Facts about thermography

FACTS THERMOGRAPHY

Brief thermography facts

The basics of thermography and emission factor (and how to set it), factors affecting measurements, tips for outdoor thermography and an emission factor table.

Measurement facts thermography

Thermography in theory.

All objects with a temperature above absolute zero (0 Kelvin = -273.15 °C) emit IR radiation. The human eye does not perceive such radiation, as we are largely blind to this wavelength. However, the thermal imaging camera is not. The critical component – the detector – is sensitive to IR radiation. By measuring the strength of the IR radiation, it determines the temperature of the surface of the object being measured and makes it visible to the human eye using a thermal image. This process is called thermography.

To make the IR radiation visible, it is registered by the detector and converted into an electrical signal. Then each signal is assigned a specific color, which is then shown on the thermal imaging camera’s display. Basically, the thermal imaging camera translates wavelengths from the infrared spectrum into wavelengths visible to the human eye (colors).

Thermography radiation spectrum

There is a common misconception that you can see inside objects with a thermal imaging camera. This is not possible, only the surface temperature can be visualized.


Emission, reflection and transmission.

The IR radiation recorded by a thermal imaging camera consists of emission, transmission and reflection.

Emission (ε)

Emission is the ability of a material to emit IR radiation. This capability is expressed using the emission factor. It depends, among other things, on the material itself and the surface texture. For example, the sun has an emission factor of 100%. This value appears nowhere else. Concrete, on the other hand, is close with an emission factor of 93%. This means that 93% of the IR radiation is emitted from the concrete itself.

Reflection (ρ)

The remaining 7% are reflections from the material/object of measurement, i.e.. the temperature reflected from the measured object. In a thermal imaging camera, the emission factor and reflected temperature can be set to obtain a thermal image with the highest possible accuracy.

Transmission (t)

Transmission is the ability of a material to allow IR radiation to pass through it (transmissivity). What appears transparent to the eye is not always transparent to IR radiation. For example, window glass is almost completely opaque to IR radiation while some thin plastic materials that are completely opaque to the eye can be almost completely transparent to IR radiation. However, most materials do not transmit IR radiation, which means that as a rule the transmissivity is almost zero in most cases and can be ignored.

Thermography emission transmission reflection

The relationship between emission and reflection

1. target with high emission factor (ε ≥ 0,8):

  • Has low reflectance (ρ): ρ = 1 – ε
  • The temperature of such objects can be measured very easily with a thermal imaging camera.

2. Measurement objects with medium emission factor (0.8 < ε < 0.6):

  • Has medium reflectance (ρ): ρ = 1 – ε
  • The temperature of such objects can be easily measured with a thermal imaging camera.

3. Target with low emission factor (ε ≤ 0.6)

  • Has high reflectance (ρ): ρ = 1 – ε
  • It is possible to use the thermal imaging camera to measure the temperature, but you should review the measurement results very carefully and ensure that the correct emission factor is used.

Setting the emission factor.

Checking that the emission factor is correctly set is particularly important when there is a large temperature difference between the measured object and the environment.

When the temperature of the measured object is higher than the ambient temperature:

  • An emission factor set too high will result in too low temperatures in the heating image.
  • An emission factor that is set too low results in too high temperatures in the heating image.

When the temperature of the measured object is lower than the ambient temperature

  • An emission factor set too high will result in too high temperatures in the heating image.
  • An emission factor set too low will result in too low temperatures in the heating image.
Thermography emission factor setting

Tips on the emission factor.

  • The greater the difference between the temperature of the measured object and the ambient temperature and the lower the emission factor, the greater the measurement error. These errors are even greater if the emission factor is incorrectly set.
  • Many materials that are transparent to the human eye (such as glass) are not permeable to IR radiation.
  • The few transmissive materials available include thin plastic materials and germanium, the material used for the lens and protective glass in a Testo thermal imaging camera.
  • If necessary, remove everything that covers the measured object, otherwise the thermal imaging camera will only measure the surface temperature of the covering material.
  • Always take into account the operating conditions of the measured object
  • If subsurface parts affect the temperature distribution on the surface of the target through thermal conduction, it is often possible to identify internal structures in the thermal image. However, the thermal imaging camera never measures anything other than surface temperatures. It is therefore not possible to draw any conclusions about temperature values inside the object being measured.
Thermal imaging camera testo 868s left

Field of view, smallest measurable object and measuring spot.

Tips on the emission factor

The field of view (FOV) describes the area visible to the thermal imaging camera. It depends on the optics used. Wide angle lenses are suitable for large fields of view, telephoto lenses for good spatial resolution. The larger the field of view, the more you see. A wide field of view (> 30°) is particularly useful indoors, as walls limit the ability to stand further away from the measurement object with the thermal imaging camera to see more.

