1. An image scanner for scanning a document to produce electrical scan signals representative of an image on the document along a plurality of parallel scan lines, comprising: a document supporting drum, defining a document supporting surface, for receiving a document to be scanned and supporting said document on said surface, means for rotating said drum such that said document supported thereby is transported past a document scanning station, a plurality of photoelectric transducers, each of said transducers providing a scan signal in response to light received thereby, optical means for directing light from each of a plurality of scan points at said scanning station to an associated one of said photoelectric transducers, whereby each of said scan points defines a scan line as said document is transported past said scanning station, and means on said document supporting surface of said drum defining a recognizable background pattern, whereby a recognizable pattern of scan signals from said transducers is produced during scanning of said document supporting surface.