Detection of printing and coating media

Methods and apparatus are provided for determining whether an optically variable material (OVM) is present at a surface 30 of a security document such as a banknote. The particular type of OVM may be identified, or a pass or fail signal is generated depending on whether a specific OVM is found. The surface is illuminated with at least two different wavelengths of light from a spectrometer at a particular angle to the surface, and the intensity of the light reflected or scattered at two or more angles is determined using photodiodes 34. The output from the photodiodes may be compared with values stored in a look-up table to produce a decision signal or 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.