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Ray Tracing (ray + tracing)
Selected AbstractsShallow Bounding Volume Hierarchies for Fast SIMD Ray Tracing of Incoherent RaysCOMPUTER GRAPHICS FORUM, Issue 4 2008H. Dammertz Abstract Photorealistic image synthesis is a computationally demanding task that relies on ray tracing for the evaluation of integrals. Rendering time is dominated by tracing long paths that are very incoherent by construction. We therefore investigate the use of SIMD instructions to accelerate incoherent rays. SIMD is used in the hierarchy construction, the tree traversal and the leaf intersection. This is achieved by increasing the arity of acceleration structures, which also reduces memory requirements. We show that the resulting hierarchies can be built quickly and are smaller than acceleration structures known so far while at the same time outperforming them for incoherent rays. Our new acceleration structure speeds up ray tracing by a factor of 1.6 to 2.0 compared to a highly optimized bounding interval hierarchy implementation, and 1.3 to 1.6 compared to an efficient kd-tree. At the same time, the memory requirements are reduced by 10,50%. Additionally we show how a caching mechanism in conjunction with this memory efficient hierarchy can be used to speed up shadow rays in a global illumination algorithm without increasing the memory footprint. This optimization decreased the number of traversal steps up to 50%. [source] Accelerating Ray Tracing using Constrained TetrahedralizationsCOMPUTER GRAPHICS FORUM, Issue 4 2008Ares Lagae Abstract In this paper we introduce the constrained tetrahedralization as a new acceleration structure for ray tracing. A constrained tetrahedralization of a scene is a tetrahedralization that respects the faces of the scene geometry. The closest intersection of a ray with a scene is found by traversing this tetrahedralization along the ray, one tetrahedron at a time. We show that constrained tetrahedralizations are a viable alternative to current acceleration structures, and that they have a number of unique properties that set them apart from other acceleration structures: constrained tetrahedralizations are not hierarchical yet adaptive; the complexity of traversing them is a function of local geometric complexity rather than global geometric complexity; constrained tetrahedralizations support deforming geometry without any effort; and they have the potential to unify several data structures currently used in global illumination. [source] ReduceM: Interactive and Memory Efficient Ray Tracing of Large ModelsCOMPUTER GRAPHICS FORUM, Issue 4 2008Christian Lauterbach We present a novel representation and algorithm, ReduceM, for memory efficient ray tracing of large scenes. ReduceM exploits the connectivity between triangles in a mesh and decomposes the model into triangle strips. We also describe a new stripification algorithm, Strip-RT, that can generate long strips with high spatial coherence. Our approach uses a two-level traversal algorithm for ray-primitive intersection. In practice, ReduceM can significantly reduce the storage overhead and ray trace massive models with hundreds of millions of triangles at interactive rates on desktop PCs with 4-8GB of main memory. [source] GPU-Based Nonlinear Ray TracingCOMPUTER GRAPHICS FORUM, Issue 3 2004Daniel Weiskopf In this paper, we present a mapping of nonlinear ray tracing to the GPU which avoids any data transfer back to main memory. The rendering process consists of the following parts: ray setup according to the camera parameters, ray integration, ray-object intersection, and local illumination. Bent rays are approximated by polygonal lines that are represented by textures. Ray integration is based on an iterative numerical solution of ordinary differential equations whose initial values are determined during ray setup. To improve the rendering performance, we propose acceleration techniques such as early ray termination and adaptive ray integration. Finally, we discuss a variety of applications that range from the visualization of dynamical systems to the general relativistic visualization in astrophysics and the rendering of the continuous refraction in media with varying density. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism [source] Simulation of X-ray frames from macromolecular crystals using a ray-tracing approachACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2009Kay Diederichs An algorithm is described which simulates a data set obtained from a protein crystal using the rotation method. The diffraction pattern of an ideal crystal is specified by the orientation of the crystal's cell axes with respect to a specified laboratory coordinate system, the distance between the crystal and the detector, the wavelength and the rotation range per frame. However, a realistic simulation of an experiment additionally requires at least a plausible physical model for crystal mosaicity and beam properties. To explore the physical basis of reflection shape and rocking-curve variation, the algorithm simulates the diffraction of a real crystal composed of mosaic blocks which is illuminated with a beam of given divergence and dispersion. Ray tracing for each reflection leads to reflection shapes and rocking curves that appear realistic. A program implementing the algorithm may be used to reproducibly generate data sets that model different physical aspects (imperfections) of the crystal and the experiment. Certain types of systematic errors of the experimental apparatus may also be simulated. Further applications include teaching and characterization of the properties of data-reduction algorithms. [source] Fast BVH Construction on GPUsCOMPUTER GRAPHICS FORUM, Issue 2 2009C. Lauterbach We present two novel parallel algorithms for rapidly constructing bounding volume hierarchies on manycore GPUs. The first uses a linear ordering derived from spatial Morton codes to build hierarchies extremely quickly and with high parallel scalability. The second is a top-down approach that uses the surface area heuristic (SAH) to build hierarchies optimized for fast ray tracing. Both algorithms are combined into a hybrid algorithm that removes existing bottlenecks in the algorithm for GPU construction performance and scalability leading to significantly decreased build time. The resulting hierarchies are close in to optimized SAH hierarchies, but the construction process is substantially faster, leading to a significant net benefit when both construction and traversal cost are accounted for. Our preliminary results show that current GPU architectures can compete with CPU implementations of hierarchy construction running on multicore systems. In practice, we can construct hierarchies of models with up to several million triangles and use them for fast ray tracing or other applications. [source] Gradient-based Interpolation and Sampling for Real-time Rendering of Inhomogeneous, Single-scattering MediaCOMPUTER GRAPHICS FORUM, Issue 7 2008Zhong Ren Abstract We present a real-time rendering algorithm for inhomogeneous, single scattering media, where all-frequency shading effects such as glows, light shafts, and volumetric shadows can all be captured. The algorithm first computes source radiance at a small number of sample points in the medium, then interpolates these values at other points in the volume using a gradient-based scheme that is efficiently applied by sample splatting. The sample points are dynamically determined based on a recursive sample splitting procedure that adapts the number and locations of sample points for accurate and efficient reproduction of shading variations in the medium. The entire pipeline can be easily implemented on the GPU to achieve real-time performance for dynamic lighting and scenes. Rendering results of our method are shown to be comparable to those from ray tracing. [source] Shallow Bounding Volume Hierarchies for Fast SIMD Ray Tracing of Incoherent RaysCOMPUTER GRAPHICS FORUM, Issue 4 2008H. Dammertz Abstract Photorealistic image synthesis is a computationally demanding task that relies on ray tracing for the evaluation of integrals. Rendering time is dominated by tracing long paths that are very incoherent by construction. We therefore investigate the use of SIMD instructions to accelerate incoherent rays. SIMD is used in the hierarchy construction, the tree traversal and the leaf intersection. This is achieved by increasing the arity of acceleration structures, which also reduces memory requirements. We show that the resulting hierarchies can be built quickly and are smaller than acceleration structures known so far while at the same time outperforming them for incoherent rays. Our new acceleration structure speeds up ray tracing by a factor of 1.6 to 2.0 compared to a highly optimized bounding interval hierarchy implementation, and 1.3 to 1.6 compared to an efficient kd-tree. At the same time, the memory requirements are reduced by 10,50%. Additionally we show how a caching mechanism in conjunction with this memory efficient hierarchy can be used to speed up shadow rays in a global illumination algorithm without increasing the memory footprint. This optimization decreased the number of traversal steps up to 50%. [source] Accelerating Ray Tracing using Constrained TetrahedralizationsCOMPUTER GRAPHICS FORUM, Issue 4 2008Ares Lagae Abstract In this paper we introduce the constrained tetrahedralization as a new acceleration structure for ray tracing. A constrained tetrahedralization of a scene is a tetrahedralization that respects the faces of the scene geometry. The closest intersection of a ray with a scene is found by traversing this