WO2013143006A1

WO2013143006A1 - Perforation-based security feature

Info

Publication number: WO2013143006A1
Application number: PCT/CH2012/000070
Authority: WO
Inventors: Martin Eichenberger; Dieter Sauter
Original assignee: Orell Füssli Sicherheitsdruck Ag
Priority date: 2012-03-29
Filing date: 2012-03-29
Publication date: 2013-10-03

Abstract

The security system comprises a first security element (5) in the form of a perforation arranged in a first substrate section (7) and a second security element (6) that e.g. carries microletters in a second substrate section (8). When the two substrate sections (7, 8) are brought to an overlap, a recognizable motif is generated, such as an enlarged version of the microletters. The system is especially suited for security documents, such as banknotes, identification documents or documents of value.

Description

Perforation-based security feature

Technical Field

The invention relates to the field of security systems, such as they are used in combination with security documents, such as banknotes, passports, documents of value, certificates, etc. In particular, the invention relates to a security system comprising a first substrate section, a first security element arranged in said first substrate section, a second substrate section, a second security element arranged in said second substrate section, wherein said first and said second security element are adapted to form a visually perceptible motif when overlaid.

Background Art

WO 2006/131831 and US 2010/0314861 describe security systems that are based on two substrate sections having differing structures printed thereon. When the substrate sections are overlaid, a new visually perceptible motif is created. This type of system can be used e.g. as a self- verifying security feature on a security document if the two substrate sections form part of the same document.

Disclosure of the Invention

It is an object of the present invention to provide a security system of the above type with improved security.

This object is achieved by the security system of claim 1. Accordingly, at least the first element comprises a first perforation with a plurality of holes extending through said first substrate section.

In an advantageous embodiment, the second substrate section comprises a transparent window and the second security element is arranged at least partially on said transparent window. This provides better visibility of the motif. In this context, the term "transparent" designates a window that has a relative transmission of at least 50%, in particular at least 80%, for at least one wavelength in the UV-VIS- NIR spectral range between 300 nm and 10 μηι, in particular in the visible spectral range between 400 nm and 800 nm. In another advantageous embodiment, the first and the second security element are periodic in at least one direction. As shown below, this allows to generate remarkable Moire-type effects. In addition, a perforated security element that is periodic in at least one dimension is easy to manufacture by means of laser perforation. For this purpose, a laser beam repetitively traverses the given substrate section along a first direction while the substrate section is advanced in a second direction, with the first and the second directions being transversal, in particular perpendicular, to each other.

Advantageously, in this case, the first perforation is periodic in a first direction and non-periodic in second direction. Such structures are, again, easily manufactured by means of a laser beam in the manner described in the previous paragraph, with the laser beam being switched on and off at irregular intervals. On the other hand, such structures are difficult to manufacture by mechanical means.

In a further advantageous embodiment, the first and the second security element are non-periodic in at least one direction. This can help to hide any traces of the motif when the first and the second security document are not in their overlaid position.

The second security element can e.g. comprise a printed pattern. Alternatively, or in addition thereto, it can comprise a second perforation with a plurality of holes extending through said second substrate section.

The invention also relates to a set of security documents comprising the security system described above. The set includes at least a first and a second security document. The above first substrate section is part of the first security document and the second substrate section is part of the second security document. Hence, the first security element is arranged on the first security document and the second security element is arranged on the second security document. However, advantageously, on the first security document, there is no second security element suitable to generate the motif with the first security element, and/or vice versa. In this case, the verification requires the first as well as the second security document.

For example, the first and the second security document are banknotes of differing denominations.

