Image analysis – Applications
Reexamination Certificate
2001-06-11
2004-11-16
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
C380S054000, C283S093000
Reexamination Certificate
active
06819775
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of anticounterfeiting and authentication methods and devices and, more particularly, to methods, security devices and apparatuses for authentication of documents and valuable articles using the intensity profile of moire patterns.
Counterfeiting of 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 same is also true for other valuable products such as CDs, DVDs, software packages, medical drugs, etc., that are often marketed in easy to falsify packages.
The present invention is concerned with providing a novel security element and authentication means offering enhanced security for banknotes, checks, credit cards, identity cards, travel documents, industrial packages or any other valuable articles, thus making them much more difficult to counterfeit.
Various sophisticated means have been introduced in prior art for counterfeit prevention and for authentication of documents or valuable articles. Some of these means are clearly visible to the naked eye and are intended for the general public, while other means are hidden and only detectable by the competent authorities, or by automatic devices. Some of the already used anti-counterfeit and authentication means 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 or goods.
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 (for example, U.S. Pat. No. 5,018,767 (Wicker), or U.K. Pat. Application No. 2,224,240 A (Kenrick & Jefferson)). In all these cases, the presence of moire patterns indicates that the document in question is counterfeit. Other prior art methods, on the contrary, take advantage of the intentional generation of a moire pattern whose existence, and whose precise shape, are used as a means of authenticating the document. One known method in which a moire effect is used to make visible an image encoded on the document (as described, for example, in the section “Background” of U.S. Pat. No. 5,396,559 (McGrew)) is based on the physical presence of that image on the document as a latent image, using the technique known as “phase modulation”. In this technique, a uniform line grating or a uniform random screen of dots is printed on the document, but within the pre-defined borders of the latent image on the document the same line grating (or respectively, the same random dot-screen) is printed in a different phase, or possibly in a different orientation. For a layman, the latent image thus printed on the document is hard to distinguish from its background; but when a reference transparency comprising an identical, but unmodulated, line grating (respectively, random dot-screen) is superposed on the document, thereby generating a moire effect, the latent image pre-designed on the document becomes clearly visible, since within its pre-defined borders the moire effect appears in a different phase than in the background. However, this previously known method has the major flaw of being simple to simulate, since the form of the latent image is physically present on the document and only filled by a different texture. The existence of such a latent image on the document will not escape the eye of a skilled person, and moreover, its imitation by filling the form by a texture of lines (or dots) in an inversed (or different) phase can easily be carried out by anyone skilled in the graphics arts.
Other moire based methods, in which the presence of moire intensity profiles indicates the authenticity of the document, have been disclosed by the present inventors in U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. These methods completely differ from the above mentioned technique, since no phase modulation is used, and furthermore, no latent image is present on the document. On the contrary, all the spatial information which is made visible by the moire intensity profiles according to the inventions of the present inventors is encoded in the specially designed forms of the individual dots which constitute the dot-screens. These inventions are based on specially designed periodic structures, such as dot-screens (including variable intensity dot-screens such as those used in real, full gray level or color halftoned images), pinhole-screens, or microlens arrays, which generate in their superposition periodic moire intensity profiles of any chosen colors and shapes (letters, digits, the country emblem, etc.) whose size, location and orientation gradually vary as the superposed layers are rotated or shifted on top of each other. In U.S. Pat. No. 5,712,731 (Drinkwater et al.) another moire based method is disclosed which, unlike the above mentioned inventions, can be combined within a hologram or a kinegram, or with parallax effects due to the varying view angles of the observer. However, this last disclosure has the disadvantage of being limited only to the case where the superposed revealing structure is a microlens array and the periodic structure on the document is a constant dot-screen with identical dot-shapes throughout. Thus, in contrast to the present authors' inventions, this disclosure excludes the use of dot-screens or pinhole-screens as revealing structures, as well as the use on the document of full, real halftoned images with varying tone levels (such as portraits, landscapes, etc.), either in full gray levels or in color, that are made of halftone dots of varying sizes and shapes—which are the core of the methods disclosed by the present inventors, and which make them so difficult to falsify.
In the present invention the present inventors disclose new methods largely improving their previously disclosed methods mentioned above, which make them even more difficult to counterfeit. These new improvements make use of the theory developed in the paper “Fourier-based analysis and synthesis of moires in the superposition of geometrically transformed periodic structures” by I. Amidror and R. D. Hersch, Journal of the Optical Society of America A, Vol. 15, 1998, pp. 1100-1113 (hereinafter, “[Amidror98]”), and in the book “The Theory of the Moire Phenomenon” by I. Amidror, Kluwer, 2000 (hereinafter, “[Amidror00]”). According to this theory it is possible, by using certain mathematical rules that will be explained in detail below, to synthesize aperiodic, geometrically transformed structures which in spite of being aperiodic in themselves, still generate, when they are superposed on top of one another, periodic moire intensity profiles with clearly visible and undistorted elements, just like in the periodic cases disclosed by the present inventors in their previous U.S. Pat. No. 6,249,588 and its continuation-in-part U.S. Pat. No. 5,995,638. Furthermore, it will be disclosed here how even cases which do not yield periodic moires can still be advantageously used for anticounterfeiting and authentication of documents and valuable articles in accordance with the present invention.
It should be noted that the approach on which the present invention is based further differs from that of prior art in that it not only provides full mastering of the qualitative geometric properties of the generated moire (such as its geometric layout), but it also enables the intensity levels of t
Amidror Isaac
Hersch Roger D.
Ecole Polytechnique Federale de Lausanne
Johns Andrew W.
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