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MCL640 Manual Principles of Thermal Imaging
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5.5 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:
6. By means of a printed table
7. 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.
8. Determination by ratio — Option 1
9. 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:
10.
11. Where, EK : ES = 1 : X
12. Determination by ratio — Option 2
13. 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.
14. Comparison with blackbody surface — Option 1
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).
EK : energy of blackbody-type source
ES: energy of measured object
X: emissivity of measured object