The present invention pertains to the field of flame detection, and is particularly adapted to prevent false alarms generated by modulated and time varying sources of background radiation such as the sun, welders, hot objects and artificial lights. Flame detectors are presently available which are solar blind to steady sunlight but will not function properly if the sunlight incident on the detector is modulated, or time varying as when an object passes between the sun and the sensor in the sensor's field of view. Modulation of reflected sunlight by ocean waves and ripple is known to cause false alarms at offshore oil platforms with existing flame detection equipment.
Conventional flame detectors also have difficulties in detecting big fires since the high optical intensity emitted by the big fire causes saturation of the electronics following the optical detector. It is known that big fires have a larger proportion of the flicker occurring at higher frequencies than small fires.
U.S. Pat. No. 4,639,598, Kern et al., and U.S. Pat. No. 3,931,521, Cinzori, disclose use of two detectors, one operating at far infrared (7 to 25 microns) and the other at near infrared (0.8 to 1.1 microns) wavelengths. This approach suffers due to several reasons, including a) reduced sensitivity due to lower infrared emission by fires in the far infrared wavelength range as compared to strong CO.sub.2 emission at 4.3 microns from hydrocarbon fires, b) reduced sensitivity of thermal detectors available for the far infrared wavelength range compared to photoconductive detectors for shorter wavelengths, and c) necessity to look at spurious signals in the near infrared, caused by a large number of sources unrelated to fires, and also in the far infrared due to sources strongly emitting such as human or animal presence, both resulting in overburdening of the processor.