Apparatus for determining inter-pen offsets in a multiple pen ink jet printer including a drop detector for measuring flight time of ink drops, an aperture plate disposed in the print media plane, and a carriage position sensor for determining carriage position when ink drop flight time exceeds a predetermined...http://www.google.com.au/patents/US5109239?utm_source=gb-gplus-sharePatent US5109239 - Inter pen offset determination and compensation in multi-pen ink jet printing systems
