Spectral Directional-hemispherical Reflectance Research Articles

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%.

Measurements of spectral emissivity, reflectance and transmittance at high temperatures using laser heating and auxiliary light source

A multi-wavelength measurement method for the high-temperature radiation properties of semi-transparent materials heated by high power laser and irradiated by the infrared source with alternating spectral distributions is introduced. The method avoids the strong dependence of existing methods on accurate temperature measurements or emissivity model of semi-transparent samples. The spectral emissivity, reflectance, transmittance and temperature of semi-transparent samples can be determined simultaneously. Furthermore, we established the experimental device, including a FTIR spectrometer (spectral range of 2.0∼16 μm), a 500 W 915 nm fiber laser, and an infrared SiC emitter irradiation source. The radiation properties of the oxidized stainless-steel sample (opaque materials) and n-type, phosphorous-doped silicon wafer sample (semi-transparent materials) were investigated experimentally at different temperatures (400∼800 °C). With the increase of laser power, the upper limit of sample temperature can be extended to higher temperatures. The measurement results show that the infrared spectral emissivity of stainless-steel increases with the increase of temperature. The spectral emissivity of semi-transparent n-type, phosphorous-doped silicon wafer increases with the increase of temperature, while the spectral directional-hemispherical reflectance decreases with the increase of temperature. The results of temperature and wavelength dependent emissivity of silicon wafer sample are consistent with the literatures, which verifies the applicability of the device. The research work provides a useful reference for measuring the high-temperature radiation properties of semi-transparent materials.

Numerical and experimental determination of optical properties and thermal conductivity of ceramic composites based on fumed silica and silica fiber

The thermal insulation material composed of fiber-reinforced fumed silica (in the raw and opacified with silicon carbide, zirconia or titania powders) was produced and analyzed. Optical properties (scattering and absorption coefficients) were experimentally examined in the spectral range of 0.9 – 4.7 µm by the solution of the inverse problem of radiative transfer. The inverse problem was based on the invariant embedding equation and the measured spectral directional-hemispherical reflectance. The effect of silicon carbide opacifier addition on the optical properties, effective and radiative thermal conductivity was estimated. The calculated radiative thermal conductivity of thermal insulations with various volume fractions of SiC opacifier was compared to the one deduced from the effective thermal conductivity using the calorimetric technique of conductive and radiative components separation. The effective thermal conductivity was measured by the quasi-stationary technique in the temperature range of 373 – 1173 K. The results of the inverse problem solution were compared to the calculations based on the Mie theory and the measured size distribution of silicon carbide particles. The experimental scattering coefficient was shown to be smaller than the calculated one. At the same time, absorption coefficients demonstrated mutual consent. Possible uncertainties in SiC powder size and the complex refractive index of the basic material were considered as a probable explanation of the obtained results. The size distribution of SiC opacifier was numerically optimized to obtain as low Rosseland mean radiative thermal conductivity as possible. The influence of the opacifier chemical composition was analyzed by collating radiative thermal conductivity deduced from the measured effective thermal conductivity to ones calculated using Mie theory. Finally, the effect of Fe2O3, Si3N4, TiO2, graphite, ZrO2, TiN, TiC, AlN opacifiers was numerically analyzed using Mie theory. Compounds based on titanium and an iron sesquioxide were shown to be promising opacifiers for high temperature applications.

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.

Determination of Scattering and Absorption Coefficients of Porous Silica Aerogel Composites

