In the late 1980s I did an electrical engineering internship in a coal-fired power station over summer vacation. The gas furnace igniters ran continuously, but how do you detect presence or absence of burner flames against semi-apocalyptic background of ignited pulverised coal dust being air-blasted into the furnace? Have a little window and photosensor pointing at the burner flame and FFT. No spectral component spike at xHz (IIRC x ~= 13? -- it's a burner flame, underlying dynamics not same as for candle wick) --> ringing alarms, flashing lights.