Specular Research Articles

Chiral surface spin textures in Cu2OSeO3 unveiled by soft X-ray scattering in specular reflection geometry

ABSTRACT Resonant elastic soft X-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small-angle X-ray scattering has been involved in the soft energy X-ray range which, however, brings in difficulties with the sample preparation that involves focused ion beam milling to thin down the crystal to below a few hundred nm thickness. We show how to circumvent these restrictions using XRMS in specular reflection from a sub-nanometer smooth crystal surface. The method allows observing diffraction peaks from the helical and conical spin modulations at the surface of a Cu 2 OSeO 3 single crystal and probing their corresponding chirality as contributions to the dichroic scattered intensity. The results suggest a promising way to carry out XRMS studies on a plethora of noncentrosymmetric systems hitherto unexplored with soft X-rays due to the absence of the commensurate Bragg peaks in the available momentum transfer range.

Reconfigurable acoustic metagrating for multiple anomalous wavefront manipulation functionalities

In this paper, we propose a reconfigurable acoustic metagrating composed of periodic rotatable slit tubes and a sound hard plane for realizing multiple anomalous wavefront manipulation functionalities: specular reflection, three-channel retroreflection, and beam splitting. The acoustic metagrating can manipulate the reflectances of different diffracted waves by changing the rotational angle of slit tubes. Both the theoretical analysis and numerical simulations demonstrate the proposed metagrating can stimulate specific diffraction orders and achieve the desired functionalities. Our research work provides a good method for designing reconfigurable acoustic devices with multiple functionalities and has potential applications in acoustic sensing, acoustic communication, and noise mitigation.

Simulation of acoustic reflection from Arctic ice

The acoustic reflection from an ice sheet in the arctic, as observed by an upward looking sonar, is simulated. The reflection from a smooth ice sheet is modeled using OASES, using material properties that may be found in the published literature. Depending on the bandwidth of the signal, specular reflections from the top and bottom of the ice, as well as one or more resonances that are identified as leaky Lamb waves should be detectable. If the speed of the Lamb wave is faster than the speed of sound in water, it will leak energy back into the water. The leaky Lamb wave has a characteristic resonant frequency that is a function of the ice thickness and shear speed. The reflection and backscatter from a rough ice sheet is modeled using SPECFEM2D. Scattering from a rough interface may obscure the expected signals, depending on the severity of the roughness. [Work supported by ONR, Ocean Acoustics Program.]

Wave acoustic solutions in small rooms

This presentation will discuss the growing application of wave based solutions due to the increased processing power of desktop and distributed cloud cluster computing. The causes and mitigation of four types of potential acoustical distortion in small rooms, below and above 200 Hz, will be presented. Below 200 Hz, approaches to minimize modal effects and the speaker boundary interference response will be presented. We will discuss limitations of traditional dimensional ratio determinations, more accurate wave-based predictions, importance of speaker and listener locations, and prediction and measurement of low frequency absorption devices. Above 200 Hz, comb filtering from strong specular reflections and poor diffusion are potential problems. We will describe the necessary ratio of surface width to incident wavelength for a specular reflection and its effect on comb filtering. Finally, we describe the importance of creating a mixing room design and the use of broad bandwidth diffusive surfaces, which can be both measured and simulated with a virtual goniometer boundary element program called VIRGO, from a 3D CAD file topology of any shape. This offers the acoustician the ability to evaluate surface topologies that are of interest in a room design quickly and inexpensively.

Volume scattering of sound signals in gassy bottom and estimation of bubbles concentration in Lake Kinneret

In the paper, results of experiments in shallow Lake Kinneret (Israel) and the corresponding data processing are presented. Wideband LFM sound signals (300 Hz–3.50 kHz and 300 Hz–15 kHz) were radiated by the source mounted directly on vertical line array (VLA) of the length 20 m placed at several locations with different water depths (from 20 to 35 m). Received sound field timeseries constitutes a sequence of pulse arrivals comprised of specular reflections from interfaces followed by reverberation codas caused by non-specular scattering from the interface roughness and volume inhomogeneity. Properties of received signals are analyzed assuming that the sediment can be modeled as a thin layer containing bubbles over a homogeneous fluid half-space. Using a set of hydrophones of VLA allowed isolation of the volume scattering component, estimating frequency-angle dependences of scattered field, and hypothesizing some properties of the gassy sediment, such as concentration of bubbles and their effective size. [Work was supported by BSF grant 2018150.].

