Theory and Evaluation Methods
The creation of thermal images, i.e., thermography - or less accurately termed thermovision - is an extremely versatile measurement procedure, and handling modern thermal cameras can be compared to widely used digital video cameras. However, this simplicity should not mislead anyone: professional (correct from a measurement perspective) thermal image creation requires professional (theoretical) knowledge, experience, and proper measurement preparation. (Otherwise, only "nice colorful" - but uninterpretable, leading to erroneous conclusions - images would be produced.)
It is a sad experience that distributors of thermal cameras and service providers creating thermal images often make serious professional mistakes related to thermal image production. Therefore, we will repeat the presentation of the most important theoretical and practical thermographic regularities so that both the creators of the images and the users of the evaluations can better utilize the information provided by the thermal images!
Theoretical Introduction
The basis of infrared temperature measurement Infrared temperature measurement based on infrared radiation, i.e., thermography, utilizes the physical phenomenon that above the absolute zero K temperature (-273.15 °C), bodies emit electromagnetic waves, such as radio waves, light, and heat (radiation). Infrared radiation is found in the electromagnetic spectrum in the wavelength range of 760 nm to 1 mm. From a technical perspective of temperature measurement, the range up to 20 µm is significant. This can be divided into the following parts:
| Wavelength | Infrared Subrange |
| 0.8 µm ... 2 µm | short-wavelength infrared |
| 2 µm ... 6 µm | mid-wavelength infrared |
| 6 µm ... 20 µm | long-wavelength infrared |
Temperature measurement is based on the electromagnetic waves (infrared radiation) emitted by the object being measured. In order to infer the temperature, the relationship between the object's temperature and the emitted radiation must be examined. This relationship is described by the Planck radiation law, which describes the spectral distribution of radiation emitted by an ideal radiator (black body), and in essence - summarized briefly - the following: the hotter an object is, the more radiation it emits, and the shorter the wavelength of the most strongly emitted radiation. It is also noteworthy that long wavelengths are always present (they strengthen with increasing temperature), while short wavelengths are emitted only by hot objects.
As an illustration of this relationship, we present the radiation maxima of a few practical materials:
| Radiating Object | Temperature | Radiation Maximum |
| Deep-frozen food | -18 °C | 11.4 µm |
| Skin | 32 °C | 9.5 µm |
| Boiling water | 100 °C | 7.8 µm |
| Dark red-hot iron | 600 °C | 3.3 µm |
| White-hot iron | 1200 °C | 2.0 µm |
Table: Radiation maxima of materials encountered in practice
Practical Issues of Non-Contact Temperature Measurement
Properties of the objects being measured - heat emission and reflection The so-called black body is the ideal physical radiation model that emits 100% of the heat radiation expected according to the Planck radiation law based on its temperature. However, the radiation capability of real bodies more or less lags behind the black body model. The emission capacity of a body in the infrared radiation range is described compared to the black body by the emissivity factor (ε), which depends primarily on the material (more precisely, its surface), the surface roughness, and the wavelength (thus, the temperature of the body). It is important to know that the incorrect consideration of the emissivity factor is the most common and, in terms of error magnitude, the most significant error affecting the results of thermographic measurements. The more the emissivity factor of a body deviates from the ideal value of 1 (i.e., the lower the emissivity), the more its reflective (radiation reflection) property is strengthened. Therefore, in addition to measuring the heat radiation emitted proportionally to the temperature of the body (in the worst case, even instead of it), the instrument also measures the heat radiation reflected on the surface of the object from the environment. To reduce this error, the emissivity value must be specified as accurately as possible, and the ambient temperature must also be taken into account when determining the object temperature. The effect of the transmission stage on the measurement result - transmission loss Since the infrared radiation forming the basis of thermography must pass through some medium (from the object being measured to the measuring device), the behavior (characteristics) of the medium in the infrared range naturally affects the measurement. In most cases, the medium is air, but other materials that transmit infrared waves (e.g., special measuring windows) also occur.
