Spectral Characteristics Research Articles

Experimental characterisation of rotor noise in tandem configuration

This paper presents a comprehensive investigation into the noise radiation characteristics of two rotors in a tandem configuration. Through an extensive experimental campaign, the study explores the effect of the separation distance between two rotors on the noise directivity patterns, spectral characteristics and temporal features of the radiated noise under both hovering and edge-wise flight conditions. The directivity patterns of the overall sound pressure level obtained from the polar and side arrays demonstrate a dependence on the separation distance and advance ratio. Spectral characteristics elucidate the influence of separation distance on tonal and broadband energy content. While the high-frequency broadband noise increases for both hovering and edge-wise flight similarly, the low-frequency broadband energy strongly depends on the advance ratio and the rotor separation distance. The results reveal a distinct envelope in the design space, where the radiated broadband noise for tandem configurations can be minimized, offering potential insights for noise reduction strategies. Additionally, the wavelet analysis for tandem configurations reveals that the emitted noise at the first and second blade passing frequency is strongly intermittent with amplitude modulation, which shall be considered for annoyance perception.

New three red-edge vegetation index (VI3RE) for crop seasonal LAI prediction using Sentinel-2 data

Leaf area index (LAI) serves as a pivotal parameter in crop monitoring, significantly impacting agricultural applications. Empirical models are one of the commonly used methods for estimating LAI, they are often dependent on vegetation indices (VIs), predominantly derived from low-to-moderate spatial resolution satellite sensors. A critical limitation of these VIs is their tendency to saturate at elevated LAI values. Additionally, the interplay between chlorophyll content (Cab), LAI, as well as average leaf inclination angle (ALA), particularly in reflectance spectra from red to red-edge regions, has been underexplored in past research. Based on Sentinel-2 satellite, with three red-edge bands and enhanced spatio-temporal resolution, this study introduced a new three red-edge vegetation index (VI3RE), comprising NDVI3RE and CI3RE, which leverages the unique spectral characteristics of these bands and related differential response to LAI and Cab variations. We investigated VI3RE’s efficacy through a threefold approach: firstly, by conducting sensitivity analyses using noise equivalent (NE)ΔLAI ((NE) ΔLAI=1.37 and 1.59 for NDVI3RE and CI3RE; (NE)ΔLAI = 1.75 – 3.25 for other VIs) and extended Fourier amplitude sensitivity test (EFAST) (the contribution of LAI: FOI > 30 % and TOI > 40 % for VI3RE; FOI ∼ 17 % − 28 % and TOI ∼ 25 % − 34 % for other VIs, except for NDVI with TOI ∼ 50 %), we demonstrated VI3RE’s heightened sensitivity to LAI and its improved capability for seasonal LAI estimation compared to other VIs. Secondly, by comprehensively considering the simplicity, interpretability, AIC and R2 values of various models, the linear regression model was selected for analyzing the relationships between LAI and various VIs. We established that VI3RE, particularly NDVI3RE, exhibits a robust correlation with LAI, thereby enhancing LAI estimation accuracy (R2 = 0.72, RMSE = 1.39). Lastly, we applied the LAI-VI regression models to generate crop LAI imageries throughout the growing season, subsequently validating by field measured LAI data. The results affirmed VI3RE’s superior performance in seasonal LAI estimation of crops, notably during peak growth and sowing phases. We conclude that the newly formulated VI3RE offers a universal, highly precise, and reliable model for LAI prediction across various crop types and phenological stages. However, the broader application of VI3RE may be limited, due to the three red-edge bands are not commonly found in most satellite platforms. In addition, some other aspects are not considered in this study, such as phenological information, meteorological factors, sampling strategies, etc., which should be meticulously considered in the future studies.

