A particle detector that determines the presence of particles in an enclosed volume includes a laser that directs a coherent optical beam to a beam splitter that produces first and second divergent beams. An optical system images the point of origin of the two divergent beams within the beam splitter...http://www.google.com.au/patents/US5192870?utm_source=gb-gplus-sharePatent US5192870 - Optical submicron aerosol particle detector
