Method 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 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 quality control and document sorting, by
employing the methods and apparatus described.