Relative Spectral Emissivity Research Articles

Soot inception in laminar coflow diffusion flames

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.

Reflectance determination of optical spectral emissivity of metal surfaces at ambient conditions

Optical pyrometry requires knowledge of both spectral radiance and spectral emissivity to accurately measure temperature. No reliable compendium of spectral emissivity data currently exists for common metals in the optical and near-infrared spectral regions where pyrometry is typically performed. Here, we measure the spectral reflectivity of numerous common metals, with known surface finishes, between 400 and 1100 nm. The measurements are carried out under ambient conditions, near 300 K at an atmospheric pressure of ∼0.78 bar. We then calculate the emissivity as a function of the wavelength for each sample. We define and use the illumination and collection geometries most practical for applying these measurements to radiometric thermometry. We also examine the effect of surface finish upon the relative spectral emissivity of a given material and find qualitative guidelines for predicting the increase in emissivity with increasing surface roughness. These measurements can both improve the accuracy and precision of optical pyrometry and provide the necessary reference value for converting emerging diagnostic measurements of relative spectral emissivity under extreme dynamic conditions into absolute dynamic spectral emissivity.

Relation between emissivity evolution during annealing and selective oxidation of TRIP steel

Spectral emissivity measurement (λ = 8–12 μm) of TRIP alloy steel during annealing in HNx atmosphere is carried out. Emissivity variation is contributed by the selective oxidation that takes place along with the surface topography from pre-annealed state. Comparing results from physicochemical characterization (Fourier transform infrared spectroscopy, scanning electron microscopy and optical surface profiler) on post-annealed samples and examining the global and relative spectral emissivity separately established correlation of roughness to the former and oxide signatures to the peak features in the latter. Intensity of emissivity peaks at 10 μm and 10.5 μm vary with ratio of Mn to Al. Meanwhile, the influence of local texture on emissivity is demonstrated.

Plasma Measurements of the Fe xvii L-shell Emission and Blending with F viii and F ix

Abstract We measured the L-shell emission spectrum of Fe xvii in a low-density, low-gradient magnetically confined laboratory plasma that contains predominately C, O, Fe, and Ni as trace elements and found excellent agreement with the relative spectral emission obtained in solar and astrophysical observations. However, we obtained spectra that appear to have an usually large Fe xvii resonance transition, which is commonly labeled , from hot plasmas that also contain fluorine. The wavelength of the Lyα feature of F ix is coincident with the wavelength of the Fe xvii line within one part in 538, and its flux, therefore, enhances the Fe xvii resonance line. Moreover, the resonance and forbidden lines of F viii are close to the transitions in Fe xvii and may further alter the inferred apparent Fe xvii line ratios, particularly in spectrometers with moderate spectral resolution. The enhanced emission of line can thus serve as a new spectral diagnostic for the detection of fluorine in astrophysical plasmas.

Determination of true temperature of opaque matter based on its thermal radiation spectrum: Using relative emissivity and the Wien displacement law

Two methods for determining of the true (thermodynamic) temperature via thermal radiation spectrum of an opaque heated object are presented. The first method is based on the relative emissivity. It is shown that in many cases, the range of the true temperature values may be narrowed down using the “convex-concave” criterion. As shown, that any relative spectral emissivity dependence can be approximated by the same parametric model. The second method is based on the use of the Wien displacement law for real materials. This method is effective when the radiation of the object close to the gray-body radiation in the region of the spectral emission maximum. It is shown that these two methods complement each other.

Herschel SPIRE FTS relative spectral response calibration

Herschel/SPIRE Fourier transform spectrometer (FTS) observations contain emission from both the Herschel Telescope and the SPIRE Instrument itself, both of which are typically orders of magnitude greater than the emission from the astronomical source, and must be removed in order to recover the source spectrum. The effects of the Herschel Telescope and the SPIRE Instrument are removed during data reduction using relative spectral response calibration curves and emission models. We present the evolution of the methods used to derive the relative spectral response calibration curves for the SPIRE FTS. The relationship between the calibration curves and the ultimate sensitivity of calibrated SPIRE FTS data is discussed and the results from the derivation methods are compared. These comparisons show that the latest derivation methods result in calibration curves that impart a factor of between 2 and 100 less noise to the overall error budget, which results in calibrated spectra for individual observations whose noise is reduced by a factor of 2-3, with a gain in the overall spectral sensitivity of 23% and 21% for the two detector bands, respectively.

