Heat, Temperature and IR Measurement Basics

Temperature is a measure of the thermal energy contained by an object; the degree of hotness or coldness of an object is measurable by a number of means and is defined by temperature scales.
There are three (3) modes of heat transfer, conduction, convection and radiation. All heat is transferred by means of one or another of these three modes—usually by two or three modes in combination. Of these three modes, infrared thermography is most closely associated with radiative heat transfer, but it is essential to understand all three in order to comprehend the significance of IR thermograms.
Temperature is expressed in either absolute or relative terms. There are two absolute scales called "Rankine" (English system) and "Kelvin" (metric system). There are two corresponding relative scales called "Fahrenheit" (English system) and "Celsius" or "centigrade" (metric system). Absolute zero is the temperature at which no molecular action takes place. This is expressed as "zero Kelvins" or "zero Rankines" (0 K or 0 R). Relative temperature is expressed as "degrees Celsius" or "degrees Fahrenheit" (°C or °F).

Conductive Heat Transfer

Conductive heat transfer is the transfer of heat in stationary media. It is the only mode of heat flow in solids, but can also take place in liquids and gases. It occurs as the result of atomic vibrations (in solids) and molecular collisions (in liquids) whereby energy is moved, one molecule at a time, from higher temperature sites to lower temperature sites.

Convective Heat Transfer

Convective heat flow takes place in a moving medium and is almost always associated with transfer between a solid and a moving fluid (such as air). Free convection takes place when the temperature differences necessary for heat transfer produce density changes in the fluid and the warmer fluid rises as a result of increased buoyancy.
Forced convection takes place when an external driving force, such as a cooling fan, moves the fluid. An illustration of convective heat transfer is what happens at the interface between the surface of a heat exchanger and air moved by an exhaust fan. The thermal energy stored in the heat exchanger is convected into the surrounding air, warming the air and cooling the surface of the heat exchanger.

The figure demonstrates the forced convective heat transfer between a flat heated plate and a moving fluid. In convective heat flow, heat transfer takes effect by means of two mechanisms; the direct conduction through the fluid and the motion of the fluid itself.
The presence of the plate causes the free fluid velocity, Vf, of the fluid, to decrease to zero at the surface and influences its velocity throughout a variable distance called the boundary layer. The thickness of the boundary layer depends inversely on the free fluid velocity, and is greater for free convection and smaller for forced convection. The rate of heat flow depends, in turn on the thickness of the convection boundary layer and the temperature difference, AT, between Ts and Too (Ts is the surface temperature, Too the free fluid temperature outside the boundary layer).

Radiative Heat Transfer

Radiative heat transfer is unlike the other two modes in several respects:

  • It can propagate through a vacuum.
  • It occurs by electromagnetic emission and absorption
  • It occurs at the speed of light and behaves in a manner similar to light.

While conductive and convective heat transferred between points is linearly proportional to the temperature difference between them, the energy radiated from a surface is proportional to the fourth power of its absolute temperature. The radiant thermal energy transferred between two surfaces is proportional to the third power of the temperature difference between the surfaces
Thermal infrared radiation leaving a surface is called radiant exitance or radiosity. It can be emitted from the surface, reflected off the surface, or transmitted through the surface, as illustrated in the figure. The total exitance is equal to the sum of the emitted component, (We), the reflected component, (Wr), and the transmitted component, (Wt). The surface temperature, however, is related only to We, the emitted component.