Thermal radiation is ubiquitous to all matter at finite temperature and controlling the radiative nature of that matter has been a key enabling factor in the development of several recent technologies, such as thermal diodes, thermal antennae, thermophotovoltaics, heat-assisted magnetic recording, and contactless cooling in microelectromechanical systems. At the micro/nano-scale, thermal radiation does not reliably behave in the way Planck's blackbody law predicts, due to near-field effects such as the diffraction, interference, and tunneling of light. In fact, the so-called blackbody limit can routinely be broken by several orders of magnitude when objects of dimensions or separation distances much smaller than the peak thermal wavelength (approximately 10 \si{\micro\meter} at room temperature) exchange thermal radiation. A deeper theory is required to understand near-field thermal radiation: Maxwell's equations. Maxwell's equations allow for a direct connection between the thermally induced current fluctuations and radiative transfer. In this dissertation, I investigate radiative transfer among spherical bodies aligned in a linear chain. The chain may be composed of any number of spheres, and the spheres themselves may be composed of any linear isotropic material, may be of any size and separation distance, and may each have any number of spherically symmetric layers. Using a dyadic Green's function formalism, I derive numerically exact formulas for heat transfer between pairs of spheres in the chain and between any sphere in the chain and its environment. My work clearly demonstrates that adding coatings to spherical objects can drastically impact the spectrum of radiative transfer, enhancing or diminishing it in various cases. This degree of tailoring makes coated spheres a flexible, yet unexplored, platform for future experiments in near-field radiative heat transfer. My work also demonstrates that, in an experiment measuring the distance dependent heat transfer between two spheres, heat transfer from the spheres to their environment can also have a strong distance dependence, which must be considered carefully when designing an experiment and analyzing its results. This demonstrates a cautious but optimistic outlook for the near-field radiative heat transfer community moving beyond traditional plane-plane and sphere-plane experimental configurations.

Many uncertainties remain as to how the most massive stars are formed and how they interact with their environment via radiative and mechanical processes. This feedback may affect future generations of star formation -- triggering it by compressing gas, or hindering it by dispersing reservoirs. These scenarios can be simulated by solving the equations of hydrodynamics and radiative transfer. However, the latter is usually simplified due to its computational expense, despite its importance in determining the dynamics. In this thesis, I describe how I increased the efficiency of the radiation hydrodynamics code, TORUS, which uses a Monte Carlo approach to solving the radiative transfer. Tens of millions of energy packets are propagated through a domain split over hundreds of processors running in parallel with Message Passing Interface (MPI). By re-examining and improving communication algorithms, I lowered the radiation run time by about a factor of ten, making it tractable to run three-dimensional simulations of massive star feedback in clusters. This includes both the stellar and diffuse radiation fields, with multiple atomic species and silicate dust grains. The full ionization states and temperatures can then be fed in to produce self-consistent synthetic observations. I applied this to clouds of 1000 and 10,000 solar masses with surface density 0.01 g/cm^2, containing a 34 solar mass star, with photoionization and radiation pressure feedback. Photoionization is efficient at shaping and dispersing clouds. The expanding ionization front forms dense, spherical knots with pillars pointing away from the emitting star. These resemble the Pillars of Creation in the Eagle Nebula, and the proplyds observed in the Orion Nebula. In the lower-mass model, almost all material is removed from the (15.5 pc)^3 grid within 1.6 Myr; the higher mass cloud is somewhat more resistant, with 25 per cent remaining inside (32.3 pc)^3 after 4.3 Myr. Radiation pressure has a negligible effect, but will be more important for denser clouds or higher luminosities.

In this thesis, I apply techniques from the field of computer graphics to ray tracing in astrophysical simulations, and introduce the grace software library. This is combined with an extant radiative transfer solver to produce a new package, taranis. It allows for fully-parallel particle updates via per-particle accumulation of rates, followed by a forward Euler integration step, and is manifestly photon-conserving. To my knowledge, taranis is the first ray-traced radiative transfer code to run on graphics processing units and target cosmological-scale smooth particle hydrodynamics (SPH) datasets. A significant optimization effort is undertaken in developing grace. Contrary to typical results in computer graphics, it is found that the bounding volume hierarchies (BVHs) used to accelerate the ray tracing procedure need not be of high quality; as a result, extremely fast BVH construction times are possible (< 0.02 microseconds per particle in an SPH dataset). I show that this exceeds the performance researchers might expect from CPU codes by at least an order of magnitude, and compares favourably to a state-of-the-art ray tracing solution. Similar results are found for the ray-tracing itself, where again techniques from computer graphics are examined for effectiveness with SPH datasets, and new optimizations proposed. For high per-source ray counts (≳ 104), grace can reduce ray tracing run times by up to two orders of magnitude compared to extant CPU solutions developed within the astrophysics community, and by a factor of a few compared to a state-of-the-art solution. taranis is shown to produce expected results in a suite of de facto cosmological radiative transfer tests cases. For some cases, it currently out-performs a serial, CPU-based alternative by a factor of a few. Unfortunately, for the most realistic test its performance is extremely poor, making the current taranis code unsuitable for cosmological radiative transfer. The primary reason for this failing is found to be a small minority of particles which always dominate the timestep criteria. Several plausible routes to mitigate this problem, while retaining parallelism, are put forward.

