Showing papers on "Reflection (physics) published in 1996"

A comparison of theoretical and empirical reflection coefficients for typical exterior wall surfaces in a mobile radio environment

Abstract: This paper presents microwave reflection coefficient measurements at 1.9 GHz and 4.0 GHz for a variety of typical smooth and rough exterior building surfaces. The measured test surfaces include walls composed of limestone blocks, glass, and brick. Reflection coefficients were measured by resolving individual reflected signal components temporally and spatially, using a spread-spectrum sliding correlation system with directional antennas. Measured reflection coefficients are compared to theoretical Fresnel reflection coefficients, applying Gaussian rough surface scattering corrections where applicable. Comparisons of theoretical calculations and measured test cases reveal that Fresnel reflection coefficients adequately predict the reflective properties of the glass and brick wall surfaces. The rough limestone block wall reflection measurements are shown to be bounded by the predictions using the Fresnel reflection coefficients for a smooth surface and the modified reflection coefficients using the Gaussian rough surface correction factors. A simple, but effective, reflection model for rough surfaces is proposed, which is in good agreement with propagation measurements at 1.9 GHz and 4 GHz for both vertical and horizontal antenna polarizations. These reflection coefficient models can be directly applied to the estimation of multipath signal strength in ray tracing algorithms for propagation prediction.

P-wave signatures and notation for transversely isotropic media: An overview

Abstract: Progress in seismic inversion and processing in anisotropic media depends on our ability to relate different seismic signatures to the anisotropic parameters. While the conventional notation (stiffness coefficients) is suitable for forward modeling, it is inconvenient in developing analytic insight into the influence of anisotropy on wave propagation. Here, a consistent description of P -wave signatures in transversely isotropic (TI) media with arbitrary strength of the anisotropy is given in terms of Thomsen notation. The influence of transverse isotropy on P -wave propagation is shown to be practically independent of the vertical S -wave velocity VS0 , even in models with strong velocity variations. Therefore, the contribution of transverse isotropy to P-wave kinematic and dynamic signatures is controlled by just two anisotropic parameters, e and δ, with the vertical velocity VP0 being a scaling coefficient in homogeneous models. The distortions of reflection moveouts and amplitudes are not necessarily correlated with the magnitude of velocity anisotropy. The influence of transverse isotropy on P -wave normal-moveout (NMO) velocity in a horizontally layered medium, on small-angle reflection coefficient, and on point-force radiation in the symmetry direction is entirely determined by the parameter δ. Another group of signatures of interest in reflection seisimology–the dip-dependence of NMO velocity, magnitude of nonhyperbolic moveout, time-migration impulse response, and the radiation pattern near vertical–is dependent on both anisotropic parameters (e and δ) and is primarily governed by the difference between e and δ. Since P -wave signatures are so sensitive to the value of e − δ, application of the elliptical-anisotropy approximation (e = δ) in P -wave processing may lead to significant errors. Many analytic expressions given in the paper remain valid in transversely isotropic media with a tilted symmetry axis. Moreover, the equation for NMO velocity from dipping reflectors, as well as the nonhyperbolic moveout equation, can be used in symmetry planes of any anisotropic media (e.g., orthorhombic).

A Study of the Interaction between Ultrasound and a Partially Contacting Solid--Solid Interface

Abstract: The measurement of the reflection of ultrasonic waves from a partially contacting solid--solid interface can be used to study the contact conditions at that interface. This paper describes measurements and predictions of the reflection of ultrasonic waves from partially contacting aluminium--aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact region the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient increased with frequency. No measurements were taken in the perfect contact region in which the reflection coefficient is known to be zero at all frequencies as this state is the same as a continuous piece of aluminium. The reflection coefficient variation with frequency was modelled using a spring model, good agreement between experiments and predictions being achieved. Reflection coefficient measurements were then used to study the contact between two aluminium surfaces under repeated loading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. Both elastic and plastic statistical contact models, as well as a numerical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the experimentally determined stiffness variations and the predicted stiffness was within an order of magnitude of the measured value in all cases.

