Observed Reflectance Spectra Research Articles

Surface oxides characterization based on hyperspectral observations

During steel sheets manufacturing, the addition of alloying elements to the steel to improve its mechanical performance can cause the formation of layers of selective oxides at the surface. These layers can significantly impact the surface properties of the steel sheets, which makes their detection an essential task. In this article, we present a novel methodology based on hyperspectral measurements to detect the presence of surface layers of selective oxides on steel sheets. Hyperspectral sensors have the ability to measure the reflectance of the steel surface over a large spectral band of the electromagnetic spectrum, usually in the infrared range. However, due to their limited spatial resolution, the observed reflectance spectra usually mix the contribution of several elementary oxides. A first step toward surface characterization is therefore to identify the elementary oxides contributing to the hyperspectral observations, a task referred to as hyperspectral unmixing in the literature. In our case, a major difficulty is that the optical model describing the formation of the electromagnetic wave coming onto the sensors is strongly nonlinear. To alleviate this difficulty, we rely on an approach that formulates the hyperspectral unmixing task as a sparse regression problem. This approach works by pre-computing the optical reflectance associated to reference layers of heterogeneous oxides that can form at the surface to construct a dictionary of reflectance spectra and by seeking to recover the hyperspectral observations as a linear combination of a small number of elements taken from this dictionary. To perform the recovery, we propose and compare two approaches based on the path and group Lasso algorithms. We illustrate our methodology on numerical experiments, which demonstrate that our approach is able to estimate the presence of selective oxides with high recall and precision. These numerical experiments also highlight the main limitations of the proposed approach. We finally apply our estimation method to characterize the surface chemistry of experimental samples. Our characterization approach is then compared to observations conducted on the samples through transmission electron microscopy and XPS analysis to demonstrate its validity.

Reproduction of Biomimetic Structural Colors via µm‐Scale Ni Textures

AbstractA µm‐scale Ni texture coated with a dielectric multilayer Bragg stack is developed to cost‐effectively reproduce a variety of biomimetic structural colors on a large scale. The reflectance of the highly reflective region by the Bragg‐stacked flat Ni‐surface is significantly reduced, while the high‐order reflection in the shorter wavelength region increases due to the interference effect inside the triangular Ni texture. This high‐order reflectance is easily adjusted to regulate the reflection color according to the shape of the Bragg stacked Ni textures. The observed reflectance spectra show that the monochromaticity increases as the apex angle of the Ni textures decreases, whereas the amount of color mixture increases as the area of the flat portion formed in them increases. Thus, this color change is quantified by analyzing the variation in the peak wavelength and bandwidth of the reflection spectrum according to the shape of the Ni textures and the thickness of the Bragg stack. Also, by fabricating the triangular Ni textures with gradually decreasing thickness of the Bragg stack from the ridges to the valleys, the continuous change of the reflection color is demonstrated effectively as the location of the incident light reflected from the texture changes.

Global Modeling of Ganymede's Surface Composition: Near‐IR Mapping From VLT/SPHERE

AbstractWe present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near‐infrared (0.95–1.65 μm) observations from the ground‐based VLT/SPHERE instrument. Observed reflectance spectra were modeled using a Markov Chain Monte Carlo method to estimate abundances and associated uncertainties of water ices, acids, salts and a spectrally flat darkening agent. Results confirm Ganymede's surface is dominated by water ice in young bright terrain (impact craters, sulci), and low‐albedo spectrally flat material in older dark terrain (e.g., Galileo Regio). Ice grain size has strong latitudinal and longitudinal gradients, with larger grains at the equator and on the trailing hemisphere. These trends are consistent with the effects of the latitudinal thermal gradient and global variations in radiation driven sputtering. Sulfuric acid has a low abundance and appears potentially spatially correlated with plasma bombardment, where Ganymede's poles are exposed to the external Jovian magnetic field. Best‐estimate abundances suggest a mixture of salts could be present, although their low abundances, spectral degeneracies and associated uncertainties mean individual salt species cannot be detected with confidence. If present, sodium magnesium sulfate and magnesium chlorate appear tentatively correlated with exogenic plasma bombardment, while magnesium chloride and sulfate appear tentatively correlated with younger terrain, implying a possible endogenic origin. MCMC modeling was also performed on Galileo/NIMS data, showing comparable distributions. The high spatial resolution of SPHERE allows the precise mapping of small scale (<150 km) surface features, which could be used along with higher spectral resolution observations to jointly confirm the presence and distribution of potential species.

