Editing Smoke detector (section)

=== Photoelectric === <!-- [[Photoelectric smoke detector]] and [[Optical smoke detector]] redirect here. -->
{{more citations needed section|date=August 2016}}
[[File:OpticalSD.jpg|thumb|Optical smoke detector with the cover removed; the angled plastic in an arc across the top is a light baffle]]
[[File:OpticalSmokeDetector.png|thumb|Optical smoke detector{{ordered list|Optical chamber|Cover|Case moulding|Photodiode (transducer)|Infrared LED}}]]

A '''photoelectric''', or '''optical smoke detector''', contains a source of [[infrared]], [[visible light|visible]], or [[ultraviolet]] light—typically an [[incandescent light bulb]] or [[light-emitting diode]] (LED)—a [[lens (optics)|lens]], and a [[photoelectric receiver]]—typically a [[photodiode]]. In spot-type detectors, all of these components are arranged inside a chamber where air, which may contain smoke from a nearby fire, flows. In large open areas such as atria and auditoriums, [[optical beam smoke detector|optical beam]] or projected-beam smoke detectors are used instead of a chamber within the unit: a wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device or reflected to the receiver by a reflector. In some types, particularly optical beam types, the light emitted by the light source passes through the air being tested and reaches the photosensor. The received [[Radiative transfer|light intensity will be reduced]] due to [[scattering]] from particulates of smoke, air-borne dust, or other substances; the circuitry detects the light intensity and generates the alarm if it is below a specified threshold, potentially due to smoke.<ref name="AirflowPatterns">
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    | title = The Effects of High Air Velocity and Complex Airflow Patterns on Smoke Detector Performance
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In other types, typically chamber types, the light is not directed at the sensor, which is not illuminated in the absence of particles. If the air in the chamber contains particles (smoke or dust), the light is [[scattering|scattered]] and some of it reaches the sensor, triggering the alarm.<ref name="AirflowPatterns" />

According to the [[National Fire Protection Association]] (NFPA), "photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering". Studies by Texas A&M and the NFPA cited by the City of Palo Alto, California state, "Photoelectric alarms react slower to rapidly growing fires than ionization alarms, but laboratory and field tests have shown that photoelectric smoke alarms provide adequate warning for all types of fires and have been shown to be far less likely to be deactivated by occupants."<ref>{{Cite web |title=Risk Analysis of Residential Fire Detector Performance |url=https://propertyevaluation.net/Texas%20A&M%2095-0%20Risk%20Analysis%20of%20Residential%20Fire%20Detector%20Performance.pdf |archive-url=https://web.archive.org/web/20160411233139/http://propertyevaluation.net/Texas%20A&M%2095-0%20Risk%20Analysis%20of%20Residential%20Fire%20Detector%20Performance.pdf |archive-date=2016-04-11 |access-date=2022-07-19 |website=Walker Property Evaluation Services}}</ref>

Although photoelectric alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the NFPA recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke or use both the ionization and photoelectric smoke sensing methods. Some combination alarms may also include a carbon monoxide detection capability.

The type and sensitivity of light source and photoelectric sensor and type of smoke chamber differ between manufacturers.