Spectral Directional Emissivity Research Articles

Controlling thermal radiation by photonic quantum well structure with zero-averaged-refractive-index gap

Based on Kirchoff's second law, we investigate the spectral-directional thermal emissivity of a photonic quantum well (PQW) structure with a zero-averaged-refractive-index (zero-n¯) gap coating on an absorbing substrate. It is shown that the thermal emission through tunneling modes having frequencies in the zero-n¯ gap is not only weakly dependent on the emission angle and polarization state but also insensitive to the unit-cell size scaling of the barrier photonic crystal. More importantly, the PQW structure promises the frequency-selective thermal emission for both TE and TM polarization states along a well-defined direction. This may result in designing thermal antennas.

Spectral and directional emittance of alumina at 823 K

Spectral–directional emittance measurements of aluminum oxide (99.5% pure), in air, were performed at 823 K using an apparatus comprised of a Fourier Transform Infrared (FTIR) spectrometer, a blackbody radiating cavity (hohlraum), and a sample holder which allows directional measurements. The data cover a wide spectral range between 2 and 25 μm, and a directional range from a surface normal to a 72° polar angle. The aluminum oxide sample used in the experiment had a nominal surface roughness of 1 μm determined by a profilometer. Directional emittance shows no departure from dielectric behavior.

Validation of the Infrared Emittance Characterization of Materials Through Intercomparison of Direct and Indirect Methods

A comparison of the spectral directional emittance of samples as a function of wavelength was performed at the Fourier Transform Infrared Spectrophotometry (FTIS) and the Advanced Infrared Radiometry and Imaging (AIRI) facilities at NIST. At the FTIS, the emittance is obtained indirectly through the measurement of near-normal directional-hemispherical reflectance (DHR) using an infrared integrating sphere. At the AIRI, the normal directional emittance is obtained directly through the measurement of the sample spectral radiance referenced to that from blackbody sources, while the sample is located behind a black plate of known temperature and emittance. On the same setup at the AIRI, the normal emittance at near ambient temperatures is also measured indirectly by a “two-temperature” method in which the sample spectral radiance is measured while the background temperature is controlled and varied. The sample emittance measurements on the comparison samples are presented over a wavelength range of 3.4 μm to 13.5 μm at several near-ambient temperatures and for near-normal incidence. The results obtained validate the two independent capabilities and demonstrate the potential of the controlled background methods for measurements of the radiative properties of IR materials.

Radiation Thermometry and Emissivity Measurements Under Vacuum at the PTB

A new experimental facility was realized at the PTB for reduced-background radiation thermometry under vacuum. This facility serves three purposes: (i) providing traceable calibration of space-based infrared remote-sensing experiments in terms of radiation temperature from −173 °C to 430 °C and spectral radiance; (ii) meeting the demand of industry to perform radiation thermometric measurements under vacuum conditions; and (iii) performing spectral emissivity measurements in the range from 0 °C to 430 °C without atmospheric interferences. The general concept of the reduced background calibration facility is to connect a source chamber with a detector chamber via a liquid nitrogen-cooled beamline. Translation and alignment units in the source and detector chambers enable the facility to compare and calibrate different sources and detectors under vacuum. In addition to the source chamber, a liquid nitrogen-cooled reference blackbody and an indium fixed-point blackbody radiator are connected to the cooled beamline on the radiation side. The radiation from the various sources is measured with a vacuum infrared standard radiation thermometer (VIRST) and is also imaged on a vacuum Fourier-transform infrared spectrometer (FTIR) to allow for spectrally resolved measurements of blackbodies and emissivity samples. Determination of the directional spectral emissivity will be performed in the temperature range from 0 °C to 430 °C for angles from 0° to ±70° with respect to normal incidence in the wavelength range from 1 μm to 1,000 μm.

