Showing papers on "Point source published in 1988"

Point source terahertz optics

Abstract: We demonstrate an ultrafast 10 μm sized, electric dipole source of terahertz radiation closely coupled to a 1 cm spherical mirror. This optical approach has the advantages of subpicosecond response times with essentially complete collection efficiency. Using this technique, we have generated and detected subpicosecond freely propagating electrical pulses.

Solutions of the bio-heat transfer equation.

Abstract: A solution of the bio-heat transfer equation for a 'step-function point source' is presented and discussed. From this basic solution one can, in principle, obtain the temperature field resulting from a general heat source distribution by superposition. As an example, the method is used to calculate the temperature on the body surface at a point where therapeutic ultrasound is applied. Comparison is made with experimental results recently published by Williams and co-workers.

The application of diffraction tomography to cross-hole seismic data

Abstract: Previously published equations for diffraction tomography do not solve the “two and one‐half dimensional problem” (point source illumination of two‐dimensional geology) if sources and receivers are confined to linear arrays. In spite of this lack of a formal solution, useful images can be formed by the application of two‐dimensional formulas to such problems. The estimation of difference fields, of crucial importance in diffraction tomography, reduces to the problem of estimating the source function. Using assumptions about the consistency of the source behavior, we extract the source function in a statistical fashion from cross‐hole data. Using this technique, the difference fields are computed directly from the recorded wave fields for two experiments and diffraction tomographic images are obtained. In the first experiment, the data are generated using a two‐dimensional finite‐difference modeling algorithm. In the second, a physical scale model of a crosshole experiment is performed in an ultrasonic mod...

Molecular outflows associated with bright far-infrared sources

Abstract: A systematic search was carried out for high-velocity CO emission associated with bright 100-micron sources from the IRAS Point Source Catalog, in an effort to increase the understanding of the nature and evolutionary status of the objects producing molecular outflows. Eighteen sources with 100-micron flux densities greater than 500 Jy were selected and maps were made in the J = 1-0 (C-12) line around each source. Almost every source observed was found to lie toward, or in the immediate vicinity of, strong CO emission and in most cases close to the maximum of this emission. Thus, all of the far-infrared sources can be associated with molecular clouds. Five of the sources show clear evidence of high-velocity molecular emission and at least three have bipolar morphologies. The newly detected outflow sources are all intrinsically luminous objects, and their outflows energetic. The infrared characteristics of all the sources in the present survey suggest that they are young stellar objects still embedded in their parent molecular clouds. Statistics on the occurrence of outflows from bright far-infrared sources are used to set an upper limit of 400,000 yr for the dispersal time scale of material around young, luminous stellar objects. 31 references.

Experimental confirmation of horizontal refraction of cw acoustic radiation from a point source in a wedge‐shaped ocean environment

Abstract: Analysis of experimental data obtained in the region of the East Australian Continental Slope has been found to be consistent with theoretical predictions of energetic horizontal refraction due to multiple reflections from a sloping bottom. The observed azimuthal characteristics are in accordance with both normal mode and ray theoretical solutions. The experiment, conducted with two ships (one towing a source and the other an array), started in water ≊400 to 500 m deep with an initial separation of 34 km and proceeded to deeper water on a diverging but constant line of bearing course. Bearing shifts deduced from beamformed data show changes of tens of degrees in as little as half an hour. These bearing shifts demonstrate the dominance of horizontally refracted arrivals for the geometry. A combination of source–array positions and critical angle effects served to limit both the azimuthal extent of energy received and the total number of modes arriving at any one position.

Far-Infrared (120--300 Micron) Observations of the Carina Nebula

Abstract: The Carina nebula has been observed in a 120-300 µm band with a 100 cm balloon-borne telescope, and maps have been generated with a spatial resolution of ~1'. More than thirty compact sources were detected in the ~0.5 deg 2 area scanned; many of these sources do not have IRAS PSC Point source Catalog counterparts. The diffuse emission accounts for nearly half of the total emission in the band. A comparison with observations at other wavelengths shows that the far-infrared emission is well correlated with other tracers of matter. The region appears to be rich in massive stars and most of the compact far-infrared sources are likely to be dust-gas clumps heated externally by these stars.

