WO2019191852A1

WO2019191852A1 - Security document with individualized window

Info

Publication number: WO2019191852A1
Application number: PCT/CH2018/000013
Authority: WO
Inventors: Sylvain Chosson; Martin Eichenberger
Original assignee: Orell Füssli Sicherheitsdruck Ag
Priority date: 2018-04-06
Filing date: 2018-04-06
Publication date: 2019-10-10

Abstract

The contour (8) of a window (2) in the carrier (1) of a security document is varied between documents. The variation is small and invisible to the naked eye, but it can be detected using a suitable detection device. The window (2) can be covered on one of both sides by transparent layers (41, 42) in order to protect the small variations of the edge from wear. The technique e.g. allows to encode a documents serial number, denomination, or bearer in the contour of the window (2).

Description

Security document with individualized window

Technical Field The invention relates to a method for manufacturing a set of security documents. It also relates to such a set of security documents. Further, it relates to a method for checking the authenticity of a security document of such a set.

Background Art

Security documents, such as banknotes, checks, vouchers, passports, ID cards, credit cards, etc., usually contain security features that render them difficult to copy.

For example, it has been known to provide security documents with windows. Such a document is e.g. described in W02004/076198.

Disclosure of the Invention The problem to be solved by the present invention is to provide a method and a set of security documents as mentioned above that are difficult to counterfeit.

This problem is solved by the method and set of security documents according to the independent claims.

Accordingly, the method relates to manufacturing a set of security documents, wherein each document comprises a window in a carrier. The method comprises, for each document, the following steps:

- Calculating, for each security document, an individualized contour path for a contour of the window. The individualized contour paths are varied be- tween individual documents in a visually imperceptible manner.

- Forming the window using the individualized contour path for its contour. In other words, the edge of the window of each document follows its individualized contour path.

In this context, a“window” designates a transparent part of the car- rier, i.e. a.part having a transmission that is at least twice as high, in particular at least ten times as high, as the part of the carrier surrounding it. The term“transmission” designates the averaged transmission over at least part of the visual spectrum of light, e.g. between 420 nm and 750 nm.

Advantageously, the transmission of the window is at least 50%, in particular at least 70%. In contrast to this, the transmission of the carrier can be less 5 than 20%, in particular less than 10%.

A“set of security documents” designates a plurality of security documents, such as the banknotes of a given series or the identification documents of a given state or of a given organization.

The term“visually imperceptible” is to be understood as follows: If :o any two of the documents in the set of documents are shown to at least 100 average persons and they are asked- to compare the shapes of the windows, no more than 10% are able to pinpoint, with the naked eye, any differences.

In particular,“visually imperceptible” differences are achieved if the individualized contour paths between any two of the documents vary by no more is than 100 pm, in particular by no more than 50 pm.

On the other hand, the individualized contour paths between any two of the documents advantageously vary, in at least some parts, by at least 10 pm in order for easier machine-based detection.

In addition, if the diameter of the window in all directions is at least 0 50 times larger than the variation of the individualized contour paths between any two of said documents, the variations are hard to discern, too.

In one embodiment, the method comprises, for each document, the following steps:

- Providing an original contour path for the contour: This is the₅ “global” contour path from which all individual window contours are calculated.

- Providing a first identifier; This is an identifier that varies over at least some of the documents.

- Modifying the original contour path using the first identifier for calculating the individualized contour path for the contour: This contour path is dif-o ferent from the original contour path.

- Forming the window using said individualized contour path for its contour. In other words, the window is formed with its contour following the modified contour path.

In order to make the variations visually imperceptible, the closest₅ distance of any point of the individualized contour path from the original contour path is e.g. no more than 100 pm, in particular by no more than 50 pm. On the other hand, for at least some points on the individualized contour path, the closest distance from the original contour path is advantageously at least 10 pm in order to generate deviations that can be measured reliably.

The method can further comprise the step of applying, in particular printing, a second identifier on each security document. The second identifier also varies over at least some of the documents.

Advantageously, the first and/or second identifier is/are unique to the document. For example, the second identifier can be a serial number.

In one embodiment, the first and the second identifier are related such that it can be checked if the first and second identifier are“matching”. For this purpose, one of the following methods can e.g. be used:

- The first and identifier is an invertible function of the second identifier: In this case, the knowledge of the first identifier allows to derive the second identifier and vice versa. Verification can be carried out directly. In one specific em- bodiment, the first and the second identifier may be equal.

- The first identifier comprises or is a hash value of the second iden- tifier or the second identifier comprises or is a hash value of this first identifier. In this case, a hash function can be used for comparing the two identifiers. In particular, the hash values can be cryptographic hash values, i.e. a cryptographic hash function is used.

In a particularly useful embodiment, the method comprises the step of establishing a database comprising the first and said second identifiers for each document. This technique has the advantage that the selection of the two identifiers can be carried out separately, which allows to easily form the window in a different processing step from selecting the second identifier and to associate them e.g. later. For example, individualized windows may be created by the manufacturer of a security paper, i.e. that manufacturer selects the first identifier. Later, the paper is processed by a security printing company, and that company may want to apply a unique serial number, i.e. the second identifier, to the documents. Once the documents are finished, they can be scanned for attributing the two identifiers in the database.

Advantageously, the individualized contour path is calculated such that the first identifier can be retrieved by analyzing the individualized contour path.

In other words, there exists a reverse function for mapping the shape of the individualized contour path to the first identifier.

