The optical air pollution detector is suitable for use with IC engines to monitor the presence of oil mist and give warning of the build-up of explosive atmospheres. The detector chamber comprises two or more identical air sampling tubes (6) mounted on a hollow cross-piece (8). The tubes have diametrically opposed drillings containing a concave mirror (18) and a light source and detector head (16). Each light emitter is supplied via optic fibre guides (33) from a single common source (32). Fibre guides also connect the detector head with separate photo-electric elements (36). The output signal from each detector is sequentially scanned and compared with the average value in an electronic circuit which provides an alarm signal when the pollution level exceeds a certain value.
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