White Paper

White Paper: Two-Color Or Ratio Thermal Imaging - Potentials And Limits

Source: FLIR Systems, Inc - Research & Science


By K.-P. Möllmann, F. Pinno, M. Vollmer, Brandenburg University of Applied Sciences, Germany

Two-color or ratio pyrometry is a well-established method to overcome the fundamental difficulties in determining and eliminating the effect of emissivity in the temperature measurement. As an approach for an "emissivity-free" radiation thermometry, the radiance within two narrow spectral bands is measured and the object temperature is determined from the signal ratio while at the same time eliminating the influence of the emissivity under certain conditions. The improved sensitivity of IR camera detectors allows the application of this method to infrared thermal imaging. Based on a discussion of the theoretical basics of this measurement principle, the paper will give a detailed analysis of the most important parameters influencing the accuracy of the temperature measurement using ratio thermal imaging:

  • the selection of the two spectral bands (wavelength and object temperature dependence, spectral width, spectral separation between the bands),
  • the influence of reflected background radiation by the object,
  • the influence of spectrally-dependent object emissivity or "non-grey" object behavior.

Following this analysis, we discuss the necessary measurement conditions as well as the potentials and the limits of ratio thermal imaging. Experimental investigations were carried out using a FLIR SC6000 research camera with narrow band pass filters.

FLIR Systems, Inc - Research & Science