Backscattering Angle Research Articles

Angle-resolved Optical Coherence Tomography with sequential angular selectivity for speckle reduction

We present a novel method for rapidly acquiring optical coherence tomography (OCT) images at multiple backscattering angles. By angularly compounding these images, high levels of speckle reduction were achieved. Signal-to-noise ratio (SNR) improvements of 3.4 dB were obtained from a homogeneous tissue phantom, which was in good agreement with the predictions of a statistical model of speckle that incorporated the optical parameters of the imaging system. In addition, the fast acquisition rate of the system (10 kHz A-line repetition rate) allowed angular compounding to be performed in vivo without significant motion artifacts. Speckle-reduced OCT images of human dermis show greatly improved delineation of tissue microstructure.

Precise measurement of the differential cross section from the O16(α,α)O16 elastic reaction at 165° and 170° between 2.4 and 6.0MeV

Non-Rutherford cross section for elastic scattering of α particles from oxygen has been measured for energies between 2.4 and 6.0MeV at commonly used backscattering angles of 165° and 170°. High precision in energy has been obtained with a 2MV Tandetron accelerator, with precise and stable energy calibration. The choice of a thin Ta2O5∕Ta∕C standard increased the sensitivity and reduced systematic errors due to the geometry. Precise data for energy, width, and ratio to Rutherford of the resonances have been extracted from nuclear shell models, and excitation levels of Ne20 have been deduced. The well-known resonance ER=3031.7±0.5keV and the narrow and intense resonance ER=5375.5±0.5keV, not included in cross-section libraries, have been applied for surface oxygen analysis in thin WCN∕SiO2∕Si multilayer samples, with improved depth resolution by optimizing the measuring geometry.

Combining acoustics and video to image fish in the acoustic dead zone

A stationary acoustical‐optical platform (AOP) has been developed to detect groundfish presence and to estimate biomass. The AOP is a new approach to fisheries science because sampling from a platform on the ocean floor overcomes limitations of traditional surface acoustic surveys, where echoes from groundfish near the seafloor are obscured by stronger reflections from the ocean bottom. The AOP capitalizes on the complementary strengths of acoustical and optical sampling techniques by using acoustics for fish enumeration and optics for species identification and measurements of fish length. Each AOP is equipped with four video cameras, two 200‐kHz sonar transducers, and a suite of environmental sensors. One sonar face is mounted in the horizontal direction to image the area directly above the seafloor, whereas the second transducer is oriented vertically to sample the water column above the platform. Acoustic backscatter, target species, size, and observation angle were quantified when fish were simultaneously detected by the sonar and optical sensors. Calculations of target strengths were then related to fish size and orientation, for incorporation into biomass estimates. The AOP is designed to be used in an observation system with multiple platforms elements, so fish populations can be monitored over large spatial scales. [Work supported by NOAA.]

Cross sections for 165° backscattering of 8.0–11.7 MeV α from carbon

Non-Rutherford cross sections for carbon have been measured at the 165° backscattering angle for α beam energies ranging from 8.0 to 11.7MeV. Thin ∼42(±4)μg/cm2 amorphous freestanding carbon foils were measured at the 0° tilt angle. Also, ∼1.8×1018at/cm2 C films e-beam deposited onto Mo substrates were measured at the 75° tilt angle. The measured cross sections from these samples are discussed and compared with values from earlier work. The cross sections from all work are shown in graphic form and those not already available in tabular form will be submitted to the Ion Beam Analysis Nuclear Data Library (IBANDL).

