Marconi TFT Power Heads
The Marconi TFT and General Microwave TFT power sensors are based on a thermo-electric calorimeter using bismuth and nichrome thermocouples. There are a few microscope photos of the actual sensor elements below in various states.
The original patent document for these power heads is available here.
(below) Marconi TFT 6421 power head 100mW DC-12GHz