Abstract: A combustion control system is operated based on the detection of smoke density. A photoelectric device measures the intensity of light directed through the smoke by a lamp in order to measure the smoke density. This measurement is employed to control fuel and/or air supply or the like. To provide a standard reference, light is directed from the lamp through an alternate path not affected by the smoke to the photoelectric device. A shutter mechanism alternately exposes the photoelectric device to light passing through or circumventing the smoke. The photoelectric measurement is compared with a standard and the comparison is used to adjust circuitry connected to the photoelectric device to stabilize the effects of the latter. A continuous monitoring of the adjustment is carried out to determine when an acceptable range of adjustment has been exceeded. A smoke density amplifier is employed to increase effectiveness.