The Nature of the 660-Kilometer Upper-Mantle Seismic Discontinuity from Precursors to the PP Phase

Abstract: Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lame's constant lambda and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lame's constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.

A normal mode model for acousto‐elastic ocean environments

Abstract: A normal mode method for propagation modeling in acousto‐elastic ocean waveguides is described. The compressional (p‐) and shear (s‐) wave propagation speeds in the multilayer environment may be constant or have a gradient (1/c2 linear) in each layer. Mode eigenvalues are found by analytically computing the downward‐ and upward‐looking plane wave reflection coefficients R1 and R2 at a reference depth in the fluid and searching the complex k plane for points where the product R1R2=1. The complex k‐plane search is greatly simplified by following the path along which |R1R2|=1. Modes are found as points on the path where the phase of R1R2 is a multiple of 2π. The direction of the path is found by computing the derivatives d(R1R2)/dk analytically. Leaky modes are found, allowing the mode solution to be accurate at short ranges. Seismic interface modes such as the Scholte and Stonely modes are also found. Multiple ducts in the sound speed profile are handled by employing multiple reference depths. Use of Airy function solutions to the wave equation in each layer when computing R1 and R2 results in computation times that increase only linearly with frequency.

Investigation on the reflection and transmission properties of complex absorbing potentials

Abstract: The reflection and transmission properties of different complex absorbing potentials (CAPs) are studied using WKB and scaling procedures which make the results transferable to any mass and kinetic energy. Explicit formulas are obtained which describe the reflection and transmission properties of monomial CAPs −iηxn with high accuracy. These properties are now well understood. The approximate results are compared to exact analytical results available for quadratic CAPs, and to numerical results obtained by wave packet propagation followed by an energy resolved analysis. The approximate, but accurate, description of the action of the CAP is finally used to determine optimal CAP parameters. CAP length, strength, and order can now be chosen in such a way that the sum of reflection and transmission is minimized. Optimal parameters are compiled for different energies and energy intervals.

Efficient optical system for a high resolution projection display employing reflection light valves

Abstract: An optical system is described consisting of reflection birefringent light valves, polarizing beam splitter, color image combining prisms, illumination system, projection lens, filters for color and contrast control, and screen placed in a configuration offering advantages for a high resolution color display. The system includes a quarter wave plate positioned to suppress stray reflection from the projection lens. The system also includes a second quarter wave plate disposed on the screen and a polarizing film disposed on the second quarter wave plate to suppress ambient light from illuminating the screen and entering the system.

Model for quantitative analysis of reflection-electron-energy-loss spectra: Angular dependence

Abstract: A model to reproduce inelastic electron scattering cross sections as determined from reflection-electronenergy-loss experiments is proposed. This model is an extension of model B from Yubero and Tougaard @Phys. Rev. B46, 2486~1992!#. Here, a more general geometry is considered where the incidence and exit angles can be varied. Then, for a given geometry and energy of the primary electrons, the dielectric function of the sample is the only input for the calculations. A systematic study of the behavior of the model is presented for the case of Si and Fe.

SBR image approach for radio wave propagation in tunnels with and without traffic

Abstract: We propose a deterministic approach to model the radio propagation channels in tunnels with and without traffic. This technique applies the modified shooting and bouncing ray (SBR) method to find equivalent sources (images) in each launched ray tube and sums the receiving complex amplitude contributed by all images coherently. In addition, the vector effective antenna height (VEH) is introduced to consider the polarization-coupling effect resulting from the shape of the tunnels. We verify this approach by comparing the numerical results in two canonical examples where closed-form solutions exist. The good agreement indicates that our method can provide a good approximation of high-frequency radio propagation inside tunnels where reflection is dominant. We show that the propagation loss in tunnels can vary considerably according to the tunnel shapes and the traffic inside them. From the results we also find a "focusing" effect, which makes the power received in an arched tunnel higher than that in a rectangular tunnel. Besides, the deep fading that appears in a rectangular tunnel is absent in an arched tunnel. The major effect of the traffic is observed to be the fast fading due to the reflection/obstruction of vehicles. Additional considerations, such as time delay, wall roughness, and wedge diffraction of radio wave propagation in tunnels are left for future studies.

