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!

Determination of continental surface emissivity and temperature from satellite observations.

http://ara.lmd.polytechnique.fr/htdocs-public/products/emissivity/emissivity.html
Surface emission depends on surface parameters, i.e. emissivity and temperature. Emissivity of land surfaces substantially varies with vegetation, soil moisture, composition, and roughness (Nerry et al. (1988); Salisbury and D'Aria (1992)). As emissivity depends on wavelength, it is referred to as spectral emissivity. Emissivity also depends on the viewing angle.

Continental surface emissivity in the thermal infrared window is a key parameter for estimating the surface radiation budget. The energy emitted from the surface is proportional to the spectrally integrated surface emissivity and depends on the surface temperature. A 10% error (from 0.9 to 1.0, for example) on the emissivity approximately corresponds to a 10% error in the energy emitted from the surface (a portion of which may be compensated by the reflected incoming radiation). Prabhakara and Dalu (1976); Ogawa et al. (2003).

Wikipedia on Emissivity

Wikipedia has an entry for emissivity that is very brief and limited as of November 2008. Perhaps some visitors will improve it.

Will check periodically and let you know.

Link: http://en.wikipedia.org/wiki/Emissivity

Prediction of the thermal radiative properties of an X-Ray µ-tomographied porous silica glass

Prediction of the thermal radiative properties of an X-Ray µ-tomographied porous silica glass
B.Rousseau, D.De Sousa Meneses, P.Echegut, M.Di Michiel, J.-F.Thovert
Prediction of the thermal radiative properties of an X-Ray µ-tomographied porous silica glass
Applied Optics 46 4266-4276, (2007)

ABSTRACT
"A Monte Carlo ray tracing procedure is proposed to simulate thermal optical processes in heterogeneous materials. It operates within a detailed 3D image of the material, and it can therefore be used to investigate the relationship between the microstructure, the constituent optical properties, and the macroscopic radiative behavior. The program is applied to porous silica glass. A sample was first characterized by 3D x-ray tomography; then, its normal spectral emittance was calculated and compared with the experimental spectrum measured independently by high-temperature infrared emittance spectroscopy. We conclude with a discussion of the light-scattering mechanisms occurring in the sample."

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