The present invention relates to a method for checking the authenticity of a security document. Within the context of the present invention, a security document is defined as having a non-conducting substrate or base material such as paper or plastic and particles functioning as an electrical dipole incorporated in the base material as security element, i. e. as element which distinguishes non-security documents from genuine security documents.

The method makes use of microwaves.

Background of the invention.

Such a method and a related apparatus is known from GB-A-2 050 664.

According to this prior art, an article to be checked for authenticity, is placed in the path of an unguided beam of microwave radiation. The microwave energy arrested by the article and the microwave energy reflected by the article are determined. The proportion of arrested energy in excess of the reflected energy, i. e. the absorbed energy, is characteristic for the type of particles functioning as an electrical dipole which are incorporated in the security documents. This absorbed energy discriminates against non-security documents.

The apparatus and method described in GB-A-2 050 664, however, have some disadvantages.

First of all, the arrested microwave energy and the reflected microwave energy are of the same magnitude and are large in comparison with their difference. This means that the difference, i. e. the amount of absorbed energy is relatively small. As explained in GB-A-2 050 664, this necessitates accurate measurement of both the arrested and reflected microwave energy.

A second drawback is that the apparatus comprises rather bulky horn antennas functioning as microwave receivers, which make the apparatus unpractical as small-size detector and impossible to be used in a hand- held device.

A third and major drawback is that the apparatus still does not always allows to distinguish security documents from non-security documents

such as wet paper documents or documents with conductive salts in their substrate.

US-A-4,820,912 discloses the so-called microwave'fingerprint'system.

It is based on the discovery that particles functioning as an electrical dipole, such as stainless steel fibres, are dispersed or distributed in a unique way in the substrate and that microwaves can measure that unique way. By measuring the microwave response coming from the document to be checked, it is possible to determine a distribution pattern which is unique for each security document. Making use of this technique, personalised access to automated entrances of factories or buildings can be organised. Such a system, however, is too complex and oversized for simple authentication operations to be performed at the point of transaction, i. e. at these points where the security documents are exchanged or realised in return for goods, services or money.

Summary of the invention.

It is an object of the present invention to mitigate the disadvantages of the prior art.

It is also an object of the present invention to provide a method which has improved facilities to discriminate between security documents and non-security documents.

It is another object of the present invention to provide a method which makes use of a small-sized and compact apparatus, which can be used at the point of transaction. The point of transaction is the most effective place to screen for counterfeits, having regard to the increase in circulation of counterfeit documents.

According to the present invention there is provided a method of checking the authenticity of a security document. The security document comprises an electrically non-conducting base material such as paper or

plastic and particles functioning as an electrical dipole are incorporated in the base material to function as security element.

Examples are bank notes, cheques, passports, credit-cards, tickets, lottery-tickets and bonds which comprise the above articles.

The particles are preferably electrically conductive fibres, e. g. stainless steel fibres or hard drawn fibres made of a semi-soft-magnetic material (see below). The length of the fibres is smaller than 40 mm, e. g. smaller than 10 mm. The diameter of the fibres ranges from 2 pm to 50 um, preferably from 2 um to 30 um.

The method comprises following steps : (a) manually performing a relative movement between a document to be checked and a microwave detector; (b) emitting microwaves to the document to be checked and receiving microwaves from the document to be checked in order to check whether the particles are present; (c) preventing human fingers or peoples'hands from interfering with the microwaves received from the document to be checked in order to avoid that human fingers are confused with the articles.

Indeed, at the point of transaction, users prefer to locate a document to be checked on the microwave measuring head with a hand behind it. It has been discovered by the inventors that the simple presence of a hand behind the document gives a major problem since the received microwave signal from a hand happens to be indistinguishable in practice from that from paper or plastic containing an area where metallic microwave reflecting particles are distributed. Attempts have been made by the inventors to put the particles in discrete bands to aid discrimination from hands. However, it has been found that because of size constraints associated with typical tickets, bonds and banknotes, and the effective scan area of a compact microwave system, the widths or usable bands are indistinguishable in practice from the signal obtained from fingers.

