Abstract

At least two photoelectric sensors, sensitive to different spectral ranges of incident light, provide two sensed output signals; the relationship of the a-c components of the sensed output signals is evaluated, and it is determined if these a-c components fall within predetermined low frequency ranges, for example, 2 to 50 Hz, preferably 5 to 25 Hz; if so, a "flame present" signal is provided, for example to give a fire alarm, or to indicate that a burner is operating. Preferably, the relationship of the signals is such that a different signal is provided between one of the sensed signals and a fraction of the other, and conversely, and the sensitivity of the sensors is adjusted to have the same output signals at a predetermined color temperature, for example about 1,400° K. Illumination signals incident on the sensors due to other sources than flames then are reliably eliminated while still providing the "flame present" signal upon coincidence of the appropriate difference signals, which coincidence preferably is determined by analyzing the phase relationships of the resulting difference signals in a phase comparator.