Patent ID: 7963450

Claim:
A security feature for authentication of an article comprising: a marking on the article, the marking comprising a dye having a substantially narrow absorption band and a maximum absorption wavelength of electromagnetic radiation in the near infrared range; a first illumination source for emitting an electromagnetic signal having a wavelength substantially identical to the maximum absorption wavelength of the dye; a second illumination source for emitting an electromagnetic signal having a wavelength less than the wavelength of the first illumination source; a third illumination source for emitting an electromagnetic signal having a wavelength greater than the wavelength of the first illumination source; and a detector configured to receive a signal comprising at least one electromagnetic radiation signal emitted by the first, second or third illumination source and reflected by the article, wherein the detector further comprises a differentiator for outputting a differential signal, which is the difference between a first reflective signal and a second reflective signal; wherein the first reflective signal comprises electromagnetic radiation emitted by the first, second and third illumination sources and the second reflective signal comprises electromagnetic radiation emitted by the second and third illumination sources.