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Emissivity
Emissivity is defined as the ratio of the energy emitted at a temperature to the energy emitted by a
perfect blackbody at that same temperature. A perfect blackbody would have an emissivity of 1.0.
However, in the real world there is no such thing as a perfect blackbody.
For example, if a perfect blackbody emits 10000 W/m2 at a given temperature and a material emits
5000 W/m
at that same temperature, then the emissivity of that material is 0.5 or 50 %. If another
2
material emits 9500 W/m
It is important to note that for any opaque material, the ratio of energy rejected plus the ratio of energy
transmitted is equal to 1.0 (this is known as Kirchhoff's Law). So if a material's emissivity is 0.95, the
material rejects 5 % of the energy radiated by objects facing it. By contrast, if an object has an
emissivity of 0.50, the material rejects 50 % of the energy radiated by objects facing it. This means this
rejected energy can contribute to measurement accuracy. This is especially true when measuring
materials with lower emissivity, and objects at lower temperatures.
Uncertainty Caused by Emissivity
A lack of knowledge of emissivity itself can contribute greatly to inaccuracy in IR temperature
measurement. Figure
For an example, say we are measuring an object at 500 °C. We assume it has an emissivity of 0.95.
However, its emissivity is really 0.93. This would cause our 8 μ m to 14 μ m IR thermometer to read the
temperature 6.7 degrees low, a -6.7 °C error in temperature measurement.
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
0.000
-50
-0.500
-1.000
-1.500
Figure 2. Effect of a 1 % Increase of Emissivity on Measured Temperature
Basic Infrared Thermometry Theory - Relating to the use of the Product
at that same temperature, it has an emissivity of 0.95.
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shows a graph of this phenomenon in the 8 μ m to 14 μ m band.
2
Effect of a 1% Increase of Emissivity on Measured Temp
TBG=23°C, =0.95, =8 to14 m
0
50
100
150
200
250
300
Target Temperature (°C)
Precision Infrared Calibrator
350
400
450
500
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