GB2130414A - Security documents and verification thereof - Google Patents

Abstract

A security document carrying a magnetisable material is verified by first saturating (11, 12) the material; obtaining (13, 14) a signal related to the strength of the remanent field from the material; subjecting the material to a second magnetic-field (15, 16) of opposite polarity to the saturating field; obtaining (17, 18) a second signal related to the magnetic field then produced by the material; and combining the first and second signals to obtain an indication characteristic of the material and substantially independent of the amount and distribution of the material carried by the document. <IMAGE>

Description

SPECIFICATION Security documents and verification thereof The present invention relates to security documents and to methods of verifying security documents, ie.

checking that they are genuine.

Magnetic materials are provided as a security feature in certain security documents. For instance some banknotes carry a thread of plastics film having a magnetic powder coated therein. The thread is incorporated within the body of the paper.

Other banknotes have a surface printing of some design in magnetic ink on their surfaces.

One of the benefits of these magnetic security features is that their presence or absence can be sensed by a document verifying machine. Such machines may need to be able to handle a large throughput of documents. For instance worn used banknotes must be checked for authenticity before being destroyed and replaced by new notes or re-issued if they still have a useful life.

At present the surface printing of security documents with a pattern, e.g. words and/or numerals, in magnetic ink confers a relatively low degree of security. The presence or absence of magnetic material can easily be checked. However, some magnetic ink pigments are easily obtained and so magnetic printing could be provided on a forgery. It would provide a much higher level of security if a magnetic material not readily available were used and means were provided to enable a document verifier to differentiate to a high standard of reliability between different types of magnetic material as opposed to merely testing if any magnetic material was present.

A first parameter that can be used to differentiate between magnetic materials is their coercivity. A magnetic security device in or on a document can be magnetised in a first direction by passing the document through a magnetic field sufficient to saturate the material e.g. between opposed north and south poles of a pair of permanent magnets.

When the material passes out of the field it will be magnetised and will exhibit a residual magnetic field of its own of a strength determined by its remanence. The application of a reverse field e.g. by a second pair of permanent magnets will reduce or reverse that remanent field. The reverse field strength necessary to bring the remanent field to zero is known as the coercivity. The coercivity is a property of the magnetic material itself and is not altered by the amount of magnetic material present or the form in which that material is present.

A further parameter that can be used to differentiate between magnetic materials in or on a security document is the remanent flux produced by the material after being subjected to a saturating magnetic field. This however will depend not only on the nature of the magnetic material but on the form and amount in which it is present.

Where magnetic material is present on a thread incorporated into the body of the paper, the amount of magnetic material and its shape will generally be constant over all genuine documents. This will remain true even when the documents have been used and are becoming worn. When the magnetic material is printed on the surface of the document, eg. as a number, the amount of material present initially and its spatial distribution will differ between genuine documents and also the material will be very subject to wear and will gradually be rubbed away.

Other known magnetic inclusions such as magnetic fibres, powders, planchettes, micro-capsules, water permeable ribbons and security threads of varying widths do not suffer from wear but the amount and distribution of magnetic material in a given location in the document may vary from one genuine document or another.

Methods of verification which involve the measurement of the magnitude of a magnetic flux density of varying amounts of magnetic material present in a security device lying within or on the surface of a document are therefore unlikely to be suitable for verifying documents because the flux will vary from one genuine document to another.

However, it is necessary to make such a measurement of the flux density in the magnetic material in checking the nature of the magnetic material to a degree giving optimum security against imitation.

Accordingly, methods of verification capable of differentiating between different magnetic materials on the basis both of coercivity and of remanence have hitherto been restricted to use with threads incorporated in the paper of the documents.

British Patent Specification No. 1534859 describes a method of distinguishing documents comprising magnetisable material of a coercivity greater than a predetermined value from those containing one of a coercivity less than a predetermined value. The document is subjected to a magnetising saturating field. The document is then subjected to a reverse field of such a strength as to reverse the sense of the magnetisation of a lower coercivity material but only to reduce the magnitude of the magnetisation of a material of higher coercivity. The document then passes a reading head containing a coil in which a signal is induced by the magnetised material. The shape of the signal will depend on whether the coercivity of the material was low or high, i.e., whether the initial residual magnetism has been reversed in direction or not.