Evaluation of micro-bubble size and gas hold-up in two-phase gas–liquid columns via scattered light measurements

In this paper, potential use of an elliptically polarized light scattering (EPLS) method to monitor both bubble size and gas hold-up in a bubble-laden medium is explored. It is shown that with the use of the new EPLS system, normalized scattering matrix elements ( M ij 's) measured at different side and back-scattering angles can be used to obtain the desired correlations between the bubble sizes and input flow parameters for a gas–liquid (GL) column, including gas flow rate and surfactant concentrations. The bubble size distributions were first evaluated experimentally using a digital image processing system for different gas flows and surfactant concentrations. These images showed that the bubbles were not necessarily spherical. We investigated the possibility of modeling the bubbles as effective spheres. The scattering matrix elements were calculated using the Lorenz–Mie theory and the results were compared against the experimentally determined values. It was observed that the change in the bubble size yields significant changes in M 11 , M 33 , M 44 , and M 34 profiles. An optimum single measurement angle of θ = 120 ∘ was determined for a gas velocity range of 0.04–0.35 cm/s ( ID = 4.5 cm ). The choice of the optimum angle depends on frit pore size, column diameter, gas pressure, and surfactant concentration. These results suggest that a simplified version of the present EPLS system can effectively be used as a two-phase flow sensor to monitor bubble size and liquid hold-up in industrial systems.

Backscatter effects of surfaces composed of dry biological particles

We present the backscattering of particulate surfaces consisting of dry biological particles using two laboratory photopolarimeters that measure intensity and degree of linear polarization in a phase-angle range 0.2–60°. We measure scattering properties from three samples composed of dry biological particles, Bacillus subtilis var. niger (BG) spores and samples of fungi Aspergillus terreus and Sporisorium cruentum spores. We find that the surfaces display a prominent brightness opposition effect and significant negative polarization near backscattering angles. The brightness and polarimetric phase curves are different for B. subtilis and the fungi.

Discrete-dipole analysis of backscatter features of agglomerated debris particles comparable in size with wavelength

We study the backscattered light of agglomerated debris particles using the discrete dipole approximation (DDA) in order to isolate specific physical components that contribute to negative polarization and the brightness opposition surge. We examine a few specific particle systems that display a prominent brightness surge and significant negative polarization over a range of near backscatter angles. In all cases, removal of the far-field interaction components results in the disappearance of the brightness surge and negative polarization branch; whereas, these phenomena remained if the near-field or radial components of the interaction fields are removed. This suggests that the mechanisms for these phenomena are embedded within the far-field interaction component.

Beyond single scattering off flat samples

Most RBS data analysis assumes straight trajectories and one single large angle backscattering event for the analysing beam, and flat sample surface and interfaces. Multiple scattering, plural scattering, and roughness are often ignored. This may lead to erroneous data analysis. In complex systems, understanding and taking into account these effects is essential. We discuss pitfalls that can occur when they are ignored, with emphasis on multilayer systems.

Comparison of asymptotic backscattering models (SSA, WCA, and LCA) from one-dimensional Gaussian ocean-like surfaces

In this paper, recent asymptotic backscattering models are compared for one-dimensional multiscale dielectric sea surfaces with Gaussian statistics and by considering the Elfouhaily et al. height spectrum. We focus on the calculations of the normalized radar cross sections (NRCS) obtained from the weighted and local curvature approximations (WCA and LCA), recently published by Elfouhaily, and of the first- and second-order small slope approximation (SSA(1) plus SSA(2) denoted as SSA), developed by Voronovich. Voronovich et al. (2002 Waves Random Media 11 247-269) presented simulations of the SSA by making an assumption over the SSA(2) contribution (the model is referred to as SSAM). The NRCS computation is then similar to SSA(1), where the sea spectrum is substituted by a modified spectrum defined as the product of the sea spectrum by the second-order polarization term. The second-order statistical moment of WCA is calculated rigorously for any two-dimensional height correlation of the surface with Gaussian statistics. The NRCS of the WCA, WCAQ (obtained from a quadratic approximation of WCA), LCA, SSA, and SSAM backscattering models are compared for moderate wind speeds, for microwave frequencies and for backscattering angles ranging from 0 (nadir) to 70/spl deg/.

