MCS640 Thermal Imager Manual Principle of Thermal Imaging • 33
4.5 Determining Emissivity
Emissivity is the ratio of energy radiated from an object to the energy radiated from a blackbody. The
emissivity varies with the surface condition of the object and also with temperature and wavelength. If this
value is not accurate, then the true temperature cannot be measured. In other words, a variation or change in
emissivity will cause a change in the indications on a thermal imager.
To approach the true temperature therefore,
Therefore, in order to perform correct measurement for true temperature, the emissivity is determined as
follows:
1. By means of a printed table
Various books and literature carry physical constants tables, but if the measuring condition is not
identical, the constants may not usable. In such cases the literature should be used only for reference.
2. Determination by ratio — Option 1
A contact-type thermometer is used to confirm that the measured object is in thermal equilibrium
and that the blackbody-type source is at the same temperature. The object and the blackbody-type
source are then measured with the radiation thermometer and the resulting energy ratio is then used
to define the emissivity as follows:
Where, EK : ES = 1 : X
3. Determination by ratio — Option 2
An object, resembling a blackbody, is attached to a heat source to make the temperature of the
blackbody part and the measuring object the same. The ratio of infrared radiation energies are then
determined as in #2 above.
EK : energy of blackbody-type source
ES: energy of measured object
X: emissivity of measured object
The emissivity must approximate 1.0 ( The measured object must be
nearly a blackbody).
The emissivity must be corrected (
The emissivity of the measured
object must be internally corrected to 1 by the thermal imager).
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