Cavity-enhanced thermal emission from semiconductor lasers

The thermal emission properties of GaAs-based semiconductor diode lasers are studied with a calibrated setup for quantitative measurements of spectrally resolved emittances. Emission spectra are measured for a stack of high-power diode-laser bars and for n-doped bulk GaAs reference samples and are compared to a calibrated blackbody radiator. The thermal radiation measurements are analyzed with a formalism providing the directional spectral emittance, transmittance, and reflectance under normal incidence. The thermal emission properties are related to the underlying materials properties such as absorption coefficients and carrier concentrations. The thermal emission from the semiconductor lasers displays a pronounced enhancement compared to the bulk reference samples. The metallic contacts and the heavily p-doped layers of the diodes are identified as the origin of this cavity enhancement. The semitransparent nature of the semiconductor materials influences the results of thermal infrared imaging substantially. We derive guidelines for extracting reliable information on device temperatures and discuss limitations of the technique.

Local estimation of directional emissivity of light-scattering volumes using the Monte-Carlo method

A method is described for the calculation of directional emissivity of volumes of arbitrary geometry filled with radiating, absorbing, and scattering medium. The high efficiency of Monte-Carlo simulation method based on local sampling algorithm is demonstrated. The problem of test solutions is discussed, which are used for estimating the Monte-Carlo simulation algorithms. Results are given of calculation of spectral directional emissivity of model jet of a solid-propellant engine.

Temperature-Resolved Infrared Spectral Emissivity of SiC and Pt–10Rh for Temperatures up to 900°C

This article reports the first comprehensive results obtained from a fully functional, recently established infrared spectral-emissivity measurement facility at the National Institute of Standards and Technology (NIST). First, sample surface temperatures are obtained with a radiometer using actual emittance values from a newly designed sphere reflectometer and a comparison between the radiometer temperatures and contact thermometry results is presented. Spectral emissivity measurements are made by comparison of the sample spectral radiance to that of a reference blackbody at a similar (but not identical) temperature. Initial materials selected for measurement are potential candidates for use as spectral emissivity standards or are of particular technical interest. Temperature-resolved measurements of the spectral directional emissivity of SiC and Pt–10Rh are performed in the spectral range of 2–20 μm, over a temperature range from 300 to 900°C at normal incidence. Further, a careful study of the uncertainty components of this measurement is presented.

Full Field Temperature Measurement of Specular Wafers During Rapid Thermal Processing

Many of the processes involved in the creation of semiconductor devices involve high-temperature processing of silicon wafers. The benefits of reduced thermal budget and faster cycle time make rapid thermal processing (RTP) a possible key technology for semiconductor manufacturing. However, the problem of nonuniform wafer temperature has prevented it from further spread among the industry. The first step in developing controls to maintain a uniform wafer temperature is accurate temperature measurement during processing. In this paper, a system was developed to exploit the specular reflectivity of silicon wafers and obtain a measurement of the wafer temperature profile. The spectral reflectivity is determined by measuring the intensity of an incident beam and the beam reflected from the wafer surface. With this measured reflectivity value the spectral-directional wafer emissivity was determined using Kirchhoff's law. The obtained emissivity then was used to calculate the wafer temperature profile from an image obtained with an infrared camera. An experimental study of the transmittance of an undoped silicon calibration wafer at an elevated temperature is also discussed

New experimental device for infrared spectral directional emissivity measurements in a controlled environment

A new experimental device for infrared spectral directional emissivity measurements in a controlled atmosphere is presented. The sample holder, which permits to measure spectral directional emissivity up to 1050K, is placed inside a stainless steel sample chamber that can be evacuated or filled with different gases. The signal detection is carried out by means of a Fourier transform infrared spectrometer. The experimental results focus on the capability of the device to perform emissivity measurements as a function of temperature, emission angle, and in situ surface state evolution. A careful study of the sample temperature homogeneity and the measurement method has been done, including the background radiation, the apparatus response function, and temperature differences between the sample and the blackbody radiator. As a consequence, a compact expression for the sample emissivity that generalizes those previously obtained for the direct radiometric measurement method is found. The error assessment shows that the main contribution to the emissivity uncertainty is related to the sample temperature. The overall uncertainty at intermediate temperature is estimated to be around 3% at short wavelengths. Emissivity measurements of Armco iron were used in order to check the accuracy of the experimental device. The experimental results show an excellent fit with direct emissivity data accessible in the literature, as well as with the theoretical emissivity obtained from the Hagen-Rubens relation.

