Magnetic detection of security articles

A method for detecting the presence of magnetic elongated particles in a substrate the base material of which has magnetic properties substantially differing from the corresponding magnetic properties of the elongated particles. The elongated particles have such a long and thin form that their demagnetisation factor N is smaller than 1/250, they have a diameter smaller than 30 micrometer and a magnetic saturation field greater than 100 A/m. The method comprising the following steps: PA1 (a) emitting an electromagnetic source signal of one or more particular base frequencies to the substrate so that any present magnetic elongated particles go into a non-linear part of their B-H curve for at least part of a cycle of the source signal; PA1 (b) detecting an electromagnetic detection signal emanating from the substrate; PA1 (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 or of the harmonics, the particular higher harmonics being indicative of the presence of the magnetic elongated particles.

FIELD OF THE INVENTION 
The present invention relates to a method and an apparatus for detecting 
the presence of particles in a substrate the base material of which has 
electromagnetic properties substantially differing from the corresponding 
electromagnetic properties of the particles. 
The invention also relates to the particles and to the substrate comprising 
such particles which allow to recognise easily a document as being a 
genuine security document in order to prevent the document from being 
copied or in order to contribute to its authentication. 
The invention is intended primarily to be used in the identification or 
authentication of all type of security documents having a paper or 
synthetic base material such as bank notes, cheques, passports, 
credit-cards, tickets, lottery-tickets and bonds which comprise the above 
particles, but it is also applicable to other applications in which 
objects need to be recognised. 
BACKGROUND OF THE INVENTION 
The prior art has already dealt in an extensive way with the identification 
of security documents. 
Some prior art solutions go in the direction of recognition of possible 
characteristic patterns printed at the surface of some security documents. 
In order to prevent genuine security documents from being falsely copied by 
means of high-resolution colour photocopying apparatus, the prior art has 
also proposed to add to the fibrous structure of the base material of the 
substrate or to the surface of the document one or more security elements 
allowing the identification and/or making difficult the manufacturing of 
the document. 
U.S. Pat. No. 4,114,032 (priority date 1973) and U.S. Pat. No. 4,218,674 
(filing date 1975) disclose a similar system where the security documents 
have fibres which are coated with a magnetic or magnetizable material 
embedded therein. The mere presence of the magnetic fibres inside the 
security documents is tested or, as an improved feature, the distribution 
of the magnetic fibres in the security document is measured so that every 
single security document can be given a unique mark. 
Up to 500 million different possible combinations may be obtained. EP-A-0 
625 766, EP-A-0 632 398 and EP-A-0 656 607 (all filing date in 1993) 
disclose a system where the fibres consist of magnetic powder as core of a 
polymer sheath. Magnetic detection is done by DC current used to excite a 
coil. Because of magnetic prehistory or disturbing of magnetic fields or 
deformations of the security documents, however, the repetitivity of such 
a magnetic scanning system is not ensured and accurate discrimination 
between genuine security documents and counterfeit documents is not always 
guaranteed. So detection is not always distinctive. 
Moreover, if characters on the security document have been printed by means 
of a magnetic ink which is detectable by means of a sorting apparatus, 
there may be interference between the magnetic fibres and the magnetic ink 
of the characters. 
Other embodiments disclosed in the prior art are based on the detection of 
particular electromagnetic properties of the security elements. FR 2 425 
937 discloses a method of dispersing metallic fibres, more particularly 
stainless steel fibres, inside the fibrous structure of paper in order to 
allow the identification by means of microwaves. 
U.S. Pat. No. 4,820,912 (priority date 1985) discloses an alternative 
system where the security documents comprise randomly distributed 
electrically conductive fibres. By scanning the documents by means of 
microwaves the unique distribution of the fibres inside the security 
document can be obtained. Up to 64.sup.320 different possible combinations 
of the mark characterising this distribution can be obtained. Application 
of this microwave technique to reproduction apparatus such as photocopying 
apparatus in order to prevent security documents from being copied, such 
as disclosed in WO-A- 95/24000 (priority date 1994) fails to distinguish 
security documents from printed circuit boards (PCB's) or from greeting 
cards having decorative metal foils on its surface. On the other hand, the 
system does not discover the presence of the fibres if a metal plate is 
put above a genuine security document. Particular cover lids of 
photocopying apparatus or metallic parts in the neighbourhood of the 
photocopying apparatus, may disturb the system. As a consequence, these 
systems are not completely reliable. 
The prior art has also provided a number of optical authentication systems. 
