<P> A photoelectric , or optical smoke detector contains a source of infrared , visible , or ultraviolet light ( typically an incandescent light bulb or light - emitting diode ) , a 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 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 back 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 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 . 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 scattered and some of it reaches the sensor , triggering the alarm . </P>
which type of fire detector uses the effect of smoke on a light source and photo-electric cell
optical smoke detector