Showing papers on "Reflection (physics) published in 1991"

Surface reflection: physical and geometrical perspectives

Abstract: Reflectance models based on physical optics and geometrical optics are studied. Specifically, the authors consider the Beckmann-Spizzichino (physical optics) model and the Torrance-Sparrow (geometrical optics) model. These two models were chosen because they have been reported to fit experimental data well. Each model is described in detail, and the conditions that determine the validity of the model are clearly stated. By studying reflectance curves predicted by the two models, the authors propose a reflectance framework comprising three components: the diffuse lobe, the specular lobe, and the specular spike. The effects of surface roughness on the three primary components are analyzed in detail. >

A comprehensive physical model for light reflection

Abstract: A new general reflectance model for computer graphics is presented. The model is based on physical optics and describes specular, directional diffuse, and uniform diffuse reflection by a surface. The reflected light pattern depends on wavelength, incidence angle, two surface roughness parameters, and surface refractive index. The formulation is self consistent in terms of polarization, surface roughness, masking/shadowing, and energy. The model applies to a wide range of materials and surface finishes and provides a smooth transition from diffuse-like to specular reflection as the wavelength and incidence angle are increased or the surface roughness is decreased. The model is analytic and suitable for Computer Graphics applications. Predicted reflectance distributions compare favorably with experiment. The model is applied to metallic, nonmetallic, and plastic materials, with smooth and rough surfaces.

Constraining object features using a polarization reflectance model

Abstract: The authors present a polarization reflectance model that uses the Fresnel reflection coefficients. This reflectance model accurately predicts the magnitudes of polarization components of reflected light, and all the polarization-based methods presented follow from this model. The authors demonstrate the capability of polarization-based methods to segment material surfaces according to varying levels of relative electrical conductivity, in particular distinguishing dielectrics, which are nonconducting, and metals, which are highly conductive. Polarization-based methods can provide cues for distinguishing different intensity-edge types arising from intrinsic light-dark or color variations, intensity edges caused by specularities, and intensity edges caused by occluding contours where the viewing direction becomes nearly orthogonal to surface normals. Analysis of reflected polarization components is also shown to enable the separation of diffuse and specular components of reflection, unobscuring intrinsic surface detail saturated by specular glare. Polarization-based methods used for constraining surface normals are discussed. >

Transparent boundary condition for beam propagation

Abstract: A new boundary condition algorithm is presented that passes outgoing radiation freely with a minimum reflection coefficient (typically 10−5) while inhibiting the flux of incoming radiation. In contrast to the commonly used absorber method, this algorithm contains no adjustable parameters and is thus problem independent. It adapts naturally to a standard Crank–Nicholson difference scheme and is shown to be accurate and robust for both two-and three-dimensional problems.

Internal reflection of diffusive light in random media

Abstract: The consequences of internal reflection of multiply scattered light at the boundaries of disordered media are studied We show that the effect of internal reflection due to index mismatch can be quantitatively accounted for with a single parameter by incorporating a reflection coefficient into the boundary condition for the diffusive light We measure the angular correlation functions in transmission and reflection at different thicknesses for both high- and low-index mismatch By including the effect of internal reflection, we are able to obtain consistent quantitative agreement between experiment and theory Extensions to other experiments including diffusing-wave spectroscopy, coherent backscattering, frequency correlations, and pulse propagation are discussed

Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces

Abstract: Surface-relief structures having the form of an array of modified pyramids (having curved rather than flat triangular shown to produce an index-matching layer that will reduce surface reflection by several orders of sides) are Such structures are equivalent to a quintic (fifth-order polynomial) gradient-index layer, which has magnitude. been shown to be near optimum for reducing reflection at dielectric interfaces.

Reflection and transmission from porous structures under oblique wave attack

Abstract: The linear theory for water waves impinging obliquely on a vertically sided porous structure is examined. For normal wave incidence, the reflection and transmission from a porous breakwater has been studied many times using eigenfunction expansions in the water region in front of the structure, within the porous medium, and behind the structure in the down-wave water region. For oblique wave incidence, the reflection and transmission coefficients are significantly altered and they are calculated here. Using a plane-wave assumption, which involves neglecting the evanescent eigenmodes that exist near the structure boundaries (to satisfy matching conditions), the problem can be reduced from a matrix problem to one which is analytic. The plane-wave approximation provides an adequate solution for the case where the damping within the structure is not too great. An important parameter in this problem is Γ 2 = ω 2 h ( s - i f )/ g , where ω is the wave angular frequency, h the constant water depth, g the acceleration due to gravity, and s and f are parameters describing the porous medium. As the friction in the porous medium, f , becomes non-zero, the eigenfunctions differ from those in the fluid regions, largely owing to the change in the modal wavenumbers, which depend on Γ 2 . For an infinite number of values of ΓF 2 , there are no eigenfunction expansions in the porous medium, owing to the coalescence of two of the wavenumbers. These cases are shown to result in a non-separable mathematical problem and the appropriate wave modes are determined. As the two wavenumbers approach the critical value of Γ 2 , it is shown that the wave modes can swap their identity.

