19,494 research outputs found
Detection of Edges in Spectral Data II. Nonlinear Enhancement
We discuss a general framework for recovering edges in piecewise smooth
functions with finitely many jump discontinuities, where . Our approach is based on two main aspects--localization using
appropriate concentration kernels and separation of scales by nonlinear
enhancement.
To detect such edges, one employs concentration kernels, ,
depending on the small scale . It is shown that odd kernels, properly
scaled, and admissible (in the sense of having small -moments of
order ) satisfy , thus recovering both the location and amplitudes of all edges.As
an example we consider general concentration kernels of the form
to detect edges from the first
spectral modes of piecewise smooth f's. Here we improve in
generality and simplicity over our previous study in [A. Gelb and E. Tadmor,
Appl. Comput. Harmon. Anal., 7 (1999), pp. 101-135]. Both periodic and
nonperiodic spectral projections are considered. We identify, in particular, a
new family of exponential factors, , with superior
localization properties.
The other aspect of our edge detection involves a nonlinear enhancement
procedure which is based on separation of scales between the edges, where
, and the smooth regions where . Numerical examples demonstrate that by coupling
concentration kernels with nonlinear enhancement one arrives at effective edge
detectors
Second-harmonic phonon spectroscopy of -quartz
We demonstrate midinfrared second-harmonic generation as a highly sensitive
phonon spectroscopy technique that we exemplify using -quartz (SiO)
as a model system. A midinfrared free-electron laser provides direct access to
optical phonon resonances ranging from to $1400\
\mathrm{cm}^{-1}T_c=846\ \mathrm{K}\alpha\beta$-quartz,
demonstrating the technique's potential for studies of phase transitions
Far-field scattering microscopy applied to analysis of slow light, power enhancement, and delay times in uniform Bragg waveguide gratings
A novel method is presented for determining the group index, intensity enhancement and delay times for waveguide gratings, based on (Rayleigh) scattering observations. This far-field scattering microscopy (FScM) method is compared with the phase shift method and a method that uses the transmission spectrum to quantify the slow wave properties. We find a minimum group velocity of 0.04c and a maximum intensity enhancement of ~14.5 for a 1000-period grating and a maximum group delay of ~80 ps for a 2000-period grating. Furthermore, we show that the FScM method can be used for both displaying the intensity distribution of the Bloch resonances and for investigating out of plane losses. Finally, an application is discussed for the slow-wave grating as index sensor able to detect a minimum cladding index change of , assuming a transmission detection limit of
A Data-Driven Edge-Preserving D-bar Method for Electrical Impedance Tomography
In Electrical Impedance Tomography (EIT), the internal conductivity of a body
is recovered via current and voltage measurements taken at its surface. The
reconstruction task is a highly ill-posed nonlinear inverse problem, which is
very sensitive to noise, and requires the use of regularized solution methods,
of which D-bar is the only proven method. The resulting EIT images have low
spatial resolution due to smoothing caused by low-pass filtered regularization.
In many applications, such as medical imaging, it is known \emph{a priori} that
the target contains sharp features such as organ boundaries, as well as
approximate ranges for realistic conductivity values. In this paper, we use
this information in a new edge-preserving EIT algorithm, based on the original
D-bar method coupled with a deblurring flow stopped at a minimal data
discrepancy. The method makes heavy use of a novel data fidelity term based on
the so-called {\em CGO sinogram}. This nonlinear data step provides superior
robustness over traditional EIT data formats such as current-to-voltage
matrices or Dirichlet-to-Neumann operators, for commonly used current patterns.Comment: 24 pages, 11 figure
Image Segmentation with Eigenfunctions of an Anisotropic Diffusion Operator
We propose the eigenvalue problem of an anisotropic diffusion operator for
image segmentation. The diffusion matrix is defined based on the input image.
The eigenfunctions and the projection of the input image in some eigenspace
capture key features of the input image. An important property of the model is
that for many input images, the first few eigenfunctions are close to being
piecewise constant, which makes them useful as the basis for a variety of
applications such as image segmentation and edge detection. The eigenvalue
problem is shown to be related to the algebraic eigenvalue problems resulting
from several commonly used discrete spectral clustering models. The relation
provides a better understanding and helps developing more efficient numerical
implementation and rigorous numerical analysis for discrete spectral
segmentation methods. The new continuous model is also different from
energy-minimization methods such as geodesic active contour in that no initial
guess is required for in the current model. The multi-scale feature is a
natural consequence of the anisotropic diffusion operator so there is no need
to solve the eigenvalue problem at multiple levels. A numerical implementation
based on a finite element method with an anisotropic mesh adaptation strategy
is presented. It is shown that the numerical scheme gives much more accurate
results on eigenfunctions than uniform meshes. Several interesting features of
the model are examined in numerical examples and possible applications are
discussed
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