Finally, the invention relates to a method for verifying the above security system. This method comprises the steps of overlapping said first and said second security element and checking for the appearance of said motif. If the motif does not appear properly, the security system is rejected as invalid. 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. Such description makes reference to the annexed drawings, wherein:

Fig. 1 shows a banknote as an example of a security document comprising a security system,

Fig. 2 shows a first example of a second security element,

Fig. 3 shows a first example of a first security element adapted to cooperate with the security element of Fig. 2,

Fig. 4 shows the security elements of Figs. 2 and 3 in overlap, Fig. 4 shows the motif generated by the overlap of Fig. 3,

Fig. 5 shows the dimensions of a hole of the perforation,

Fig. 6 shows the dimensions of a part of the second security element,

Fig. 8 shows the distances between holes of the first security element,

Fig. 9 shows a second example of a second security element, Fig. 10 shows a second example of a first security element adapted to cooperate with the security element of Fig. 9,

Fig. 11 shows the security elements of Figs. 9 and 10 in overlap, Fig. 12 shows a checkerboard pattern generated by two different dye layers,

Fig. 13 shows examples of the first security element,

Figs. 14 - 17 show further examples of the first security element,

Fig. 18 shows another embodiment of a security document with the first and the second security element,

Fig. 19 shows the motif generated by an overlap of the security elements of Fig. 18 under a first angle of 0°,

Fig. 20 shows the motif generated by an overlap of the security elements of Fig. 18 under a second angle of 3°,

Fig. 21 shows a further example of a security document to be folded about a folding line non-perpendicular to the edges of the security document,

Fig. 22 shows a set of security documents,

Fig. 23 shows a security system with two first and two second security elements, Fig. 24 shows a security system using elongate holes,

Fig. 25 illustrates the effect of the system of Fig. 24 under a first viewing direction,

Fig. 26 illustrates the effect of the system of Fig. 24 under a second viewing direction,

Fig. 27 shows a further example of a first security element,

Fig. 28 shows a second security element adapted to cooperate with the security element of Fig. 27,

Fig. 29 shows the motif generated by an overlap of the security elements of Figs. 27 and 28,

Fig. 30 shows a sectional view of a first security document,

Fig. 31 shows a sectional view of a second security document, and

Fig. 32 shows a sectional view of a third security document.

Modes for Carrying Out the Invention Definitions:

The term "system" relates to an apparatus in the broadest sense, not to a method. The present security system can e.g. be formed by parts of one or more security document(s) or by a security document and a testing device.

The term "paper" includes paper based on wood pulp as well as on cotton.

Fig. 1 shows a banknote as an example of a security document. The document comprises a substrate 1, which can be of any type known to the skilled person, such as a paper substrate or a plastics substrate.

Substrate 1 carries a plurality of markings, as known to the skilled person, such as security features 2, artwork 3 and textual information 4. In addition, the document comprises a security system formed by at least a first and a second security element. In the example of Fig. 1, there are two first security elements 5, 5' and two second security elements 6, 6'. The first security elements 5, 5' are arranged in first substrate sections 7, T and the second security elements 6, 6' are arranged in second substrate sections 8, 8' of substrate 1.

Examples of a first security element 5 (or 5') and a second security element 6 (or 6') are shown in Figs. 2 and 3. First security element 5 comprises a number of holes 9 forming a perforation. Each hole of the perforation extends all the way through is substrate section 7 and through substrate 1. The holes have small diameter, advantageously in a range below 200 μπι, in particular between 50 μπι and 200 μιη. In order to provide good contrast between the perforated and non-perforated parts, first substrate section 7 is advantageously non-transparent. In the present text, the term "non-transparent" designates a substrate section or other structure that has a relative transmission of less than 50%, in particular less than 20%, for at least one wavelength in the UV-VIS-NIR spectral range between 300 nm and 10 μιη, in particular in the visible spectral range between 400 nm and 800 nm.

In the example of Fig. 3, the holes 9 are arranged in a regular two- dimensional array. The hole spacings ax, ay along the two primary directions x, y of the array can be equal or non-equal.

