Thermography / Infrared temperature measurement: theory and practice

Non-contact temperature measurement using a thermal imager is essential for many applications. If you follow a few basic rules, you can now use infrared measurement more effectively. Learn more about the most important theoretical bases of thermography. And enjoy useful tips for your daily work with the thermal imager.

Measuring object

1. Material and emissivity

A thermal camera measures long wavelength infrared radiation emitted by an object. The intensity of the infrared radiation emitted (by the object itself) depends on the surface of the material.

Note: Each surface has a specific emissivity.

2. Color

The color of the surface has no significant influence on the long wavelength infrared radiation emitted by the measurement object. It is the temperature that is decisive. A black lacquered radiator emits, for example, infrared radiation of long wavelength identical to a white lacquered radiator at the same temperature.

Note: The color of a surface hardly plays a role.

3. The surface of the measurement object

The properties of the surface of the measured object play a decisive role in thermography. This is because the emissivity of the surface changes depending on the structure of the surface, fouling and coating.

• Surface structure

Smooth, shiny, reflective and / or polished surfaces generally have a slightly lower emissivity than matte, structured, raw, weathered and / or scratched surfaces of the same material.

Note: Pay particular attention to the presence of possible sources of radiation in the environment (eg sun, heaters, etc.) when measuring on smooth surfaces.

• Moisture, snow, frost on the surface

Water, snow and frost have a relatively high emissivity (approx. 0.85

Note: Avoid, if possible, taking measurements on wet surfaces or surfaces covered with snow or frost.

• Contamination and foreign bodies on the surface

Dirt or foreign objects, such as dust, soot or lubricating oil, on the surface of the object being measured generally increase the emissivity of the surface. Therefore, measuring dirty objects is usually no problem. However, your thermal imager still measures the temperature of the surface, that is, that of the dirt and not the precise temperature of the surface of the object measured under it.

Note: Avoid taking measurements on surfaces with particles on the surface (temperatures distorted by air pockets).

Measurement environment

1. Ambient temperature

In order for your thermal imager to measure surface temperature correctly, the reflected temperature (RTC) must be considered in addition to the emissivity (ε) setting.

In many measurement applications, the reflected temperature corresponds to the ambient temperature.

Accurate adjustment of the emissivity is important when the temperature differences between the measured object and the measurement environment are large.

2. Radiation and parasitic sources

Every object whose temperature exceeds absolute zero (0 Kelvin = 273.15 ° C) emits infrared radiation. Objects with a large temperature difference from the temperature of the measured object can influence infrared measurements due to their own radiation. Such interference should, if possible, be avoided or stopped.

Isolate interference sources, eg with canvas or cardboard.

You can measure the reflected radiation, for example, with a Lambertian reflector in combination with your thermal imager.

3. Weather conditions

• Clouds

A very cloudy sky provides the ideal conditions for infrared measurements outdoors. Reason: The measuring object is protected against solar radiation and "cold sky radiation".

• Precipitation

Water, ice and snow have high emissivity and are impermeable to infrared radiation. Measuring moist objects can also cause measurement errors because the surface of the measured object cools with evaporation.

Note: Heavy precipitation (rain, snow) can distort the measurement results.

4. Air / Air humidity

When the lens (or protective glass) of the thermal imager becomes fogged due to high relative humidity in the air, not all of the infrared radiation can be received. The water prevents all radiation from reaching the lens of the infrared camera. Very dense fog can also influence the measurement; in fact, the water droplets in the transmission line allow less infrared rays to pass.

Note: Pay attention to low relative humidity of the air in the measurement environment. This will prevent condensation in the air (fog), on the object to be measured, on the protective glass or the lens of the thermal imager.

• Air Currents

Due to heat exchange (convection), the air near the surface has the same temperature as the object being measured. The wind or the air currents "move" this layer of air and a new layer of air, not being adapted to the temperature of the measured object, is found at this place. Convection absorbs heat from hot measuring objects and supplies heat to cold measuring objects until the air temperature and that of the surface of the measuring object have harmonized. This heat exchange effect increases as a function of the temperature difference between the surface of the measurement object and the ambient temperature.

Note: Wind or air currents in the room can influence the temperature measurement using a thermal camera.

• Atmospheric pollution

Some suspended matter such as dust, soot, smoke, and some vapors have high emissivity and low transmissivity. This means that they can interfere with measurements because they themselves emit infrared radiation, which is also perceived by the thermal camera. In addition, the infrared radiation from the measured object can only partially penetrate to the thermal camera because it is dispersed and absorbed by the suspended matter.

5. Light

Light or illumination plays no significant role when measuring with a thermal imager. Measurements can also be performed in the dark because thermal cameras measure long wavelength infrared radiation. However, some light sources themselves emit infrared thermal radiation and can therefore influence the temperature of objects in their environment.

Therefore, do not take measurements in direct sunlight or near a hot bulb.

