In a raster output scanner, a system for using one original beam and one facet of a rotating polygon to generate a scan line which can be turned on to have three levels of intensity. The original beam is first separated into two beams in a beam splitter. The resultant beams are polarized ninety degrees...http://www.google.de/patents/US5251058?utm_source=gb-gplus-sharePatent US5251058 - Multiple beam exposure control
