How to measure extremely high temperatures?
Depends what you mean by &quot;extreme&quot; but base metal thermocouples (eg. K, N) work okay up to &gt;1000°C depending on atmosphere, precious metal thermocouples (eg. R, S, B) to perhaps 1600°C and tungsten/rhenium thermocouples to well over 2000°C and briefly to perhaps 3000°C.
There are also optical pyrometers. In the olden days one would match the appearance of a filament against the light from a furnace, which works up to quite high temperatures (perhaps 3000°C).