LumaSense Technologies MC320 Manual - Manual (Page 21)

MC320 Thermal Imager Manual Principle of Thermal Imaging 21
Where,
W o = total radiant energy emitted by a body at a given
temperature T.
W
bb = total radiant energy emitted by a blackbody at the
same temperature T.
If all energy falling on an object were absorbed (no transmission
or reflection), the absorptivity would equal to 1. At a steady
temperature, all the energy absorbed could be re-radiated
(emitted) so that the emissivity of such a body would equal 1.
Therefore in a blackbody,
absorptivity = emissivity = 1
Practical real life objects do not behave exactly as this ideal, but
as described with transmissivity and reflectivity,
absorptivity + transmissivity + reflectivity = 1
Planck’s Law
Energy radiated from the blackbody is described as follows
[“Planck’s Law”.]
1)
Stefan Bolzmann’s
equation
In order to obtain total radiant emittance of the blackbody,
integrate the equation (1) through all wavelengths (0 to infinity).
The result is as follows and is called “Stefan-Bolzmann equation.”
2)
Wien’s displacement
law
The temperature of blackbody can be obtained directly from the
radiant energy of the blackbody by this equation. In order to
find out the wavelength on the maximum spectral radiant
emittance, differentiate Planck’s law and take the value to 0.
3)
The equation is called “Wien’s displacement law”.
Note:
A blackbody is a
theoretical surface,
which absorbs and re-
radiates all the IR
energy it receives. It
does not reflect or
transmit any IR energy.
Perfect blackbody
surfaces do not exist in
nature.