Some of them have been disclosed already in U.S. Pat. No. 3,313,941 
(filing date 1963) and in U.S. Pat. No. 3,449,585 (filing date 1966). All 
optical systems, however, suffer from the major drawback that wear or 
damage or dirt on the surface of genuine security documents can cause the 
security documents as being no longer recognised as authentic. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid the drawbacks of the 
prior art. 
It is another object of the present invention to provide for a robust 
recognition system that allows to distinguish genuine security documents 
from other objects or documents. 
It is also an object of the present invention to provide for a system which 
prevents genuine security documents from being copied. 
It is still an object of the present invention to provide for a system that 
does not interfere with conventional magnetic character readers. 
It is a further object of the present invention to provide for a substrate 
such as a security document, more particularly a banknote, including 
security elements, easily detectable in an anti-photocopy system. 
According to a first aspect of the present invention, there is provided a 
method for detecting the presence of magnetic elongated particles in a 
substrate the base material of which has magnetic properties substantially 
differing from the corresponding magnetic properties of the elongated 
particles. Preferably the base material is made of a non-magnetic 
material. The elongated particles have such a long and thin form that 
their demagnetisation factor N is smaller than 1/250, preferably smaller 
than 1/1000. Their cross-sectional diameter is smaller than 30 micrometer 
and their magnetic saturation field H.sub.s is greater than 100 A/m, 
preferably greater than 200 A/m and most preferably greater than 300 A/m. 
The magnetic saturation field H.sub.s is preferably smaller than 1000 A/m. 
The terms "magnetic saturation field H.sub.s " are herein defined as the 
magnetic field at the onset of the saturation flux density B.sub.s. The 
terms "cross-sectional diameter" herein refer to the maximum 
cross-sectional dimension. 
The method comprises the following steps: 
(a) emitting an electromagnetic source signal of one or more particular 
base frequencies to the substrate so that any present magnetic elongated 
particles go into a non-linear 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 
substrate; 
(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 
particles. 
Using the non-linearity of the magnetisation properties of the labelling 
material, i.e. the change in magnetic flux density B with applied magnetic 
field H as an effective parameter for detection is a technique which is 
known as such in electronic article surveillance (EAS) or anti-theft 
systems. The signals which can be obtained from this approach are very 
distinctive and the electronics and signal processing can be straight 
forward. EAS systems have been disclosed in an extensive way in the patent 
literature. Some examples are FR 763 681 (filing date 1933), U.S. Pat. No. 
3,631,442 (filing date 1967), U.S. Pat. No. 3,990,065 (filing date 1975) 
and EP-A-0 153 286 (priority date 1984). 
A number of substantial differences between EAS systems and the present 
invention are, however, apparent. 
In EAS systems anti-theft labels are used to trigger alarms at the exit 
areas of shops if the products have not been offered at the pay-desk. The 
exit area of a shop is much larger than the volume required for detection 
of magnetic elongated particles in security documents. A typical exit gate 
has a width of about 1 m, while in the present invention distances of only 
a few cm, e.g. of only 0.5 to 5 cm, between the magnetic field and the 
magnetic elongated particles are sufficient to perform the detection. This 
basic difference leads to a number of properties which are different for 
application of the present invention: 
1) The magnetic material of EAS labels is rather bulky, since it must be 
present in a sufficient volume to trigger off the alarm in the relatively 
large exit area; a typical cross-sectional dimension is about 1 mm and the 
length can be several cm long. In contrast herewith, the magnetic 
elongated particles according to the present invention have a much smaller 
volume. Their demagnetisation factor N is smaller than 1/250, preferably 
smaller than 1/1000 and their cross-sectional diameter is smaller than 30 
micrometer, preferably smaller than 15 micrometer and most preferably 
ranging from 1 to 10 micrometer. The maximum value for the demagnetisation 
factor N is chosen so that the magnetic elongated particles can be 
detected by means of an apparatus with acceptable coil dimensions and 
power dissipation so that they can be installed on e.g. a photocopier or a 
bank note counting machine. 
