Infrared Thermography

Anita Roth, winter semester 2015/16

Infrared thermography is a technique used for measuring differences in thermal radiation from objects. It is a non-destructive testing method. Possible applications are the detection of heat bridges in walls (passive thermography) and fractures in concrete elements (active thermography).

Infrared Thermography

Infrared thermography uses the property that every object at a temperature above 0 K emits thermal radiation. A special camera sensitive to the infrared wavelengths is used to measure the heat emitted by a body. The recorded image is displayed in a colour scale chosen by the user. Knowledge of the filter created by using a different scale is indispensable when interpreting infrared thermography images. For example a scale in which red is 10 °C and blue is 8 °C may be used. This will create the impression that the areas of 10 °C are very warm (bright red) and the areas with 8 °C are much colder. However, when choosing a different scale in which 20 °C is red and 0 °C is blue, the difference between 8 °C and 10 °C will barely be noticeable. The effect of the scale can be seen in Figure 1. Some areas are coloured red, indicating heat even though their actual temperature is around 6 °C. A big advantage of this method is that it is contact free, quick and yields an easy interpretation. Low quality infrared cameras are cheaply available as smartphone accessories. However, professional equipment is very expensive and the interpretation of the images demands a solid education. Infrared cameras can also be attached on drones for testing in bigger heights or hard to access areas.

Infrared thermography distinguishes between active and passive applications. Passive thermography observes the heat emissions of an object without influencing its temperature. It is just observing its “natural” heat emissions, i.e., taking the image of a building to observe whether there are warmer areas where heat is emitted to the surroundings. Active thermography is mostly used in mechanical engineering. The object in question is heated for a defined amount of time and the cooling process is observed. Due to the poor heat capacity of concrete this method is today rarely used in the construction industry. ^[1]

Theoretical background

Thermal radiation

Thermal radiation is part of the electromagnetic wave spectrum and was discovered around 1800 by William Herschel. He realized that most of the heat transport takes place in a non-visible area of the electromagnetic spectrum, bordering the red frequencies. As an electromagnetic wave, thermal radiation has the same properties as visible light with respect to reflection, deflection and refraction. The infrared spectrum covers wavelengths from 700 nm to 1 mm and is therefore located in between the red part of the visible spectrum and the microwave wavelengths as can be seen in Figure 2. ^[1]

Radiation laws

When an electromagnetic wave meets an object, it will split up into an absorbed, a transmitted and a reflected part. This process is characterized by the transmission rate $//<![CDATA[ \begin{array}{l}t\end{array} //]]>$, absorption rate $//<![CDATA[ \begin{array}{l}a\end{array} //]]>$ and reflection rate $//<![CDATA[ \begin{array}{l}r\end{array} //]]>$. These properties obey the following relation: $//<![CDATA[ \begin{array}{l}r + t + a = 1\end{array} //]]>$. Most solid bodies do not transmit radiation. If all energy is reflected ($//<![CDATA[ \begin{array}{l}r = 1\end{array} //]]>$) a body appears to be white and it will barely heat up in the sun. Consequently if all energy is absorbed ($//<![CDATA[ \begin{array}{l}a = 1\end{array} //]]>$) it will appear to be black. ^[1]

An example characterizing the different emission properties of surfaces is the Leslie cube. It is made out of brass and has a black, a white and 2 neutral coloured side walls. If filled with hot water it can be used to show that the black side wall emits more heat energy than the white side wall. In Figure 3 the infrared image on the left shows how high the hot water is already filled into the cube. The main laws governing infrared radiation are briefly described here. The Stefan-Boltzmann law gives a relation linking the infrared radiation (which is recorded by the infrared camera) to the temperature of an object. Kirchhoff’s law was the basis for Planck’s quantum hypothesis. It provides a link between the absorption capacity and emission capacity of an object. By knowing how much the object is heated and how much energy is emitted again, the objects emissivity can be determined. The third important law is Planck’s emission law. He succeeded in finding a way to describe the intensity of heat radiation from the blackbody as a function of the wavelength and the temperature. For a given temperature we can therefore determine the wavelengths at which the maximum heat transfer takes place.

