LumaSense Technologies BoilerSpection-SD - Manual (Page 47)

BoilerSpection HD/SD Manual Principles of Thermal Imaging 47
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 from 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.
6.5 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:
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 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).