This method only allows a distinction to be drawn between documents on the basis of coercivity. The additional safeguard of checking that the remanence is correct cannot be made. Also the coercivity can only be checked as being either above or below some threshold value. No particular value for or related to the coercivity can be obtained.

In a verification apparatus which we have produced, a unidirectional magnetic field is applied to a document by a pair of permanent magnets. This field saturates any magnetic material in or on the document. The remanent field so produced is then detected by a pair of reading heads sited on each side of the document's path. The amplitude of the resulting signal is checked to be above a preset threshold. The document is then subjected to a reverse field of a strength chosen to reduce the flux density of the particular magnetic material which should be present in genuine documents to zero. A second pair of reading heads then checks that the flux density is in fact substantially zero.

This system works well if the magnetisable material is present in a strip of known size in the body of the paper. It provides a high level of security because (a) both the remanence and the coercivity of the magnetisable material are involved in the check and (b) the coercivity is checked within upper and lower levels, not merely with reference to a single threshold.

However, this method cannot be applied to documents in which the magnetisable material is present on or within the document in a pattern varying from document to document. This is because the remanent field detected would then be subject to large variation. The remanent field has to be checked because the coercivity check alone would be defeated by a document with either no magnetic character or only an extremely weak magnetic character since the signal looked for at that stage is a null signal.The present invention now provides a method of verifying a security document having thereon and/or therein a magnetisable material which method comprises temporarily applying a first unidirectional magnetic field to the magnetisable material thereby to magnetise the magnetisable material, then obtaining a first signal related to the strength of the remanent magnetic field produced by the material, applying to the magnetisable material a second temporary unidirectional magnetic field of polarity opposite to the first, then obtaining a second signal related to the strength of the magnetic field then produced by the magnetisable material, combining the said first and second signals to obtain a signal characteristic of the magnetisable material and substantially independent of the amount and distribution of the magnetisable material present in or on the document, and comparing that signal with a predetermined standard.

The magnetic material is identified by this methd by a signal which is not determined by either coercivity or remanence alone and which is therefore a very specific means of identification. The signal used for the vertification is substantially independent of the amount of magnetic material present and its pattern. Accordingly, this method may be used for checking the genuineness of documents by virtue of a magnetisable security feature on the surface of the document, eg. a printing of magnetic ink and/or a magnetisable security feature within the document comprising, for example, magnetic fibres or powder, planchettes or capsules, water permeable ribbons or security threads of varying widths.

There are some magnetisable materials available of which supplies are relatively secure. Such materials have a very high coercivity compared to the magnetisable materials which have up to now been used for printing on security documents. Printing in these high coercivity materials would provide a higher degree of security than the present practice of printing using easily available low coercivity materials. Up until now however, there has been no satisfactory method for verifying such a document by closely checking the nature of magnetisable material on the surface thereof. Such a method is provided by the present invention so that it now becomes useful to place high coercivity materials on the surface of a security documents and/or within the document in a form such that theamount of magnetic material may vary from one genuine document to another.

Accordingly the present invention provides, a security document comprising as a security feature or features, magnetisable materials where amount and distribution varies as described above which magnetic materials have a coercivity of at least 48,000 ampere/metre (600 oersteds).

For carrying out the above method of document verification, the present invention provides a document verifying machine comprising means for temporarily applying a first unidirectional magnetic field to a security document comprising a magnetisable material, means for thereafter obtaining a first signal related to the strength of the remanent magnetic field produced by said material, means for temporarily applying a second unidirectional magnetic field of opposite polarity to the first, means for thereafter obtaining a second signal related to the strength of the remanent magnetic field then produced by said material, and means for combining said first and second signal to produce a third signal characteristic of the magnetisable material and substantially independent of the amount and distribution of the magnetisable material present in or on the document.

Preferably, in the method of the invention, the first magnetic field is sufficiently strong to saturate the magnetisable material. Generally, the first and second signals are time dependent signals. Preferably, the signal characteristic of the material is obtained by taking a ratio involving the first and second signals such as to eliminate the common dependency of each signal on the amount and distribution of magnetic material.