tetrahedralization along the ray, one tetrahedron at a time. We show that constrained tetrahedralizations are a viable alternative to current acceleration structures, and that they have a number of unique properties that set them apart from other acceleration structures: constrained tetrahedralizations are not hierarchical yet adaptive; the complexity of traversing them is a function of local geometric complexity rather than global geometric complexity; constrained tetrahedralizations support deforming geometry without any effort; and they have the potential to unify several data structures currently used in global illumination. [source] ReduceM: Interactive and Memory Efficient Ray Tracing of Large ModelsCOMPUTER GRAPHICS FORUM, Issue 4 2008Christian Lauterbach We present a novel representation and algorithm, ReduceM, for memory efficient ray tracing of large scenes. ReduceM exploits the connectivity between triangles in a mesh and decomposes the model into triangle strips. We also describe a new stripification algorithm, Strip-RT, that can generate long strips with high spatial coherence. Our approach uses a two-level traversal algorithm for ray-primitive intersection. In practice, ReduceM can significantly reduce the storage overhead and ray trace massive models with hundreds of millions of triangles at interactive rates on desktop PCs with 4-8GB of main memory. [source] GPU-Based Nonlinear Ray TracingCOMPUTER GRAPHICS FORUM, Issue 3 2004Daniel Weiskopf In this paper, we present a mapping of nonlinear ray tracing to the GPU which avoids any data transfer back to main memory. The rendering process consists of the following parts: ray setup according to the camera parameters, ray integration, ray-object intersection, and local illumination. Bent rays are approximated by polygonal lines that are represented by textures. Ray integration is based on an iterative numerical solution of ordinary differential equations whose initial values are determined during ray setup. To improve the rendering performance, we propose acceleration techniques such as early ray termination and adaptive ray integration. Finally, we discuss a variety of applications that range from the visualization of dynamical systems to the general relativistic visualization in astrophysics and the rendering of the continuous refraction in media with varying density. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Picture/Image Generation I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism [source] A practical grid-based method for tracking multiple refraction and reflection phases in three-dimensional heterogeneous mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2006M. De Kool SUMMARY We present a practical grid-based method in 3-D spherical coordinates for computing multiple phases comprising any number of reflection and transmission branches in heterogeneous layered media. The new scheme is based on a multistage approach which treats each layer that the wave front enters as a separate computational domain. A finite-difference eikonal solver known as the fast-marching method (FMM) is reinitialized at each interface to track the evolving wave front as either a reflection back into the incident layer or a transmission through to the adjacent layer. Unlike the standard FMM, which only finds first arrivals, this multistage approach can track those later arriving phases explicitly caused by the presence of discontinuities. Notably, the method does not require an irregular mesh to be constructed in order to connect interface nodes to neighbouring velocity nodes which lie on a regular grid. To improve accuracy, local grid refinement is used in the neighbourhood of a source point where wave front curvature is high. The method also provides a way to trace reflections from an interface that are not the first arrival (e.g. the global PP phase). These are computed by initializing the multistage FMM from both the source and receiver, propagating the two wave fronts to the reflecting interface, and finding stationary points of the sum of the two traveltime fields on the reflecting interface. A series of examples are presented to test the efficiency, accuracy and robustness of the new scheme. As well as efficiently computing various global phases to an acceptable accuracy through the ak135 model, we also demonstrate the ability of the scheme to track complex crustal phases that may be encountered in coincident reflection, wide-angle reflection/refraction or local earthquake surveys. In one example, a variety of phases are computed in the presence of a realistic subduction zone, which includes several layer pinch-outs and a subducting slab. Our numerical tests show that the new scheme is a practical and robust alternative to conventional ray tracing for finding various phases in layered media at a variety of