Showing papers on "Reflection (physics) published in 1990"

Artificially soft and hard surfaces in electromagnetics

Abstract: A transversely corrugated surface as used in corrugated horn antennas represents a soft boundary. A hard boundary is made by using longitudinal corrugations filled with dielectric material. The concept of soft and hard surfaces is treated in detail, considering different geometries. It is shown that both the hard and soft boundaries have the advantage of a polarization-independent reflection coefficient for geometrical optics ray fields, so that a circularly polarized wave is circularly polarized in the same sense after reflection. The hard boundary can be used to obtain strong radiation fields along a surface for any polarization, whereas the soft boundary makes the fields radiated along the surface zero. >

Shape of the red edge as vitality indicator for plants

Abstract: The shift of the red edge in the reflection spectra of vegetation targets is a known phenomenon documenting changes in the biological status of plants. In our study we analysed the variability of red edge reflection in dependency of differently managed field plots, The results indicate that the red edge is not fully described by the shift of the main inflection point, but has to be considered as a collection of several different and possibly independent features, each of them influenced by biological parameters of the plants. Thus, taking all features, the red edge as derived from high resolution spectra may provide enough information to detect small differences in the chemical and morphological status of plants.

Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic Shield

Abstract: Plate tectonics provides the linking framework for all tectonic and magmatic activity seen today, but it is not known when plate tectonics first developed on Earth. New deep seismic reflection and ...

Angle-dependent reflectivity by means of prestack migration

Abstract: Most present day seismic migration schemes determine only the zero‐offset reflection coefficient for each grid point (depth point) in the subsurface. In matrix notation, the zero‐offset reflection coefficient is found on the diagonal of a reflectivity matrix operator that transforms the illuminating source‐wave field into a reflected‐wave field. However, angle dependent reflectivity information is contained in the full reflectivity matrix. Our objective is to obtain angle‐dependent reflection coefficients from seismic data by means of prestack migration (multisource, multioffset). After downward extrapolation of source and reflected wave fields to one depth level, the rows of the reflectivity matrix (representing angle‐dependent reflectivity information for each grid point at that depth level) are recovered by deconvolving the reflected wave fields with the related source wave fields. This process is carried out in the space‐frequency domain. In order to preserve the angle‐dependent reflectivity in the im...

On the cloud absorption anomaly

Abstract: This paper provides an overview of the subject of absorption of solar radiation by water clouds in the earth's atmosphere. the paper summarizes the available evidence which points to disagreements between theoretical and observed values of both cloud absorption and reflection. the importance of these discrepancies, particularly to remote sensing of clouds as well as to studies of cloud physics and earth radiation budgets, is emphasized. Existing cloud absorption and reflection measurements are reviewed and the persistent differences that exist between calculated and measured near-infrared cloud albedos are highlighted. Various explanations for these reflection and absorption discrepancies are discussed and a simple outline of the theory of cloud absorption is provided. This outline is used to examine the large-droplet hypothesis as well as the effects of absorbing aerosol and enhanced water vapour continuum absorption. A further hypothesis regarding the effects of cloud inhomogeneities is also examined. While the theory of cloud absorption is not completely understood, especially with regard to inhomogeneous clouds, the underlying conclusion of this paper points to the need for better measurements of solar radiation in clouds, water vapour absorption and microphysics properties of clouds.

Polarization-based material classification from specular reflection

Abstract: A computationally simple yet powerful method for distinguishing metal and dielectric material surfaces from the polarization characteristics of specularly reflected light is introduced. The method is completely passive, requiring only the sensing of transmitted radiance of reflected light through a polarizing filter positioned in multiple orientations in front of a camera sensor. Precise positioning of lighting is not required. An advantage of using a polarization-based method for material classification is its immunity to color variations, which so commonly exist on uniform material samples. A simple polarization-reflectance model, called the Fresnel reflectance model, is developed. The fundamental assumptions are that the diffuse component of reflection is completely unpolarized and that the polarization state of the specular component of reflection is dictated by the Fresnel reflection coefficients. The material classification method presented results axiomatically from the Fresnel reflectance model, by estimating the polarization Fresnel ratio. No assumptions are required about the functional form of the diffuse and specular components of reflection. The method is demonstrated on some common objects consisting of metal and dielectric parts. >

A model for anisotropic reflection

Abstract: A reflection and refraction model for anisotropic surfaces is introduced. The anisotropy is simulated by small cylinders (added or subtracted) distributed on the anisotropic surface. Different levels of anisotropy are achieved by varying the distance between each cylinder and/or rising the cylinders more or less from the surface. Multidirectional anisotropy is modelled by orienting groups of cylinders in different direction. The intensity of the reflected light is computed by determining the visible and illuminated portion of the cylinders, taking self-blocking into account. We present two techniques to compute this in practice. In one the intensity is computed by sampling the surface of the cylinders. The other is an analytic solution. In the case of the diffuse component, the solution is exact. In the case of the specular component, an approximation is developed using a Chebyshev polynomial approximation of the specular term, and integrating the polynomial.This model can be implemented easily within most rendering system, given a suitable mechanism to define and alter surface tangents. The effectiveness of the model and the visual importance of anisotropy are illustrated with some pictures.