Second security element 6 comprises a number of repeating symbols 10, e.g. in the form of a letter "F" as shown. In the embodiment of Fig. 2, the symbols are printed on second substrate section 8, in a color or luminance of sufficient contrast to its surroundings such that the second security element 6 is readily recognizable. Advantageously, the symbols 10 are small, e.g. formed by microwriting with letters of a height of less than 2 mm, such that they are hard to discern.

For improved contrast, and as shown in Fig. 1, second substrate section 8 comprises a transparent window 11, and second security element 6 is arranged at least partially on said transparent window. In this case, second security element 6 should be non-transparent.

Alternatively, the symbols 10 can be formed by transparent regions in an otherwise non-transparent second substrate section 8.

In the example of Fig. 2, the symbols 10 are arranged in a regular two-dimensional array. The symbol spacings sx, sy along the two primary directions x, y of the array can be equal or non-equal.

The first and second security elements 5 and 6 of Fig. 1 differ from the first and second security elements 5' and 6' by their spacings sx and sy, such that they give rise to similar effects at different sizes factors.

The first and second security elements 5, 5' and 6, 6' can be brought to an overlap by folding the document along a folding line 14 (Fig. 1). By doing so, each first security element 5 (or 5') comes to rest on top of each its matching second security element 6 (or 6'), as it is illustrated Fig. 4. When viewing this structure from the side of first security element 5, only a part of second security ele- ment 6 will be visible through the holes 9. This part forms the motif 14 as shown in Fig. 5.

As can be seen, for the present embodiment, motif 14 shows a zoomed version of symbol 10. This is due to the fact that the spacings sx, ax and sy, ay differ by a small amount. In that case, the holes 9 "scan" different locations of the symbols 10, thereby generating a lens effect.

To achieve such a zooming, the ratio a s of the spacing s of the symbols and the spacing a of the holes should, in the overlaid position of the two security elements 5, 6 and along at least one direction, be in the range [0.5, 0.99] or [1.01, 2]. Large zooming is achieved if a/s is close to 1.

This effect is observed best if the dimensions of the holes 9, as compared to the structures of the symbols 10, are small. This is illustrated in Figs. 6 and 7 and can generally be described as follows: The second security element comprises line or dot structures (such as the lines of the letter F) that, at least in one direction, have a width b larger than the diameter d of the holes 9 of the first security element 5.

In addition, in order to be able to scan reasonably large symbols 10, the holes 9 should not be too close together. This is illustrated in Fig. 8, which shows the median distance a (corresponding to the above ax, ay) between the centers of the holes 9 in relation to the median diameter d of the holes 9 in the same direction. For good scanning properties, the following should apply: d < 0.2 · a.

Another embodiment of first and second security elements 5, 6 is shown in Figs 9 and 10. Here, first security element 5 is again a series of holes. (In contrast to Fig. 3, Fig. 10 shows the first substrate section in black and the holes in white color.) Second security element 6 is a checkerboard pattern. Fig. 11 shows the motif generated when overlaying first security element 5 and second security element 6. As can be seen, an enlarged part of the checkerboard pattern is shown.

In a particularly useful embodiment of the invention, second motif 6 is created by a combination of different dye layers. In this respect, the term "dye layer" represents a layer of dye applied in a single printing step, and the term "different dye layers" designates layers applied in different printing steps. For example, one dye layer can be applied in an offset printing step, and the other can be applied in an intaglio printing step or screen printing step. Since it is difficult to apply the different printing steps in register (i.e. in alignment), the degree of alignment of the different dye layers in an indication of the printing quality, and a misalignment is indicative of a counterfeited product. Hence, if the security element 6 is crated from different dye layers, the motif generated by an overlay of the first and second security elements 5 and 6 will be distorted if the dye layers are not in register. For example, the checkerboard pattern of the second security element of Fig. 9 can be built up from squares of different day layers 16, 17, as shown in Fig. 12.

