Method to design a security feature on the substrate of security documents using sub wavelength grating
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United States of America Patent
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Dec 29, 2015
Grant Date -
Apr 30, 2015
app pub date -
Oct 24, 2013
filing date -
Oct 24, 2013
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Abstract
Method of designing a security thread in security documents with better color contrast at any viewing angle is claimed. The invention discloses designing of security thread in security documents like bank notes using subwavelength gratings with period and thickness less than the wavelength of light. The sub wavelength gratings preferably asymmetric are designed such that the Oth order reflections are of longer wavelength (red) and the higher order reflections (diffracted orders) are of shorter wavelengths (blue/green). The security thread so designed gives better color contrast unlike the rainbow colors of prior art thus allowing better and easier distinction of authentic documents.
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