Certain flame monitoring requirements call for greater reliability than what a simple flame rod can provide. (Key feature: These are designed and agency approved as ‘stand alone’ devices to work with any externally wired combustion control) No amplifier module is required! This is a UV-sensitive photocell output that then must pass one leg of a dual channel microprocessor. The other channel has an algorithm programmed for irregularity, as a flame is. This is a safety feature to cut off errant pattern light, such as ambient or hot refractory following a flame-out. Commonly called a flicker sensor.