US 5,995,638 A · Methods and apparatus for authentication of documents by using the intensity profile of moire patterns

Abstract

New method and apparatus for authenticating security documents such as banknotes, passports, etc. which may be printed on any support, including transparent synthetic materials and traditional opaque materials such as paper. The invention is based on moire patterns occuring between superposed dot-screens. By using a specially designed basic screen and master screen, where at least the basic screen is comprised in the document, a moire intensity profile of a chosen shape becomes visible in their superposition, thereby allowing the authentication of the document. If a microlens array is used as a master screen, the document comprising the basic screen may be printed on an opaque reflective support, thereby enabling the visualization of the moire intensity profile by reflection. Different variants of the invention are disclosed, some of which are specially adapted for use as covert features. Automatic document authentication is supported by an apparatus comprising a master screen, an image acquisition means such as a CCD camera and a comparing processor whose task is to compare the acquired moire intensity profile with a prestored reference image. Depending on the match, the document handling device connected to the comparing processor accepts or rejects the document. An important advantage of the present invention is that it can be incorporated into the standard document printing process, so that it offers high security at the same cost as standard state of the art document production.

Claims27

Dependent claims

A method for authenticating documents by using at least one Moire intensity profile, the method comprising the steps of:

a) creating on a document a basic screen with at least one basic screen dot shape;
b) creating a master screen with a master screen dot shape;
c) superposing the master screen and the basic screen, thereby producing a Moire intensity profile;
and
d) comparing said Moire intensity profile with a prestored Moire intensity profile and depending on the result of the comparison, accepting or rejecting the document;
where the produced Moire intensity profile is a normalized T-convolution of the basic screen and of the master screen and where the orientation and period of the produced Moire intensity profile are determined by the orientations and periods of the basic screen and of the master screen.

3 more elements

A method for Authenticating documents by using at least one Moire intensity profile, the method comprising the steps of:

i) creating on a document a basic screen with at least one basic screen dot shape;
ii) creating a master screen with a master screen dot shape;
iii) superposing the master screen and the basic screen, thereby producing a Moire intensity profile;
and
iv) comparing said Moire intensity profile with a prestored Moire intensity profile and depending on the result of the comparison, accepting or rejecting the document;
where comparing a Moire intensity profile with a prestored Moire intensity profile is done by computer-based matching, which requires an acquisition of a Moire intensity profile and a geometrical correction of a rotation angle error and of a scaling error in the acquired Moire intensity profile, comprising the steps of:
a) acquiring a Moire intensity profile by an image acquisition means;
b) intersecting the acquired Moire intensity profile with a straight line parallel to a main axis of the prestored Moire intensity profile;
c) computing a discrete straight line segment representing the acquired Moire intensity profile along the straight line by resampling the straight line intersecting the acquired Moire intensity profile at the resolution of the acquired Moire intensity profile;
d) checking the considered discrete straight line segment as well as parallel instances of it for valid intensity variations defined as intensity variations with a quasi-period not smaller than .sigma..sub.min times the smallest of the two periods P.sub.1, P.sub.2 of the prestored Moire intensity profile and not larger than .sigma..sub.max times the largest of the two periods P.sub.1, P.sub.2 of the prestored Moire intensity profile;
e) rejecting the document in the case where no valid intensity variations occur in any of the parallel discrete straight line segment instances;
f) in the case of valid intensity variations, rotating the discrete straight line segment showing valid intensity variations until an angle .delta. is reached in which the rotated discrete straight line segment comprises successive identical quasi-periods P of intensity variations;
g) computing the scaling error .sigma.=P/P.sub.1 ;
h) using the angle .delta. and the scaling error .sigma. to rotate by angle .delta. and to scale by factor 1/.sigma. a window of the acquired Moire intensity profile containing at least one period of said acquired Moire intensity profile, thereby obtaining a geometrically corrected Moire intensity profile;
i) matching the so-obtained geometrically corrected Moire intensity profile with the prestored Moire intensity profile and obtaining a proximity value giving the proximity between the acquired Moire intensity profile and the prestored Moire intensity profile;
and
j) rejecting the document if the proximity value is lower than an experimentally determined threshold.

14 more elements

Dependencies are inferred from the claim text.

Methods and apparatus for authentication of documents by using the intensity profile of moire patterns

Life of this patent

Methods and apparatus for authentication of documents by using the intensity profile of moire patterns

Life of this patent

Abstract

Claims27