To overcome difficulties in measuring temperatures beyond 700°, or in places unapproachable by traditional techniques, an optical fiber is used to pick-up the corresponding temperature radiation. This radiation is sent to an optoelectronic system and split into two channels by an achromatic optical fiber coupler. Each of the two channels is defined by its specific spectral domain with the help of silicon photodiodes equipped with appropriated filters. Signals are then amplified and dicitized in order to be processed.
"High Temperature Measurements With A Fiber Optic Pyrometric Sensor", Proc. SPIE 0586, Fiber Optic Sensors I, (7 August 1986); doi: 10.1117/12.951157; https://doi.org/10.1117/12.951157