Hemispherical Reflectance Measurements Research Articles

Processing and optical behavior of dense (Hf,Zr)B2 solid solutions for solar energy receivers

While individual borides and, more recently, quinary High Entropy Transition Metal Borides have been investigated, the fabrication and characterization of bulk binary to quaternary solid solutions are barely explored. In this work, dense (Hf0.5Zr0.5)B2 is produced by Spark Plasma Sintering (SPS) from powders prepared by Self-propagating High-temperature (SHS). SHS produced a multiphase product, whose secondary phases are fully converted into the desired diborides during the subsequent SPS step. Optical properties of (Hf0.5Zr0.5)B2 are evaluated for the first time with focus on the possible use as novel high-temperature solar thermal absorber, by hemispherical reflectance measurements and calculation of solar absorptance, temperature-dependent spectral selectivity and absorber opto-thermal efficiency at various solar concentration ratios. To optimize the material, a chemically etched surface texture was realized to modify the optical properties. The etched sample showed a higher solar absorptance (0.71) and a lower spectral selectivity than the unetched one, with consequent higher opto-thermal efficiency at all temperatures for solar concentration ratios 1000 ÷ 3000, while at lower concentration ratios and temperatures >1400 ÷ 1600 K, the unetched sample shows the highest efficiency. These results show the promising properties of binary diborides for solar thermal applications.

Two-color pyrometry for backface temperature and emissivity measurement of burning materials

Two-color (2C) pyrometry has long been used for flame temperature and soot concentration studies and is now becoming more widely used to measure backface temperatures of burning materials. With the obvious advantage of being a contact-free method that requires only minimal optical access, 2C pyrometry combined with high-speed acquisition is a promising diagnostic tool to obtain exceptional temporal and spatial resolution of thermally degrading samples. However, its conceptual simplicity relies on a set of basic assumptions that when violated can result in large errors. In this work, we used an experimental configuration representative for fire resistance testing for aerospace and naval applications to analyze the impact of camera parameters and test setup on the accuracy of the surface temperature results obtained. Two types of fiber reinforced polymer composites and a steel plate are used to investigate material specific aspects that affect the measurements. An improved workflow for camera calibration is presented that takes into account the actual experimental setup. The temperature and emissivity mapping obtained through in-situ IR measurements is compared to data acquired through thermocouples and post-fire hemispherical directional reflectance measurements at room temperature. Excellent agreement is found for post-processed 2C pyrometry measurements and TC probe measurements with an uncertainty of 30 °C at 500 °C. In-situ emissivity mappings of burnt specimens are consistent with data from the literature for degraded composite samples and oxidized steel plates. Our results remind the importance of proper post-processing and demonstrate the potentials of 2C pyrometry for fire testing applications. We also present a detailed error budget and suggestions for calibration measurements to improve measurement accuracy.

Laboratory intercomparison of solar absorptance and thermal emittance measurements at room temperature

Solar thermal absorber coatings play an important role in the opto-thermal efficiency of receivers in Concentrated Solar Power (CSP). Two standard figures of merit are the solar absorptance αsol and thermal emittance εth, derived from spectral directional hemispherical reflectance measurements at room temperature. These two figures of merit allow comparing coating formulations in terms of performance and durability. In this study, a black coating and a solar selective coating are optically characterized by different laboratories to compare spectral datasets, solar absorptance αsol and thermal emittance εth calculations. The comparison includes various benchtop spectrophotometers operating in the UV-VIS-NIR and Infrared spectral ranges as well as three commercial portable reflectometers/emissometers. A good agreement is found between the nine parties participating in this intercomparison campaign. The black coating αsol value is 96.6 ± 0.2%, while the solar selective coating αsol value is 94.5 ± 0.4%. For the thermal emittance, spectral data is concatenated and integrated from 0.3 to 16 μm. The black coating εth value calculated at 650 °C is 80.8 ± 3.8%, while the solar selective coating εth value calculated at 650 °C is 25.0 ± 0.5%.