Stefan-Boltzmann Law

Every object at a temperature above the absolute zero point (-273.1 5 °C = 0 K) radiates thermal energy. The Stefan-Boltzmann law states, that there is an upper limit to the thermal radiation emitted by an object (of unit surface area) described by:

$//<![CDATA[ \begin{array}{l}P = \sigma * T^4\end{array} //]]>$ with $//<![CDATA[ \begin{array}{l}\sigma = 5.67 * 10^{-8} \frac{W}{m^2 K^4}\end{array} //]]>$ (Stefan-Boltzmann Constant).

This is the energy that would be emitted by a black body. In reality however, black bodies do not exist. Every material has an emissivity constant between 0 and 1 that depends on the temperature of the object. The formula then reads:

$//<![CDATA[ \begin{array}{l}P = \varepsilon (T) * \sigma * T^4\end{array} //]]>$

Some typical values of
ϵ

are:

^[2]

Kirchhoff's Law

Kirchhoff's law links emission capacity and absorption capacity of an object. It states that

$//<![CDATA[ \begin{array}{l}P = a * P_\text{s}\end{array} //]]>$

with $//<![CDATA[ \begin{array}{l}P\end{array} //]]>$ = emission capacity of an object

$//<![CDATA[ \begin{array}{l}a\end{array} //]]>$ = absorption capacity of the object

$//<![CDATA[ \begin{array}{l}P_s\end{array} //]]>$ = theoretical emission capacity of a black body.

^[2]

Planck's emission law

For a very long time it was impossible to fully describe the radiation spectrum of a black body. All known laws described only parts of the spectrum correctly. This changed when Planck had the idea to describe electromagnetic waves as particles. It was also the start of the theory of quantum mechanics. He started by describing the energy of a particle as proportional to its frequency.

$//<![CDATA[ \begin{array}{l}E = h * f = \frac{h * c}{\lambda}\end{array} //]]>$

with

$//<![CDATA[ \begin{array}{l}E\end{array} //]]>$ = Energy

$//<![CDATA[ \begin{array}{l}H\end{array} //]]>$ = $//<![CDATA[ \begin{array}{l}6.626 * 10^{- 34}\end{array} //]]>$ [Js], Planck’s constant

$//<![CDATA[ \begin{array}{l}f\end{array} //]]>$ = frequency

$//<![CDATA[ \begin{array}{l}c\end{array} //]]>$ = speed of light in a vacuum

$//<![CDATA[ \begin{array}{l}λ\end{array} //]]>$ = wavelength.

Using this, he could derive an emission law that fully describes the emission spectrum of a black body.

$//<![CDATA[ \begin{array}{l}\rho (f,T) df = \frac{8 \pi f^3}{c^3} * \frac{1}{e^{\frac{hf}{kT}} - 1} df\end{array} //]]>$

with

$//<![CDATA[ \begin{array}{l}k\end{array} //]]>$ = Boltzmann constant.

His findings show, that the intensity of the radiation depends on the temperature of the object as well as the wavelength. Figure 4 is a plot of the above law and shows that as temperature rises, the location of the maximum radiation intensity moves towards shorter wavelengths. At room temperature (300 K), the maximum radiation intensity is at a wavelength of around 10 μm

and therefore in the infrared domain. As temperature increases however, it moves towards the visible wavelengths, which explains why we see hot iron glowing red or even yellow. ^{[1] [2]}

Infrared thermography in civil engineering

Passive Thermography

The typical usage of passive infrared thermography is the imaging of houses to detect heat bridges or poorly insulated windows/walls (see Figure 5). However there are several other applications that this technology can be used for:

• Detection of hidden structures like wooden beams of studwork structure below a painted surface (see Figure 6).
• Localisation of leaks in pipes (i.e. for floor heating, warm water pipes)
• Localisation of damp areas due to the different heat properties of wet and dry material (i.e. in an attic with leaky areas).

