Radiative transfer

About: Radiative transfer is a research topic. Over the lifetime, 43287 publications have been published within this topic receiving 1176539 citations.

Iterative reconstruction scheme for optical tomography based on the equation of radiative transfer.

Abstract: We report on the development of an iterative image reconstruction scheme for optical tomography that is based on the equation of radiative transfer. Unlike the commonly applied diffusion approximation, the equation of radiative transfer accurately describes the photon propagation in turbid media without any limiting assumptions regarding the optical properties. The reconstruction scheme consists of three major parts: (1) a forward model that predicts the detector readings based on solutions of the time-independent radiative transfer equation, (2) an objective function that provides a measure of the differences between the detected and the predicted data, and (3) an updating scheme that uses the gradient of the objective function to perform a line minimization to get new guesses of the optical properties. The gradient is obtained by employing an adjoint differentiation scheme, which makes use of the structure of the finite-difference discrete-ordinate formulation of the transport forward model. Based on the new guess of the optical properties a new forward calculation is performed to get new detector predictions. The reconstruction process is completed when the minimum of the objective function is found within a defined error. To illustrate the performance of the code we present initial reconstruction results based on simulated data.

Theory of the optical properties of snow

Abstract: The optical properties of a snowpack are calculated, giving relations between the asymptotic flux extinction coefficient and albedo under diffuse illumination on the one hand and the density, grain size, and wavelength, on the other. A geometrical optics calculation of the properties of a single grain is used with approximate solutions of the radiative transfer problem. Most of the scattering is the result of change in direction of the light beam upon transmission through the grain, rather than reflection. The asymptotic flux extinction coefficient is directly proportional to the snowpack density and inversely proportional to the square root of the grain size. The albedo under diffuse illumination is independent of density and proportional to the square root of the grain size. The derived relations agree with experimental data to within 20% for the flux extinction coefficient and to better than 2% for the albedo without using adjustable parameters. Higher values of the flux extinction coefficient will be measured if the snow is confined within a finite cylinder with absorbing sides. An analysis of this effect is given.

High‐resolution solar spectral irradiance from extreme ultraviolet to far infrared

Abstract: [1] This paper presents new extremely high-resolution solar spectral irradiance (SSI) calculations covering wavelengths from 0.12 nm to 100 micron obtained by the Solar Irradiance Physical Modeling (SRPM) system. Daily solar irradiance spectra were constructed for most of Solar Cycle 23 based on a set of physical models of the solar features and non-LTE calculations of their emitted spectra as function of viewing angle, and solar images specifying the distribution of features on the solar disk. Various observational tests are used to assess the quality of the spectra provided here. The present work emphasizes the effects on the SSI of the upper chromosphere and full-non-LTE radiative transfer calculation of level populations and ionizations that are essential for physically consistent results at UV wavelengths and for deep lines in the visible and IR. This paper also considers the photodissociation continuum opacity of molecular species, e.g., CH and OH, and proposes the consideration of NH photodissociation which can solve the puzzle of the missing near-UV opacity in the spectral range of the near-UV. Finally, this paper is based on physical models of the solar atmosphere and extends the previous lower-layer models into the upper-transition-region and coronal layers that are the dominant source of photons at wavelengths shorter than ∼50 nm (except for the He II 30.4 nm line, mainly formed in the lower-transition-region).

Thermal conductivity of thermal barrier coatings

Abstract: In thermal barrier coatings and other ceramic oxides, heat is conducted by lattice waves, and also by a radiative component which becomes significant at high temperatures. The theory of heat conduction by lattice waves is reviewed in the equipartition limit (above room temperature). The conductivity is composed of contributions from a spectrum of waves, determined by the frequency dependent attenuation length. Interaction between lattice waves (intrinsic processes), scattering by atomic scale point defects and scattering by extended imperfections such as grain boundaries, each limit the attenuation length in different parts of the spectrum. Intrinsic processes yield a spectral conductivity which is independent of frequency. Point defects reduce the contribution of the high frequency spectrum, grain boundaries and other extended defects that of the low frequencies. These reductions are usually independent of each other. Estimates will be given for zirconia containing 7wt% Y 2 O 3 , and for yttrium aluminum garnet. They will be compared to measurements. The effects of grain size, cracks and porosity will be discussed both for the lattice and the radiative components. While the lattice component of the thermal conductivity is reduced substantially by decreasing the grain size to nanometers, the radiative component requires pores or other inclusions of micrometer scale.