2) The magnetic material of EAS labels can be classified as very soft 
magnetic material, i.e. material having a very small coercive force 
H.sub.c and a relatively high dynamic permeability .mu..sub.d, since small 
magnetic fields H covering the exit area of a shop must be able to 
saturate the EAS labels. In contrast herewith, the magnetic elongated 
particles according to the present invention, although still being 
classified as soft magnetic materials, have such a shape and/or 
composition and/or structure that they are effectively magnetically hard 
enough to stay below the saturation point of their B-H loop in the fields 
used in the shop systems so that they do not generate high enough signals 
to activate the shop alarms. In comparison with EAS labels, the magnetic 
elongated particles according to the present invention have preferably a 
lower magnetic dynamic permeability .mu..sub.d and therefore require a 
substantially higher magnetic field to reach saturation. The magnetic 
saturation field H.sub.s of magnetic elongated particles according to the 
present invention is greater than 100 A/m, preferably greater than 200 A/m 
and most preferably greater than 300 A/m. This lower value is chosen not 
to trigger off EAS alarms. Preferably the magnetic saturation field 
H.sub.s is smaller than 1000 A/m so that it can be achieved by means of a 
detector apparatus with acceptable coil dimensions and power dissipation 
that it can be built in a photocopier or a bank note counting machine or 
an automatic vending machine. The inventors have experienced so far that 
anything over 1000 A/m will be difficult to achieve with an air cored 
coil. It is possible, however, to obtain a magnetic saturation field 
greater than 1000 A/m when making use of a ferrite cored coil or when 
making use of ferromagnetic powders in the core. The magnetic saturation 
flux density is preferably greater than 0.1 Tesla and typically ranges 
from 0.1 Tesla to 1.0 Tesla, and the dynamic permeability .mu..sub.d (for 
definition see below) ranges from 10 to 10000, e.g. from 100 to 10000. 
Within the context of the present invention, all these magnetic properties 
have been determined by use of an alternating current magnetometer at 
frequencies from 10 kHz to 100 kHz. 
3) Due to the bulky material of EAS labels, the frequencies applied are 
limited in order to reduce eddy current losses. In contradistinction 
herewith, much higher frequencies (higher than 1 kHz) can be applied in 
the present invention, since the magnetic elongated particles have a much 
lesser volume. The corresponding harmonics also have a much higher 
frequency (higher than 10 kHz) and typical harmonics have an order of ten 
or higher. 
4) In EAS systems the problem of covering the large volume of the exit area 
of a shop and the problem of orientation-sensitive EAS labels has led to a 
number of embodiments where two or more base frequencies are used or to 
the use of an additional rotating magnetic field in order to create a 
global spatial magnetic field which is insensitive to the orientation of 
the EAS. Due to the much more limited sizes of the volumes required for 
the present detection method, such complications are not necessary for the 
present invention. A source signal of a single base frequency has proved 
to be sufficient. 
In an embodiment of the present invention, the method comprises the 
additional step of: (d) generating a signal which prevents from taking a 
true copy in case said particular harmonics are present. 
According to a second aspect of the present invention, there is provided a 
detection apparatus for detecting the presence of magnetic elongated 
particles in a substrate the base material of which has magnetic 
properties substantially differing from the corresponding magnetic 
properties of the elongated particles. Preferably the base material is 
made of a non-magnetic material. The elongated particles have such a long 
and thin form that their demagnetisation factor N is smaller than 1/250. 
Their cross-sectional diameter is smaller than 30 micrometer and their 
magnetic saturation field H.sub.s is greater than 100 A/m (preferably 
greater than 200 A/m and most preferably greater than 300 A/m). 
The apparatus comprises: 
(a) an oscillator for emitting an source signal of one or more base 
frequencies to the substrate; 
(b) a detector for detecting a detection signal emanating from the 
substrate; 
(c) a signal processor for examining the detection signal for the presence 
of any particular higher harmonics of the base frequencies or of any 
linear combination of the base frequencies, where the particular higher 
harmonics are indicative of the presence of the magnetic elongated 
particles. 
According to a particular embodiment of the apparatus, both the source 
signal and detection signal are electrical signals and the apparatus 
further comprises a drive coil for converting the source signal into a 
magnetic drive field, and a detection coil for converting a detection 
magnetic field into the detection signal. The coils are so arranged to 
null out the magnetic drive field in the detection coil in order to avoid 
saturating the amplifier and to minimise any cross-coupling which can 
occur with conducting materials. 
According to a preferable embodiment of the apparatus the drive coil is 
arranged around a ferrite core. 
The ferrite core has a U-shape and a drive coil is arranged around each leg 
of the U-shaped ferrite core. A detection coil is also arranged around 
each leg of the U-shaped ferrite core. Each detection coil is preferably 
divided into two parts, with one part at both sides of the drive coil. 
These two parts of a detection coil are wired in anti-phase in order to 
null out the drive signal. 
Next to a drive coil and a detection coil, a third coil may be present 
around the ferrite core in order to detect the presence of any ferrous 
metals. 
