556 research outputs found
Comparison of the estimation of the degree of polarization from four or two intensity images degraded by speckle noise
Active polarimetric imagery is a powerful tool for accessing the information
present in a scene. Indeed, the polarimetric images obtained can reveal
polarizing properties of the objects that are not avalaible using conventional
imaging systems. However, when coherent light is used to illuminate the scene,
the images are degraded by speckle noise. The polarization properties of a
scene are characterized by the degree of polarization. In standard polarimetric
imagery system, four intensity images are needed to estimate this degree . If
we assume the uncorrelation of the measurements, this number can be decreased
to two images using the Orthogonal State Contrast Image (OSCI). However, this
approach appears too restrictive in some cases. We thus propose in this paper a
new statistical parametric method to estimate the degree of polarization
assuming correlated measurements with only two intensity images. The estimators
obtained from four images, from the OSCI and from the proposed method, are
compared using simulated polarimetric data degraded by speckle noise
An Ultra Fast Image Generator (UFig) for wide-field astronomy
Simulated wide-field images are becoming an important part of observational
astronomy, either to prepare for new surveys or to test measurement methods. In
order to efficiently explore vast parameter spaces, the computational speed of
simulation codes is a central requirement to their implementation. We introduce
the Ultra Fast Image Generator (UFig) which aims to bring wide-field imaging
simulations to the current limits of computational capabilities. We achieve
this goal through: (1) models of galaxies, stars and observational conditions,
which, while simple, capture the key features necessary for realistic
simulations, and (2) state-of-the-art computational and implementation
optimizations. We present the performances of UFig and show that it is faster
than existing public simulation codes by several orders of magnitude. It allows
us to produce images more quickly than SExtractor needs to analyze them. For
instance, it can simulate a typical 0.25 deg^2 Subaru SuprimeCam image (10k x
8k pixels) with a 5-sigma limiting magnitude of R=26 in 30 seconds on a laptop,
yielding an average simulation time for a galaxy of 30 microseconds. This code
is complementary to end-to-end simulation codes and can be used as a fast,
central component of observational methods relying on simulations.Comment: Submitted to Astronomy and Computing. 13 pages, 9 figure
Off-equilibrium fluctuation-dissipation relation in a spin glass
We report new experimental results obtained onthe insulating spin glass
CdCrInS_4. Our experimental setup allows a quantitative comparison between
thethermo-remanent magnetization and the autocorrelation of spontaneous
fluctuations of magnetization, yielding acomplete determination of the
fluctuation-dissipationrelation. The dynamics can be studied both in
thequasi-equilibrium regime, where the fluctuation-dissipation theorem holds,
and in the deeply aging regime. The limit of separation of time-scales, as used
in analytical calculations, can be approached by use of a scaling procedure.Comment: Minor changes, accepted for publication in EPJ-
Quantum limits in image processing
We determine the bound to the maximum achievable sensitivity in the
estimation of a scalar parameter from the information contained in an optical
image in the presence of quantum noise. This limit, based on the Cramer-Rao
bound, is valid for any image processing protocol. It is calculated both in the
case of a shot noise limited image and of a non-classical illumination. We also
give practical experimental implementations allowing us to reach this absolute
limit.Comment: 4 pages, two figure
Cramer-Rao lower bound for the estimation of the degree of polarization in active coherent imagery at low photon level
International audienceThe degree of polarization (DOP) is an important tool in many optical measurement and imaging applications. We address the problem of its estimation in images that are perturbed with both speckle and photon noises, by determining the Cramer-Rao Lower Bound (CRLB) when the illuminated materials are purely depolarizing. We demonstrate that the CRLB is simply the sum of the CRLBs due to speckle noise and to Poisson noise. We use this result to analyze the influence of different optical parameters on DOP estimation
Implementation of arbitrary real-valued correlation filters for the shadow-casting incoherent correlator
International audienceWe describe an incoherent correlator, based on the shadow-casting principle, that is able to implement any real-valued linear correlation filter. The correlation filter and the input image are displayed on commercial liquid-crystal television ~LCTV! panels. Although it cannot handle high-resolution images, the incoherent correlator is lensless, compact, low cost, and uses a white-light source. A bipolar technique is devised to represent any linear filter, computed from a single reference image or composite, in the correlator. We demonstrate experimentally the efficiency of the design in the case of optimal trade-off ~OT! filters and optimal trade-off synthetic discriminant function ~OT-SDF! filter
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