Abstract: The invention relates to paper, in particular a sheet of paper, preferably security paper such as a bank note, check or the like, the paper (1) comprising at least one window (2) therethrough bridged by a transparent film (3), such as a plastics film or a transparent paper.

Abstract: To test the genuineness and/or condition of dollar bills or other substantially identical papers, one or more thickness gauges are positioned in the path of these papers and determine the deviations, if any, of the thickness of a passing specimen from a predetermined value or from the thickness of a reference specimen scanned concurrently therewith. The measured deviations are fed into a processor which counts positive or negative deviations within a multiplicity of incremental periods and determines the nature of any irregularity from the count of deviations of either sign. With three gauges measuring thickness along as many parallel tracks, the system may discriminate among such irregularities as gaps, overlappings, adhesive tape and dog-eared corners.