SECURITY THREADS FOR BANKNOTES USING HIGH-CONTRAST SUBWAVELENGTH GRATINGS

摘要

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 0" order reflections are of longer wavelength (red) and the higher order reflections (diffracted orders) are of shorter wavelengths (bluetgreen). 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.