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MCL640 Manual Principles of Thermal Imaging
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22
Transmission, Absorption, and Reflection of Infrared Energy
The figure above shows the three modes by which the radiant energy striking an object may be
dissipated. These modes of dissipation are:
a = absorption
t = transmission
r = reflection
The fractions of the total radiant energy, which are associated with each of the above modes of
dissipation, are referred to as the absorptivity (a) transmissivity (t) and the reflectivity (r) of the
body. According to the theory of conservation of energy, the extent to which materials reflect,
absorb and transmit IR energy is known as the emissivity of the material.
5.3 Blackbody Radiation
The emissivity of a body is defined formally by the equation below as the ratio of the radiant
energy emitted by the body to the radiation, which would be emitted by a blackbody at the
same temperature.
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.
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.