High temperature, spectral-directional emittance of high purity nickel oxidized in air

Spectral-directional emittance measurements for 99.99% nickel, thermally oxidized in air, were performed at temperatures of 673, 773, and 873 K using an apparatus comprised of a Fourier transform infrared (FTIR) spectrometer, a blackbody radiating cavity (hohlraum), and a sample holder which allows directional measurements. The data cover the spectral range between 2 and 20 μm, and the directional range from a surface normal to 72° polar angle. The Ni sample had a nominal surface roughness of 4.1 μm and was heated for 1 h at the measurement temperatures prior to emission measurements. X-ray diffraction and EDS analyses were performed in order to characterize the sample surface. It was found that the normal emittance of oxidized nickel increases with temperature for the temperature range considered. Directional emittance shows slightly departure from pure metal behavior.

Armco Iron Normal Spectral Emissivity Measurements

Directional spectral emissivity data in different environments are needed in a great number of scientific and technological applications. In this work, the normal spectral emissivity of Armco iron is studied as a function of temperature under a controlled atmosphere. Emissivity values are calculated by the direct radiometric method. The evolution with thermal cycling, the dependence on temperature, and the effect of surface roughness are considered. Additionally, the electrical resistivity is calculated by using the Hagen–Rubens emissivity relation. This work makes progress in the use of Armco iron as an emissivity reference.

Spectral-Directional Emittance of 99.99% Aluminum, Thermally Oxidized Below Its Melting Point

Spectral-directional emittance measurements for 99.99% aluminum, thermally oxidized in air, were performed using a radiometric technique. The apparatus is comprised of a Fourier transform infrared spectrometer and a blackbody-radiating cavity. The sample holder is held on a slotted arc rack, which allows directional measurements from normal to grazing angles. The aluminum sample was heated for an extended period of time (150 h) at high temperature below its melting point prior to performing measurements. The data presented here cover the spectral range between 3 and 14 μm, directional range from surface normal to 72° polar angle, and temperatures from 673 to 873 K. The complex index of refraction is also reduced from emittance data. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and Auger depth profiling were used as surface techniques to characterize the thickness and composition of aluminum oxide film that formed on the metallic surface.

Modeling the infrared radiance of rough sea surface in the infrared windows: 3–5 μm and 8–12 μm

The general formulations for spectral directional emissivity and spectral bi-directional reflectivity are analyzed by using Fresnel's formula and Snell's law in rough sea surface which is simulated by wave facets, whose slopes are changing according to the isotropic Gaussian distribution with respect to surface wind. Then, shadowing effect is taken into account in both emission and reflection of sea surface. On this basis, the mathematic expression is obtained for spectral radiance of rough sea surface, in which the radiance of rough sea surface is considered as a composition of self-emission, reflection of sky and reflection of sun. Finally, calculations for infrared radiance of rough sea surface in bands of 3–5 μm and 8–12 μm are given to illustrate their significant different, which are caused mostly by the dispersion of sun radiation.

In-situ measured spectral directional emissivity of snow and ice in the 8–14 μm atmospheric window

The directional emissivity of snow and ice surfaces in the 8–14 μm thermal infrared (TIR) atmospheric window was determined from spectral radiances obtained by field measurements using a portable Fourier transform infrared spectrometer in conjunction with snow pit work. The dependence of the directional emissivity on the surface snow type (grain size and shape) was examined. We obtained emissivity spectra for five different surface types, i.e., fine dendrite snow, medium granular snow, coarse grain snow, welded sun crust snow, and smooth bare ice. The derived emissivities show a distinct spectral contrast at wavelengths λ = 10.5–12.5 μm which is enhanced with increasing the snow grain size. For example, emissivities at both 10.5 μm and 12.5 μm for the nadir angle were 0.997 and 0.984 for the fine dendrite snow, 0.996 and 0.974 for the medium granular snow, 0.995 and 0.971 for the coarse grain snow, 0.992 and 0.968 for the sun crust, and 0.993 and 0.949 for the bare ice, respectively. In addition, the spectral contrast exhibits a strong angular dependence, particularly for the coarser snow and bare ice, e.g., the emissivity at λ = 12.5 μm for the off-nadir angle of 75° reaches down to 0.927, 0.896, and 0.709 for the coarse grain snow, sun crust, and bare ice cases, respectively. The angular dependent emissivity spectra of the bare ice were quite consistent with the spectra predicted by the Fresnel reflectance theory. The observed results firmly demonstrate that the directional emissivity of snow in the TIR can vary depending upon the surface snow type. The high variability of the spectral emissivity of snow also suggests the possibility to discriminate between snow and ice types from space using the brightness temperature difference in the atmospheric window.