New approaches to nonlinear diffractive field propagation

Abstract: In many domains of acoustic field propagation, such as medical ultrasound imaging, lithotripsy shock treatment, and underwater sonar, a realistic calculation of beam patterns requires treatment of the effects of diffraction from finite sources. Also, the mechanisms of loss and nonlinear effects within the medium are typically nonnegligible. The combination of diffraction, attenuation, and nonlinear effects has been treated by a number of formulations and numerical techniques. A novel model that incrementally propagates the fields of baffled planar sources with substeps that account for the physics of diffraction, attenuation, and nonlinearity is presented. The model accounts for the effects of refraction and reflection (but not multiple reflections) in the case of propagation through multiple, parallel layers of fluid medium. An implementation of the model for axis symmetric sources has been developed. In one substep of the implementation, a new discrete Hankel transform is used with spatial transform tec...

Confocal scanning optical microscope

Abstract: A confocal scanning optical microscope in which a specimen under test is simultaneously scanned with two distinct spots or slits of illumination (S1, S2), and two output beams emitted from the specimen due to reflection or fluorescence are descanned and passed to separate stationary confocal apertures (A1, A2) and detectors (D1, D2).

Effects of surface magnetism on optical second harmonic generation

Abstract: We report on the first experiments showing the influence of surface magnetization on optical second harmonic generation in reflection at a Fe(110) surface. The magneto-optical Kerr effect modifies the hyperpolarizability of the surface in the optical field, leading to a dependence of the second harmonic yield on the direction of magnetization relative to the light fields. For the clean surface an effect of 25% was determined, which decays exponentially with surface contamination by the residual gas, thus demonstrating the high surface sensitivity of this technique.

Oblique reflection of turbidity currents

Abstract: Turbidity currents meeting obstacles, for example, the margins of a confined basin, are subject to reflection. The consequent change in flow direction is expressed in the sequence of depositional structures of the resulting bed of sediment. Putative examples of orthogonal reflection have been described, based on 180° opposed current directions. We present field evidence for the more general case of oblique reflection of turbidites, and we report the results of flume experiments indicating a mechanism involving generation of internal solitary waves at an oblique ramp. These propagate normal to the ramp, regardless of the angle of incidence. Flow directions in reflected turbidites may indicate the orientation of reflecting surfaces, such as basin margin slopes, and thus may be of considerable help in paleogeographic and tectonic reconstructions.

Measurement of Regular Wave Reflection

Abstract: The present paper describes the measurement of regular wave reflection. Three methods involving the use of two and three fixed probes are described and developed so as to provide explicit solutions for the incident wave height, reflection coefficient, and the phase of the reflected wave train. Extensions of the methods to the case of oblique wave reflection are described. A comparison is made of the range of application and accuracy of the methods. A method involving a least‐squares fit to measurements from three fixed probes is found to be the most accurate, whereas a method involving three height measurements is found to be generally unsuitable except under certain conditions. For methods involving the use of three probes, recommendations are made for the relative probe spacing so as to avoid conditions at which the methods fail or become inaccurate.

Optical mapping of crack tip deformations using the methods of transmission and reflection coherent gradient sensing - A study of crack tip K-dominance

Abstract: A new full field optical technique-‘Coherent Gradient Sensing’ (CGS)-is developed and used to study crack tip deformations in transparent as well as opaque solids. A first order diffraction analysis is provided for the technique and its feasibility is demonstrated both in transmission and reflection modes. Preliminary results from the dynamic crack growth experiment clearly demonstrate the capability of CGS to be an effective experimental alternative to other optical methods used in dynamic fracture studies. Notably, it is a full field technique which works with optically isotropic materials.

An inverse‐scattering model for an all‐optical logic gate

Abstract: Electromagnetic inverse scattering theory is used to model the ON and OFF states of an all‐optical logic gate. The scattering data are two phase‐conjugate reflection coefficients that were obtained by pole reflection.

Reflection/transmission coefficients and azimuthal anisotropy in marine seismic studies

Abstract: SUMMARY introduce two methods for inferring the direction of vertical fractures from marine seismic data. First we derive the necessary reflection and transmission coefficients for an interface between a liquid and an azimuthally anisotropic solid. Next we show that multicomponent ocean bottom seismometer data from surface airgun sources along two perpendicular shot lines can be rotated into the principal directions of azimuthal anisotropy to determine the orientation of vertical fractures. Finally we show that P-wave amplitude versus offset (AVO) depends on the orientation of the shot line with respect to the vertical fractures. Thus P-wave AVO can also be used to determine fracture orientation.

