Light shown through a space containing smoke or an aerosol is detected fromeveral paths through the space. The light may be gathered from physically separated optical fibers feeding a common photodetector, or several physically separated, series-connected photodetectors may be used. The total amount...http://www.google.es/patents/US4018534?utm_source=gb-gplus-sharePatente US4018534 - Aerosol average-density determining system
