Message Board Thread - "Emissivity values "

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Emissivity values JOHN 4/17/2001
How are emissivity values derived on common material emissivity charts? (especially low E materials)
 
Deriving Emissivity Values Bernie Lyon 1/9/2004
Emissivity values may be obtained using infrared spectrometry. This is done by comparing radiation from an area of a plane surface of a specimen with radiation from a similar area of a blackbody radiator at the same wavelength λ, temperature, and viewfactor. This usually yields a graph known as the Normal Spectral Emissivity. "Normal" refers to the fact that the specimen is observed at a normal angle, or perpendicular to the surface. You can find samples of these graphs on page 673 of the Nondestructive Testing Handbook "Infrared and Thermal Testing," edited by Xavier Maldague, published by the American Society for Nondestructive Testing, ISBN 1-57117-044-8.
The graphs show the spectral emissivity of the sample over a broad waveband. For example, there is a graph of anodized (not black) aluminum illustrating the emissivity from about 3 to 14 micrometers. At three micrometers the emissivity is about 0.9, and at 12 micrometers the emissivity is about 0.65. When this information is sent to a table, the value may be displayed as the average emissivity over 3 to 14 micrometers, which would be about 0.77. If the average emissivity was taken from 3 to 5 micrometers it would be 0.87; for 8 to 12 micrometers it would be 0.72. Emissivity can vary considerably over broad wavebands. If a table lists the value as "total" normal emissivity, it is the average emissivity over a broad waveband, which may or may not be close to the inband sensitivity of your infrared camera.
Some emissivity tables define specific wavebands and may even include temperatures. You can create emissivity tables for your own infrared camera, or cameras, by making careful emissivity measurements of specimens. If you compile an emissivity table for one infrared camera, you can not assume that it will be the same for another model infrared camera, even if it operates in the same waveband. There are variations in the detector responses of cameras. A shortwave HgCdTe detector does not have the same response as a shortwave PtSi FPA detector. So, you would derive different emissivity values using these cameras.
 
Re:Emissivity values GRP 2/4/2004
Yes, the previous response was quite on the money, but still only part of the answer. We've got a special section on our website at http://www.temperatures.com/eindex.html called the E-missivity Trail, where we've started down the road to simplfying the facts about emissivity. Sort of getting them in the proper pecking order for those not familiar with all the details.

There is also an excellent reference, long out of print, from the NBS (the original name for NIST) called NBS Special Publication 300,Volume 7 "Precision Measurement and Calibration, Radiometry and Photometry" November 1971. It shows many of the methods used to measure emissivity in the laboratory and provides much of the fundamental data on several ceramic emissivities covered in the large compendium on emissivity published later in three huge volumes edited by Toloukian and DeWitt. These and other resources are listed in detail also on our website at http://www.temperatures.com/temprefs.html

There is a huge amount of information out there on emissivity, but applying it often takes more understanding than it does to find it.
 
Re:Emissivity values Incinerador 5/17/2004
My name is Juan Pablo Sáenz, and I bought an IR measuring device. I want to measure the temp. of a flame, but I do not know how the emissivity range of a flame.
I am requiring a flame emissivity range or something like it.

Thank you very much in advance.

Juan Pablo Sáenz
 


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