The use of contrast variation in the specular reflection of neutrons from interfaces

Abstract: Several of the useful results of the kinematic approximation are presented and applied to experimental data. From the kinematic equations it is shown that contrast variation in neutron reflection can be used to obtain a scattering length density profile across an interface unique within the limitations of resolution. It is shown to be more important for two component systems to obtain actual density profiles. Contrast variation is the only way to achieve this and an illustrative example is analysed in terms of the kinematic approximation.

Crustal-scale structure of the southern Rhine Graben from ECORS-DEKORP seismic reflection data

Abstract: Two seismic lines with a total length of 207 km were shot in 1988 by the French and German deep reflection seismic groups (ECORS and DEKORP) across the Rhinegraben. The southern profile shows the following characteristics. (1) The Moho discontinuity is marked by a strong reflector, except beneath the eastern part of the graben and at the western end of the profile. Its depth is only 8 s (two-way traveltime) below the graben and increases progressively to 10 s below the Lorraine basin. (2) The layered lower crust reveals strong variations in the seismic signature and apparent thickness. (3) East-dipping reflectors in the middle crust may be attributed to Variscan features. (4) The Lorraine basin is characterized by Carboniferous and Permian-Triassic strata onlapping the Vosges crystafline basement. (5) The Rhinegraben is markedly asymmetric, possibly owing to extensional movement along a listric shear zone, which appears to merge at depth with a flat-lying detachment in the ductile lower crust. The Oligocene sedimentary infill was controlled by an east-dipping normal fault whose vertical throw is about 3 km.

Wave interactions with generalized Cantor bar fractal multilayers

Abstract: The reflection and transmission properties of finely divided fractal layers are investigated and characterized. The results for electromagnetic or optical waves normally incident upon generalized Cantor bar fractal multilayers are found for various fractal dimensions and stages of growth. A new exact self‐similar algorithm is described which makes use of the self‐similarity of the structures to clearly display the underlying physics. This fractal computational scheme provides the reflection and transmission coefficients for fractally distributed layers with extreme economy when compared to traditional approaches. Finally, a method for extracting fractal descriptors from scattered data is discussed. High‐ and low‐ frequency regimes are examined.

Liquid crystal display and reflective diffuser therefor including a reflection cavity section and an illumination cavity section

Abstract: A diffuser for backlighting a display panel consists of a shell having a flat surface for positioning against the backside of the panel. The shell has a cavity with two sections. A reflection cavity section extending into the shell from the flat surface has highly reflective walls one of which opposite the mouth of the cavity defines a ramp. An illumination cavity section extending into the shell intercepts the reflection cavity section at a boundary near the foot of the ramp. When light propagates from the illumination cavity section into the reflection cavity section, the walls of the latter cavity section distribute that light by reflection and the ramp redirects that light to the mouth of the reflection cavity section from which the light propagates to the display panel as a uniform diffuse emanation.

Total reflection X-ray fluorescence spectrometry for quantitative surface and layer analysis

Abstract: Measurements of X-ray fluorescence spectra versus grazing incident angles provide information on elemental composition as well as density and thickness of near surface layers. Calculations of fluorescence intensities are presented, which are used for the evaluation of data obtained by total reflection X-ray fluorescence (TXRF) spectrometry. The calculation is based on a matrix formalism to account for standing wave phenomena due to transmission and reflection in layered material. For the determination of concentrations the model makes additional use of the fundamental parameter technique in order to include absorption and enhancement effects of the fluorescence radiation. On the basis of experimental data some capabilities of this nondestructive and contactless probing technique are presented.

Long-wave anisotropy in stratified media; a numerical test

Abstract: When a seismic signal propagates in a stratified earth, there is anisotropy if the dominant wavelength is long enough compared to the layer thickness. In this situation, the layered medium can be replaced by an equivalent nondispersive transversely isotropic medium. Theoretical and experimental analyses of the required minimum ratio of seismic wavelength to layer spacing based on kinematic considerations yield different results, with a much higher value in the experimental test.The present work investigates the effects of layering by wave simulation and attempts to establish quantitatively the minimum ratio for which the long-wave approximation starts to be valid. We consider two-constituent periodically layered media and analyze the long-wave approximation for different material compositions and different material proportions in 1-D and 2-D media. The evaluation of the minimum ratio compares snapshots and synthetic seismograms visually and through a measure of coherence.Layering induces scattering with wave dispersion or anisotropy depending upon the wavelength-to-layer thickness ratio. The modeling confirms the dispersive characteristics of the wave field, the scattering effects in the form of coda waves at short wavelengths, and the smoothed transversely isotropic behavior at long wavelengths. 1-D numerical tests for different media indicate that the minimum ratio is highest for the midrange of compositions, i.e., equal amount of each material, and for stronger reflection coefficients between the constituents. For epoxy-glass, the value is around R = 8, while for sandstone-limestone, it is between R = 5 and R = 6. Recent wave-propagation experiments done in epoxy-glass also imply a highest minimum ratio for midrange of composition; however, the 1-D numerical tests confirm the long-wave approximation at shorter wavelengths than experimentally. The 2-D case shows that the more anisotropic the equivalent medium, the higher the minimum ratio, and that the approximation depends upon the propagation angle with longer wavelengths required in the direction of the layering.