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| Figure: Spectral transmission factor of air [source: Infratec] |
The figure above shows that the air transmission properties depend greatly on the wavelength. The regions with good transmission capability (shaded) are commonly referred to as atmospheric windows. While the transmission factor in the 8...14 μm range - the long-wave atmospheric window - provides almost perfect transmission even over long distances, in the 3...5 μm range - the mid-wave atmospheric window - the atmosphere causes measurable losses even at distances of several tens of meters.
Transmission of heat radiation from the background of the object This error occurs when the object is partially transparent, especially in terms of infrared radiation. In such cases, the background of the object must be taken into account just as much as the foreground in terms of heat radiation reflection. This can be particularly problematic when strong heat emitters (e.g., technologically necessary heating devices) are located directly behind the object being measured. Conditions for building and HVAC thermography measurements Considering what has been discussed in the theoretical introduction, we already know that due to the low temperatures to be measured, we need to choose a long-wave thermal camera. This is good for us because thanks to the favorable transmission properties of the long-wave atmospheric window, we can detect heat radiation from distances of up to a hundred meters almost "losslessly." Furthermore, it is favorable that typical building materials (except for window glass and shiny metal coatings!) have relatively high emissivity, and since the temperatures of the objects to be measured typically differ only slightly from the ambient temperature, we only need to consider the effect of reflection to a small extent. (It is a completely different situation with glass surfaces or brand-new aluminum claddings for insulation! In such conditions, they simply cannot be measured!)
Special Features of Building Thermography
The primary goal of building thermography is the objective and comprehensive assessment of building insulation. However, never forget that thermographic measurement serves to capture the momentary surface temperatures, which are influenced by various measurement conditions. Regarding buildings, the following thermographic procedures are distinguished: Quantitative thermographic inspections: The aim of quantitative building thermography is to evaluate the complete surface temperature distribution of the building and determine the heat transfer coefficient (e.g., calculating heat loss or heating energy requirements). Since this can only be calculated based on very accurate (absolute accuracy) temperature data, very strict conditions must be met regarding data collection with the thermal camera. The procedure is characterized by:
Qualitative thermographic inspections:
The purpose of qualitative thermal imaging building inspections is to search for and document the building's thermal bridges and insulation "defects" (qualitative differences). Most problems can be detected based on temperature differences that can be displayed with a thermal camera with sufficiently high temperature resolution, with absolute (numerically accurate) temperature data playing a minor role in such cases. The characteristics of the procedure are:
Both quantitative and qualitative building thermographic inspections should include both indoor and outdoor inspections. Moreover, for quantitative inspections, indoor measurements are practically "mandatory" because this is the only way to calculate the heat flow properties of individual surfaces. The following table provides an overview of the measurement conditions and differences (difficulties) between indoor and outdoor measurements that need to be considered during the inspections.
Generally Recommended Measurement Conditions and Requirements
In order to not only capture beautiful colorful images of the building to be inspected with the thermal camera but also to produce thermal images that can be evaluated by architects, energy experts, structural engineers, and the operator - allowing for correct conclusions - the following MINIMUM requirements must be met:
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| Figure: Condition assessment of an industrial hall - air tightness defects and condensation [source: PIM] |
Special Application: Leak Detection with Thermography
Thermographic leak detection is based on the physical laws of heat conduction. If the temperature of the medium flowing in the pipe (usually water) is higher than its surroundings (heating or hot water pipes, underfloor heating, etc.), heat conduction occurs through the surrounding materials to the external (observable) surface. Thus, during the heating process, the position of the conduit becomes visible first using thermographic devices. At the location of the leak, the warm liquid escapes, so it can flow along the conduit and between the layers of surrounding materials laterally and downward. Heat conduction now also starts from the warm liquid in all directions, including towards the visible surface. With sufficiently sensitive thermographic devices, it can be determined that the visible heat distribution on the surface differs from the "normal" heat distribution of an intact conduit. It is always valid that leaks can only be detected with thermographic devices if there is or can be created a temperature difference at the leak site, which can be observed on the surface through heat conduction with a thermal camera. It is advisable to repeat the thermographic recording from multiple viewing angles at suspected leak locations to safely eliminate reflections. Requirements for a thermal imaging system:
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| Figure: Theory of leak detection [source: PIM] |
Evaluation of Building Thermographic Measurements with Special Software
In the previous parts of our article, we primarily dealt with the qualitative thermographic examination of buildings. However, in the following, we aim to introduce a special building thermography image evaluation software that is capable of supporting both qualitative examination and quantitative evaluation. Obviously, the primary requirement for all evaluations is that the thermal images are taken under the mentioned measurement conditions, and important environmental parameters for evaluation are also recorded. In addition, precise knowledge of the building's material structure (e.g., materials used, layer thicknesses, as well as elements of heating, ventilation, air conditioning) is necessary.