Determination of Spectral and Broadband Albedos in Visible-near Infrared Bands for Different Phenophases of Wheat Using Hemispherical Directional Reflectance Measurements

Albedo is an important parameter in radiation balance, energy balance, atmospheric correction of remote sensing data and weather forecasting models. The values of albedo of agricultural area vary with changes in biophysical parameters at different phenophases of the crop. Spectral and broadband albedo of visible-infrared bands for wheat canopy was estimated with numerical and spectral integration of hemispherical directional reflectance factors (HDRFs). The HDRF measurements on wheat were acquired at 54 view angles (6 azimuthal planes × 9 view zenith angles) to cover the hemispheric span up to 60° view zenith at 7-8 days’ interval during early growth stages to physiological maturity. Canopy and atmospheric parameters were measured parallel to HDRFs of the crop. The spectral and broadband albedo of wheat varies in decreasing pattern with advancement of growth stages. Visible-infrared albedo of crop was maximum at early growth stages (tillering and booting; 0.23) and minimum at late growth stages (late milking and dough; 0.15). The albedo values retrieved from HDRF in this investigation can be useful for phenophase specific applications in cropland fluxes algorithms, crop phenotyping, regional climate modeling and other remote sensing studies.

Application of hydrosilane-free atmospheric pressure chemical vapor deposition of SiOx films in the manufacture of crystalline silicon solar cells

In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup. As SiOx films are deposited at room temperature without complex vacuum systems, they can be a good candidate for the use in commercial c-Si solar cell production lines. The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed. In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells. Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell's electrode on the underlying transparent conducting oxide. In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterized by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.

Parameter estimation and evaluation of Ross-Li and RPV models for wheat phenophases using hemispherical directional reflectance measurements

ABSTRACT Hemispherical directional reflectance factor (HDRF) measurements of wheat crop acquired with field goniometer system are used to estimate parameters and evaluate anisotropic reflectance models. The HDRFs were sampled at 54 view angles (6 azimuthal planes × 9 view zenith angles) to cover the hemispheric span up to 60° zenith. The observations were made at 7–8 days’ interval during different phenophases of the crop. Biophysical parameters were also measured parallel to HDRF. Kernel driven Ross-Li variants and RPV (Rehman Pinty Verstrate) models parameters are generated by fitting HDRF data. RossThin-H/LiSparse-R model was fitted best for all visible-near infrared bands in early phenophases while the RossThick-H/LiSparse-R model was best fitted for heading and flowering phases in most cases. In late grain development phases (Late milking and Dough) of the wheat, HDRFs of visible bands were best fitted with RossThin-H/LiDense-R model in most instances. The HDRF simulation of wheat by RPV models is identical to Ross-Li models. Both models are adequate to account reflectance anisotropy of wheat canopy. Phenophase specific model parameters can be used to reconstruct reflectance anisotropy of wheat for different applications.

Design and Fabrication of Dual-Scale Broadband Antireflective Structures on Metal Surfaces by Using Nanosecond and Femtosecond Lasers.

Antireflective surfaces, with their great potential applications, have attracted tremendous attention and have been the subject of extensive research in recent years. However, due to the significant optical impedance mismatch between a metal surface and free space, it is still a challenging issue to realize ultralow reflectance on a metal surface. To address this issue, we propose a two-step strategy for constructing antireflective structures on a Ti-6Al-4V (TC4) surface using nanosecond and femtosecond pulsed lasers in combination. By controlling the parameters of the nanosecond laser, microgrooves are first scratched on the TC4 surface to reduce the interface reflection. Then, the femtosecond laser is focused onto the sample surface with orthogonal scanning to induce deep air holes and nanoscale structures, which effectively enhances the broadband absorption. The antireflection mechanism of the dual-scale structures is discussed regarding morphological characterization and hemispherical reflectance measurements. Finally, the modified sample surface covered with micro-nano hybrid structures is characterized by an average reflectance of 3.1% over the wavelengths ranging from 250 nm to 2250 nm.