A relative, lie-value image normalisation (LVIN) procedure was investigated as a means of estimating surface reflectances from sequences of Landsat TM and ETM+ imagery, and standardising image data for change detection studies when there are uncertainties in sensor calibration and atmospheric parameters over time. The basis of the LVIX procedure is that for an A-date sequence, the digital numbers (DNs) of N-1 overpass images can be mapped to the reflectance values of a reference image for a set of pseudo- invariant targets (PITs) common to all images in the sequence. The robust M-estimator was employed to provide the transformation function that achieved the mapping. The investigation also showed that in some instances the LVIN procedure could incorporate the modelled Path DN-the modelled DN for a target of zero surface reflectance. A lack of surface validation data was a limitation in the investigation. However, a qualitative evaluation of the LVIN procedure was possible by examining the pre- and post-normalisation image histograms. In a comparison with the results of the 6S radiative transfer code, it war observed that when both overpass and reference images were acquired with the same sensor, the LVIK procedure appeared t o correct for atmospheric effects; and when overpass and reference images were with different sensors, the LVIN procedure also corrected for between-sensor differences. Moreover, it was demonstrated for the more "temporally-invariant" PITs that the procedure retrieved surface reflectances that were on average within ±0.02 reflectance units. / The ability of the LVIK procedure to standardise sequences of image data was further demonstrated in the study of vegetation change. The normalised difference vegetation index (NDVI) was calculated from LVIN estimates of surface reflectance for a selection of sites around the township of Mt. Barker, Western Australia. NDVI data had characteristics consistent with data that have been corrected for atmospheric effects. A modification to the LVIN procedure was also proposed based on an investigation of some empirically-derived vegetation reflectance relationships. Research into the robustness of the relationships for a greater range of vegetation types is recommended.

One method of estimating the longwave radiative heating of the atmosphere is to combine satellite observations of emitted radiances with those computed from synoptic temperature and humidity profiles. Modeled and observed radiances are brought into agreement by altering cloud properties or even by adjusting the temperature and water vapor profiles. Here this strategy is examined in an exploratory study using global meteorological data sets and a radiative transfer model typical of those found in general circulation models. Calculated radiances are compared to those observed by the Earth Radiation Budget Satellite (ERBS). Input for the model is obtained from the National Meteorological Center (NMC) in the form of vertical profiles of temperature and relative humidity. The comparisons are limited to clear sky as deduced by ERBE algorithms, and additional filtering which requires homogeneous surface type for a 3 x 3 array of ERBS scanner fields of view. Observations are obtained from 60° N to 60° S that lie within 30 minutes of the NMC analysis time. Following the work of Ramanathan and Downey (1986), comparisons are separated into climatologically distinct groups as well as by satellite viewing angle. This separation is an attempt to distinguish between biases in the radiation model and those in the NMC data set. Results are presented for the months of July 1985, and January 1986. A comparison of the present radiation model's output with that obtained from a Geophysical Fluid Dynamics Laboratory (GFDL) model shows a bias of nearly 3% in the present model for a standard mid-latitude summer profile. Global results show a negative bias in the modeled values for nearly all scenes, except for nighttime desert. The nighttime desert bias may be a result of a skin-air temperature difference not resolved by the NMC analyses. The overall negative bias may be a result of an overestimation of water vapor for regions with low relative humidity. / Graduation date: 1991

Lateral light scattering in fibrous media is investigated by computing the modulation transfer function (MTF) of 22 paper samples using a Monte Carlo model. The simulation tool uses phase functions from infinitely long homogenous cylinders and the directional inhomogeneity of paper is achieved by aligning the cylinders in the plane. The inverse frequency at half maximum of the MTF is compared to both measurements and previous simulations with isotropic and strongly forward single scattering phase functions. It is found that the conical scattering by cylinders enhances the lateral scattering and therefore predicts a larger extent of lateral light scattering than models using rotationally invariant single scattering phase functions. However, it does not fully reach the levels of lateral scattering observed in measurements. It is argued that the hollow lumen of a wood fiber or dependent scattering effects must be considered for a complete description of lateral light scattering in paper. / PaperOpt

