Scattered Field Research Articles

Image reconstruction of a perfectly conducting cylinder by the genetic algorithm

The paper presents a computational approach to reconstruct the shape of a perfectly conducting cylinder. Based on the boundary condition and the measured scattered field, a set of nonlinear integral equations is derived and the imaging problem is reformulated into an optimum one. By using the genetic algorithm, the shape of the object can be reconstructed. The genetic algorithm will always converge to a global extreme solution no matter what the initial estimate. Numerical results are given to demonstrate that, even when the initial guess is far away from the exact one, good reconstruction has been obtained. In such a case, the calculus-based method often becomes trapped in a local extreme.

Iterative angular‐mode inversion of a conducting cylinder

AbstractFor the iterative reconstruction of the shape function of a two‐dimensional conducting object, the angular modal representation of the scattered field gives effective choice of measurement points especially in the presence of noise in the scattered field. It is shown that the object center and its initial shape may be estimated from the effective propagating modes of the measured scattered field. By employing N effective propagating modes excluding the exponentially small higher‐order modes, numerical calculation shows that the reconstruction of the shape function with 2N unknowns is possible. When the noise is present, the characteristics of the cost function show that the effective propagating modes with multiple incident waves eliminating the shadow region of the object are needed for the stable inversion. © 1995 John Wiley & Sons, Inc.

Diffraction tomographic imaging with a circular array

A new approach of imaging an object profile from the scattered wavefield, which is measured using a large (far field approximation) circular array of sensors surrounding the object, is described. It is shown that the measured field is itself proportional to the Fourier transform of the object. Consequently, as compared to the existing methods, such as back propagation method for a linear array, our method overcomes the well known effects of the finite window in numerical evaluation of the Fourier transform of the measured scattered field. This results into improved reconstruction. >

The Observation of Optical Vortices in the Thermal Rayleigh Scattering Field of a Liquid Crystal Film

Optical vortices are found in the thermal Rayleigh scattering light field of a nematic liquid crystal film. The optical vortices change their positions on the wave front quickly, and at a certain point, the vortices with different topological charges appear in a random way.

Full Multiple Scattering and Crystal Field Multiplet Calculations Performed on the Spin Transition FeII(phen)2(NCS)2 Complex at the Iron K and L2,3 X-ray Absorption Edges

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFull Multiple Scattering and Crystal Field Multiplet Calculations Performed on the Spin Transition FeII(phen)2(NCS)2 Complex at the Iron K and L2,3 X-ray Absorption EdgesV. Briois, Ch. Cartier dit Moulin, Ph. Sainctavit, Ch. Brouder, and A.-M. FlankCite this: J. Am. Chem. Soc. 1995, 117, 3, 1019–1026Publication Date (Print):January 1, 1995Publication History Published online1 May 2002Published inissue 1 January 1995https://doi.org/10.1021/ja00108a018RIGHTS & PERMISSIONSArticle Views540Altmetric-Citations81LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

"Blind" shape reconstruction from experimental data

A method for reconstructing the shape of a bounded impenetrable object from measured scattered field data is presented. The reconstruction algorithm is, in principle, the same as that used before for reconstructing the conductivity of a penetrable object and uses the fact that for high conductivity the skin depth of the scatterer is small, in which case the only meaningful information produced by the algorithm is the boundary of the scatterer. A striking increase in efficiency is achieved by incorporating into the algorithm the fact that for large conductivity, the contrast is dominated by a large positive imaginary part. This fact together with the knowledge that the scatterer is constrained in some test domain constitute the only a priori information about the scatterer that is used. There are no other implicit assumptions about the location, connectivity, convexity, or boundary conditions. The method is shown to successfully reconstruct the shape of an object from experimental scattered field data in a blind test.

Full Wave Single and Double Scatter from Rough Surfaces

Using the full wave approach, the single and double scattered electromagnetic fields from deterministic one-dimensional rough surfaces are computed. Full wave expressions for the single and double scattered far fields are given in terms of multidimensional integrals. These integrals are evaluated using the Cornell National Supercomputer IBM/3090. Applying the steepest descent approximation to the double scattered field expressions, the dimensions of the integrals are reduced from four to two in the case of one-dimensional rough surfaces. It is shown that double scatter in the backward direction is significant for near normal incidence when the rough surface is highly conducting and its mean square slope is very large. Even for one-dimensional rough surfaces, depolarization occurs when the reference plane of incidence is not parallel to the local planes of incidence and scatter. A geometrical optics approximation is used to interpret the results of the double scattered fields for normal incidence near backscatter. The physical interpretation of the results could shed light on the observed fluctuations in the enhanced backscatter phenomenon as the angle of incidence increases from near normal to grazing angles. The results show that double scatter strongly depends upon the mean square slope, the conductivity of the rough surface and the angle of incidence.

