WO1997039429A1

WO1997039429A1 - Method and apparatus for authentication of a sheet having a security thread embedded therein

Info

Publication number: WO1997039429A1
Application number: PCT/NO1997/000104
Authority: WO
Inventors: Einar Gotaas
Original assignee: Einar Gotaas
Priority date: 1996-04-18
Filing date: 1997-04-18
Publication date: 1997-10-23
Also published as: NO961527D0; NO961527L

Abstract

A bank note genuineness sensor is based on detection of a metallic security thread embedded in the bank note. Electrode pairs provide input coupling and (possibly) output coupling of an electrical AC signal transmitted between the electrode pairs in the short moment where a security thread in a bank note is pulled past the electrode pairs. The arrangement of electrodes in pairs, i.e. one part electrode of a pair on each side of the slit through which the bank note is pulled, ensures good input and output coupling of the electrical signal.

Description

METHODANDAPPARATUS FORAUTHENTICATION OF ASHEETHAVINGASECURITYTHREAD EMBEDDEDTHEREIN

The invention relates to recognition and approval or disapproval of a security thread in a dielectric sheet, particularly a bank note or a document. The security thread used today in bank notes and security paper is most often a metal thread or a metallized thread of plastics. The plain plastics threads used as security threads, are no longer in use in European bank notes and documents. A metal thread or a metallized plastics thread will, such as described in this invention, exhibit quite special electrical properties. The invention can easily distinguish a "genuine" security thread from copies made by means of printing methods, as well as copies/forgeries made by means of tape, plastics etc. In the market today there are numerous different methods for rapidly, cheaply and simply providing a good indication as to whether a bank note is genuine or counterfeit. However, most methods turn out to be too time- consuming, require training, have a large uncertainty, or to be too costly.

The invention is based upon the fact that in most genuine bank notes, there is a metal thread or a metallized thread. This thread is very difficult to "insert" in the paper before a counterfeit bank note is printed. The security thread lies embedded, partly in the center of the note paper. Further, it is difficult to find a suitable material for use in such forgeries.

Since most bank notes have this special security thread, one and the same genuineness detector can be used for almost any bank note. The user does not need large reference works to find out what the genuineness sensor is intended to indicate for the various notes.

The genuineness detector will distinguish between various thread types. This can of course in itself be used as a security criterium.

However, for the simplest application, an inexpensive, reliable sensor for detecting colour photocopied counterfeit notes, it is only necessary to detect if there is "some" metal or metallized security thread inside the paper. It is of course possible to envision that a counterfeiter manages to embed a "genuine" security thread in his forgery. This would be very time consuming, unless the counterfeiter has access to paper production machines. Producing a forgery, one by one, having a counterfeit security thread, will be so time consuming that it is not profitable to the counterfeiter.

The forgeries flooding the market today, are "amateur copies" produced in a copying machine, one by one. It is primarily against this type of forgery that the present invention finds its strongest use.

If the counterfeiter manages to embed a genuine security thread, he will also probably use better printing techniques than a copying machine. In reality, one is then facing a professional, skilled and qualified counterfeiter. Such a person must be stopped with the aid of quite different methods.

There are several technical measuring methods for evaluating the presence of a security thread. For instance optical methods are used. However, these methods are not reliable, since it is possible, in technical printing, to produce a note which is mistakably similar to a genuine bank note as to visual/optical quality.

Measuring the dielectric property of the bank note paper is also previously known. A metal thread /metallized plastics thread can be distinguished in a relatively simple manner using this technique. However, this technique has a few other disadvantages, which means that it is not always usable in practice.

From DE 43 39417 is known a bank note testing means of a hand-held type, where the conductive security thread of a bank note is used as a capacitive coupling element between an input coupling electrode and an output coupling electrode for a high frequency voltage signal. The thread itself then works as an ohmic conductor between two capacitive coupling points.

The present invention substantially uses the same measurement principle as mentioned regarding DE 43 39 417, but utilizes a technique which provides better defined signals, thereby providing an increased possibility for being able to distinguish rapidly between genuine and false bank notes. With the method and apparatus in accordance with the present invention, a genuineness sensor is achieved which is quite useful in practice, can be manufactured at a correct price, and not least has a reliability which is very good. In accordance with the invention a method and an apparatus are provided for authentication of a dielectric sheet having a security thread embedded therein, the method and the apparatus including the features disclosed in the appended independent patent claims 1 and 4. Advantageous embodiments of the invention appear from the dependent patent claims 2, 3 and 4-7 attached to the indepen¬ dent claims.

