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Radiation principles

Infrared (IR) temperature measurement is based on physical principle that said every material is emitting electromagnetic radiation if its own temperature above -273,15°C. A section of this radiation is measured by infrared thermometers and cameras.

Figure shows the spectrum of radiation but visible light is just a narrow section of it. Only this radiation can be detected by human eyes. In order to utlize the IR radiation an IR sensitive equipment is necessery.
Planck’s radiation principle is demonstrated on diagram below showing the quantity and quality (wavelength) of radiation emitted from the object in function of its temperature.

Based on Planck’s theory – among others – Stefan and Boltzmann made a step forward focusing on the measurement of the territory under the curves - determinable by meters applied. Wien discovered the correlation between temperature, wavelength and maximum intensity of radiation and made the diagram below to present that the cooler the object the longer the wavelength of the maximum radiation intensity.

Background of measurement

In measurement process radiation is collected by lens of IR camera and forwarded to the detector. It generates electric signals to compose an image. Advanced IR detectors (cameras) record radiation intensity level to each pixels (image points). To colour them is upon your choice - even later can be done in the course of processing.

Collected radiation quantity consists of three items (Kirchoff law)- see following picture. Object temperature is expressed only by emitted radiation - consequently the bigger the emission factor the higher is the certainty of the temperature defined.


Relation between the three quantities is expressed by the following equation :

Some special cases :

Collected radiations arrive to the camera lens through the air. By this transmission medium some of the radiations won’t be let through according to their wavelength.

Diagram shows the transmission factor of air in function with wavelength. No absorption occurs when transmission factor equals to 1. Six windows can be seen (grey fields) where camera operation may be advantageous. By low wavelength only high temperatures can be measured (see Planck principle). Thats why cameras commonly applied are using 8 to 14 µm range of wavelength.

Some world famous success reached through IR camera application:

  • Adidas
  • BASF
  • BMW
  • Soyuz
  • Heathrow airport London
  • Saint Peter Cathedral in Vatican
  • bird flu
  • discovering pipeline faults
  • medical applications
  • veterinary
  • electronic componen