For instance, the ratio taken may be the ratio of the peak value of the first signal to the peak value of the second. Alternatively, the ratio taken may be of an instantaneous value of the first signal and of a corresponding instantaneous value of the second signal. A further option is for the ratio to be of a value obtained by integrating the first signal and a value obtained by integrating the second signal.

A further option is for the ratio to be of the peak value of the first signal and the sum of the peak values of the first and second signals.

A further option is for the ratio to be of an instantaneous value of the first signal and the sum of that value and a corresponding instantaneous value of the second signal. A further option is for the ratio to be of a value obtained by integration of the first signal and the sum of that value and a value obtained by integration of the second signal. Many other ways of combining the first and second signals so as to eliminate dependency on the amount and spatial distribution of the magnetic material will readily occur to those skilled in the art. Preferably the first and second signals are each obtained by passing the security document between a pair of facing magnetic reading heads.

In a document verifying machine according to the invention, means may be provided for comparing the third signal with a predetermined standard.

Alternatively, the machine may simply provide a readout of the signal for an operator to use in making the comparison with the standard.

The method of document verification provided by the invention may be carried out in machines in which a succession of documents are transported past the vertification equipment. In machines of this kind, documents which are rectangular in shape may move so that either their long or short edges lead.

The line of travel may be straight or curved.

Alternatively, the method of the invention may be carried out in machines in which the document to be verified is temporarily stationary while the repective magnetising and detection processes are carried out.

If the machine is capable of dealing with a rapid succession of documents, means may be provided for sorting the documents automatically into "genuine" and "non-genuine" groups according to the result of a comparison between the third signal and the predetermined value carried out by the machine.

Preferably, the means for applying each of the unidirectional magneticfields is one or more permanent magnets providing a pair of poles of opposite polarity separated by a gap, and means is provided for conveying the document through said gap.

Preferably, the means for deriving the first and second signals each comprise a pair of magnetic reading-heads separated by a gap, and means if provided for coveying the document through the gap. The magnetic material applied to the surface of a document and/or within the document according to this invention preferably has a coercivity of at least 48,000 ampere/metre, more preferably. at least 80,000 ampere/metre.

When the document is to be verified while stationary, the first and second magnetisations may be applied by two permanent magnets which are moved past the document in succession. For example, the magnets may be supported by means which move linearly or in a curved path so as to approach the document at a point wherein the magnetisable material is located. Such means, together with the permanent magnets, may be duplicated and moved in synchronism one on each side of the document.

The same or additional supporting means may be used to move the two read heads past the document.

The read heads may move at uniform or nonuniform velocity, for example, with a high frequency oscillatory motion superimposed on a relatively lower forward velocity.

Each read head may, of course, be duplicated, one on each side of the document.

Alternatively, the first and second stage magnetisations may be carried out by one or two electromagnets. The first magnetisation may be induced into the magnetisable material by a strong current in one direction. The second, reverse magnetisation may be induced by a current in the opposite direction. After the first magnetisation, the document may be moved forward a short distance and brought to rest over a read head. To obtain the first signal while both the document and the read head are stationery, the read head will, preferably, comprise a flux sensitive device of known kind excited by an external oscillator. Alternatively, the signal may be obtained from other known devices exploiting, for example, the Hall, Wiegand or magneto resistive effects.

The document may, after first magnetisation and detection be moved backforsecond magnetisation and forward again for detection of the second signal, the same device being used for detecting both the first and second signals.

Other security features may of course be present in addition to the high coercivity material. For instance, other magnetic materials may be present in known forms providing predetermined or varying fluxes and having coercivities different to that of the high coercivity material characteristic of the invention. For example, thread of uniform or varying width of magnetisable material may be present in the thickness of the DaDer.

Alternatively, or additionally, non-magnetic security features may be present such as water marks, metals detectable by for instance their conductivity, flourescent materials, strong X-ray absorbers, dichroic materials, holographic materials, phosphorescent materials, security threads or strips having a characteristic shape, eg. sinusoidal, or security threads or strips which are visible at spaced locations on the document. The term "security documents" covers all documents of value and in particular includes bank notes, cheques, warrants, credit cards, cash vouchers, etc.