The invention will now be described in closer detail while referring to the drawings, where: fig. 1 shows part of a paper note having a genuine security thread, fig. 2 shows the upper part of a sensor, fig. 3 shows the lower part of a sensor, fig. 3b shows a bank note passing the sensor, fig. 4 shows the upper and lower parts of the sensor mounted together, fig. 5 shows an example of a simple electrical signal processing circuit, fig. 6 shows a form of the output signal from part of the signal processing circuit of fig. 4, fig. 7 shows a bank note having two security threads, as well as one side of a sensor, and fig. 7b shows a bank note having a double thread, as well as one side of the sensor.

Fig. 1 shows part of a paper note 1 having a security thread 2.

Fig. 2 shows one side of the sensor through which the paper note is to be passed. 3 is an insulating material, 4 is a metal contact, about 2 x 2 mm, 5 is a metal contact about 5 x 5 mm. Fig. 3 shows the other side of the sensor though which the paper note is to be guided. 7 is a metal contact, about 2 x 2 mm, provided in alignment with the corresponding contact 4 in fig. 2. 8 is a metal contact, about 5 x 5 mm, provided in alignment with the opposite and corresponding contact 5 in fig. 2.

Fig. 4 shows the sensor in an assembled state. The two sides of the sensor (3 and 6) are placed at a distance d from each other, (d will typically be about 0,2 - 0,5 mm). The metal electrodes 7 and 8 are spring-loaded, via the device 11a and 11b, so that the metal electrodes 7 and 4 "are pressed together" via 11a. Similarly for electrodes 8 and 5, via the device 11b.

The paper note to be tested is guided in between plates 3 and 6 as indicated in fig. 3b. The bank notes must be pulled through the sensor, so that during a short moment the security thread will pass the small electrode pair 7 and 4, at the same time as the thread is also covered by the somewhat larger electrode pair 5 and 8.

It must be noted that spring loading of the electrodes (11a and 11b) is not necessary, however a better measuring result is obtained in this manner. The electrode 4 has a connecting lead 9a, the electrode 5 has a connecting lead 10a, the electrode 7 has a connecting lead 9b and electrode 8 has a connecting lead 10b. In the further description it is assumed that 9a and 9b are connected together, and 10a and 10b are connected together.

Fig. 3b shows a bank note, including a security thread, which is placed in the sensor, a small section of the security thread being covered by electrode pair 7 and 4, and at the same time the thread is also covered by the somewhat larger electrode pair 5 and 8.

Fig. 5 shows a simple electrical circuit for demonstrating a genuine thread. An oscillator O generates an AC voltage, designated U (0,1-1 MHz). This voltage is connected, via 9a, 9b, to one electrode pair, in the drawing figure denoted E. The other electrode pair, denoted D, is connected, via 10a, 10b, to amplifier A. The output signal from A is designated U_sτ.

The security thread in the bank note is most often completely covered by bulk paper. From the electrode pair in E, there will be a capacitive coupling of the AC voltage into the security thread. This connecting capacitance is drawn in the figure as C1.

It should be noted that this coupling in many cases will be purely ohmic, that is a direct electrical contact between electrodes and security thread. However, this does not alter the inventive idea. The security thread will conduct the applied voltage well between the to electrode pairs. A metallized plastics thread usually conducts an electrical current. If there are microscopic "fractures" in the metallization, so that there is not a purely ohmic electrical connection throughout the thread, the invention will still work. The electrical coupling through a thread with such microscopic "fractures" turns out to conduct the signal very well. The ohmic resistance of the thread is denoted Rs in fig. 5.

There will be a capacitive (possibly a purely ohmic) coupling, denoted C2, from the security thread into the amplifier A.

The size of the electrodes can in principle be selected relatively freely. The inventive idea is that a genuine security thread will conduct an electrical signal across the bank note. (There are genuine security threads made merely from plastics material. Such a thread will not be easily detected in this invention.) However, per se previously known techniques with shape-adapted electrodes, will also be able to increase the quality of the genuineness detection somewhat in this invention.

It turns out that a good result is achieved when the electrode pair D has a size about the same as the width of the security thread (2 x 2 mm). However, the electrode may well be given a rectangular shape, so that the coupling capacitance from the thread into the electrodes is as large as possible. In principle the electrodes may be "thread shaped" , for instance 1 x 20 mm. This would result in a large coupling signal, but the geometry would also be very sensitive to how the bank note is guided through the sensor. If the electrodes are long and narrow, and the note thread is not passed through the sensor with such an angle that optimum adaptation occurs, the sensor will give a reduced performance.

