What is the theory of heat?
In order to understand the capabilities of thermal imaging, a knowledge of basic physics is required.
There are four major forms of heat transfer-
- Latent Heat (Evaporation/Condensation)
Of these, generally standard thermal imaging in buildings is mainly concerned with conduction and to a lesser extent, convection. However all of the energy transfer mechanisms are encountered in various ways. In this paper, which concentrates on moisture, latent heat is an important element.
Conduction, where the proximity of atoms or molecules are said to “touch” each other, pass on the heat from the hotter one to the cooler. This is the only method of heat transfer that is found in a solid.
The heat transfer rate can be verbally expressed as-
The rate of heat flow under steady state conditions is directly proportional to the thermal conductivity of the object through which the heat flows, and the temperature difference between the two ends of the object. It is inversely proportional to the length or thickness of the object. (Infrared Training Center, 2010).
From this the mathematical formula can be constructed as-
Q/t = kA (T1-T2)
In this case Q is energy (J) and t is time (s). The conductivity of the material, k is measured in the expression (W/m*K). The cross sectional area A is in m2 with the temperature difference T1-T2 in (K). Finally the length of the conductive path is L (m). (Infrared Training Center, 2010)
The conductivity of water is considered to be 0.6W/m*K compared to brick which is 1. (Infrared Training Center, 2010). This has a bearing on how thermal imagery can identify moisture. However most important is thermal capacity, which is the ability of a substance to hold stored energy.
The thermal capacity (measured in kJ/kg*K) of water is extremely high, as seen in table 1, when compared to other substances. This is the main reason for thermal imaging being able to detect moisture. As water heats up within the bounds of atmospheric conditions, it becomes higher.
||Specific Heat Capacity (kJ/kgK)
|Water @ 40 C
|Water @ 200 C
Table 1 – Specific heat capacities (Engineeringtoolbox.com, n.d.)
The higher the specific heat capacity, the harder it is for the substance to gain energy as well as to lose energy. Therefore it can be seen that water will require a far greater amount of energy to heat up to the same temperature as brick, for example. Therefore for this investigation, water will heat up and cool down more slowly than the dry brick, stone or other substrate. This is how thermal imaging identifies moisture. This can be seen in figure 1 where the silos containing liquid show the level of the liquid inside due to the differing heat capacities of the liquid interior and the metallic exterior.
Figure 1 – Thermal image of Silos containing a liquid
A reasonable definition of convection is-
….. a heat transfer mode where a fluid is brought into motion, by either gravity or another force, thereby heat ( is transferred) from one place to another. (Infrared Training Center, 2010)
For convection to take place it must be within liquid or gas. The movement is caused by the differing densities of the molecules or atoms. The higher the temperature the less dense the fluid as the molecules are moving faster and are further away from each other (and taking up more volume). Gravity will have greater effect on the slower/ cooler bodies causing them to sink thus creating circulation. Thermal imagery can look at the patterns on solids that have been created by the convection currents via the boundary layer where the heat transfer is through conduction. Figure 2 shows convective warm air at the top of the wind falling down the window as it cools by conduction. By the time it reached the bottom of the window we can see via the boundary layer conduction, how the now cooler air falls, illustrated by the darker swirling areas below the windowsill.
Figure 2 – Thermal image of convection currents of cold air near a window
Latent heat is the heat transfer associated with evaporation and condensation. We all understand that evaporation cools down the body as it extracts heat. An example is sweating. We can see, once a kettle is boiling all further energy from the heat source increases evaporation and therefore the water cannot heat the water any further due to the increased cooing effect of the evaporation. The opposite is true with condensation.
Figure 3 shows a flat roof with a patch of wet. The evaporation is cooling the surface of the roof giving a lower temperature. It can be noted that sometimes the area that has evaporation might be hotter than the dry contiguous area but the evaporative effect is greater than the underlying heat and therefore will be seen in the thermograph as cooler.
Figure 3 – Thermal image of evaporating water on a roof
Heat transfer by emission and absorption of thermal radiation is called radiation heat transfer. (Infrared Training Center, 2010)
This is a factor that needs to be taken into account. However the main affect for thermography is incidental reflective radiation that skews the temperature of a body within the picture taken. This is the emissivity effect and will be mention later.
The electromagnetic spectrum encompasses all of the radiation wavelengths between the shortest waves of gamma radiation at around 10-14 m to the longest radio waves at around 107 m with the infrared sector or the spectrum roughly in the centre and will be explained in more detail later on.
Electromagnetic Spectrum (Ibarra-Castanedo, 2007)
Thermal radiation does not just occur in the infrared but across the spectrum. This radiation transfers heat by emission and absorption. The intensity is the important element so that in our surrounding temperatures, the most intense radiation is in the infrared range and the hotter it gets, the intensity moves to shorter wavelengths. An example of this is glowing hot metal has the greatest radiation in the shorter wavelength that become visible. Cooler objects have the radiation intensity further into the long wavelengths. The intensity of the radiation of different wavelengths has differing effects on us, for example we feel the heat from microwaves (in this case it is the energy exciting water molecules that create the heat) or X rays that we cannot feel initially but will cause illness over time.
Thermal infrared radiation wavelengths are longer than that of visible light in the electromagnetic spectrum. Visible light is considered to be between 0.4 and 0.7µm length and at this point it needs to be noted that the wavelength bands are not sharply defined. Thermal infrared wavelengths lie between 1-14µm. This part of the spectrum can be further broken up into near, mid and far infrared.
Within the infrared spectrum that the thermal camera covered there is a “blind spot” due to the effect of the atmosphere. The atmosphere attenuates the radiation between about 5 and 8 µm. Therefore there is no radiation to pick up. THe graph below shows the actual attenuation of radiation at 1km in the infrared band. This occurs between the mid wave infrared (MWIR) and the long wave infrared (LWIR) wavelengths. The result is that this attenuated area cannot be used in infrared imaging technology.
Transmission of air in the infrared part of the electromagnetic spectrum (University of Virginia, 2011)