Imaging reflections in elliptically anisotropic media

Abstract: In an isotropic medium, waves reflected from a mirror form a virtual image of their source. This property of planar reflectors is generally not true in the presence of anisotropy. In their short note, Blair and Korringa (1987) show that for the special case of SH waves from a point source in a transversely isotropic medium, an aberration‐free image is formed for any orientation of the mirror. While their proof is mathematical, we show the same result in an intuitive, pictorial fashion and in the process discover that although the image is indeed aberration free, it is still distorted.

X-ray imaging with coded masks

Abstract: How can one image objects from which high-energy X rays emanate Actually the imaging is made possible by a technique that is similar to the one applied in making medical X-ray photographs. To make such photographs a film with an X-ray sensitive emulsion is placed directly behind a body part that is irradiated from the front by a small point source of X rays. The film in effect records the shadow cast by those components of the part that absorb X rays. A somewhat old-fashioned word for the technique is skiagraphy, from the Greek skia, meaning shadow. Now suppose the X rays come not from a point source such as a medical X-ray tube but from an extended source of unknown shape-perhaps a huge cloud of hot intergalactic plasma. Because the extended source can be envisioned as consisting of numerous point sources, the shadow of any intervening object is blurred. The reason is that each component point source causes a slightly different shadow to be cast on the film. Yet if one knows the shape of the intervening object precisely, one can easily predict the form of the shadow it would cast when illuminated by a single point source ofmore » X rays. By comparsing the shadows produced by all possible combinations of point sources with the actual recorded shadow, it might then be possible to reconstruct the shape of the extended source. That, in fact, is the operating principle of coded-mask X-ray imaging. The known object between the X-ray source and the detector is a coded mask: an X-ray-opaque plate that has a pattern of holes. The key to this type of imaging lies in the selection of a pattern that enables one to reconstruct an image of the X-ray source from the form of the mask's shadow.« less

Measurement of the Landsat Thematic Mapper modulation transfer function using an array of point sources

Abstract: This paper presents a method for measuring the Thematic Mapper (TM) imaging system point spread function (PSF) using TM imagery or a specially constructed target consisting of a two-dimensional array of approximate point sources of known dimensions and radiometric qualities. The target allows 16 separate point sources to be imaged simultaneously by the TM. The point sources were carefully placed on the ground so that their relative positions were known. Owing to sample-scene phasing, each imaged point source exhibits a different amount of blur in the digital image. The target pixels may then be recombined according to their known relative positions to form a single, sampled, nonaliased imaging system PSF. The modulation transfer function is then obtained as the modulus of the discrete Fourier transform of the PSF.

Waveguide characterization and source localization in shallow water waveguides using the Prony method

Abstract: Horizontal samples of a point source in a shallow water waveguide are processed by the Prony method. First, when the source position is known, a high resolution estimation of the normal mode parameter Km (the horizontal component of mode wavenumber) can be obtained. Owing to the weak damping process, βm, the modal attenuation coefficient, can be obtained by using a modified Prony method. In this article, an ‘‘interval average and iterative’’ approach is proposed. Numerical examples illustrate that a 10−4 resolution of Km (150‐Hz frequency) is provided by a 500‐m data length, roughly an order of magnitude smaller than the data length required by the linear spectrum analysis technique. Second, when the source position is unknown but the waveguide characters (Km, βm) are known, the same procedure can be used for source position estimation. Some numerical examples in typical shallow water are presented.

Modelling meter‐scale acoustic intensity fluctuations from oceanic fine structure and microstructure