Reflection and transmission characteristics at the edge of shore fast sea ice

Abstract: The reflection and transmission of ocean waves at a sea ice boundary is reconsidered. The sea ice is modelled as a continuous, thin elastic plate of uniform thickness, floating on water of arbitrary constant depth. Unlike earlier solutions, matching of potentials between the free surface domain and the ice-covered domain is done at all depths; previous solutions were incompletely matched, as the potentials in each domain were deficient. In the present solution a number of the infinity of evanescent modes, which were hitherto ignored, are included in the solution and allow matching to be carried out by minimization of an integrated error term from surface to seafloor. Reflection and transmission are found to be markedly influenced by the inclusion of these modes, suggesting that conclusions based on the incomplete potentials may be substantially in error.

Analysis of multilayer thin-film structures containing magneto-optic and anisotropic media at oblique incidence using 2×2 matrices

Abstract: A complete analysis of multilayer structures containing an arbitrary number of dielectric, metal, magnetic, and birefringent/dichroic layers is presented. An algorithm, based on simple 2×2 matrices, is derived which allows reflection, transmission, absorption, magneto‐optic conversion, birefringence, and dichroism of the structure to be computed on a personal computer. The incident beam is assumed to be plane monochromatic with arbitrary angle of incidence. There are no approximations involved, and the results are direct consequences of Maxwell’s equations.

The von Neumann paradox for the diffraction of weak shock waves

Abstract: We present results from our experiments with the irregular reflection of shock waves in argon. We compare the data with the results we obtained numerically; the assumptions for the computational code were that we had unsteady, two-dimensional, compressible, inviscid, flow of a perfect gas. When precautions were taken to reduce the effects of the gas viscosity on the experimental data, we obtained very good agreement between the numerical and the experimental results for the ramp Mach number and the trajectory path triple-point angle, but there were discrepancies with the wave-angle data. The discrepancies were ascribed to the sensitivity of the data to both viscosity and to a singularity. We show that there are actually two weak irregular wave reflections, namely a classic Mach reflection (MR) and a new type, that we call a von Neumann reflection (NR). The structure of the NR is discussed in some detail, and so are the transition criteria for the various wave systems.

Saturated and Unsaturated Spectra of Gravity Waves and Scale-Dependent Diffusion

Abstract: For a highly idealized condition, the spectrum of saturated and unsaturated gravity waves at each height is calculated directly from the wave equation. A principal feature of this wave equation is the inclusion of wave dissipation, although in an approximate form. In the absence of wave absorption, reflection, radiation, wind shears, resonant wave–wave interactions and other sources and sinks, this dissipation at each height is determined solely by the “turbulent” or chaotic state caused by off-resonant wave–wave interactions and instability of the (broad) wave spectrum at that height. The dissipation is approximately accounted for by a diffusion term. The appropriate diffusivity is self-consistent with the continuum of spectral waves that cause the chaotic state and is argued to be scale dependent. An inverse calculation is also made of what the observed spectra imply for wave dissipation—again assuming that many wave dissipations can be approximately described by a scale-dependent diffusion pro...

Normal-incidence reflection of slow atoms from an optical evanescent wave

Abstract: We demonstrate specular reflection for laser-cooled atoms dropped onto a gradient-light-force mirror. Transit times of 264 msec are observed for atoms dropped from a height of ~2.0 cm and bouncing twice on the evanescent wave before being detected. Of the ~107 atoms initially dropped, ~3 × 103 are detected after 264 msec.

Reflection and transmission of elastic waves from a fluid‐saturated porous solid boundary

Abstract: Based on the classical work of Biot [J. Acoust. Soc. Am. 28, 168 (1956)] which predicts that three different kinds of bulk waves may propagate in the fluid‐saturated porous solid, the wave equation is solved to determine the energy reflection and transmission coefficients of plane elastic waves at oblique incidence on an interface between a fluid and a fluid‐saturated porous solid. For this purpose, the necessary formalism of the energy equation, the Poynting energy flux vector, and the sound intensity of elastic waves in fluid‐saturated porous media are presented. Two general cases of mode conversion have been investigated: (1) The initial wave is incident from the fluid to the interface and generates three transmitted bulk waves in the fluid‐saturated porous solid, and (2) the initial wave is incident from the fluid‐saturated porous solid to the interface and generates three reflected bulk waves in the same medium. Furthermore, the transmission of sound through a fluid‐saturated porous solid plate immer...

