Image analysis – Applications
Reexamination Certificate
1995-08-28
2001-06-19
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
C380S054000, C283S093000
Reexamination Certificate
active
06249588
ABSTRACT:
BACKGROUND OF THE INVENTION
Counterfeiting documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems (see, for example, “Making Money”, by Gary Stix, Scientific American, March 1994, pp. 81-83).
The present invention is concerned with providing a novel security element and authentication means of enhanced security for banknotes, cheques, credit cards, travel documents and the like, which is even more difficult to counterfeit than present banknotes and security documents.
Various sophisticated means have been introduced in prior art for counterfeit prevention and for authentication of documents. These include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms, etc. Nevertheless, there is still an urgent need to introduce further security elements, which do not considerably increase the cost of the produced documents.
Moire effects have already been used in prior art for the authentication of documents. For example, United Kingdom Pat. No. 1,138,011 (Canadian Bank Note Company) discloses a method which relates to printing on the original document special elements which when counterfeited by means of halftone reproduction show a moire pattern of high contrast. Similar methods are also applied to the prevention of digital photocopying or digital scanning of documents. In all these cases, the presence of moire patterns indicates that the document in question is counterfeit. However, in prior art no advantage is taken of the intentional generation of a moire pattern having a particular intensity profile, whose existance, and whose precise shape, are used as a means of authentication of the document. The approach on which the present invention is based further differs from that of prior art in that it not only provides fulll mastering of the qualitative geometric properties of the generated moire (such as its period and its orientation), but it also permits to determine quantitatively the intensity levels of the generated moire.
SUMMARY OF THE INVENTION
The present invention relates to a new method and apparatus for authenticating documents such as banknotes, trust papers, securities, identification cards, passports, etc. This invention is based on the moire phenomena which are generated between two specially designed dot-screens, at least one of which being printed on the document itself. Each dot-screen consists of a regular lattice of tiny dots, and is charactrized by three parameters: its repetition frequency, its orientation, and its dot shapes. The dot-screens used in the present invention are similar to dot-screens which are used in classical halftoning, but they have specially designed dot shapes, frequencies and orientations, in accordance with the present disclosure. Such dot-screens with simple dot shapes may be produced by classical (optical or electronic) means, which are well known by people skilled in the art. Dot-screens with more complex dot shapes may be produced by means of the method disclosed in co-pending U.S. patent application Ser. No. 08/410,767 filed Mar. 27, 1995 (Ostromoukhov, Hersch).
When the second dot-screen (hereinafter: “the master screen”) is layed on top of the first dot-screen (hereinafter: “the basic screen”), in the case where both screens have been designed in accordance with the present disclosure, there appears in the superposition a highly visible repetitive moire pattern of a predefined intensity profile shape. For example, the repetitive moire pattern may consist of any predefined letters, digits or any other preferred symbols (such as the country emblem, the currency, etc.).
As disclosed in U.S. Pat. No. 5,275,870 (Halope et al.) it may be advantageous in the manufacture of long lasting documents or documents which must withstand highly adverse handling to replace paper by synthetic material. Transparent sheets of synthetic materials have been successfully introduced for printing banknotes (for example, Australian banknotes of 5 or 10 Australian Dollars).
The present invention concerns a new method for authenticating documents which may be printed on various supports, including (but not limited to) such transparent synthetic materials. In one embodiment of the present invention, the moire intensity profile shapes can be visualized by superposing a basic screen and a master screen which are both printed on two different areas of the same document (banknote, etc.). In a second embodiment of the present invention, only the basic screen appears on the document itself, and the master screen is superposed on it by the human operator or the apparatus which visually or optically validates the authenticity of the document. In a third embodiment of this invention, the basic screen appears on the document itself, and a sheet of microlenses (hereinafter: “microlens array”) whose frequency is identical to that of the master screen is used by the human operator or by the apparatus instead of the master screen. An advantage of this third embodiment is that it applies equally well to both transparent support, where the moire is observed by transmittance, and to opaque support, where the moire is observed by reflection. (The term “opaque support” as employed in the present disclosure also includes the case of transparent materials which have been made opaque by an inking process or by a photographic or any other process.)
The fact that moire effects generated between superposed dot-screens are very sensitive to any microscopic variations in the screened layers makes any document protected according to the present invention practically impossible to counterfeit, and serves as a means to easily distinguish between a real document and a falsified one.
It should be noted that the dot-screens which appear on the document itself in accordance with the present invention may be printed on the document like any screened (halftoned) image, within the standard printing process, and therefore no additional cost is incurred in the document production.
Furthermore, the dot-screens printed on the document in accordance with the present invention need not be of a constant intensity level. To the contrary, they may include dots of gradually varying sizes and shapes, and they can be incorporated (or dissimulated) within any halftoned image printed on the document (such as a portrait, landscape, or any decorative motif, which may be different from the motif generated by the moire effect in the superposition). To reflect this fact, the terms “basic screen” and “master screen” used hereinafter will include also cases where the basic screens (respectively: the master screens) are not constant and represent halftoned images. (As is well known in the art, the dot sizes in halftoned images determine the intensity levels in the image: larger dots give darker intensity levels, while smaller dots give brighter intensity levels.)
The terms “print” and “printing” in the present disclosure refer to any process for transferring an image onto a support, including by means of a lithographic, photographic or any other process.
The disclosures “A generalized Fourier-based method for the analysis of 2D moire envelope-forms in screen superpositions” by I. Amidror, Journal of Modem Optics, Vol. 41, 1994, pp. 1837-1862 (hereinafter, “Amidror94”) and U.S. patent application Ser. No. 08/410,767 (Ostromoukhov, Hersch) have certain information and content which may relate to the present invention and aid in understanding thereof, they are therefore entirely incorporated herein this disclosure by reference.
REFERENCES:
patent: 4278755 (1981-07-01), Liu
patent: 4999006 (1991-03-01), Hamanaka
patent: 5018767 (1991-05-01), Wicker
patent: 5275870 (1994-01-01), Halope et al.
patent: 5396559 (1995-03-01), McGrew
patent: 5487567 (1996-01-01), Volpe
patent: 5537486 (1996-07-01), Stratigos et al.
patent: 5995638 (1999-11-01), Amidror et al.
patent: 1138011 (1968-12-01), None
patent: 2 224 240 (1990-05-01), None
Völkel et al., “Image Prope
Amidror Isaac
Hersch Roger D.
Ecole Polytechnique Federale de Lausanne
Johns Andrew W.
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