Hemispherical Reflectance Research Articles

Factors influencing starch accumulation in microalga Chlorella sorokiniana

The spectral absorption cross-section of microalgae is an essential parameter in modeling the microalgae metabolism and growth kinetics as well as in estimating the productivity and efficiency of photobioreactors. This paper presents a simple experimental procedure for retrieving the average spectral absorption cross-section of concentrated microalgal suspensions. The method combines experimental measurements of the normal–hemispherical transmittance and reflectance of the suspensions in conventional cuvettes with an inverse method and analytical expressions obtained from the modified two-flux approximation accounting for absorption and multiple scattering. The method was validated with direct measurements of the scattering phase function and of the absorption and scattering cross-sections of freshwater microalgae Chlorella vulgaris. It was able to retrieve the absorption cross-section with acceptable accuracy. The latter was then used to estimate successfully the fluence rate using a simplified light transfer model, the local rate of photon absorption, and the biomass productivity in flat plate PBRs.

Near-infrared optical properties of a porous alumina ceramics produced by hydrothermal oxidation of aluminum

The measured spectral normal–hemispherical reflectance of an optically thick sample of porous alumina ceramics produced by hydrothermal oxidation of aluminum and subsequent high-temperature treatment of boehmite (or böhmite), AlO(OH), is used in a combination with the published data for absorption coefficient of alumina to retrieve the near-infrared optical properties of the alumina ceramics at both room and elevated temperatures. The spectral emissivity of porous alumina ceramics is also determined. An approximate model based on the Mie theory for single grains is suggested to estimate the transport scattering coefficient and the radiative conductivity of the material at high temperatures.

Mathematical framework for predicting the thermal behaviour of spectrally selective coatings within an industrial near-infrared furnace

AbstractA transient finite difference thermal model based on the heat equations is developed, valid for spectrally selective surface coatings on any substrate material within a near-infrared (NIR) furnace. Spectral radiative heat transfer equivalent to a blackbody provides the heat source. Both radiative and natural convective cooling are accounted for. A Monte Carlo ray tracing algorithm is formulated and used to determine the radiation view factor. The variance of the algorithm in relation to mesh resolution and sample size is tested against published exact solutions. The radiative flux is divided into absorbed and reflected bands using hemispherical reflectance spectra measured within the 250–15,000 nm wavelength range, enabling the model to predict the thermal build-up of coatings with very different radiative properties. Results show that the transient temperature distribution of spectrally selective surface coatings within an NIR furnace can be modelled, with good agreement observed between experime...

72-1:Invited Paper: Reflection Measurements and Uncertainties Using Sampling Spheres on Flat, Convex, and Concave Displays

A sampling sphere was designed to measure hemispherical diffuse reflection on flat and curved displays. Interchangeable sample ports eliminate the gap between sample port and convex or concave displays. We investigate the measurement uncertainties by comparing sampling spheres to a large integrating sphere, flat to curved display shapes, and results from different labs. The hemispherical diffuse reflectance of flexible e-paper does not significantly depend on curvature.

Simultaneous retrieval of multiparameters in a frequency domain radiative transfer problem using an improved pdf-based aco algorithm

ABSTRACTInspired by the particle swarm optimization algorithm, an improved ant colony optimization algorithm based on probability density function (IPDF-ACO) was developed for solving the inverse problem of frequency domain radiative transfer in a one-dimensional participating medium. The plural hemispherical reflectance and transmittance simulated by the finite-volume method served as input of the inverse analysis. The extinction coefficient, scattering albedo, and asymmetry parameter were retrieved simultaneously by using the amplitude and phase information as measured values. Through five test cases, the IPDF-ACO proved to be more accurate and efficient than the basic PDF-ACO even with noisy data.

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.

Compositional dependence of optical properties of zirconium, hafnium and tantalum carbides for solar absorber applications

Besides ultra-refractoriness and favorable mechanical and chemical characteristics, carbides of early transition metals show intrinsic spectral selectivity, making them appealing for high-temperature solar absorber applications. However these kinds of ceramics can be produced using many processing methods resulting in different compositions, density and surface finishing. Thus the present work reports on the systematic study of microstructural, mechanical and optical properties of dense zirconium, hafnium and tantalum carbides as a function of the sintering method (high pressure or pressureless), implying use of 10 or 20vol% of MoSi2 as sintering aid. The spectral hemispherical reflectance of Zr-, Hf- and Ta-carbides has been measured in the 0.25–16.5μm wavelength range and correlated to the surface microstructure and roughness. Room and high temperature fracture strength has been measured as well.

