Absolute zero

Absolute zero is -273.15 °C (0 Kelvin = -459.69 °F). All bodies whose temperature is at the absolute zero point, emit no infrared radiation.


When electromagnetic infrared radiation hits an object, the object absorbs some of this energy. The absorption of infrared radiation means that the object heats up. Warmer objects emit more infrared radiation than colder objects. The absorbed infrared radiation is thus converted into emitted infrared radiation (radiating from the object). The absorptivity corresponds to the emissivity. The incident infrared radiation on the object that is not absorbed is reflected and/or transmitted (let through).

Adjustment time

The adjustment time is the time needed by the camera to adapt to the ambient temperature of the measurement site in order to measure within the specifications. See the instruction manual for the adjustment time of your thermal imager.


Black body radiator

An object that absorbs all of the energy from the incident infrared radiation, converts it into its own infrared radiation and emits it in full. The emissivity of black body radiators is exactly one. There is therefore no reflection or transmission of the radiation. Objects with properties of this nature do not occur in the field. Devices for calibrating thermal imagers are known as black body radiators. However, their emissivity is only just under one.



Procedure in which the measurement values of an instrument (actual values) and the measurement values of a reference instrument (nominal values) are determined and compared. The result provides clues as to whether the actual measuring values of the instrument are still within a permissible limit/tolerance range. In contrast to an adjustment, the identified deviation from the actual measuring value is merely documented in a calibration and not adjusted to the nominal measuring value. The intervals at which a calibration is to be performed depends on the respective measuring tasks and requirements.

Celsius (°C)

Temperature unit. Under normal pressure, the zero point of the Celsius scale (0 °C) is the freezing temperature of water. A further fixed point for the Celsius scale is the boiling point of water at 100 °C.

°C = (°F - 32) / 1.8 or °C = K - 273.15.

Coldspot and hotspot

The coldest spot of an area on the thermal image is referred to as a “coldspot”, and the hottest spot is referred to as a “hotspot”.

Using the function “Auto Hot/Cold Spot Recognition”, you can display these two spots directly on your thermal image in the imager display. This function is also available in many of the analysing software packages. e.g. Testo IRSoft. In this software you can also display these two spots for any areas of the thermal image you wish to define.

Coloured body radiator

Coloured body radiators are materials whose degree of emissivity is dependent on the wavelength. If one views the same object with a thermal imager in the long-wave infrared range (LIWR, 8 – 14 μm), and one in the medium-wave infrared range (MIWR, 3 – 5 μm), it can be necessary to set different emissivities in the thermal imager.

Colour palette

Selection of colours for the thermal image in the imager (e.g. colour palette “rainbow”, “iron”, “grey scale”). The contrasts of the thermal images can be shown with varying quality depending on the measuring task and the colour palette set. The colour palette can also be set individually using analysing software (e.g. Testo IRSoft) after the thermal image has been saved. Pay heed to the interpretability of your thermal image when choosing the colour palette. Red and yellow colours are intuitively associated by the viewer with heat, green and blue colours with cold.


Transition of a substance from gaseous to liquid state. Air humidity can condense on surfaces if the surface temperature, and therefore the temperature of the air on the surface, is lower than the dew point temperature.


Heat conduction. Transfer of thermal energy between neighbouring particles. Here, energy is always transferred from the warmer to the colder particle. Unlike convection, there is no mass transport of particles in conduction.


Heat transfer, whereby thermal energy moves from one fluid or gas to another as a result of the mass transport of particles.



The detector receives the infrared radiation and converts it into an electrical signal. The geometric resolution of the detector is given in pixels, and the thermal resolution with the NETD.

Dew point/dew point temperature

Temperature at which water condenses. At dew point temperature, the air is saturated with more than 100 % water vapour. Once the air cannot absorb any more water vapour, condensate forms.


Emissivity (ε)

A measure of the ability of a material to emit (give off) infrared radiation. The emissivity varies according to the surface properties, the material and, for some materials, also according to the temperature of the object.


Fahrenheit (°F)

Temperature unit used mainly in North America.

°F = (°C x 1.8) + 32.

Example 20 °C in °F: (20 °C x 1.8) + 32 = 68 °F.

Field of view

Cf. “FOV”, p. 43.

FOV (Field Of View)

Field of view of the thermal imager. This is specified as an angle (e.g. 32°) and defines the area that can be seen with the thermal imager. The field of view is dependent on the detector in the thermal imager and on the lens used. Wide-angle lenses have a large field of view for the same detector, whereas telephoto lenses (e.g. Testo 9° telephoto lens) have a small field of view.


Grey body radiator

As no ideal black body radiator (ε = 1) exists in nature, the concept of a grey body radiator (ε < 1) is used as an alternative. Many building materials and organic materials can be described as grey body radiators in a narrow spectral range. The wavelength-dependency of the emissivity is negligible here (cf. “Coloured body radiators”), as the spectral sensitivity of common thermal imagers only records a small spectral excerpt from the infrared spectrum. This thus presents an acceptable approach.

Grey body radiators, in contrast to black body radiators, never absorb the infrared radiation immitting on to them to 100%, and for this reason, the intensity of the emitted radiation is lower.



Cf. “Coldspot and hotspot”, p. 40.


Ideal radiator

Cf. “Black body radiator”, p. 39.

