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
Source: International Journal of Thermophysics, Volume 25, Number 2, March 2004 , pp. 547-560(14)
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: email@example.com 2: Los Alamos National Laboratory, Physics Division (P-23), MS H803, Los Alamos, New Mexico 87545, U.S.A.
The Heat Island Group at Lawrence Berkeley National Laboratory, Berkeley, CA have measured solar reflectance of roofing samples with an UV-VIS-NIR Spectrometer with an integrating sphere and they measured the spectral emittance of the samples with a FTIR Spectral Emissometer. The following writeup and graphs are from their webpageat eetd.lbl.gov/HeatIsland/CoolRoofs/Samples.html
“Below are examples of complete reflectance and emittance data for several metal roofing samples made of cool roofing materials. These measurements show examples of complete laboratory information needed to determine radiative heat exchange by a roof which, in turn, can be used to estimate peak roof temperatures.
“The spectral solar reflectance is the total reflectance (diffuse and specular) as a function of wavelength, across the solar spectrum (wavelengths of 0.3 to 2.5 µm). It is used to compute the overall solar reflectance, using a standard solar spectrum as a weighting function. It also contains the information in the visual range (0.4 to 0.7 µm) which is sufficient to compute the color coordinates for color matching with other materials.
“The spectral thermal emittance (the graphs on the right) contains the information for computing the overall thermal emittance, using a blackbody curve as the weighting function. The spectral range is about 5 to 40 µm. If the spectral thermal emittance is approximately a horizontal line (a “gray” body), then the overall emittance is adequate for computing longwave radiative radiative exchange between the roof and the atmosphere. If the spectral thermal emittance deviates markedly from a horizonal line, then the details of the spectral emittance and the atmospheric emittance are necessary for a complete computation.
|Note that the hunter green sample (middle graph) looks green to the eye because of the reflectance “bump” at 0.5 µm. The average solar reflectance, at 0.086, is almost as low as black (zero).””The burgundy sample (bottom graph) looks red due to the increase in reflectance near 0.7 µm. The visible reflectance is only about 0.1, but the relatively high reflectance in the near infrared (0.7 to 2.5 µm) yields an overall solar reflectance of 0.226.”The emittance for all these samples is roughly 0.9, with an abrupt fall-off near 6 µm. Link to: Roof Heat Transfer > Emittance”
|Galvalume (top graph), due to the inclusion of aluminum metal in the zinc anti-corrosion coating, is more reflective to sunlight than traditional galvanized steel which has a solar reflectance around 0.5.A further coating, with a clean acrylic material (low graph), can be used to raise the infrared emittance without significantly changing the solar reflectance.
Location: Industrial Research Limited (IRL): Measurement Standards Laboratory of New Zealand
Their Client: New Zealand Refining Company
The Topic: Infrared radiation thermometry for furnace tube temperature measurement
Client benefit: High quality measurement leads to better plant control, reduced risk and increased profitability
Most of this article is from the writeup on the IRL website.
Industrial Research undertook furnace radiation modeling for the New Zealand Refining Company which operates several large furnaces in many stages of the production of gasoline (Also known around the world variously as “petrol” and “benzene”).
Thermal infrared radiation thermometry, or IR Thermometry, is the only feasible method for obtaining temperature measurements of many furnaces and the process tubes within.
The technology is not new, it was known that the measurements suffer from errors stemming from several environmental factors such as reflection of background thermal radiation and absorption and emission of the emitted & reflected radiation (from the surfaces being measured, by furnace gases.
Variations in the spectral emissivity and reflectivity of the materials comprising the surfaces being measured also influence the resulting temperature measurements.
The New Zealand Refining Company used the knowledge and background offered by Industrial Research’s Measurement Standards Laboratory in furnace radiation modelling to obtain more accurate measurements.
The service gave increased confidence that safe and efficient operating parameters were being maintained. As a result, the plant can operate more efficiently through the operators being able to better predict plant life and tube life.
Effects of Preoxidation Treatments on Spectral Normal and Total Normal Emittance of Inconel, Inconel-X and Type 347 Stainless Steel
Authors: Wayne S. Slemp; NATIONAL AERONAUTICS AND SPACE ADMINISTRATION HAMPTON VA LANGLEY RESEARCH CENTER
The spectral/normal-emittance values of several oxidized surfaces prepared by varying the preoxidation treatments or oxidation time for inconel, Inconel-X, and type 347 stainless steel were determined at temperatures of 900, 1,200, l,500, and 1,800 F over a wavelength range of 1 to l5 microns. Polishing, grit blasting, etching, or combinations of these preparations were used as preoxidation treatments. These values were compared for 900 and 1,800 F to determine the effects of these treatments on the spectral-normal-emittance values. Significant effects of preoxidation treatments and oxidation times on the spectral normal emittances of oxidized inconel, Inconel-X, and type 3k7 stainless steel are presented. In general, if a grit-blasted surface is etched before being oxidized, the final oxidized surface will have a lower emittance but will be more adherent and uniform. Of the two types of grit used in this study, the coarser grit provided the higher emittance. Polishing provided the lowest emittance of all specimens tested. In the one set of tests in which oxidation time was varied (on the inconel specimens), increasing oxidation time increased the emittance; however, increasing the time beyond 2 hours produced no further effect.
On the 13-15 June, 2001, in Budapest, Hungary, the 12th International Conference on Thermal Measurements and Thermogrammery (THERMO) was held.
Among the papers were two by Prof. Dr.-Ing. W. Bauer, Dipl.-Phys., A. Moldenhauer, Dipl.-Phys. & M. Rink of the Gerhard Mercator Universität Duisburg, Germany
The first presentation was entitled:
“New System for Spectral Emissivity Measurements at the University of Duisburg”
(Click on the link to access the Abstract in PDF Format)
The second was:
“Spectral emissivities of metals dependent on heat-treating processes”.
(Click on the link to access the Abstract in PDF Format)
Other papers by the members of the Duisburg Universitat have their abstracts listed on this Conference information page, also.
Contacts for more information are:
Prof. Dr. Ing. W. Bauer, Gerhard-Mercator-Universität, Duisburg, Germany
Fachbereich 8, Fachgebiet Energieeinsatz(Germany)
Tel.: +49 203/379-3629
Fax.: +49 203/379-3464
Prof. Dr. Ing. W. Bauer
Email: bauer [at] ihg.uni-duisburg.de
Thomas Funke Dipl.-Ing.
Email: Thomas.Funke [at] uni-duisburg.de