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Studies of mid-ocean ridges have provided evidence of magma chambers beneath both fast and intermediate spreading ridges. However no such features have been observed to date beneath slow spreading ridges. These contradictory observations are in direct conflict with seismic studies which reveal that the resulting crustal structures are similar and hence crustal structure is independent of the spreading rate. These latter observations in turn lead to the implication that the accretionary processes operating at all ridge types must also be similar. The aim of this study is to attempt to resolve between this discrepancy in geophysical observations of magma chambers at fast, intermediate and slow spreading ridges and investigate the nature of accretionary processes operating such that the same crustal structure is achieved. Therefore an apparently currently magmatically active section of the slow spreading Mid-Atlantic Ridge at 57 45'N on the Reykjanes Ridge, was selected as the target of a multidisciplinary geophysical experiment to be conducted aboard the RRS Charles Darwin in 1993. Wide-angle seismic data recorded using 10 digital ocean bottom seismometers were used to generate models of the crustal structure along and across-axis. These models were confirmed and further constrained by modelling of normal incidence seismic and gravity data and by comparison with the results of modelling controlled source electromagnetic data. The resultant models indicate that a magma chamber exists beneath the axial volcanic ridge studied, providing the first geophysical observation of such a feature at any slow spreading ridge. This magma chamber is similar in dimensions to those observed beneath fast and intermediate spreading ridges and consists of a thin, narrow sill-like body which appears to be continuous along-axis, and which is underlain by a region of partial melt extending almost to the Moho. This latter feature also appears to be both longer-lived and more extensive than the magma chamber. The 2.5 km depth to the top of the magma chamber is only slightly greater than that observed at fast spreading ridges, which indicates that magma chamber depth does not vary significantly with spreading rate. However, there ore insufficient data available to fully constrain and develop this relationship to its fullest. Therefore the results of this study indicate that the processes of crustal accretion occurring at all spreading ridges are similar, with the lack of observations of magma chambers being due to the fact that the periods of magmatic activity at slow spreading ridges are considerably more widely separated in both space and time than for fast and intermediate spreading ridges. The main difference however, appears to occur in the process of emplacement of layer 2A, which is observed to thicken off-axis at fast spreading ridges due to the less viscous lavas produced at these ridges being able to flow further off-axis. The results of this study, and of two other studies at slow spreading ridges, show that layer 2A is completely formed on-axis and thins off-axis due to extensional faulting. The remainder of the crust is completely emplaced, and the Moho formed, on-axis at all spreading rates.

Fisheye lenses refer to ultra-wide angle lenses that have field of view equal or larger than 180 degrees. Such lenses introduce large amount of barrel distortion to capture at least the entire hemisphere in front of the lens. Fisheye lenses were initially designed for scientific use, such as cloud recording and angle measuring, and were widely used for commercial purposes later. The development of photographic fisheye lenses started in 1960s. However, the lack of detailed references on photographic fisheye lens design makes such design challenging. This thesis provides detailed introduction of photographic fisheye lens design for 35mm format DSLR cameras. A discussion on the history of fisheye lenses is provided to describe the development of fisheye lenses. The tangential and sagittal magnifications are mathematically derived for each fisheye lens projection mapping method to show their differences. The special properties and design issues of photographic fisheye lenses are described in detail. Along with each design issue, some solutions suggested by the author are also provided. The performance of the current diagonal fisheye lenses for 35mm DSLR cameras are evaluated in detail. Then a new diagonal fisheye lens designed by the author is presented and compared with the current diagonal fisheye lenses on the market. Finally, a zoom fisheye lens designed for 35mm DSLR cameras is presented and discussed.

In this thesis, a modified BRDF model for wide-angle scattering is presented. The proposed model is developed from empirical observations of several BRDF models. The model is an extention of the classical microfacet models. By replacing the two cosines of elevation angles with functions and exponent parameters, our model is able to give a special characteristic which we have not found in any other BRDF models. The characteristic at wide-angle scattering can be, for example, seen on the polyethylene material. In addition, our proposed model can greatly improve relative error from the reference model. The average relative error improvement is about 20 ercent for a cosine weighted error metric,E1 , and 10 percent for a logarithmic error metric, E 2,. Moreover, we also introduce a new optimization approach for the proposed terms. This approach can do optimization so that our proposed model gives at least an equivalent error to the reference model.

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QC 351 A7 no. 40 / The aim of this study was to determine to what extent a constant spectral response can be obtained for wide-passband interference filters used with wide -angle lenses. We investigated the possibility of using the curvature of the lens surfaces to reduce the shift in the filter passband for large field angles and found that locating the filter on the proper surface will considerably reduce the shift of the passband. Specifically, we determined the distribution of angles of incidence for full aperture pencils incident at several field angles on the second and fourth surfaces of the 90° Geocon IV, the 90° Paxar, and the 125° Pleogon. We then calculated the spectral transmittance of each lens when a wide passband interference filter was located on its second or fourth surface. We also calculated the degree of polarization introduced. From the cases considered, we found that the tracing of an upper and lower marginal (rim) ray at maximum field angle is sufficient to determine the suitability of a surface, the criterion being that, the smaller the angle of incidence at the surface, the better. In addition, we found that, with the filter on the second surface of the Paxar, spectral transmittance changes with field angle were negligible and the modulation due to polarization was about 1 %.

