Apparatus and method to detect and measure, in a non-image area on a toner-image-bearing member, a low coverage of toner particles relating to a background coverage of toner particles in an image frame, and in particular, to detect and measure by an optical detector a low toner coverage in excess of...http://www.google.es/patents/US7149445?utm_source=gb-gplus-sharePatente US7149445 - Detection of background toner particles