A physical model of sound diffraction and reflections in the human concha

Abstract: An approximated physical model of the frequency transfer function of the human concha is developed in this paper. This formulation includes diffraction, reflection, and interference phenomena in the concha cavity. The performance of the proposed diffraction/reflection model is compared with that of the single‐delay‐and‐add approximation by checking their predictions against the experimental transfer function of a metal spiral‐shaped diffracting/reflecting system. Results show that the diffraction/reflection model performs considerably better at predicting both the absolute center frequency of spectral minima and the relative frequency spacing between them. The diffraction/reflection model is then applied to a realistic concha shape and its predictions are compared with experimental head‐related transfer functions for azimuth‐ and elevation‐varying sound sources. In this case, the model predicts the elevation‐dependent spectral features related to the transverse dimensions of the concha. Additionally, the diffraction/reflection model predicts that, because of sound diffraction, similar spectral features must be generated in the concha for sources at all azimuths within the frontal part of the ipsilateral hemisphere. Experimental and theoretical evidence supporting this prediction is presented.

A reflection-dominated X-ray spectrum discovered by ASCA in the Circinus galaxy

Abstract: We report the rst pointed X{ray observation, made by ASCA, of the nearby Seyfert 2 galaxy in Circinus. The spectrum consists of a very prominent (EW2 keV) uorescent 6.4 keV iron line, a very at continuum (photon spectral index less than one) and other lines, including one at 7 keV probably due, at least in part, to iron K uorescent emission. The spectrum is consistent with Compton scattering and uorescent emission from cold matter illuminated by an obscured active nucleus.

Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz

Abstract: Using transient terahertz reflection spectroscopy, the far‐infrared (0.1–1.1 THz) reflection amplitude and associated phase change upon reflection from undoped InSb was measured between 80 and 260 K. Using the Drude model, the transient terahertz data were fit by adjusting the values for the electron mobility and carrier concentration. The close agreement between the data and model validates using the Drude model to describe the electronic and optical properties of InSb below 1 THz. These results suggest a possible application of transient terahertz reflection spectroscopy for measuring semiconductor properties of thick samples without surface contact.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis

Abstract: A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 μ∊ The spatial resolution depends on the strain gradient; ie, the higher the gradient, the better the resolution A resolution of 08 mm was achieved for a 5-mm grating with a gradient of 250 μ∊/mm

Room acoustic prediction based on a unified treatment of diffuse and specular reflection

Abstract: A new general algorithm for room acoustic prediction is presented. The algorithm, based on approximate cone tracing, handles diffuse reflection by a splitup of cones incident on diffusing surfaces. The splitup of cones treats the interaction between specular and diffuse reflection in a physically sensible manner. A ‘‘brute‐force’’ implementation of such an algorithm inevitably creates an exponential dependence of the calculation time on the number of reflection combinations between diffusely reflecting surfaces, and therefore results in extremely long processing times. By exploiting the properties of diffuse reflection, the described algorithm displays an essentially linear dependence resulting in processing times short enough also for personal computers. The algorithm adapts its behavior to the properties of each specific environment, relies on very few nonverifiable assumptions, and is designed to meet the specific requirements of auralization. The splitup of cones creates a myriad of weak diffuse reflections resulting in a very smooth late decay. The algorithm is described in detail and sample predictions are shown.

A unified approach to 3-D seismic reflection imaging, Part I: Basic concepts

Abstract: Given a 3-D seismic record for an arbitrary measurement configuration and assuming a laterally and vertically inhomogeneous, isotropic macro‐velocity model, a unified approach to amplitude‐preserving seismic reflection imaging is provided. This approach is composed of (1) a weighted Kirchhoff‐type diffraction‐stack integral to transform (migrate) seismic reflection data from the measurement time domain into the model depth domain, and of (2) a weighted Kirchhoff‐type isochrone‐stack integral to transform (demigrate) the migrated seismic image from the depth domain back into the time domain. Both the diffraction‐stack and isochrone‐stack integrals can be applied in sequence (i.e., they can be chained) for different measurement configurations or different velocity models to permit two principally different amplitude‐preserving image transformations. These are (1) the amplitude‐preserving transformation (directly in the time domain) of one 3-D seismic record section into another one pertaining to a different...

