Eric Rahne, B.Sc. in Electrical Engineering, Level 3 accredited thermography expert (PIM Ltd.)
One of the most important elements of buildings in terms of energy is their external "cladding" - the walls, windows, and roof. It is crucial that these elements have the best possible thermal insulation properties (lowest thermal conductivity and lowest thermal emissivity). Thermal bridges can be relatively easy to identify: where the highest temperature is observed in an outdoor image, in most cases a thermal bridge (or crack) can be found. In indoor images, the coldest spots usually indicate thermal bridges. It is also possible to determine which building element has better or worse insulation properties. The execution of joints and connections, structural thermal bridges caused by building elements, as well as "defects" from HVAC or electrical installations in external walls can be examined.
A basic requirement for every measurement is that the temperature difference between indoor and outdoor is at least 15 K, the wall is not wet from rain, there is no wind blowing, and the measurement is done during a sun-free period. The accuracy of thermographic (thermal imaging) temperature sensing primarily depends on the emissivity of the measured surface. The smaller the emissivity of a surface, the more important the correction calculation, which, in addition to this factor, can only be calculated accurately with the precise knowledge of the background temperature (ambient temperature) and (in the case of thermally transparent bodies) the background temperature for radiative heat transfer. The emissivity of materials depends on the material itself, surface roughness, wavelength, and viewing angle. It follows from this relationship that in some cases, the temperature of an object cannot be measured:
Similar challenges arise when inspecting glass surfaces: it is characteristic that glass transmits radiation with wavelengths shorter than 3.5 µm (such as sunlight and shortwave thermal radiation), but not with longer wavelengths (e.g. the thermal radiation from lower temperature bodies). In the long-wave range (long-wave atmospheric window: 8 ... 14 µm), glass does not have transmission capability, yet it has only a 60% emissivity factor due to its reflective properties. Moreover, as a "polished" surface, the degree of reflection increases further if the image is not taken at a right angle. Naturally, the presence of dirt (dust) on the glass slightly increases the emissivity, but this is an uncertain and practically negligible factor. In summary, long-wave thermal cameras (such as the currently predominantly marketed so-called real-time matrix - bolometer - cameras, which are the most common for building thermography surveys) detect glass as a mirror, therefore the temperature of glass surfaces cannot be measured with such devices. (Note: With short-wave cameras - depending on the additional filter - we can either see through the glass or measure the surface temperature of the glass, if it exceeds 100°C. In the case of very high temperatures, it is also possible to measure the temperature of glass surfaces with long-wave thermal cameras using very specific filters, but this is rarely done.) During thermographic inspections, temperature decreases due to evaporative heat loss caused by moisture can also be detected. This naturally requires a particularly good thermal resolution camera. This method can help identify issues such as leaks due to faulty roof connections, gutters or sewage pipes, as well as moisture seeping up from the ground or infiltrating, and moisture accumulated in building materials due to condensation.
Building thermographic images must meet certain measurement conditions and thermal camera quality parameters in order to evaluate the measurement results. Very often (also in the mentioned audit report), there are issues with the temperature resolution, geometric resolution, and pixel resolution of the thermal image. (Note: the cause of this is usually the selection of inadequate - too low-power - thermal cameras and the incomplete professional qualifications of the contractor.) Temperature resolution Especially when the range to be measured falls between room temperature and the lower limit of the measurement range, the temperature resolution predominantly determines the image quality. The "NETD" (noise equivalent temperature difference) represents the effective value of the camera's own noise, expressed in the temperature difference that results in the same electrical signal magnitude (usually measured at 30°C). (In other words: the "NETD" is the value of the temperature change that results in the same electrical signal change in the camera due to its own noise.) This value increases significantly when the object's temperature decreases, especially for short-wave devices. Since in building thermographic measurements temperature differences smaller than 0.5°C must be recognizable in the thermal image, and the "noise level" required for this should be less than half of this, i.e., less than 0.25°C, or more precisely +/-0.12°C. As thermal camera manufacturers define the aforementioned NETD value at 30°C and this value deteriorates significantly with decreasing temperature, for typical outdoor temperatures during building thermographic measurements (-15°C ... +5°C), a thermal camera with at least +/-0.08°C NETD value (or in other words: with thermal resolution) must be chosen. With lower-quality thermal cameras, the aforementioned building defects cannot be identified! Geometric resolution In addition to temperature resolution, geometric resolution also significantly influences the achievable image quality and the authenticity of the image's temperature data. The IFOV parameter influencing this (given typically in mrad) provides the smallest elemental viewing angle that has been mapped with a unique sensor (pixel). To ensure good reproduction of details, it is important for this value to be as small as possible.
