There are a few adjustments to what you'd expect from this, but it is a good 1st pass approx.
@THH: Then you should ask Optris to correct the manual... What they certainly not will do!
They clearly state:
Since per the Stefan Boltzmann law, the electric signal of the detector is as follows:
U ~ εTobj4
I propose before You, next time, write up something, read first the manual of the tools, that were used...