59 research outputs found
Multi-View Polarimetric Scattering Cloud Tomography and Retrieval of Droplet Size
Tomography aims to recover a three-dimensional (3D) density map of a medium or an object. In medical imaging, it is extensively used for diagnostics via X-ray computed tomography (CT). We define and derive a tomography of cloud droplet distributions via passive remote sensing. We use multi-view polarimetric images to fit a 3D polarized radiative transfer (RT) forward model. Our motivation is 3D volumetric probing of vertically-developed convectively-driven clouds that are ill-served by current methods in operational passive remote sensing. Current techniques are based on strictly 1D RT modeling and applied to a single cloudy pixel, where cloud geometry defaults to that of a plane-parallel slab. Incident unpolarized sunlight, once scattered by cloud-droplets, changes its polarization state according to droplet size. Therefore, polarimetric measurements in the rainbow and glory angular regions can be used to infer the droplet size distribution. This work defines and derives a framework for a full 3D tomography of cloud droplets for both their mass concentration in space and their distribution across a range of sizes. This 3D retrieval of key microphysical properties is made tractable by our novel approach that involves a restructuring and differentiation of an open-source polarized 3D RT code to accommodate a special two-step optimization technique. Physically-realistic synthetic clouds are used to demonstrate the methodology with rigorous uncertainty quantification
Single View Refractive Index Tomography with Neural Fields
Refractive Index Tomography is an inverse problem in which we seek to
reconstruct a scene's 3D refractive field from 2D projected image measurements.
The refractive field is not visible itself, but instead affects how the path of
a light ray is continuously curved as it travels through space. Refractive
fields appear across a wide variety of scientific applications, from
translucent cell samples in microscopy to fields of dark matter bending light
from faraway galaxies. This problem poses a unique challenge because the
refractive field directly affects the path that light takes, making its
recovery a non-linear problem. In addition, in contrast with traditional
tomography, we seek to recover the refractive field using a projected image
from only a single viewpoint by leveraging knowledge of light sources scattered
throughout the medium. In this work, we introduce a method that uses a
coordinate-based neural network to model the underlying continuous refractive
field in a scene. We then use explicit modeling of rays' 3D spatial curvature
to optimize the parameters of this network, reconstructing refractive fields
with an analysis-by-synthesis approach. The efficacy of our approach is
demonstrated by recovering refractive fields in simulation, and analyzing how
recovery is affected by the light source distribution. We then test our method
on a simulated dark matter mapping problem, where we recover the refractive
field underlying a realistic simulated dark matter distribution
Learned 3D volumetric recovery of clouds and its uncertainty for climate analysis
Significant uncertainty in climate prediction and cloud physics is tied to
observational gaps relating to shallow scattered clouds. Addressing these
challenges requires remote sensing of their three-dimensional (3D)
heterogeneous volumetric scattering content. This calls for passive scattering
computed tomography (CT). We design a learning-based model (ProbCT) to achieve
CT of such clouds, based on noisy multi-view spaceborne images. ProbCT infers -
for the first time - the posterior probability distribution of the
heterogeneous extinction coefficient, per 3D location. This yields arbitrary
valuable statistics, e.g., the 3D field of the most probable extinction and its
uncertainty. ProbCT uses a neural-field representation, making essentially
real-time inference. ProbCT undergoes supervised training by a new labeled
multi-class database of physics-based volumetric fields of clouds and their
corresponding images. To improve out-of-distribution inference, we incorporate
self-supervised learning through differential rendering. We demonstrate the
approach in simulations and on real-world data, and indicate the relevance of
3D recovery and uncertainty to precipitation and renewable energy
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Cloud tomography applied to sky images: a virtual testbed
Two tomographic techniques are applied to two simulated sets of sky images with different cloud fraction. The Algebraic Reconstruction Technique (ART) is applied to optical depth maps from sky images to reconstruct 3-D cloud extinction coefficients without considering multiple scattering effects. Reconstruction accuracy is explored for different products, including surface irradiance and extinction coefficients, and as a function of the number of available sky imagers and setup distance. Increasing the number of imagers improves the accuracy of the 3-D reconstruction: for surface irradiance, the error decreases significantly up to four imagers at which point the improvements become marginal. But using nine imagers gives more robust results in practical situations in which the circumsolar region of images has to be excluded due to poor cloud detection. The ideal distance between imagers was also explored: for a cloud height of 1 km, increasing distance up to 3 km (the domain length) improved the 3-D reconstruction. An iterative reconstruction technique that iteratively updated the source function improved the results of the ART by minimizing the error between input red radiance images and reconstructed red radiance simulations. For the best case of a nine-imager deployment, the ART and iterative method resulted in 53.4% and 33.6% relative mean absolute error for the extinction coefficients, respectively.The authors acknowledge funding from the California Energy Commission EPIC program. Felipe Mejia was supported by the National Science Foundation Graduate Research Fellowship under Grant No. (DGE-1144086). In addition, Íñigo de la Parra has been partially supported by the Spanish State Research Agency (AEI) and FEDER-UE under grants DPI2016-80641-R and DPI2016-80642-R
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