Wind Speed Effects on Sea Surface Emission and Reflection for the Along Track Scanning Radiometer

Abstract: The emission and reflection properties of a rough sea surface are investigated, with particular emphasis on the wavelengths and viewing geometry relevant to the Along Track Scanning Radiometer. The authors start from Fresnel's equations for a flat water surface and calculate the effect of changing sea state on direct emissivity and reflectivity. The authors then investigate the role of surface-emitted surface-reflected (SESR) radiation, which enhances emissivity at high wind speeds. The effect of foam and whitecaps at high wind speeds is referred to briefly in the appendix but essentially remains an unknown quantity. Radiative transfer calculations that employ emissivity models also have to consider the reflection of downwelling radiance from the atmosphere. Although energy conservation requires that reflectivity is 1 minus emissivity, the variability of the sky brightness with zenith demands that one consider also the angular distribution of the reflected radiance. Additionally, the extended sta...

Dynamics of MHD wave propagation in the low-latitude magnetosphere

Abstract: The propagation of MHD waves depends on a local Alfven speed and ambient geometry. The dynamical properties of MHD waves in the plasmasphere and magnetosphere are investigated by assuming a realistic Alfven speed profile in a dipole field. The WKB approximation is used to determine the cutoff boundaries and estimate the wave dispersion over a whole meridional plane. It is found that most wave energy may be transmitted effectively into the inner magnetosphere near the equatorial region, since the reflection of incoming waves at the plasmapause becomes weakest at the equator. It is also examined how the transmission from the outer magnetosphere to the inner magnetosphere depends on wave frequencies and azimuthal wavenumbers. The cutoff boundaries of wave propagation are quantitatively determined for each mode and wavenumber, which show various structures on the meridian. The results suggest that the propagation region may consist of two separable domains of the inner and outer magnetosphere for a relatively low-frequency wave. The inner region of propagation appears to be a distorted torus around the dipole axis. Such spatial separation of the two regions becomes weak and gradually interconnected for the waves with a relatively small azimuthal wavenumber or high frequency. The wave spectra and energy distribution are also investigated for different azimuthal wavenumbers. The numerical results show that the cavity modes may exist in the plasmasphere even at the absence of the outer magnetospheric boundary, which is found to be strongly associated with the characteristics of wave parameters. In particular, it is suggested that the plasmaspheric cavity modes, in the nightside region, may play a crucial role in producing Pi 2 pulsations. In addition, theories of waveguide and cavity modes based on the dipole model are discussed in detail.

Reflection of thermoelastic waves at a solid half-space with two relaxation times

Abstract: The effect of two relaxation times, following Green and Lindsay's theory, on the reflection of thermoelastic waves at a homogeneous, isotropic, and thermally conducting elastic solid half-space is studied. The results for the partition of the energy for various values of the angle of incidence are presented and compared with that of Lord and Shulman's theory to show the importance of the second relaxation time.

Optical properties of isotropic chiral media

Abstract: A review is given of the optical properties of isotropic chiral media, based on the symmetrized constitutive relations of Condon. The review includes discussion of wave propagation in chiral media, and derivation of the reflection and transmission amplitudes of an isotropic optically active medium, and of a layer resting on a substrate. Boundary conditions and energy conservation relations are derived. For the chiral layer, simple formulae are given for the reflection and transmission coefficients at normal incidence, in the weak chirality limit, near the critical angles, and for a thin layer. Analytic expressions are given for all the reflection and transmission amplitudes in the general case. An ellipsometric method of measuring the chirality of very small sample volumes is suggested.