The effect of returning radiation on relativistic reflection

ABSTRACT We study the effect of returning radiation on the shape of the X-ray reflection spectrum in the case of thin accretion discs. We show that the returning radiation mainly influences the observed reflection spectrum for a large black hole spin (a > 0.9) and a compact primary source of radiation close to the black hole at height h < 5rg, and that it dominates the reflected flux for extreme values of spin and compactness. The main effect of the returning radiation is to increase the irradiating flux on to the outer parts of the accretion disc, leading to stronger reflection and a flatter overall emissivity profile. By analysing simulated observations we show that neglecting returning radiation in existing studies of reflection-dominated sources has likely resulted in overestimating the height of the corona above the black hole. An updated version of the publicly available relxill suite of relativistic reflection models which includes returning radiation is also presented.

Hazy Blue Worlds: A Holistic Aerosol Model for Uranus and Neptune, Including Dark Spots.

We present a reanalysis (using the Minnaert limb‐darkening approximation) of visible/near‐infrared (0.3–2.5 μm) observations of Uranus and Neptune made by several instruments. We find a common model of the vertical aerosol distribution i.e., consistent with the observed reflectivity spectra of both planets, consisting of: (a) a deep aerosol layer with a base pressure >5–7 bar, assumed to be composed of a mixture of H2S ice and photochemical haze; (b) a layer of photochemical haze/ice, coincident with a layer of high static stability at the methane condensation level at 1–2 bar; and (c) an extended layer of photochemical haze, likely mostly of the same composition as the 1–2‐bar layer, extending from this level up through to the stratosphere, where the photochemical haze particles are thought to be produced. For Neptune, we find that we also need to add a thin layer of micron‐sized methane ice particles at ∼0.2 bar to explain the enhanced reflection at longer methane‐absorbing wavelengths. We suggest that methane condensing onto the haze particles at the base of the 1–2‐bar aerosol layer forms ice/haze particles that grow very quickly to large size and immediately “snow out” (as predicted by Carlson et al. (1988), https://doi.org/10.1175/1520-0469(1988)045<2066:CMOTGP>2.0.CO;2), re‐evaporating at deeper levels to release their core haze particles to act as condensation nuclei for H2S ice formation. In addition, we find that the spectral characteristics of “dark spots”, such as the Voyager‐2/ISS Great Dark Spot and the HST/WFC3 NDS‐2018, are well modelled by a darkening or possibly clearing of the deep aerosol layer only.

Heritable variation in needle spectral reflectance of Scots pine (Pinus sylvestris L.) peaks in red edge

Foliar reflectance is readily used in evaluating physiological status of agricultural crops and forest stands. However, in the case of forest trees, underlying genetics of foliar spectral reflectance and pigment content have rarely been investigated. We studied a structured population of Scots pine, replicated on two sites, with the selected trees´ pedigree reconstructed via DNA markers. This allowed us to decompose phenotypic variance of pigment and reflectance traits into its causal genetic components, and to estimate narrow-sense heritability (h2).We found statistically significant h2 ranging from 0.07 to 0.22 for most of the established reflectance indices. Additionally, we investigated the profile of heritable variation along the reflectance curve in 1 nm wavelength (WL) bands. We show that the maximum h2 value (0.39; SE 0.13) across the 400 to 2500 nm spectral range corresponds to the red edge inflection point, in this case to 722 nm WL band. Resultant h2 distribution indicates that additive gene effects fluctuate along the reflectance curve.Furthermore, h2 of the most widely used formats of reflectance indices, i.e. the simple ratio and the normalized difference, was estimated for all WL bands combined along the observed reflectance spectrum. The highest h2 estimates for both formats were obtained by combining WL bands of the red edge spectrum.These new genetically driven pigment- and spectral reflectance- based markers (proxies of adaptive traits) may facilitate selection of stress resistant plant genotypes. Recent development of high-resolution spectral sensors carried by airborne and spaceborn devices make foliage spectral traits a viable technology for mass phenotyping in forest trees.