The apparatus according to the second aspect of the present invention can 
be used in automatic vending machines, bank note counting machines and 
reproduction apparatus. 
With respect to the use in reproduction apparatus, in order to detect the 
presence of any security documents in the whole scanning region, the 
following embodiments can be used: 
1) the use of more than one drive coil and more than one detection coil; 
2) drive coils and detection coils forming a daisy chain; 
3) the use of only one drive coil and one detection coil, both having an 
elongated form; 
4) the use of one drive coil and more than one detection coil. 
According to the third aspect of the present invention, there is provided 
an elongated magnetic particle for being incorporated in a base material 
of a substrate where the base material has magnetic properties differing 
substantially from the corresponding magnetic properties of the particle. 
Preferably the base material is made of a non-magnetic material. The 
particle has such a long and thin form that its demagnetisation factor N 
is smaller than 1/250, preferably smaller than 1/1000. The diameter of the 
particle (i.e. its maximum cross-sectional dimension) is smaller than 30 
micrometer, preferably smaller than 15 micrometer, preferably ranging from 
1 to 10 micrometer and its magnetic saturation field H.sub.s is greater 
than 100 A/m, preferably greater than 200 A/m, and most preferably greater 
than 300 A/m. 
The magnetic field strength inside the material is given by 
EQU H.sub.in =H.sub.app -N.times.M 
where M is the magnetisation of the material, H.sub.app is the applied 
magnetic field and N is the demagnetisation factor. 
With uniform magnetisation this reduction in the internal field strength 
can be considered as a reduction in the apparent permeability from its 
true value of .mu.r, which is the so-called bulk magnetic permeability or 
magnetic permeability of the material, to the magnetic apparent or 
effective permeability .mu..sub.r ', where 
EQU 1/.mu..sub.r =1/.mu..sub.r '-N, 
or 
EQU .mu..sub.r '=.mu..sub.r /(1+N.mu..sub.r) 
The effect of the reduction in permeability therefore causes the B-H loop 
to shear into a shape which has a higher saturation field and lower 
remanence. In case of a sphere, the demagnetisation factor N=1/3. Whereas 
for long, thin ellipsoids (approximating to cylinders represented by the 
elongated particles such as fibres) N is given by: 
EQU N=[ln (2p)-1]/p.sup.2 
where p is the length to diameter ratio. 
For a fibre of 8 micrometer diameter and 3 mm length, N is equal to 
1/25000. 
Based on these equations if, as an example, we take a material with a bulk 
permeability .mu..sub.r of 100000 then a sphere of identical material 
would appear to have an magnetic apparent permeability .mu..sub.r ' 
approximately 7000 times smaller than a fibre with the dimensions shown 
above. This will then have a direct effect on the magnitude of the field 
required to saturate the material in each case. Thus spheres, or powders 
of approximately spherical form would not be suitable for the application 
described herein. 
Preferably, the magnetic saturation flux density B.sub.s of the magnetic 
elongated particle is greater than 0.1 Tesla and typically ranges from 0.1 
Tesla to 1.0 Tesla, for example from 0.1 Tesla to 0.5 Tesla. 
The apparent or effective magnetic permeability .mu..sub.r ' is measured at 
d.c. The magnetic dynamic permeability .mu..sub.d. parameter is an 
indicator of the sensitivity of the particle in practical situations 
taking account of bulk permeabilities, shape factors, the a.c. frequency 
of the drive fields and the field limits which are typical in EAS gates 
and which would be practical in our proposed new invention system. The 
magnetic dynamic permeability .mu..sub.d is therefore herein defined as 
the ratio of the saturation flux density B.sub.s to the magnetic 
saturation field H.sub.s multiplied by .mu..sub.o., measured at an a.c. 
frequency. If the materials do not saturate at the fields used in the 
magnetometer then the magnetic dynamic permeability .mu..sub.d is defined 
as the ratio of the flux density B to .mu..sub.o H at the maximum field 
used in the experiment (e.g. about 1000 A/m). The magnetic dynamic 
permeability .mu..sub.d is clearly related to the apparent magnetic 
permeability .mu..sub.r ' and both parameters would have the same or close 
to the same value at d.c. in a low loss material in which sheer due to 
demagnetisation dominates the shape of the measured B-H-loop. The magnetic 
dynamic permeability .mu..sub.d of the magnetic elongated particle 
preferably ranges from 10 to 10000, e.g. from 100 to 10000. 
