LumaSense Technologies MC320 Manual - Manual (Page 23)

MC320 Thermal Imager Manual Principle of Thermal Imaging 23
Absorptivity equals emissivity, thus emissivity can be described by
reflectivity and transmissivity.
e + t + r = 1
In order to obtain the true temperature of an object, it is
necessary to obtain the emissivity correctly. Therefore, the
emissivity of the object has to be measured by using a blackbody-
type source which is closest to an ideal blackbody as possible. The
blackbody-type source can be designed to meet the conditions
pointed out by Kirchoff where “the radiation within an
isothermal enclosure is blackbody radiation.”
As a blackbody-type source for a measurement must radiate
outside of the enclosed surface, a small hole is cut through the
wall of the enclosure small enough not to disturb the blackbody
condition. The radiation leaving this hole should closely
approximate that of a blackbody. When the diameter of the hole
is as 2r and the depth is as L, if L/r is equal or more than 6, it is
used as a blackbody-type source for practical use. The following
figure shows an example of a blackbody-type source based on
blackbody conditions.
4.5 Determining Emissivity
Emissivity is the ratio of energy radiated from an object to the
exterior and 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:
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).