Polarimetric classification of land cover for Glen Affric radar project

In recent years two polarimetric decompositions methods, the A/E method (Cloude and Pottier, 1997) and the three-component decomposition method (Freeman and Durden, 1998) have become the main methods of the polarimetric synthetic aperture radar data classification in radar imagery applications. In both methods the structural composition of the land cover is modelled using the main backscatter responses, as a function of the polarised channels, the level of energy received and the associated backscatter angles. To enhance the coherence in the polarimetric decompositions, data are generally multi-looked and de-speckled. The authors have applied and compared the 'Freeman' three-component decomposition followed by the Wishart complex classifier with the A/E decomposition algorithms to the de-speckled and edge-enhanced Glen Affric, fully polarimetric L-band radar data. The Glen Affric radar site was chosen for its extensive and diverse semi-natural woodlands of native Scots Pine, and its varied topography. The application results demonstrate the response of the radar classification from a semi-natural land cover on a rapid topography.

HF radar observations of high-aspect angle backscatter from the E-region

Abstract. We present evidence for the observation of high-aspect angle HF radar backscatter from the auroral electrojets, and describe the spectral characteristics of these echoes. Such backscatter is observed at very near ranges where ionospheric refraction is not sufficient to bring the sounding radio waves to orthogonality with the magnetic field; the frequency dependence of this propagation effect is investigated with the Stereo upgrade of the CUTLASS Iceland radar. We term the occurrence of such echoes the "high-aspect angle irregularity region" or HAIR. It is suggested that backscatter is observed at aspect angles as high as 30°, with an aspect sensitivity as low as 1dB deg–1. These echoes are distinguished from normal electrojet backscatter by having low Doppler shifts with an azimuthal dependence that appears more consistent with the direction of the convection electric field than with the expected electron drift direction. This is discussed in terms of the linear theory dispersion relation for electrojet waves. Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)

Transmitted and reflected scattering matrices from an English oak leaf.

The complete Mueller matrix for an English oak (Quercus robur) leaf for a fixed azimuth angle (90 degrees) was determined immediately after plucking and a day following exposure to normal room temperature and pressures. The Mueller matrices were determined for transmitted light at observation angles ranging from 0 degrees to 24 degrees and for reflected backscattering angles from 153 degrees to 170 degrees. All the measurements were taken with a He-Ne laser light source at 0.63 microm. Since positive eigenvalues were obtained for the coherence matrix, the polarimetric measurements were physically realizable. The anisotropy parameters were determined from the Jones matrices by use of the decomposition theorem. From the M33 and M44 components of the Mueller matrices, it was found that nonspherical structures within the leaf were primarily responsible for observed transmitted light scatter, and spherical structures were mostly responsible for observed backscatter. Variations in backscatter Mueller matrix elements from a fresh leaf to a second day of observation were assumed because of changes to water vapor concentration in the leaf.

Double-Scattering Approximation Theory of Enhanced Backscatterings of Light Produced from Aggregated Particles

We theoretically analyze the spatial anisotropy and polarization dependences of enhanced backscattering of light using a double-scattering approximation. The polarization of light is calculated in terms of the Jones vector and the scattering amplitude matrix. We discuss the cases where the individual scattering event is isotropic and governed by Rayleigh scattering for the media in which scatterers are homogeneously dispersed and aggregated. The spatial properties of the far-field intensity distribution can be demonstrated by asymptotic expansions in the region of the large backscattering angle. As a result, we theoretically reveal the polarization anisotropy of the decreasing variation in the far-field intensity distribution dependent on the fractal dimension, the azimuth in the backscattering angle, and the polarization state.

Backscattering from bioturbated sediments at very high frequency

Recent backscattering measurements made in the Gulf of La Spezia, Italy, using a sonar operating at 140 kHz combined with thorough characterization of seabed interface and volume properties illustrate the importance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which are attributed to the level of bioturbation (e.g., sea shell fragments, burrows, pockets of water) have been quantified using X-Ray computed tomography. Two-dimensional interface roughness spectra have also been determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence.