A comparative study on the spectral characteristics of nanosecond pulsed discharges in atmospheric He and a He+2.3%H2O mixture

Nanosecond pulsed discharges at atmospheric pressure in a pin-to-pin electrode configuration are well reproducible in time and space, which is beneficial to the fundamentals and applications of low-temperature plasmas. In this experiment, the discharges in helium (He) and He with 2.3% water vapor (H2O) are driven by a series of 10 ns overvoltage pulses (~13 kV). Special attention is paid to the spectral characteristics obtained in the center of discharges by time-resolved optical emission spectroscopy. It is found that in helium, the emission of atomic and molecular helium during the afterglow is more intense than that in the active discharge, while in the He+2.3%H2O mixture, helium emission is only observed during the discharge pulse and the molecular helium emission disappears. In addition, the emissions of OH(A-X) and Hα present similar behavior that increases sharply during the falling edge of the voltage pulse as the electrons cool down rapidly. The gas temperature is set to remain low at 540 K by fitting the OH(A-X) band. A comparative study on the emission of radiative species (He, He2, OH and H) is performed between these two discharge cases to derive their main production mechanisms. In both cases, the dominant primary ion is He+ at the onset of discharges, but their He+ charge transfer processes are quite different. Based on these experimental data and a qualitative discussion on the discharge kinetics, with regard to the present discharge conditions, it is shown that the electron-assisted three-body recombination processes appear to be the significant sources of radiative OH and H species in high-density plasmas.

Temporal and Spectral Characteristics of Persistent Emission and Special Bursts of Magnetar SGR J1935+2154 Based on Insight-HXMT

In October 2022, the magnetar SGR J1935+2154 entered the active outburst state. During the episode, the Insight-HXMT satellite carried out a long observation that lasted for 20 days. More than 300 bursts were detected, and a certain amount of persistent radiation signals were also accumulated. This paper mainly introduces the results of persistent radiation profile folding and period search based on Insight-HXMT data. At the same time, the burst phase distribution characteristics, spectral lag results of burst, the spectral characteristics of zero-lag bursts and the time-resolved spectral evolution characteristics of high-flux bursts are reported. We found that there is no significant delay feature during different energy bands for the bursts of SGR J1935+2154. The observed zero-lag burst does not have a unique spectrum. The time-resolved spectrum of the individual burst has consistent spectral types and spectral parameters at different time periods of the burst. We also find that the burst number phase distribution and the burst photon phase distribution have the same tendency to concentrate in specific regions of the persistent emission profile.

Background-oriented schlieren experimental and simulation study of the effect of sidewalls on optical turbulence anisotropy in a thermal convection turbulence simulator

The anisotropic optical structure characteristics within the Rayleigh-Bénard (RB) turbulence simulator have been investigated through experiment and simulation. We collected data using background-oriented schlieren technology and extracted displacements using optical flow algorithms. The wavefront phase can be recovered using the conjugate gradient iterative algorithm. We analyzed the optical structural characteristics within the turbulence simulator under different temperature differences. It is observed that with an increased temperature difference, the spatial correlation of the refractive index field decreases at an accelerated rate as the separation distance increases, and the degree of anisotropy characterized by the phase structure function increases. Large eddy simulation and split-step beam propagation method complement experimental results. Numerical simulations demonstrate consistency with experimental data. The results show that the von Karman power spectral density fits the simulation and experimental phase structure function very well relative to the Kolmogorov power spectral density, which demonstrates that the simulation system combining dynamics simulation and optical simulation can achieve power spectral characteristics that are consistent with reality. The experimental and simulation results are crucial for understanding the flow field's optical structural properties inside the turbulence simulator. They will help to construct a suitable turbulence simulator for optical engineering applications and establish a rational scheme for analyzing its internal optical structural properties.

Selective laser spectroscopy of yttrium scandate doped with ytterbium ion

The spectral-kinetic properties of Yb3+ optical centers in YScO3 crystal fiber were studied using selective laser spectroscopy. Three distinct types of Yb3+ optical centers were identified. Various spectral characteristics of the Yb3+ optical centers were determined, including homogeneous and inhomogeneous broadening values, Stark level splitting, and the Yb3+ ions lifetime. The first optical center, with a lifetime of τI77K = 4.05 ms, was formed due to the substitution of Y3+ for Yb3+ ions in the local site of С3i. The other two Yb3+ optical centers, with lifetimes of τI77K = 1.0 ms (СII2(Y3+) center) and τII77K = 0.620 ms (СII2(Sc3+)), respectively, were formed as a result of the substitution of Y3+ and Sc3+ for Yb3+ in the local site of C2.