IFOVgeo (Instantaneous Field Of View)

The IFOVgeo states the resolution of the imager system. It states which details the imager system, dependent on the detector and the lens, can resolve. The resolution of the imager system (IFOVgeo) isgiven in mrad (=Milliradiant), and describes the smallest object that, depending on the measuring distance, can still be depicted on the thermal image. The size of this object corresponds to one pixel on the thermal image.

IFOVmeas (Measurement Instantaneous Field Of View)

Designation of the smallest object whose temperature can be accurately measured by the thermal imager. It is 2–3 times larger than the smallest identifiable object (IFOVgeo).The rule of thumb is: IFOVmeas ≈ 3 x IFOVgeo. IFOVmeas is also known as the smallest measurable measurement spot.

Image refresh rate

Specification in hertz of how often per second the displayed image is refreshed (e.g. 9 Hz / 33 Hz / 60 Hz). An image refresh rate of 9 Hz means that the thermal imager updates the thermal image in the display nine times per second.

Infrared radiation

Infrared radiation is electromagnetic radiation. Every object with a temperature above absolute zero (0 Kelvin = -273.15 °C) emits infrared radiation. Infrared radiation covers the wavelength range from 0.78 mm up to 1,000 mm (= 1 mm) and therefore borders on the wavelength range for light (0.38 – 0.78 μm). Thermal imagers often measure the long-wave infrared radiation in the range from 8 mm to 14 mm (like the testo 875i and testo 882, for example), as the atmosphere in this wavelength range is extremely permeable to infrared radiation.


Lines of the same temperature. You can display isotherms using analysis software (e.g. Testo IRSoft) or with high-quality thermal imagers. All measurement points in the thermal image whose temperature evalues are within a pre-defined range are then marked in colour.



Kelvin (K)

Temperature unit.

0 K corresponds to the absolute zero point (-273.15 °C).

The following applies: 273.15 K = 0 °C = 32 °F.

K = °C + 273.15.

Example 20 °C in K: 20 °C + 273.15 = 293.15 K.


Lambert radiator

A Lambert radiator is an object that reflects incident radiation with the optimum diffusion; in other words the incident radiation is reflected with equal strength in all directions.You can measure the temperature of the reflected radiation on a Lambert radiator using the thermal imager.

Laser marker

With the laser marker, the laser sighting is shown parallax-free, enabling you to see the exact position of the laser spot on the thermal imager display. This function is included in the testo 885 and testo 890 cameras.

Laser pointer

A laser pointer supports homing in on the measuring surface (a red dot is projected onto the measuring object). The laser sighting and the centre of the image do not correspond exactly, as they are on different optical axes. This is why the laser dot is not suitable for the marking exact locations that were aimed at in the display using the crosshairs. It only serves as a guide.


Laser class 2: never direct the laser at persons or animals and never look into the laser! This can damage the eyes!


The size of the field of view of the thermal imager, and thus the size of the measuring spot, change according to the lens used.

A wide-angle lens (e.g. 32° – standard lens for the testo 875i) is particularly suitable if you want an overview of the temperature distribution across a large surface. You can use a telephoto lens (e.g. Testo 9° telephoto lens) to measure small details with precision, even from a great distance away.


Measuring spot

Cf. “IFOVmeas”, p. 44.


NETD (Noise Equivalent Temperature Difference)

Key figure for the smallest possible temperature difference that can be resolved by the camera. The smaller this value, the better the measurement resolution of the thermal imager.





Relative humidity (%RH)

Percentage specification of the water vapour saturation level of the air. For example, at 33 %RH the air contains only approx. 1/3 of the maximum volume of water vapour that the air could absorb at the same temperature and the same air pressure. At an air humidity in excess of 100 %, condensate starts to form as the air is fully saturated and cannot absorb any more moisture. The gaseous water vapour in the air liquefies. The warmer the air, the more water vapour it can absorb without condensation being formed. For this reason, condensation always occurs first on cold surfaces.

Real body

Cf. “Grey body radiator”, p. 43.

Reflectance (ρ)

The ability of a material to reflect infrared radiation. The reflectance depends on the surface properties, the temperature and the type of material.

RTC (Reflected Temperature Compensation)

With real bodies, some of the thermal radiation is reflected. This reflected temperature must be factored into the measurement of objects with low emissivity. Using an offset factor in the camera, the reflection is calculated out and the accuracy of the temperature measurement is thus improved. This is generally done by means of a manual input into the camera and/or via the software.

In most cases, the reflected temperature is identical to the ambient temperature (mainly in indoor thermography). If the infraredradiation from sources of interference is reflected on the surface of the measuring object, you should determine the temperature of the reflected radiation (e.g. using a Lambert radiator). The reflected temperature has little effect on objects with very high emissivity.




State variable for the energy contained in a body.


Imaging procedure using measuring technology that visualises thermal radiation or the temperature distributions of object surfaces using a thermal imager.


Cf. “Thermal image”, p. 49.

Transmittance (τ)

Measure of the ability of a material to allow infrared radiation to pass through it. It depends on the thickness and type of the material.

Most materials are not permeable to long-wave infrared radiation.

Thermal image

Image that shows the temperature distributions of the surfaces of objects using different colours for different temperature values.

Thermal images are taken using a thermal imager.

Thermal imager

Camera that measures infrared radiation and converts the signals into a thermal image. Using a thermal imager, surface temperature distributions can be shown that are not visible to the human eye. Typical areas of application are found, for example, in building thermography and in electrical and industrial thermography.

Two-point measurement

The two-point measurement has two crosshairs in the camera display, which can be used to read off individual temperatures