Detection of airborne pollutants

A smoke detector comprising a detector chamber, an inlet for introducing an airflow to be sampled into the chamber, an outlet for said airflow from the chamber, means for generating a beam of light within the chamber, a scattered light detector responsive to the presence of scattered light within a zone of the chamber consequent on the presence of smoke particles within the sampled airflow in said zone, a light absorber at an end of the chamber remote from the light source for receiving and absorbing said beam after passage through the chamber, first collimator means for collimating the beam, and second collimator means beyond the first collimator means, said second collimator means comprising a collimator disc having an aperture of a size such that the beam collimated by the first collimator means passes through the aperture in the disc without contacting the edge of the aperture, said disc serving to trap glints of light arising from the first collimator means to thereby reduce stray light in the said zone at which detection of light scatter occurs, said zone being beyond the second collimator means.