Lighting apparatus with a light guiding body having projections in the shape of a trapezoid

Abstract: A reflection layer is provided on a surface of a light guiding body on which projections are formed with intervals, except the side surfaces of the projections. A light emitting element is provided close to one end surface of a light guiding element. Most of the light from the light emitting element is input to the light guiding body through the light input surface. The light travels through the light guiding body to be output from the side surfaces of the projections. After passing through a directional change element, the light is input to the light receiving element in a direction substantially perpendicular to a light receiving element. Outside light passes through the light receiving element and is then input to a lighting apparatus. In the lighting apparatus, the outside light passes through the directional change element to be reflected by a reflection layer on the light-output-side surface of the light guiding body and then again passes perpendicularly through the directional change element toward the light receiving element. When the light passes through the light receiving element, a displayed image can be viewed. The reflection layer can reflect the light with a small loss. In addition, if the outside light is input at an angle of other than 90° to the light receiving element at first, the light can be reflected by the reflection layer with a small loss.

CRRES Poynting vector observations of electromagnetic ion cyclotron waves near the plasmapause

Abstract: Electromagnetic ion cyclotron waves have been observed by the Air Force Geophysics Laboratory triaxial fluxgate magnetometer on the CRRES satellite in the vicinity of the plasmapause. Specific wave events in the 1300–0100 MLT local time sector and over L = 4.8 – 6.7 at magnetic latitudes between ± 20° have been studied with respect to energy propagation. Using the wave magnetic field data and the orthogonal two-component Berkeley electric field experiment data allows the computation of the Poynting vector. Nine events with frequencies below 0.6 Hz have been analyzed. General wave properties showed intensities in the range 4–18 μW m−2. In most events, energy propagation was predominantly away from the equator, supporting the existence of an equatorial source region. There are exceptions and one event, observed 3° from the equator, also showed some propagation back to the equator with equal energy. Four periodic wave packets were seen in another event. This event showed unidirectional propagation away from the equator over all packets with periodicity approximately one-half of the calculated time for a typical bouncing wave packet. Consequently, the periodicity is not the signature of a classical periodic structured Pc 1–2 pulsation event of the type seen on the ground. Generally, the results for these waves with f < fHe+ may be interpreted by assuming an ion cyclotron instability source located near the equator and irregularly emitting wave trains of energy into one or both hemispheres, with minimal reflection at the ionosphere.

High-order optical harmonic generation from solid surfaces

Abstract: During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.

Reflection of shear Alfven waves at the ionosphere and the divergent Hall current

Abstract: A general expression for the reflection and mode conversion of MHD waves at the ionosphere is derived. On the basis of the expression, the effect of ionospheric divergent Hall current (Hall part of the induction current associated with the inductive electric field of fast magnetosonic wave) on localized toroidal oscillation is examined. When the horizontal scale of localized oscillation is of the order of several times of the height of ionosphere, the inductive electric field can play a significant role in the reflection of shear Alfven waves with longer periods (e.g., ∼100 s) in the high-latitude region, especially, in the auroral zone. Then, the contribution of the divergent Hall current to the field-aligned one can be no longer neglected and so the eigenfrequency of localized toroidal oscillation is effectively controlled by the height-integrated Hall conductivity in the ionosphere.

Probability of reflection by a random laser.

Abstract: A theory is presented (and supported by numerical simulations) for phase-coherent reflection of light by a disordered medium which either absorbs or amplifies radiation. The distribution of reflection eigenvalues is shown to be the Laguerre ensemble of random-matrix theory. The statistical fluctuations of the albedo (the ratio of reflected and incident power) are computed for arbitrary ratio of sample thickness, mean free path, and absorption or amplification length. On approaching the laser threshold all moments of the distribution of the albedo diverge. Its modal value remains finite, however, and acquires an anomalous dependence on the illuminated surface area.

Radar apparatus for detecting a direction of a center of a target

Abstract: A radar apparatus of an automotive vehicle includes a radar unit which radiates an electromagnetic wave to a target in a forward direction of the vehicle and receives reflection beams from the target to detect the target. A scanning control unit performs a beam scanning of the radar unit to the target so that the reflection beams during the beam scanning are received. A center direction determining unit detects a distribution pattern of the received reflection beams with respect to respective scanning angles of the radar unit, performs a similarity approximation of the distribution pattern by using an antenna directional gain pattern of the radar unit to produce an approximated distribution pattern, and determines a direction of a center of the target based on a peak of the approximated distribution pattern.