External and internal reflection near field microscopy: experiments and results

Abstract: Two configurations of a scanning near field optical microscope working in reflection are presented. Results exhibiting nanometric resolution are given and discussed.

Matrix formalism for calculation of electric field intensity of light in stratified multilayered films

Abstract: A new algorithm has been proposed for the calculation of the electric field intensity in stratified multilayered films when light is incident on the system. The algorithm utilizes matrix formulas based on Abeles’s formulas for the calculation of reflectance and transmittance. Equations for calculating patial absorptance due to a certain depth in the films are also derived. Some examples of the application of the electric field description are given for the analysis of three kinds of reflection spectroscopic methods which use metal surfaces: reflection–absorption, surface electromagnetic wave, and metal overlayer ATR methods. The algorithm given here offers a useful tool in understanding the mechanism of light absorption in various spectroscopic methods, and is convenient to use where intensity of the IR spectrum is of interest.

Spatial Fourier transform method of measuring reflection coefficients at oblique incidence. I: Theory and numerical examples

Abstract: A new method using spatial Fourier transform has been developed to measure reflection coefficients at oblique incidence. The method involves the measurement of complex pressure distributions on two parallel planes lying close to the surface of a test material and decomposing each of the complex pressure distributions into plane‐wave components by using two‐dimensional spatial Fourier transform. The incident and reflected plane‐wave components on the surface of the test material can be mathematically separated by the use of plane‐wave propagation theory. This separation leads to the determination of reflection coefficients at arbitrary angles of incidence. Investigation has been made into the error due to the finite size of the measurement area to show that the magnitude of the error can be reduced by using a dipole source instead of a monopole source. Numerical examples are given to illustrate the validity of the method.

Theoretical estimates of light reflection and transmission by spatially complex and temporally varying sea ice covers

Abstract: The reflection, absorption, and transmission of light at visible and near-infrared wavelengths by snow and ice covers is important for a number of geophysical problems. The focus of this paper is on the reflection and transmission of light by spatially inhomogeneous and temporally varying sea ice covers. This is investigated using a two-stream, multilayer radiative transfer model in the wavelength region from 400 to 1000 nm. The model is computationally simple and utilizes the available experimental data on the optical properties of sea ice. The ice cover is characterized as a layered medium composed of selections from nine distinct snow and ice types. Three case studies are presented illustrating values of spectral albedo, transmittance, and transmitted photosynthetically active radiation (PAR) for (1) a spatially inhomogeneous ice cover, (2) a uniform ice cover as it undergoes a melt cycle, and (3) a temporally changing spatially variable ice cover. Results indicate that small-scale horizontal variations in snow depth and ice thickness can cause light transmission to change over 3 orders of magnitude. Dramatic changes in light reflection and transmission are predicted in the early part of the melt season as the ice cover evolves from an opaque, snow-covered medium to translucent bare or ponded ice.

Solution of the equations of dynamic elasticity by a Chebychev spectral method

Abstract: We present a spectral method for solving the two‐dimensional equations of dynamic elasticity, based on a Chebychev expansion in the vertical direction and a Fourier expansion for the horizontal direction. The technique can handle the free‐surface boundary condition more rigorously than the ordinary Fourier method. The algorithm is tested against problems with known analytic solutions, including Lamb’s problem of wave propagation in a uniform elastic half‐space, reflection from a solid‐solid interface, and surface wave propagation in a haft‐space containing a low‐velocity layer. Agreement between the solutions is very good. A fourth example of wave propagation in a laterally heterogeneous structure is also presented. Results indicate that the method is very accurate and only about a factor of two slower than the Fourier method.