The terms "magnetic elongated particle" refer to an elongated particle 
itself made of a magnetic material and possibly of a magnetic material and 
a non-magnetic material. In particular the magnetic material can be coated 
or encapsulated with a non-magnetic material or the elongated particle can 
be made of a non-magnetic material being coated with a magnetic material 
or incorporating a magnetic material. The thickness of the coating may 
range from 1 to 5 micrometer. 
The magnetic material can be made starting from an alloy comprising 
components chosen among Fe, Cr, Co, Cu, Ni, Mo, Mn, Nb, B, V, C, Si and P, 
more particularly among Ni, Fe, Mo, Mn, Cu. Soft magnetic materials have 
been disclosed for example in EP-A-0 295 028 and in U.S. Pat. No. 
4,298,862. 
A suitable alloy composition responds to the general formula: 
EQU Ni.sub.a Fe.sub.b Cr.sub.c Co.sub.d Cu.sub.e Mo.sub.f Mn.sub.g P.sub.h 
Nb.sub.i B.sub.j V.sub.k Si.sub.l Cm, 
where a to m represent integers. 
More particular alloy compositions have 52 to 85% of nickel (Ni) and 
varying amounts of other components. 
An example of a good working alloy composition is: 80.00% Ni, 4.20% Mo, 
0.50% Mn, 0.35% Si; 0.02% C, the balance being Fe. 
Other typical compositions are: 
Ni.sub.82 Fe.sub.14 Mo.sub.3 Mn.sub.1 
Ni.sub.79 Fe.sub.16 Mo.sub.4 Mn.sub.1 
Ni.sub.70 Fe.sub.11 Cu.sub.12 Mo.sub.2 Mn.sub.5 
Ni.sub.71 Fe.sub.11 Cu.sub.13 Mo.sub.2 Mn.sub.3 
Ni.sub.71 Fe.sub.11 Cu.sub.12 Mo.sub.2 Mn.sub.4. 
Some of these compositions are commercialised under names as .mu.-metal, 
Permafi, Permalloy, Supermalloy, Vitrovac and Metglas. 
As non-magnetic and non-metallic material glass, carbon or synthetic 
material such as polymers especially polypropylene and polyethylene can be 
mentioned. 
According to a preferable embodiment of this third aspect of the present 
invention, the elongated particle is a fibre which can be a metallic fibre 
or a non-metallic fibre coated with a magnetic substance. 
The fibres can be uniformly dispersed and distributed all over the 
substrate and therefore not easily missed by the detection system. The 
fibres can be preferably uniformly and individually dispersed all over the 
substrate in order to prevent the formation of agglomerates of fibres. 
Additionally, since the fibres are dispersed inside the substrate, they 
are not easily removable by the counterfeiters who would like to remove 
them before making a photocopy and reinstate them inside the substrate 
after photocopy thereof. 
The fibres are preferably hard drawn or work hardened metal fibres, e.g. 
manufactured according to the technique of bundled-drawing which is well 
known as such. This manufacturing technique has the advantage of yielding 
a much higher production rate than hot melt production techniques. Hard 
drawing makes the magnetic fibres also `harder` from a magnetic point of 
view, i.e. less soft-magnetic so that a higher magnetic saturation field 
H.sub.s is required. This is particularly useful in the present invention 
since it helps to distinguish from EAS tags and prevents from setting the 
alarms in EAS gates. The inventors have also found that the magnetic 
dynamic permeability .mu..sub.d of the hard drawn fibres can be doubled by 
annealing. This still keeps the saturation field H.sub.s sufficiently 
high, but makes the fibres more sensitive. 
The magnetic elongated particles may also be amorphous metal fibres. 
According to a fourth aspect of the present invention, there is provided a 
substrate comprising a base material and elongated particles inside the 
base material. The magnetic properties of the elongated particles differ 
from the corresponding magnetic properties of the base material. 
Preferably the base material is made of a non-magnetic material. The 
elongated particles have such a long and thin form that their 
demagnetisation factor N is smaller than 1/250. Their diameter is smaller 
than 30 micrometer and their magnetic saturation field ranges from 100 to 
1000 A/m, preferably from 200 to 1000 A/m and most preferably from 300 to 
1000 A/m. Preferably the base material is a non-magnetic material such as 
plastic or a fibrous structure like paper. 
Preferably the elongated particles have a magnetic saturation flux density 
being greater than 0.1, and typically ranging between 0.1 Tesla and 1.0 
Tesla, for example between 0.1 Tesla to 0.5 Tesla and a magnetic dynamic 
permeability .mu..sub.d ranging from 10 to 10000, for example between 100 
and 10000. 