In more general terms, in an advantageous embodiment, the second security element comprises distinct, advantageously non-overlapping, areas 16, 17 covered by at least two different dye layers. In particular, the dye layers are from different members of the group comprising offset dye layers (i.e. dye layers applied by means of offset printing), intaglio dye layers (i.e. dye layers applied by means of intaglio printing) and screen dye layers (i.e. dye layers applied by means of screen printing). For better visibility of the motif, the second security element 6 is again applied to a window 11, with transparent regions 18 of the window being interspersed with the distinct areas 16, 17, as shown in Fig. 12.

In yet a further embodiment, the distinct areas 16, 17 can be applied to different sides of the window 11. This allows to check the register between the printing processes that were applied to the opposite sides of substrate 1. A further embodiment of this type is shown in more detail below.

The arrangement of the holes 9 can be varied in numerous ways. Fig. 13 shows several possible arrangements of the holes 9:

- The holes 9 form a regular array with equal spacing along x and y

(Fig. 13A)

- The holes 9 form rows, with the hole spacing ax along the row being smaller, advantageously at least twice as small, than the spacing ay between the rows (Fig. 13B). As shown, the holes can even touch, thereby forming continuous lines.

- The holes 9 can form rows along one direction, while they are mutually offset when comparing the rows (Fig. 13C).

- The holes 9 can form at least two different row patterns, one with a smaller hole spacing than the other (Fig. 13D).

- The holes can enclose a non-perforated symbol (Fig. 13E) or they can form a perforated symbol (Fig. 13G).

- The holes can be continuous stripes (Fig. 13H).

The shapes of the holes can vary. In most of the above examples, the holes had circular cross-section. Figs. 14 - 17 show some alternatives and further embodiments:

The holes can be elongate (Fig. 14). 12 000070

8

The holes can be a mix of elongate and circular holes (Fig. 15). The holes can be star-shaped (Fig. 16) or have any other polygonal shape.

The holes can be formed along rows extending non- perpendicular to the edges of substrate 1 (Fig. 17).

The present invention can also be applied using the techniques shown in US 2010/0314861. In particular, the holes 9 of the perforation may form lines, as shown in Fig. 18. Such lines may be continuous lines of non-varying thickness (as shown in Fig. 13H), dotted lines of touching holes (as shown in Fig. 13B or 13C) or dotted lines of non-touching holes. When overlaying such lines with a second security element 6 having repeating symbols 10, that second security element is "scanned" along a direction perpendicular to the lines, which allows e.g. to obtain a zoom-effect when, in overlay, the lines of the repeating symbols 10 are parallel to the lines formed by the holes 9, such as shown in Fig. 19. When folding the document along a an oblique folding line, thereby generating an overlap of the two security elements 5, 6 under an angle, a characteristic distortion such as shown in Fig. 20 is observed.

In general, and as shown in Fig. 21, the first and second security elements 5, 6 can be arranged on substrate 1 at positions symmetric to a folding line 14 that extends non-perpendicular and non-parallel to the edges 20 of the substrate. To achieve this, the locations of the two security elements 5, 6 should be substantially symmetric to such a folding line. Such an oblique arrangement renders counterfeiting even more difficult. In particular, a counterfeiter may not even be able to detect that the security system exists.

In yet a further embodiment, the invention relates to a set of security documents as described above. Such a set is shown in Fig. 22. The shown embodiment comprises several banknotes 22, 23, 24 of the same series but with differing denominations. One denomination (e.g. banknote 23) carries a first security element 5 structured to verify the second security element 6 on a different denomination (banknote 22). Similarly, a third denomination (banknote 24) may carry the first security element 5' structured to verify the second security element 6' on one of the first denominations (banknote 23), while there may be a further matching pair 5", 6" of security elements on the remaining two denominations (banknotes 22 and 24) in order to "close the circle". If a single banknote does not contain matching first and second security elements 5, 6 suitable to generate the motif with each other, a verification always needs two different documents. This can e.g. also be of advantage for "key type" systems, where one document forms a key and the other a lock. In general, and also as embodied in Fig. 22, one document may form a testing device for verifying another document. For example, banknote 23 forms a testing device for banknote 22. The testing device can e.g. also be of a different nature than the tested document. For example, the testing device can be formed by a foil which is to be overlaid with a banknote to be tested. The foil in that case comprises one of the security elements, while the banknote comprises the other.