Abstract: Using model spectra of sound speed fluctuations, we have calculated the intensity variance and the wave number spectrum of intensity for acoustic propagation from a point source. We consider solutions in the unsaturated (Rytov) approximation, which is accurate when the intensity variance, normalized to unity mean intensity, is less than about 0.3. Our model medium spectra are consistent with observed vertical profiles of sound speed. Model fine-scale sound speed inhomogeneities are characterized by the Garrett-Munk (GM) internal wave spectrum or a simpler anisotropic (A) inverse power law spectrum. Microscale sound speed structure is modelled by a band-limited isotropic Kolmogorov inertial subrange spectrum. With the GM model medium, the Rytov approximation is typically valid when the combination σR2 of frequency σ and range R is less than that given by 90 kHz and 1.1 km. Experiments within this window can determine characteristics of the fine scale medium spectra. In order to probe microstructure effects, frequencies near the high limit are necessary so that the Fresnel radius is 4 m or less. Previously published unsaturated vertical wave number intensity spectra recorded in the Kane Basin northwest of Greenland are compared with theory. The intensity spectral shapes from the GM and A fine structure models correspond well with those from the experiment. GM models give spectra low in level, by up to a factor of 20. Predictions from A type models can match the experimental spectral level if they are given an anisotropy ratio of sound speed inhomogeneities of 0.15, compared with the GM value of ƒ/N ≈ 0.02. Calculations show that microstructure may have intermittently influenced the Kane Basin transmission at small Fresnel radius Rƒ and that high intensity variance at large Rf must result from fine-structure variability.

Geophysical imaging with arbitrary source illumination

Abstract: Geophysical diffraction tomography is a technique for quantitative, high-resolution, subsurface imaging. This approach to imaging is a generalization of the conventional backprojection algorithm of X-ray tomography accounting for the diffraction effects that result from longer wavelength seismic or electromagnetic waves necessary for geophysical remote sensing. A diffraction tomography algorithm is presented for a configuration in which a finite number of sources of arbitrary character are distributed along one line and a finite number of receivers are distributed along a line having an arbitrary orientation with respect to the source line. Since most geophysical sources may be reasonably represented as point sources, the two-dimensional form of the algorithm is implemented for cylindrical-beam (a point source in two dimensions) illumination. Numerical experiments are performed to investigate a range of source-receiver configurations. It is found that parallel source and receiver arrays, a cross-borehole configuration, provide better image quality than orthogonal arrays. >

Point source diffraction and its use in an encoder.

Abstract: Diffraction from a point light source (a laser diode) was studied. The diffraction images were found to be magnified without using a lens and to move corresponding to the displacement of the slit aperture. A digital encoder using this diffraction is proposed to obtain very high resolution without a lens.

Numerical ray tracing in the atmospheric surface layer

Abstract: A numerical ray‐tracing model, including calculation of the sound pressure, was developed. It is valid for propagation from a point source in a moving, stratified atmosphere. Numerical integration of the ray equations was performed, and all rays reaching a specific point were found. Several expressions for the height dependency of the wind speed and the temperature were used, e.g., Monin–Obukhov similarity theory functions, the parameters of which were determined by use of a least‐squares method. Measurements of sound propagation from a point source over finite impedance ground were made. Meteorological parameters were monitored simultaneously, wind direction and relative humidity at a single height, wind speed and temperature at five elevations. Comparison with the model was made out to a distance of 150 m. The agreement between the model values and those measured was good. The influence of the directional characteristics of the source was studied, and found to be very important.

Encapsulated high brightness electron beam source and system

Abstract: An encapsulated high brightness source for use in or with an electron beam system such as an electron beam microscope. The source preferably includes a field emitter. The source includes source enclosure means which defines an ultra high vacuum enclosure for the field emitter. A lens which serves as part of the ultra high vacuum enclosure for the source defines a differential pressure aperture. Other lens elements draw electrons from the field emitter and form a focus on axis in the vicinity of the differential pressure aperture, which serves as an effective point source for the associated electron beam system. The source may be permanently built-in or modular; if modular, it may be assembled, tested, and stored in an ultra high vacuum operative condition for OEM assembly or as a replacement part.

