UV ( Ultraviolet) Flame Detector.

Flame Detector which contains an ultraviolet photo tube that responds to radiation in the 185 to 260 nano meter region when radiation from a flame strikes the cathode plate within a UV detector tube, electrons are ejected from the cathode plate. These electrons are accelerated towards the positively charged anode of the tube. They collide with molecules of an ionization gas, with which the tube is filled. This emits more electrons and produces an avalanche condition. More   electrons are released which creates a momentary electron flow from the cathode to the anode. This momentary current (pulse) recurs at a rate proportional to the intensity of the UV radiation. This output converted to standard mA  output.