Biomedical Engineering Reference
In-Depth Information
14.3.5 Thermal Radiation
Thermal radiation is electromagnetic radiation emitted from a material that is due to the
heat of the material, the characteristics of which depend on its temperature. An example of
thermal radiation is the infrared radiation emitted by a common household radiator or electric
heater. A person near a raging bonfire will feel the radiated heat of the fire, even if the sur-
rounding air is very cold. Thermal radiation is generated when heat from the movement of
charges in the material (electrons and protons in common forms of matter) is converted to elec-
tromagnetic radiation. Sunshine, or solar radiation, is thermal radiation from the extremely
hot gasses of the sun, and this radiation heats the earth. The earth also emits thermal radiation
but at a much lower intensity because it is cooler. The balance between heating by incoming
solar thermal radiation and cooling by the earth's outgoing thermal radiation is the primary
process that determines the earth's overall temperature. As such, radiation is the only form
of heat transfer that does not require a material to transmit the heat. Radiative heat is trans-
ferred from surface to surface, with little heat absorbed between surfaces. However, the sur-
faces, once heated, can release the heat via conduction or convection to the surroundings.
Thermal radiation is conducted via electromagnetic waves. As such, this form of heat
transfer is not only a function of the temperature difference between the two surfaces but
also the frequency range of the emitted and received energy. As an example, sunlight is
composed of the visible light spectrum as well as infrared energy and ultraviolet energy.
Figure 14.47 depicts the effects of temperature and wavelength of the thermal energy on
the heat transfer rate.
When radiant energy reaches a surface, the energy can be absorbed, transmitted
(through), or reflected (or any combination). The sum of these three effects equals the total
energy transmitted, and the parameters that describe these three phenomena are given by
a
þ
r
þ
t
¼
1
;
FIGURE 14.47
Peak wavelength and total radiated amount vary with temperature. Although the plots show
relatively high temperatures, the same relationships hold true for any temperature down to absolute zero. Visible
light is between 380 and 750 nm.
