| 摘要 |
<p>In fire-detection apparatus of the kind in which a radiation receiver is exposed to a source of radiation through an intervening gaseous medium, the beam of radiation overlaps the receiver so that the beam continues to cover the receiver despite minor changes in beam direction due, for example, to movement of the surfaces supporting the emitter and receiver. Discrimination against ambient radiation is improved by causing the source, which is a solid-state emitter, to emit pulses of radiation and by having a frequency-selective circuit for selecting from the output of the radiation-sensitive device in the receiver a pulse signal resulting from the received pulses of radiation; an alarm circuit is responsive to an amplitude modulation in this pulse signal at a frequency which is typical of the modulation produced by a fire heating the gaseous medium between the source and the receiver.</p> |
