| CPC G01N 21/658 (2013.01) | 12 Claims |
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1. A method of authenticating a marking applied on a substrate and having a composition comprising a first material including a SERS taggant, or a SERRS taggant, comprising the following steps performed by a system comprising a light source, a Raman spectrometer, an imaging unit and a control unit having a processing unit and a memory, the light source being controlled by the control unit via a current loop to deliver a calibrated excitation light:
storing in the memory a full model of a Raman spectrum of a genuine marking applied on a genuine substrate and having a composition comprising a genuine first material including a genuine SERS taggant, or a genuine SERRS taggant, as a first weighted sum of a reference Raman spectrum of the genuine taggant, a reference Raman spectrum of a reference genuine substrate that is not marked with the genuine taggant, and a reference Raman spectrum of a reference genuine first material not including the genuine taggant, collected upon respective illumination of the genuine taggant, the reference genuine substrate, and the reference genuine first material with excitation light;
storing in the memory a reduced model of a Raman spectrum of a reduced marking, the reduced marking differing only from the genuine marking by its composition not including the genuine taggant, as a second weighted sum of the reference Raman spectrum of the reference genuine substrate, and the reference Raman spectrum of the reference genuine first material;
upon illuminating the marking with the excitation light, measuring a corresponding Raman light signal scattered by the marking via a Raman spectrometer to obtain a measured Raman spectrum of the marking;
by the processing unit:
fitting the measured Raman spectrum with the full model of the Raman spectrum by calculating values of the weights in the full model that minimize, under non-negativity constraint of said weights, a difference between the full model and the measured Raman spectrum and obtaining a corresponding first residual;
fitting the measured Raman spectrum with the reduced model of the Raman spectrum by calculating values of the weights in the reduced model that minimize, under non-negativity constraint of said weights, a difference between the reduced model and the measured Raman spectrum and obtaining a corresponding second residual;
calculating a F-value corresponding to a F-test of comparison of the full model and the reduced model for the measured Raman spectrum from the obtained first residual and second residual; and
deciding whether the taggant is present or not in the marking based on the calculated F-value.
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