Effect of Co2+ substitution on MgAl2O4 studied by infrared reflectance spectroscopy

Infrared reflectivity spectra are measured in the frequency range between 10 and 15,500cm−1 at room temperature for cobalt substituted nanocrystalline magnesium aluminates MgAl2−xCoxO4 (x=0.0, 0.5, 1.0, 1.5, 2.0). Four optical phonon modes manifesting the spinel structure have been observed and analyzed using the Lorentz oscillator model. The resonant frequency, oscillator strength and the damping factor of these modes were extracted from Lorentz oscillator fit to measured spectra. The observed reflectivity spectra exhibit a typical semiconductor behavior. Born and Szigeti effective charges have been estimated and the results are discussed to understand polarization and ionicity of MgAl2−xCoxO4 system.

Scattering-enhanced absorption and interference produce a golden wing color of the burnished brass moth, Diachrysia chrysitis.

Here we report how interference and scattering-enhanced absorption act together to produce the golden wing patches of the burnished brass moth. The key mechanism is scattering on rough internal surfaces of the wing scales, accompanied by a large increase of absorption in the UV-blue spectral range. Unscattered light interferes and efficiently reflects from the multilayer composed of the scales and the wing membranes. The resulting spectrum is remarkably similar to the spectrum of metallic gold. Subwavelength morphology and spectral and absorptive properties of the wings are described. Theories of subwavelength surface scattering and local intensity enhancement are used to quantitatively explain the observed reflectance spectrum.

Multiple Resonances Induced by Plasmonic Coupling between Gold Nanoparticle Trimers and Hexagonal Assembly of Gold-Coated Polystyrene Microspheres.

Optical properties of a gold nanoparticle trimer assembly coupled with gold-coated hexagonally close-packed polystyrene microspheres were investigated by linear and nonlinear spectroscopy. The observed reflection spectrum shows multiple peaks from the visible to near-infrared spectral regions. The spectroscopic properties were also examined by a finite-difference time-domain simulation. We found that the optical response of plasmons excited in the gold nanoparticle trimers was significantly modulated by strong coupling of the plasmons and the photonic mode induced in the gold-coated polystyrene assembly. Two-photon induced photoluminescence and Raman scattering from the sample were investigated, and both signals were significantly enhanced at the gold nanoparticle assembly. The simulations reveal that the electric fields can be enhanced site-selectively, not only at the interstitial sites in the nanoparticle assembly but also at the gaps between the particle and the gold film due to plasmonic interactions, by tuning the wavelength and are responsible for the strong optical responses.

PRECISION VITICULTURE FROM MULTITEMPORAL, MULTISPECTRAL VERY HIGH RESOLUTION SATELLITE DATA

In order to exploit efficiently very high resolution satellite multispectral data for precision agriculture applications, validated methodologies should be established which link the observed reflectance spectra with certain crop/plant/fruit biophysical and biochemical quality parameters. To this end, based on concurrent satellite and field campaigns during the veraison period, satellite and in-situ data were collected, along with several grape samples, at specific locations during the harvesting period. These data were collected for a period of three years in two viticultural areas in Northern Greece. After the required data pre-processing, canopy reflectance observations, through the combination of several vegetation indices were correlated with the quantitative results from the grape/must analysis of grape sampling. Results appear quite promising, indicating that certain key quality parameters (like brix levels, total phenolic content, brix to total acidity, anthocyanin levels) which describe the oenological potential, phenolic composition and chromatic characteristics can be efficiently estimated from the satellite data.

PRECISION VITICULTURE FROM MULTITEMPORAL, MULTISPECTRAL VERY HIGH RESOLUTION SATELLITE DATA

Abstract. In order to exploit efficiently very high resolution satellite multispectral data for precision agriculture applications, validated methodologies should be established which link the observed reflectance spectra with certain crop/plant/fruit biophysical and biochemical quality parameters. To this end, based on concurrent satellite and field campaigns during the veraison period, satellite and in-situ data were collected, along with several grape samples, at specific locations during the harvesting period. These data were collected for a period of three years in two viticultural areas in Northern Greece. After the required data pre-processing, canopy reflectance observations, through the combination of several vegetation indices were correlated with the quantitative results from the grape/must analysis of grape sampling. Results appear quite promising, indicating that certain key quality parameters (like brix levels, total phenolic content, brix to total acidity, anthocyanin levels) which describe the oenological potential, phenolic composition and chromatic characteristics can be efficiently estimated from the satellite data.

