189 research outputs found
Aberrated dark-field imaging systems
We study generalized dark-field imaging systems. These are a subset of linear
shift-invariant optical imaging systems, that exhibit arbitrary aberrations,
and for which normally-incident plane-wave input yields zero output. We write
down the theory for the forward problem of imaging coherent scalar optical
fields using such arbitrarily-aberrated dark-field systems, and give numerical
examples. The associated images may be viewed as a form of dark-field Gabor
holography, utilizing arbitrary outgoing Green functions as generalized
Huygens-type wavelets, and with the Young-type boundary wave forming the
holographic reference
Phase-and-amplitude recovery from a single phase contrast image using partially spatially coherent X-ray radiation
A simple method of phase-and-amplitude extraction is derived that corrects
for image blurring induced by partially spatially coherent incident
illumination using only a single intensity image as input. The method is based
on Fresnel diffraction theory for the case of high Fresnel number, merged with
the space-frequency description formalism used to quantify partially coherent
fields and assumes the object under study is composed of a single material. A
priori knowledge of the object's complex refractive index and information
obtained by characterizing the spatial coherence of the source is required. The
algorithm was applied to propagation-based phase contrast data measured with a
laboratory-based micro-focus X-ray source. The blurring due to the finite
spatial extent of the source is embedded within the algorithm as a simple
correction term to the so-called Paganin algorithm and is also numerically
stable in the presence of noise
Aberrations in shift-invariant linear optical imaging systems using partially coherent fields
Here the role and influence of aberrations in optical imaging systems
employing partially coherent complex scalar fields is studied. Imaging systems
require aberrations to yield contrast in the output image. For linear
shift-invariant optical systems, we develop an expression for the output
cross-spectral density under the space-frequency formulation of statistically
stationary partially coherentfields. We also develop expressions for the output
cross{spectral density and associated spectral density for weak-phase,
weak-phase-amplitude, and single-material objects in one transverse spatial
dimension
A fast implicit X-ray diffusive-dark-field retrieval method using a single mask and exposure
Complementary to conventional and phase X-ray radiography, dark-field imaging
has become central in visualizing diffusive scattering signal due to the
spatially-unresolved texture within an object. To date most
diffusive-dark-field retrieval methods require either the acquisition of
multiple images at the cost of higher radiation dose or significant amounts of
computational memory and time. In this work, a simple method of X-ray diffusive
dark-field retrieval is presented, applicable to any single-mask imaging setup,
with only one exposure of the sample. The approach, which is based on a model
of geometric and diffusive reverse-flow conservation, is implicit and
non-iterative. This numerically fast methodology is applied to experimental
X-ray images acquired using both a random mask and a grid mask, giving high
quality reconstructions that are very stable in the presence of noise. The
method should be useful for high-speed imaging and/or imaging with low-flux
sources
360-degree Video Stitching for Dual-fisheye Lens Cameras Based On Rigid Moving Least Squares
Dual-fisheye lens cameras are becoming popular for 360-degree video capture,
especially for User-generated content (UGC), since they are affordable and
portable. Images generated by the dual-fisheye cameras have limited overlap and
hence require non-conventional stitching techniques to produce high-quality
360x180-degree panoramas. This paper introduces a novel method to align these
images using interpolation grids based on rigid moving least squares.
Furthermore, jitter is the critical issue arising when one applies the
image-based stitching algorithms to video. It stems from the unconstrained
movement of stitching boundary from one frame to another. Therefore, we also
propose a new algorithm to maintain the temporal coherence of stitching
boundary to provide jitter-free 360-degree videos. Results show that the method
proposed in this paper can produce higher quality stitched images and videos
than prior work.Comment: Preprint versio
Effective brilliance amplification in neutron propagation-based phase contrast imaging
Propagation-based neutron phase-contrast tomography was demonstrated on an
insect sample, using the ISIS pulsed spallation source. In our proof-of-concept
low-fluence experiment the tomogram with Paganin-type phase-retrieval filter
applied exhibited an effective net boost of in the signal-to-noise
ratio as compared to an attenuation-based tomogram, implying an effective boost
in neutron brilliance of well over two orders of magnitude. The phase-retrieval
filter applies to monochromatic as well as poly-energetic neutron beams.
Expressions are provided for the optimal phase-contrast geometry as well as
conditions for the validity of the method. The underpinning theory is derived
under the assumption of the sample being composed of a single material, but
this can be generalized. The effective boost in brilliance may be employed to
give reduced acquisition time, or may instead be used to keep exposure times
fixed while improving contrast and spatial resolution
Azimuthally Differential Pion Femtoscopy in Pb-Pb Collisions at root s(NN)=2.76 TeV
We present the first azimuthally differential measurements of the pion source size relative to the second harmonic event plane in Pb-Pb collisions at a center-of-mass energy per nucleon-nucleon pair of root(NN)-N-s = 2.76 TeV. The measurements have been performed in the centrality range 0%-50% and for pion pair transverse momenta 0.2 <k(T) <0.7 GeV/c. We find that the R-side and R-out radii, which characterize the pion source size in the directions perpendicular and parallel to the pion transverse momentum, oscillate out of phase, similar to what was observed at the Relativistic Heavy Ion Collider. The final-state source eccentricity, estimated via R-side oscillations, is found to be significantly smaller than the initial-state source eccentricity, but remains positive-indicating that even after a stronger expansion in the in-plane direction, the pion source at the freeze-out is still elongated in the out-of-plane direction. The 3 + 1D hydrodynamic calculations are in qualitative agreement with observed centrality and transverse momentum R-side oscillations, but systematically underestimate the oscillation magnitude.Peer reviewe
D-meson production in p-Pb collisions at root S-NN=5.02 TeV and in pp collisions at root S=7 TeV
Peer reviewe
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