In one embodiment, along at least part of the contour, the deviation between the original contour path and the individualized contour path is a periodic function. In this context, a“periodic function” is a function that is repetitive in the sense that it has a constant period length or a period length that varies no more than 25%, in particular by no more than 10%, from one period to the next, i.e. whose period varies slowly or not at all. Deviations described by this type of function can be detected easily and have inherent redundancy

Advantageously, said“part of the contour” extends over at least 10 periods of the function for good redundancy.

For example, at least one of the following parameters of the periodic function can depend on (i.e. can be calculated as a function of) the first identi fier:

- The period length of the function, i.e. its frequency.

- The change of the period length in case that the frequency of the function varies, e.g. in a chirp function.

- The amplitude of the function.

- The phase offset of the function, i.e. the function can be displaced along the contour as a function of the first identifier.

Advantageously, the period length of the function is at least 10 pm and/or no more than 100 pm in order to make it easily detectable.

Similarly, the amplitude of the function can e.g. be at least 10 pm and/or no more than 100 pm in order to make it easily detectable while keeping it in- visible to the naked eye.

In a particularly advantageous embodiment, each document com prises a transparent foil spanning the window. Such a foil protects the edges of the window from wear and thereby makes the feature more robust.

The contour can e.g. be formed by cutting, in particular laser cut- ting, the carrier.

In another embodiment, the carrier can be transparent and the method comprises the step of printing said contour with opaque ink on said carrier.

The invention also relates to set of security documents manufactura ble, in particular manufactured, by. the method described above.

In another aspect, the invention also relates to a set of security documents. Each security document of the set comprises a carrier and a window arranged in the carrier. Between any two of the security documents, the contours of the windows differ in visually imperceptible (but machine-detectible) manner. The security documents of this set can comprise the features as they are described in relation to the method above. As mentioned, the invention also relates to a method for checking the authenticity of a security document of this type. This method comprises the following steps:

- Measuring at least part of the contour of the window.

- Testing for the presence of the deviations of said contour from an original contour path.

The original contour path can e.g. be the original contour path mentioned above and it can e.g. be stored in the detection device. Alternatively, it may also be derived, at least in part, from the measured contour, e.g. by low-pass filtering of the measured contour in order to eliminate the individual deviations. Or the devia tions may be obtained directly by high-pass filtering the measured contour.

In one embodiment, the method can comprise the step of calculating a Fourier descriptor or Fourier transform of at least part of the contour. Using Fourier descriptors or Fourier transforms has the advantage of robustness, in particular in view of possible degradations of the contour as well as in view of an imperfect recording of the same.

The method can also comprise the step of recording, in addition to the contour, a feature different from the window (i.e. from the contour) on the security document. This allows to determine the position where the window or its original contour should be. Hence, the deviations can be derived more accurately by using the position of the feature.

The relevant part(s) of the contour can be measured by various means, such as:

- By means of a camera.

- By means of a scanner, in particular a line-scanner.

- By means of a touch screen.

The invention also relates to a data processing device comprising means for carrying out the method above. This device may e.g. be a dedicated hardware, or it may e.g. be a smartphone equipped with suitable software.

The invention also relates to a computer program comprising instructions which, when the program is executed by a computer, such as the device mentioned above, cause the computer to carry out the method as described.

Accordingly, the invention also relates to a computer-readable data carrier having stored thereon such a computer program.

In yet a further aspect, the invention may relate to a method for manufacturing a set of security documents comprising the steps of calculating, for the document, a first and a second contour path, in particular (but not necessarily) a first and a second individualized contour path, wherein said first and second contour paths intersect in several locations,

forming a first opening in a first non-transparent layer along the first contour path,

forming a second opening in a second non-transparent layer along the second contour path,

arranging the first and second non-transparent layers on opposite sides of a transparent layer.

Brief Description of the Drawings The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. This description makes reference to the annexed drawings, wherein:

Fig. 1 shows a security document,

Fig. 2 is a sectional view along line II-II of Fig. 1,

Fig. 3 shows three windows with different contours,

Fig. 4 shows a function used to generate a window of the type of

Fig. 3,

Fig. 5 shows an original contour path for a window contour, Fig. 6 shows a first embodiment of an individualized contour path, embedding the same signal with three various amplitudes

Fig. 7 shows a second embodiment of an individualized contour path,

Fig. 8 shows a third embodiment of an individualized contour path, Fig. 9 shows the signals or functions used for generating the contour paths of Figs. 6 - 8,

Fig. 10 shows a first embodiment of a device for checking the validity of a security document,

Fig. 11 shows a second embodiment of a device for checking the validity of a security document,

Fig. 12 shows a third embodiment of a device for checking the validity of a security document. Fig. 13 shows a block diagram of such a device,

Fig. 14 shows steps of checking a security document,

Fig. 15 shows a window contour,

Fig. 16 shows a detail of Fig. 15,

Fig. 17 shows the Fourier spectrum of the contour of Fig. 16,

Fig. 1 8 shows a further embodiment of a security document with a transparent foil spanning the window,

Fig. 19 shows an embodiment of a security document with two transparent foils,

Fig. 20 shows an embodiment of an identification document,

Fig. 21 shows an embodiment of a security document with e.g. a printed window,

Fig. 22 shows an embodiment of a security document with an individualized patch,

Fig. 23 shows a sectional view, along line XXIII-XXIII of Fig. 24, of yet another security document with two differing contours on opposite sides,

Fig. 24 shows a top view of the embodiment of Fig. 24,

Fig. 25 shows a view of the window of the document of Fig. 23 when illuminated in transmission at a wavelength at which the window is transparent,

Fig. 26 shows a view of the window of the document of Fig. 23 when illuminated in reflection at a wavelength at which the window is absorbing, and

Fig. 27 shows the areas where the views of Figs. 25 and 26 differ. Note: The deviations of the individualized contour path of the window contour from the original contour path are enlarged in some of the figures for better visibility.

