Some of my articles have contained thermal pictures of ferrite cored inductors and transformers.
I have been asked several times recently about the assumed emissivity and the accuracy questioned, I assume this has been discussed online somewhere.
When first measuring ferrites with non-contact thermometers, I performed some experiments to discover whether the default emissivity ε=0.95 applied. It would be convenient if it did, and permit use of some instruments that do not allow adjustment of ε.
In the past, I have compared the reading of non-contact thermometers with several K thermocouple meters and a Thermomelt indicator, and observed insignificant difference (ie less than the variance of repeated measurements).
The following experiment is a thermal pic of a FT240-43 core on the black plastic case of the instrument. The setup has had hours to stabilise thermally.
Above is a combined thermal image and faint visual image. This instrument has only one readout point, and by moving it around, only 0.1° variation was observed between the background and the core.
The shape of the core cannot be reliably discerned in the thermal image, the variation is mostly measurement noise… but lets assume there was 0.1° difference between the black plastic case and the core when they are both at 23° and calculate the implied ε that would account for that indication.
Above, the calculated actual ε that would account for 0.1° error is 0.949. Such an error in ε does not translate to the same error in T at different T.
Above, calculating the actual temperature of a measurement when the actual ε is 0.949 and the instrument is calibrated for 0.95 gives 100°, the error is not significant in a three digit precision answer.
If we find what value of ε is required to cause a -1° error at 100°, it is 0.940… well below the estimated 0.949.
I sleep easy that the thermographs I have presented of ferrite cored inductors and transformers do not have significant error due to the assumed black body emissivity of the ferrite.