For example, a 1.5 mrad IFOV indicates that each individual measurement point assigned to each pixel (projected measurement spot) has a diameter of 1.5 mm at a distance of 1m.
Since the position of the "projected" image point on the object to be measured is unknown and the sensor matrix itself (due to manufacturing technology requirements) has gaps, the above point size must be multiplied by 3 to determine the smallest measurable object size. Failure to do so may result in the measurement spot containing not only the radiation from the object's surface but also from its background. Due to averaging within the measurement spot, the measurement result can be either lower or higher than the object's actual temperature due to the background temperature. The greater the temperature difference between the object and the background, the greater the measurement error will be! Of course, this rule applies not only to small objects (e.g., thin wires, filaments, etc.) but also when measuring large objects (e.g., large cross-sectional cables, doors, etc.). Obviously, different dimensions are involved: for small objects, we are talking about measuring surfaces on the order of millimeters, which can be measured from distances of up to several tens of centimeters based on the geometric resolution capability of the applied thermal camera and optics; for large objects, we are talking about measuring surfaces on the order of centimeters from distances of several meters (up to 10 meters). In all cases, the use of equipment that allows compliance with the above rule is necessary! Concrete example:If we want to measure a ten-story panel building, then to measure the upper floors of the built structure (at about 30 meters height), we need to work from a distance of about 60-70 m to minimize geometric distortion of the image (to avoid perspective effects). According to Pythagoras, the distance between the thermal camera and the object in this case is 67 - 76 m, so with a thermal camera resolution of 1.3 - 1.4 mrad (the geometric resolution achievable with most professional cameras' standard lenses), the elementary measurement point has a diameter of 87 - 106 mm, thus the smallest measurable object must be larger than 261 or 318 mm! (As a reminder: a window frame is rarely wider than 70 mm). Therefore, the use of a telephoto lens is necessary, which (depending on the camera type) provides a geometric resolution of 0.2 - 0.5 mrad.Number of pixels In addition to geometric resolution, the image quality achievable with a thermal camera - or more precisely, the detail of the measurement - is determined by the number of pixels of the thermal camera. The reason for this is that for graphic recognition, a certain minimum number of pixels must fall on different parts of the object being measured - just as we are used to in digital photography. It is easy to understand that with more pixels, we can represent the object surface with greater detail, or the same level of detail over a larger object surface in a single thermal image. If the number of pixels is low, many images need to be taken, and for the evaluation of continuous objects and the preparation of reports, it often becomes necessary to montage the images (which is a very time-consuming task). This question is not insignificant for thermal cameras. While in digital cameras we talk about resolutions of 5, 6, 7, or even more than 10 megapixels (10 million pixels), in the case of matrix thermal cameras, the number of pixels is typically 320x240 (thus 76,800 pixels). There are also cameras with lower capabilities (a common type is 160x120, with only 19,200 pixels), which therefore can only display smaller areas with acceptable detail, which naturally strongly limits their application area (in return, their price is very favorable). Thanks to the development of thermal camera sensors, more and more thermal cameras with higher numbers of pixels are being produced. Thermal cameras with a sensor matrix containing 384x288 elements are available at an acceptable price, and even devices with sensor matrices of 640x480 pixels are available (with a frame rate of 50 or 60 Hz). In building thermography, acceptable image quality (or more precisely: detail) is achieved by using a 320x240 pixel thermal camera to take "overview" thermal images of up to 2 floors of wall details, as in this case one pixel is captured every 25 millimeters (remember: with averaging!). Depending on the camera lens - with a standard lens - this typically means a measurement distance of 15 m (with a geometric resolution of 1.5 mrad, the pixel size is then 22.5 mm, so the smallest - accurately measurable - object size is 67.5 mm!). For capturing details, additional shots need to be taken from closer or with a telephoto lens.