If one electrode is prepared to "match" the thread, being long and narrow, e.g. 1 x 20 mm, it is important that the other electrode, E, should be significantly larger. This in order to ensure that the electrodes cover the thread in an optimum manner.

Fig. 6 shows a signal observed in an oscilloscope, U is the voltage from the oscillator, Ust is the signal when a thread is located "correctly" in the sensor, and Uo is the signal when there is no security thread in the paper. The ratio of Ust to Uo is quite large for most types of security thread, e.g.

30/1. It can also be shown that the shape (and the amplitude) of curve Ust provides good information regarding the properties of the thread that is tested.

So far the invention has mostly been described as a sensor for recognizing an "accidental" electrically conductive thread in the paper. Security threads may have various widths. E.g. in a Norwegian NOK 200 bank note, two threads are embedded next to each other. The technique of the invention can be improved significantly by shape-adapting the electrodes to the thread it is desirable to optimize the sensor to. One electrode pair should e.g. have the same width as the thread. In fig. 7 there is a sketch of a bank note having a double thread, denoted

12. There is also shown a suggestion for a shape-adapted electrode which will improve the sensor significantly, for demonstrating such a double thread. The electrode 5 (electrode pair 5 and 8) is maintained as previously, about 5 x 5 mm. This electrode pair will cover a section of the complete double thread. Electrode 7 (electrode pair 7 and 4) is now prepared as two somewhat elongate electrodes. Each one of the electrodes has the same width as the bank note thread. The distance between the electrodes is made equal to the distance between the threads. This double electrode may very well be connected together to provide one common electrode. Fig. 7b shows the position of the bank note which results in the maximum coupling in position D (fig. 5), and thereby the largest signal output from the electrical signal processing circuit.

It should also be pointed out that the material of which the bank note is made, in principle may be any non-conductive material. The invention may very well be used to detect electrically conductive materials inside e.g. a plastic foil.

Claims

1. A method for authenticating a dielectric sheet, e.g. a paper bank note, having an embedded security thread which is placed in a known manner inside the sheet, which comprises metallic material and which is substantially continuous along its length, thereby to exhibit substantially low resistance along its length, said sheet being guided through a sensor having a plurality of electrodes and in such an angular position of the sheet, that the longitudinal direction of the security thread is substantially constant and predetermined during through passage, characterized in that an electrical AC signal is applied to an electrode pair where the two electrodes of the pair are situated mutually opposite and on respective sides of a slit in the sensor through which the sheet can be passed, and that the signal is attempted detected by a further electrode pair arranged correspondingly, however shifted in the slit in a direction substantially along the predetermined longitudinal direction of the security thread during the pass- through.

2. The method of claim 1 , characterized in that the electrical AC signal applied, can be altered in frequency, and that the variable form of the detected signal, depending on frequency, is used to determine the properties of the security thread.

3. The method of claim 1 or 2, characterized in that good contact between the dielectric sheet and the electrodes is ensured by having at least one electrode in an electrode pair provided with spring-loading for pressure toward the opposite and corresponding electrode.

4. An apparatus for authenticating a dielectric sheet, e.g. a paper bank note, having an embedded security thread which is placed in a known manner inside the sheet, which comprises metallic material and which is substantially continuous along its length, thereby to exhibit substantially low resistance along its length, o

said apparatus comprising a sensor having a plurality of electrodes, said sensor being adapted for passing the sheet therethrough with such an angular position of the sheet that the longitudinal direction ofthe security thread is substantially constant and predetermined during the passing through, characterized in that the sensor electrodes comprise an electrode pair for applying an electrical AC signal, the two electrodes in the pair being situated mutually opposite and on respective sides of a slit in the sensor through which the sheet can be guided, and a further electrode pair for detecting a transmitted signal, said further electrode pair being arranged in a corresponding manner as the first mentioned electrode pair, however shifted in the slit in a direction substan¬ tially along the predetermined longitudinal direction of the security thread during the pass-through.

5. The apparatus of claim 4, characterized in that at least one of the electrodes in a pair is spring- loaded so that it is urged toward the opposite and corresponding electrode.

6. The apparatus of claim 4 or 5, characterized in that the electrode pairs are shaped with the same width as a security thread in a particular sheet which the sensor has been optimized to recognize.

7. The apparatus of one of claims 4-6, the dielectric sheet having two parallel security threads, characterized in that each electrode pair is shaped as a double electrode, with width and spacing corresponding to the width and spacing of a double thread in a sheet that the sensor is optimized to recognize.