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.

Spectral-Directional Emittance of CuO at High Temperatures

Spectral-directional emittance measurements for cupric oxide (CuO) are presented. The data cover polar angles of 0-84deg from the surface normal, wavelengths between 1.5 and 8μm, and temperatures between 400 and 700°C. The data were generated using a radiometric, direct emission measurement method. The oxide was grown on a very clean, smooth, and mirror-like copper surface, heated in air at 700°C until emission measurements became constant (270h). X-ray diffraction and EDS analyses were performed to characterize the spatial and molecular composition of the copper oxide layer. It is generally found that CuO emittance decreases with increasing polar angle, increases with increasing wavelength, and increases with increasing temperature. Spectral-directional emittance values calculated from the Fresnel relations show good agreement with the measurements up to polar angles of 72deg.

Directional spectral emissivities of clear skies

From the total spectral emissivities determined for clear skies in a preceding paper their directional values are obtained. The Bliss formulation still used for this doing is justified. The results point out the inefficiency of a large band around the horizon for cooling purposes. As an application, a natural focusing system to obtain dew is explained.

Numerical simulation of the cooling of a hot disk rapidly subjected to combined convective and radiant heat losses

The development of a transient method for measuring the spectral directional emissivity of solid materials at high temperature is currently being completed by the analysis of the influence of various sources of error. A hot disk is subjected to mixed convective and radiant heat losses on one face after the rapid withdrawal of a shutter. The cooling is investigated during a short period of observation through numerical simulations of the transient temperature. The thermal problem is simulated using 3D and axisymmetric 2D codes. The progressive withdrawal of the shutter results in a delay for the transient temperature in comparison with the instantaneous withdrawal. For an insulating material, the influence of the side wall boundary conditions remains negligible at the center of the front face. The strong influence of the convective heat transfer is demonstrated.

Study of an Optical Device Used to Homogenize a Laser Beam. Application to Emissivity Measurements on Semitransparent Materials at High Temperature

On an experimental apparatus designed to measure the directional spectral emissivity of semitransparent materials at high temperature (2000°C), a specific optical device (kaleidoscope) is mounted in order to homogenize the energy distribution of a CO2 laser beam, which is used as a heating source. An objective of this work, for operating in the best conditions, is to obtain a uniform temperature of the tested sample. This study, based on Fourier optics, focuses on a square aperture kaleidoscope. A model was developed to simulate the energy distribution of the laser beam, at any transverse plane on the optical path. The final objective is to simulate the energy distribution on the sample surface to optimize the homogenization device taking into account the surface temperature gradient induced by the local energy distribution. To validate this model, quantitative comparisons of theoretical simulations and experimental thermal spots are performed.

Directional Spectral Emittance of a Packed Bed: Influence of the Temperature Gradient in the Medium

Recently a new experimental set-up for measuring the directional spectral emittance has been developed. Both sides of packed opaque spheres are simultaneously heated with identical power laser beams (CO2 10.6 μm) and the isothermal condition in the medium is assumed. In this paper we consider the coupled conduction-radiation equations to investigate the effect of the temperature nonuniformity on the calculated value of the emittance and to verify if the isothermal assumption is valid. Moreover, the sensitivity of the emittance and the temperature profile to the values of parameters (such as the porosity, the face temperatures, and the particle hemispherical reflectivity, etc.) is studied.