Alfven wave trapping, network microflaring, and heating in solar coronal holes

Abstract: Fresh evidence that much of the heating in coronal holes is provided by Alfven waves is presented. This evidence comes from examining the reflection of Alfven waves in an isothermal hydrostatic model coronal hole with an open magnetic field. Reflection occurs if the wavelength is as long as the order of the scale height of the Alfven velocity. For Alfven waves with periods of about 5 min, and for realistic density, magnetic field strength, and magnetic field spreading in the model, the waves are reflected back down within the model hole if the coronal temperature is only slightly less than 1.0 x 10 to the 6th K, but are not reflected and escape out the top of the model if the coronal temperature is only slightly greater than 1.0 x 10 to the 6th K. Because the spectrum of Alfven waves in real coronal holes is expected to peak around 5 min and the temperature is observed to be close to 1.0 x 10 to the 6th K, the sensitive temperature dependence of the trapping suggests that the temperature in coronal holes is regulated by heating by the trapped Alfven waves.

Reflection and transmission of acoustic plane waves on an immersed orthotropic and viscoelastic solid layer

Abstract: This paper presents a method of characterizing orthotropic and viscoelastic behavior of some composite materials. The purpose is to measure the nine coefficients of the complex stiffness matrix, by using ultrasonic waves transmitted through a plate-shaped sample immersed in water. From a study or propagation in principal planes that has been reported in a previous publication [B. Hosten et al., J. Acoust. Soc. Am. 82, 1763–1770 (1987)], this paper emphasizes heterogeneous mode conversion and propagation through a nonprincipal plane, inside the composite. A numerical method is proposed for computing reflection and transmission coefficients of plane waves for any incidence plane through an orthotropic, lossy solid layer. Numerical calculations of reflection and transmission are compared to experimental data: Amplitudes, wave vectors, and damping vectors are given for a unidirectional graphite/epoxy composite.

An ultrasonic interface layer model for bond evaluation

Abstract: An interface layer model for adhesive bonding weakness evaluation is described to assist in ultrasonic oblique incidence analysis of waves traveling across the interface. This model assumes that the interface between the substrate and adhesive can be treated from a wave propagation point of view as a homogenous isotropic interface layer. The interface bonding quality can then be estimated by predetermined properties of the interface layer. The smooth bond can be formed by a thin nonviscous liquid layer between the two solid media. Numerical results show a frequency dependence of the reflection factors as well as a direct influence of the interface layer properties on reflection. Experiments on bonded structures were conducted to confirm the theoretical prediction. Experiments on smooth bond simulation structures showed the same trends as the theoretical curves of longitudinal wave reflection vs. frequency. Higher sensitivities of shear waves at oblique incidence for bonding weakness detection were found c...

Experimental studies of high‐power microwave reflection, transmission, and absorption from a plasma‐covered plane conducting boundary

Abstract: Experimental studies of the reflection, transmission, and absorption of high‐power microwave pulses from a plasma‐covered plane conducting boundary are presented. Under optimum conditions, backscattered rf power is attenuated by more than 30 dB over values measured in the absence of the plasma. Measurements of the radial and axial plasma density profiles and the neutral gas pressure near the plane conductor indicate that collisional absorption processes are not the primary source of the observed attenuation in the backscattered microwave signal, and that the plasma density exceeds the critical density over much of the volume nearest the conductor. The effects of a tenfold reduction in the microwave power density on the reflection and absorption characteristics of the system are also reported.

Fresnel zone plate antennas at millimeter wavelengths

Abstract: The performance characteristics of Fresnel zone plates are described with emphasis on applications at millimeter wavelengths. Recent results are included along with a summary from numerous earlier investigations. Parameters described include design information, efficiency, bandwidth, focal characteristics, off-axis performance, axial intensity dependence, aberrations, and far-field pattern. Both transmission and reflection configurations are discussed, mostly for cases where the focal length and diameter are comparable. This type of zone plate has advantages (compared to a lens or paraboloid) of reduced loss, weight, volume, and cost, as well as simple planar construction, with similar diffraction-limited beamwidth and major sidelobe performance, but lower efficiency.