Development of a multispectral stereo-camera system comparable to Hayabusa2 optical navigation camera (ONC-T) for observing samples returned from asteroid (162173) Ryugu

Hayabusa2 collected 5.4 g of samples from the asteroid (162173) Ryugu and brought them back to Earth. Obtaining multiband images of these samples with the spectral bands comparable to those used for remote-sensing observations is important for characterizing the collected samples and examining how representative the sample is compared with spacecraft observations of Ryugu as a whole. In this study, we constructed a multiband microscopic camera system that enables both visual multispectral imaging at 390 (ul), 475 (b), 550 (v), 590 (Na), 700 (w), and 850 nm (x), and three-dimensional (3D) shape reconstruction of individual grain samples based on stereo imaging. The imaging system yields the images of 4096 x 2160 pixels with the pixel resolution of 1.93 {\mu}m/pix and field of view of 7.9 x 4.2 mm. Our validation measurements demonstrate that our multispectral imaging system, which observes the samples with the spectral bands comparable to those on the telescopic optical navigation camera (ONC-T) on Hayabusa2, yields reflectance spectra with a relative error of 3% and a 3D model with an error of 5%. These results indicate that the multiband imaging system with a 3D shape reconstruction capability yields accurate spectral and shape data on the returned samples. Using this instrument, we conducted multispectral measurements of two Ryugu samples (grains in the dishes A3 and C1) acquired from two locations on the asteroid. The average spectra of the measured Ryugu samples were flat and consistent with the global averaged spectrum of Ryugu. The 550-nm band (v-band) reflectance of the returned grains in the dishes was 2.4% on average, higher than that of the global averaged spectrum of Ryugu observed with ONC-T. This apparent difference could be because the returned grains have greater specular reflectance.

Analysis of an acoustic propagation model for sources of noise with directivity in indoor environments

Small caliber firearm (SCF) noise sources are typically impulsive in nature, possess a large amount of acoustic energy (consequently a large hearing damage risk), and are not omnidirectional. These sources are often operated in indoor shooting ranges where the potential for noise exposure risk is greater due to the reflective surfaces in the room. Indoor sound propagation models require inputs such as geometry, wall material properties, and some quantified source level description. One such room acoustic modeling technique is the Image-Source Method (ISM). ISM typically assumes specular reflections off the walls and represents those reflections as image sources. Many ISM algorithms can operate with high computational efficiency for simple omnidirectional source models, usually represented by a single quantity: sound power. However, ISM models using an omnidirectional source assumption can produce high errors in some scenarios involving highly directional sources, such as SCFs. In this work, an ISM algorithm has been modified to predict listener exposure levels from non-omnidirectional sources in complicated room designs, and has been validated against measured data from an SCF on an indoor Air Force shooting range.

Appearance estimation and reconstruction of glossy object surfaces based on the dichromatic reflection model

AbstractWe propose an approach for estimating and reconstructing the material appearance of objects based on the spectral image data acquired in complex illumination environments with multiple light sources. The object appearance can be constructed with various material properties such as spectral reflectance, glossiness, and matteness under different geometric and spectral illumination conditions. The objects are assumed to be made of an inhomogeneous dielectric material with gloss or specularity. The color signals from the object surface are described by the standard dichromatic reflection model, which consists of diffuse and specular reflections, where the specular component has the same spectral composition as the illuminant. The overall appearance of objects is determined by a combination of chromatic factors, based on the reflectance and illuminant spectra, and shading terms, which represent the surface geometries of the surface illumination. Therefore, the appearance of a novel object can be reconstructed by modifying the chromatic factors and shading terms. The method for appearance estimation and reconstruction comprises four steps: (1) illuminant estimation, (2) spectral reflectance estimation, (3) shading term estimation and region segmentation, and (4) appearance reconstruction based on the reflection model. The proposed approach is validated in an experiment in which objects of different materials are illuminated using different light sources. We demonstrate typical reconstruction results with novel object appearances.