scales. [source] Traveltime approximation for a reflected wave in a homogeneous anisotropic elastic layerGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002M. Zillmer Summary An approximation to the traveltime field is calculated for an elastic wave that propagates in a homogeneous anisotropic layer and is reflected at a plane boundary. The traveltime is approximated by a Taylor series expansion with the third derivative of the traveltime being taken into account. The coefficients of the series refer to the seismic ray, which is locally the fastest ray. Simple formulae are obtained for orthorhombic media in the crystal coordinate system, which relate the traveltimes of the reflected waves to the elastic constants of the medium. A numerical example is presented for wave propagation in orthorhombic olivine, which is a constituent of the Earth's mantle. A second example is given by an isotropic host rock with a set of parallel cracks, which is an important model for wave propagation in the Earth's crust. The elastic parameters can be determined by measuring the reflection times as a function of source,receiver offset. The approximate traveltime,distance curves are compared with traveltimes obtained from seismic ray tracing. [source] A Maslov-propagator seismogram for weakly anisotropic mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002Georg Rümpker Summary We introduce a formalism to calculate shear-wave seismograms for weakly-anisotropic and inhomogeneous media. The method is based on the combination of the forward-propagator method, which accounts for shear-wave interaction along a single reference ray, and the Maslov ray-summation, which incorporates amplitude and phase information from neighbouring rays to account for waveform and diffraction effects at caustics and in shadow regions. The approach is based on the assumption that the multiply split shear waves, on the way to a given receiver, travel along a common ray path that can by obtained from ray tracing in an isotropic reference medium (i.e. the common-ray approximation). The forward propagator and the Maslov amplitude are expressed with respect to radial and transverse coordinates (perpendicular to the ray propagation direction) that are defined uniquely by the initial conditions. Local polarizations and slownesses of the fast and slow shear-waves in the direction of propagation are obtained from the eikonal equation. The Maslov-propagator phase is given by the average shear-wave traveltime along the reference ray. Phase advances and delays of individual shear wave components are accounted for by the propagator. The geometrical-spreading information required for the Maslov integration is supplied by dynamic ray tracing in the isotropic reference medium. In the high-frequency limit effective phase functions are defined to assess the validity of the Maslov propagator phase information. For a homogeneous isotropic reference medium, we find good agreement with exact Maslov phase functions for anisotropic perturbations of up to 20 per cent. As a numerical application we consider effects of inhomogeneous anisotropy in a shear-wave cross-hole survey. The variations of the transversely-isotropic medium require 2-D slowness integrals. The method can handle discontinuities of the fast polarization along the ray path and also for neighbouring rays which is important for the slowness integration. Smooth transitions between isotropic and anisotropic regions along the ray path can be accounted for without the need to switch between numerical formulations. [source] Regional teleseismic tomography of the western Lachlan Orogen and the Newer Volcanic Province, southeast AustraliaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2002Frank M. Graeber Summary From 1998 May to September a portable array of 40 short-period digital seismograph stations was operated in western Victoria, southeast Australia, across the western end of the mid-Paleozoic Lachlan Foldbelt and the Newer Volcanic Province. Consisting of four parallel, almost W,E-oriented receiver lines, the array covered an area of about 270 × 150 km2. The major aim of the LF98 (Lachlan Foldbelt survey 1998) project is to map lateral variations in P -wave speeds (Vp) in the crust and upper mantle using teleseismic arrival time tomography, primarily in order to investigate whether the major surface structural zones are associated with seismic velocity signatures at depth. Little a priori information from seismic profiling is available. We invert 4067 relative arrival time residuals for a minimum structure Vp model in the upper few hundred km using non-linear iteration and 3-D ray tracing. The most prominent negative anomaly (,3.8 per cent) in Vp is found at a depth of about 45 km underneath the eastern part of the Newer Volcanic Province. It correlates spatially with the highest