Modes for Carrying Out the Invention First Embodiment:

The security document of Figs. I and 2 is e.g. a banknote. It comprises a carrier 1 and a window 2.

In the embodiment shown, carrier 1 is a multi-layer substrate e.g, comprising one or two layers of paper 3 and a transparent layer 4.

For example, and as shown, there may be two opaque layers of paper 3 with the transparent layer 4 arranged between them.

Other embodiments of carrier 1 will be described below. In addition to window 2, the security document typically has further security features, which may e.g. be printed thereon or affixed thereto.

In the embodiment of Fig. 1, the document comprises a unique serial number 5, which is e.g. printed thereon. Also, it may e.g. have a denomination 6, which may also be printed thereon.

In the example of Fig. 1 , window 2 has a substantially circular contour 8. However, its contour path is modulated to deviate from a perfect circle in order to encode information therein.

The security document of Fig. 1 is one of a set of security docu- ments, and the contour paths of the contours 8 of their windows 2 will differ.

By way of example, Fig. 3 shows e.g. three different contour paths A, B, and C for contour 8 of window 2 as they can be found on different specimens of the set of security documents.

In the example of Fig. 3, the deviation of the individualized contour paths 1 1 of the contours from the circular original contour path 10 (shown in a dotted line in Fig. 3 A) corresponds to a chirped sine signal as shown in Fig. 4.

Such a chirped sine signal can e.g. be described by

with the parameter A, B, C, and D describing the amplitude, the period (or frequency), the change of the period (or frequency), and the offset, respectively, and x e.g. describing the polar angle along the contour path as seen from the center of the window. In the example of Fig. 3, the parameters A = 0.25, B = 2, C = 1.4, and D = 0 were used.

One or more of the parameters A, B, C, and/or D can be modified from document to document and can be used to encode information.

For example, the serial number 5 or a hash value thereof can be used to calculate one or more of the parameters in order to have window contours that differ between at least part of the security documents.

As mentioned above, the deviations of individualized contour path 11 and original contour path 10 are so small that they are invisible to the unaided eye.

Second embodiment:

Figs. 5 - 8 illustrate a second group of embodiments where the con

Fig. 5 shows the unmodulated, original contour path 10 of the con tour. Figs. 6, 7, and 8 show examples of individualized contour paths 11 as compared to original contour path 10.

The deviations between the individual contour paths 11 and the original contour path 10 are, in the second group of embodiments sums of one or more of the functions illustrated in Fig. 9.

For example, individualized contour path 11 of the embodiment of Fig. 6 is calculated from original contour path 10 by adding the function or signal 1 of Fig. 9. Individualized contour path 11 of the embodiment of Fig. 7 is calculated from original contour path 10 by adding the functions or signals 1 + 2 of Fig. 9. Individualized contour path 1 1 of the embodiment of Fig. 8 is calculated from original contour path 10 by adding the functions or signals 1 + 3 of Fig. 9.

The various functions or signals 1 , 2, and 3 again have parameters, such as their amplitude, period, chirp, and phase, which can be used to encode infor- mation as in the example above.

Third embodiment:

Figs. 15 and 16 illustrate a third embodiment, again for a window 2 with a substantially rectangular contour 8 with rounded comers.

In Fig. 15, the modulations that have been carried out along contour

8 are not visible. Therefore, a part 14 of contour 8 is shown in enlarged manner in Fig. 16.

In this embodiment, the deviation of the individualized contour path 11 from the original contour path 10 (which is e.g. a straight line at this magnifica- tion) are a sum of several sine functions at e.g. frequencies 4, 15, and 37 (arbitrary units), which may have equal or different amplitudes and/or phases.

Method and device for testing the document:

In order to test the document, in a first step the actual contour 8 of window 2 is measured by a suitable device. Examples of such devices 16 are shown in Figs. 10 - 12, and Fig. 13 shows block diagram of such a device 16.

In the embodiment of Fig. 10. device 16 is a scanner which scans contour 8 of a security document that is positioned on a defined plane, such as on the bed of a flatbed scanner. This has the advantage that there is a defined distance and tilt between the camera and the security document while recording contour 8. In the embodiment of Fig. I I, device 16 is a device having a touch screen 17, which may be a capacitive or force-based touch screen. A pen 18 (or fin ger) is used for rubbing along at least part of contour 8 of window 2 in order to record the contour.

The pen moving over the step-wise contour 18 will generate a de tectable signal on touchscreen 17 demarking the position of the contour.

In the embodiment of Fig. 12, device 16 is a smartphone or other type of hand-held camera used to take at least one image of at least part of contour 8. In this embodiment, the distance and/or tilt and/or rotation between the camera and the security document are undefined and need to be corrected for (see below).

Fig. 13 shows block diagram of such a device 16, which forms a data processing device for carrying out the authenticity check described here. It corn- prises a sensor 20,· which may e.g. be the camera of the device 16 of the embodiments of Figs. 10 or 12 or the touch screen 17 of the embodiment of Fig. 11.

Device 16 further comprises a controller 21 , such as a microprocessor, and a memory 22, e.g. for storing data and a computer program for carrying out the method described here.

Device 16 also comprises an inpu output-device 23, such a as a touchscreen.

