Method for checking value documents
View Patent ↗The invention relates to a method for checking value documents by which the similarity of the intensity spectrum of a value document to be checked to a reference spectrum can be tested. To check the similarity of the intensity spectrum recorded from the value document to be checked to the reference spectrum, not the recorded intensity spectrum itself is compared with the reference spectrum, but rather a corrected intensity spectrum adapted to the reference spectrum, which arises from the recorded intensity spectrum by a linear correction. The correction parameters of the linear correction function are ascertained by adapting the linearly corrected intensity spectrum to the reference spectrum. Through the adapting it is achieved that fluctuations of the recorded intensity spectrum that are due to measuring technology are compensated at least partly.
1. A method for checking value documents, comprising the steps of:
detecting at several different wavelengths the light emanating from a value document for recording an intensity spectrum S(λ) of the value document;
forming a corrected intensity spectrum S′(c,a,λ)=c+a·S(λ) by correcting the recorded intensity spectrum using a linear correction function, so that the corrected intensity spectrum S′(c,a,λ)=c+a·S(λ) is a linear transform of the recorded intensity spectrum S(λ) and implements a first correction parameter c and a second correction parameter a;
adapting the corrected intensity spectrum S′(c,a,λ)=c+a*S(λ) to a reference spectrum R(λ) in order to reduce a deviation, defined by a distance measure, which the corrected intensity spectrum S′(c,a,λ) has from the reference spectrum R(λ), there being ascertained through the adapting an adapted corrected intensity spectrum S′(λ)=c*+a*·S(λ) adapted to the reference spectrum, whose first correction parameter c assumes a value c* and whose second correction parameter a assumes a value a*,
calculating a distance which the adapted corrected intensity spectrum adapted to the reference spectrum has from the reference spectrum; and
employing the calculated distance for assessing the similarity between the recorded intensity spectrum and the reference spectrum.
2. The method according to claim 1 , wherein the first and second correction parameters are ascertained through the adapting of the linearly corrected intensity spectrum to the reference spectrum.
3. The method according to claim 1 , wherein the adapting of the corrected intensity spectrum to the reference spectrum is carried out individually for each value document to be checked individually for each recorded intensity spectrum of each value document to be checked.
4. The method according to claim 1 , wherein, through the adapting of the corrected intensity spectrum to the reference spectrum, fluctuations due to measuring technology that have influenced the recorded intensity spectrum upon its recording are compensated at least partly.
5. The method according to claim 1 , wherein the adapting of the corrected intensity spectrum to the reference spectrum is carried out within the framework of a compensation calculation by which a least-squares fit of the corrected intensity spectrum to the reference spectrum is carried out.
6. The method according to claim 1 , wherein the distance that is calculated between the adapted corrected intensity spectrum and the reference spectrum is the chessboard distance of the adapted corrected intensity spectrum from the reference spectrum.
7. The method according to claim 1 , wherein by the method there is checked a spectral characteristic of a security feature of the value document for its similarity to a spectral characteristic specified for the security feature with which the security feature was specified before the value document was equipped with the security feature.
8. The method according to claim 7 , wherein, for assessing the similarity of the recorded intensity spectrum to the reference spectrum, the calculated distance, in particular the chessboard distance, is compared with a specified fluctuation range with which the spectral characteristic of the security feature was specified.
9. The method according to claim 1 , wherein the adapting of the corrected intensity spectrum to the reference spectrum is carried out in a spectral portion of the recorded intensity spectrum which comprises only wavelengths in the infrared.
10. The method according to claim 1 , wherein, upon calculating the distance, the distance is calculated only for a partial portion of that spectral portion for which the adapting of the corrected intensity spectrum to the reference spectrum was carried out.
11. The method according to claim 1 , wherein, upon calculating the distance, the distance is calculated only for a spectral portion of the adapted corrected intensity spectrum from the reference spectrum, in which the value document is checked for a spectral characteristic of a security feature, said spectral portion preferably lying completely in the infrared.
12. The method according to claim 1 , wherein the adapting of the corrected intensity spectrum to the reference spectrum is carried out in a constrained manner such that the adapted corrected intensity spectrum is not falsified in comparison to the recorded intensity spectrum.
13. The method according to claim 1 , wherein the adapting is carried out so as to be constrained by one or several certain side conditions being prescribed for two parameters derived from the first and second correction parameters.
14. A sensor for checking value documents which is configured for carrying out the method of claim 1 .
15. An apparatus for checking value documents which has a sensor for checking value documents according to claim 14 .