Heating of the nighttime D region by very low frequency transmitters

VLF signals propagating in the Earth‐ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low frequency (VLF, 3‐30 kHz) transmitters. Ionospheric cooling and heating are observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0‐kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/on episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4‐kHz NSS probe wave propagating from Annapolis, Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3°, respectively. In 6 of these 41 episodes, the amplitude of the 28.5‐kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpected due to the > 770 km distance between NAA and the NAU‐Gander great circle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth‐ionosphere waveguide. A three‐dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent and shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The enhancements are annular, with a geomagnetic north‐south asymmetry and a radius at the outer half‐maximum of the collision frequency enhancement of about 150 km. Heating by the NAA transmitter is predicted to increase the nighttime D region electron temperature by as much as a factor of 3. The calculated changes in the D region conductivity are used in a three‐dimensional model of propagation in the Earth‐ionosphere waveguide to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the observed signal perturbations. Discrepancies in the predictions are attributed to lack of knowledge of the D region electron density profile along the probe wave great circle paths.

Wavefront amplitude distribution in the female breast

Ultrasound measurements of a large population of wavefronts transmitted through female breasts at 3 and 4 MHz show that the wavefront amplitude distribution is close to Rayleigh. This finding is consistent with a fully developed scatter field, implying that the scatter energy removed from the coherent incident beam dominates the wavefront. The wavefront received from an inhomogeneous medium is the superposition of an incident wave plus a scattered wave. If the scattered field is weak, the received field is dominated by the incident field and the wavefront amplitude distribution is Rician. If the scattered field is strong, the received field is primarily the scattered field and the wavefront amplitude distribution is Rayleigh. If, in addition to scattering, refraction between bodies of different refractive indexes occurs, the total net effect on the wavefront amplitude distribution is the same as for strong scattering. This is what we have observed in the highly refractive female breast. This result has implications for the design of high lateral-resolution echo scanners that will incorporate adaptive phase deaberration algorithms. The published algorithms were developed for weak scattering and therefore may not be powerful enough. Alternatives have to be found to deaberrate the severe wavefront distortion in the breast.

Two‐dimensional location and shape reconstruction

A method for reconstructing the location and the shape of a bounded impenetrable object from measured scattered field data is presented. The algorithm is, in principle, the same as that used for reconstructing the conductivity of a penetrable object and uses the fact that for high conductivity the skin depth of the scatterer is small, in which case the only meaningful information produced by the algorithm is the boundary of the scatterer. A striking increase in efficiency is achieved by incorporating into the algorithm the fact that for large conductivity the contrast is dominated by a large positive imaginary part. This fact, together with the knowledge that the scatterer is constrained in some test domain, constitute the only a priori information about the scatterer that is used. There are no other implicit assumptions about the location, connectivity, convexity, or boundary conditions. Some refinements of the algorithm which reduce the number of points at which the unknown function is updated are incorporated to further increase efficiency. Results of a number of numerical examples are presented which demonstrate the effectiveness of the location and shape reconstruction algorithm.

Formulation of short-wavelength bistatically scattered fields in terms of monostatic returns

The relation between short-wavelength bi- and monostatically scattered fields of smooth scatterers, e.g., the sphere, is governed by the bistatic theorem. This relation is extended here to scatterers with marked edges, viz. finite cylinders and rectangular baffles. The formulation of the bistatic field in terms of the monostatic return does however depend on two parameters not relevant to the classical bistatic theorem: The product of the wave number times the characteristic dimension, and the bistatic angle between the incident and scattered wave.

Application of Generalized Near-Field Acoustical Holography to Scattering Problems

Generalized near-field acoustical holography (GENAH) is applied to scattering problems, making use of its wavelength-limitation-free characteristic in the reconstruction process. GENAH signal processing is directly adapted for the reconstruction of the scattered field, while a method to extract the scattered-pressure hologram is independently proposed to create scattering GENAH. The measurement of the near field of a scatterer yields the superimposed incident and scattered fields. Their decomposition is carried out by subtracting the incident-pressure hologram from the superimposed-pressure hologram in K space. The validity of this decomposition method was demonstrated by comparing the experimental and theoretical results of finite submerged shells. Reconstruction of shell-surface vibration induced by scattering displayed a reasonable result.

A regional convolution kernel algorithm for scatter correction in dual‐energy images: Comparison to single‐kernel algorithms

Single kernel scatter correction algorithms are based on the model that the scatter field can be predicted by convolution of the primary intensity (Iprim) with a spatially invariant scatter point-spread function (PSF). Practical limitations (Iprim unknown) suggest the substitution of the total detected intensity (Idet) for Iprim as the source image in the convolution. In regions of high scatter fraction (SF), Idet is a poor approximation of Iprim, thereby causing an overestimation of scatter originating in the region. This contributes to errors in estimating detected scatter in the mediastinum and neighboring regions. A technique using a regionally variable point-spread function that significantly reduces RMS error in estimation of the primary image as compared to the single PSF method is investigated. The regionally variable convolution method employs a larger PSF in the mediastinum and a smaller PSF in the lungs to reduce the error in estimating the scatter throughout the image. The method to allow for patient differences has also been expanded and various implementations of these methods have been compared. Results show that the dual-kernel algorithm is always more effective than an equivalent single-kernel algorithm. The dual-kernel algorithm using a predicted scatter fraction curve gives an overall RMS error in the primary of as low as 20.8% which is equivalent to 8.7% RMS error in the scatter. The dual-kernel method using a predicted scatter fraction curve approaches the accuracy of the single-kernel method using patient specific scatter measurements. Because using individual scatter measurements is a less desirable method for clinical use, we feel that the dual-kernel algorithm which uses two regions specific convolution kernels and a variable scatter fraction curve is the preferable method.