It has therefore been found that a practicable design of a microwave system for the application at the point of transaction is to have a method to ensure that false signals are not obtained from peoples'hands holding the document to be verified. The most convenient way to do this is with a slot which is large enough to make insertion of the document to be checked easy, but small enough to exclude peoples'hands and particularly fingers.

In a preferable embodiment of the invention, both the microwaves reflected from said document to be checked and the microwaves transmitted through said document to be checked are received. This is because it has further been found that if the measurement is only by microwave reflectance then spurious signals, which could be interpreted as coming from a security document containing the metallic particles, can be obtained from other articles such as wet paper, metal foils and edges of thick documents.

In another preferable embodiment of the invention, characteristic values which are a function of microwaves reflected from a security document and microwaves transmitted through said security document are stored in memory or are used to set threshold values for a set of window comparators. These values are characteristic for a security document, which means that other documents, although similar to a security document from an electrically conducting point of view, do not produce these values. The method further compares the same function of microwaves reflected from and transmitted through said document to be checked with said values in memory or with said threshold values. If the comparison is positive, a security document is authenticated, if the comparison is negative, the document to be checked is not a security document.

As a first example, the method according to the invention determines the reduction AT in the transmitted microwaves caused by the presence of the document to be checked, determines the increase in reflected microwaves AR caused by the presence of the document to be checked and compares the determined values AR and AT with threshold values or with values in memory which are characteristic for a security document. Indeed, considering a AR-AT plane, it has been observed that the oR and AT values measured from a security document enter into one or more particular characteristic subregions where the AR and AT values measured from various non-security documents do not enter.

These subregions are unique for security documents containing the electrically conducting particles. These subregions can be used in the decision algorithm, e. g. by storing the characteristic values of these regions in memory and comparing them with the values measured from the document to be checked. It is therefore possible to provide a high level of discrimination from articles which cause spurious detection events.

As a second example, a method according to the invention determines an amount T of microwaves transmitted through a document to be checked, determines an amount R of microwaves reflected from a document to be checked, calculates the values T+R and T-R, and compares the values T+R and T-R with threshold values or values in memory which are characteristic for a security document. Indeed, the inventors have found that calculated values of T+R and T-R can be used with, or instead of AT and AR, to provide further discrimination between the signals obtained by moving a document containing the metal fibres across the microwave path, and those obtained with other common articles e. g. paper with metal foils, metal strips or conducting salts.

In still another preferable embodiment of the invention, the method further detects whether the microwave path is completely covered by the

document to be checked, in order to avoid that due to edge effects a security document is confused with a pile of non-security documents or with thick non-security documents. Indeed the microwave signal received from an electrically conductive particle of a limited size, such as a metal fibre with a limited length and limited diameter, is very similar to the microwave signal of a thick non-security document, or of a pile of metal foils or wet papers that only partially covers the microwave path.

In a practical embodiment of the invention, microwaves are emitted by means of a Gunn diode and the microwaves reflected from the document to be checked as well as the microwaves transmitted through the document to be checked are received by means of Schottky diodes placed in wave-guide cavities. The antennas in this invention are just flat flanges on the end of the wave-guides so that the need for bulky horn antennas is no longer present. Plastic end covers are used at the end of the wave-guides to stop the ingress of dirt.

In order to minimise the deleterious effects of microwave reflections on the operation of the oscillator and detector diodes caused by impedance mis-match between the wave-guides and the detection gap an attenuator is used in the wave-guides. This attenuator is conveniently formed from a conducting foam but it could also be formed from a suitably thin sheet of resistive material or by using a PIN diode across the wave-guide.

As a practical feature of the invention, the method preferably generates an authentication signal when detecting the presence of particles functioning as an electrical dipole in a non-conducting substrate and/or generates an alarm signal when the presence of such particles is not detected. The signal can be an audible, a visual or a vibratory signal, or a combination of these signals.

In a lot of applications the authentication signal will be generated when a genuine security document is presented.

