Developing robust and traceable temperature measurements using thermal imaging
Quantitative thermal imaging is the reliable, traceable temperature measurement of an object's surface, on a pixel-by-pixel basis. It is based on the principle of blackbody radiation, where the wavelength (or colour) of the light emitted by an object depends on its temperature. Each pixel of the thermal imager detects the radiation incident upon it from an object of interest, similar to non-contact thermometry.
A thermal imager is a very complex system and hence difficult to calibrate with a meaningful uncertainty. There are a high number of pixels involved in capturing an image, typically tens to hundreds of thousands. In addition, the speed at which the images are taken and the very sensitive dependence on the relative position of all objects in the room increase the complexity further. When performed correctly, the resulting images are informative and invaluable for mapping temperature across objects, and comparing results to thermal models.
2D thermal image calibration techniques
To underpin thermal image calibrations, NPL has developed world-class, state-of-the-art, blackbody calibration sources for thermal imager calibrations from -40 °C to +3000 °C for all short-, mid- and long-wavelength systems, with very low uncertainties. This calibration is UKAS-accredited and can be used to determine a reliable correction factor for each pixel of the imager, which can then be used with confidence to observe surfaces, eliminating the influence of differing pixel sensitivity.
The importance of this has demonstrated in a medical setting where thermal images can be used to detect foot ulceration. This is critical for people with diabetes where, if not treated quickly, the damage can be life-threatening.
3D thermal image calibration techniques
The understanding of 2D calibration is now the basis for further research into techniques to account for the geometry of the object of interest, and other effects that influence the image, such as reflection of radiation originating from neighbouring objects.
Our work with the European Space Agency (ESA), PhotoCore and PSI-tran has demonstrated a new technique for taking 3D thermal images of satellites. It allows the geometrical form to be accounted for, and the measurement data to be compared with thermal models, which verifies their accuracy.
Thermal imaging systems
Saving lives through early foot ulcer detection