Silica aerogels are porous ultralight materials with exceptional physical properties that are promising materials for thermal insulation applications. This paper theoretically and experimentally investigates the spectral scattering and absorption coefficients of a porous silica aerogel. Silica aerogel samples were prepared with the same compositions and various thicknesses using the sol-gel technique and supercritical drying. The spectral normal-hemispherical transmittances and reflectances of the silica aerogel samples with various thicknesses were measured for wavelengths of 0.38–15 μm. The reflectance and transmittance are higher at short wavelengths than in the infrared region due to the strong scattering and weak absorption at short wavelengths. The thicker samples strongly attenuate the spectral normal-hemispherical transmittance, but have little effect on the spectral normal-hemispherical reflectance. A modified two-flux radiative transfer model was used to analyze the radiation propagation in the silica aerogel with a rough surface morphology and millimeter thicknesses to develop theoretical expressions for the spectral directional-hemispherical reflectance and transmittance. Then, the optical constants, including the absorption coefficient and the scattering coefficient, were determined for wavelengths of 0.38–15 μm based on experimental data by the least-squares algorithm. The results show that when considering the radiation propagation inside the sample, the surface reflection at the air–aerogel interface can be neglected for aerogel thicker than 1.1 mm when the absorbing and scattering effects inside the sample are quite important. The analysis provides valuable data for the optical properties for silica aerogel applications.

Scattering and polarization properties of the scarab beetle Cyphochilus insulanus cuticle.

Optical properties of natural photonic structures can inspire material developments in diversified areas, such as the spectral design of surfaces for camouflage. Here, reflectance, scattering, and polarization properties of the cuticle of the scarab beetle Cyphochilus insulanus are studied with spectral directional hemispherical reflectance, bidirectional reflection distribution function (BRDF) measurements, and Mueller-matrix spectroscopic ellipsometry (MMSE). At normal incidence, a reflectance (0.6-0.75) is found in the spectral range of 400-1600 nm and a weaker reflectance <0.2 in the UV range as well as for wavelengths >1600 nm. A whiteness of W=42 is observed for mainly the elytra of the beetle. Chitin is a major constituent of the insect cuticle which is verified by the close similarity of the measured IR spectrum to that of α-chitin. The BRDF signal shows close-to-Lambertian properties of the beetle for visible light at small angles of incidence. From the MMSE measurement it is found that the beetles appear as dielectric reflectors reflecting linearly polarized light at oblique incidence with low gloss and a low degree of polarization. The measured beetle properties are properties that can be beneficial in a camouflage material.

Caractérisation radiative de liquides volatils semi-transparents dans le domaine infrarouge

The LNE has developed a technique to measure infrared radiative properties of volatile liquids. LNE uses a homemade reflectometer for the measurement of the spectral directional hemispherical reflectance from 0.8 μm to 14 μm and a Fourier transform spectrometer for the measurement of the regular spectral transmittance from 1 μm to 25 μm. For volatile liquids, a tight cell with two infrared windows was constructed. Optical models were developed to calculate the spectral complex refractive index of the liquid from reflectance and transmittance results. Results were validated for water by comparison to data found in literature.

Direct and Indirect Methods for Calculating Thermal Emission From Layered Structures With Nonuniform Temperatures

The determination of emissivity of layered structures is critical in many applications, such as radiation thermometry, microelectronics, radiative cooling, and energy harvesting. Two different approaches, i.e., the “indirect” and “direct” methods, are commonly used for computing the emissivity of an object. For an opaque surface at a uniform temperature, the indirect method involves calculating the spectral directional-hemispherical reflectance to deduce the spectral directional emissivity based on Kirchhoff’s law. On the other hand, a few studies have used a combination of Maxwell’s equations with the fluctuation-dissipation theorem to directly calculate the emissivity. The present study aims at unifying the direct and indirect methods for calculating the far-field thermal emission from layered structures with a nonuniform temperature distribution. Formulations for both methods are given to illustrate the equivalence between the indirect and the direct methods. Thermal emission from an asymmetric Fabry–Pérot resonance cavity with a nonuniform temperature distribution is taken as an example to show how to predict the intensity, emissivity, and the brightness temperature. The local density of states, however, can only be calculated using the direct method.