Yet another security system is shown in Fig. 23. Here, substrate 1 comprises at least two non-adjacent first security elements si 1 and sl2 and at least two non-adjacent second security elements s21 and s22. They are arranged such that, when folding the document along folding line 14, security element si 1 can be brought to overlap with security element s21 and security element sl2 can be brought to overlap with security element s22 at the same time.

Fig. 24 shows an embodiment of the system where the first security element 6 comprises elongate holes 9, 9', similar to the embodiments if Figs. 14 and 15. In general, for achieving strong effects, the maximum diameter of the holes (i.e. the diameter along the longitudinal extension of the holes) should advantageously be at least 1.5 times, in particular at least 2 times, in particular at least 10 times, larger than a minimum diameter of the elongate holes.

If first substrate section 7 is sufficiently thick, in particular at least equal to the minimum diameter of said holes, a strong tilting effect is achieved, as taught by WO 2004/011274.

This can e.g. be exploited by the system shown in Fig. 24, where first security element 5 comprises a first and a second set 26, 27 of elongate holes with non-parallel, in particular perpendicular, axes of maximum diameter. (Alternatively, and as explained by WO 2004/011274, the first security element may also comprise a combination of elongate holes and non-elongate holes.) If a perforation of this type is overlaid with a second security element 6 and viewed from an angle oblique to the plane of the security elements 5, 6, a rotation of the security elements 6 about an axis perpendicular to their plane will alternatively reveal, to the viewer, different motifs, as illustrated in Figs. 25 and 26. In general, when viewing from an oblique angle, a hole whose maximum diameter points towards the viewer will have larger transmission than a hole whose minimum diameter points towards the viewer.

Hence, in the situation of Fig. 25, the viewer will be well able to view through the set 26 of the holes, but not through the set 27, hence he will see a motif created by the overlay of set 26 of the holes with section 28 of second security element 6. On the other hand, in the situation of Fig. 26, the viewer will be able to view through the set 27 of the holes and therefore see a motif created by the overlay of set 27 of the holes with section 29 of the second security motif 6.

Yet a further system is shown in Figs. 27 and 28. Fig. 27 shows first security element 5 (with the holes 9 being shown as black dots and first substrate 7 as white background), while Fig. 28 shows second security element 6. Fig. 29 shows the motif generated by the overlay of the two security elements. Even though no global motif is visible from the two security elements 5, 6 individually, their combination reveals a global motif in the form of a cross. This is due to the fact that, within the contours of the motif (shown with dotted lines 29 in Figs. 27 and 28), the holes 9 of first security element 5 and the symbols 10 of second security element 6 coincide, while they do not coincide outside the contours of the motif. Hence, the symbols 10 inside the motif will appear in the overlay, while the symbols 10 outside the motif will be hidden.

In order to hide the motif in the first and second security elements 5, 6, the spacing of the holes 9 and symbols 10 in the first and second security elements 5, 6 should, in at least one direction, be non-periodic. In more general terms, and as mentioned, the first and the second security element 5, 6 should advantageously be non-periodic in at least one direction.

As mentioned above, second security element 6 can be a printed pattern. However, it can also be applied by other means, e.g. by lamination. Or it can be formed by a second perforation. For example, the second motif 6 of Fig. 28 can be formed by a second perforation.

Fig. 30 shows a cross-sectional view through yet another embodiment of a security system. The figure shows the holes 9 of first security element 5. They extend through first substrate section 7 of substrate 1 as well as a dye layer 30. Dye layer 30 is advantageously non-transparent and dark in order to improve the contrast of the motif.

