| 摘要 |
In a system for security marking a product, a surface of the product 10 is covered by a layer 14 of light-transmitting material which exhibits variations in refractive index throughout the material such that for most angles of view the material acts as a conventional diffuser of light and thereby conceals or obscures a marking 12 applied to a surface of the product and covered by layer 14, but for a critical angle of view, or a limited range of angles close to that critical angle, the layer is effectively transparent allowing unimpeded viewing of the marking. In a variant, the marking is itself incorporated in the layer in the form of refractive index variations in the layer. The layer 14 is provided by applying to the product, for example over a conventional printed marking, a coating of a photopolymerisable material and causing said material to polymerise, by appropriate exposure to collimated ultraviolet light at a predetermined angle of incidence, to establish the desired refractive index variations within the layer. |
