In the arrangement shown above, the smoke detectors are placed in line with the pulsed infrared light source. In the absence of smoke, the masking disk casts a shadow over the circular region of diameter ‘AB’ and prevents light from emitting diode from falling onto the detecting photocell. In the presence of smoke, light is scattered and some of it falls onto the photocell. The evaluation circuit is arranged to trigger an alarm if the threshold value is exceeded for a predetermined number of consecutive pulses.
“Fire Safety at Sea” by Dr. James Cowley