Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
1999-07-02
2001-07-10
Lee, Michael G. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S439000, C235S493000, C235S450000
Reexamination Certificate
active
06257488
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to 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 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 fibers which are coated with a magnetic or magnetizable material embedded therein. The mere presence of the magnetic fibers inside the security documents is tested or, as an improved feature, the distribution of the magnetic fibers 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 fibers 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 fibers 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 fibers, more particularly stainless steel fibers, 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 fibers. By scanning the documents by means of microwaves the unique distribution of the fibers inside the security document can be obtained. Up to 64
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. In addition, with a system mounted on existing designs of colour photocopier, it is not practicable to measure the microwave transmission through the platen area and document as this would require an area sensor mounted on the photocopier lid, and this is generally incompatible with their design. Measuring the microwave signal in reflection, it has been found that the signals from people's hands are indistinguishable from those from documents containing metal fibers. On the other hand, the system does not discover the presence of the fibers 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.
OBJECTS AND 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 another 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 {fraction (1/250)}, preferably smaller than {fraction (1/1000)}. Their cross-sectional diameter is smaller than 30 micrometer and their magnetic saturation field H
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
s
is preferably smaller than 1000 A/m. The terms “magnetic saturation field H
s
” are herein defined as the magnetic field at the onset of the saturation flux density B
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 or magnetization 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 1
Burrell Jon
Fisher John
Robertson Paul
Franklin Jamara A.
Lee Michael G.
N.V. Bekaert S.A.
Shlesinger & Arkwright & Garvey LLP
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