Near-field Effects on Partially Coherent Light Scattered by an Aperture

We investigate how the near field affects partially coherent light scattered from an aperture in an opaque screen. Prior work on this subject has focused on the role of surface plasmons, and how they affect spatial coherence is well documented. Here, we consider other near-field effects that might impact spatial coherence. We do this by examining the statistics of the near-zone field scattered from an aperture in a perfect electric conductor plane—a structure that does not support surface plasmons. We derive the near-field statistics (in particular, cross-spectral density functions) by applying electromagnetic equivalence theorems and the Method of Moments. We find, even in the absence of surface plasmons, that near-field physics can affect the coherence of the scattered field. The analysis and findings presented herein complement the existing coherence-related surface plasmons literature, and could find use in the design of photonic devices built to engineer spatial coherence

A Clamped Dual-Ridged Waveguide Measurement System for the Broadband, Nondestructive Characterization of Sheet Materials

A novel two-port probe which uses dual-ridged waveguides for the nondestructive, broadband characterization of sheet materials is presented. The new probe is shown to possess approximately 2 to 3 times the bandwidth of traditional coaxial and rectangular/circular waveguide probe systems while maintaining the structural robustness characteristic of rectangular/circular waveguide probe systems. The theoretical development of the probe is presented, namely, by applying Love’s equivalence theorem and enforcing the continuity of transverse fields at the dual-ridged waveguide apertures, a system of coupled magnetic field integral equations is derived. The system of coupled magnetic field integral equations is solved using the method of moments to yield theoretical expressions for the reflection and transmission coefficients. The complex permittivity and permeability of the unknown material under test are then found by minimizing the root-mean-square difference between the theoretical and measured reflection and transmission coefficients. Experimental results of two magnetic absorbing materials are presented to validate the new probe. The probe’s sensitivity to measured scattering parameter, sample thickness, and flange-plate thickness errors is also investigated

Broadband, Non-destructive Characterisation of PEC-backed Materials Using a Dual-ridged-waveguide Probe

A new probe which utilises a dual-ridged waveguide to provide broadband, non-destructive (ND) material characterisation measurements of a perfect electric conductor (PEC)-backed material is introduced. The new probe possesses a bandwidth similar to existing coaxial probes and is structurally robust like rectangular waveguide probes. The combinations of these two qualities make it especially attractive for ND inspection/evaluation applications in the field. The theoretical development of the dual-ridged-waveguide probe is discussed. A magnetic field integral equation is derived by applying Love’s equivalence theorem and enforcing the continuity of transverse fields at the dual-ridged-waveguide aperture. The magnetic field integral equation is then solved for the theoretical reflection coefficient using the method of moments. The permittivity and permeability of the material under test are found by minimising the root-mean-square difference between the theoretical and measured reflection coefficients using non-linear least squares. To validate the new probe, experimental results are presented of a magnetic absorbing material comparing results obtained using the new probe with those obtained using a traditional, destructive technique. The probe’s sensitivity to sample thickness, flange-plate thickness, cutoff wavenumber and measured S-parameter uncertainties is also investigated

A Geometrical Optics Polarimetric Bidirectional Reflectance Distribution Function for Dielectric and Metallic Surfaces

A polarimetric bidirectional reflectance distribution function (pBRDF), based on geometrical optics, is presented. The pBRDF incorporates a visibility (shadowing/masking) function and a Lambertian (diffuse) component which distinguishes it from other geometrical optics pBRDFs in literature. It is shown that these additions keep the pBRDF bounded (and thus a more realistic physical model) as the angle of incidence or observation approaches grazing and better able to model the behavior of light scattered from rough, reflective surfaces. In this paper, the theoretical development of the pBRDF is shown and discussed. Simulation results of a rough, perfect reflecting surface obtained using an exact, electromagnetic solution and experimental Mueller matrix results of two, rough metallic samples are presented to validate the pBRDF.© 2009 Optical Society of America

Nondestructive Characterization of Salisbury Screen and Jaumann Absorbers Using a Clamped Rectangular Waveguide Geometry

A nondestructive technique to characterize Salisbury screen and Jaumann absorbers is presented. The proposed method utilizes two flanged rectangular waveguides to unambiguously determine the permittivities of two-layer dielectric absorbers. The derivation of the theoretical reflection and transmission coefficients, necessary to determine material under test permittivities, is presented. The derivation makes use of Love’s equivalence principle and the continuity of transverse magnetic fields to formulate a system of coupled magnetic field integral equations. These integral equations are solved using the Method of Methods to yield theoretical scattering parameters. The unknown permittivities are then found using nonlinear least squares. To validate the proposed nondestructive technique, measurement results of three two-layer dielectric absorbers are presented and analyzed. In addition, an extensive error analysis is performed on the extracted permittivity values. The results of the proposed method are found to be in good agreement with the results returned by traditional, destructive waveguide transmission/reflection approaches

Scattering of a Partially-coherent Wave from a Material Circular Cylinder

The case of a partially-coherent wave scattered from a material circular cylinder is investigated. Expressions for the TMz and TEz scattered-field cross-spectral density functions are derived by utilizing the plane-wave spectrum representation of electromagnetic fields and cylindrical wave transformations. From the analytical scattered-field cross-spectral density functions, the mean scattering widths are derived and subsequently validated via comparison with those computed from Method of Moments Monte Carlo simulations. The analytical relations as well as the simulation results are discussed and physically interpreted. Key insights are noted and subsequently analyzed

Nondestructive Electromagnetic Characterization of Uniaxial Sheet Media Using a Two-Flanged Rectangular Waveguide Probe

