ANTICOUNTERFEITING METHOD

A covert method of distinguishing between genuine and counterfeit articles by means of applying an optical filter to genuine articles as a mark of authenticity. The filter for use in the method may comprise a multi layer stack having at least one maximum and at least one minimum in the spectral characteristic and a selective absorber over a range of visible wavelengths. The spectral characteristic is such that the filter has a neutral appearance when viewed by an unaided eye under normal lighting conditions, but gives rise to characteristic optical effects when illuminated with selected wavelengths. The characteristic effects provide an authenticating signature for the filter. In the absence of the filter, the illumination would not give rise to the characteristic effects. The intensity of radiation reflected or transmitted by the filter at two or more wavelengths may be used to provide the authenticating characteristic. In an alternative embodiment, the filter may be illuminated with radiation at two or more angles of incidence. The resulting intensity or spatial distribution of radiation reflected or transmitted by the filter may be used to verify the authenticity of the article by comparing the observed properties with known properties for the filter.