Cold light sources, such as LEDs or neon tubes, are not critical: they convert most of the energy used into visible light, not infrared radiation.

Special features of thermographic measurements outdoors

Infrared radiation from a clear sky is, in common parlance, called "cold sky radiation". When the sky is clear, “cold sky radiation” (~ -50… -60 ° C) and hot sun radiation (~ 5500 ° C) are reflected throughout the day. The surface of the sky is greater than that of the sun; the reflected temperature is therefore often below 0 ° C during outdoor thermographic measurements, even when the sun is shining. In the sun, objects heat up due to absorption of solar radiation. This clearly influences the surface temperature - up to several hours after solar radiation.

Tips and tricks for thermographic measurements outdoors

• Take measurements early in the morning and / or when the sky is very overcast. However, avoid rain or snow. Fog or strong wind are also unfavorable.
• Change your position during measurement to identify reflections. The reflections move, the thermal peculiarities of the measurement object remain in the same place - even if the viewing angle changes.
• Avoid measurements near very hot or very cold objects or insulate them.
• Avoid direct sunlight, even a few hours before the measurement. Also observe the cloud cover a few hours before the measurement.
• Do not take measurements if moisture in the air condenses on the thermal imager.
• Do not carry out measurements when the air is highly polluted (eg when dust has just been raised).

Theoretical bases of thermography

Learn more about the physical basics of thermography in our compact tutorial. This knowledge will give you a head start, eg in setting the emissivity correctly for each surface.

Every object whose temperature exceeds absolute zero (0 Kelvin = -273.15 ° C) emits infrared radiation (IR rays). The human eye cannot see it, however, because it is almost blind at this wavelength range. This is not the case with a thermal camera. Its heart, the infrared detector is sensitive to IR rays. Depending on the intensity of the IR rays, it determines the temperature at the surface of the object and makes it visible to the human eye thanks to a thermal image. This process is called thermography.

To make the IR rays visible, the detector records them, converts them into electrical signals, then assigns a defined color to each signal and displays it on the thermal camera screen. Basically, thermal cameras translate the wavelengths of the infrared spectrum into wavelengths perceptible to the human eye (colors).

Contrary to a relatively widespread misconception, thermal imaging cameras do not allow us to see inside objects, but only to identify their surface temperature.

Emissivity, reflexivity, transmissivity

It is helpful to know these terms so that you can use a thermal imager as an effective tool.

The radiation recorded by a thermal camera consists of the emission, transmission and reflection of infrared radiation emitted by objects within the visual field of the thermal camera.

Transmissivity (t)

Transmittance describes the ability of a material to pass (transmit) IR rays. A thin plastic film, for example, has a very high transmissivity. If we therefore want to measure the temperature of a thin plastic film hung in front of a wall with a thermal camera, we do not measure the temperature of the film, but that of the wall. Most materials do not pass IR rays so the transmissivity of a material is usually almost 0 and can therefore be neglected.

Emissivity (ε)

Emission is the ability of a material to emit IR rays. This capacity is indicated by the emissivity. This depends, among other things, on the material itself and the properties of its surface. The sun, for example, has an emissivity of 100 %. However, this value is never found on a daily basis. Concrete still has an emissivity of 93 %. This means that 93% of IR radiation is emitted by the concrete itself.

Reflexivity (ρ)

The 7 % that are missing are reflections from the environment of the material / object being measured, i.e. the temperature reflected on the object. Both emissivity and reflected temperature can be adjusted in thermal imaging cameras to achieve the most accurate thermal image possible.

The relationship between emission and reflection

• Measuring objects with high emissivity (ε ≥ 0.8):

have a low reflectivity (ρ): ρ = 1 - ε

allow very reliable measurements of their temperature by means of a thermal camera

• Measurement objects with an average emissivity (0.6

have an average reflectivity (ρ): ρ = 1 - ε

allow reliable measurements of their temperature by means of a thermal camera

• Measurement objects with low emissivity (ε ≤ 0.6):

have a high reflectivity (ρ): ρ = 1 - ε

allow temperature measurements using a thermal imager, but their results must be critically analyzed

require a correct adjustment of the reflected temperature compensation, as this contributes to a large extent to the temperature calculation

Emissivity adjustment

Each material has a different emissivity. To obtain optimal thermal images, the emissivity must be adjusted on the camera.

Correct emissivity adjustment is especially important when the temperature differences between the measured object and the measurement environment are large.

• If the object to be measured has a temperature higher than the ambient temperature:

an emissivity set too high will result in too low temperatures being displayed in the thermal image.

an emissivity set too low will result in too high temperatures being displayed in the thermal image.

• If the object to be measured has a temperature below ambient temperature:

an emissivity set too high will result in too high temperatures being displayed in the thermal image.

an emissivity set too low will result in the display of too low temperatures in the thermal image.