Simple method for measuring the spectral absorption cross-section of microalgae

The spectral absorption cross-section of microalgae is an essential parameter in modeling the microalgae metabolism and growth kinetics as well as in estimating the productivity and efficiency of photobioreactors. This paper presents a simple experimental procedure for retrieving the average spectral absorption cross-section of concentrated microalgal suspensions. The method combines experimental measurements of the normal–hemispherical transmittance and reflectance of the suspensions in conventional cuvettes with an inverse method and analytical expressions obtained from the modified two-flux approximation accounting for absorption and multiple scattering. The method was validated with direct measurements of the scattering phase function and of the absorption and scattering cross-sections of freshwater microalgae Chlorella vulgaris. It was able to retrieve the absorption cross-section with acceptable accuracy. The latter was then used to estimate successfully the fluence rate using a simplified light transfer model, the local rate of photon absorption, and the biomass productivity in flat plate PBRs.

Estimation of radiative properties of thermal protective clothing

Thermal protective clothing provides more safety and time to allow wearer to complete task or escape from external high heat or fire exposures. Radiation heat transfer is significant in such high temperature conditions. In the present work, radiative properties of various fabrics used for thermal protective clothing are estimated. Experimentally measured spectral directional–hemispherical reflectance and transmittance data available from literature are used to predict some of the radiative properties. A coupled finite volume radiative transfer equation solver along with genetic algorithm is used for this purpose. Radiative properties of some commonly used fabrics in thermal protective clothing at various heat source temperatures are predicted. Effects of pyrolysis on these properties are also analyzed. It is found that the extinction coefficient of the outermost layer of thermal protective clothing is very high as compared to the other inner layers, and it plays a significant role in blocking heat transfer to the skin. Scattering in fabrics used for thermal protective clothing is also found significant.

A physically-based model for retrieving foliar biochemistry and leaf orientation using close-range imaging spectroscopy

Abstract Radiative transfer models have long been used to characterize the foliar content at the leaf and canopy levels. However, they still do not apply well to close-range imaging spectroscopy, especially because directional effects are usually not taken into account. For this purpose, we introduce a physical approach to describe and simulate the variation in leaf reflectance observed at this scale. Two parameters are thus introduced to represent (1) specular reflection at the leaf surface and (2) local leaf orientation. The model, called COSINE (ClOse-range Spectral ImagiNg of lEaves), can be coupled with a directional–hemispherical reflectance model of leaf optical properties to relate the measured reflectance to the foliar content. In this study, we show that, when combining COSINE with the PROSPECT model, the overall PROCOSINE model allows for a robust submillimeter retrieval of foliar content based on numerical inversion and pseudo-bidirectional reflectance factor hyperspectral measurements. The relevance of the added parameters is first shown through a sensitivity analysis performed in the visible and near-infrared (VNIR) and shortwave infrared (SWIR) ranges. PROCOSINE is then validated based on VNIR and SWIR hyperspectral images of various leaf species exhibiting different surface properties. Introducing these two parameters within the inversion allows us to obtain accurate maps of PROSPECT parameters, e.g., the chlorophyll content in the VNIR range, and the equivalent water thickness and leaf mass per area in the SWIR range. Through the estimation of light incident angle, the PROCOSINE inversion also provides information on leaf orientation, which is a critical parameter in vegetation remote sensing.

A New Method for the Estimation of Broadband Apparent Albedo Using Hyperspectral Airborne Hemispherical Directional Reflectance Factor Values

The broadband albedo values retrieved from satellite sensors are usually compared directly to ground measurements. Some authors have noted the necessity of high spatial resolution albedo estimates to fill the gap between ground measurements and satellite retrievals. In this respect, hyperspectral airborne data with high spatial resolution is a powerful tool. Here, a new operational method for the calculation of airborne broadband apparent albedo over the spectral range of 350–2500 nm is presented. This new method uses the Hemispherical Directional Reflectance Factor (HDRF) as a proxy for the narrowband albedo, assuming a Lambertian approximation. The broadband apparent albedo obtained is compared to that estimated using theapparent albedo equation devised for the Moderate Resolution Imaging Spectroradiometer (MODIS). Airborne data were collected using the Airborne Hyperspectral Scanner (AHS). Field data were acquired at three sites: a camelina field, a green grass field, and a vineyard. The HDRF can be used to approximate the narrowband albedo for all View Zenith Angle (VZA) values for flights parallel to the solar principal plane (SPP); for flights orthogonal to the SPP, discrepancies are observed when the VZA approaches −45°. Root Mean Square Error (RMSE) values in the range 0.009–0.018 were obtained using the new method, improving upon previous results over the same area (RMSEs of 0.01–0.03). The relative error in the albedo estimation using the new method is 12% for −36.2° < VZA < 40.8° in the case of flights parallel to the SPP and less than 15% for −13° < VZA < 45° and 45% for VZA = −45° for flights orthogonal to the SPP. The good performance of the new method lies in the use of the at-surface solar irradiance and the proposed integration method.