Excerpt: Recent advancements in fabrication capabilities have renewed interest in the electromagnetic characterization of complex media, as many metamaterials are anisotropic and/or inhomogeneous. Additionally, for composite materials, anisotropy can be introduced by load, strain, misalignment, or damage through the manufacturing process [1], [2]. Methods for obtaining the constitutive parameters for isotropic materials are well understood and widely employed [3]–[8]. Therefore, it is crucial to develop a practical method for the electromagnetic characterization of anisotropic materials

Nondestructive Electromagnetic Material Characterization using a Dual Waveguide Probe: A Full Wave Solution

A nondestructive technique for determining the complex permittivity and permeability of a perfect electric conductor backed magnetic shielding material using a dual waveguide probe is presented. The dual waveguide probe allows for the simultaneous collection of reflection and transmission coefficients which distinguishes it from single probe methods common in the literature. Theoretical development of these coefficients, which is accomplished through a coupled magnetic field integral equations formulation using Love\u27s equivalence principle and solved via the method of moments (MOM), is discussed. Evaluation of the resulting MOM impedance matrix elements is performed using complex plane integration leading to enhanced computational efficiency and physical insight. Comparison of the theoretical and measured reflection and transmission coefficients using a root finding algorithm leads to the desired permittivity and permeability. Measurement results of a magnetic shielding material are presented and compared to traditional methods for the purpose of validating the new technique. The probe\u27s sensitivity to aperture alignment, sample thickness, and flange thickness is also investigated.Abstract © AGU

Global application of an unoccupied aerial vehicle photogrammetry protocol for predicting aboveground biomass in non‐forest ecosystems

P. 1-15Non-forest ecosystems, dominated by shrubs, grasses and herbaceous plants, provide ecosystem services including carbon sequestration and forage for grazing, and are highly sensitive to climatic changes. Yet these ecosystems are poorly represented in remotely sensed biomass products and are undersampled by in situ monitoring. Current global change threats emphasize the need for new tools to capture biomass change in non-forest ecosystems at appropriate scales. Here we developed and deployed a new protocol for photogrammetric height using unoccupied aerial vehicle (UAV) images to test its capability for delivering standardized measurements of biomass across a globally distributed field experiment. We assessed whether canopy height inferred from UAV photogrammetry allows the prediction of aboveground biomass (AGB) across low-stature plant species by conducting 38 photogrammetric surveys over 741 harvested plots to sample 50 species. We found mean canopy height was strongly predictive of AGB across species, with a median adjusted R2 of 0.87 (ranging from 0.46 to 0.99) and median prediction error from leave-one-out cross-validation of 3.9%. Biomass per-unit-of-height was similar within but different among, plant functional types. We found that photogrammetric reconstructions of canopy height were sensitive to wind speed but not sun elevation during surveys. We demonstrated that our photogrammetric approach produced generalizable measurements across growth forms and environmental settings and yielded accuracies as good as those obtained from in situ approaches. We demonstrate that using a standardized approach for UAV photogrammetry can deliver accurate AGB estimates across a wide range of dynamic and heterogeneous ecosystems. Many academic and land management institutions have the technical capacity to deploy these approaches over extents of 1–10 ha−1. Photogrammetric approaches could provide much-needed information required to calibrate and validate the vegetation models and satellite-derived biomass products that are essential to understand vulnerable and understudied non-forested ecosystems around the globe.S

Parental Factors Associated With the Decision to Participate in a Neonatal Clinical Trial

Importance: It remains poorly understood how parents decide whether to enroll a child in a neonatal clinical trial. This is particularly true for parents from racial or ethnic minority populations. Understanding factors associated with enrollment decisions may improve recruitment processes for families, increase enrollment rates, and decrease disparities in research participation. Objective: To assess differences in parental factors between parents who enrolled their infant and those who declined enrollment for a neonatal randomized clinical trial. Design, setting, and participants: This survey study conducted from July 2017 to October 2019 in 12 US level 3 and 4 neonatal intensive care units included parents of infants who enrolled in the High-dose Erythropoietin for Asphyxia and Encephalopathy (HEAL) trial or who were eligible but declined enrollment. Data were analyzed October 2019 through July 2020. Exposure: Parental choice of enrollment in neonatal clinical trial. Main outcomes and measures: Percentages and odds ratios (ORs) of parent participation as categorized by demographic characteristics, self-assessment of child's medical condition, study comprehension, and trust in medical researchers. Survey questions were based on the hypothesis that parents who enrolled their infant in HEAL differ from those who declined enrollment across 4 categories: (1) infant characteristics and parental demographic characteristics, (2) perception of infant's illness, (3) study comprehension, and (4) trust in clinicians and researchers. Results: Of a total 387 eligible parents, 269 (69.5%) completed the survey and were included in analysis. This included 183 of 242 (75.6%) of HEAL-enrolled and 86 of 145 (59.3%) of HEAL-declined parents. Parents who enrolled their infant had lower rates of Medicaid participation (74 [41.1%] vs 47 [55.3%]; P = .04) and higher rates of annual income greater than $55 000 (94 [52.8%] vs 30 [37.5%]; P = .03) compared with those who declined. Black parents had lower enrollment rates compared with White parents (OR, 0.35; 95% CI, 0.17-0.73). Parents who reported their infant's medical condition as more serious had higher enrollment rates (OR, 5.7; 95% CI, 2.0-16.3). Parents who enrolled their infant reported higher trust in medical researchers compared with parents who declined (mean [SD] difference, 5.3 [0.3-10.3]). There was no association between study comprehension and enrollment. Conclusions and relevance: In this study, the following factors were associated with neonatal clinical trial enrollment: demographic characteristics (ie, race/ethnicity, Medicaid status, and reported income), perception of illness, and trust in medical researchers. Future work to confirm these findings and explore the reasons behind them may lead to strategies for better engaging underrepresented groups in neonatal clinical research to reduce enrollment disparities