Characteristics of Sizing Materials Used in Persian Medieval Manuscripts: Physical, Optical, Spectral Imaging, and Fungicidal Properties

ABSTRACT In this study, we investigated the diverse range of materials used for sizing in Iranian paper manuscripts during the Timurid (fifteenth century) to Safavid (sixteenth century) and Qajar (nineteenth century) periods. Our approach combined historical analysis with scientific examination of reconstructed sizings. We reconstructed 15 sizing materials based on identified Persian historical recipes and analysed their physical, optical, and spectral characteristics. Additionally, we assessed their behaviour against the mould fungus Aspergillus flavus. The results revealed distinctive properties for each sizing material, shedding light on their potential applications in paper preservation. Furthermore, our investigation demonstrated variations in hygroscopicity, thickness, grammage, and ash content post-sizing. The sizing materials also exhibited different effects on paper reflectance properties. Additionally, our study revealed insights into the impact of sizing on burnished papers, indicating that the mechanical process of burnishing did not significantly alter the chemical composition or spectral properties of the paper, with only minor changes in brightness observed in specific cases. All tested sizing materials supported varying levels of mould growth, indicating potential implications for paper conservation. Our findings provide valuable insights into the historical practices of Iranian paper sizing and offer practical considerations for the preservation of paper manuscripts.

Unveiling reflectance spectral characteristics of floating plastics across varying coverages: insights and retrieval model

Unveiling reflectance spectral characteristics of floating plastics across varying coverages: insights and retrieval model

Design and Application of Bionic Camouflage Materials Simulating Spectral Reflection Characteristics of Plants: A Review

Hyperspectral remote sensing (RS) has rapidly developed in recent years and has been widely used in the military field. This technology not only brings huge opportunities for military reconnaissance but also poses unprecedented challenges to military camouflage, severely complicating the development of plant hyperspectral camouflage materials and technology. In this review, the spectral reflectance characteristics of plants and the application of hyperspectral RS in plant RS and military operations are reviewed. The development status of bionic camouflage materials that simulate the spectral reflection characteristics of plants is analyzed. With the existing hyperspectral camouflage materials and technology, bionic camouflage technology is limited by the inability of bionic materials to accurately imitate the characteristic absorption peaks of green vegetation, low stability and durability, and the large overall material thickness, which complicate actual large-scale application. On this basis, a future development direction and a trend of plant hyperspectral bionic camouflage materials and technology are proposed.

Determination of butylated hydroxytoluene in edible oil using near-infrared spectroscopy combined with multivariate analysis

Edible oil is a fundamental component of human daily diet and has a wide impact on global agriculture and economy. The issue of additives in edible oil has always been a concern for its quality and safety, as excessive use of additives can be harmful to human health. This study suggests a quantitative detection technique for determining the concentration of butylated hydroxytoluene (BHT) in edible oil using near-infrared spectroscopy (NIRS) technology. Firstly, the spectral characteristics of edible oil samples with different concentrations of BHT are characterized using a near-infrared spectrometer. Then, particle swarm optimization (PSO), butterfly optimization algorithm (BOA), and fox optimizer (FOX) are employed to optimize the preprocessed spectral wavelengths. Finally, a detection model based on the optimized wavelengths is established using backpropagation neural network (BP-ANN) to achieve quantitative detection of BHT content in edible oil. The results show that the model based on the feature wavelengths optimized by the FOX algorithm has the best detection performance. This model takes 7 features as inputs, with a root mean square error of prediction (RMSEP) of 1.6116 g·kg−1 and a prediction determination coefficient (RP) of 0.9900 on the prediction set. The results demonstrate that near-infrared spectroscopy technology can accurately predict the BHT content in edible oil.

Selenium Nanoparticles Stabilized by Amphiphilic Molecular Brushes with Varying Degrees of Polymerization of Side Chains: Spectral and Structural-Morphological Characteristics

Selenium Nanoparticles Stabilized by Amphiphilic Molecular Brushes with Varying Degrees of Polymerization of Side Chains: Spectral and Structural-Morphological Characteristics

The development of methods for determination the total flavonoids in buds of Populus alba L.

