Importance

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Abstract

There is a large difference in frequency dependent, complex electrical permittivity between genuine and counterfeit documents, including paper currency. Genuine bills have a substantially different complex permittivity which is distinct from the permittivity of counterfeit bills. The invention broadly measures the complex permittivity, .epsilon., of the `paper` bills to determine if the bill(s) is genuine or counterfeit. In one aspect, the present invention comprises a device and method for determining the genuineness of a document by measuring the complex permittivity based on the frequency dependent dielectric properties of the document. The complex electric permittivity of a dielectric material is typically read by measuring complex impedance using a capacitor to sense material properties. The document may include an enhanced region comprising an additive that establishes a known permittivity in the enhanced region. For example, the enhanced region may be established by using a 'smart ink' that is applied to the document, or a dopant that may be built into the document itself. The 'smart ink' or dopant may include carbon nanotubes, barium titanate, barium strontrium, or in general ferroelectric materials having a relatively high permittivity and a ferroelectric transition point that is above room temperature (e.g., lithium niobate, rochelle salt, sodium niobate, lithium tantalate). To determine the authenticity of the document, the permittivity of the enhanced region is measured. Advantageously, providing the enhanced region changes the local properties of the document to prevent document counterfeiting.

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First Claim