So what’s the problem? The problem is that the flat-plate calibrators also need to be calibrated, and the traditional way these calibrators have been calibrated is in most cases critically flawed. To calibrate the calibrator you need to know two things, the surface temperature of the flat-plate, and the surface emissivity of the flat-plate.
Traditionally neither of these are measured. In fact, traditionally the only thing measured is the internal temperature of the flat-plate. Temperature losses at the surface, inhomogeneity in the surface temperature, and a lack of traceable measurement of the emissivity at the time of calibration all contribute to errors that can far exceed the stated uncertainties of the calibration.
This leads to a situation where the laboratory staff may take the rather unscientific approach of changing the position of the thermometer in relation to the flat plate until they find the “sweet spot” where the thermometer passes calibration. This is not good metrology.
To solve this problem, Fluke Calibration developed a Radiation Thermometry Laboratory. Its mission is to improve and assure measurement accuracy and traceability in radiometric calibration of infrared thermometers. To make a reliable flat-plate calibrator that is easy to use, a lot of work has to be done behind the scenes in the calibration lab to ensure measurement quality.
For an in depth look at how this was done, see the paper by Frank Liebmann and Tom Kolat, published in NCSLI Measure Magazine titled Traceability and Quality Control in a Radiation Thermometry Laboratory. It describes the quality control measures that were taken at Fluke Calibration to improve and assure measurement accuracy for radiometric calibrations.