The sensor 20 should have sufficient resolution to record the deviations in the contour’s individualized contour path 1 1 from the original contour path

10.

For example, if the a camera is able to image an area of 1 cm x 1 cm onto 2000 x 2000 pixels, each pixel represents an area of roughly 5 pm x 5 pm, he. the features of the deviations of the individualized contour path 1 1 from the origi nal contour path 10 should be at least 10 pm for being resolved well,

It must be noted that periodic deviations are particularly well suited because of their redundancy that allows them to be detected even if their size is at the limit of the sensor’s resolution.

To check the authenticity of the security document, at least part of contour 8 is measured. Then the individualized contour path 1 1 of the measured part of the contour 8 is tested for its deviations from original contour path 10.

Measuring contour 8 typically consists of creating an image of win dow 2 along at least part of its edge.

Image processing techniques can then be used to detect the location of the edge of the window. Techniques for edge detection are known to the skilled person. Examples can e.g. be found at https ://en.wikipedia.org/wiki/Edge_detection. Once the contour is known, it can be analyzed for the presence of the deviations.

In one embodiment, such analysis can e.g. comprise the calculation of a Fourier descriptor of at least part of the contour. A technique for doing so is de- 5 scribed by F. P. Kuhl et al. Computer Graphics and Image Processing 18, 236 - 258 (1982) and is based on an analysis of the contour’s chain code.

In another example, which is illustrated in reference to Figs. 16 and 17, the individualized contour path 1 1 of the contour can be expressed as a set of co ordinates and be subject to a Fourier transform in order to analyze its spectrum. Theo frequency components, their amplitude, as well as their phases can be derived from the complex Fourier transform. Fig. 17 shows the amplitude of the Fourier transform.

Using a Fourier transform and looking at its higher-frequency com ponents (at frequencies that correspond to the structural size of the deviations) is an example of high-pass filtering, which allows to detect the deviations withouts knowledge of the exact position of original contour path 10 of the contour.

Hence, depending on the nature of the deviations, the deviations can be determined without the exact knowledge of the position of original contour path 10. However, e.g. if the deviations have a curvature and spatial frequency comparable to the original contour path 10, a_. knowledge of the location of original contour pathc 10 can improve the reliability of detection.

If the position of original contour path 10 needs to be known, it can e.g. be obtained by low-pass filtering the measured contour, in particular if the deviations have high frequencies only. For example, a low-pass filtering of the signal shown in Fig. 16 (e.g. by running a linear regression on the curve representing indi-5 vidualized contour path 1 1) will yield the position of original contour path 10.

In yet another embodiment, the position of original contour path 10 can be derived by recording, in addition to at least part of contour 8, a feature on the security document that is different from window 2 but in register (i.e. in a known spa tial relation) with original contour path 10. Such a feature is shown, by way of exam-₀ pie, as a line 30 in Fig. 15, which may e.g. be a dotted line printed onto the document’s surface and aligned with a section of the original contour path of contour 8.

In yet a further embodiment, parts of contour 8 may not be varied during the calculation of the individualized contour path 11. For example, in the em bodiment of Fig. 15, it may be defined that the deviation of individualized contour path 1 1 and original contour path 10 at the locations 31 is zero for all security docu ments of a set. This allows to detect the position of original contour path 10 by means of sensor 20. In more general terms, in an advantageous embodiment, a deviation between individualized contour path 11 and original contour path 10 is zero in at least at two locations of contour 8, for all the documents of the set.

In particular, said deviation is zero in at least three, in particular in at least four such locations, with the locations being non-collinear. This is of particu- lar advantage if the camera of the measuring device is at an undefined position in relation to the security document because these locations can then be used for calculating the tilt, rotation and/or distance between the camera and the security document, thus allowing a more accurate determination of the deviations between individualized contour path 11 and original contour path 10. Further such locations are shown in Fig. 15 under reference number 3 G .

Hence, advantageously, the method for checking the authenticity of a security document comprises the step of determining the tilt, rotation, and/or distance between a camera and the security document from said at least two locations 31, 31’.

Fig. 14 shows yet another example of the method for checking the authenticity of the document.

In a first step El , the image to be processed is recorded by sensor 20. From this, the locations of the original contour path 10 and the individualized contour path i 1 are determined, e.g. using the techniques as described above.

In a second step E2, the distance between individualized contour path 11 and the original contour path 10 is calculated, i.e. the“overall signal”, e.g. as a function of the polar coordinate as seen from the center of the window.

In a third step E3, the low-frequency component of the deviation is calculated, e.g, using low-pass filtering. In the present case, different low-frequency signals are attributed to different denominations of the banknotes. Hence, the denomi nation of the banknote can be obtained in this step.

In a fourth step E4, the high-frequency component of the deviation is calculated, e.g. using high-pass filtering. In the present case, the high-frequency signal encodes the serial number of the banknote or a hash value thereof.

This example illustrates some of the data that can be encoded in contour 8. This is further discussed in the next section.

Data encoded in the contour:

in general, individualized contour path 11 is calculated from original contour path 10 using a“first identifier” attributed to the document. This first identifier is advantageously (but not necessarily) unique to the document.

For example, it can depend on the serial number 5 of the document if a serial number is atributed to the document. Two variants are possible:

- The first identifier can e.g. be equal to the serial number, or, more generally, it can by an invertible function of the serial number. Hence, the knowledge of individualized contour path 1 1 allows to retrieve the serial number of the document (and vice versa). This function can e.g. be a mathematical function or it can be a lookup-function implemented as a database.