Thermography measuring distance

Minimum measurable object / measurement spot (IFOVmeas)

The smallest measurable object describes the smallest object that can not only be identified, but whose temperature can be measured with precision. At a spatial resolution of the lens of 3.5 mrad and a measuring distance of 1 m, the smallest measurable object has an edge length of 3.5 mm and appears in the display as a pixel. For an accurate measurement, the measuring object should be 2 to 3 times higher than the smallest measurable object. As a rule of thumb for the smallest measurable object (IFOVmeas): IFOVmeas ≈ 3 x IFOVgeo.

Minimum identifiable object (IFOVgeo)

The smallest identifiable object is the smallest dimension that can be identified by a pixel. A pixel is an element on the thermal imaging camera’s detector that detects IR radiation and converts it into electrical signals. Each pixel corresponds to a measurement value.

Thermography measuring spot

Thermography in practice.

Non-contact temperature measurement with a thermal imaging camera is indispensable in many applications. If you follow some basic rules, you will be able to use infrared measurement even more effectively from now on.

  • Learn more about the main theoretical principles of thermography
  • Benefit from valuable tips for your daily work with a thermal imaging camera.

Measurement object.

1. material and emission factor

A thermal imaging camera measures the long-wave IR radiation emitted by all objects. The amount of IR radiation emitted by the measured object depends on the surface of the material.

ATTENTION! Each surface has a specific emission factor.

Thermography material emission factor
Colour thermography

2. color

The color of the surface has no significant effect on the long-wave IR radiation emitted by the target. The crucial factor is temperature. For example, a black-coated heating element emits exactly the same amount of long-wave IR radiation as a white-coated one at the same temperature.

ATTENTION! The color of the surface almost doesn’t matter.

3. Surface of the object to be measured

In thermography, the surface structure of the measured object plays a crucial role. The surface emission factor varies depending on the surface structure. Coatings and dirt also play a role.

Thermography surface of the object to be measured

Surface structure

As a rule, the emission factor on smooth, shiny, reflective and/or polished surfaces is slightly lower than on textured or matt, rough, weathered and/or scratched surfaces, even if the material is the same.

ATTENTION! When measuring on smooth surfaces, particular attention should be paid to the presence of other sources of radiation in the environment (e.g. sunlight, heating elements, etc.).

Moisture, snow and frost on the surface

Water, snow and frost have relatively high emission factors (about 0.85 < ε < 0.96). Therefore, measurement on such surfaces is normally not a problem. However, it must be remembered that the temperature of the measured object can be disturbed by natural coatings of this type. This is because moisture cools the surface of the measured object as it evaporates and snow has good insulating properties. Hoarfrost does not normally form a dense surface. Therefore, the emission factor of both the frost and the underlying surface needs to be taken into account in measurements.

ATTENTION! If possible, avoid measuring on wet, snow- or frost-covered surfaces.

Dirt and foreign objects on the surface

Dirt and foreign objects such as dust, soot or lubricants on the surface usually increase the emission factor. Therefore, measuring on dirty objects is usually not a problem. However, the thermal imaging camera always measures the surface temperature, i.e. dirt and not the exact temperature of the underlying surface of the measured object.

ATTENTION! Avoid measuring on loose dirt (error measurement if air is included in the material).


The measurement environment.

1. Ambient temperature

Another factor you need to consider is the reflected temperature (RTC) and the emission factor setting (ε), so that your thermal imaging camera can calculate the surface temperature correctly.

  • In many cases, the reflected temperature is identical to the ambient air temperature.
  • Checking that the emission factor is correctly set is important when there is a large temperature difference between the measured object and the environment.
Thermography ambient temperature
Thermography radiation sources

2. Sources of radiation and interference

All objects with a temperature above absolute zero (0 Kelvin = -273.15 °C) emit IR radiation. Objects whose temperature is very different from the target may interfere with the IR measurement, due to their own radiation. You should avoid or screen out such sources of interference whenever possible.

  • For example, use a screen or a corrugated cardboard box to shield the sources of interference.
  • You can measure the reflected radiation, e.g. using a Lambert radiator, in combination with your thermal imaging camera.

3. Weather

Clouds
Ideally, outdoor IR measurements should take place when cloud cover is thick. Reason: The target is shielded from solar radiation and ‘cold sky’ radiation.

Precipitation
Water, ice and snow have a high emission factor and are impervious to IR radiation. In addition, measuring on wet objects can result in measurement errors, as the surface of the measured object cools down when the moisture evaporates.

ATTENTION! Heavy precipitation (rain, snow) can disturb the measurement result.

4. air / humidity

If there is condensation due to high relative humidity on the thermal imaging camera’s optics (or protective glass), the IR radiation cannot be fully received. Because of the water, the radiation does not reach the optics. Extremely dense fog can also affect the measurement because the droplets between the camera and the object to be measured transmit less IR radiation.

ATTENTION! Check that the relative humidity of the environment is low. This allows you to avoid condensation in the air (fog), on the measuring object, the protective glass or the lens of the thermal imaging camera.