The invention will be illustrated by the following description of preferred embodiments thereof with reference to the accompanying drawings in which: The Figure shows in schemetic manner an apparatus for verifying a security document such as a bank note.

A path for the conveyance of a document through the apparatus is provided. Means such as pairs of rollers 10 to serve to convey the document along the path.

The document first comes to a powerful permanent magnet or, preferably, pair of magnets 11 and 12. They are arranged to that the document path passes between facing north and south poles. The document will thereby experience a strong unidirectional magnetic field.

The document next comes to a magnetic read head 13 or preferably a pair of read heads 13 and 14.

These are situated each side of the path, facing the path so as to detect any magnetised material present or on in the document. Each read head contains at least one coil in which a moving magnetised material may induce a voltage signal. When a pair of read heads 13 and 14 is used they are electrically connected so that the passage of a magnetised material between them produces a single signal.

It has been found that the use of a pair of heads on each side of the document path enables a signal to be produced that is relatively independent of the exath path followed by the document. If only one head is used then a slight variation in the document path and hence the distance of the document from the head produces a significant change in the signal from the head. In the arrangement shown, if the actual path followed by a particular document deviates from the ideal by approaching head 14, the signal produced by head 14 will be misleading increased but this will be compensated by a corresponding loss in the signal form from head 13 so that the signal obtained from the pairs of heads will be substantially unaffected by slight divergences from the ideal path.

The document next passes a permanent magnet 15 or through the gap between permanent magnets 15 and 16 arranged similarly to magnets 11 and 12 but with reverse polarity. The strength of these magnets may be the same as or different to that of magnets 11 and 12.

The document then passes a second read head 17 or, preferably a second pair of read heads 17 and 18 which may be of the same kind as heads 13 and 14.

These are arranged in the same way with respect to the document path. Indeed, rather than provide a second pair of heads it would be possible to reverse the path of the document after magnet 15 and 16 so that the document passes back past heads 13 and 14.

It would then of course be necessary to take account of the effect on the signal produced by reversing the direction. Also it will generally be preferable to have an uninterrupted passage of documents through the machine giving a higher throughput. Accordingly the provision of a second set of heads is preferred.

The passage of a document 20 through the apparatus will now be considered. By way of example, document 20 has a surface printing of a design such as a number particular to that document in magnetisable pigmented ink. The magnetic pigment has a coercivity of about 120,000 ampere metre (1500 oersteds). Document 20 is one of a series which each have a different number printed in the same magnetisable ink.

The document 20 passes between magnets 11 and 12. These have a field strength of about 0.75T (7500 Gauss) which is sufficient to saturate the magnetisable ink printing. When document 20 passes out of the magnetic field between the magnets, the magnetisable ink printing has acquired a residual magnetism and the flux density of this will be a characteristic of the particular magnetisable material employed.

It is preferably a substantial residual magnetism.

When the document passes between heads 13 and 14, the signal produced will be dependent on the flux density of the ink. The signal produced by heads 13 and 14 is referred to hereafter as the first signal.

The magnetic material on the document is next subjected to the field between magnets 15 and 16.

This opposes the residual magnetism of the magnetic material. The field strength of the magnets 15 and 16 may be chosen to reduce the residual magnetism of the ink, to reduce it to zero, or to reverse it. The field strength needed to achieve a given change in the residual magnetism will depend on the coercivity of the magnetisable material. Preferably, the field strength will be adjusted for example, by adjustment of the gap between magnets 15 and 16.

When the document passes read heads 17 and 18 therefore, the field exerted by the magnetisable material will have been changed by magnets 15 and 16 in a manner which will depend upon the coercivity of the material and the actual reverse field which has been applied. The signal, which may be a null signal, produced by the heads 15 and 16 together is hereinafter termed the second signal.

Means are provided for electronically combining the first and second signals to provide a third signal.

The first and second signals will be time dependent signals except that the second signal may in theory be a null signal.