Theory of the detection of the field surrounding half-dressed sources

Abstract: Half-dressed sources are defined as sources deprived partially or totally of the cloud of virtual quanta which surrounds them in the ground state of the total system. Two models of a half-dressed point source S are considered, the first in the framework of the theory of massive scalar fields and the second in quantum electrodynamics (QED). In both cases the detector is modeled by a second fully dressed source T of the same field, which is also bound to an oscillation center by harmonic forces. It is shown that when S at time t = 0 is suddenly coupled to or decoupled from the field, the detector T, which is initially at rest, is set in motion after a time t = R/sub 0//c, where R/sub 0/ is the S-T distance. Neglecting the reaction back on the field due to the oscillatory motion of T, the amplitude of oscillation for t = infinity is obtained as a function of R/sub 0/. Thus the time-varying virtual field of S is shown to be capable of exerting a force which excites the model detector. For the QED case, this force is related to the properties of the energy density of the virtualmore » field. This energy density displays a singularity at r = ct, and the mathematical nature of this singularity is studied in detail. In this way it is shown that the energy density of the time-dependent virtual field is rather different from that of a pulse of radiation emitted by a source during energy-conserving processes. The differences are discussed in detail, as well as the limitations of the model.« less

Two-scale solution for atmospheric scintillation from a point source

Abstract: In a previous paper [ J. Opt. Soc. Am. A2, 2133 ( 1985)] we presented three-dimensional numerical solutions for the scintillation index versus the range for a plane wave propagating in the atmosphere, based on the two-scale approximate solution to the fourth-moment equation. In that paper the refraction-index fluctuations were represented by a modified Kolmogorov spectrum, which included an inner scale. In the present paper we present the equivalent results for a point source. The spectrum here is modeled by a function that is somewhat different from that used in the previous paper, and we discuss the relation between the two. The results of the present paper are of interest because they can be compared with recent experimental data.

A computer program for calculating Ge(Li) detector counting efficiencies with large volume samples

Abstract: A novel computing method has been developed to calculate the absolute photopeak efficiency of a Ge(Li) detector for cylindrical samples of different heights and diameters and with variable density. For each point in the cylindrical sample the detection efficiency is calculated taking into account the distance from the detector and gamma ray attenuation and the efficiency is integrated numerically over the volume of the sample. The detector is approximated as a point detector with an experimentally determined effective interaction depth, it is necessary to measure the absolute efficiency for a point source located on the detector axis at a known distance. Then the computer program calculates the absolute counting efficiency for any cylindrical sample and for any density. The measured and calculated values for two different cylinders and three different densities give a good (−12%) overall agreement.

Electronic distance measuring network monitoring during the Rabaul seismicity/deformational crisis of 1983–1985

Abstract: Following a dramatic increase in the seismicity and rate of ground deformation at Rabaul Caldera in September 1983, an electronic distance measuring (EDM) network was established in November–December 1983 to provide additional surveillance of the developing volcanic hazard. The network and operational procedure were designed to monitor frequently a large area and to obtain results rapidly. Initially, data reduction was carried out using the conventional refractive index technique to correct for observed atmospheric variables, but the principle of line ratios using modeled atmospheric variables showed promise for accelerating and simplifying data reduction by obviating meteorological considerations. A standard error of ±1.31 ppm using the line ratio reduction technique favorably compares with results obtained by other researchers. Horizontal strain in the east central part of the caldera increased at monthly rates of 10–80 ppm, reaching a peak in April 1984. This peak coincided with the highest monthly rate of earthquake occurrence (13,800) and the largest monthly rate of ground uplift (approximately 80 mm). The greatest amount of horizontal ground deformation recorded during the period 1983–1985 was about 400 mm. After April 1984 the strain rates declined gradually and, in relation to other components of deformation, probably resumed a precrisis level by mid-1985. Horizontal deformation in the southern part of the caldera was relatively large in comparison to the amount of ground tilt and uplift recorded there. Because of the relatively small number of reflector stations, modeling of the EDM data is somewhat inconclusive. However, a Mogi point source model provides a reasonable fit to the observed deformation in the northern part of the caldera. The point source model and simple axisymmetric models do not provide a good match to the deformation recorded in the southern part of the caldera. The dominance of horizontal deformation there could be explained by a dyke intrusion.

Acoustic modeling and migration of stacked cross-hole data

Abstract: Prestack computations for cross‐hole data are relatively expensive, as they are for prestack surface survey data. It is therefore of interest to develop methodologies for modeling and processing stacked cross‐hole data. In this context, stacking is over sources, not midpoints. Modeling with a line source produces data that are equivalent (by Huygen’s principle) to those obtained by stacking over a line of point sources. Reverse‐time finite‐difference migration may be applied to the resulting stacked section by generalizing the excitation‐ time imaging condition for a point source to a line source. Illustrations include successful applications to both synthetic data and scale‐model data.

