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What The Heck is Emissivity? (Part 2)
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I would like to know :
How can I make correction for different emissivities at same target temperature?
(For example: Blackbody (Target) temperature is 1000 degree Celsius,Emissivity of blackbody is 1.00,
infrared thermometer emissivity is 0.95)
temperature @emissivity 0.95 = x degree Celsius
temperature @ emissivity 1.00=?
Awaiting for your kind reply.
Thank & have a nice day!
K.M.Shein
This is a multi-faceted question. It depends not only on the temperature but also on the spectral response of the radiation thermometer.
One quick way of getting an estimate of the answer would be to look at the fine technical note that used to be published by Ircon, but it seems to be no longer available online.
Another would be to review the table of temperature errors induced by 1% emissivity changes in ASTM Standard E 1256. One can easily scale the change in temperature caused by either a 5% change in the emissivity correction or a change in emissivity of the object by 5%; they are nearly the same but can be in opposite directions.
Once one knows the temperature and the waveband center for an instrument, the approximate error, close enough for most industrial purposes or about 1% of temperature or less can be calculated from first principles. The formula for doing so is provided in the ASTM standard.
For instance, if one sites an accurate radiation thermometer into a blackbody running at 1000 Degrees C (assuming the entire field of view of the instrument is filled), and sets its emissivity correction to 0.95, the blackbody temperature will appear higher than 1000 Degrees. The size of the difference will depend on the waveband of the thermometer used.
A thermometer which has a waveband sensitivity on the long wavelength side of the Planck Curve for 1273 K, say centered on about 11 millimicrons, will show a very large difference, about 50 Degrees C.
A thermometer having a 1 millimicron waveband center, on the short wavelength side of the Planck Curve would exhibit a difference of only about 5 or 6 Degrees C. An instrument with a wavelength band centered at about 0.65 millimicrons would exhibit even less difference, closer to 4 Degrees C.
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