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EMISSIVITY EVALUATION OF FIXED POINT BLACKBODIES

A paper by Sergey Mekhontsev, Vladimir Khromchenko, Alexander Prokhorov, Leonard Hanssen
National Institute for Standards and Technology, Gaithersburg, MD, USA

Presented at the 9th International Symposium on Temperature and Thermal Measurements in Industry and Science (TEMPMEKO 2004), June 22-25, 2004, Dubrovnik, Croatia, Proceedings, Vol. 1, ed. by D. Zvizdic (2004), pp. 581-586.

ABSTRACT

A new facility for the characterization of infrared spectral emittance of materials has recently been developed at NIST. The facility operation is based on measurements of a sample’s spectral radiance and surface temperature with help of a set of variable temperature blackbodies and a spectral comparator. For highest accuracy, variable temperature blackbodies are calibrated in spectral radiance against a pair of fixed-point blackbodies with interchangeable crucibles of In, Sn, and Zn, and Al, Ag, and Cu, respectively. The spectral emissivity of the fixed-point blackbodies also needs to be accurately characterized. We employ a multi-prong approach: (1) Monte Carlo ray-trace modeling and calculations, (2) hemispherical reflectance measurements of the crucible cavity material flat sample, as well as the cavity itself, (3) direct spectral emittance measurements of the same samples using the facility, and (4) comparison of the fixed point blackbodies with each other as well as with variable temperature heat pipe blackbodies, using filter radiometers and the facility’s Fourier transform spectrometer. The Monte Carlo code is used to predict the cavity emissivity with input of the cavity shape and the emissivity and specularity of the cavity material. The reflectance measurements provide emissivity data of both the material and the cavity at room temperature. The results are used to compare with and validate the code results. The direct emittance measurements of the material provide the temperature dependence of the material emittance as code input. The code predicted results for the cavities at their operating temperature (freeze points) are then compared with the relative spectral radiance measurements. Use of this complete set of evaluation tools enables us to obtain the spectral emissivity of the blackbodies with reliably determined uncertainties.

It presently can be downloaded in PDF format from the NIST website by CLICKING HERE

Emissivity Coefficients of Some Common Materials

We’ve been trying to preach to the inexperienced about Spectral Emissivity vs (just plain) emissivity.

The former is the subject used in Thermal Infrared Radiation Thermometry (Pyrometry, to some) and users of Thermal Infrared Imagers (Thermographic or Thermography Cameras) while the latter is the domain of radiation heat transfer considerations (except of course when spectral issues, like windows and atmospheres get in the way of the radiation transfer).

See the Emissivity Trail Pages at About Temperature Sensors if you’d like a brief rant or two.

But popular ignorance of details not withstanding, it is still a bit of a shock to see the term ‘Emissivity” a prominent feature on both instrumentation and engineering websites, Here’s another one with some sample text (no numbers here) from the Engineering Toolbox website. (Note: we corrected their misspelling of “emissivity” – as mentioned in our semi – rant pages on About Temperature Sensors, the word seems to be misspelled as often as it the term and the values are misunderstood and misused!)

The radiation heat transfer emissivity coefficient of some common materials as aluminum, brass, glass and many more

The emissivity coefficient – ? – indicates the radiation of heat from a ‘grey body’ according the Stefan-Boltzmann Law, compared with the radiation of heat from a ideal ‘black body’ with the emissivity coefficient ? = 1.

The emissivity coefficient – ? – for some common materials can be found in the table below. Note that the emissivity coefficients for some products varies with the temperature. As a guideline the emisivities below are based on temperature 300 K.
Surface Material

After Note: We have tried over the past ten years or so, with very limited success, to point out to organizations that should know better, including at least one each manufacturer of “Infrared Thermometers” and One Prominent Maker of Blackbody calibration furnaces, that they need to mend their errant ways and get with the one true religion of Spectral Emissivity.

Heck, the Church of the Flying Spaghetti Monster got a better response and Rodney Dangerfield gets more respect.

There are a few bright lights at the end of the emissivity “black hole”, the new facilities at several national Metrology Laboratories, such as the one at NIST dealing with Infrared Optical Properties of Materials and the “Modern emissivity measuring facility for industry-orientated calibrations developed at PTB“.

Hope springs eternal!

Texture and porosity effects on the thermal radiative behavior of alumina ceramics

Texture and porosity effects on the thermal radiative behavior of alumina ceramics
Int. J. Thermophys. (in press) [view]

New Article
By: O.Rozenbaum, D.De Sousa Meneses; P.Echegut
Texture and porosity effects on the thermal radiative behavior of alumina ceramics
Int. J. Thermophys. (in press) [view]

ABSTRACT
“Thermal and optical properties of ceramics are dependent on radiation scattering and cannot be determined by the only knowledge of their chemical composition as for single crystals. In this paper, we investigate extrinsic effects such as roughness, porosity and texture on spectral emissivity of alumina ceramics. Roughness effects have an influence mainly in the opaque zone; an important porosity dependence and the presence of a critical porosity threshold were also pointed out in the semi-transparent zone. Furthermore, it was shown that two ceramics with similar total porosity but with different textures possess radically different emissivities, showing that grain size, pore size and spatial repartition of the grains is also crucial for the comprehension of the ceramics thermal properties”

Work performed at and reported by: Centre National de la Recherche Scientifique (CNRS), France.

Spectral emissivity from 2 micrometers to 15 micrometers

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