White poplar (Populus alba L., family Saliaceae ) has a similar chemical composition with other species of the genus Populus L. and it can consider as a promising source of raw materials containing phenolic compounds, in particular flavonoids. The pharmacological activity of the buds of pharmacopeial species of genus Populus L., and also of the white poplar is caused by biologically active compounds, mainly of a phenolic nature, including flavonoids (pinostrobin, piriocembrin, quercetin, etc.), phenylpropanoids (caffeic acid, etc.) and simple phenols (salicin).. One of the most well-known biologically active compounds of white poplar is quercetin, for which an antihistamine, anti-inflammatory action has been demonstrated. Differential spectrophotometry carried out in accordance with the OFS was used as a research method.1.2.1.1.0003.15 "Spectrophotometry in the ultraviolet and visible regions". Spectral characteristics of water-alcohol extracts were evaluated on a Specord 40 spectrophotometer (Analytik Jena AG, Germany) in cuvettes with a layer thickness of 10 mm. It was determined that in all electronic spectra of the water-ethanolic extractions from the buds of the white poplar there are two absorption maxima in the area of 290 nm and 370 nm, due to flavanones and flavonols, respectively. It was established that in the electronic spectra of the water-ethanolic extractions from the buds of the white poplar, a significant bathochromic shift of the long-wavelength band in the presence of aluminum chloride is observed, which confirms the presence of flavonoids. Under the conditions of differential spectrophotometry UV spectrum of the water-ethanolic extractions from the buds of the white poplar, an absorption maximum is observed at wavelength of 430 nm, which indicates the expediency of using quercetin in the analysis technique, which has an absorption maximum at a wavelength of 430±2 nm. As a result of the study, the method has been developed for the quantitative determination of the amount of flavonoids in the buds of white poplar using differential spectrophotometry calculated on quercetin at an analytical wavelength of 430 nm. The optimal parameters of extraction of raw materials were determined: extractant – 90% ethyl alcohol, the ratio of "raw material-extractant" – 1:30, extraction time – 60 minutes. The content of the total flavonoids calculated on quercetin in the buds of the white poplar is varied from 0,45±0,02 % to 0.69 ± 0.03%

Effect of arsenate and phosphate substitution on hydroxyl group in libethenite Cu2PO4OH - olivenite Cu2AsO4OH solid solution series

Libethenite and olivenite have been regarded as intriguing photocatalysts because of their singular structures. Copper hydroxy-phosphates and -arsenates exhibit distinctive properties: they incorporate bridging hydroxyl (OH) groups shared by neighboring copper atoms. In materials employed for photoelectrochemical applications, the significance of surface OH groups and OH-related defects frequently varies and is contingent upon the specific material system and the reaction under consideration. Hydroxyl groups can enhance photocatalytic performance by generating OH radicals or serving as crucial agents in catalytic reactions. Therefore, the presence of an OH group within the crystal structure of the material can give rise to potentially interesting behaviors. Seven compounds within the solid solution series of libethenite Cu2PO4OH and olivenite Cu2AsO4OH with stoichiometric ratio (Cu2(Px,As1- xO4)OH, x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1 were synthesized at 70 °C via aqueous solutions, and their properties were examined using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. The effect of substituting (PO4)3– ions with (AsO4)3– ions on the alteration of the lattice parameters and spectral characteristics was elucidated by analyzing the correlation between the chemical composition and X-Ray diffraction peak positions. The substitution process leads to modifications in the unit cell parameters and volume. These isomorphic substitutions can be also discerned through alterations in the infrared and Raman spectra, which exhibit variations in the position and intensity of the bands corresponding to phosphates, arsenates, and hydroxyl ions. The incorporation of As into the solid solution resulted in altered bond lengths and geometries, which is consistent with the symmetry transition from orthorhombic libethenite to monoclinic olivenite. The variations in the unit cell parameters and the shifts in the peak position exhibited non-linear trends, and a discernible step was observed between the Cu2(P0·3As0.7)O4OH and Cu2(P0·5As0.5)O4OH intermediate phases. The findings of this study will prove beneficial in evaluating the feasibility of the libethenite-olivenite isometric series as a photocatalysts.

