Patent ID: 8159657

Claim:
A method of authenticating an article using a hyperchromic color system embodied by a corresponding set of dyes or pigments and/or a corresponding ink set, and comprising a number m of at least 3 base colors which are chosen such as to have at least one absorption maximum which is substantially different from the absorption maxima of the base colors of the CIEXYZ-system located at 450 nm (blue), 550 nm (green), and 600 nm (red), each color being embodied by a corresponding single chemical compound having one or several absorption bands in the 300 to 1100 nm spectral region, said dyes or pigments of the hyperchromic color system being incorporated into a printing ink or coating composition, or into a set of printing inks or coating compositions and used for providing on said article a marking, the base colors being freely chosen and having absorption spectra which are substantially different from each other in the 300 to 1,100 nm wavelength range, such that the statistical correlation of two spectra represented by two vectors s 1 and s 2 , as expressed by the normalized scalar product (s 1 *s 2 )/(|s 1 |*|s 2 |), shall not exceed 0.95; said method comprising the steps of: a) measuring an absorption spectrum of said marking of the article, assessing its light reflection or transmission properties by using optical densities OD=log(Io/I), and expressing said spectrum s(λ) as a vector s=(s 1 , s 2 , s 3 , . . . , s n ), wherein s 1 , s 2 , s 3 . . . , s n are the measured optical densities OD at n different wavelengths λ 1 , λ 2 , λ 3 , . . . , λ n ; b) expressing the measured absorption or reflection spectrum y 1 as a vector of hypercolor coordinates x=(A′*A) −1 *A′*y 1 , wherein the rectangular matrix A of dimensions n times m is formed by the vectors s representing the absorption or reflection spectra of m different dyes or pigments of said hyperchromic color system; and c) comparing the information derived from the spectrum y 1 in step b) with information derived from a corresponding spectrum of an authentic item, and rejecting the article under test if one of the hypercolor coordinates x i lies outside a corresponding predefined concentration fork.