Validation of MISR land surface broadband albedo

Land surface broadband albedo is a critical variable for many scientific applications. Due to the scarcity of spectral albedo measurements of the Earth's surface environments, it is useful to construct broadband albedo from spectral albedo data obtained by multi‐angle satellite observations. The Multi‐angle Imaging SpectroRadiometer (MISR) onboard NASA's Earth Observing System (EOS) Terra satellite provides land surface albedo products from multi‐angular observations; however, the products have not been comprehensively validated. We convert MISR spectral albedos to total shortwave albedos and validate them using ground measurements at different validation sites. For most surface types, a published narrowband to broadband conversion formula was used, but a new conversion formula for snow and ice covered sites is developed in this study where the spectral range of the instrument is different. Several comparisons are made: (1) between MISR directional‐hemispherical reflectance (DHR) or albedo and MODIS (Moderate Resolution Imaging Spectroradiometer) DHR; and (2) between MISR spectral DHR and bi‐hemispherical reflectance (BHR). The results show that: (1) both the value and the temporal trends of the MISR shortwave albedo and the ground measured shortwave albedo are in good agreement, with the exception of the snow and ice sites; (2) the MISR DHR conforms well to MODIS DHR; and (3) the values of MISR DHR and BHR are nearly identical.

Measurement of Infrared Spectral Directional Hemispherical Reflectance and Emissivity at BNM-LNE

An infrared reflectometer has been designed by BNM-LNE (Bureau National de Metrologie–Laboratoire National d’Essais) to measure the spectral directional hemispherical reflectance of solid materials at ambient temperature. For opaque materials, the spectral directional emissivity can be calculated from the measured reflectance. The reflectance can be measured from 0.8 to 14 μm in five directions with an angle of 12°, 24°, 36°, 48°, and 60° with respect to the normal to the surface of the sample. The optical arrangement to collect the reflected flux is based on the Coblentz arrangement (hemispherical mirror). In fact, four mirrors cut in an hemisphere are used to collect the flux reflected by the sample. This optical arrangement was chosen to limit the angle of incidence of rays on the detector (38° instead of 90° for the Coblentz arrangement). The final expanded uncertainty (level of confidence 95%) of the reflectance is estimated to be about ±0.03 for wavelengths between 0.8 and 10 μm and ±0.04 for wavelengths over 10 μm. The values of the spectral reflectance measured on a black paint and on a white ceramic tile are compared to those measured by the two laboratories PTB (Physikalisch Technische Bundesanstalt) and NIST (National Institute of Standards and Technology). The results validate the measurements performed at BNM-LNE.

Thermal infrared directional emissivity of powdered quartz

Thermal infrared directional emissivity of quartz was measured for several particle size ranges and packing fractions. The samples measured were (1) a polished quartz slab; (2) a 75–250 μm powdered quartz sample, cleansed of clinging fines; (3) a 0–75 μm powdered quartz sample, sifted into a fairy castle structure; and (4) the same sample compressed to minimize porosity. The spectra of the particulate samples showed a strong dependence on exitance angle, particle size and packing fraction. In addition, thermal gradient effects significantly affected the measured emissivity of the fine, sifted sample. The measured directional emissivity was modeled by first using Mie theory to calculate single‐particle scattering properties of a quartz spheres of appropriate size at a single wavelength and radiative transfer theory to calculate the flux reflected from an optically thick, plane‐parallel “atmosphere” composed of particles with these scattering properties. For the 75–250 μm sample, close packing of individual particles was accounted for by subtracting the diffraction contribution to the scattering cross section. These calculations are repeated wavelength‐by‐wavelength to determine the spectral directional hemispherical reflectance of the quartz sample. Kirchhoff s law was then used to obtain spectral directional emissivity. This model, which uses optical constants derived from widely used oscillator parameters for quartz, reproduces the directional emissivity spectrum of the large powdered quartz to better than 10%. However, model calculations for 0–75 μm particle size quartz were less successful. This could be due to the several approximations used in the model, or to a possible error in the oscillator parameters. More successful calculations based on different optical constants suggest that the widely used oscillator parameters may well be wrong.

Measurement of the spectral directional-hemispherical reflectance of materials irradiated perpendicularly to their surfaces

Measurement of the spectral directional-hemispherical reflectance of materials irradiated perpendicularly to their surfaces