NO178049B

NO178049B - Detection of watermarks in measuring objects of paper, such as banknotes and security documents

Info

Publication number: NO178049B
Application number: NO922387A
Authority: NO
Inventors: Jan Waaler
Original assignee: Norges Bank Kobe
Priority date: 1992-06-18
Filing date: 1992-06-18
Publication date: 1995-10-02
Also published as: NO922387L; NO922387D0; NO178049C

Description

Foreliggende oppfinnelse angir en fremgangsmåte og et system for optisk deteksjon av vannmerker i måleobjekter av papir, eksempelvis sedler eller sikkerhetsdokumenter. The present invention specifies a method and a system for the optical detection of watermarks in paper measuring objects, for example notes or security documents.

Vannmerker beskrives som kontrollerte lokale variasjoner av tykkelse og/eller tetthet i papir. Vannmerker genereres under papir-produksjonen og har gjerne form som portretter, bokstaver eller geometriske mønstre. Generering av vannmerker er en del av selve papirproduksjonsprosessen. Watermarks are described as controlled local variations of thickness and/or density in paper. Watermarks are generated during paper production and often take the form of portraits, letters or geometric patterns. Generating watermarks is part of the paper production process itself.

Vannmerkers natur forårsaker tilhørende endringer i flere av papirets fysiske egenskaper; lokale termiske egenskaper, absorpsjon av lys, ultralyd og radioaktiv stråling, lysspred-ning, tykkelse etc. The nature of watermarks causes associated changes in several of the paper's physical properties; local thermal properties, absorption of light, ultrasound and radioactive radiation, light scattering, thickness etc.

Tidligere kjente teknikker for deteksjon av vannmerker baserer seg vanligvis på bruk av synlig eller nær-infrarødt lys. Ulemper ved disse tidligere kjente teknikker er imidler-tid at det kan opptre forstyrrelser i målingen på grunn av absorpsjon/refleksjon av trykkmaterialer, smuss, tape etc, og det er behov for en fremgangsmåte for deteksjon som er slik at disse feilkildenes bidrag kan elimineres eller i hvert fall gjøres svært små. Previously known techniques for the detection of watermarks are usually based on the use of visible or near-infrared light. Disadvantages of these previously known techniques are, however, that disturbances can occur in the measurement due to absorption/reflection of printing materials, dirt, tape etc., and there is a need for a method for detection which is such that the contribution of these sources of error can be eliminated or in any case, very small ones are made.

Fra EPO-søknad nr. 92,691 er det kjent en metode for å detektere en sikkerhetstråd i sedler ved gjennomlysning med IR-lys av bølgelengde 4,5-6,0 nm. Videre beskriver EPO-søknad nr. 408,337 en metode for optisk undersøkelse av sedler vedrø-rende graden av slitasje. Her oppgis at bølgelengden under visse forhold kan være 3-10 /xm. Ingen av de forannevnte publikasjonene viser dog deteksjon av vannmerker. Beslektet teknikk er også å finne i de britiske patentsøknadene GB 2,107,911 og 2,153,070. From EPO application no. 92,691, a method is known for detecting a security thread in banknotes by transillumination with IR light of wavelength 4.5-6.0 nm. Furthermore, EPO application no. 408,337 describes a method for the optical examination of banknotes regarding the degree of wear. It is stated here that the wavelength under certain conditions can be 3-10 /xm. However, none of the aforementioned publications show detection of watermarks. Related art is also found in British patent applications GB 2,107,911 and 2,153,070.

Ifølge oppfinnelsen er det således tilveiebrakt en fremgangsmåte for optisk deteksjon av vannmerker i måleobjekter av papir, så som sedler og sikkerhetsdokumenter, med benyttelse av belysning fra en strålingskilde og med anvendelse av et optisk deteksjonssystem, hvor det spesielle ved fremgangsmåten er at det benyttes mellom-infrarød transmisjonsmåling i deler av det elektromagnetiske spektrum mellom 2 og 12 /im, slik det forøvrig angis i det vedføyde patentkrav 1. Ifølge oppfinnelsen er det også tilveiebrakt et system for optisk deteksjon av vannmerker, og systemet defineres nøyaktig i det vedføyde patentkrav 8. According to the invention, a method has thus been provided for the optical detection of watermarks in paper measuring objects, such as banknotes and security documents, with the use of illumination from a radiation source and with the use of an optical detection system, where the special feature of the method is that it uses intermediate infrared transmission measurement in parts of the electromagnetic spectrum between 2 and 12 µm, as stated otherwise in the attached patent claim 1. According to the invention, a system for optical detection of watermarks is also provided, and the system is precisely defined in the attached patent claim 8.