Second motif 6 is shown symbolically as a layer 31 on top of window 11 of second substrate section 8. When folding substrate 1 along a direction as indicated by an arrow 32, the motif generated by the overlay of the holes 9 and layer 31 will appear. Assuming that layer 31 represents symbols such as "AB ABAB" and the first and second security elements 5, 6 are structured to generate the zooming effect as illustrated in Figs. 2 - 4, the resulting motif will be an enlarged version of the symbols, such as a large "AB".

In the embodiment, second security element 6 comprises a first substructure in the form of layer 31 arranged on a first side 34 of second substrate section 8 and a second substructure in the form of another layer 33 arranged on a second side 35 of first second substrate section 8, with side 35 being opposite to side 34. When substrate 1 is therefore folded in opposite direction, as indicated by arrow 37, the motif generated by the overlay of the holes 9 and layer 33 will appear. For example, if second layer 33 forms the symbols "OFOFOF", the motif will be an enlarged "OF".

The first and second substructure or layer 31 and 33 can be applied to the same window or to different windows. Advantageously, they are offset laterally, at least if second substrate section 8 is transparent.

As has already been mentioned above, the security system allows to verify the register of structures printed to the front and backside of a document. This is illustrated in Fig. 31, where first substructure 31 and second substructure 33 are applied to opposite sides 35, 36 of the same window 11. In the example of Fig. 31, first substructure 31 forms the symbols "O O 0 " on one line and " F F F" on the next line, while second substructure 33 forms the symbols " F F F" on the first line and "O O O " on the next line. When folding substrate 1 as indicated by arrow 32, the combination of both substructures 31 and 33 is viewed through the holes 9, and therefore a zoomed "OF" will appear.

Fig. 32 shows yet another embodiment, where the first security element 5 comprises holes 9 extending non-perpendicularly through the first substrate section 7. In particular, first security element 5 comprises a first set 35 of holes extending along a first direction through first substrate section 7 and a second set 36 of holes extending under a second direction through first substrate section 7, with the first and second directions being non-parallel and advantageously being under an angle of at least 30° to each other. Further, advantageously, the diameter of the holes 9 (or, for elongate holes, their minimum diameter), is smaller than the thickness of first substrate section 7. The transmission of such holes depends on the viewing direction and will, for an arbitrary viewing direction, be different for the first and second set 35, 36 of the holes.

Hence, when folding substrate 1 as indicated by arrow 32, the motif generated by the overlay of first set 35 of holes with second security element 6 is best seen from a first viewing direction, while the motif generated by the overlay of second set 36 with second security element 6 is best seen from a second viewing direction. Advantageously, second security element 6 therefore comprises first and second regions 38 and 39 with different symbols, such that the two motifs differ. For example, first region 38 can carry the letters "OFOFOF", while second region 39 can carry the letters ("ABABAB"). Depending on the viewing direction, an enlarged OF or AB will become visible. While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

1. A security system comprising

a first substrate section (7),

a first security element (5) arranged in said first substrate section

(7),

a second substrate section (8),

a second security element (6) arranged in said second substrate section (8),

wherein said first and said second security element (6) are adapted to form a visually perceptible motif (14) when overlaid,

characterized in that said first security element (5) comprises a first perforation with a plurality of holes (9) extending through said first substrate section (7).

2. The security system of claim 1 wherein said second substrate section (8) comprises a transparent window (11) and said second security element (6) is arranged at least partially on said transparent window (11).

3. The security system of any of the preceding claims wherein said second security element (6) comprises a first substructure (31) arranged on a first side (34) of said second substrate section (8) and a second substructure (33) arranged on a second side (35) of said second substrate section (8), with said second side (35) being opposite said first side (34).

4. The security system of any of the preceding claims wherein the first and the second security element (6) are periodic in at least one direction.

