| 摘要 |
A method is disclosed to provide a unique indicator of authenticity of currency or a document, optically responsive to a local magnetic field and which employs material difficult to duplicate. The indicator is in the form of magnetic viewing material added to the document either by adherence of a patch or strip of the material, or by printing it directly onto the document's surface. The basic ingredient of the magnetic viewing material is a multitude of minute micro-capsules, each having tiny nickel flakes suspended in a viscous fluid encapsulated by a tough, transparent plastic shell. A layer consisting of these micro-capsules has the property that the planes of the nickel flakes will align themselves in the direction of an applied magnetic field. If the field is perpendicular to the layer, the flakes will be seen edge on and will reflect a minimum amount of light. When the field is parallel to the plane of the layer, the planes of the nickel flakes will be parallel to the layer and the nickel flakes now appear as little mirrors reflecting incident light and the layer will appear bright. Hence, the validity of the document can be readily tested by moving a permanent magnet in the vicinity of the patch or strip and noting whether the light patterns reflected from the patch or strip change in response to the magnetic field. Only authentic documents, processed in accordance with the method taught by the invention, will be appropriately responsive.
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