Ytterligere mer spesifikke trekk ved oppfinnelsen er angitt i de uselvstendige patentkrav tilknyttet de ovennevnte patentkrav 1 og 8. Further, more specific features of the invention are indicated in the independent patent claims associated with the above-mentioned patent claims 1 and 8.

Oppfinnelsen angår således en spesiell anvendelse av lys-absorpsjon, hvor det benyttes et spektralområde betegnet som mellom-infrarødt, som her er definert som bølgelengdeområdet mellom 2 og 12 ^m. Fordelen med dette området er at det kan velges ut deler av dette spektralbåndet der andre forstyrrelser som absorpsjon/refleksjon av trykkmaterialer, smuss, tape etc. gir små bidrag. The invention thus relates to a special application of light absorption, where a spectral range designated as mid-infrared is used, which is defined here as the wavelength range between 2 and 12 µm. The advantage of this area is that parts of this spectral band can be selected where other disturbances such as absorption/reflection of printing materials, dirt, tape etc. make small contributions.

Oppfinnelsen skal i det følgende beskrives nærmere med bruk av utførelseseksempler, og med henvisning til den ved-føyde figur 1 som viser en skisse av prinsippet ifølge oppfinnelsen. In the following, the invention will be described in more detail with the use of exemplary embodiments, and with reference to the attached Figure 1, which shows an outline of the principle according to the invention.

Prinsippet er beskrevet som en installasjon i en eksiste-rende maskin for seddelbehandling hvor objektet som skal karakteriseres, transporteres forbi sensor-systemet, og hvor det detekterte signalets tidsforløp gjenspeiler punkter på dokumentet langs transportbanen. The principle is described as an installation in an existing machine for banknote processing where the object to be characterized is transported past the sensor system, and where the time course of the detected signal reflects points on the document along the transport path.

Prinsippet realiserer måling av infrarød (IR) transmisjon ved at stråling fra en passende kilde på den ene siden av papiret blir detektert på motsatt side av papiret ved hjelp av en detektor som er følsom for det ønskede spektralområde. The principle realizes the measurement of infrared (IR) transmission by radiation from a suitable source on one side of the paper being detected on the opposite side of the paper by means of a detector that is sensitive to the desired spectral range.

Fig. 1 viser et måleobjekt, eksempelvis en seddel 1 orientert vinkelrett på papirplanet, i bevegelse mot venstre med hastighet "V". Seddelen 1 passerer mellom en strålingskilde 2 og et optikk/spektralfilter/detektor-arrangement 3, 4. Optikk 3 og detektor 4 avbilder et tilpasset synsfelt for å detektere de ønskede strukturer i måleobjektet 1. Måleobjektets bevegelse gir et tidsvarierende signal som representerer de tilhørende måle-parametrene langs bevegelsesretningen. Fig. 1 shows a measurement object, for example a note 1 oriented perpendicular to the plane of the paper, moving to the left with speed "V". The bill 1 passes between a radiation source 2 and an optics/spectral filter/detector arrangement 3, 4. Optics 3 and detector 4 image an adapted field of view to detect the desired structures in the measurement object 1. The movement of the measurement object produces a time-varying signal that represents the associated measurements -the parameters along the direction of movement.

Prinsippet kan også realiseres stasjonært, f.eks. ved at man benytter et infrarødt kamera som er følsomt innenfor ønsket spektralområde. Denne metoden kan gi en to-dimensjonal avbildning av vannmerket. The principle can also be realized stationary, e.g. by using an infrared camera that is sensitive within the desired spectral range. This method can provide a two-dimensional representation of the watermark.

Sensor-prinsippet består av en kilde for elektromagnetisk stråling som emitterer med ønsket spektral-fordeling. I sin enkleste form kan dette være en termisk strålingskilde med en temperatur tilpasset behovet. En passende infrarød laser er et annet eksempel på en egnet strålingskilde. The sensor principle consists of a source of electromagnetic radiation that emits with the desired spectral distribution. In its simplest form, this can be a thermal radiation source with a temperature adapted to the need. A suitable infrared laser is another example of a suitable radiation source.

Strålingskilden innrettes i seddeltransporten slik at den stråler mot passerende sedler, på tvers av seddelens trans-portretning, se fig. 1. The radiation source is arranged in the note transport so that it radiates towards passing notes, across the note's trans-portrait, see fig. 1.