Ion reflection and dissipation at quasi‐parallel collisionless shocks

Abstract: Large scale one-dimensional hybrid simulations have been performed of a quasi-parallel (ΘBn = 20°) high Mach number collisionless shock. It is found that backstreaming reflected ions, i.e., upstream ions with velocities exceeding the shock ram velocity, originate from the outer part (v≳ 1.7vth) of the velocity space of the incident distribution. The backstreaming ions produce very low-frequency magnetosonic waves which propagate upstream with about 1.3VA (Alfven speed). As the wave crests convect toward the shock, they steepen up and the shock reforms itself. During shock reformation a large part of the incident ions are reflected. This, in turn, slows the incident ions down. The slowed down incident particle distribution and the reflected particle distribution merge and constitute the new thermalized downstream distribution. In the interval of a relatively stationary shock low-frequency whistler waves stand at the shock front. During these time intervals the whistler waves are probably responsible for dissipation by nonadiabatic compression of the incident ions. The whistler waves are destroyed by the incoming large amplitude wave crest and reemerge at the new shock front. The reapparance seems to be due to the nonlinear steepening of the incoming wave crest at the upstream side.

Wave diffraction through offshore breakwaters

Abstract: The interaction of water waves with a long linear array of offshore breakwaters is examined to determine the reflection and transmission coefficients for these structures, providing data on the sheltering afforded by these structures. Two variational methods and an eigenfunction expansion method are used to determine the reflection coefficients for waves with wavelengths longer than the distance from gap to gap in the breakwater array. The eigenfunction method is also used for breakwaters, where the spacing between the gaps is longer than the water wavelength. For this case, analogous to scattering of light by a grating, numerous monochromatic directional wave trains can be generated in the region behind (and in front of) these breakwaters, which can lead to the generation of rip currents, beach cusps, and other periodic phenomena on beaches behind the structures.

Dependence of Stimulated Electromagnetic Emission on the Ionosphere and Pump Wave

Abstract: Electromagnetic emission, stimulated by a powerful high frequency radio wave injected into the ionospheric F region, exhibits a rich spectral structure in the sidebands of the reflected pump wave. Results from such experiments at the Heating facility near Tromso, Norway, are presented. Spectral features in a steady state, a few seconds after the onset of the pump, are shown to depend on ionospheric conditions, the pump frequency, and power. Different types of stimulated radiation spectra may be obtained when the ionospheric critical frequency is well above or near the pump frequency. Further, the appearance of the spectral features is very sensitive to the pump frequency near harmonics of the F region electron cyclotron frequency. Our interpretation is that the considered stimulated electromagnetic emissions are generated primarily in two interaction regions, the plasma resonance region at the pump reflection height and the upper hybrid resonance region, typically a few kilometers below the reflection height.

Radiant heating apparatus for providing uniform surface temperature useful in selective laser sintering

Abstract: A radiant heater, and an apparatus for performing selective laser sintering using the same, are disclosed. The radiant heater is ring-shaped with its dimensions and distance from the target surface preferably defined to provide radiant energy to a target surface, with the rate of energy received per unit area of the target surface being substantially uniform. A frusto-conical ring-shaped radiant heater is also disclosed, which may be disposed closer to the surface to provide improved efficiency of heat transfer in a uniform fashion. A cooling element similarly shaped is also disclosed, which operates in the same manner to uniformly transfer radiant heat from the target surface. Zoning of the ring-shaped radiant heaters, to allow for non-uniform radiant energy emission to the target surface, is also disclosed. Monitoring of the temperature of the target surface is done by way of infrared sensors. For monitoring of cool temperatures, a second infrared sensor is directed at the radiant heater, so that compensation for reflection of the radiant heat from the surface may be accomplished.

Relationship between leaky Lamb modes and reflection coefficient zeroes for a fluid‐coupled elastic layer

Abstract: It is shown in this article how the zeroes of the plane‐wave reflection coefficient for a fluid‐coupled elastic layer are modified under fluid loading, throughout the entire range of fluid density. Transformation and exchange behavior of branches of the reflection coefficient zero spectrum with increasing fluid density has been found and discussed. This study extends an earlier investigation of the complex mode spectrum (reflection coefficient poles). The behavior of the zeroes of the reflection coefficient is compared with that of the poles as a function of the fluid density. In addition, we derive an analytical expression for the intersection of the fluid‐modified fundamental zero branches and the Rayleigh wave speed. This number can be useful as a measure of the sensitivity of a solid material to fluid loading. More significantly, the results of the current study have immediate relevance for an important class of advanced materials: polymer‐matrix composites. It is demonstrated that the spectral modifi...

Backlight for an electronic display

Abstract: A backlight having a reflection plate, a diffusion plate, a transparent light guide plate held between the diffusion plate and the reflection plate and having a reflection and diffusion surface adjacent to the reflection plate and a recessed light emitting surface adjacent to the diffusion plate, so that a space is defined between the recessed light emitting surface and the bottom surface of the diffusion plate adjacent thereto, a holder connected to the diffusion plate to define accommodation spaces between the opposite ends of the transparent light guide plate and the holder, and line light sources located in the accommodation spaces.