LumaSense Technologies BoilerSpection-SD - Manual (Page 44)

BoilerSpection HD/SD Manual Principles of Thermal Imaging 44
6.2 Emissivity
Infrared radiation is energy radiated by the motion of electronics, atoms and molecules on the
surface of an object, where the temperature of the object is more than absolute zero. The intensity
of the emittance is a function of the temperature of the material. In other words, the higher the
temperature, the greater the intensity of infrared energy that is emitted. As well as emitting
infrared energy, materials also reflect infrared, absorb infrared and, in some cases, transmit infrared
energy. When the temperature of the material equals that of its surroundings, the amount of
thermal radiation absorbed by the object equals the amount emitted by the object.
Figure 29: 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 principle of conservation of energy, the extent to which materials reflect,
absorb and transmit IR energy is known as the emissivity of the material.