- The first identifier can be or comprise a hash function of the serial number (or vice versa), Le. it is a non-invertible function of the serial number. Advan tageously, it is a cryptographic hash function such that a third party is unable to guess the first identifier from the serial number. However, the third party may be able to test if a given first identifier matches the serial number. Using a hash function also has the advantage of reducing the amount of data that has to be encoded in the individualized contour.

The serial number, which is a“second identifier” as defined in the claims, is advantageously unique to the document, such as a serial number. In may be separately applied to the document, e.g. by printing. In particular, it can be applied to the document in a human-readable form.

In other embodiments, the second identifier may not necessarily have to be unique to each document, i.e. some documents may share the same second identifier.

As mentioned, in a particularly advantageous embodiment, a data- base may be established for storing the first and second identifiers of each document, such that one identifier can be retrieved by knowing the other one. This has the advantage that the two identifiers can be selected independently and at different times in the manufacturing process.

In particular, in one step, window 2 is formed using the first identi- fier. In another step, the second identifier is selected and applied to the document, e.g. by printing. In a third step, the first and second identifier are associated in the database. The order of the steps may vary.

In the example of Fig. 14, the security document is a banknote, and individualized contour path 1 1 encodes the denomination of the banknote as well as its serial number (or a hash value of the serial number). Hence, the“first identifier” depends on both the serial number as well as the denomination. In another example, as illustrated in Fig. 20, the document may be an identification document attributed to a bearer. It has a photograph 35 and/or a bearer’s name 36 and/or a storage device 37 with biometric data of the bearer, such as his fingerprint, applied thereto. The document also has a window 2.

in this case, window 2 is generated using the“first identifier” and the photograph and/or name and/or biometric data is the“second identifier”.

In particular, the first identifier can be an invertible function or hash function of the bearer’s name and/or the first identifier can be a hash function of the photograph and/or of the biometric data.

This allows to cross-check the authenticity of the identification document and renders tampering with the bearer’s name, photograph and/or biometric data difficult.

Physical design:

In the embodiment of Fig. 1, carrier 1 is a multi-layer substrate e.g. comprising one or two layers of paper 3 and a transparent layer 4, such as the Duras- afe® substrate by Landqart, Switzerland.

However, carrier 1 may e.g. also be a simple opaque substrate, such as a paper substrate, where window 2 is an opening formed by laser cutting.

In yet a further embodiment, as shown in Fig. 18, carrier 1 comprises a first layer 40 of non-transparent material with an opening forming the win dow 2 therein. A second layer 41 of a transparent material, such as a transparent foil, covers at least part of window 2 and at least part of contour 8 and is affixed on a first side of first layer 40. This provides better physical protection of contour 8 against wear at least in the region where it is covered by the second layer 41.

In yet another embodiment, as shown in Fig. 19, there is an additional third layer 42, of a transparent material, such as a transparent foil, covering at least par of window 2 and at least part of contour 8. It is affixed on a second side to first layer 40, with the second side being opposite the first side. Contour 8 is arranged between the second and third layers 41, 42 along at least part of its length, in particu lar everywhere. Thus, it is even better protected against wear.

In particular, the layers 41 and/or 42 of the embodiments of Figs. 18 and 19 may be adhesively bonded to layer 40.

One or both of the transparent layers 41 , 42 in the embodiments of Figs. 18 and 19 may be patches manufactured separately and applied to first layer 40 after forming the opening for window 2 therein. Such patches may e.g. comprise further security features, such as diffractive and/or metallized structures.

In yet another embodiment, as shown in Fig. 21, carrier 1 may com prise a transparent layer 45 covered by a non-transparent layer 46 except in the loca- iron of window 2. For example, layer 46 can be formed at least in part of a non-trans- parent ink applied to transparent layer 45, such as Guardian (TM) by COL Secure, Australia, by means of a printing technique.

In this case, contour 8 may e.g. be formed during the application of layer 46, in particular during printing. Alternatively, it may be formed after the appli- cation of layer 46 by material removal, e.g. by means of laser ablation.

In one advantageous embodiment, carrier 1 comprises at least one layer of paper, such as first layer 40, and contour 8 of window 2 is formed by an edge cut into the paper, in particular by means of a laser. As mentioned, though, the nontransparent layer may also be formed of another non-transparent material or of a multi-layer structure having at least one non-transparent sub-layer.

Patch:

In yet a further embodiment, each security document may comprise at least one patch applied to it, such as a foil adhesively applied a side of carrier 1. This design may e.g. correspond to the embodiment of Figs. 18 or 19.

Fig. 22 shows an embodiment of such a security document. In this embodiment, the security document is a banknote and e.g. again comprises a unique serial number 5, which is e.g. printed thereon. Also, it may e.g. have a denomination 6, which may also be printed thereon.

One or more patches 48, 49 are applied to the security document.

Advantageously, each such patch 48, 49 is adhesively attached thereto, e.g. by means of gluing.

The patches may comprise security features 50, such as holograms, metallized regions, optically variable devices, etc.

In an advantageous embodiment, at least part of the edge of the patch 48, 49 is modulated, as illustrated by reference number 51 in Fig. 22, in order to encode information that is invisible to the naked eye, using e.g. one or more of the techniques described above for modulating the window’s contour 8.

Hence, similar to what is described above, the method of manufac- turing can comprise the following steps:

- Applying a patch 48, 49 to each of the security documents. - Calculating, for each patch 48, 49, an individualized patch path 51 for the edge of the patch 48, 49. The individualized patch paths 51 are varied between individual patches in visually imperceptible manner.