Summarizing, the combination of shape, composition and structure of the 
magnetic elongated particles is such that 
the magnetic field required to achieve saturation of the flux density in 
the particle is sufficiently greater than that produced in EAS systems and 
sufficiently lower than that required to saturate hard ferromagnetic 
material such as iron, steel or plate, and 
the magnetic remanent flux density is sufficiently lower than those of 
magnetic ink used in the magnetic coding system as defined in the 
international standard for magnetic ink character recognition ISO 1004. 
These properties are fulfilled when the combination of shape, composition 
or structure of the magnetic elongated particles is such that the 
elongated particles have: 
i) a saturation field ranging from 100 to 1000 A/m 
ii) a saturation flux density being greater than 0.1 Tesla, and typically 
ranging between 0.1 Tesla and 1.0 Tesla; 
iii) a magnetic dynamic permeability Pd ranging from 10 to 10000, for 
example ranging from 100 to 10000. 
The magnetic elongated particles, especially the fibres, have a mean 
cross-sectional diameter ranging from 1 to 30 micrometer (.mu.m), 
preferably from 5 to 15 micrometer, and a length ranging from 1 to 20 mm, 
preferably ranging from 2 to 10 mm. 
Most preferably the elongated particles are hard drawn or work hardened 
metal fibres, but can also be amorphous metal fibres. 
The magnetic elongated particles can be made of an alloy comprising 
components chosen among Ni, Fe, Cr, Co, Cu, Mo, Mn, P, Nb, B, V, C, Si, 
and more particularly Fe, Ni, Mo, Mn, Si and C. 
The base material of the substrate can be made of paper or of a synthetic 
material, especially a plastic such as polypropylene or polyethylene. 
The magnetic elongated particles can also be made of magnetic and 
non-magnetic material. 
Preferably the substrate according to the fourth aspect of the present 
invention, has a quantity of magnetic elongated particles, especially 
fibres, ranging from 0.1 to 5 per cent, preferably from 0.2 to 2 per cent, 
most preferably from 0.5 to 1.5 per cent by weight relative to the weight 
of the substrate. If the substrate is a paper sheet, its thickness 
commonly varies from 20 to 300 micrometer. Bank notes usually have a 
thickness varying between 80 and 120 micrometer. 
The elongated particles may be uniformly or randomly dispersed in the whole 
substrate and/or may be present only in selected parts of the substrate. 
The fibres may be distributed within a substrate in selected parts thereof 
according to methods known in the art and especially in methods disclosed 
in WO 96/14469 (PCT/FR95/01405). Preferably the fibres are included only 
in parts of bank notes corresponding to printed areas so that the fibres 
are less visible. More particularly, the fibres are included outside any 
watermark area. Most preferably, the fibres are outside the areas which 
are printed with magnetic ink so as to avoid any possible electromagnetic 
interference. 
In one preferred embodiment the fibres are present within the substrate in 
areas having the form of tapes of a width of at least 20 mm. 
Preferably, the elongated particles, especially the fibres have a colour 
near to the colour of the base material. This can be realised by the 
deposit of a covering or a coating providing to the fibres the wished 
colour. Methods of depositing such a coating have been disclosed in French 
patent application FR 95 02866 and in international application PCT/FR/96 
00390.

DETAILED DESCRIPTION OF AN EMBODIMENT 
Reference number 10 in FIG. 1 refers to a B-H curve of an EAS label which 
can be designated as "very soft magnetic". It is characterised by a very 
low saturation field H.sub.s and a rather high level of magnetic dynamic 
permeability. Reference number 12 refers to a B-H curve of a magnetic 
elongated particle which is to be embedded in a substrate according to the 
present invention. Although being also a soft magnetic material, it is not 
that "very soft" as is an EAS label. The saturation field H'.sub.s is 
higher than the corresponding values of an EAS label. Reference number 14 
refers to the B-H curve of a mild steel plate clearly showing a saturation 
field that is much larger than H.sub.s and H'.sub.s. 
It will be clear from FIG. 1 that the low magnetic fields applied in EAS 
systems to saturate the EAS labels do not saturate the magnetic elongated 
particles according to the present invention and do not trigger the alarm 
systems in shops. It will also be clear from FIG. 1 that magnetic fields 
applied in the present invention to saturate the magnetic elongated 
particles are still in the relatively linear part of a B-H curve of a mild 
steel plate and will not create the same series of higher harmonics. This 
difference can be used in order to discriminate between the two types of 
materials and even to detect the marker tag in the presence of large 
ferromagnetic objects. 