Surface emissivity retrieval from Digital Airborne Imaging Spectrometer data

A study has been carried out on the most recent algorithms for the estimation of land surface emissivity (ε) using high‐resolution data (Digital Airborne Imaging Spectrometer, DAIS) over the Rhine Valley (France) and Castilla La Mancha (Spain). Three published methods have been applied for extracting absolute spectral emissivity information from images recorded during the DAISEX experiment in 1999. They are NDVI Thresholds Method (NDVITHM), Normalized Emissivity Method (NEM) and Temperature/Emissivity Separation (TES). These lather two methods were originally designed to work over geological surfaces. Five methods have been used for extracting relative spectral emissivity. They are temperature‐independent spectral indices method (TISI), reference channel method (REF), emissivity normalization method (NOR), emissivity re‐normalization method (RE), and alpha emissivity method (ALPHA), respectively. NDVITHM and NEM give the same absolute emissivity values with differences between 1% and 0.2% depending on the thermal channel considered, while NDVITHM and TES give the greatest differences, around 2%. The comparison with in situ values shows that NDVITHM gives the best results for vegetation plots, while NEM gives the best ones for bare soil and water plots. The TISI and NOR relative methods give the same relative emissivity values within less than 0.4%, in accordance with Becker and Li's [1990] conclusion as regards the superiority of these methods compared to the others.

Evaluation of Six Methods for Extracting Relative Emissivity Spectra from Thermal Infrared Images

The performance of six published methods for extracting relative spectral emissivity information from thermal infrared multispectral data has been evaluated. In the first part of this article, we recall those six methods and show mathematically that they are almost equivalent to each other. Then, using simulated data for the TIMS (Thermal Infrared Multispectral Scanner) instrument, we analyze the sensitivity of those methods to different sources of error which may occur in real data such as errors due to 1) method simplification, 2) instrumental noise and systematic calibration error, 3) uncertainties on the estimation of downwelling atmospheric radiance, and 4) uncertainties of atmospheric parameters in atmospheric corrections. In terms of resulting errors in relative emissivity, the results show that: a) all methods are very sensitive to the uncertainties of atmosphere. An error of 20% of water vapor in midlatitude summer atmosphere (2.9 cm) may lead to an error of 0.03 (rms) for Channel 1 (worst case) of TIMS. b) The effect of the atmospheric reflection term is very important. If this term is neglected in method development, this may lead to an error of 0.03 (rms) for Channel 1 and midlatitude summer atmosphere. This is the case for the alpha method. c) Instrumental noise commonly expressed by noise equivalent difference temperature (NEΔ T) from 0.1 K to 0.3 K results in an error on relative emissivity ranging from 0.002 to 0.005 for all methods. d) Error on relative emissivity due to the instrumental calibration error (systematic error) is negligible. The study also shows that the relative emissivity derived with deviate atmosphere is linearly related to its actual value derived with correct atmospheric parameters. Based on this property, we propose three methods to correct for the errors caused by atmospheric corrections under horizontally invariant atmospheric conditions. A practical analysis with the real TIMS data acquired for Hapex-Sahel experiment in 1992 supports the results of this simulation.

Evaluation of Different Methods for Extracting Relative Spectral Emissivity Information from Simulated Thermal Infrared Multispectral Scanner Data

The performance of five published methods for extracting relative spectral emissivity information from thermal infrared multispectral data has been evaluated. In the first part of this article, we have recalled those five methods and shown mathematically that they are almost equivalent to each other. Then, using simulated data built up with the TIMS (Thermal Infrared Multispectral Scanner) instrument, we have analyzed the sensitivity of those methods to different sources of error which may occur in real data such as errors due to 1) method simplification, 2) instrumental noise and systematic calibration error, 3) uncertainties on the estimation of downwelling atmospheric radiance, and 4) uncertainties of atmospheric parameters in atmospheric corrections. In terms of resulting errors in relative emissivity, the results show that: a) All methods are very sensitive to the uncertainties of atmosphere. An error of 20% of water vapor in midlatitude summer atmosphere (2.9 cm) may lead to an error of 0.03 (rms) for Channel 1 of TIMS. b) The effect of the atmospheric reflection term is very important. If this term is neglected in method development, this may lead to an error of 0.03 (rms) for Channel 1 and midlatitude summer atmosphere. This is the case for the alpha method. c) Instrumental noise commonly expressed by noise equivalent difference temperature (NEΔT) from 0.1 K to 0.3 K results in an error of relative emissivity ranging from 0.002 to 0.005 for all methods. d) Error in relative emissivity due to the instrument calibration error (systematic error) is negligible. The study also shows that the relative emissivity derived with an incorrect atmosphere is linearly related to its actual value derived with correct atmospheric parameters. Based on this property, we propose three methods to correct for the errors caused by atmospheric corrections under horizontally invariant atmospheric conditions. A practical analysis with the real TIMS data acquired for Hapex-Sahel experiment in 1992 supports the results of this simulation.

