| 摘要 |
A covert mark (1) which may be applied to an article (2) comprises a fluorescent or phosphorescent material and has a unidirectionally aligned structure. Upon illumination of the mark (1) with ultraviolet radiation (3a), the mark (1) emits polarised fluorescent or phosphorescent radiation (3a) of characteristic radiation. The mark (1) may therefore be used in a system for covertly marking and checking the authenticity of a genuine article (2) whereby polarised fluorescent or phosphorescent radiation (3b) emitted from the mark (1) is incident on linearly polarising material. Detecting polarised fluorescent or phosphorescent radiation (3b) of characteristic wavelength and polarisation provides an indication of the authenticity of the article (2). By varying the orientation of the linearly polarising material, the plane of polarisation of radiation transmitted by the material is varied and a characteristic flashing effect is observed. The observation of fluorescent or phosphorescent radiation (3b) of characteristic wavelength in addition to the characteristic flashing effect provides an indication of the authenticity of the article (2). In an alternative arrangement, the system may comprise a liquid crystal cell, containing a twisted nematic material, to which a variable voltage is applied. The voltage applied across the liquid crystal cell may be varied to produce the required characteristic flashing effect.
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