The Vlasov–Poisson–Landau System with the Specular-Reflection Boundary Condition

We consider the Vlasov–Poisson–Landau system, a classical model for a dilute collisional plasma interacting through Coulombic collisions and with its self-consistent electrostatic field. We establish global stability and well-posedness near the Maxwellian equilibrium state with decay in time and some regularity results for small initial perturbations, in any general bounded domain (including a torus as in a tokamak device), in the presence of specular reflection boundary condition. We provide a new improved \(L^{2}\rightarrow L^{\infty }\) framework: \(L^{2}\) energy estimate combines only with \(S_{ }^{p}\) estimate for the ultra-parabolic equation.

Modeling specular and diffuse reflection of UV LEDs for microbial inactivation in air ducts

A method for modeling UV LED irradiance in air sanitization ducts including non-uniform LED emission profile and surface reflection is described and used in conjunction with simple flow and pathogen inactivation models to calculate overall microbial inactivation. The Discrete Ordinates Method (DOM) with Legendre-Chebyshev quadrature sets is used to represent both non-uniform LED emission profiles and combined specular and diffuse surface reflection. Evaluation of DOM sensitivity to spatial and directional discretization indicated high order quadrature schemes (P36-T36) are required to accurately represent emission and reflection profiles. Predicted microbial inactivation compares well with limited measurements from a previous test system. Modeling shows that for a similar number of LEDs, highly reflective surfaces (diffuse or specular) produce higher inactivation and that for similar reflectivities, specular reflection produces higher inactivation than diffuse reflection. Positioning LEDs on two or four duct walls slightly increases inactivation compared to single wall positioning.

Optimisation of satellite geometries in Very Low Earth Orbits for drag minimisation and lifetime extension

The utilisation of the Very Low Earth Orbit (VLEO) region offers significant application specific, technological, operational, and cost benefits. However, attaining sustained and economically viable VLEO flight is challenging, primarily due to the significant, barely predictable and dynamically changing drag caused by the residual atmosphere, which leads to a rapid deterioration of any spacecraft’s orbit unless mitigated by a combination of active and passive techniques. This article addresses one passive method by optimising satellite shapes in order to achieve a minimisation of the atmospheric drag force and thus extension of operational lifetime. Contrary to previous investigations in the field, a constant internal volume is maintained to account for the placement of satellite instruments and payload inside the structure. Moreover, the satellite geometry is not varied heuristically but optimised, via a numerical 2D profile optimisation specifically developed for this purpose. From the resulting optimal satellite profiles, 3D satellite bodies are derived, which are then verified via the Direct Simulation Monte Carlo method within the open-source particle code PICLas. In addition, rather unconventional designs, i.e. ring geometries, which are based on the assumption of fully specular particle reflections, are proposed and assessed. The optimised satellite geometries offer pure passive lifetime extensions of up to 46% compared to a slender reference body, while the above-mentioned ring geometries achieve passive lifetime extensions of more than 3000%. Finally, the article presents design recommendations for VLEO satellites in dependence of different surface properties.

Investigation of the light output of 3D-printed plastic scintillators for dosimetry applications

