Obscuration type smoke detector.

摘要

A sensor for sensing the presence in a medium of species which absorb or scatter radiation, such as smoke particles in air, by means of changes in the attenuation of radiation transmitted through the medium, in which visible or infra-red radiation is transmitted from two sources (1 and 2), along dissimilar paths (3 and 4), to a receiver (5). A control means (6) causes the sources to emit and accurately controls the ratio of the emitted energies. The signal received from one source is subtracted from the signal received from the other source by another means (7). The difference signal is used by the control means as the basis for controlling the ratio of the energies emitted by the sources. Changes in the absorption co-efficient are measured by sensing changes in the difference signal and/or by changes in the ratio of the emitted energies.