Spectrally selective CuS solar absorber coatings on stainless steel and aluminum

CuS and ZrO2/CuS thin films have been deposited at 200 °C in ambient atmosphere by spray pyrolysis onto stainless steel S304 and aluminum AA1050 sheets in order to be used as prospective solar absorbers for thermal energy harvesting. Zirconia has been deposited as a top layer working as an antireflection coating increasing the solar absorption and protecting the underlying copper sulfide layer, which works as the solar light absorbing material. Solar selectivity of the samples was evaluated measuring the total hemispherical reflectivity in the ultraviolet‐visible infrared range and calculating solar absorptance and thermal emittance. The solar selectivity was optimized by tuning the film thickness of the coating stack. X‐ray diffraction, SEM and X‐ray photoelectron spectroscopy analyses have been used to proof crystal phase, coverage, film thickness, and surface contamination of optimized samples, revealing that the copper sulfide film is CuS covellite and the optimized film thicknesses are in the range of 50 nm for the ZrO2 top layer and 140 nm for the CuS layer. Copyright © 2016 John Wiley & Sons, Ltd.

Modeling the appearance of metal‐like packaging printing

AbstractA study of the agreement between a visual assessment and the measurement of the reflectivity of printed metallic inks was performed. A psychophysical property for this percept was identified as visual brilliance. The visual judgments of a series of printed panels were compared with measurements from various types of specular reflectance instruments. The results showed that gloss does not linearly follow the appearance of highly reflective specular surfaces. It was observed that the visual assessment of brilliance was highly correlated to the normalized hemispherical diffuse reflectance factor difference. Further, visual brilliance could be mapped to a logarithmic function of the luminous specular reflectance factor and the measurement scale of log(Y) agreed with the scale of hemispherical diffuse reflectance. It was, thus, concluded that either of these measures may be utilized to establish aims and tolerances for production control of the reproduction of foil‐like prints. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 38–49, 2017

Determination of the spectral complex refractive indices of microalgae cells by light reflectance-transmittance measurement

Two inverse models, namely as single and double-layer inverse models, are developed on the basis of a light scattering measurement technique to determine the spectral complex refractive indices of microalgae from the total hemispherical reflectance and transmittance measured in-situ. Combined with the Mie theory, a direct problem is solved by the finite volume method with the forward simulation of radiative transfer equation under the incidence of a collimated laser. An inverse problem is solved by an improved Quantum-behaved Particle Swarm Optimization (IQPSO) algorithm. The IQPSO algorithm shows a faster convergence speed and a higher accuracy within a smaller number of generations in retrieving the spectral complex refractive indices than the standard Particle Swarm Optimization (PSO) algorithm and the QPSO algorithm. The spectral complex refractive indices of Chlamydomonas reinhardtii at different wavelengths are estimated on the basis of the IQPSO algorithm with the single-layer inverse model. The influence of measurement errors on the inverse results is also investigated. Results indicate that IQPSO is a potential and effective optimal algorithm to estimate the spectral complex refractive indices of microalgae. The estimated results of the imaginary part of the refractive indices demonstrate a higher accuracy than those of the real part, especially with noisy measurement data. To improve the accuracy of single-layer estimated results, a double-layer inverse model is proposed. The investigation reveals that the double-layer inverse model can obtain more accurate results than the single-layer inverse model, especially with random measurement errors, which is more important for directing the experiment of retrieving optical refractive index of microalgae in the further research.

Optical indices and transport scattering coefficient of pyrolytic boron nitride: a natural thermal barrier coating for solar shields

Absorption and scattering properties of pyrolytic boron nitride (pBN) have been characterized by infrared spectroscopy. The strong dielectric anisotropy predicted by first principles calculations is confirmed by measurements performed on a highly oriented pBN sample. Optical properties of textured samples elaborated by chemical vapor deposition were identified from normal hemispherical reflectance and transmittance spectra by applying modified two-flux and four-flux transport models. It is also shown that coating carbon–carbon composites used to build solar shields with a pBN layer having an optimal thickness could improve the protection performance.