The following table shows an experimental comparison of actual markers and 
examples of common magnetic objects measured in a magnetometer at between 
20 Hz to 20 kHz. 
TABLE 
______________________________________ 
saturation dynamic 
flux saturation permeability 
Type of material + geometric density .sub.B.sub.s field H.sub.s 
.mu..sub.d 
dimensions (Tesla) (A/m) [B.sub.s /(.mu..sub.o H.sub.s)] 
______________________________________ 
EAS label 0.35 30 10000 
(200 Hz) 
32 mm .times. 0.8 mm .times. 25 .mu.m 80 
(11 kHz) 
paper clip &gt;1000 60 
mild steel plate &gt;&gt;1000 30 
12 mm .times. 9 mm .times. 1 mm 
hard drawn magnetic fibre 0.55 500-600 730 
3 mm length .times. 8 .mu.m diameter (11 kHz) 
______________________________________ 
.mu..sub.o = 40 .times. 10.sup.-7 Vs/Am 
The EAS label in the table has a volume and mass which is about 3000 times 
that of the metal fibre of the table. 
The above figures represent the relative differences between the materials. 
It should be appreciated, however, that in practical cases for EAS tags 
and the invention system, the actual magnetisation of the scanning or 
interrogating field needs to be taken into account at the orientation of 
the material in the field, the bulk of material present and the 
frequencies used. 
The magnetic metal fibre had an a.c. remanence of 0.3 Tesla in the 
measurement. In practice the d.c. remanence would be lower than this so 
that no significant electromagnetic noise signals are generated which 
interfere with other magnetic code systems. In particular, the fibres did 
not interfere with a standard magnetic character reader reading characters 
made for magnetic inks. In other words, using the measurement method and 
definition of maximum residual signal levels as defined in the 
International Standard for magnetic ink character recognition, ISO 1004, 
the effect of remanent flux density is acceptable. 
Experimental results show that it is possible to detect a good signal 
amplitude at high harmonics from the fibre mentioned in the above table 
and that at high frequencies there is very low interference from harmonics 
from the drive electronics. With the small cross sectional area of the 
fibres the eddy current losses are small up to quite high frequencies and 
the output signals are increased by the fact that the detected voltage is 
proportional to the rate of change of flux density. With bulk 
ferromagnetic materials the eddy current losses are much higher at high 
frequencies and so they don't generate very high harmonics. Using a base 
frequency to sweep the fibres (which are characterised in the table above) 
around their B-H loop at 20 kHz, and a peak field greater than 600 A/m it 
was found that at frequencies between 100 kHz and 1 MHz there was a stream 
of harmonics from the fibres and very much smaller signals from other 
common electrically conducting objects. In practice the base frequency and 
the detection frequency or frequencies can be selected to maximise the 
signal from the particular fibre marker and minimise signals from other 
common objects and signals generated from the apparatus in which the 
system is installed. 
Tests carried out by the inventors have indicated that the invention system 
provides a good discrimination between a security document with magnetic 
elongated particles and paper, books, hands, printed circuit boards, 
metallic foil greeting cards, non metallic bindings of documents, spiral 
metal bindings of documents, paper clips, metal plates and photocopier lid 
materials. A security document lying under a non-magnetic metal plate 
could be easily identified (this in great contrast with a microwave system 
where the metal plate conceals the magnetic fibres for the microwaves) 
A suitable drive and detection circuit is shown in FIG. 2. A resonant drive 
power oscillator 16 is used to minimise harmonic generation and the 
oscillator 16 is driven by a frequency which is divided down from the 
selected harmonic. As an example, the inventors have found that, amongst 
other harmonics, the 19th harmonic of 20 kHz, at 380 kHz, or the 21st 
harmonic or a higher harmonic may be a good choice as it gives good 
signals from fibres with very small signals from common ferromagnetic 
materials such as mild steel. The oscillator 16 generates an electrical 
source signal which is fed to a drive coil 18 which transforms the 
electrical source signal into a magnetic drive signal. A detection coil 20 
suitably arranged with respect to the drive coil 18, detects any field 
emanating from magnetic elongated particles and transforms this into an 
electrical detection signal. A high pass filter 22 is used to reduce the 
fundamental frequency, as this can be coupled between the coils by 
conducting metals and overload the amplifiers. A phase sensitive detector 
24 is used to provide good signal-to-noise ratio. Oscillator 26 operates 
at the frequency of the selected harmonic, and frequency divider 28 
divides the frequency in order to obtain the base frequency. Other high 
harmonics are also suitable and it is an advantage to combine several to 
derive the final detection signal. 