density of Pliocene and Pleistocene eruption centres northwest of Melbourne, and is therefore interpreted as a hotspot-related high-temperature anomaly causing reduced mantle velocities. The related coherent volume of significantly lower than average velocities extends down to depths greater than 100 km in the east, and extends west underneath the Newer Volcanic Province. A strong velocity contrast, with average velocities ,2 per cent greater in the west, is found down to about 100 km across the Moyston Fault Zone, which forms the major structural boundary between the early-Paleozoic Delamerian Orogen in the west and the Lachlan Orogen in the east. This result suggests that the Moyston Fault Zone should be seen as a major lithospheric boundary. In the south this boundary is also expressed by a distinct discontinuity in Sr-isotopic ratios of xenoliths (the so-called Mortlake discontinuity) and a change in the geochemistry of plutons of similar age. However, if the east to west velocity contrast originally existed in this southern zone, it is now overprinted by the thermally reduced mantle velocities beneath the Newer Volcanic Province. [source] Application of a three-dimensional ray-tracing technique to global P, PP and Pdiff traveltime tomographyGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001A. Gorbatov Summary A 3-D ray-path tracing algorithm was successfully applied to global P -wave traveltime tomography. The inversion was conducted iteratively using the resultant P -wave velocity model as the initial model for the subsequent iteration. The LSQR method was adopted to solve a large and sparse system of equations. This iteratively linearized inversion with 3-D ray tracing increased wave-speed anomalies, located heterogeneities better and reduced smearing as compared to those derived from a conventional one-step inversion using 1-D ray tracing, although the general pattern of velocity anomalies was similar. A major difference was found in the lowermost mantle, where the departure of a ray path from the great circle path tends to be in general greatest. In particular, a pronounced high-velocity anomaly develops beneath the Indian Ocean, a feature not obvious in the result of 1-D inversion. The final P -wave velocity model was obtained by including reported PP and Pdiff traveltime data. The addition of the PP data sharpened the images and enhanced velocity anomalies in the upper mantle, especially at latitudes above 45° of the Northern Hemisphere. The addition of the Pdiff data sharpened and amplified velocity anomalies in the lowermost mantle in general. [source] Off-great-circle propagation of intermediate-period surface waves observed on a dense array in the French AlpsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2000N. Cotte Array analysis is performed on surface waves recorded in the French Alps using a small-aperture (25 km) temporary array of six broad-band stations. The analysis shows that both Rayleigh and Love waves deviate relative to the great-circle path. The deviations are particularly strong, up to 30°, between 20 and 40 s period. To interpret these observations, we first study the effect of large-scale structures using ray tracing in a smooth, laterally heterogeneous model of the Earth. Second, we evaluate the local effect by considering a model for the French Alps including strong lateral heterogeneities around the array that were not taken into account in the ray tracing. By combining these two possible causes of the observed deviations, we propose an explanation for the general trend in the observed deviations. Finally, we show that by taking into account azimuthal deviations, phase velocities measured at a regional scale can be significantly improved. [source] Paraxial ray methods for anisotropic inhomogeneous mediaGEOPHYSICAL PROSPECTING, Issue 1 2007Tijmen Jan Moser ABSTRACT A new formalism of surface-to-surface paraxial matrices allows a very general and flexible formulation of the paraxial ray theory, equally valid in anisotropic and isotropic inhomogeneous layered media. The formalism is based on conventional dynamic ray tracing in Cartesian coordinates along a reference ray. At any user-selected pair of points of the reference ray, a pair of surfaces may be defined. These surfaces may be arbitrarily curved and oriented, and may represent structural interfaces, data recording surfaces, or merely formal surfaces. A newly obtained factorization of the interface propagator matrix allows to transform the conventional 6 × 6 propagator matrix in Cartesian coordinates into a 6 × 6 surface-to-surface paraxial matrix. This matrix defines the transformation of paraxial ray quantities from one surface to another. The redundant non-eikonal and ray-tangent solutions of the dynamic ray-tracing system in Cartesian coordinates can be easily eliminated from the 6 × 6 surface-to-surface paraxial matrix, and it can be reduced to 4 × 4 form. Both the 6 × 6 and 4 × 4 surface-to-surface paraxial matrices satisfy useful properties, particularly the symplecticity. In their 4 × 4 reduced form, they can be used to solve important boundary-value problems of a four-parametric system of paraxial rays, connecting the two surfaces, similarly as the well-known surface-to-surface matrices in isotropic media in ray-centred coordinates. Applications of such boundary-value problems include the two-point eikonal, relative geometrical spreading, Fresnel zones, the design of migration operators, and more. [source] Traveltime computation by wavefront-orientated ray tracingGEOPHYSICAL PROSPECTING, Issue 1 2005Radu Coman ABSTRACT For multivalued traveltime computation on dense grids, we propose a wavefront-orientated ray-tracing (WRT) technique. At the source, we start with a few rays which are propagated stepwise through a smooth two-dimensional (2D) velocity model. The ray field is examined at wavefronts and a new ray might be inserted between two adjacent rays if one of the following criteria is satisfied: (1) the distance between the two rays is larger than a predefined threshold; (2) the difference in wavefront curvature between the rays is larger than a predefined threshold; (3) the adjacent rays intersect. The last two criteria may lead to oversampling by rays in caustic regions. To avoid this oversampling, we do not insert a ray if the distance between adjacent rays is smaller than a predefined threshold. We insert the new ray by tracing it from the source. This approach leads to an improved accuracy compared with the insertion of a new ray by interpolation, which is the method usually applied in wavefront construction. The traveltimes computed along the rays are used for the estimation of traveltimes on a rectangular grid. This estimation is carried out within a region bounded by adjacent wavefronts and rays. As for the insertion criterion, we consider the wavefront curvature and extrapolate the traveltimes, up to the second order, from the intersection points between rays and wavefronts to a gridpoint. The extrapolated values are weighted with respect to the distances to wavefronts and rays. Because dynamic ray tracing is not applied, we approximate the wavefront curvature at a given point using the slowness vector at this point and an adjacent point on the same wavefront. The efficiency of the WRT technique is strongly dependent on the input parameters which control the wavefront and ray densities. On the basis of traveltimes computed in a smoothed Marmousi model, we analyse these dependences and suggest some rules for a correct choice of input parameters. With suitable input parameters, the WRT technique allows an accurate traveltime computation using a small number of rays and wavefronts. [source] Seismic modelling study of a subglacial lakeGEOPHYSICAL PROSPECTING, Issue 6 2003José M. Carcione ABSTRACT We characterize the seismic response of Lake Vostok, an Antarctic subglacial lake located at nearly 4 km depth below the ice sheet. This study is relevant for the determination of the location and morphology of subglacial lakes. The characterization requires the design of a methodology based on rock physics and numerical modelling of wave propagation. The methodology involves rock-physics models of the shallow layer (firn), the ice sheet and the lake sediments, numerical simulation of synthetic seismograms, ray tracing, ,,p transforms, and AVA analysis, based on the theoretical reflection coefficients. The modelled reflection seismograms show a set of straight events (refractions through the firn and top-ice layer) and the two reflection events associated with the top and bottom of the lake. Theoretical AVA analysis of these reflections indicates that, at near offsets, the PP-wave anomaly is negative for the ice/water interface and constant for the water/sediment interface. This behaviour is shown by AVA analysis of the synthetic data set. This study shows that subglacial lakes can be identified by using seismic methods. Moreover, the methodology provides a tool for designing suitable seismic surveys. [source] Interval velocity and thickness estimate from wide-angle reflection dataGEOPHYSICAL PROSPECTING, Issue 4 2001Roberto De Franco A method to estimate interval velocities and thickness in a horizontal isotropic layered medium from wide-angle reflection traveltime curves is presented. The method is based on a relationship between the squared reflection traveltime differences and the squared offset differences relative to two adjacent reflectors. The envelope of the squared-time versus offset-difference curves, for rays with the same ray parameter, is a straight line, whose slope is the inverse of the square of the interval velocity and whose intercept