The amplitude of the signals will be dependent upon the amount of magnetisable material present.

This will change as the document ages as some will be rubbed off in handling. The wave form ortime dependency ofthe signal will depend on the printed pattern and so will be different for every document in the series of which document 20 is a member.

The first and second signals may however be combined in such a way as to eliminate the dependency which they share on the amount and spatial distribution of the magnetisable material. There are many ways in which this may be done.

The first signal may be represented as V(t) = k1f(t) where f(t) is a time dependentfunction expressing the dependence of V(t) on the amount and spatial distribution of the magnetisable material and ka is a constant expressing the dependence of V1 (t) on the inherent nature of the magnetisable material, in particular its remanence.

Similarly, the second signal may be expressed as V2(t) = k2f(t) and k2 will be a constant which is also an expression of the nature of the magnetic material but is related to the coercivity of the material.

It can be seen that a ratio of the two signals can eliminate the function f(t) to produce a third signal k2/k1 which expresses the nature of the magnetic material and is dependent upon both remanence and conductivity.

The actual ratio taken need not be simply of the first and second signals. For instance the ratio may be of the second signal to the sum of the first and second signals.

The signals may be treated in various ways to enable the desired ratio to be taken. For instance the maximum amplitude of each of the first and second signals may be determined and used in the desired ratio. Alternatively, some instantaneous values that correspond between the first and second signals may be used. Again, the signals may each be integrated to provide a value for each to be used in the ratio. Other specific ways for eliminating the dependence of each function on the amount and spatial distribution of the magnetic material will readily occur in those skilled in the art.

Suitable electronic means for carrying out these operations will also be known to those skilled in the art. For instance, conventional integrator circuits may be used to integrate each of the first and second signals and the desired ratio can then easily be taken using conventional methods. The two integrated figures could even be displayed for an operator to ratio himself. Generally, however the ratio will be taken electronically.

The third signal will be compared with a predetermined standard to determine whether the document is to be accepted as genuine. This may be done automatically or by an operator. Generally, it will be done automatically and means will be provided to act upon the document in a manner appropriate to its status as either genuine or non-genuine. For instance, if the apparatus of the invention is embodied in one station of a high speed document sorting installation the genuine documents might be passed on to further stations or machines designed to separate worn notes for destruction from newer notes destined for recirculation whilst non-genuine notes would be automatically diverted to be held for investigation. In a note accepting vending machine genuine notes would trigger the dispensing of goods or otherwise be accepted as payment, whilst nongenuine notes would not.

Many other methods for combining the first and second signals may be used in accordance with the invention. The signals may be processed as time dependent, analogue signals or, alternatively may be converted into digital form for further processing.

Computer or micro-processor techniques may with advantage, be used.

For example, the first and second signals described herein above may be integrated and added together algebraically to provide a sum signal. The first integrated signal may then be successfully attenuated by known amounts. The sum signal and each successive attenuated signal may then be compared in a series of comparator circuits such that each circuit provides a high or low output. The pattern of high and low outputs from the comparators may be interpreted as a binary code. This pattern or code may then be compared with its own external standard or compared with the code produced by another magnetic feature within the same document.

The verifying apparatus of this invention may be used to check the character of more than one magnetic feature on or in a single document. The magnetisable materials in the features may have the same or different coercivities. If they are different, then the third signal for each will be different. The two third signals may be each compared with its own external standard or else they may be compared with each other so that they each act as a standard forthe other. They need not of course be equal.

To imitate a given security document according to this invention to a degree adequate to satisfy the verification apparatus described above, it would be necessary to either obtain supplies of the specific magnetisable material used in the genuine document or else to produce a material closely resembling it not only in coercivity but also in remanence.

The high coercivity materials employed in the document according to this invention are very difficult to obtain. Accordingly, the security of the documents and the reliability of the method of verification provided by this invention as specifically described above may therefore be expected to be very high.

Afurther advantage of the apparatus described is that it can be set to recognise different magnetic materials without changing the permanent magnets.

A system which depends on the second pair of magnets precisely cancelling the remanentfield produced by the first pair such as has been used previously, cannot be set up for a different magnetisable material without changing the second pair of magnets.