Approximation function of spectral irradiance of standard lamps

Band filter radiometry has recently become popular. Its application for detector-based spectral-irradiance measurements requires improved characterization of standard lamps. This paper presents an approximation function for the spectral irradiance of NPL/GEC and FEL lamp data, reported in the 1990 international comparison measurements of the Consultative Committee for Photometry and Radiometry (CCPR). Characterization with a minimum number of parameters, applicable within the visible spectral range, is attempted. Since the spectral emissivity of tungsten is nearly linear in the spectral range from 400 nm to 900 nm, the irradiance of the standard lamps was approximated with three parameters, representing a linear polynomial multiplied by the Planck function. The same linear polynomial was used for each lamp of the same type, and was fitted only for temperature and intensity. The average difference between the measured and the approximated data was found to be less than 0.085 % for the NPL/GEC lamps and less than 0.095 % for the FEL lamps. This means that the relative spectral emissivity of each lamp of the same type is practically the same. The temperature fitting was characterized by the average difference between the measured and approximated data in the ultraviolet and infrared spectral ranges.

Temperature-independent spectral indices in thermal infrared bands

In order to perform spectral analysis in the thermal infrared bands, temperature-independent thermal infrared spectral indices (TISI) are derived from observable thermal infrared radiances. Being temperature-independent and simply related to spectral emissivities of the observed surface, these indices are as easy to use to perform spectral analysis as reflectances in the visible and near infrared spectral domains. Examples of such indices are given for TIMS and AVHRR. Several properties of these indices are discussed, particularly their sensitivity to spectral emissivity variation, their efficient combination, and the derivation of relative spectral emissivity. These indices are quite complementary to NDVI and can lead to stronger results if used together rather than separately. For instance, in the examples worked out, these indices are more sensitive to bare soils characteristics than NDVI. Furthermore, these indices can be tailored to weight certain bands more heavily than others, giving to these indices a wide range of application. They may, for instance, be used for geologic compositional mapping. Practical examples from TIMS and AVHRR data are shown and briefly discussed with a possible applications to the determination from AVHRR data of the surface temperature and surface emissivity.

Excitation temperature, degree of ionization of added iron species, and electron density in an exploding thin film plasma

Time and spacially resolved spectra of a cylindrically symmetric exploding thin film plasma were obtained with a rotating mirror camera and astigmatic imaging. These spectra were decouvolved to obtain relative spectral emissivity profiles for nine Fe(II) and two Fe(I) lines. The effective (electronic) excitation temperature at various positions in the plasma and at various times during the first current halfcycle was computed from the Fe(II) emissivity values using the Boltzmann graphical method. The Fe(II)/Fe(I) emissivity ratios together with the temperature were used to determine the degree of ionization of Fe. Finally, the electron density was estimated from the Saha equilibrium. Electronic excitation temperatures range from 10,000–15,000 K near the electrode surface at peak discharge current to 7000–10,000 K at 6–10 mm above the electrode surface at the first current zero. Corresponding electron densities range from 10 17-10 18 cm −3 at peak current to 10 15-10 16cm −3 near zero current. Error propagation and criteria for thermodynamic equilibrium are discussed.

Absolute radiation dosimetry in photochemotherapy

The construction and properties of a detector which can be used to measure absolute irradiance from extended UV sources are described. It is shown how the use of this detector coupled with a knowledge of the relative spectral emission from a photochemotherapy unit can lead to accurate estimation of absolute spectral irradiance incident upon a patient's skin.

Spectral composition of the light of the Lantern-fish,Myctophum punctatum

Relative spectral emission curves are available for the lights of many invertebrate animals, but none exists for fish. It seems likely that many bony fish use their photophores to signal to one another. Information about the spectral composition of fish luminescence is desirable to permit comparisons with the spectral sensitivities of fish eyes, to enable calculations to be made of luminous intensities, and to allow estimates to be made of the rate of attenuation of such lights in sea water. To further these ends, measurements were made of the spectral composition of the luminescence of the lantern-fish,Myctophum punctatum.