Advice on emissivity

• The greater the difference between the temperature of the measurement object and the ambient temperature and the lower the emissivity, the greater the measurement errors. These errors are amplified when the emissivity is poorly adjusted.
• Many materials transparent to the human eye, such as glass, are not permeable to long wavelength infrared rays.
• Materials with low transmissivity include, for example, thin plastic films and germanium, the material from which the lenses and protective glass of Testo thermal imaging cameras are made.
• If necessary, remove any covers from the measuring object; otherwise, the thermal camera will only measure the temperature at the surface of the cover.
• Always observe the instructions for use of the measuring object.
• When elements below the surface influence, by conduction, the temperature distribution at the surface of the measurement object, it is often possible to identify the internal structures of the measurement object on the thermal image. However, the thermal imager only measures the surface temperature. It is therefore not possible to draw precise conclusions about the temperature values of the elements inside the measurement object.

Visual field and measuring spot

Fundamental information to be able to evaluate the technical properties of a thermal camera.

The visual field (FOV)

The visual field (also called “Field of View” or “FOV”) describes the surface visible with a thermal camera. This depends on the lens used. A wide angle lens offers a large field of vision, a telephoto lens offers good spatial resolution. The larger the visual field, the larger the area you can see. A wide field of view (> 30 °) is especially interesting when you use your thermal camera indoors; this is because walls often do not allow you to move far enough away from the object of measurement to see more.

The smallest measurable object / The measuring spot (IFOVmeas)

Smallest measurable object describes the smallest object that is not only detected, but whose temperature can be measured reliably. When the spatial resolution of the objective is 3.5mrad and the measuring distance is 1m, the smallest detectable object has 3.5mm sides and is displayed as one pixel at the bottom. 'screen. To get accurate measurements, the object to be measured should be 2-3 times the size of the smallest detectable object. The following rule therefore applies for the smallest measurable object (IFOVmeas): IFOVmeas ≈ 3 x IFOVgeo

Smallest detectable object (IFOVgeo)

The smallest detectable object is the smallest dimension that can be identified by a pixel. A pixel is an element on the thermal camera's detector that records IR rays and converts them into electrical signals. Each pixel corresponds to a measurement value.

The emissivities of the most common materials

The following table serves as a reference for adjusting the emissivity in thermography. It indicates the emissivity ε for some common materials. Since the emissivity varies according to the temperature and the properties of the surfaces, the values given here can only be considered as reference values. To measure the absolute value of temperature, the emissivity of the material must be determined accurately.

Material and material temperature Emissivity

Aluminum, laminated (170 ° C) 0.04

Aluminum, unoxidized (25 ° C) 0.02

Aluminum, unoxidized (100 ° C) 0.03

Aluminum, strongly oxidized (93 ° C) 0.2

Aluminum, extremely polished (100 ° C) 0.09

Cotton (20 ° C) 0.77

Concrete (25 ° C) 0.93

Lead (40 ° C) 0.43

Lead, oxidized (40 ° C) 0.43

Lead, gray oxidized (40 ° C) 0.28

Chromium (40 ° C) 0.08

Chrome, polished (150 ° C) 0.06

Ice, smooth (0 ° C) 0.97

Iron, emery polished (20 ° C) 0.24

Iron with casting rind (100 ° C) 0.8

Iron with rolling crust (20 ° C) 0.77

Glass (90 ° C) 0.9

Plaster (20 ° C) 0.94

Granite (20 ° C) 0.45

Rubber, hard (23 ° C) 0.94

Rubber, flexible, gray (23 ° C) 0.89

Cast iron, oxidized (200 ° C) 0.64

Wood (70 ° C) 0.94

Cork (20 ° C) 0.7

Heat sink, black galvanized (50 ° C) 0.98

Copper slightly tarnished (20 ° C) 0.04

Copper, oxidized (130 ° C) 0.76

Copper, polished (40 ° C) 0.03

Copper, laminated (40 ° C) 0.64

Plastics: PE, PP, PVC (20 ° C) 0.94

Varnish, blue, on aluminum foil (40 ° C) 0.78

Varnish, black, matt (80 ° C) 0.97

Varnish, yellow, 2 coats, on aluminum foil (40 ° C) 0.79

Varnish, white (90 ° C) 0.95

Marble, white (40 ° C) 0.95

Masonry (40 ° C) 0.93

Brass, oxidized (200 ° C) 0.61

Oil paints (all colors) (90 ° C) 0.92-0.96

Paper (20 ° C) 0.97

Porcelain (20 ° C) 0.92

Stoneware (40 ° C) 0.67

Steel, hot-treated surface (200 ° C) 0.52

Steel, oxidized (200 ° C) 0.79

Steel, cold rolled (93 ° C) 0.75-0.85

Clay, fired (70 ° C) 0.91

Transformer varnish (70 ° C) 0.94

Brick, mortar, plaster (20 ° C) 0.93

Zinc, oxidized 0.1