Impact of assimilation of absolute dynamic topography on Arctic Ocean circulation

The ocean circulation is typically constrained in operational analysis and forecasting systems through the assimilation of sea level anomaly (SLA) retrievals from satellite altimetry. This approach has limited benefits in the Arctic Ocean and surrounding seas due to data gaps caused by sea ice coverage. Moreover, assimilation of SLA in seasonally ice-free regions may be negatively affected by the quality of the Mean Sea Surface (MSS) used to derive the SLA. Here, we use the Regional Ice Ocean Prediction System (RIOPS) to investigate the impact of assimilating Absolute Dynamic Topography (ADT) fields on the circulation in the Arctic Ocean. This approach avoids the use of a MSS and additionally provides information on sea level in ice covered regions using measurements across leads (openings) in the sea ice. RIOPS uses a coupled ice-ocean model on a 3-4 km grid-resolution pan-Arctic domain together with a multi-variate reduced-order Kalman Filter. The system assimilates satellite altimetry and sea surface temperature together with in situ profile observations. The background error is modified to match the spectral characteristics of the ADT fields, which contain less energy at small scales than traditional SLA due to filtering applied to reduce noise originating in the geoid product used. A series of four-year reanalyses demonstrate significant reductions in innovation statistics with important impacts across the Arctic Ocean. Results suggest that the assimilation of ADT can improve circulation and sea ice drift in the Arctic Ocean, and intensify volume transports through key Arctic gateways and resulting exchanges with the Atlantic Ocean. A reanalysis with a modified Mean Dynamic Topography (MDT) is able to reproduce many of the benefits of the ADT but does not capture the enhanced transports. Assimilation of SLA observations from leads in the sea ice appears to degrade several circulation features; however, these results may be sensitive to errors in MDT. This study highlights the large uncertainties that exist in present operational ocean forecasting systems for the Arctic Ocean due to the relative paucity and reduced quality of observations compared to ice-free areas of the Global Ocean. Moreover, this underscores the need for dedicated and focused efforts to address this critical gap in the Global Ocean Observing System.

Upconversion Emission and Dual-Mode Sensing Characteristics of NaYF4:Yb3+/Er3+ Microcrystals at High and Ultralow Temperatures.

In this study, we investigate micrometer-sized NaYF4 crystals double-doped with Yb3+/Er3+ lanthanide ions, designed for temperature-sensing applications. In contrast to previous studies, which focused predominantly on the high-temperature regime, our investigation spans a comprehensive range of both high and ultralow temperatures. We explore the relationship between temperature and the upconversion luminescence (UCL) spectra in both frequency and time domains. Our findings highlight the strong dependence of these spectral characteristics of lanthanide-doped NaYF4 crystals on temperature. Furthermore, we introduce a dual-mode luminescence temperature measurement technique, leveraging the upconversion emission intensity ratio for both green and red emissions. This study also examines the correlation between temperature sensing, energy level disparities, and thermal coupling in Er3+ ions across various temperature scales. Our research contributes to advancing the understanding and application of lanthanide-doped materials, setting a foundation for future innovations in temperature sensing across diverse fields.

Unsupervised Learning for Improved Gamma-Ray Spectrometry in Pixelated Cadmium Zinc Telluride (CZT) Detectors

Machine learning has been found to be ubiquitously useful across many industries, presenting an opportunity to improve radiation detection performance using data-driven algorithms. Improved detector resolution can aid in the detection, identification, and quantification of radionuclides. In this work, a novel, data-driven, unsupervised learning approach is developed to improve detector spectral characteristics by learning, and subsequently rejecting, poorly performing regions of the pixelated detector. Feature engineering is used to fit individual characteristic photo peaks to a Doniach lineshape with a linear background model. Then, principal component analysis is used to learn a lower-dimension latent space representation of each photo peak where the pixels are clustered, and subsequently ranked, based on the cluster mean distance to an optimal point. Pixels within the worst cluster(s) are rejected to improve the full-width at half-maximum (FWHM) by 10% to 15% (relative to the bulk detector) at 50% net efficiency when applied to training data obtained from measurements of a 100 μCi 154Eu source using a H3D M400i pixelated cadmium zinc telluride detector. These results compare well with, but do not outperform, a greedy algorithm that accumulates pixels in order of FWHM from lowest to highest used as a benchmark. In the future, this approach can be extended to include the detector energy and angular response. Finally, the model is applied to newly seen natural and enriched uranium spectra relevant for nuclear safeguards applications.