Round Robin Test for the comparison of spectral emittance measurement apparatuses

CSP (Concentrated Solar Power) plants technologies use the concentration of solar energy on a receiver to produce heat and then electricity by a thermodynamical process. A solar absorber material is used to convert the energy carried by light into heat. This type of material works at high temperatures (up to 1000 °C) under a highly concentrated solar flux (up to x1000 or more). Optical properties determine the performance of absorbers and it is thus necessary to measure their spectral absorptance and emittance. Solar absorptance is directly linked to the capacity of the absorber material to convert the solar flux into heat. Emittance drives the radiative thermal losses for the heated absorber and depends on the absorber temperature. The characterization of a material in operational conditions at high temperatures requires advanced apparatuses, and different measurement methods exist for the characterization of these two quantities of relevance regarding an absorber. A Round Robin Test (RRT) was conducted with the objective of comparing different new optical apparatuses and methods for measuring the emittance or luminance of various solar absorbers in air. Measurements were carried out directly at temperatures up to 560 °C while heating the samples, and also indirectly by hemispherical reflectance measurements at room temperature. In this paper, the Round Robin Test procedure to compare apparatuses is described, as well as the corresponding reflectance and emittance results on four types of materials. In addition, a discussion of some factors of influence over high temperature measurements in air and of the observed discrepancies among results from the evaluators is presented. The reliability of reflectance/emittance measurements is also demonstrated and statistics of deviations from the mean value are analysed. These allow us to infer information about measurement reproducibility. The reflectance spectra of all samples after high temperature measurements in air (up to 500 °C) do not show any significant changes.

Geometry of the hemispherical radiometric footprint over plant canopies

Radiometric measurements of hemispherical surface reflectance and long-wave irradiance are required to quantify the broadband albedo and the outgoing thermal radiation. These observations are typically integrated with eddy covariance measurements of sensible and latent heat fluxes to characterize the surface energy budget. While the aerodynamic footprint has been widely investigated, the geometry of the hemispherical radiometric footprint over plant canopies has been rarely tackled. In the present work, the size and shape of the hemispherical radiometric footprint are formalized for a bare surface and in presence of a vegetation cover. For this purpose, four idealized canopies are analyzed and the dependency of the radiometric footprint on leaf area index and canopy height is explored. Besides, the radiometric footprint is compared with the aerodynamic footprint in conditions of neutral stability. It was observed that almost 100% of the hemispherical radiometric signal originates within a distance of a few radiometer heights, while only about 50–80% of the cumulative aerodynamic signal is generated within a distance of about 20 sensor heights. In order to achieve comparable extensions of the footprint areas, hemispherical radiometric measurements should therefore be taken about 6–15 times higher than turbulent flux ones, depending on the vegetation type. The analysis also highlights that the size of the radiative footprint decreases at increasing leaf area index, whereas the aerodynamic footprint shows an opposite behavior. For the abovementioned reasons, this work may support the interpretation of energy flux measurements and the optimal design of eddy covariance stations located in heterogeneous sites.

Radiative Characteristics of High-Porosity Media Containing Randomly Oriented Fibers in Space

This paper proposed radiative characteristics' expressions for media containing randomly oriented fibers in space. In deriving these simple radiative characteristics' expressions, the fibrous medium effective extinction coefficient is defined to match with the one of large particle obtained by combining geometric optics and Fraunhofer diffraction theory. Fibrous media radiative characteristics are then derived as an average over all incident radiation angles of single fiber radiative characteristics. Theoretical hemispherical reflectance and normal transmittance predictions using the proposed fibrous media radiative characteristics based on the Mie theory agreed well with literature experiments. Therefore, media containing fiber randomly oriented in space can be scaled to a suitable equivalent media such that scattering mechanisms behave similarly to that occurring in a participating media containing spherical particles. Numerical investigations show that a theoretical model which assumes Henyey–Greenstein (HG) scattering phase function can conveniently be used for the estimation of equivalent fibrous media radiative characteristics using hemispherical reflectance measurements. On the other hand, the estimated equivalent fibrous media radiative characteristics from hemispherical measurements and using a two-flux model with isotropic scaling radiative characteristics may be subjected to serious errors in the case of semitransparent media for which the absorption is significant.

