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Report Title: Normal Spectral Emittance Measurements (600–2000 nm) of Polycrystalline Alumina (PCA) at High Temperatures
Author(s) L. Bigio, of the Component Ceramics Laboratory
GE Report Number: 99CRD128
Date: November 1999
Number of Pages: 8
Key Words: spectral emittance, 600-2000 nm, polycrystalline alumina, PCA, high temperatures, fiber optic radiance probe
“A novel method is shown for measuring the spectral emittance of polycrystalline alumina (PCA) in the temperature range ~1100-1400 C, from 600-2000 nm. The method utilizes a CO2 laser at 10.6 microns to heat a ~4 mm size spot on a small sample (~6×4 mm) from one side, while the temperature and emission are measured optically from the other side. A fiber optic radiance probe focuses on a 2 mm spot in the middle of the larger heated region, and directs the collected emission to a spectrometersystem. Radiance measurements are conducted relative to a blackbody at 1000 C. The temperature is measured at the same location viewed by the radiance probe using a thermal imaging pyrometer with a narrow bandpass at 10.6 microns. Since this is the same wavelength as the laser, care must be taken to avoid the scattered laser radiation that partially overfills the sample. An emittance at 10.6 microns of 0.97 is assumed for the temperature measurements. In this spectral region, the emittance is found to decrease with increasing wavelength and increase with increasing temperature. This method can be used on other ceramic materials as well.”
Here’s a summary of what the Field Guide is all about in the words of its authors:
“Because of the rapid advance of airborne and satellite sensor technology in providing higher spectral resolution over progressively broader wavelength regions, there is a need for more (and more accurate) field measurements to complement overhead data. The purpose of this field guide is to facilitate such ground-based measurements, first through a review of the environmental factors affecting such measurements, second through an evaluation of the instrumentation involved, and third through a suggested approach to the measurement process.
“In evaluating environmental factors affecting spectral measurements in the field, the sources of radiance from a target are discussed in both the reflectance and emittance regions of the spectrum, as well as how those sources are modified by atmospheric attenuation and scattering, and the presence of clouds and wind.
“Another factor affecting all spectral measurements in the field is the computer typically used for instrument control and data storage. Computers tend to be the universal weak link in field spectrometers, because of their typical low tolerance for bright sunlight, temperature extremes, windblown dust, and rain. Various solutions to the computer problem are discussed, including the acquisition of hardened computers.
“The most commonly used field spectrometers are described, with advice on how to get the most out of each instrument. Then the pros and cons of each instrument are discussed with regard to different applications.
“Finally, how to approach field measurements is described, beginning with a thorough testing of a field instrument (and the field instrument user) in the laboratory. Approaches to data collection, record keeping, data reduction, and data analysis are discussed. A major conclusion is that much greater support for data analysis is necessary to reach the full potential of spectroscopic remote sensing for target identification”.
The absorption features of sulfate (gypsum and anhydrite) and phosphate (apatite) result from vibrations of the S-O and P-O bonds in the sulfate and phosphate anions, respectively. The strong ionic bonding of the chlorides inhibits independent vibration between the individual diatomic pairs (e.g., Na and Cl) but rather requires the entire crystal lattice to vibrate as a whole.
This figure is from Lane, M.D. and P.R. Christensen, Thermal infrared emission spectroscopy of salt minerals predicted for Mars, Icarus, 135, 528-536, 1998.
CLICK ON IMAGE TO ENLARGE –SOURCE: Arizona State University website: www.mars.asu.edu/~lane/sulfphoschl.html
“The InfraRed Sea Surface Emissivity (IRSSE) model was developed for use in the Global Data Assimilation System (GDAS) at NCEP/EMC. Previously, the GDAS used an IRSSE model based on Masuda et al (1988).
“The Masuda model doesn’t account for the effect of enhanced emission due to reflection from the sea surface (only an issue for larger view angles) and the implementation was based on coarse spectral resolution emissivity data making its application to high resolution instruments, such as AIRS, problematic.
“The emissivity spectra are computed assuming the infrared sensors are not polarised and using the data of Hale for the refractive index of water, Segelstein (1981) for the extinction coefficient, and Friedman (1969) for the salinitiy/chlorinity corrections.
“Instrument spectral response functions (SRFs) are used to reduce the emissivity spectra to instrument resolution. These are the quantities predicted by the IRSSE model.”
A Comparison of Laser Polarimetry and Integrating Sphere Reflectometry: Proceedings of the Fifteenth Symposium on Thermophysical Properties, Part I
Authors: Seifter A.1; Boboridis K.2; Obst A.W.2
Both integrating sphere reflectometry (ISR) as well as laser polarimetry have their advantages and limitations in their ability to determine the normal spectral emissivity of metallic samples. Laser polarimetry has been used for years to obtain normal spectral emissivity measurements on pulse-heated materials. The method is based on the Fresnel equations, which describe reflection and refraction at an ideally smooth interface between two isotropic media. However, polarimetry is frequently used with surfaces that clearly deviate from this ideal condition. Questions arise with respect to the applicability of the simple Fresnel equations to non-specular surfaces. On the other hand, reflectometry utilizing integrating spheres provides a measurement of the hemispherical spectral reflectance, from which the normal spectral emissivity can be derived. ISR provides data on spectral-normal-hemispherical reflectance and, hence, normal spectral emissivity for a variety of surfaces. However, the resulting errors are minimal when both the sample and the reference have a similar bidirectional reflectance distribution function (BRDF). In an effort to explore the limits of polarimetry in terms of surface roughness, room temperature measurements on the same samples with various degrees of roughness were performed using both ISR and a laser polarimeter. In this paper the two methods are briefly described and the results of the comparison are discussed.
Keywords: emissivity; integrating sphere; laser polarimetry; reflectometry; rough surfaces; roughness
Document Type: Research article
Affiliations: 1: Los Alamos National Laboratory, Physics Division (P-23), MS H803, Los Alamos, New Mexico 87545, U.S.A., Email: firstname.lastname@example.org 2: Los Alamos National Laboratory, Physics Division (P-23), MS H803, Los Alamos, New Mexico 87545, U.S.A.