Detection of printing and coating media

Methods and apparatus are described for determining if an optically variable material (OVM) is present in or on a surface, typically of a security document such as a banknote. In some embodiments the particular type of OVM is identified. In other embodiments a pass or fail signal is generated depending 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 angle to the surface and the intensity of the light reflected and/or scattered light from the surface is determined using photodiodes. The analogue output from the photodiodes may be digitised and the digital values processed according to one or another of different algorithms described, so as to produce an output signal whose value can be compared witli 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 quality control and document sorting, by employing the methods and apparatus described.