Infrared Theory – Heat Transfer

Infrared theory


In order to understand the capabilities of thermal imaging, a knowledge of basic physics and heat transfer is required.

There are four major forms of heat transfer-


Of these, generally standard thermal imaging in buildings is mainly concerned with conduction and to cheapest pharmacy a lesser extent, convection. However all of the energy transfer mechanisms are encountered in various ways. viagra online 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

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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.


Material Specific Heat Capacity ( kJ/kgK)
Water @ 40 C


Water @ 200 C








Table 1 – cialis and l-arginine together Specific heat capacities (, 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 one a day cialis 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 below where the silos containing liquid show the level of the liquid inside due to the differing

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heat capacities of the liquid interior and the metallic exterior.


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. The picture below 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 why 2 tubs cialis 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.

The picture below 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

Finally, radiation-

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 (below) 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.


Figure 4 – Electromagnetic Spectrum (Ibarra-Castanedo, 2007)


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