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 Infrared Theory
Forms of Heat Transfer
Heat (thermal
energy) can be transferred between objects or materials in the solid,
liquid, or gas form by one or more of three transfer mechanisms:
conduction, convection, and radiation.
Conduction
During conduction, thermal
energy is transferred through a solid object or between two solid objects
when they are in physical contact. For example, when heat is
transferred along the length of the metal fire poker.
Convection
During convection, thermal energy is transferred from a solid object to a
gas, such as air, with which it is in contact. The gas then transfers that
energy to other objects with which it is in contact. A gas heated by
convection is not static but will rise in relation to the cooler gas
surrounding it. Coals in a fire transfer
heat to the air above which rises and heats the hands.
Radiation
During radiation, thermal energy is transferred from an object (solid,
liquid, or gas) by converting its heat energy into electromagnetic energy.
As an example, a light bulb filament becomes very hot as electrical current
passes through it. Some of the filament’s thermal energy is converted into
electromagnetic energy, a fraction of which is emitted as light energy that
can be seen with the human eye. Thermal radiation is the only mechanism
capable of transferring thermal energy through a vacuum and is the mechanism
by which the sun heats the earth. The human body is able to sense (feel)
radiant thermal energy emitted from hot objects such as the sun, a fire, or
a hot stove.
All
objects transfer thermal energy by radiation and the majority of that
radiated energy occurs in the infrared region of the electromagnetic
spectrum, which is invisible to the human eye. Thermal imagers and
infrared cameras detect
infrared energy and create images of infrared energy
that is radiated by objects.
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