A diagrammatic arrangement of a light obscuration smoke detector is shown in figure above. An infrared light beam, at an operating frequency of 1000 pulses per second, is received by a photoelectric cell and analysed. In a fire, smoke rises and spreads below ceiling level and the intensity of the light falling on the receiver is reduced due to light scattering or absorption. The fire alarm sounds when the signal strength is reduced to between 40 – 90 % for a period of about 5 seconds. Using a pulsed light source saves power and allows the use of a receiver turned to the pulse frequency to reduce false alarm caused by sunlight or illumination. Light obscuration smoke detector is suitable for covering large areas with flat ceilings. They are not suitable for outside use.
“Fire Safety at Sea” by Dr. James Cowley