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4180, 4181
Technical Guide
The emissivity of an object is not an easy thing to determine. One method is to take a calibrated
variable-emissivity IR thermometer, aim it properly at the object in question, and adjust its emissivity
until its reading matches the known temperature of the object. This gives an average emissivity of the
object over the IR thermometer's bandwidth. This method assumes the measured object is gray.
We term an object as having constant emissivity over all wavelengths as being gray or a gray body. It
should be noted that most objects tend not to have a constant emissivity over all bandwidths, thus
these objects are not perfect gray bodies. Uncertainties in emissivity values need to be considered in
the total uncertainty of measurements with IR thermometers.
One widely used and accepted method in determining the spectral dependence on emissivity (or how
emissivity varies with wavelength) is Fourier Transform Infrared (FTIR) testing. A graph from such a
test is shown in Figure 6.
Effect of Background Temperature
Another effect related to emissivity is that of background temperature. Background temperature is the
temperature of objects facing the measured surface. Remember that when an object has an emissivity
of 0.95, it is rejecting 0.05 of background object's radiation energy. Of course the amount of
background radiation is a function of the background objects' temperature. An example of this effect is
shown in Figure 3.
0.140
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0.080
0.060
0.040
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0.000
-50
Figure 3. Effect of a 1 % Increase in Background Temperature in the 8 μm to14 μm Band
14
Effect of a 1°C Increase in Background Temp on Measured Temp
TBG=23°C, =0.95, =8 to14 m
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Target Temperature (°C)
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