Thermography air humidity weather clouds

Air currents
As a result of heat exchange (convection), the air near the surface has the same temperature as the object being measured. If it is windy or draughty, this layer of air will “blow away” and be replaced by a new layer that has not yet adapted to the temperature of the measured object. As a result of convection, heat will be carried away from a warm target or absorbed by a cold target, until the air temperature and the surface of the target have adapted to each other. This heat exchange effect increases as the temperature difference between the surface of the target and the ambient air increases.

ATTENTION! Air currents or drafts in the room can affect temperature measurements with thermal imaging cameras.

Air pollution
Some particles such as dust, soot and smoke and even some fumes have high emission factors and are barely permeable. This means that they can interfere with the measurement, as they emit their own IR radiation which is received by the thermal imaging camera. In addition, only part of the IR radiation from the target can reach the thermal imaging camera, as it is scattered and absorbed by the particles in the air.

5. Light

Light or illumination has no significant impact on measurements made with a thermal imaging camera. It can also be measured in the dark, as the thermal imaging camera measures long-wave IR radiation. However, there are light sources that themselves emit IR thermal radiation and thus can affect the temperature of adjacent objects.

  • For example, do not measure in direct sunlight or near a hot light bulb.
  • Cold light sources such as LEDs or neon lights do not interfere significantly with the measurements: they convert most of the energy into visible light and not into IR radiation.
Thermography light exposure

Considerations for outdoor thermography.

The IR radiation from a clear sky is called ‘cold sky radiation’. On a cloudless day, cold celestial radiation (~ -50 … -60 °C) and warm solar radiation (~ 5500 °C) are reflected. The sun only occupies a small part of the entire sky, which means that the reflected temperature is normally below 0°C, even on a sunny day. Objects are heated by the sun because they absorb sunlight. It has a major impact on the surface temperature – sometimes several hours after being exposed to solar radiation.

Outdoor thermal imaging camera

Tips & Tricks for outdoor thermography

  • The ideal time for outdoor thermography is in the early morning hours and/or under dense cloud cover. It should not rain or snow. Fog and strong winds are also best avoided.
  • During the measurement, it is useful to change position in order to detect reflections. Reflections change while thermal anomalies of the target remain the same – even when the viewing angle changes.
  • Avoid measurements near very hot or cold objects, or shield them.
  • Avoid direct sunlight, even if the sun was shining several hours before the measurement. Take into account cloud cover, even if it appeared several hours before the measurement.
  • Do not measure if there is condensation on the thermal imaging camera.
  • Do not measure in highly polluted air (e.g. just after dust has been stirred up).

Emission factor table.

MaterialMaterialtemperaturEmissionsfaktor
Aluminium, blankvalsad170 °C0,04
Aluminium, ej oxiderad 25 °C0,02
Aluminium , kraftigt oxiderad 93 °C0,20
Aluminium, högpolerad100 °C0,09
Bomull 20 °C0,77
Betong 25 °C0,93
Bly, grovt 40 °C0,43
Bly, oxiderat40 °C0,43
Krom40 °C0,08
Krom, polerad150 °C0,06
Is, slät0 °C0,97
Järn, smärglat 20 °C0,24
Järn med gjutbeläggning 100 °C0,80
Järn med valsbeläggning20 °C0,77
Gips20 °C0,90
Glas90 °C0,94
Granit20 °C0,45
Gummi, hårt 23 °C0,94
Gummi, mjukt grått23 °C0,89
Gjutjärn, oxiderat200 °C0,64
Trä70 °C0,94
Kork20 °C0,70
Radiatorelement, svarteloxerat50 °C0,98
Koppar, något matt20 °C0,04
Koppar, oxiderad130 °C0,76
Koppar, polerad 40 °C0,03
Koppar, valsad40 °C0,64
Plast: PE, PP, PVC20 °C0,94
Blå färg på aluminiumfolie 40 °C0,78
Mattsvart färg80 °C0,97
Vit färg90 °C0,95
Vit marmor40 °C0,95
Murverk40 °C0,93
Mässing, oxiderad200 °C0,61
Oljefärg (alla färger)90 °C0,92 ... 0,96
Papper20 °C0,97
Porslin20 °C0,92
Sandsten40 °C0,67
Stål, värmebehandlad yta200 °C0,52
Stål, oxiderat200 °C0,79
Stål, kallvalsat93 °C0,75 ... 0,85
Bränd lera70 °C0,91
Transformatorfärg70 °C0,94
Tegelsten, murbruk, puts 20 °C0,93
Zink, oxiderad-0,1


Thermal imaging cameras and thermography courses from Nordtec.

Entry-level model testo 865s

Testo has a wide range of good and affordable thermal imaging cameras in different classes. The entry-level 865s has a resolution of 19,200px.

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testo 890 professional thermal imaging camera

Professional and expert class

The testo 890 series meters have exceptionally high resolution and high thermal sensitivity. They also have a number of important features such as voice recording, interchangeable lenses, etc.

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testo 883 courses

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Nordtec has a basic course in thermography included with the purchase of a thermal imaging camera. The course is run by a qualified and experienced trainer who mixes theory and practice for optimal learning.

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