Retrievals of atmospheric information from satellite observations permit the investigation of otherwise inaccessible atmospheric phenomena. The recovery of this information from optical instrumentation located in orbit requires both an inversion algorithm like the Saskatchewan Multiplicative Algebraic Reconstruction Technique and a forward model like the SASKTRAN radiative transfer model. These are used together at the University of Saskatchewan to retrieve sulphate aerosol extinction profiles from the radiance measurements made by the Canadian built OSIRIS instrument. Although these retrievals are highly successful the process currently does not consider the polarization of light or OSIRIS's polarization sensitivities because SASKTRAN is a scalar model. In this work the development of a vector version of SASKTRAN that can perform polarized radiative transfer calculations is presented.<p> The vector SASKTRAN's results compare favorably with vector SCIATRAN, another polarized model that is in development at the University of Bremen. Comparisons of the stratospheric aerosol retrieval vectors generated from the scalar and vector SASKTRAN results indicate that the polarized calculations are an important factor in future work to improve the aerosol retrievals and to recover particle size or composition information.

Carbon Monoxide (CO) was discovered in the stratosphere of Neptune from the detection of the J=3-2 and J=2-1 rotational transitions in emission at 345.8 and 230.5 GHz respectively. It was conventionally thought that all of the atmospheric carbon should be in its reduced form of methane (CH₄). Two sources of stratospheric CO have been postulated: CO transported from the interior by convection due to Neptune's strong internal heat source (internal source); or, CO produced through photochemical reactions from an external supply of water (external source). <p>In this research project the J=3-2 transition of CO was observed to find the CO profile in Neptune's atmosphere and determine the mechanism producing CO. Three instruments were used at the James Clerk Maxwell Telescope (JCMT) to measure the CO line: the heterodyne receiver B3; the University of Lethbridge Fourier Transform Spectrometer (FTS); and, the Submillimeter Common User Bolometer Array (SCUBA). <p>The high resolution (1.25 MHz) of the heterodyne observations over a large frequency range (~20 GHz) produced a very powerful result because the narrow emission core from the stratosphere and the broad absorption feature arising in the lower atmosphere were measured simultaneously. The CO abundance profile was determined using a model of the J=3-2 CO transition in Neptune's atmosphere developed for this project. Calculations indicate a CO abundance of 1.9^+0.5_-0.3x10^-6 in the upper stratosphere and (0.8±0.2)x10^-6 in the lower stratosphere and troposphere. <p>The moderate resolution of the FTS data allowed the broad absorption feature to be measured. Uranus was originally chosen as the calibration source, but the discovery of CO in Uranus by Encrenaz et al. (2004), while this project was in progress, prompted both Neptune and Uranus to be examined for CO absorption. Two data sets (1993 and 2002) were analyzed and it was found that the 1993 spectra produced superior results, giving a CO mole ratio in the lower atmosphere between 0.8x10^-6 and 2x10^-5; this agrees, within the uncertainty limit, with the lower atmosphere heterodyne result. A tentative detection of CO in Uranus was also obtained from the 1993 data, with a CO abundance profile constrained to pressures greater than 0.5 bar with an abundance between 5x10^-7 and 1x10^-5. The 2002 data were found to be inferior to the 1993 data because of imperfect cancellation of thermal emission from the terrestrial atmosphere. <p> The 850ìm SCUBA filter profile is well matched to the width of the CO feature. Photometric observations of Neptune and Uranus were used to determine if the reduction in integrated flux due to CO absorption could be detected using SCUBA. A CO mole ratio in the range (1.2-1.7) x10^-6 was found for Neptune, calibrated against Uranus and assuming no CO in Uranus. Calibration of the Neptune and Uranus SCUBA data against Mars to produce an independent estimate of the CO abundance in both planets did not produce a useful result because of large calibration errors. <p>Comparison of the results from the three techniques determined that the heterodyne measurement was superior and the derived CO profile was used to determine the source of neptunian CO. It was concluded that the source of CO in Neptune is both internal and external. The lower atmosphere result indicates an interior dominated by water ice. The most likely mechanism for the upper atmosphere CO involves meteoritic ablation, photolysis of H₂O, and chemical reaction with by-products of methane photochemistry. The required H₂O influx for this mechanism is at least two orders of magnitude higher than previously observed, indicating either that the observed H₂O abundance is too small or that CO is produced by a different mechanism.

