting-edge
nanoplasmonic technologies, including ultracompact photonic circuits,
quantum-optics components, enhanced lab-on-a-chip devices, efficient
light-absorbing surfaces and advanced optical filters, while additionally
affording a niche platform to explore the fundamental science of plasmon
excitations and their interactions. In this Review, we provide a research
status update of plasmons in tapered grooves, starting with a presentation of
the theory and important features of GSPs and CPPs, and follow with an overview
of the broad range of applications they enable or improve. We cover the
techniques that can fabricate tapered groove structures, in particular
highlighting wafer-scale production methods, and outline the various photon-
and electron-based approaches that can be used to launch and study GSPs and
CPPs. We conclude with a discussion of the challenges that remain for further
developing plasmonic tapered-groove devices, and consider the future directions
offered by this select yet potentially far-reaching topic area.Comment: 32 pages, 34 figure
Interference-based Investigation of Microscopic Objects Near Surfaces: a View From Below
Phenomena occurring when microscopic objects approach planar surfaces are challenging to probe directly because their dynamics cannot be resolved with a sufficiently high spatial/temporal resolution in a non-invasive way, and suitable techniques/methods involve complex instrumentation/computations of limited accessibility/applicability. Interference-based techniques can overcome these barriers. However, because most set-ups and analysis methods are ideal for planar-like geometries, their accurate application for studying microscopic objects has been difficult. Reflection interference contrast microscopy (RICM) has shown particular promise allowing objects in close proximity to a surface to be observed from below, producing interferograms that inherently embed detailed information about the objects’ topography near the substrate. Because precise extraction of this information has been challenging, this study seeks to develop analysis methods applicable to RICM to facilitate its practical implementation for accurate investigation of interfacial phenomena between microscopic objects and surfaces.
The most sophisticated theory of RICM was significantly improved and coupled with a general method to simulate the interference pattern from arbitrary convex geometries. Experimental results revealed that accurate reconstruction of an object’s contour is possible by fitting its interferogram; however, this is computationally intensive and of limited applicability, motivating the formulation of a simplified and accurate RICM model. This facilitated a major breakthrough: an innovative analysis of RICM interferograms provides the inclination angles of the geometry under study and a mathematical procedure allows near-instantaneous reconstruction of the contour with nanometer-scale resolution, applicable to arbitrarily shaped convex objects under different experimental conditions.
A method for extracting nanometer-scale topographic information from RICM interferograms has been proposed; in particular, microspheres can be conveniently analyzed to measure surface roughness based on fringe visibility. Also, precise and accurate measurements of microspheres’ size were performed by means of optimized and robust fringe spacing analysis. Finally, RICM’s distinctive “view-from-below” perspective was applied in simple experiments involving the deposition of microspheres on surfaces, directly revealing the existence of different scenarios depending on deposition media and unique femtoliter-scale capillary condensation dynamics underneath micron-sized glass beads. Results show that RICM has a clear potential for near real-time analysis of ensembles of objects near surfaces so that statistical/probabilistic behavior can be realistically captured
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