| 摘要 |
PURPOSE:To remove noise owing to scattered light and eliminate the possibility for failure in announcement by utilizing the difference in arrival times at a photodetector based on the optical path difference between the measuring light arriving at the same by reflecting on a retroreflective plate and the scattered light by smoke. CONSTITUTION:Pulselike measuring light is emitted from a light emitting device 1 by the periodic pulses of a pulse oscillator 4. This measuring light reflects on a retroreflective plate D, and is detected after a constant time t0. Here, the delay time of a delay circuit C connected to the oscillator 4 is set at t0, and a switch 6 is closed after the t0 time since the generation of the pulses. Consequently, the scattered light by smoke arrives at a photodetector 2 before this t0 time and therefore the switch 6 is held open, and the output of the photodetector 2 is not transmitted to an amplifier circuit 7, a comparator 8, etc., thus the noise by the scattered light which is detected before the measuring light is removed. Upon lapse of the delay time t0, the switch 6 closes, and the measuring light by smoke density is detected. |
