SIGNATURE MARK RECOGNITION SYSTEMS

A signature recognition system for identifying an article with a distinctive diffractive element (or elements) and verifying the presence of that element or elements comprising; an article with one or more diffraction gratings impressed thereon, the gratings) exhibiting periodic wave surface profiles having a depth-to-pitch ratio .delta. of between 0.1 and 0.5, a source of polarised electromagnetic radiation of wavelength .lambda., such that the pitch G of the periodic wave surface profile of the grating(s) is comparable to an integer multiple n of that wavelength means for directing the source of polarised electromagnetic radiation to the surface of the grating(s) at a plane of incidence substantially normal to the plane of the surface of the diffraction grating and at an angle of approximately 45 ° azimuth to the alignment of the grooves on the surface, and means for detecting radiation reflected from the grating(s) surface which is oppositely polarised to the incident radiation.