is the square of the interval time. The method yields velocity and thickness estimates without any knowledge of the overlying stratification. It can be applied to wide-angle reflection data when either information on the upper crust and/or refraction control on the velocity is not available. Application to synthetic and real data shows that the method, used together with other methods, allows us to define a reliable 1D starting model for estimating a depth profile using either ray tracing or another technique. [source] Entry dynamics and acoustics/infrasonic/seismic analysis for the Neuschwanstein meteorite fallMETEORITICS & PLANETARY SCIENCE, Issue 10 2004D. O. REVELLE This includes ground-based photographic and radiometer data as well as infrasound and seismic data from this very significant bolide event (Spurný et al. 2002, 2003). We have also used these data to model the entry of Neuschwanstein, including the expected dynamics, energetics, panchromatic luminosity, and associated fragmentation effects. In addition, we have calculated the differential efficiency of acoustical waves for Neuschwanstein and used these values to compare against the efficiency calculated using available ground-based infrasound data. This new numerical technique has allowed the source height to be determined independent of ray tracing solutions. We have also carried out theoretical ray tracing for a moving point source (not strictly a cylindrical line emission) and for an infinite speed line source. In addition, we have determined the ray turning heights as a function of the source height for both initially upward and downward propagating rays, independent of the explicit ray tracing (detailed propagation path) programs. These results all agree on the origins of the acoustic emission and explicit source heights for Neuschwanstein for the strongest infrasonic signals. Calculated source energies using more than four different independent approaches agree that Neuschwanstein was certainly <500 kg in initial mass, given the initial velocity of 20.95 km/s, resulting in an initial source energy ,0.0157-0.0276 kt TNT equivalent (4.185 times 1012 J). Local source energies at the calculated infrasonic/seismic source altitudes are up to two orders of magnitude smaller than this initial source energy. [source] SUPRACENTER: Locating fireball terminal bursts in the atmosphere using seismic arrivalsMETEORITICS & PLANETARY SCIENCE, Issue 9 2004W. N. EDWARDS A computer program, SUPRACENTER, calculates travel times by ray tracing through realistic atmospheres (that include winds) and locates source positions by minimization of travel time residuals. This is analogous to earthquake hypocenter location in the solid Earth but is done through a variably moving medium. Inclusion of realistic atmospheric ray tracing has removed the need for the simplifying assumption of an isotropic atmosphere or an approximation to account for "wind drift." This "drift" is on the order of several km when strong, unidirectional winds are present in the atmosphere at the time of a fireball's occurrence. SUPRACENTER-derived locations of three seismically recorded fireballs: 1) the October 9, 1997 El Paso superbolide; 2) the January 25, 1989 Mt. Adams fireball; and 3) the May 6, 2000 Morávka fireball (with its associated meteorite fall), are consistent with (and, probably, an improvement upon) the locations derived from eyewitness, photographic, and video observations from the respective individual events. If direct acoustic seismic arrivals can be quickly identified for a fireball event, terminal burst locations (and, potentially, trajectory geometry and velocity information) can be quickly derived, aiding any meteorite recovery efforts during the early days after the fall. Potentially, seismic records may yield enough trajectory information to assist in the derivation of orbits for entering projectiles. [source] Where is the radiation edge in magnetized black hole accretion discs?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Kris Beckwith ABSTRACT General relativistic (GR) magnetohydrodynamic (MHD) simulations of black hole accretion find significant magnetic stresses near and inside the innermost stable circular orbit (ISCO), suggesting that such flows could radiate in a manner noticeably different from the prediction of the standard model, which assumes that there are no stresses in that region. We provide estimates of how phenomenologically interesting parameters like the ,radiation edge', the innermost ring of the disc from which substantial thermal radiation escapes to infinity, may be altered by stresses near the ISCO. These estimates are based on data from a large number of three-dimensional GRMHD simulations combined with GR ray tracing. For slowly spinning black holes (a/M < 0.9), the radiation edge lies well inside where the