Smoke sensor

摘要

A separate type photoelectric smoke sensor having a light emitting section for emitting a light beam to a reflecting plate disposed at a certain distance from the light emitting section, a light receiving section for receiving reflected light from the reflecting plate, and a judgement section for outputting a sense signal if a received light output from the light receiving section is smaller than a threshold value previously set. The quantity of reflected light from a shielding object is obtained from the difference between or the ratio of the quantity of received light measured during lighting of the light emitting section in a situation where there is no shielding object and the quantity of received light measured during lighting of the light emitting section in a situation where there is the shielding object, and the difference between these quantities of reflected light and the quantity of received light measured during lighting of the light emitting section is compared with the threshold value to determine whether or not there is a fire. It is thereby possible to correctly discriminate the existence of any shielding object other than smoke in an observed region. Even if there is a shielding object, the influence of the shielding object is cancelled to obtain the true quantity of reflected light from the reflecting plate no matter what the reflectivity thereof, thereby ensuring accurate determination as to whether or not there is a fire.