Abstract: A filter having a cut-off frequency of the order of 50Hz is included in the circuit of flame monitoring apparatus to remove signals below the cut-off frequency. In the circuit shown, a silicon p-n junction photo-electric pick-up is capacitively connected to an amplifier, the amplifier is connected to a high-pass filter, the output of the filter is connected to a further amplifier, the output of the further amplifier is applied to a detector integrator, the output of the detector integrator is connected to a Schmitt trigger, and the output of the trigger is applied to an output circuit giving a warning signal if the flame being monitored is extinguished. The use of sighting tubes for the photo-electric pick-ups is disclosed.