standard model predicts, particularly when the system is viewed at high inclination. For more rapidly spinning black holes, the contrast is smaller. At fixed total luminosity, the characteristic temperature of the accretion flow increases between a factor of 1.2 and 2.4 over that predicted by the standard model, whilst at fixed mass accretion rate, there is a corresponding enhancement of the accretion luminosity which may be anywhere from tens of per cent to order unity. When all these considerations are combined, we find that, for fixed black hole mass, luminosity and inclination angle, our uncertainty in the characteristic temperature of the radiation reaching distant observers due to uncertainty in dissipation profile (around a factor of 3) is greater than the uncertainty due to a complete lack of knowledge of the black hole's spin (around a factor of 2) and furthermore that spin estimates based on the stress-free inner boundary condition provide an upper limit to a/M. [source] Determining the zone of reflection for posterior corneal surface comparison phakometryOPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 1 2009Andrew Carkeet Abstract Although comparison phakometry has been used by a number of studies to measure posterior corneal shape, these studies have not calculated the size of the posterior corneal zones of reflection they assessed. This paper develops paraxial equations for calculating posterior corneal zones of reflection, based on standard keratometry equations and equivalent mirror theory. For targets used in previous studies, posterior corneal reflection zone sizes were calculated using paraxial equations and using exact ray tracing, assuming spherical and aspheric corneal surfaces. Paraxial methods and exact ray tracing methods give similar estimates for reflection zone sizes less than 2 mm, but for larger zone sizes ray tracing methods should be used. [source] Limiting efficiency of crystalline silicon solar cells due to Coulomb-enhanced Auger recombinationPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 2 2003Mark J. Kerr Excitonic effects are known to enhance the rate of intrinsic recombination processes in crystalline silicon. New calculations for the limiting efficiency of silicon solar cells are presented here, based on a recent parameterization for the Coulomb-enhanced Auger recombination rate, which accounts for its dopant type and dopant density dependence at an arbitrary injection level. Radiative recombination has been included along with photon recycling effects modeled by three-dimensional ray tracing. A maximum cell efficiency of 29.05% has been calculated for a 90-,m-thick cell made from high resistivity silicon at 25°C. For 1,,,cm p -type silicon, the maximum efficiency reduces from 28.6% for a 55-,m-thick cell in the absence of surface recombination, down to 27.0% for a thickness in the range 300,500,,m when surface recombination limits the open-circuit voltage to 720,mV. Copyright © 2002 John Wiley & Sons, Ltd. [source] 3124: Algorithms and instrumentation for quantified retinal oximetry in a snapshotACTA OPHTHALMOLOGICA, Issue 2010AR HARVEY Purpose To develop an instrument and techniques for useful absolute and relative clinical oximetry of the retina. Methods A novel snapshot multispectral imaging system has been optimised for retinal oximetry. Eight monochromatic images of the retina are recorded with a field of view of 24 degrees. Algorithms calculate the absorption of light at each point along delineated blood vessels and for each of the eight wavebands. Data inversion using an analytical model for light propagation enables oximetry at along the blood vessels. Spectral inversion has been refined using ray tracing and Monte Carlo modelling of light propagation using a realistic phantom eye. Results A comparison of Monte Carlo modelling of light propagation in the phantom retina with recorded images for blood of various oxygenations indicates the influence of several unknowns, including scatter from optical surfaces within the ophthalmoscope and eye, the geometry of the blood vessels and eye, the optical constants of the ocular media and the complexity of light propagation in the retinal structure. Relative oximetry within the retina is possible with repeatability of about 1% in the phantom and 5% in a real eye but the influence of various systematic effects can introduce systematic differences between actual and calculated oxygenation that can significantly exceed these values. Conclusion Although oximetry using only a very small number of spectral bands is possible, this is prone to systematic errors that can effect both absolute and relative oximetry. The ability to record eight spectral images in a single snapshot offers promise to provide an enhanced clinically useful and validated oximetry technique. [source] | ||||||||||||||||