Et detektor-system med ønsket spektral følsomhet plasse-res rett overfor strålingskilden, slik at objektet passerer mellom strålingskilde og detektor-system. Detektor-systemet kan bestå av et enkelt element eller en rekke (array) av detektor-elemeneter. Hvert detektor-element vil idet objektet passerer, motta en andel stråling fra strålingskilden som vil variere med materialets beskaffenhet, f.eks. tykkelse, sammen-setning, evt. påførte stoffer. For vannmerke-deteksjon er det vesentlig å minimalisere virkningen av uønskede materialer og maksimalisere virkningen av variasjoner i tykkelse x tetthet. Signal som genereres av detektor-element(ene) når objektet transporteres forbi, vil være et uttrykk for vannmerkets beskaffenhet. A detector system with the desired spectral sensitivity is placed directly opposite the radiation source, so that the object passes between the radiation source and the detector system. The detector system can consist of a single element or an array of detector elements. As the object passes, each detector element will receive a proportion of radiation from the radiation source which will vary with the nature of the material, e.g. thickness, composition, possibly applied substances. For watermark detection, it is essential to minimize the effect of unwanted materials and maximize the effect of variations in thickness x density. The signal generated by the detector element(s) when the object is transported past will be an expression of the nature of the watermark.

Signalene kan behandles analogt eller i en datamaskin med hensikt å redusere rå-data til et meningsfylt uttrykk for vannmerkets beskaffenhet, objektets ekthet etc. The signals can be processed analogically or in a computer with the intention of reducing raw data to a meaningful expression of the nature of the watermark, the authenticity of the object, etc.

Claims

1. Method for optical detection of watermarks in measuring objects (1) made of paper, such as banknotes and security documents, using illumination from a radiation source (2) and using an optical detection system (3, 4), characterized in that it is used mid-infrared transmission measurement in parts of the electromagnetic spectrum between 2 and 12 /jm.

2. Method according to claim 1, characterized by spectral filtering of the radiation source (2) or detection system (optics, filter/detector) (3, 4) so that the system's sensitivity is adapted to the spectral properties of the measurement object (1) and is as insensitive to unwanted factors as possible.

3. Method according to claim 1 or 2, characterized in that the object (1) is illuminated with low divergence from the radiation source (2), to ensure that the radiation intensity on the surface of the measuring object (1) is as high as possible independent of the measuring object's distance from the radiation source (2 ).

4. Method according to one of the preceding claims, characterized in that the object (1) is imaged with low divergence and high depth of field using adapted optics (3) in the detection system, to ensure that detected radiation from the measurement object's (1) surface is as much as possible regardless of the measurement object's distance from the detection system (3, 4).

5. Method according to one of the preceding claims, characterized in that two or more identical but spatially separated detection systems are used which provide a corresponding number of parallel detection channels on the measurement object, each detection system having a spatial resolution that is adapted to the geometric sizes it is desirable to detect, as the typical size of a resolution element for portrait watermarks is approx. LxW mm<2>.

6. Method according to one of the preceding claims, characterized in that the signals from the transmission measurement are processed using analog electronics or digital data processing systems to extract information regarding the existence and quality of watermarks and paper, as well as to further verify existence, dimension and position of any embedded safety wire.

7. Method according to claim 5 or claim 6 when this depends on claim 5, characterized by the fact that information is extracted from the parallel detection channels for the characterization of object thickness, as well as in addition single versus multiple feeding of measurement objects as well as the presence of adhesive tape and other foreign elements on the measurement objects.

8. System for optical detection of watermarks in paper measuring objects, such as banknotes and security documents, comprising a radiation source (2) for illumination towards the object (1) as well as an optical detection system (3, 4), characterized in that the radiation source (2) is arranged to emit infrared radiation comprising at least parts of the electromagnetic spectrum between 2 and 12 /xm, and that the optical detection system (3, 4) is adapted to detect corresponding areas of the spectrum.

9. System according to claim 8, characterized in that the radiation source (2) or the detection system (pickup, filter/detector) (3, 4) includes a device for spectral filtering, so that the system's sensitivity is adapted to the spectral properties of the measurement object (1) and is as insensitive to unwanted factors as possible.

10. System according to claim 8 or 9, characterized in that the radiation source (2) is arranged to illuminate the object (1) with low divergence to ensure that the radiation intensity on the surface of the measurement object is as high as possible regardless of the measurement object's distance from the radiation source (2) .

11. System according to one of claims 8-10, characterized in that the detection system's optics (3) are adapted for imaging the object (1) with low divergence and high depth of field, to ensure that detected radiation from the surface of the measurement object is as independent as possible of the measurement object's distance from the detection system (3, 4).

12. System according to one of claims 8-11, characterized in that it comprises two or more identical but spatially separated detection systems which provide a corresponding number of parallel detection channels on the measurement object, each detection system having a spatial resolution which is adapted to the geometric sizes it is desirable to detect, as the typical size of a resolution element for portrait watermarks is approx. LxW mm<2>.

13. System according to one of claims 8-12, characterized in that it comprises analog electronics or digital data processing systems for processing signals from the transmission measurement, to extract information regarding the existence and quality of watermarks and paper, as well as to further verify the existence, dimension and position of any embedded safety wire.