<p>This dissertation examines the use of artificial structured materials -- known as metamaterials -- in two antenna applications in which conventional dielectric materials are otherwise used. In the first application, the use of metamaterials to improve the impedance matching of planar phased array antennas over a broad range of scan angles is explored. A phased array antenna is composed of an array of antenna elements and enables long-distance signal propagation by directional radiation. The direction of signal propagation is defined as the scan angle. The power transmission ratio of a phased array is the ratio of the radiated power to the input power, and depends on the scan angle. The variation in the power transmission ratio is due to the different mutual coupling contributions between antenna elements at different scan angles. An optimized stack of dielectric layers, known as a wide-angle impedance matching layer (WAIM), is used to optimize the power transmission ratio profile over a broad range of scan angles. In this work, the use of metamaterials to design anisotropic WAIMs with access to a larger range of constitutive parameters -- including magnetic permeability -- to offer an improved power transmission ratio at a broad range of scan angles is investigated. </p> <p>In the second antenna application, a strategy to create maximally transmissive and minimally reflective electromagnetic radome materials using embedded metamaterial inclusions is introduced. A radome is a covering used to protect an antenna from weather elements or provide structural function such as the prevention of aerodynamic drag. A radome should be made from a fully transparent and non-refractive material so that radiated fields from and to the enclosed antenna are not disrupted. The aim of this research was to demonstrate that embedded metamaterial inclusions can be used to isotropically adjust the dielectric properties of a composite material to a desired value. This strategy may lead to the creation of a structural material with electromagnetic properties close to air, thus reducing the detrimental scattering effects often associated with conventional radome materials.</p> <p>Chapter 1 introduces the concept of metamaterials and discusses the use of subwavelength metallic structures to artificially engineer constitutive parameters such as permeability of permittivity. In Chapter 2, the analytical formulations that enable the characterization of the transmission performance of a planar phased array covered with anisotropic impedance matching layers are developed. Chapter 3 discusses the design rules that must govern the design parameters of anisotropic WAIMs realizable using metamaterials, and also presents examples of anisotropic impedance matching layers that provide a maximum power transmission ratio for most scan angles. In addition, numerical and experimental results on a metamaterial placed over a phased array are presented. In Chapter 4, the feasibility of using metamaterials to realize a minimally transparent and fully transmissive radome material is numerically investigated. In Chapter 5, experimental results that corroborate earlier numerical simulation results are analyzed.</p> / Dissertation

Composite propellants are mainly composed of: crystalline oxidizer, metallic fuel, and polymeric binder. Additives, such as plasticizers, catalysts, bonding agents and curing agents may also be incorporated to propellant compositions in small amounts. These ingredients should be mixed rigorously in order to obtain a uniform microstructure throughout the cast propellant profile. The quality of the propellant mixture has to be determined quantitatively to improve the product quality and to reduce costs. In this study, it was aimed to develop an easy, cost effective and rapid test method for determining the optimum mixing conditions for the manufacturing process of solid rocket propellants. An analytical method used in the literature for assessing mixing quality in highly filled polymeric systems is wide-angle x-ray diffractometry (WA-XRD). After finding out the concentration distribution of the components indirectly by WA-XRD, degree of mixing was identified using statistical methods. To accomplish this, series of samples were taken from various locations of the mixing chamber and analyzed by WA-XRD. Degree of mixing calculations based on ratio of intensity arising from aluminum phase over total crystal intensity, and the ratio of intensity arising from ammonium perchlorate phase over total crystal intensity gave satisfactory results. Radial mixing efficiency of planetary mixer was determined, and it was found that mixing at the center was more effective than mixing at the outer regions. Also, by scanning electron microscopy technique (SEM), interactions between binder and solid loading during mixing process were observed. It was seen that polymeric matrix gradually encloses solid particles during mixing.

During the DARPA SCENICC program, J. Ford, et al., demonstrated that CFBs provide a compelling route to compact, wide angle imagers. Monocentric lenses readily provide diffraction-limited images over wide field but onto a hemispherically curved image surface. They demonstrated visible CFBs can be tapered, cut and polished to relay curved images to flat sensors. We have shown that this provides a volumetric imaging efficiency a hundredfold larger than bulk optics can produce; a hundred times the resolution in the same volume or a hundred times less volume for the same resolution. Ford's work leveraged commercial fiber bundles available for the visible spectrum based on silica. We have developed hybrid fiber bundles using step-index confinement between chalcogenide glass cores and polymer cladding with high index-contrast. The high contrast is necessary to provide tight confinement to the high-index As-Se core with minimal crosstalk between closely spaced cores. Tight confinement also minimizes absorption losses in the PEI polymer cladding. The high contrast of this system also provides a large NA to optimize coupling into the CFB from fast lenses. We introduce disorder into the core radius as a mechanism to further decouple adjacent cores, reduce crosstalk and increase fill-factor. We present coupled-mode theory, modal crosstalk superposition, and finite-element modelling to quantify coupling losses and crosstalk as a function of geometry and disorder. We fabricated preforms, drew small fiber bundles and characterized optical properties of the bundles to aid scale-up to megapixel MWIR CFBs.