Facts about IR temperature


Measuring with IR thermometers

This article provides a brief description of the principles of IR radiation, what an IR instrument measures and why it is important to consider the emission factor.

Facts about IR temperature
IR thermometer with beam


It is a well-known fact that all bodies emit electromagnetic waves, or radiation, depending on their temperature. When radiation is scattered, energy is also transported. This means that the radiation can be used to measure temperature without touching the object being measured. The radiation energy and its characteristic wavelengths depend primarily on the temperature of the radiating body.

3rd atmospheric window.

Infrared (IR) radiation covers a relatively wide range of frequencies. From ca. 750 nm which is directly below the visible red light to approx. 1 mm adjacent to the microwave range. Different materials give off different amounts of emission in different parts of the IR range. Most manufacturers of IR temperature meters have chosen the so-called 3rd atmospheric window which is between 8 μm and 14 μm. The advantage is that most materials have a high emission factor, over a wide range of temperatures in this area, which allows the instruments to be used in many different applications.

Comparable to visible light.

Can IR radiation be compared to regular visible light? Yes and no. What is comparable is the general behavior, such as, for example. that the angle of incidence is equal to the angle of reflection. What appears shiny to the eye is often also shiny to the IR radiation and creates reflection. However, it can be misleading to take the comparison too far, e.g. an ordinary window glass that is almost completely transparent to visible light is not at all transparent to IR radiation. In contrast, there are materials that are opaque to the eye but partially transparent to IR radiation.

What does the instrument measure?

IR temperature emission transmission reflection

In total, there are three factors affecting the measurement. The most important and usually largest factor is the emission, which is the radiation emitted by the target itself. The most common interference factor is reflection, which is the radiation coming from other sources and reflected to the instrument via the target. The third is transmission, which is radiation passing through the object being measured.

What is an emission factor?

Since different materials emit and reflect different amounts, a correction is needed to make the IR measurement accurate. This correction is called an emission factor. The starting point is a so-called theoretical blackbody which has zero reflection and transmission and thus has an emission factor of 1, which is written “e = 1”.

Emission factors for different materials are given in tables and range from e 0.01 to 0.99. Most IR temperature meters have a pre-set emission factor of 0.95 which gives a good accuracy on most measurements below +100 °C because almost all non-metallic materials such as paper, plaster, wood, rubber, stone, paint, food etc. have an emission factor very close to 0.95.

Calibration testo 845 measuring spot
For instruments that do not mark the entire measuring surface, it is important to keep track of the relationship between distance and the diameter of the measuring surface.

Measuring instruments and practical tips.

Measurement facts testo 835 T1 temperature

Simple hand instruments.

We have a wide range of portable IR instruments from Testo. Both for non-contact temperature measurement and non-destructive measurements of incoming raw materials in the food industry.

See the range

Moisture logger testo 176-h1

Things to consider when measuring.

IR measurement is a useful and effective method in many situations. However, there are some things to consider in order to get reliable temperature values. Learn how to measure correctly and avoid common sources of error.

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testo Saveris software

Use tape on shiny surfaces.

Shiny surfaces are often tricky to measure with an IR thermometer. A good trick is to put a piece of tea on the surface. We have a specially developed emission tape to help you perform reliable IR measurements.

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