The primary purpose of building thermography is the objective and comprehensive assessment of building insulation. But never forget that thermographic measurement serves to capture the momentary surface temperatures, influenced by various measurement conditions. Regarding buildings, the following thermographic procedures are fundamentally distinguished from each other: Quantitative thermographic inspections The aim of quantitative building thermography is to evaluate the complete surface temperature distribution of a building and determine the heat transfer coefficient (e.g., when calculating heat loss or heating energy requirements). Since the coefficient can only be calculated based on very accurate (absolute accuracy) temperature data, very strict conditions must be met regarding the 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" (quality differences). Most problems can be detected based on heat differences that can be displayed with a thermal camera with sufficiently high temperature resolution capability, where the absolute (numerically exact) temperature data play a minor role. The characteristics of the procedure are
Both quantitative and qualitative thermographic building inspections should include indoor and outdoor inspections. In fact, for quantitative inspections, indoor measurements are practically mandatory, as this is the only way to calculate the heat flow properties of individual surfaces. Our table provides an overview of the measurement conditions to consider during indoor and outdoor measurements, as well as the differences (difficulties) between conducting the measurements. Differences between indoor and outdoor thermographic inspections
| Outdoor thermography | Indoor thermography |
| + Generally, the entire wall surface is well visible (good visibility). | - Often only certain parts of the walls can be measured, and essential surfaces may be inaccessible. |
| + Adjustable recording positions. | - Visibility is often limited due to existing furniture or building services elements. |
| + Requires little time, does not disturb the building's occupants. | - Requires a long preparation time, disturbs the building's occupants. |
| - Highly weather-dependent, can only be done in sunny weather. | + Weather has no direct impact (although the effect is significantly weakened). Under certain conditions, it can also be done during the day. |
| - Problematic for roof structures and ventilated building claddings. | + More assessable for roof structures and ventilated building claddings. |
| - Difficulty in covering external surfaces with trees, balconies, building decorations. | + Lack of air insulation (drafts) is more noticeable in windy conditions. |
Table 1: Differences between indoor and outdoor thermographic inspections General recommended measurement conditions and requirements In order to produce thermal images with a thermal camera that can be evaluated by architects, energy experts, structural engineers, and operators - allowing for correct conclusions - the following minimum requirements must be met:
From the above, it follows that building thermography can only be carried out during the heating season, in appropriately cold (below 5°C) weather.
When creating high-quality, evaluable thermal images, there are significant expectations for the thermal camera itself. Without meeting these requirements, we would obtain unrecognizable thermal images that cannot detect faults. The thermal camera must minimally meet the first six points listed below:
Let's look at examples of thermal images to demonstrate the importance of adhering to the above rules:
- Solar radiation reflects off the building walls - the walls appear warm, although there is no heating!!!
- The heating effect of daytime sunlight is barely noticeable, so measurements can now be taken.
Thermographic survey conducted in strong winds - the wind carries heat away from the right wall, making it cooler - it appears as if the insulation on the right wall is better
Measurement conducted in calm winds: - it is evident that the insulation of the right wall is as poor as the wall surface in the middle of the thermal image (and has the same strong thermal bridges) Note for both thermal images: the left part of the building is not heated (staircase)
Left image: ideal temperature gradient of an external wall (stationary state) Right image: post strong daytime heating - indoor (measurable) wall temperature: 21°C - indoor (measurable) wall temperature: 21°C - outdoor (measurable) wall temperature: 3°C - outdoor (measurable) wall temperature: 8°C Note: the horizontal segment on the right image is the result of the temporary heating effect of the day. The image illustrates the evening (after the external temperature has cooled down) temperature gradient. The increased temperature of the external wall leads to the mistaken conclusion that there is poor insulation or a thermal bridge, although the same wall structure is assumed as in the left image.
On the left side, the window was left open in a tilted position before the measurement, showing the effect of warm air flowing out on the window frame and the wall above it. It is not possible to make a correct evaluation of the window's condition or its bridging insulation properties.
The room or apartment shown in the picture (top left) is not heated. It is naturally not possible to detect insulation and other architectural defects in such cases (of this room or apartment).
Unfortunately, since the emissivity factor depends on the viewing angle as well, we must consider that the more the viewing angle deviates from the right angle, the more increasing reflection is observed. This effect is most noticeable in the measurement of curved surfaces, but it is also encountered when measuring the upper floors of tall buildings: seemingly, the upper floors appear cooler and cooler (although in reality, they are getting warmer). The explanation is that the sky (without clouds -273°C) reflects more strongly on the outer surface of the building, despite the emissivity factor of approximately 95% for wall surfaces made of silicate-based building materials. Beyond geometric resolution, the image quality achievable with a thermal camera - or more precisely, the detail of the measurement - is determined by the number of pixels in the thermal camera image. While digital cameras are discussed in terms of 5, 6, 7, or even more than 10 megapixels (10 million pixels) resolution, matrix thermal cameras typically have a number of pixels of 320x240 (thus 76,800 pixels). There are also cameras with lower capabilities - the 160x120 resolution type is common (with only 19,200 pixels), which can therefore only display smaller areas with acceptable detail. The following thermal images vividly demonstrate the effect of the number of pixels on the detail of the thermal image.
(To be continued) Eric Rahne B.Sc. in Electrical Engineering (BME) Vibration Diagnostic Expert Thermography Expert (Thermograph Level3) pim-kft.hu, termokamera.hu
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