Optical beam smoke detectors work on the principle of light obscuration, where the presence of smoke blocks some of the light from the beam,  typically through either absorbance or light scattering. Once a certain percentage of the transmitted light has been blocked by the smoke, a fire is signalled.
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Optical beam smoke detectors are typically used to detect fires in large commercial and industrial buildings, as components in a larger fire alarm system. Optical beam smoke detectors consist of at least one light transmitter and one receiver, which is photosensitive. The photosensitive receiver monitors light produced by the transmitter under normal conditions.
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In the absence of smoke, light passes from the light transmitter to the receiver in a straight line. In a fire, when smoke falls within the path of the beam detector, some of the light is absorbed or scattered by the smoke particles. An end-to-end optical beam smoke detector is a system that has a separate light transmitter and receiver. They are used in applications where there is little available room to install a wide area detector — as the receiver is on a separate element each individual unit is quite small.
Aesthetic considerations are especially important for cultural and heritage sites. UV and IR wavelengths of light react to smoke differently, and the comparative difference helps to verify real smoke by comparing the reflections and seeing a difference in the profile.
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UV and IR respond identically to things like blockage ladder in front to detector path , bugs blocking beam , fog, steam, and other things that commonly cause false alarms, so the two wavelengths of light are used together to detect smoke accurately. A reflective optical beam smoke detector incorporates a light transmitter and the detector on the same unit. The light path is created by reflecting light emitted from the transmitter off a retroreflector that is placed opposite the detector. A motorised optical beam smoke detector automatically aligns itself during installation and can compensate for alignment 'drift' i.
Early examples of optical beam smoke detectors were prone to false alarms ,  which were caused by many different factors. Most commonly, the build-up of dust, dirt and other debris would lower the detection threshold for the detector, causing the system to enter alarm when no fire was present. Light is projected through the chamber and will be scattered if it strikes smoke.
Optical beam smoke detector
This light, reflected off the smoke in the chamber, is detected by a photocell. Ionization smoke detectors the most common in home use detect the particles in smoke. As smoke passes through the chamber, the particles are ionized. These particles may then be detected by charged plates in the detector.
Smoke detectors are also available in combination with a heat detector Figure 4. The beam type detector Figure 5 operates when the beam is interrupted by obscuring smoke between the laser emitter and receiver. These detectors are most often used in areas of large open spaces. An air sampling detection system uses tubing placed throughout the protected area. The tubing has small holes spaced out along the length of the tube and air is constantly drawn into the unit, which can detect extremely low levels of combustion products.
Supervisory and Notification Devices Numerous supervisory devices can be connected to the fire alarm control panel. For example, a tamper switch Figure 6 may be placed on water control valves for automatic sprinkler systems. If this valve is closed by an unauthorized person, the tamper switch will send a supervisory signal to the control panel, alerting your people to the problem.
Supervisory devices are available for a wide variety of applications. Systems may be addressable or nonaddressable. In the first type, all of the detectors on the system have a unique digital identifier.
The fire alarm control panel can communicate individually with each device. In non-addressable systems, detectors may be divided into zones based on all of the detectors being on the same pair of wires, but the control panel cannot determine any information about an individual detector. Addressable systems offer several advantages.
The first is that a specific indication of the location of an activation is available during a fire. Would you rather know that a detection has occurred somewhere in the west wing of your building or that it has occurred in office number ? The latter is clearly more informative.
This specific location capability is also part of the second major advantage of these systems: trouble signals can specifically identify the component with a problem. If a single detector fails, for example, an addressable system will provide a trouble signal that indicates the specific detector.
In non-addressable systems, the zone will be identified, but a repair person will have to check each detector in that zone to determine which one is not working. Notification appliances are the audible, visual, and other devices located throughout the facility that warn occupants when the system has detected a fire. Horns, strobes, combination units Figure 7 , and bells are examples of these devices.
Fire alarm control panels often have features available that allow alarms to be activated in selected locations within the facility based upon the location of the detector that activates. This feature can be used to permit staged evacuations, for example. Alarms should be supplemented with communications devices that allow you to provide specific information and instructions to building occupants.
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