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Appendix: How Infrared Thermometry Works
A
Wien’s Displacement Law describes the exact mathematical
relationship between the temperature of a blackbody and the
wavelength of the maximum intensity radiation.
where λ
m
= wavelength measured in microns
T = temperature in Kelvin
Calculating Temperature
The net thermal power radiated by an object has been shown to
depend on its emissivity, its temperature and that of the ambient
temperature around the object. A very useful equation known today
as the Stefan-Boltzmann Law
has been shown both theoretically and
empirically to describe the relationship.
I = thermal power in watts/meter
2
ε = Emissivity
σ = 5.6703 x 10
-8
watts/meter
2
x K
4
(Stefan’s constant)
T = temperature of object in Kelvin
T
a
= temperature of ambient surroundings in Kelvin
The infrared thermometer uses this equation directly in calculating
the temperature of an object. The incident power is measured by the
infrared detector. The emissivity of the object is determined by the
user. The ambient temperature is measured by a sensor inside the
thermometer. With all quantities known, the thermometer uses the
Stefan-Bolzmann Law
to calculate and output the temperature of the
object.
λ
m
=
2.898
T