Reflectance spectroscopy of oxalate minerals and relevance to Solar System carbon inventories

The diversity of oxalate formation mechanisms suggests that significant concentrations of oxalic acid and oxalate minerals could be widely distributed in the Solar System. We have carried out a systematic study of the reflectance spectra of oxalate minerals and oxalic acid, covering the 0.2–16µm wavelength region. Our analyses show that oxalates exhibit unique spectral features that enable discrimination between oxalate phases and from other commonly occurring compounds, including carbonates, in all regions of the spectrum except for the visible. Using these spectral data, we consider the possible contribution of oxalate minerals to previously observed reflectance spectra of many objects throughout the Solar System, including satellites, comets, and asteroids. We find that polycarboxylic acid dimers and their salts may explain the reflectance spectra of many carbonaceous asteroids in the 3µm spectral region. We suggest surface concentration of these compounds may be a type of space weathering from the photochemical and oxidative decomposition of the organic macromolecular material found in carbonaceous chondrites. The stability and ubiquity of these minerals on Earth, in extraterrestrial materials, and in association with biological processes make them useful for many applications in Earth and planetary sciences.

Signatures of X-ray reverberation in the power spectra of AGN

We compute fully relativistic disc response functions in the case of the "lamp-post" geometry using the full observed reflection spectrum for various X-ray source heights, disc inclination, and spin values of the central black hole. Since the observed PSD is equal to the product of the intrinsic power spectrum with the "transfer function" (i.e. the Fourier transform of the disc response function), we are able to predict the observed PSDs in the case of X-ray illumination of the inner disc. The observed PSD should show a prominent dip at high frequencies and an oscillatory behaviour, with a decreasing amplitude, at higher frequencies. The reverberation "echo" features should be more prominent in energy bands where the reflection component is more pronounced. The frequency of the dip is independent of energy, and it is mainly determined by the black hole mass and the X-ray source height. The amplitude of the dip increases with increasing black hole spin and inclination angle, as long as the height of the "lamp" is smaller than ~10 gravitational radii. The detection of the X-ray reverberation signals in the PSDs can provide further evidence for X-ray illumination of the inner disc in AGN. Our results are largely independent of the assumed geometry of the disc-corona system, as long as it does not change with time, and the disc response function is characterized by a sharp rise, a "plateau", and a decline at longer times. Irrespective of the geometry, the frequency of the main dip should decrease with increasing "mean time" of the response function, and the amplitude of the dip should increase with increasing reflection fraction.

Dynamical studies of the mechanisms for optical nonlinearities of methyl-red dye doped blue phase liquid crystals.

Dynamical grating diffraction experiments and reflection/transmission polarization spectroscopy have been conducted on azo-dye doped Blue-Phase Liquid Crystal (BPLC) to investigate the mechanisms responsible for laser induced refractive index changes. The underlying mechanisms for the transient grating diffraction components are attributed to thermal indexing and lattice distortion, whereas the persistent component is due to lattice distortion/expansion caused by laser excited dye molecule isomerization. These mechanisms were distinguishable by their response dynamics and gave rise to the observed reflection spectra and photonic bandgap shift, polarization dependency and optical activity. Some preliminary studies have demonstrated the feasibility of using these mechanisms for coherent holographic and direct image writing operations.

