MCS640 Thermal Imager Manual Principle of Thermal Imaging • 32
Spectral radiant emittance of a blackbody
The graphs show that wavelength and spectral radiant emittance vary with the temperature. They also show
that as the temperature rises, the peak of spectral radiant emittance is shifting to shorter wavelengths. This
phenomenon is observable in the visible light region as an object at a low temperature appears red, and as the
temperature increases, it changes to yellowish and then whitish color—thus shifting to shorter and shorter
wavelengths as the temperature increases.
4.4 Blackbody Type Source and Emissivity
Although a blackbody is actually only a theoretical ideal, an object can be manufactured which approximates
it. A law closely related to the blackbody is Kirchhoff’s law that defines reflection, transmission, absorption and
radiation.
a = e = 1
Key: a = absorptivity t = transmissivity r = reflectivity e = emissivity
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.
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