Three-dimensional (3D) printing, specifically digital light processing (DLP) technique, can be used to manufacture plastic scintillators of any shape. The purpose of this study was to determine the light output of DLP 3D-printed scintillators for dosimetry applications. Two types of plastic scintillators with dimensions 10 mm × 10 mm × 10 mm were fabricated using DLP 3D-printing at Hanyang University, South Korea. The light output of these DLP 3D-printed samples was measured and compared to that of a commercial plastic scintillator of the same dimensions, RP-408, produced by casting. The 3D-printed scintillators emitting violet and blue light had a lower relative light output by 49% and 43%, respectively, compared to the RP-408 reference scintillator. We also investigated three types of scintillator surface finishing methods: the original surface made by the 3D printer, a sanded surface, and a polished surface. Furthermore, three wrapping configurations were tested: bare scintillator, diffuse-type polytetrafluoroethylene tape, and specular-type enhanced specular reflector foil. Both reflector types, diffuse and specular, reflected blue light with comparable efficiency. Additionally, emission and transmission spectra of the samples were measured. Emission maxima were located at 430 nm for RP-408, and 438 and 475 nm for two 3D-printed samples. Transmittance at the wavelength of maximum emission was equal to 89% for RP-408, and 73% and 66% for the two DLP-printed samples. Although the light output of the 3D-printed scintillators was about 50% lower than that of the commercial plastic scintillator, due to characteristics of 3D-printed plastic scintillators, i.e. fast, low-cost production, and easy customization of the printed shape, they are promising as an active part of dosimeters for use in high intensity gamma radiation fields produced by medical linear accelerators with acceptable signal-to-noise ratio level.

Three-dimensional measurement for specular reflection surface based on deep learning and phase measuring profilometry

ObjectivePhase measuring profilometry (PMP) is a non-contact 3D measurement method. However，it is still a very challenging task when it is applied to an object with a specular reflection surface. In this paper, a 3D measurement method for specular reflection surface based on a deep learning framework is proposed to reduce the measurement error caused by highlights. MethodsFirstly, a dual-channel U-Net network (DC-UNet) is used to identify the highlight areas in the fringe image. Secondly, the dilated residual networks (DRN) combined with the features of the neighborhood of the highlight pixels are used to correct the highlight pixels, and the predicted fringe image without highlight can be obtained. Thirdly, the predicted fringe image is blended with the original image, regarded as the final result of the highlight removal. Fourthly, phase unwrapping is implemented using the fringe image of highlight removal, and the 3D coordinates values can be obtained by combining the calibrated parameters with the phase values unwrapped. ResultsSimulated results show that the absolute maximum deviation, absolute mean error and standard deviation of reconstructed 3D height coordinates are reduced by 98.27 %, 68.70 %, and 91.11 %, respectively. The real experiment results show that the maximum error of the absolute phase is reduced by more than 87.00 %, and the mean error of the absolute phase is reduced by more than 92.72 %. Finally, more than 99.14 % of the pixels with intensity saturated caused by highlights can be corrected by the proposed method.

Visual hydrogenation of palladium membranes on an elastic substrate and their applications in hydrogen sensing

AbstractThe study of metal‐hydrogen interaction is essential for hydrogen‐related applications. Due to the small permittivity variation of palladium (Pd), it is difficult to visualize its hydrogenation processes. Soft substrates can release the expansion stress of Pd membrane and form wrinkling during hydrogenation, which can be utilized to track the hydrogen diffusion. Here, we investigate the hydrogenation processes of Pd membranes with various shapes and sizes on polydimethylsiloxane (PDMS) substrates. It was found that the Pd membranes priorly wrinkles from the edges during hydrogenation and the saturation time of hydrogen absorption reduces with the decreasing size of Pd membranes. With these measurements, we fabricate a high‐performance optical hydrogen sensor consisted of a Pd nanostripe array on PDMS. Due to the transition from specular reflection to scattering during the surface wrinkling of the Pd nanostripe, when exposed to 4% hydrogen gas mixed with nitrogen gas, the sensor shows a high‐reflectance contrast of more than 800% with the absolute reflectance change exceeding 32% over the visible band, and the response time t90 is about 1.5 seconds. These results suggest that the Pd membranes on an elastic substrate is a promising platform for visualization of hydrogen diffusion and hydrogen detection.