Method and apparatus for optical examination of an object particularly by moire ray deflection mapping

Abstract: A method and apparatus for moire ray deflection mapping for determining properties of an object in which a point source of light producing a diverging beam of direct light is passed through a first optical system including the object to be examined, which system retraces the light in the form of a converging beam of reflected light from the examined object back towards the point source. The converging beam of reflected light is intercepted before reaching the point source and is passed through a second optical system which collimates the beam of reflected light. The collimated beam is then directed through first and second gratings at a preselected angular orientation with respect to each other to produce moire fringe patterns providing an indication of the properties of the examined object. An important advantage in the above novel method is that the same setup can be used for measurements of both phase objects and specular surfaces.

Acceleration of point source fire to equilibrium spread

Abstract: ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS iv LIST OF TABLES vi LIST OF FIGURES vii 1.0 INTRODUCTION 1 1.1 Objectives 4 2.0 LITERATURE REVIEW 6 2.1 Field Observations 6 2.2 Theoretical Approximations 9 3.0 METHODS 21 3.

Classical Diffusive Photon Transport in a Slab

Abstract: The problem of localization of light in a random atmosphere requires, for comparison, a knowledge of the classical transport of light by an isotropically scattering medium in a bounded medium such as a slab. [See V. Sobolev, Light Scattering in Planetary Atmospheres, Pergamon (1975)] We consider the time-dependent Milne problem generalized to a slab with absorbing boundary conditions. We can adapt results obtained for phonon transport in GaAs at low temperatures to the corresponding optical problem. Transport of a pulse across the slab is evaluated by Monte Carlo methods, as well as by analytic approximations. Although the behavior is diffusive, when the detector is moved transversely, the mean time of arrival is found to be linear in the distance from point source to point detector!

Application of Fermi scattering theory to a magnetically scanned electron linear accelerator.

Abstract: This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick leadcollimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root‐mean‐square spatial Gaussian spread σ(z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size.Measurement of the variation in broad beam penumbra width with source–surface distance (SSD) for a 7‐MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photoncollimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photoncollimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.

P-sv wave responses for a point source in two-dimensional heterogeneous media:finite-difference method

Abstract: A method for producing approximate point source P-SV wave responses in laterally varying model is introduced. The method is based on the first-order system of wave equations for the velocities and stresses. At first, we introduce the linearly distributed stress tensors as the source and by means of two-dimentional finite-difference scheme to produce the response of this source, then the point source response is generated by correcting Loth the wave forms and geometrical spreading for the linear response. The accuracy of this approximation is justified by comparing the numerical results with the more accurate results. Because and of the dislocation source, explosion source or concentrated body force may be simulated by the sources mentioned above, also, because the finite-difference method has the advantage that arbitrary velocity and density structure in the media may be specified, the method seems to be effective for the problems such as the near field strong ground motions, explosive vibrations and exploration seismology.

Diffraction of sound above a curved surface having an impedance discontinuity

Abstract: This article describes theory and experiments in the case of propagation of sound in the presence of an impedance discontinuity and a curved surface (or sound‐speed gradients). The theory is based on a residue series, or creeping wave, solution developed for the propagation of sound in a stratified medium under conditions favorable to the formation of a shadow zone. The creeping wave solutions on each side of the impedance discontinuity are coupled heuristically by assuming a virtual source above discontinuity. The theory is compared to controlled measurements made indoors above a curved surface having an impedance jump (hard to soft). The measurements were made using a point source driven by frequencies in the range between 0.3 and 10 kHz. The theory is also compared to measurements made outdoors above a flat ground having a mixed path (asphalt to grass) in the presence of realistic gradients. Pure tones in the frequency range between 0.5 and 5 kHz were generated from a point source and the resulting sound‐pressure levels measured on a sound level meter after suitable filtering. In all cases, there is good agreement between measurements and theory.