Scientific Principles of Thermal Testing
Energy can come in many forms, and it can change from one form
to another but can never be lost. This is the First Law of Thermodynamics.
A byproduct of nearly all energy conversion is heat, which is
also known as thermal energy. When there is a temperature difference
between two objects or two areas within the same object, heat
transfer occurs. Heat energy transfers from the warmer areas to
the cooler areas until thermal equilibrium is reached. This is
the Second Law of Thermodynamics. When the temperature of an object
is the same as the surrounding environment, it is said to be at
Heat Transfer Mechanisms
Thermal energy transfer occurs through three mechanisms: conduction,
convection, and/or radiation. Conduction occurs primarily in solids
and to a lesser degree in fluids as warmer, more energetic molecules
transfer their energy to cooler adjacent molecules. Convection
occurs in liquids and gases, and involves the mass movement of
molecules such as when stirring or mixing is involved.
The third way that heat is transferred is through
electromagnetic radiation of energy. Radiation needs no medium
to flow through and, therefore, can occur even in a vacuum. Electromagnetic
radiation is produced when electrons lose energy and fall to
a lower energy state. Both the wavelength and intensity of the
radiation is directly related to the temperature of the surface
molecules or atoms.
Wavelength of Thermal Energy
The wavelength of thermal radiation extends from
0.1 microns to several hundred microns. As highlighted in the image,
this means that not all of the heat radiated from an object will
be visible to the human eye
but the heat is detectable. Consider
the gradual heating of a piece of steel. With the application
of a heat source, heat radiating from the part is felt long
before a change in color is noticed. If the heat intensity is
great enough and applied for long enough, the part will gradually
change to a red color. The heat that is felt prior to the part
changing color is the radiation that lies in the infrared frequency
spectrum of electromagnetic radiation. Infrared (IR) radiation
has a wavelength that is longer than visible light or, in other
words, greater than 700 nanometers. As the wavelength of the radiation
shortens, it reaches the point where it is short enough to enter
the visible spectrum and can be detected with the human eye.
An infrared camera has the ability to detect and
display infrared energy. Below is an infrared image of an ice
cube melting. Note the temperature scale on side, which shows
warm areas in red and cool areas in purple. It can be seen that
the ice cube is colder than the surrounding air and it is absorbing
heat at its surface. The basis for infrared imaging technology
is that any object whose temperature is above 0°K radiates
infrared energy. Even very cold objects radiate some infrared energy.
Even though the object might be absorbing thermal energy to warm
itself, it will still emit some infrared energy that is detectable
by sensors. The amount of radiated energy is a function of the
object's temperature and its relative efficiency of thermal radiation,
known as emissivity.
(Photo courtesy of NASA/JPL-Caltech/IPAC)
A very important consideration in radiation heat transfer is the
emissivity of the object being evaluated. Emissivity is a measure
of a surface's efficiency in transferring infrared energy. It
is the ratio of thermal energy emitted by a surface to the energy
emitted by a perfect blackbody at the same temperature. A perfect
blackbody only exists in theory and is an object that absorbs
and reemits all of its energy. Human skin is nearly a perfect
blackbody as it has an emissivity of 0.98, regardless of actual
If an object has low emissivity, IR instruments will indicate
a lower temperature than the true surface temperature. For this
reason, most systems and instruments provide the ability for the
operator to adjust the emissivity of the object being measured.
Sometimes, spray paints, powders, tape or "emissivity dots"
are used to improve the emissivity of an object.