apparatus for sensing the exstence of a fire and providing a warning , if desired , with improved discrimination against the possibility of false alarms . dual channel detectors are used , one detector being set to respond to incident radiation having a wavelength in the range of 0 . 8 to 1 . 1 microns while the other wavelength range is significantly displaced therefrom , being selected for wavelengths in the range from 14 to 25 microns . reliability of true signal detection is further improved by the provision of separate flame flicker bandpass filters in the respective channels , these bandpass filters being set for different passbands . circuits providing ratio discrimination , threshold detectors and delay circuitry are combined with the dual spectrum detectors and disparate flicker frequency filters to achieve improved performance . in addition , the dynamic range of instrument sensitivity is substantially increased by utilizing preamplifiers with wide gain variability controlled by automatic gain control circuits in the dual channel circuitry .