A method to retrieve the spectral complex refractive index and single scattering optical properties of dust deposited in mountain snow

ABSTRACTDust deposition to snow can have regionally important climatic and hydrologic impacts resulting from direct reduction of surface albedo and indirectly from the initiation of snow albedo feedbacks. Modeling the radiative impacts of dust deposited in snow requires knowledge of the optical properties of both components. Here we present an inversion technique to retrieve the effective optical properties of dust deposited in mountain snow cover from measurements of hemispherical dust reflectance and particle size distributions using radiative transfer modeling. First, modeled reflectance is produced from single scattering properties modeled with Mie theory for a specified grain size distribution over a range of values for the imaginary part of the complex refractive index (k = 0.00001–0.1). Then, a multi-step look-up table process is employed to retrieve kλ and single scattering optical properties by matching measured to modeled reflectance across the shortwave and near infrared. The real part of the complex refractive index, n, for dust aerosols ranges between 1.5 and 1.6 and a sensitivity analysis shows the method is relatively insensitive to the choice of n within this range, 1.525 was used here. Using the values retrieved by this method to update dust optical properties in a snow + aerosol radiative transfer model reduces errors in springtime albedo modeling by 50–70%.

Goniometric and hemispherical reflectance and transmittance measurements of fused silica diffusers

Goniometric and hemispherical reflectance and transmittance measurements of fused silica diffusers

Determination of radiative properties of polyester batting insulation material from hemispherical transmittance and reflectance measurements

This work presents a study on the thermal characterization of semi-transparent polyester batting insulation material for building application. This material is consisting of recycled plastic waste based polyester fibers with constant diameters. Thus, the effective thermal conductivity of this material is determined using fluxmeter devices. The radiative properties of the polyester batting (i.e. albedo, optical thickness, absorptance, etc.) have been obtained for different thicknesses of the material with the inverse method, which is based on infrared experimental measurements of reflection and transmission in the wavelengths range of 2–21μm using a Fourier Transform Infrared Spectrometer. Thus, the radiative conductivity is determined by the Rosseland model. The radiative conductivity varies as a function of material thickness; this variation is due to the fiber density effect.

Using spectral chlorophyll fluorescence and the photochemical reflectance index to predict physiological dynamics

Abstract Observations of terrestrial chlorophyll fluorescence and the Photochemical Reflectance Index ( PRI ) from space have the potential to improve estimates of global carbon exchange. However the relationship between photosynthetic rate and these measurements is complicated by several factors that relate to the dissipation of absorbed light energy via both photochemical (photosynthesis) and non-photochemical pathways. Numerical simulations of physiological remote sensing signals require the coupling of physically-based radiative transfer models with models of physiological dynamics. These schemes provide the quantitative frameworks from which physiological information can be extracted from remote sensing observations of vegetation, helping to resolve the aforementioned complexities. We present such a framework that links physiological fluorescence theory with spectral remote sensing type measurements at the leaf scale. We show how a simple expression can be used to predict the quantum yield of photochemistry (Φ PSII ), a proxy for photosynthetic efficiency, from spectral measurements and modelled non-photochemical quenching ( NPQ ). We tested two alternate models of NPQ ; one process-based ( PHOTOII ) and the other empirical, based on visible region reflectance changes (the PRI ). We used a Monte Carlo Radiative Transfer ( MCRT ) model to retrieve the separated yields of chlorophyll fluorescence from photosystems II and I. Measurements of dynamic spectral fluorescence, the PRI , hemispherical reflectance and transmittance, saturation pulse integrated fluorescence and pigment contents were collected from maple leaves and used to calibrate and validate the modelling framework. Both NPQ models reproduced the observed photochemical and non-photochemical dynamics. Future work is recommended to scale the framework across space, time and species.

Tantalum diboride-based ceramics for bulk solar absorbers

Ultra-high temperature ceramics (UHTCs) are the ideal material for extreme conditions thanks to very high melting points, high refractoriness and good thermo-mechanical properties at high temperatures. This paper reports, for the first time to the best of our knowledge, on the microstructural and optical characterization of several tantalum diboride (TaB2) samples with density values from 67% to full density. Pristine samples have been investigated at room temperature by means of SEM, XRD and spectral hemispherical reflectance measurements. Thermal emittance in the temperature range 1050–1800K has been measured. Structural, compositional and optical properties after high temperature exposure have been characterized as well and property changes have been explained. The obtained results favorably compare TaB2 over conventional solar absorbers for novel solar plants operating at higher temperatures.