ReDDLE-Net: Reflectance Decomposition for Directional Light Estimation

The surfaces of real objects can visually appear to be glossy, matte, or anywhere in between, but essentially, they display varying degrees of diffuse and specular reflectance. Diffuse and specular reflectance provides different clues for light estimation. However, few methods simultaneously consider the contributions of diffuse and specular reflectance for light estimation. To this end, we propose ReDDLE-Net, which performs Reflectance Decomposition for Directional Light Estimation. The primary idea is to take advantage of diffuse and specular clues and adaptively balance the contributions of estimated diffuse and specular components for light estimation. Our method achieves a superior performance advantage over state-of-the-art directional light estimation methods on the DiLiGenT benchmark. Meanwhile, the proposed ReDDLE-Net can be combined with existing calibrated photometric stereo methods to handle uncalibrated photometric stereo tasks and achieve state-of-the-art performance.

3-D pre-stack diffraction separation by extending the PWD method with parametrized local slope

SUMMARY Diffractions comprise the seismic response of subsurface geological discontinuities and thus can provide detailed geological information during 3-D seismic exploration. The separation of weak diffractions from specular reflections is challenging, especially when the diffractions and reflections have similar kinematical characteristics in the 3-D pre-stack case. Conventional separation methods often estimate the local slope based on optimization or the Hilbert transform, which directly depends on the distribution behaviour of seismic events and may lead to the aliasing effects of the hyperbolic reflected and diffracted slopes in the shot domain. In this study, a different method is employed: local slopes are parametrized and constrained to the normal moveout velocity and ray parameter, which can distinguish reflections and diffractions in the shot domain and enhance the stability and accuracy of local slopes. Interestingly, when the receiver line and the geological edge are coplanar, the corresponding edge diffractions and reflections exhibit extremely similar behaviour, rendering them indistinguishable. Considering this phenomenon, a 3-D pre-stack diffraction separation strategy is proposed based on the estimation of the local slopes in two orthogonal directions. Thus, the accurate local slope can be used for plane-wave destruction when separating diffractions in the 3-D pre-stack domain. Synthetic and field data applications demonstrate that the proposed separation strategy is effective and can obtain high-quality diffraction wavefields for detecting the subsurface discontinuous structure.

Robust and Swift Iris Recognition at distance based on novel pupil segmentation

One of the most common and successful biometric frameworks is iris recognition, which has yielded promising results in systems of access control and identity authentication. Recent systems of iris recognition have concentrated on photos taken in non-ideal circumstances. Non-ideal imaging environments involve the capture of iris pictures in motion, at a distance, or under near-infrared (NIR) or visible wavelength illumination, which introduces noise factors such as gaze deviation, lack of focus, and obstruction by hair, eyelids, lighting, specular reflections, and eyeglasses. Segmenting irises is still a challenging task for eye images taken under non-ideal circumstances for iris recognition, and is strongly dependent on pupil segmentation; to estimate a line of sight using the pupil centre method, pupil segmentation is essential. The quality of eye pictures often changes due to noise effects and the individual variance between human eyes, making segmentation of the pupil a tricky problem. To address this issue, we propose an efficacious, swift, and robust iris recognition method that is convenient for eye pictures under non-ideal conditions. In this paper, we present a novel pupil segmentation method based on a fusion of the multi-scale gray-level co-occurrence matrix (MSGLCM) and the multi-range circle Hough transform (MRRCHT) methods. Using this segmentation approach, the pupil texture region is extracted correctly and a Hough transform is then used for the outer iris region. To support the improvements in segmentation accuracy, we also apply pre- and post-processing operations. An evaluation of the proposed method is conducted in this paper using the CASIA v4.0 (Distance) and MMU V2 databases, which reflect the challenges of iris recognition from non-ideal pictures. Extensive experimental findings clarify the effectiveness of the suggested method for non-ideal iris pictures.