Thermal-wave slice tomography using wave-field reconstruction

A new matrix-equation-based theoretical computation method for detection of subsurface defects by thermal-wave tomographic imaging is presented. The method uses the measured scattered temperature field to solve the Helmholtz thermal pseudowave equation, by solving the equivalent integral equation. Numerical results are presented and compared with the ray optic calculation. The numerical calculations of the inverse problem were carried out using the Born approximation, however the theoretical technique is capable of handling the general tomographic inverse case without recourse to the Born approximation.

Scattering investigation with cylindrical acoustic holography

The objective of this paper is to investigate sound scattering of a rigid scatterer using a two‐plane cylindrical acoustic holography method. A piston source, which is vibrating sinusoidally, illuminates a scatterer. The superimposed incident and scattered fields are measured in the near field of the scatterer using cylindrical acoustic holography. These two fields are decomposed in the wave‐vector domain in order to extract the scattered field. Numerical simulations are performed and the effect of various parameters is investigated. Specifically, the distance between the two‐holography cylinder, the sampling rate, and the aperture size are investigated in the field separation technique. In addition, experimental studies were conducted inside an anechoic chamber with a baffled loudspeaker as a source and a cylinder as a scatterer. The decomposed scattered field is then used to estimate the far‐field target strength. The near‐field scattering is also investigated.

An extended range‐modified gradient technique for profile inversion

A method for reconstructing the complex index of refraction of a bounded inhomogeneous object from measured scattered field data is presented. The index and the unknown fields within the object are simultaneously reconstructed in an iterative algorithm. The method is a refinement of earlier work which incorporates a more effective way to update the unknowns at each stage of the iteration. Considerable efficiency in the algorithm is achieved. Some numerical examples are given indicating the limits on the contrasts which can be reconstructed. These limits show that the range of contrasts that may be reconstructed is extended over that achievable with the earlier work.

Investigation of the enhanced efficiencies of small superconducting loop antenna elements

The fields backscattered from arrays of electrically small copper and YBCO superconducting loops are measured and compared with moment method predictions to confirm the gain enhancements and changes in Q factors due to increasing the efficiencies of the antenna elements alone, without the influence of feed and matching networks. Measurements on coplanar and parallel loop arrays show only small increases in the backscattered fields of the YBCO scatterers, with the largest increase in efficiency being for the single loop. The mutual resistance between elements reduces the benefit of adding more elements to a YBCO loop, to a greater extent than when copper elements are added to a copper loop.

Ultrasonic four-wave mixing mediated by a suspension of microspheres in water: Comparison between two scattering theories

Two ultrasonic pump waves are used to produce a grating in a suspension of 25-μm-diam latex particles. A higher frequency ultrasonic wave is used to probe the established grating to produce ultrasonic Bragg scattering. The scattering depends strongly on the pump waves and is an unusual class of nonlinearity. A previously summarized model of the interaction [H. J. Simpson and P. L. Marston, J. Acoust. Soc. Am. 90, 2244 (A) (1991) and 91, 2351 (A) (1992)] uses an Epstein layer approximation and effective medium approximation. This previous analysis is compared with a Fourier series approximation of the grating and the resulting predicted scattered wave fields. The Fourier series expansion is only valid when the Bragg condition is satisfied, thus giving no information off the Bragg peak. The two models match at higher pump pressures and predict similar onset pressures, but differ slightly in the medium pressure region. Both results are compared to experimentally measured scattering amplitudes for a range of probe frequencies and pump pressures. [Work supported by ONR.]

Measurement of the temporal coherence of an underwater optical scattered field

AbstractThe coherence of a laser signal propagating through a turbid medium deteriorates due to scattering, affecting the performance of undereater coherent systems. The loss in coherence due to the scattering has been evaluated from the autocorrelation funcion obtained with an interoferometer followed by digital signal processing. © 1993 John Wiley & sons, Inc.

ISAR Image Formation Using Bistatic Data Computed from the Shooting and Bouncing Ray Technique

The feasibility of utilizing bistatic scattered field data to obtain the microwave ISAR image of a target is investigated. The principle and results of the bistatic imaging algorithm are presented. It is shown that the ISAR image can be reconstructed by Fourier inversion of the bistatic scattered field data under the physical optics approximation. Bistatic scattered field data is faster to simulate using the shooting and bouncing ray technique than monostatic scattered field data hence the bistatic scheme results in time savings. Monostatic and bistatic ISAR images are presented for different targets. The tradeoff between time savings and image fidelity for the two schemes is discussed.