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| Image: FORNAX evaluation software "in action" [source: Infratec] |
Geometrical Image Correction
No matter how well buildings are surveyed, the images (as in traditional photography) have certain parallax errors, distortions, and perspective errors. For evaluations, orthogonal image data is usually required. The FORNAX software performs this correction with one or two button presses.
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| Image: Thermal image geometric correction before and after [source: Infratec] |
Temperature Statistics
Temperature statistics provide assistance for assessing the severity of even the most severe thermal bridge problems (i.e., the answer to the question "Is it worth dealing with?"), as well as for assessing thermal stresses occurring in the structure.
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| Image: Temperature statistical evaluation [source: Infratec] |
Condensation, Mold Growth
Once we know at what temperature objects (walls) will experience condensation (due to reaching the dew point) under a given air temperature and humidity, based on the listed environmental parameters, condensation and mold growth can be predicted from internal thermal images.
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| Image: Marking condensation, mold growth risk on thermal image [source: Infratec] |
Essential parameters for evaluation:
Moisture Infiltration of Wall Structure
Depending on the knowledge of the wall structure, not only the risk of condensation (and mold growth) can be identified, but also how long it will take (while maintaining the current use of the room) for the building material or insulation to become moist. (This would lead to a complete loss of its insulating properties, so the process must be stopped!) The evaluation requires precise knowledge of climatic conditions, room usage, and the layer structure of the wall.
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| Image: Prediction of Moisture Ingress in Wall Structure on Thermal Image [source: Infratec] |
Risk of Frost Damage
All building elements are at risk of frost damage where moisture has accumulated for some reason and their temperature falls below freezing point. Based on climatic conditions, the program can analyze the occurrence of frost damage risk in four categories: frost damage risk only in very cold winter frost damage risk in cold winter frost damage risk in fall and winter no frost damage risk
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| Image: Indication of Frost Damage Risk in Wall Structure on Thermal Image [source: Infratec] |
Numerical Determination of Heat Flow
The most important evaluation of quantitative building thermography technology is the numerical determination of heat flow. The program assumes (very simplified) that the radiated heat quantity is proportional to the quantity of heat "transported" from inside to outside through heat flow. However, it is crucial to adhere to very strict measurement conditions and capture fixed thermal images! (Otherwise, completely erroneous data will be obtained!)
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| Image: Numerical Determination of Heat Flow Based on Thermal Image [source: Infratec] |
Numerical Determination of "U" Factor (Heat Loss Factor)
Based on heat flow, the so-called "U" factor (the heat loss factor) can be determined. Of course, indoor and outdoor temperatures must also be provided for this calculation.
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| Image: Numerical Determination of the "U" Factor (Heat Loss Factor) [source: Infratec] |
Numerical Determination of Heating Costs
Calculating heating costs is not a big step from determining the heat loss factor. Based on the heat loss factor, it is possible to determine how much energy is needed to heat the building (considering climatic conditions, desired indoor temperature, and ventilation habits). If the energy proportional costs of different heating technologies and fuels are known, the expected annual heating costs can be determined with a "simple" multiplication.
Rahne Eric (PIM Ltd.) pim-ltd.com, engineeringexpert.com
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