Using the system described herein it would be possible for a single document verifying machine to be set automatically to recognise different types of documents bearing different magnetic materials provided that each document also has a machine readable code in some form to tell the verifying machine the nature of the document and hence the type of magnetic material to accept as genuine for that document.

Claims (22)

1. A method of verifying a security document having thereon and/ortherein a magnetisable material which method comprises temporarily applying a first unidirectional magnetic field to the magnetisable material thereby to magnetise the magnetic material and then obtaining a first signal related to the strength of the remanent magnetic field produced by the material, applying to the magnetisable material a second temporary unidirectional field of polarity opposite to the first and then obtaining a second signal related to the strength of the magnetic field then produced by the magnetisable material, combining the said first and second signals to obtain a signal characteristic of the magnetisable material and substantially independent of the amount and distribution of the magnetisable material present in or on the document, and comparing that signal with a predetermined standard.

2. A method as claimed in claim 1 wherein the first magnetic field is sufficiently strong to saturate the magnetisable material.

3. A method as claimed in claim 1 or claim 2 wherein the first and second signals are time dependent signals.

4. A method as claimed in claim 3 wherein the signal characteristic of the material is obtained by taking a ratio involving the first and second signals such as to eliminate the common dependency of each signal on the amount and distribution of magnetic material.

5. A method as claimed in claim 4 wherein the ratio taken is the ratio of the peak value of the first signal to the peak value of the second.

6. A method as claimed in claim 4 wherein the ratio taken is of an instantaneous value of the first signal and of a corresponding instantaneous value of the second signal.

7. A method as claimed in claim 4 wherein the ratio taken is of a value obtained by integrating the first signal and a value obtained by integrating the second signal.

8. A method as claimed in claim 4 wherein the ratio taken is of the peak value of the first signal and the sum of the peak values of the first and second signals.

9. A method as claimed in claim 4 wherein the ratio taken is of an instantaneous value of the first signal and the sum of that value and a corresponding instantaneous value of the second signal.

10. A method as claimed in claim 4 wherein the ratio taken is of a value obtained by integration of the first signal and the sum of that value and a value obtained by integration of the second signal.

11. A method as claimed in any one of the preceding claims wherein the first and second signals are each obtained by passing the security document between a pair of facing magnetic reading heads.

12. A method of verifying a security document substantially as hereinbefore described with reference to the accompanying drawing.

13. Adocumentverifying machine comprising means for temporarily applying a first unidirectional magnetic field to a security document comprising a magnetisable material means for thereafter obtaining a first signal related to the strength of the remanent magnetic field produced by said material, means fortemporarily applying a second unidirectional magnetic field of opposite polarity to the first, means for thereafter obtaining a second signal related to the strength of the remanent magnetic field then produced by said material, means for combining said first and second signals to produce a third signal characteristic of the magnetisable material and substantially independent of the amount and distribution of the magnetisable material present in or on the document.

14. A machine as claimed in claim 13 further including means for comparing the third signal with a predetermined standard.

15. A machine as claimed in claim 13 or claim 14 wherein the means for applying each of the unidirectional magnetic fields is one or more permanent magnets providing a pair of poles of opposite polarity separated by a gap, and means is provided for conveying the document through said gap.

16. A machine as claimed in any one of claims 13 to 15 wherein the means for deriving the first and second signals each comprise a pair of magnetic reading-heads separated by a gap, and means if provided for conveying the document through the gap.

17. Apparatus for verifying a security document substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

18. A security document comprising as a security feature a magnetisable material applied to the surface thereof, which magnetic material has a coercivity of at least 48,000 ampere/metre (600 oersteds).

19. A security document as claimed in claim 18 wherein the coercivity of the magnetic material is at least 80,000 ampere/metre (1000 oersteds).

20. A security document as claimed in claim 18 or claim 19 but comprising one or more additional magnetic security features.

21. A security document as claimed in claim 20 wherein the additional security features include magnetisable material of a coercivity different from that of the first magnetisable material.

22. A security document as claimed in claim 20 wherein the additional security features include magnetisable material of a coercivity equal to that of the first magnetisable material.