Optically tunable metal-dielectric diffractive structures

Hybrid metal-polymer micropillar arrays are elaborated by solvent-assisted embossing techniques and subsequent gold deposition. The polymer is poly(methyl methacrylate) (PMMA) grafted with Disperse Red 1 (DR1) azobenzene derivatives. This photochromic material exhibits spectacular photomechanical properties. Under off-normal $p$-polarized illumination in the absorption band of the DR1, the metal-polymer hybrid micropillars bend in a direction determined by the light polarization. The deformation remains stable when the light excitation is turned off. The optically driven modification of the pillar shape allows us to tune the optical properties of the pillar arrays. Very large intensity changes are measured in the diffraction spectra at zero order (specular reflection), first order, and second order, with almost on and off switching of the diffraction efficiency. The photoinduced bending of the micropillars can be fully reversed by switching the linear light polarization to the orthogonal $s$-polarization state. This allows us to restore the initial optical properties of the pillar array. The polarization-controlled reversibility of the pillar deformation suggests a complex photoinduced deformation mechanism involving stress and stress release assisted by the change in the viscoelastic properties of the polymer under illumination in the absorption band of the DR1 chromophore.

Structural stability of a lightsail for laser-driven interstellar flight

The structural stability of a lightsail under the intense laser flux necessary for interstellar flight is studied analytically and numerically. A sinusoidal perturbation is introduced into a two-dimensional thin-film sail to determine if the sail remains stable or if the perturbations grow in amplitude. A perfectly reflective sail material that gives specular reflection of the laser illumination is assumed in determining the resulting loading on the sail, although other reflection models can be incorporated as well. The quasi-static solution of the critical point between shape stability and instability is found by equating the bending moments induced on the sail due to radiation pressure with the restoring moments caused by the strength of the sail material and the tension applied at the edges of the sail. From this quasi-static solution, analytical expressions for the critical value of elastic modulus and boundary tension magnitude are found as a function of sail properties (e.g., thickness) and the amplitude and wave number of the initial sinusoidal perturbation. These same expressions are also derived from a more formal variational energy (virtual work) approach. A numerical model of the complete lightsail dynamics is developed by discretizing the lightsail into rectangular finite elements. By introducing torsional and rectilinear springs between the elements into the numerical model, a hierarchy of models is produced that can incorporate the effects of bending and applied tension. The numerical models permit the transient dynamics of a perturbed lightsail to be compared to the analytic results of the quasi-static analysis, visualized as stability maps that show the rate of perturbation growth as a function of sail thickness, elastic modulus, and applied tension. The analytic theory is able to correctly predict the stability boundary found in the numerical simulations. The stiffness required to make a thin lightsail stable against uncontrolled perturbation growth appears to be unfeasible for known materials, however, a relatively modest tensioning of the sail (e.g., via an inflatable structure or spinning of the sail) is able to maintain the sail shape under all wavelengths and amplitudes of perturbations.

An advanced bidirectional reflectance factor (BRF) spectral approach for estimating flavonoid content in leaves of Ginkgo plantations

As a key phenolic pigment concentrated in the surface tissues of leaves, flavonoids (Flav) are the major bioactive ingredients in Ginkgo leaf extracts. Flav are also marked natural antioxidants and significant indicators of biotic and abiotic stresses, critical for determining cultivation quality and enhancing Flav yield. In particular, area-based Flav (Flavarea) is related to the shortwave-blue light interaction within leaves per unit leaf area, whereas mass-based Flav (Flavmass) is useful for the quantitative assessment of Flav yield. In order to accurately estimate the contents of Flavarea and Flavmass in leaves of Ginkgo plantations, in this study, we developed an advanced bidirectional reflectance factor (BRF) spectra-based approach by reducing the effects of specular reflection and enhancing the absorption signals of Flav (in the shortwave-blue region of spectrum), using a suite of new spectral indices (SIs) (i.e., flavonoid index (FI), modified flavonoid index (mFI) and double difference index (DD)) calculated from the leaf clip equipped spectrometers-collected data. The results demonstrated that most of the SIs derived from the developed BRF spectra-based approach obtained relatively high performance for Flav estimation by alleviating adverse effects of specular reflection to different extents (CV-R2 = 0.60–0.76). In specific, DDnir434,421 selected from DD-type indices performed (CV-R2 = 0.76 for Flavarea; CV-R2 = 0.69 for Flavmass) better than other indices. These findings represent marked potentials of the developed BRF spectra-based approach for non-destructively estimating leaf Flav content, as well as improving the understanding of the mechanisms of specular effects on Flav estimations in leaves of Ginkgo plantations.