- Shaping at least part of the edge of the patch 48, 49 according to the individualized patch path 51. In other words, the edge is shaped to follow the patch path 51.

It must be noted that the order of these steps is arbitrary in the sense that the edge of the patch can first be shaped whereupon the patch is applied to the document. Alternatively, the patch can first be applied and then its edge may be shaped. However, the former variant allows to use a wider range of edge-shaping techniques.

The edge of the patch 48, 49 may e.g. be shaped using laser cutting.

The embodiment of Fig. 22 shows examples for two such patches 48, 49. Patch 48 forms part of the second, transparent layer 41 (as shown in the em- bodiment of Fig. 18 and/or 19). It covers at least part of the window 2, in particular all of the window 2.

Patch 49 is a patch independent of window 2.

The patch may also contain the bearer’s photograph 35 for an iden tification document. An example for an individualized patch path 51 is shown in Fig. 20.

A“third identifier” can be used for calculating the individualized patch path 51 from an original patch path, using one of the techniques described above for generating the individualized contour path 11 from the original contour path 10. And the same techniques can be used to test its presence and decode the data embedded therein.

The“third identifier” used for calculating the individualized patch path 51 is advantageously, but not necessarily, unique to the security document.

For example, third identifier can depend on the first and/or second identifier mentioned above. Two variants are possible:

- It can e.g. be equal to the first and/or second identifier, or, more generally, it can by an invertible function of the first and/or second identifier. Hence, the knowledge of individualized patch path 51 allows to retrieve the first and/or second identifier (and vice versa). This function can e.g. be a mathematical function or it can be a look-up-function implemented as a database.

- It can be or contain a hash value of the first and/or second identifier, i.e. it is a non-invertible function of the serial number, or the first and/or second identifier can be or contain a hash value of the third identifier. Advantageously, a cryptographic hash function is used for calculating the hash value. This allows a third party to test if a given third identifier matches the first and/or second identifier, or vice versa, but not to manufacture new combinations. Using a hash function also may have the advantage of reducing the amount of data that has to be encoded in one of the individualized paths.

Using a database for matching the third identifier to the first and/or second identifier has again simplifies the use of separate manufacturing steps for the patch and for the rest of the document because the association between the identifiers can be established at a later time.

The individualized patch path 51 may encode the denomination of the security document and/or its serial number or a hash value thereof.

Different contours on opposite sides:

Figs. 23 and 24 shows an embodiment similar to the one of Fig. 2, where two non-transparent layers 3a, 3b are arranged on opposite sides of a transparent layer 4.

The non-transparent layers 3a. 3b may e.g. be paper (as in the embodiment of Fig. 2) or ink. The transparent layer 4 may e.g. be a polymer.

Openings 60a, 60b are formed in each of the non-transparent layers 3a, 3b. The openings 60a, 60b partially overlap and form, at their intersection, the window 2 of the document.

The two openings 60a, 60b have contours, i.e. edges, 8a, 8b. These contours 8a, 8b are different in the sense that they follow different paths.

The contours 8a, 8b intersect at a plurality of locations along the edge of window 2, e.g. at the locations 61a, 61b, 6 lc of Fig. 24. Hence, at some places along the edge of window 2, the first contour 8a lies closer to the center of window 2 while, at other places, the second contour 8b lies closer to the center of window 2.

Depending on illumination and viewing conditions, one or both of the contours 8a, 8b may be visible. This is illustrated in Figs. 25, 26,

For example, as illustrated in Fig. 26, window 2 can viewed in transmission with light of a wavelength at which transparent layer 4 is transparent but the non-transparent layers 3a. 3b are not transparent, e.g. at a wavelength in the visible spectrum. In this case, an observer or camera is located on one side of the docu- ment and a light source is located on the other side, and the observer or camera observes the light from the source propagating through the window. Hence, the window 2 will appear bright, and its edge or contour 8 is given by the intersection of the two openings 60a, 60b. In this case, the observer or camera sees parts of both contours 8a and 8b.

In another example, as illustrated in Fig. 26, window 2 can be viewed in reflection with light of a wavelength at which transparent layer 4 is absorb- ing and at least one of the non-transparent layers 3a, 3b (namely the one facing the camera and light source) is reflecting, e.g. at a wavelength in the near ultra-violet spectral range. In this case, the observer or camera is located on the same side of the document as the· light source, and the observer or camera observes the light reflected from the document. Hence, the window 2 will appear black at all locations not cov- ered by the non-transparent layer 3a, 3b on the side of the camera or observer, e.g. first non-transparent layer 3a, while the other non-transparent layer (e.g. second non transparent layer 3b) will remain invisible. The camera or window will only see one of the contours 8a, 8b, e.g. first contour 8a.

By comparing the two images taken in this maimer, it can be checked if the two contours 8a, 8b are present, in a sufficiently accurate register, on opposite sides of the transparentlayer 4. For example, Fig. 27 shows the areas where the contours of the central window region differ between the images as recorded from Fig. 25 and 26. Authenticity can e.g. be checked by comparing the differences between the two images to an expected image.

In more general terms, the method for manufacturing a set of security documents can comprise the steps of

calculating, for the document, a first and a second contour path, in particular (but not necessarily) a first and a second individualized contour path, wherein said first and second contour paths intersect in several locations 61a, 61b, 61c,

forming a first opening 60a in a first non-transparent layer 3a along the first contour path,

forming a second opening 60b in a second non-transparent layer 3b along the second contour path,

arranging the first and second non-transparent layers 3a, 3b on op posite sides of a transparent layer 4.

