The survey instrument and the inspection methodological null bank note

摘要

In apparatus for detecting counterfeit bank-notes, an electronic camera (20) forms a digital representation of a small area of the bank-note. The digital image is stored in an image memory (24). The Fourier transform of the digital image is then computed and analyzed to determine if it contains high spatial frequency components, and if so, the note (14) is identified as counterfeit. The analyzer (30) may use pattern recognition techniques. In a modification, Fourier transforms of a plurality of small areas are formed and averaged and the average is analyzed. This invention is particularly suitable for identifying counterfeit bank-notes having regular patterns e.g. of lines or dots, therein, such as are produced by colour photocopiers or offset lithography printing. MATH