FIG. 3 illustrates how the drive coil 18 may be advantageously arranged 
with respect to the detection coil 20. The direction of the magnetic field 
generated by the drive coil is shown in hatched lines, except for that 
part of the magnetic field that goes through the detection coil 20, which 
is shown by means of arrows 30 and 32. Drive coil 18 and detection coil 20 
partially overlap and are so arranged that the part of the magnetic flux 
density which goes in one direction (arrow 30) through detection coil 20 
is almost equal to the part of the flux density which goes in the other 
direction (arrow 32) in order to null-out the drive field in the detection 
coil whilst providing a region above the overlapping coils in which the 
magnetic field is effective in coupling into the magnetic elongated 
particles. An equivalent nulling effect could also be provided 
electronically by a negative feedback of the fundamental frequency. FIG. 
4, FIG. 5, FIG. 6 and FIG. 7 all show embodiments of arrangement of drive 
coil and detection coil to be used in reproduction apparatus such as 
high-resolution colour photocopying apparatus. The arrangement is such 
that a bank note with a width of only 7 cm can be detected on a scanning 
area of 21 cm.times.29.7 cm (if it contains magnetic elongated particles). 
According to FIG. 4, four pairs of a drive coil 18 with a detection coil 20 
are arranged on a suitable carrier 34 at regular distances along the width 
of the scanning area so that the presence of any genuine bank note will be 
detected irrespective of its position on the scanning area. 
In the embodiment of FIG. 5 a plurality of drive coils 18 and a plurality 
of detection coils 20 form a daisy chain wherein a drive coil 18 is 
alternated with a detection coil 20 and vice versa. 
In the embodiment of FIG. 6 the drive coil 18 takes the form of an 
elongated eight with the height of the eight equal to the width of the 
scanning area. The detection coil takes the form of an elongated ellipse 
with the length of the longitudinal axis equal to the width of the 
scanning area. Drive coil 18 and detection coil 20 are arranged one above 
the other so that here also the part of the magnetic flux density which 
goes in one direction through detection coil 20 is almost equal to the 
part of the flux density which goes in the other direction in order to 
null-out the drive field in the detection coil. FIG. 6 shows for 
didactical reasons a drive coil 18 and a detection coil 20 at a distance 
apart from each other, but they are to be arranged next to one another. 
FIG. 7 shows schematically an embodiment with only one drive coil 18 and 
four detection coils 20 so arranged that the drive magnetic field balances 
out in detection coils 20. 
DESCRIPTION OF A PREFERABLE EMBODIMENT 
Referring to FIG. 8, drive coils 18 and detector coils 20' and 20" are 
arranged around a ferrite core 36. For use in a reproduction apparatus, 
the ferrite core 36 is positioned at a few mm from a glass platen 38. A 
security document 40 comprising elongated magnetic particles 41 is 
positioned on the glass platen. The ferrite core 36 is used to ensure a 
higher magnetic field at the level of the security document 40 for a given 
drive current. 
The ferrite core 36 must not saturate in order to avoid from creating 
additional non-linearities and harmonics. 
The ferrite core 36 is preferably U-shaped. This means it has two legs 42 
connected by a "bridge" 43. The bridge 43 ensures that the flux flow is 
kept away from any neighbouring metal of the reproduction apparatus. 
A drive coil 18 is wired somewhere in the middle of each leg 42. The 
detector coil is divided into two parts 20' and 20". One part 20' is wired 
at the side of the glass platen 38 around the leg 42, the other part 20" 
is wired at the down side around the leg 42. Both parts 20' and 20" are 
wired in anti-phase, as designated by reference number 44, in order to 
null out the received drive signal and other sources of interference such 
as the presence of a lamp in the reproduction apparatus. The wiring in 
anti-phase, however, does not null out the signals received from any 
elongated magnetic particles 41 since one part 20', the top coil part, of 
the detection coil is positioned much nearer to the magnetic particles 
than the other part 20", the bottom coil part. 
Next to the drive coil and the detection coil, a third coil may be wired 
around the ferrite core in order to detect the presence of any ferrous 
metal on the glass platen 38. As is known in the art, the presence of any 
ferrous metal may disturb the magnetic flux pattern so that a ferrous 
metal could be used to hide the presence of any security documents with 
elongated magnetic particles. For reason of simplification this third coil 
is not shown in FIG. 8.