5. The security system of claim 4 wherein a spacing of said holes (9) is periodic in a first direction and non-periodic in second direction.

6. The security system of any of the preceding claims wherein said first and the second security element (6) are non-periodic in at least one direction.

7. The security system of any of the preceding claims wherein said second security element (6) comprises line or dot structures having, at least in one direction, a width (b) larger than a diameter (d) of the holes (9) of said first security element (5).

8. The security system of any of the preceding claims wherein the holes (9) of said first security element (5) have, in at least one direction, a median diameter b and a median distance a, with d < 0.2 · a.

9. The security system of any of the preceding claims wherein said holes (9) are arranged in rows, wherein a spacing ax between the holes (9) along a tow is smaller than a spacing ay between rows, and in particular wherein ax is at least twice as small as ay.

10. The security system of any of the preceding claims wherein said first security element (5) comprises elongate holes (9) in said first substrate section (7), wherein said elongate holes (9) have a maximum diameter that is at least 1.5 times, in particular at least 2 times, in particular at least 10 times, larger than a minimum diameter of said elongate holes (9).

11. The security system of claim 10 wherein a thickness of said first substrate section (7) is at least equal to said minimum diameter.

12. The security system of claim 10, wherein said first security element (5) comprises

- elongate holes (9) and non-elongate holes (9) and/or

- a first and a second set of elongate holes (9) with non-parallel, in particular perpendicular, axes of maximum diameter.

13. The security system of any of the preceding claims wherein said first security element (5) comprises holes (9) extending non-perpendicularly through said first substrate section (7), and in particular wherein said first security element (5) comprises a first set (35) of holes (9) extending along a first direction through said first substrate section (7) and a second set (36) of holes (9) extending along a second direction through said first substrate section (7), with said first and second directions being non-parallel.

14. The security system of any of the preceding claims wherein said second security element (6) comprises a second perforation with a plurality of holes (9) extending through said second substrate section (8).

15. The security system of any of the preceding claims wherein said second security element (6) comprises a printed pattern.

16. The security system of any of the preceding claims comprising at least two non-adjacent first security elements si 1 and si 2 and at least two non- adjacent second security elements s21 and s22, wherein said security system is structured such that said security element si 1 can be brought to overlap with said security element s21 and said security element sl2 can be brought to overlap with said security element s22 at the same time.

17. The security system of any of the preceding claims wherein said first substrate section (7) and said second substrate section (8) are parts a single security document.

18. The security system of claim 17 wherein said first and second security elements (5, 6) are arranged on the security document at positions symmetric to a folding line (14) that extends non-perpendicular to edges of said document.

19. The security system of any of the claims 1 to 16 wherein said first substrate section (7) is part of a security document (22) and wherein said second substrate section (8) is part of a testing device (23) separate from said security document, or vice versa.

20. The security system of any of the preceding claims wherein the second security element (6) comprises distinct areas (16, 17) covered by at least two different dye layers.

21. The security system of claim 20 wherein said different dye layers are different members of the group comprising offset dye layers, intaglio dye layers and screen dye layers.

22. The security system of any of the claims 20 or 21 wherein said second motif (14) is arranged on a window (11) in said second substrate section (8) (8) and wherein transparent regions (18) of said window (11) are interspersed with said distinct areas (16, 17).

23. A set of security documents comprising the security system of any of the preceding claims, wherein said set includes at least a first and a second security document (22, 23, 24), wherein said first substrate section (7) is part of said first security document (22) and said second substrate section (8) is part of said second security document (23), and in particular wherein on the first security document (23) there is no second security element (6) suitable to generate the motif (14) with the first security element (5).

24. The set of security documents of claim 23 wherein said first and said second security documents (22, 23, 24) are banknotes of differing denominations.

25. A method for verifying the security system of any of the preceding claims comprising the steps of overlapping said first and said second security elements (5, 6) and checking for the appearance of said motif (14).