There may be applications, however, e. g. in case of a bank employee counting and checking several hundreds banknotes, where the series of subsequent signals is highly undesirable so that the function is reversed: in this case an alarm signal is produced if a non-security document is presented.

The present method may also comprise features of other systems for determining the authenticity of a security document.

In combination with the microwave means, the method may comprise a magnetic detection system such as disclosed in European patent application No. 96203529.1. Such a magnetic system detects the presence or absence of particles of a semi-soft-magnetic material with particular magnetic features in a security document. These particular magnetic features are essentially characterised by a demagnetisation factor N smaller than 1/250 and a magnetic saturation field Hs greater than 100 A/m (soft-magnetic materials as are typically used in tags for electronic article surveillance have a magnetic saturation field Hs smaller than 100 A/m) The magnetic detection method comprises the following steps : (a) emitting an electromagnetic source signal of one or more particular base frequencies to the document to be checked so that any present magnetic elongated particles go into a non-liner part of their B-H curve for at least part of a cycle of the source signal; (b) detecting an electromagnetic detection signal emanating from the document to be checked; (c) testing the detection signal for the presence of particular higher harmonics of the base frequencies or of any linear combination of the base frequencies as well as the harmonics, where the particular higher harmonics are indicative of the presence of the magnetic elongated articles.

As these magnetic particles are also electrically conductive, they may also be detected by means of microwaves.

In a sophisticated version of this combination, a security document may comprise regions where there are only stainless steel fibres and regions where there are only soft-magnetic fibres. The microwave system would then produce a positive signal for all regions, whereas the magnetic system would only produce a positive signal for the regions with magnetic fibres.

In combination with the microwave means, the method may also comprise optical features for measuring the optical reflection and/or optical transmission from an article to be checked in order to further provide better discrimination with metallic strips, foils, PCBs,... and all kind of documents or articles with electrically conductive parts on their surface. Indeed, within the context of this invention, a genuine security document contains particles incorporated in its substrate and does not contain any particles on its surface. The optical features could also be a simple image recognition system for the expected print design of the security document.

In an alternative design of the invention, the method uses a micro- controller for auto calibration and for signal processing. This compensates variations in the functioning of microwave elements due to a temperature change and aging.

In still another alternative design of the invention, the polarization orientation of the microwaves is so selected that the electrical field of the microwaves is oriented at a substantially right angle with respect to the orientation of the metallic strip (s) across a typical bank note inserted in the most convenient way in the apparatus. This minimizes the possibility of spurious signals produced by these strips.

In yet another design of the whole system, the electrically conductive fibres may also be partially or completely oriented in a security document

(e. g. as a consequence of the manufacturing process) so that they respond in different ways to two different angles of polarization of the microwaves.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawings wherein -FIGURE 1 is a schematic representation of an invention apparatus which uses a transmitter/receiver Tx/Rx module and a receiver Rx module; -FIGURE 2 and FIGURE 3 illustrate an invention apparatus using microstrips; -FIGURE 4 illustrates an invention apparatus where wave-guides are used; -FIGURE 5 shows a curve of transmitted and reflected microwaves and the effect of inserting a security document in the apparatus ; -FIGURE 6 illustrates how a detection algorithm according to the invention works.

Description of the preferred embodiments of the invention.

Referring to FIGURE 1, a document 10 is presented for checking its authenticity in a microwave path created by means of a transmitter/receiver Tx/Rx module 11. The Tx/Rx module 11 comprises a Gunn diode 12 placed in a resonant or oscillator cavity. In order to prevent excessive reflections upsetting the Gunn diode oscillator a conducting absorber foam 13 is placed in the wave-guide. The Tx/Rx module 11 also comprises a Schottky detector diode 14. The end of the wave-guide is covered by means of a plastic sheet 15 in order to stop ingress of dirt. Next to a Tx/Rx module 11, there is also a receiver Rx module 18 for measuring the microwaves T transmitted through the

document 10. In the Rx module 18 there is a Schottky detector diode 19 also placed in the wave-guide. There is not only a conducting foam 13 in the wave-guide of the Tx/Rx module 11 but also a conducting foam 20 in the wave-guide of the Rx module 18. A plastic sheet 21 covers the wave-guide of the Rx module 18. Plastic sheets 15 and 21 form together with a suitable spacer 22 a slot for the insertion of a document. For easy insertion of the document 10, the plastic sheet 21 may be slightly bent upward or the plastic sheet 15 bent downward.