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.

The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2‐ES coupled climate model

AbstractThe Earth's hemispheric reflectances are equivalent to within ± 0.2 Wm−2, even though the Northern Hemisphere contains a greater proportion of higher reflectance land areas, because of greater cloud cover in the Southern Hemisphere. This equivalence is unlikely to be by chance, but the reasons are open to debate. Here we show that equilibrating hemispheric albedos in the Hadley Centre Global Environment Model version 2‐Earth System coupled climate model significantly improves what have been considered longstanding and apparently intractable model biases. Monsoon precipitation biases over all continental land areas, the penetration of monsoon rainfall across the Sahel, the West African monsoon “jump”, and indicators of hurricane frequency are all significantly improved. Mechanistically, equilibrating hemispheric albedos improves the atmospheric cross‐equatorial energy transport and increases the supply of tropical atmospheric moisture to the Hadley cell. We conclude that an accurate representation of the cross‐equatorial energy transport appears to be critical if tropical performance is to be improved.

Thermal emissivity of avian eggshells

The hypothesis has been tested that evolution has resulted in lower thermal emissivity of eggs of birds breeding openly in cold climates than of eggs of birds that nest under protective covering or in warmer climates. Directional thermal emissivity has been estimated from directional–hemispherical reflectance spectra. Due to several methodological difficulties the absolute emissivity is not accurately determined, but differences between species are obvious. Most notably, small waders of the genus Calidris, breeding in cold climates on the tundra, and in most cases with uniparental nest attendance, have low directional emissivity of their eggshells, about 0.92 when integration is carried out for wavelengths up to 16μm. Species belonging to Galloanserinae have the highest directional emissivity, about 0.96, of their eggs. No differences due to climate or breeding conditions were found within this group. Eggs of most other birds tested possess intermediate emissivity, but the values for Pica pica and Corvus corone cornix are as low as for Calidris. Large species-dependent differences in spectral reflectance were found at specific wavelengths. For instance, at 4.259μm the directional–hemispherical reflectance for galliforms range from 0.05 to 0.09, while for Fratercula arctica and Fulmarus glacialis it is about 0.3. The reflection peaks at 6.5 and 11.3μm due to calcite are differentially attenuated in different species.In conclusion, the hypothesis that evolution has resulted in lower thermal emissivity of bird eggs being exposed in cold climates is not supported by our results. The emissivity is not clearly related to nesting habits or climate, and it is unlikely that the small differences observed are ecologically important. The spectral differences between eggs that nevertheless exist should be taken into account when using infrared thermometers for estimating the surface temperature of avian eggs.

Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties.

Silicon nanowire possesses great potential as the material for renewable energy harvesting and conversion. The significantly reduced spectral reflectivity of silicon nanowire to visible light makes it even more attractive in solar energy applications. However, the benefit of its use for solar thermal energy harvesting remains to be investigated and has so far not been clearly reported. The purpose of this study is to provide practical information and insight into the performance of silicon nanowires in solar thermal energy conversion systems. Spectral hemispherical reflectivity and transmissivity of the black silicon nanowire array on silicon wafer substrate were measured. It was observed that the reflectivity is lower in the visible range but higher in the infrared range compared to the plain silicon wafer. A drying experiment and a theoretical calculation were carried out to directly evaluate the effects of the trade-off between scattering properties at different wavelengths. It is clearly seen that silicon nanowires can improve the solar thermal energy harnessing. The results showed that a 17.8 % increase in the harvest and utilization of solar thermal energy could be achieved using a silicon nanowire array on silicon substrate as compared to that obtained with a plain silicon wafer.

Carbon nanotube net as a conductive and transparent film for solar energy conversion

Vertically aligned silicon nanowires arrays have been grown through a metal-assisted chemical etching method, giving a heavily absorbing surface. Over this surface, a transparent and conductive net of carbon nanotubes has been formed by chemical vapour deposition. The optical characterisation of the net has been performed by the study of its hemispherical reflectance, and the electrical properties have been obtained by four-point probe method. A high transmittance of the net (over 99%) in the 300-900 nm range is reported. Also, a good sheet resistance value has been obtained (around 3 kΩ/□) for such a thin carbon nanotube net.