Examining the Percent Canopy Cover and Health of Winter Wheat in No-Till and Conventional Tillage Plots Using a Drone

The percent canopy cover and health of winter wheat are important crop performance indicators. Thus, understanding how tillage management practices affect these indicators is beneficial for improving crop performance and consequently yield. The availability of high-resolution drone data with spectral characteristics provides an opportunity to examine the percent canopy cover and health of winter wheat in different tillage systems. This is because the use of drones provides real-time high spatial resolution and temporal images to effectively monitor winter wheat conditions throughout the growing season. Nonetheless, very limited studies have utilized drone data for assessing the percent canopy cover and health conditions of winter wheat for different tillage practices. This study aimed to examine the percent canopy cover and health of winter wheat in no-till and conventional tillage plots using a drone. We used the mean Normalized Difference Vegetation Index (NDVI) ± Standard Deviation (SD) (0.89 ± 0.04) of winter wheat for the growth stages of tillering, jointing, and boot/heading to generate the percent wheat canopy cover. The Normalized Difference Red-Edge (NDRE) produced for winter wheat at the middle and late growth stages was used as a proxy for wheat health condition. We found that the mean percentage canopy cover of winter wheat was about 4% higher in no-till compared to conventional tillage plots in most of the growing season. The mean NDRE ± standard error (SE) of winter wheat was about 0.44 ± 0.01 and 0.43 ± 0.01 for no-till and conventional tillage plots, respectively, during the mid- and late growth stages. There was no significant difference in either the percent canopy cover or health of winter wheat between no-till and conventional tillage plots. The results generated in this study could be used to support farmers’ decision-making process regarding tillage practices and wheat crop performance.

Polymer-based cascaded waveguide Bragg grating for blood glucose sensing.

Waveguide Bragg grating (WBG) blood glucose sensing, as a biological sensing technology with broad application prospects, plays an important role in the fields of health management and medical treatment. In this work, a polymer-based cascaded WBG is applied to glucose detection. We investigated photonic devices with two different grating structures cascaded-a crossed grating and a bilateral grating-and analyzed the effects of the crossed grating period, bilateral grating period, and number of grating periods on the sensing performance of the glucose sensor. Finally, the spectral reflectance characteristics, response time, and sensing specificity of the cascaded WBG were evaluated. The experimental results showed that the glucose sensor has a sensitivity of 175 nm/RIU in a glucose concentration range of 0-2 mg/ml and has the advantages of high integration, a narrow bandwidth, and low cost.

The influence of temperature on spectral characteristics of silicon photovoltaic cells

The paper presents the results of research on commercial photovoltaic cells made of crystalline silicon. In particular, the focus was on the description of the elaborated by the authors measuring system with measurements methodology used for assessment of the influence of temperature on spectral characteristics of the tested cells, describing the dependence of the current sensitivity (spectral response, responsivity) and the external quantum efficiency on the wavelength of optical radiation. The investigations carried out in the proposed test system made it possible to evaluate the properties of the cells in the conditions similar to the operating conditions.

Capture of volatile iodine by aromatic amines solutions

Abstract The presence of an excessive amount of iodine, especially radioactive iodine, is dangerous to the environment. Amine solutions are the most common and technically mature class of chemical sorption used for the capture of the pollutants. The iodine vapor diffusion and release capabilities of the aromatic amine solutions have been investigated. The iodine diffusion and release experiments were examined by UV-visible spectroscopy. Many electronic spectrophotometric studies have been reported in the coordination chemistry field on complexes. Generally, these complexes were obtained by using different electron donors with various organic or metallic electron acceptors in polar and non-polar solvents. The absorption spectra of the donor, acceptors, and the resulted complexes were carried out in methanol in the region of 200–600 nm. The correlation between the spectral characteristics of molecular complexes of iodine with various aromatic amines and the ionization potentials of the donor molecules is discussed. The concentrations of the diffused iodine and the formed complex were calculated using mathematical models and calibration curves. The values of the formation constant (k AD), molar extinction coefficient (ε AD), and absorption band energy of complexes were estimated. The ionization potential of the donor I D was calculated from the complex band energies. The kinetic of the above association and reverse reactions was studied, and some kinetic parameters have been estimated.