(The order of these steps may vary depending on the manufacturing process involved.)

The invention also relates to a security document generated in this manner. Such a security document e.g. comprises

a first non-transparent layer 3 a having a first opening 61a with a first contour (edge) 8 a, a second non-transparent layer 3b having a second opening 61 b with a second contour (edge) 8b,

a transparent layer 4 arranged between the two non-transparent layers 3 a, 3 b,

wherein the first and second contours 8a, 8b intersect in several lo- cations 61a, 61 b, 61c,

Advantageously, in at least some places, the first and second contour paths, i.e. the contours 8a, 8b, deviate by at least 10 pm in order to render the differences well visible at least for camera-based authentication equipment. In particular, there are at least some locations where the first contour path lies within the second contour path by at least 10 pm and/or there are at least some locations where the sec ond contour path lies within the first contour path by at least 10 pm.

Advantageously, the first and second contour paths deviate at no place by more than 100 pm in order to make the feature hard to see by the unaided eye.

Advantageously, the register between the first and the second nontransparent layer is better than 100 pm, in particular better than 10 pm, in order to make the feature distinct on all documents of a given set of documents.

This invention may or may not use individualized contour paths, i.e, it may also be used independently of the features of claim 1.

A method for checking an authenticity of a security document of this types with a carrier 1 having a window 2 can e.g. comprise the following step:

measuring at least part of the first contour 8a and at least part of the second contour 8b.

In particular, it can include the following steps

taking a first image in a first viewing mode where both said contours 8a, 8b are visible at least in part (e.g. as illustrated in Fig. 25),

taking a second image in a second viewing mode where only one of said contours 8a, 8b is visible (e.g. as illustrated in Fig. 25),

comparing the first and the second image (e.g. as illustrated in Fig.

27).

The first image can e.g. be taken in transmission at a wavelength where the transparent layer 4 is transparent and the first and second non-transparent layers 3a, 3b are not transparent. Alternatively, it may be taken in reflection at a wavelength where the layer 4 is transparent and weakly reflecting and the first and second non-transparent layers 3a, 3b are strongly reflecting. The second image can e.g. be taken in reflection at a wavelength there the transparent layer 4 is absorbing and the first and/or second non-transparent layers 3a, 3b are/is reflecting. Alternatively, it may e.g. be taken in reflection at a wavelength where the transparent layer 4 is reflecting and the first and/or second non- transparent layer are/is non-reflecting.

In addition to taking a first and a second image as described above, the method for checking the authenticity can further comprise the step of taking a third image in a third viewing mode where only the other one (i.e. not the one of the second image) of said contours 8a, 8b is visible (e.g. again as illustrated in Fig. 25, but from the other side). This allows to individually check the other contour, thereby further adding to the reliability of the method.

In the embodiment of Figs. 23, 24, the two contours 8a, 8b were both circular with a sine-modulation in radius. They differ by a phase offset as well as by radius.

Alternatively, though, the first and second contours may also differ in frequency, or they may be non-periodic, or e.g. one may be smooth where the other is modulated, and/or vice versa. Any of the contour shaping techniques described above can be applied.

Notes:

For ease of manufacturing and robustness, window 2 advanta geously has a size of at least 0.5 cm^, in particular at least 1 cm^, such as 2 cm^.

For the same reason, it advantageously has a diameter of at least 0.5 cm in all directions.

Window 2 can have rounded comers, as shown, which improves robustness of the document. Advantageously, the radius of the curvature of any comer in the window should be at least 2 , in particular at least 5 mm.

Due to inaccuracies during the manufacturing process, the whole contour of the window may be offset by a certain distance. It is to be noted that such an offset of the contour is considered to leave the calculated individualized contour path unchanged.

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not lim- ited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. A method for manufacturing a set of security documents, wherein each document comprises a carrier (1) and a window (2), said method comprising, for each document, the steps of,

calculating, for the document, an individualized contour path (1 1) for a contour (8) of said window (2), wherein the individualized contour paths (11) are varied between individual documents in visually imperceptible manner, and

forming said window (2) in said document with said individualized contour path (1 1) for its contour (8).

2. The method of claim 1 wherein said individualized contour paths (1 1) vary between any two of said documents by no more than 100 pm, in particular by no more than 50 pm and/or in at least some parts by at least 10 pm.

3. The method of any of the preceding· claims wherein a diameter of said window (2) in any direction is at least 50 times larger than a variation of said individualized contour paths (11) between any two of said documents.

4. The method of any of the preceding claims wherein said method comprises, for each document, the steps of

providing an original contour path ( 10) for said contour (8), providing a first identifier,

modifying the original contour path (10) using said first identifier for calculating said individualized contour path (1 1),

forming said window (2) using said individualized contour path (1 1 ) for said contour (8).

5. The method of claim 4 wherein a closest distance of any point of said individualized contour path (11) from said original contour path (10) is no more than 100 pm, in particular no more than 50 pm.

6. The method of any of the claims 4 or 5 wherein, in at least some parts of the individualized contour path (11), a closest distance from the original contour path (10) is at least 10 pm.

7. The method of any of the claims.4 to 6 wherein said first identifier is unique to said document.

8. The method of any of the claims 4 to 7 further comprising the step of applying, in particular printing, a second identifier onto said security document.

