a document to be verified or tested as a possible counterfeit is scanned by a system that optically senses a specific target area or aperture on the document . this area may include material deposited on the document or may be simply the document material . a reference grid is imprinted on the document to locate the same target area to be sensed each time the document is scanned , independent of the transport used to do the scanning . the reference grid is used to locate the target area both vertically and horizontally and signals sensed from the target area are combined to a representative composite .