Spectral Emissivity & Emittance

Measurement of spectral emissivity from 2 micrometers to 15 micrometers
Charles D. Reid and E. D. McAlister
JOSA, Vol. 49, Issue 1, pp. 78- (1959)

Citation
C. D. Reid and E. D. McAlister, “Measurement of spectral emissivity from 2 micrometers to 15 micrometers,” J. Opt. Soc. Am. 49, 78- (1959)
http://www.opticsinfobase.org/abstract.cfm?URI=josa-49-1-78

San Diego CA, USA –Surface Optics’ ET10 measures emissivity values in two most commonly used spectral regions, 3 to 5 and 8 to 12 microns.

Its main application is to produce emissivity values for the infrared cameras.

Advanced IR cameras require the input of an emissivity value for accurate temperature calculations. The emissivity values obtained from tables can be far from real leading to large temperature uncertainties.

The ET10 can be used in the lab or in the field and on small or large objects. With the ET10 one can measure emissivity of any surface in just a few seconds.

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Title: STANDARDIZATION OF THERMAL EMITTANCE MEASUREMENTS. PART 5. NORMAL SPECTRAL EMITTANCE, 800-1400 K (ABSTRACT BELOW)
DOWNLOAD FULL REPORT IN PDF FORMAT

Corporate Author : NATIONAL BUREAU OF STANDARDS GAITHERSBURG MD

Personal Author(s) : Harrison, William N. ; Richmond, Joseph C. ; Shorten, Frederick J. ; Joseph, Horace M.

Handle / proxy Url : http://handle.dtic.mil/100.2/AD426846

Report Date : NOV 1963

Pagination or Media Count : 99

Abstract: Equipment and procedures were developed to measure normal spectral emittance of specimens that can be heated by passing a current through them, at temperatures in the range of 800 to 1400 K, and over the wavelength range of 1 to 15 microns. A data-processing attachment for the normal spectral emittance equipment was designed to (1) automatically correct the measured emittance for ‘100% line’ and ‘zero line’ errors on the basis of previously-recorded calibration tests; (2) record the corrected spectral emittance values and wavelengths at preselected wavelength intervals on punched paper tape in form suitable for direct entry into an electronic digital computer; and (3) to compute during a spectral emittance test on a specimen the total normal emittance, or absorptance for radiant energy of any known spectral distribution of flux, of the specimen. Working standards of normal spectral emittance having low, intermediate and high emittance values, respectively, were prepared and calibrated for use in other laboratories to check the operation of equipment and procedures used for measuring normal spectral emittance.

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”

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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.