9. The method of claim 8 wherein said second identifier is unique to said document.

10. The method of any of the claims 8 or 9 wherein said first and said second identifier are related, and in particular wherein

said first and identifier is an invertible function of said second identifier, or

said first identifier comprises or is a hash value, in particular a cryp tographic hash value, of said second identifier or said second identifier comprises or is a hash value, in particular a ciyptographic hash value, of said first identifier.

1 1. The method of any of the claims 4 to 10 further comprising the step of establishing a database comprising said first and said second identifiers for each document.

12. The method of any of the claims 4 to 1 1 wherein said individualized contour path (1 1) is calculated such that said first identifier can be retrieved by analyzing said individualized contour path (1 1).

13. The method of any of the claims 4 to 12 wherein, along at least part of said contour (8), a deviation between said original contour path (10) and said individualized contour path (11) is a periodic function, and in particular wherein said part of said contour (8) extends over at least 10 periods of said periodic function.

14. The method of claim 13 wherein a period length and/or a change of the period length and/or an amplitude and/or a phase offset of said function depend on said first identifier.

15. The method of any of the claims 13 or 14 wherein a period length of said function is at least 10 pm and/or no more than 100 pm.

16. The method of any of the claims 13 to 15 wherein an amplitude of said function is at least 10 pm and/or no more than 100 pm.

17 The method of any of the claims 4 to 16 wherein said documents are identification documents, wherein each document has its bearer’s photograph (35), name (36) and/or biometric data (37) applied to it, and wherein said sec ond identifier is said photograph (35), name (36) and/or biometric data (37),

and in particular wherein said first identifier is an invertible func- tion or is or comprises a has value of the name (36) and/or the first identifier is or comprises a hash value of the photograph (35) and/or of the biometric data (37).

18. The method of any of the claims 4 to 17 wherein a deviation be tween said individualized contour path (1 1) and said original contour path (10) is zero in at least at two locations (31 , 3 G) of said contour (8), for all the documents of the set,

and in particular wherein said deviation is zero in at least three, in particular in at least four locations (31, 3 G) for all the documents of the set, with said locations (31, 3 ) being non-colhnear.

19. The method of any of the claims 4 to 18 wherein said documents are banknotes and said first identifier encodes a denomination of its document.

20. The method of any of the preceding claims wherein each docu- ment comprises a transparent foil (4, 41, 42) extending over at least part of said win dow (2).

21. The method of claim 20 wherein each document comprises a first, non-transparent layer (40) with an opening forming said window (2) and a sec- ond, transparent layer (41) covering at least part of said window (2) and at least part of said contour (8) and affixed on a first side of said first layer (40).

22. The method of claim 21 wherein each document further comprises a third, transparent layer (42) covering at least part of said window (2) and at least part of said contour (8) and affixed on a second side of said first layer (40).

23. The method of any of the preceding claims further comprising the steps of

applying a patch (48, 49) to each of said security documents, calculating, for each patch (48, 49), an individualized patch path (51) for an edge of said patch (48, 49), wherein the individualized patch paths (51) are varied between individual patches (48, 49) in visually imperceptible manner, shaping an edge of said patch (48, 49) according to said individualized patch path (51).

24. The method of any of the claims 21 or 22 and of claim 23 wherein said patch (48, 49) comprises said second, transparent layer (41).

25. The method of any of the preceding claims comprising the step of forming said contour (8) by cutting, in particular laser cutting, said carrier (1), and/or wherein said carrier (1) comprises at least one layer of paper.

26. The method of any of the claims 1 to 24 wherein said carrier (1) is transparent and said method comprises the step of printing said contour (8) with opaque ink (46) on said carrier (1).

27. The method of any of the preceding claims comprising the steps of

calculating, for the document, a first and a second contour path, in particular a first and a second individualized contour path, wherein said first and sec- ond contour paths intersect in several locations (61a, 61b, 61c),

forming a first opening (60a) in a first non-transparent layer (3 a) along the first contour path,

forming a second opening (60b) in a second non-transparent layer (3b) along the second contour path,

arranging the first and second non-transparent layers (3a, 3b) on opposite sides of a transparent layer (4).

28. A set of security documents manufacturable, in particular manufactured, by the method of any of the preceding claims.

29. A set of security documents, in particular the set of claim 28, wherein each security document comprises a carrier (1) and

a window (2) arranged in said carrier (1),

wherein, between any two of said security documents, contours (8) of said windows (2) differ in. visually imperceptible manner.

30. A method for checking an authenticity of a security document with a carrier (1) having a window (2), in particular of a security document of any of the claims 28 or 29, said method comprising the step of

measuring at least part of a contour (8) of said window (2), and testing for the presence of deviations of said contour (8) from an original contour path (10).

31. The method of claim 30 comprising the step of obtaining at least part of said original contour path (10) by low-pass filtering of the measured con- tour (8).

32. The method of any of the claims 30 or 31 comprising the step of high-pass filtering of at least part of the measured contour (8).

33. The method of any of the claims 30 to 32 comprising the step of calculating a Fourier descriptor or a Fourier transform of at least part of said contour (8)·

34. The method of any of the claims 30 to 33 comprising the step of recording, in addition to said at least part of said contour (8), a feature (30) different from said window (2) on said security document and deriving said deviations using a position of said feature (30).

35. The method of any of the claims 30 to 34 comprising the step of measuring said at least part of said contour (8) by means of a camera and/or by means of a scanner and/or by means of a touch screen.

36. A data processing device comprising means for carrying out the method of any of the claims 30 to35.

37. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any of the claims 30 to 35.

38. A computer-readable data carrier having stored thereon the computer program of claim 37.