A normalisation framework for (hyper-)spectral imagery

It is well known that the topography has an influence on the observed reflectance spectra. This influence is not compensated by spectral ratios, i.e. the effect is wavelength dependent. In this work, we present a complete normalisation framework.The surface temperature is estimated based on the measured surface reflectance. To normalise the spectral reflectance with respect to a standard illumination geometry, spatially varying reflectance parameters are estimated based on a non-linear reflectance model. The reflectance parameter estimation has one free parameter, i.e. a low-pass function, which sets the scale of the spatial-variance, i.e. the lateral resolution of the reflectance parameter maps. Since the local surface topography has a major influence on the measured reflectance, often neglected shading information is extracted from the spectral imagery and an existing topography model is refined to image resolution.All methods are demonstrated on the Moon Mineralogy Mapper dataset. Additionally, two empirical methods are introduced that deal with observed systematic reflectance changes in co-registered images acquired at different phase angles. These effects, however, may also be caused by the sensor temperature, due to its correlation with the phase angle.Surface temperatures above 300K are detected and are very similar to a reference method. The proposed method, however, seems more robust in case of absorptions visible in the reflectance spectrum near 2000nm. By introducing a low-pass into the computation of the reflectance parameters, the reflectance behaviour of the surfaces may be derived at different scales. This allows for an iterative refinement of the local surface topography using shape from shading and the computation reflectance parameters. The inferred parameters are derived from all available co-registered images and do not show significant influence of the local surface topography. The results of the empirical correction show that both proposed methods greatly reduce the influence of different phase angles or sensor temperatures.

Optical properties of CuCl microcavities with fluctuations in their refractive index profiles along the cavity structures

We report on the optical properties of the CuCl microcavities that exhibit fluctuations in their refractive index profiles along the cavity structures. The angle-resolved reflectance spectra of the CuCl microcavities with a cavity length of half-integer multiples for the ${Z}_{3}$ exciton wavelength clearly show three cavity-polariton modes and minor modes. The Rabi splitting energies observed in the cavity-polariton modes are widely controlled by changing the cavity length of the microcavity. To analyze the spectral profile and the Rabi splitting energies qualitatively, the refractive index profiles of the constituent layers in the CuCl microcavities are evaluated using spectroscopic ellipsometry, and the reflectance spectra of the CuCl microcavities with the fluctuations in the refractive index profiles of the microcavity structures are calculated using the nonlocal response theory. The calculated reflectance spectra reproduce very well the cavity-polariton modes and the minor modes, including their energies and spectral profiles, in the observed reflectance spectra, which indicates an essential role of the fluctuation effect of the refractive indices of constituent layers in controlling the Rabi splitting energies and the quality factor of the microcavity.

Spectroscopy of stop bands in magnetic photonic crystals

The shape of the spectra of continuous visible-range radiation reflection from the surface (111) of a globular photonic crystal filled with magnetic materials was determined. The bands observed in the reflectance spectrum correspond to the appearance of stop bands of the photonic crystal. It was shown that position of the intensity maximum in the observed reflectance spectra depends on the type of a magnetic material introduced into photonic crystal pores.

THz REFLECTIVITY SPECTROSCOPY OF TUBULAR PZT NANOSTRUCTURES

ABSTRACT We report the time domain THz reflectivity spectroscopy results of lead zirconate-titanate nanotubes arrays standing normal to the Si subtrates. The observed reflectivity spectra correspond to the resonance dielectric dispersion, which is attributed mainly to the lowest frequency optical phonon typical for bulk PZT. The transverse phonon frequency is lower than in bulk PZT, most probably due to the influence of the substrate.

Vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ northern spring equinox from 2006 to 2008

Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s northern spring equinox in December 2007. The observed reflectance spectra in the Long J (1.17–1.31 μm) and H (1.45–1.65 μm) bands, obtained with the slit aligned along Uranus’ central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6–8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6–8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309–319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624–647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488–489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527–557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus’ atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6–8 bar is unclear, but the absence of the expected methane cloud at 1.2–1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987–15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577–593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527–557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus). The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from ∼1–2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions.

Complex Organic Materials in the HR 4796A Disk?

The red spectral shape of the visible to near-infrared reflectance spectrum of the sharply edged ringlike disk around the young main-sequence star HR 4796A was recently interpreted as the presence of tholin-like complex organic materials which are seen in the atmosphere and surface of Titan and the surfaces of icy bodies in the solar system. However, we show in this Letter that porous grains composed of common cosmic dust species (amorphous silicate, amorphous carbon, and water ice) also closely reproduce the observed reflectance spectrum, suggesting that the presence of complex organic materials in the HR 4796A disk is still not definitive.