15,738 research outputs found
Infrared thermograms applied to near-field testing
Electromagnetic fields close to radiant structures can be measured quickly using an infrared camera. Examples of induced fields by wire antennas over a detection screen at distances shorter than one wavelength are presented. The measured thermograms agree with simulations that take into account heat propagation on the detection screenPeer ReviewedPostprint (published version
Infrared thermograms applied to near-field testing
Electromagnetic fields close to radiant structures can be measured quickly using an infrared camera. Examples of induced fields by wire antennas over a detection screen at distances shorter than one wavelength are presented. The measured thermograms agree with simulations that take into account heat propagation on the detection screen.Peer ReviewedPostprint (published version
Telecommunications in cometary environments
Propagation effects on telecommunications in a cometary environment include those due to dust, the inhomogeneous plasma of the coma and tail, and ionization generated by impact of neutral molecules and dust on the spacecraft. Attenuation caused by dust particles is estimated to be on the order of 10 to the minus 5th power dB for the Halley Intercept Mission. Ionization generated by impact on the spacecraft is estimated to result in an electron content of 10 to the 12th power to 10 to the 13th power el/sq meters (3 eV electrons) along the telecommunications path. An estimate of the electron content due to Comet Halley itself is 10 to the 16th power to 10 to the 17th power el/sq meters, compared to a content of 10 to the 16th power to 10 to the 18th power el/sq meters for the Earth's ionosphere and 10 to the 17th power to 10 to the 18th power el/sq meters for the interplanetary medium. The electron content of the plasma near Comet Halley will cause excess range delay, and a Doppler shift of the signal from the spacecraft will occur in propagation to the rate of change of the path electron content. It is recommended that S and X down-link frequencies by employed to monitor the path electron content and amplitude scintillation and spectral broadening of the received signals. These measurements will provide a quantitative base of knowledge that will be valuable for radio science and telecommunications system design purposes
Scattering of gravitational radiation: second order moments of the wave amplitude
Gravitational radiation that propagates through an inhomogeneous mass
distribution is subject to random gravitational lensing, or scattering, causing
variations in the wave amplitude and temporal smearing of the signal. A
statistical theory is constructed to treat these effects. The statistical
properties of the wave amplitude variations are a direct probe of the power
spectrum of the mass distribution through which the waves propagate. Scattering
temporally smears any intensity variations intrinsic to a source emitting
gravitational radiation, rendering variability on time scales shorter than the
temporal smearing time scale unobservable, and potentially making the radiation
much harder to detect. Gravitational radiation must propagate out through the
mass distribution of its host galaxy before it can be detected at the Earth.
Plausible models for the distribution of matter in an host galaxy suggest
that the temporal smearing time scale is at least several milliseconds due to
the gas content alone, and may be as large as a second if dark matter also
scatters the radiation. The smearing time due to scattering by any galaxy
interposed along the line of sight is a factor times larger.
Gravitational scattering is an excellent probe of matter on parsec and
sub-parsec scales, and has the potential to elucidate the nature of dark
matter.Comment: A&A accepted, 19 pages, 4 fig
Analytical Approximations for Calculating the Escape and Absorption of Radiation in Clumpy Dusty Environments
We present analytical approximations for calculating the scattering,
absorption and escape of nonionizing photons from a spherically symmetric
two-phase clumpy medium, with either a central point source of isotropic
radiation, a uniform distribution of isotropic emitters, or uniformly
illuminated by external sources. The analytical approximations are based on the
mega-grains model of two-phase clumpy media, as proposed by Hobson & Padman,
combined with escape and absorption probability formulae for homogeneous media.
The accuracy of the approximations is examined by comparison with 3D Monte
Carlo simulations of radiative transfer, including multiple scattering. Our
studies show that the combined mega-grains and escape/absorption probability
formulae provide a good approximation of the escaping and absorbed radiation
fractions for a wide range of parameters characterizing the medium. A realistic
test is performed by modeling the absorption of a starlike source of radiation
by interstellar dust in a clumpy medium, and by calculating the resulting
equilibrium dust temperatures and infrared emission spectrum of both the clumps
and the interclump medium. In particular, we find that the temperature of dust
in clumps is lower than in the interclump medium if clumps are optically thick.
Comparison with Monte Carlo simulations of radiative transfer in the same
environment shows that the analytic model yields a good approximation of dust
temperatures and the emerging UV to FIR spectrum of radiation for all three
types of source distributions mentioned above. Our analytical model provides a
numerically expedient way to estimate radiative transfer in a variety of
interstellar conditions and can be applied to a wide range of astrophysical
environments, from star forming regions to starburst galaxies.Comment: 55 pages, 27 figures. ApJ 523 (1999), in press. Corrected equations
and text so as to be same as ApJ versio
The Iray Light Transport Simulation and Rendering System
While ray tracing has become increasingly common and path tracing is well
understood by now, a major challenge lies in crafting an easy-to-use and
efficient system implementing these technologies. Following a purely
physically-based paradigm while still allowing for artistic workflows, the Iray
light transport simulation and rendering system allows for rendering complex
scenes by the push of a button and thus makes accurate light transport
simulation widely available. In this document we discuss the challenges and
implementation choices that follow from our primary design decisions,
demonstrating that such a rendering system can be made a practical, scalable,
and efficient real-world application that has been adopted by various companies
across many fields and is in use by many industry professionals today
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