In this paper, we present a new flame monitor that uses three photovoltaic cells covering the ultraviolet (UV), visible, and infrared (IR) spectral bands. A gain-adjustable amplifier is incorporated into the monitor so that it is applicable to the coal, oil, or gas-fired flames. Self-checking of the monitor is implemented through cross correlation of the signals from the three cells, and hence, no additional self-checking hardware is required. Both the oscillation frequency and the brightness of the flame are used to monitor flame stability and to detect flame presence as well as sighting-tube blockage. Unlike conventional single-cell flame detectors, the new multicell devices can still be in operation before being repaired, after a cell-failure alarm has gone off. Experiments were carried out on an industrial-scale combustion test facility in order to demonstrate the operability and efficacy of the new flame monitor.