Decrease in reflectance of vertically-aligned carbon nanotubes after oxygen plasma treatment

Oxygen plasma treatment was used to lower the reflectance of multiwall vertically-aligned carbon nanotubes (VACNT). Specular reflectance measurements at an incident angle of 65° from normal show a decrease in reflectance over the wavelength range of 250-2500 nm, with up to 5 times lower reflectance at long wavelengths. Total hemispherical reflectance measurements show a decrease in reflectance from 300-2400 nm, with up to 8 times lower reflectance at short wavelengths. Etch rate measurements and helium ion microscopy reveal that the oxygen plasma reduces the reflectance of VACNT by modifying the surface morphology, characterized by etching away the top layer and inducing agglomeration of nanotube tips.

Mid-infrared absorption properties of green wood

There is a lack of quantitative data on the penetration depth and the amount of energy absorbed by green wood under infrared (IR) radiation. This lack of knowledge is a potential barrier to the development of IR heating as an alternative to soaking as a means of warming logs prior to peeling in the manufacture of plywood. Experimental measurements of normal hemispherical spectral reflectance and transmittance over the range 550–5,500 cm−1 wavenumbers on four wood species, beech, birch, Douglas-fir and spruce have brought new knowledge on mid-infrared absorption properties of green wood and removed some uncertainties. For instance, it is not possible to deliver energy deeper than up to 0.3 mm below the wood surface because 70–90 % of all incident IR radiation on the wood surface is absorbed in this layer. Some wood features, such as surface quality, the presence of knots and of free water in wood (the latter two having a more significant effect) influence the amount of energy absorbed. These results illustrate that IR radiation can heat the surface layers, but then heat penetrates deeper into the inside layers of wood by conduction.

Biomimetic broadband antireflection gratings on solar-grade multicrystalline silicon wafers

We report a simple and scalable bottom-up technique for fabricating broadband antireflection gratings on solar-grade multicrystalline silicon (mc-Si) wafers. A Langmuir-Blodgett process is developed to assemble close-packed silica microspheres on rough mc-Si substrates. Subwavelength moth-eye pillars can then be patterned on mc-Si by using the silica microspheres as structural template. Hemispherical reflectance measurements show that the resulting mc-Si gratings exhibit near zero reflection for a wide range of wavelengths. Both experimental results and theoretical prediction using a rigorous coupled-wave analysis model show that close-packed moth-eye arrays exhibit better antireflection performance than non-close-packed arrays due to a smoother refractive index gradient.

Radiative properties of extruded polystyrene foams: Predictive model and experimental results

The radiative properties of extruded polystyrene foams (XPS) were predicted using morphological data and optical properties of bulk medium. A particular attention was paid on the modelling of the radiative properties of struts, which are located at the junctions between cell walls. The radiative properties of XPS foams were determined by adding the contributions of each particle (walls and struts) using the independent scattering hypothesis. Hemispherical reflectance and transmittance measurements were made in addition to total thermal conductivity measurements. Comparisons were made between theoretical and experimental results and some improvements were suggested to better improve modelling of foam radiative properties.

Soil emissivity and reflectance spectra measurements

We present an analysis of the laboratory reflectance and emissivity spectra of 11 soil samples collected on different field campaigns carried out over a diverse suite of test sites in Europe, North Africa, and South America from 2002 to 2008. Hemispherical reflectance spectra were measured from 2.0 to 14 microm with a Fourier transform infrared spectrometer, and x-ray diffraction analysis (XRD) was used to determine the mineralogical phases of the soil samples. Emissivity spectra were obtained from the hemispherical reflectance measurements using Kirchhoff's law and compared with in situ radiance measurements obtained with a CIMEL Electronique CE312-2 thermal radiometer and converted to emissivity using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) temperature and emissivity separation algorithm. The CIMEL has five narrow bands at approximately the same positions as the ASTER. Results show a root mean square error typically below 0.015 between laboratory emissivity measurements and emissivity measurements derived from the field radiometer.