How does an infrared thermometer work

Function and structure of infrared thermometers

Infrared thermometers are all well and good - but how do the practical measuring devices actually work?
Image: @ lightsource

Infrared thermometers are able to determine the temperature of an object without touching it. The infrared radiation that the respective object emits to its environment is used as the basis for temperature measurement.

You should keep in mind that all objects with a temperature of over 0 degrees Kelvin emit a certain infrared radiation. However, this infrared radiation is only part of the radiation emitted. The rest of the electromagnetic radiation is of no further interest to us at this point.

A temperature of 0 degrees Kelvin corresponds to absolute zero of -273.15 degrees Celsius. Correct: Even objects with such a low temperature give off a certain amount of infrared radiation to their environment.

The higher the temperature of an object, the more infrared radiation it emits.

Table of Contents

Structure of an infrared thermometer

An infrared thermometer basically consists of a lens, a sensor, the electronics for calculations and a display. But what exactly happens at which station? How can it be that the emitted infrared radiation becomes a number at the end?

Schematic structure of an infrared thermometer.

  1. object
    The temperature of a certain point of an object should be determined with the help of a pyrometer. There should be no other object (for example a pane of glass) between the object and the infrared thermometer, otherwise a correct measurement is not possible.
  2. lens
    The lens is there to capture the emitted infrared radiation of the object and to image and focus on the sensor.
  3. sensor
    The sensor receives the focused infrared radiation, amplifies it and converts it into a corresponding electronic signal. The electronic signal is then passed on to the electronics.
  4. electronics
    The electrical signal is converted into an output variable according to the settings of the IR thermometer, which should correspond to the object temperature.
  5. Display
    In the end, the display is only there to show the calculated object temperature.

Basically, the following applies: the warmer an object is, the more infrared radiation it emits, the stronger the signal that the IR thermometer detects and the higher the temperature that is shown on the display.

Temperature measurement is not always that easy!

However, there is a problem with non-contact temperature measurement: not every object is made of the same material and emits infrared radiation to the same extent. This is where the so-called emissivity comes into play. The emissivity is the ratio of the radiation value of an object and the radiation value of a "black body“.

In order to understand the term emissivity, one must therefore refer to the model of the "black body" be aware. This is a theoretical object that emits thermal radiation that actually only depends on its own temperature. The material and the size of the surface are irrelevant. The emissivity of a "black body“Is always 1 - a value that cannot be achieved in reality.

You can find out which emissivity you have to set for which material by looking at the corresponding tables. If you buy an infrared thermometer that offers the option of setting the emissivity, the corresponding tables should be enclosed. Otherwise you will find such a table for metals here, for example.

The setting of the correct emissivity is crucial for some materials, as otherwise completely wrong temperature specifications are made.


This article and the included graphic of the structure of an infrared thermometer are basically based on an article from

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