Carbon Monoxide (CO) was discovered in the stratosphere of Neptune from the detection of the J=3-2 and J=2-1 rotational transitions in emission at 345.8 and 230.5 GHz respectively. It was conventionally thought that all of the atmospheric carbon should be in its reduced form of methane (CH₄). Two sources of stratospheric CO have been postulated: CO transported from the interior by convection due to Neptune's strong internal heat source (internal source); or, CO produced through photochemical reactions from an external supply of water (external source). <p>In this research project the J=3-2 transition of CO was observed to find the CO profile in Neptune's atmosphere and determine the mechanism producing CO. Three instruments were used at the James Clerk Maxwell Telescope (JCMT) to measure the CO line: the heterodyne receiver B3; the University of Lethbridge Fourier Transform Spectrometer (FTS); and, the Submillimeter Common User Bolometer Array (SCUBA). <p>The high resolution (1.25 MHz) of the heterodyne observations over a large frequency range (~20 GHz) produced a very powerful result because the narrow emission core from the stratosphere and the broad absorption feature arising in the lower atmosphere were measured simultaneously. The CO abundance profile was determined using a model of the J=3-2 CO transition in Neptune's atmosphere developed for this project. Calculations indicate a CO abundance of 1.9^+0.5_-0.3x10^-6 in the upper stratosphere and (0.8±0.2)x10^-6 in the lower stratosphere and troposphere. <p>The moderate resolution of the FTS data allowed the broad absorption feature to be measured. Uranus was originally chosen as the calibration source, but the discovery of CO in Uranus by Encrenaz et al. (2004), while this project was in progress, prompted both Neptune and Uranus to be examined for CO absorption. Two data sets (1993 and 2002) were analyzed and it was found that the 1993 spectra produced superior results, giving a CO mole ratio in the lower atmosphere between 0.8x10^-6 and 2x10^-5; this agrees, within the uncertainty limit, with the lower atmosphere heterodyne result. A tentative detection of CO in Uranus was also obtained from the 1993 data, with a CO abundance profile constrained to pressures greater than 0.5 bar with an abundance between 5x10^-7 and 1x10^-5. The 2002 data were found to be inferior to the 1993 data because of imperfect cancellation of thermal emission from the terrestrial atmosphere. <p> The 850ìm SCUBA filter profile is well matched to the width of the CO feature. Photometric observations of Neptune and Uranus were used to determine if the reduction in integrated flux due to CO absorption could be detected using SCUBA. A CO mole ratio in the range (1.2-1.7) x10^-6 was found for Neptune, calibrated against Uranus and assuming no CO in Uranus. Calibration of the Neptune and Uranus SCUBA data against Mars to produce an independent estimate of the CO abundance in both planets did not produce a useful result because of large calibration errors. <p>Comparison of the results from the three techniques determined that the heterodyne measurement was superior and the derived CO profile was used to determine the source of neptunian CO. It was concluded that the source of CO in Neptune is both internal and external. The lower atmosphere result indicates an interior dominated by water ice. The most likely mechanism for the upper atmosphere CO involves meteoritic ablation, photolysis of H₂O, and chemical reaction with by-products of methane photochemistry. The required H₂O influx for this mechanism is at least two orders of magnitude higher than previously observed, indicating either that the observed H₂O abundance is too small or that CO is produced by a different mechanism.

Retrievals of atmospheric information from satellite observations permit the investigation of otherwise inaccessible atmospheric phenomena. The recovery of this information from optical instrumentation located in orbit requires both an inversion algorithm like the Saskatchewan Multiplicative Algebraic Reconstruction Technique and a forward model like the SASKTRAN radiative transfer model. These are used together at the University of Saskatchewan to retrieve sulphate aerosol extinction profiles from the radiance measurements made by the Canadian built OSIRIS instrument. Although these retrievals are highly successful the process currently does not consider the polarization of light or OSIRIS's polarization sensitivities because SASKTRAN is a scalar model. In this work the development of a vector version of SASKTRAN that can perform polarized radiative transfer calculations is presented.<p> The vector SASKTRAN's results compare favorably with vector SCIATRAN, another polarized model that is in development at the University of Bremen. Comparisons of the stratospheric aerosol retrieval vectors generated from the scalar and vector SASKTRAN results indicate that the polarized calculations are an important factor in future work to improve the aerosol retrievals and to recover particle size or composition information.