Temperature stability of sputtered niobium–oxide films

The effect of annealing temperature on the structural and optical properties of sputtered niobium–oxide films has been investigated. The temperature dependence of structure, density, and optical constants has been studied by Rutherford backscattering, x-ray diffraction, x-ray reflection, optical spectroscopy, and variable angle spectroscopic ellipsometry techniques. Rutherford backscattering measurements show no variation in the stoichiometry of the films upon annealing of amorphous Nb2O5 films, while amorphous NbO is oxidized to Nb2O5. X-ray diffraction studies show that as-deposited films are amorphous and only crystallize at around 500 °C. X-ray reflectivity studies reveal a continuous increase of film density with increasing annealing temperature. Optical spectroscopy and spectroscopic ellipsometry confirm that the refractive index n and the band gap Eg increase upon increasing annealing temperature. The variation of the refractive index with density of the niobium–oxide films is observed to follow the Clausius–Mossotti relation and the molecular electronic polarizability has been deduced from the straight line fit of Lorentz–Lorentz law.

Estimating Deep Seafloor Interface and Volume Roughness Parameters Using the Multibeam-Hydrosweep System

Utilizing a hull-mounted, multinarrow beam echosounder onboard RV Polarstern, we measured variation of acoustic backscatter with incidence angles at two different sites in the Southern Oceans (Agulhas Plateau and the Riiser Larsen Sea). We modeled the data, using a composite roughness model, including water-sediment interface roughness and sediment volume roughness parameters. The model effectively uses the near normal incidence angle backscatter to determine the seafloor interface roughness parameters employing Helmholtz-Kirchhoff theory. Beyond 20° incidence angles, an application of Rayleigh-Rice theory is made by using a necessary splicing technique (combining both of the theories at 20° incidence angle). The estimated interface and volume roughness parameters are found to be in accordance with the known area geology.

Measurement of non-Rutherford cross sections of [formula omitted]He ions on [formula omitted]N at a laboratory backscattering angle of 171° between 7.5 and 9.8 MeV

The ratio of the non-Rutherford to the Rutherford cross section of 14 N was measured using 4 He ions in the energy range between 7.5 and 9.8 MeV at a laboratory backscattering angle of 171° in order to complete the data published by Foster et al. (Nucl. Instr. and Meth. B 79 (1993) 454), who measured the ratio at a lower backscattering angle of 167°. This is important because of the interest in using high energy helium ion backscattering to determine the nitrogen content in heavy substrates. Non-Rutherford backscattering spectra and 14 N depth profiles of nitrogen-implanted titanium alloys, that serve as an application of the new cross sections, are shown in this work as well. The measurements were done at beam energies of 7.5–8.2 MeV and from 9 to 9.8 MeV. Results show a resonance around 9.3 MeV with cross sections that are about 100 times higher than the values for Rutherford backscattering.

Derivation of the ionospheric structure over extended geographical regions from azimuth and elevation scan backscatter ionograms: A modular neural network approach

This paper describes a new real‐time inversion method based upon multifrequency sounding schemes and variable azimuth and elevation angle backscatter ionograms for the derivation of the three‐dimensional structure of the ionosphere. The inversion technique which has been developed for this purpose employs neural network models in a modular structure and is fully explained. Tests on simulated data demonstrate the potential accuracy of the method for reconstructing the electron density profile over remote geographical regions.

In situ investigation of AlAs/GaAs interfaces during ion implantation at 15 K

Ion implantation and Rutherford backscattering spectrometry (RBS) were performed in situ at 15 K without temperature change between implantation and measurement. N, Ar and Xe ions were implanted into AlAs/GaAs and GaAs/AlAs heterostructures to study the damage formation at the interface between the two materials. Measurements with 1.4 MeV He ions and a backscattering angle of 110° result in an improved depth resolution. The thickness of the affected GaAs adjacent to the AlAs was found to be about 20 nm nearly independent on ion species and ion fluence. This suggests that defects are mobile within the collision cascades even at temperatures as low as 15 K. In AlAs an ion-beam-induced interfacial amorphisation starts from the adjacent amorphised GaAs. At low concentrations of the implanted ions the amorphisation proceeds proportional to the total number of lattice displacements per unit volume, but with a higher rate at the AlAs/GaAs interface than at the GaAs/AlAs interface. The amorphisation mechanism changes into an ion-controlled process after a critical ion concentration is exceeded, corresponding to the amorphisation mechanism found in bulk AlAs without interfaces to GaAs.

Brillouin scattering study of molten zinc chloride.

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.