Optical sensors are more responsive to smouldering fires producing large particle smoke typical of fires involving furniture and bedding. They are more immune to invisible smoke produced by "burning the toast" and similar cooking fumes. this makes them ideal for siting in hallways close to kitchens where false alarms from ionisation alarms may be a particular problem. The BS 5839: Pt 6: 2004 recommends the use of optical alarms in circulation spaces of a dwelling, such as hallways and landings. Optical alarms are prone fo false alarms if exposed to steam and should not be located too close to poorly ventilated bathrooms or shower rooms.
- A light beam is pulsed in the sensor chamber every 10 seconds to "look" for smoke. Any smoke present has to be visible to the naked eye so that the receptor can "see" it. If no smoke is detected, the alarm woill remain in standby state.
- When large particles of smoke are detected, the light beam will be scattered onto the light receptor.
- This will then send an electrical signal to the IC (integrated circuit).
- This causes the alarm to sound.