| 摘要 |
A smoke detector is shown in which blue light is directed through a scattering volume (9) from a radiation emitter (3) and infra-red radiation is also directed through the scattering volume (9) from an infra-red source (3A). Radiation forward-scattered by any particles in the scattering volume (9) is directed by a mirror (13) onto a photodiode (15) which produces an output to control means (16). The emitters (3,3A) are pulsed at different frequencies, enabling the control means (16) to produce separate signals (21,23) corresponding respectively to the scattered blue light and the scattered infra-red radiation. For smoke particles, significantly more blue light is scattered than infra-red radiation, but this is not so much the case for non-smoke particles. A comparator (25) takes the ratio of the two signals (21,23) to produce a smoke-dependent warning output. In order to reduce power consumption and increase the life of the blue light emitter (3), the apparatus normally operates in a monitoring mode in which the infra-red emitter (3A) is pulsed intensively but at a low flashing rate, and the blue light emitter (3) is maintained inoperative, until infra-red radiation scattered by particles in the volume (9) cause the photodiode (15) to produce a sufficient output, whereupon the blue light emitter (3) is rendered operative. |