FIGURE 2 and FIGURE 3 give an illustration of a microstrip embodiment of an invention apparatus. It comprises two PCB's: A Tx/Rx board 23 and a Rx board 24. On the Tx/Rx board 23, a microwave emitter comprises a GaAs FET element 26, a 24 GHz dielectric resonator 28 and a resonant microstrip resonant line 30. The microwaves are emitted by means of antenna 32. The microwave emitter elements are covered by a metal screen 33. The Tx/Rx board 23 further comprises an antenna 34 for receiving reflected microwaves and a Schottky detector diode 36.

The Rx board 24 comprises an antenna 37 for receiving microwaves transmitted through the document 10 and a Schottky detector diode 38.

The use of relatively high frequencies above 20 GHz, such as 24 GHz, allow the use of a small and low cost system with optimal interaction with fibres of a length varying between 10 mm and 30 mm.

FIGURE 4 gives a disassembled view of an invention apparatus where wave-guides are used instead of striplines or instead of microstrips. It comprises a housing 40 upon which is mounted a plastic plate or lid 21 to form with the help of a spacer 22 a slot where a document 10 can be inserted and where the insertion of human fingers is prevented. The plate 21 is bent upward so that insertion of a document 10 is facilitated.

The microwave emitter comprises a Gunn diode 12, a wave-guide 46, and a slot antenna 48 placed at a resonant wave guide cavity. A

Schottky detector 14 receives microwaves reflected from the document 10. An optical detector 52 checks whether or not the document 10 completely covers the microwave path. Indicator LEDs 54 and 56 give a visual signal if a security document is identified or not, e. g. a red signal in case the presented document is not a security document, and a green signal in case the presented document is a security document. A Schottky detector 19 is built in the lid 21 in order to receive microwaves transmitted through the document 10.

FIGURE 5 shows two curves: The transmitted microwaves T and the reflected microwaves R. When a security document is inserted for authentication, the level of the transmitted microwaves T decreases with an amount AT and the level of the reflected microwaves R increases with an amount AR.

It has been discovered that security documents give rise to a series of (AT, AR) values, some of which are very characteristic since they are never found with non-security documents such as metal foils, or wet paper documents or documents containing conductive salts or metal strips. These characteristic values can be determined in advance and used to set the switching thresholds for window comparator circuits which provide a signal when the values for the ATand AR are in the required range. The output of the window comparator may also be combined in a boolean AND function with a document detector, such as an optical sensor. so that the microwave measurement is only taken when the document is covering the microwave sensing path.

Alternatively the values of AT and AR can be compared with threshold values stored in the memory of a micro-controller and these may also be combined with another type of document sensor as described above.

The authentication procedure therefore simply provides a check of the measured values of AT and AR against the threshold values.

These characteristic values can be determined in advance and stored in a memory of the apparatus. The authentication procedure then simply

comprises a check of the measured (AT, AR) values with the stored values.

FIGURE 6 schematically illustrates how a detection algorithm according to the invention may work. A series of (AT, AR) values of a typical security document are registered. In a AT-AR plane this forms a variable and changeable curve 58. This series of (AT, AR) values is then compared with (AT, AR) values of articles which may come very close to a security document. Examples of such articles are a sheet of wet paper, a metal foil, a PCB, a paper sheet with salt particles in it, etc... A comparison is then made and a subregion or sub-area, e. g. a rectangular area referred as 60, is determined where all the similar but different articles do not produce (AT, AR) values but where a typical security document always produces (AT, AR) values. The outcome of this comparison study is then straightforward. The values of the typical characteristic area 60 are then used to set the thresholds for identification. When checking a document for authenticity the measured (AT, AR) values are then compared with those stored values.