Difficulties/Sources of error

As the thermography camera measures all heat waves it receives, there can be several artefacts when evaluating images. It is always advisable to take a “normal” photo of the configuration in order to be able to compare the two images later on. Some typical things to be careful about are:

• It is not possible to “see” through a window with an infrared camera, but the reflection of a person in a window will be visible on the thermal image.
• Images of buildings should ideally be taken before sunrise in winter in order to avoid the reflection of the sun on the windows and heating up of the walls.
• The temperature difference of the surroundings and the building examined should be > 10 °C.
• When taking a thermography image of a house, all indoor doors should be opened hours before the measure in order to ensure a uniform heat distribution within the house.
• The weather should be dry and calm.

The most important thing to keep in mind when interpreting infrared images is that the object is seen through a filter. An image without an explained colour scale and information about when and under what weather conditions it was taken provides no scientifically resilient information. ^[1]

Active Thermography

Active thermography can be used to detect in-homogeneities close to the surface of an object. It can be used as a complement to other non-destructive testing methods like radar (Radar (engl.)) and ultrasound (Ultraschall) that have trouble detecting irregularities at very small depths. The two methods currently in use are Lock-In thermography and Pulse thermography.

Lock-In Thermography

The emitted energy of the source (i.e. halogen spotlight) is varied periodically, for example in sinusoidal or rectangular form. The variation frequency ($//<![CDATA[ \begin{array}{l}\gamma\end{array} //]]>$) determines together with the material properties the depth of the thermal stimulation within the object. A typical setup for Lock-In thermography can be seen in Figure 7. The object is placed such that if is in full view of the energy source (halogen light) and the infrared camera. In Lock-In thermography, the time (phase) lag and the amplitude of the response signal for the same frequency is measured. Due to the changed reflection properties of defects/material borders one can gain information about geometry and depth of a defect. This is done via the calculation of the thermal diffusion length (or thermal penetration depth) $//<![CDATA[ \begin{array}{l}\mu\end{array} //]]>$. Lock-In thermography is especially well suited for measurements in a depth of a few millimetres.

$//<![CDATA[ \begin{array}{l}\mu = \sqrt{\frac{2\lambda}{\omega * \rho * c_\text{p}}}\end{array} //]]>$

with

$//<![CDATA[ \begin{array}{l}c_\text{p}\end{array} //]]>$ = specific heat capacity $//<![CDATA[ \begin{array}{l}\left[\frac{J}{kg * K}\right]\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}\lambda\end{array} //]]>$ = thermal conductivity $//<![CDATA[ \begin{array}{l}\left[\frac{W}{m * K}\right]\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}\omega\end{array} //]]>$ = angular frequency $//<![CDATA[ \begin{array}{l}\left[\frac{1}{s}\right]\end{array} //]]>$

^[3]

Pulse Thermography

When using pulse stimulation the length of the impulse can be varied according to material and depth of the defect to be detected. Short impulses of some Milliseconds are usually generated with a flash lamp. For detection of defects in concrete at depths of up to 10 cm, the impulse duration may be up to 30 min. The recorded image material can either be evaluated in the time domain, or in the frequency domain after applying a Fast Fourier transform to the recorded data. This yields, for every discrete frequency, an amplitude and phase image which can be evaluated analogous to Lock-In thermography. The phase images create a sort of cross section through the object in question. The higher the frequency the closer under the surface the location of the depth profile and vice versa. A sketch of the general set up for pulse thermography can be seen in Figure 8. ^[3]

Further applications of Infrared Thermography

• Infrarot-Thermographie an Kohlefaserverbundwerkstoffen
• Infrared Thermography with Unmanned Aerial Vehicles in Non-Destructive Testing

Literature

1. Grosse, C.: Grundlagen der zerstörungsfreien Prüfung. Skript, Lehrstuhl für Zerstörungsfreie Prüfung der TU München. München 2015.
2. Baehr H.; Stephan, K.: Wärme- und Stoffübertragung. Springer Verlag, Berlin. Heidelberg, 2003.
3. DGZfP-B-05: Merkblatt über das aktive Thermographieverfahren zur Zerstörungsfreien Prüfung im Bauwesen. Merkblatt B 05. 2013.