very complex
It is very complex. First you use a sensor that has one element exposed to the outside world and one that is not. The readings from each are put into a very complex math function to get the total energy on both. Then the difference is put to another very complex math function. I have seen the printout of the computer code for a hand held unit. It was 50 pages of small print.
The sources of error are very complex. You have to have the sensors far away from where the user holds the unit because of different thermal time constans from the outside world to the two sensors.