DETECTION OF PRINTING AND COATING MEDIA

摘要

Method and apparatus are described for determining if an optically variable material (OVM) is present in or on a surface, typically of a security docume nt such as a banknote. In some embodiments the particular type of OVM is identified. In other embodiments a pass or fail signal is generated dependin g on whether a specific OVM is found on the surface. The surface is subjected to at least two different wavelengths of illuminating light at a particular ang le to the surface and the intensity of the light reflected and/or scattered lig ht from the surface is determined using photodiodes. The analogue output from t he photodiodes may be digitised and the digital values processed according to o ne or another of different algorithms described, so as to produce an output signal whose value can be compared with one or more stored reference values in a look-up table, to produce a decision signal. Comparison of the analogue photodiode output signal values against a look-up table can be used to indicate the concentration of the OVM on the surface. Methods and apparatus are described for document authentication and identification, coating qualit y control and document sorting, by employing the methods and apparatus describ ed.