Technical Field

The invention relates to the field of security documents, such as banknotes, passports, documents of value, certificates, etc. In particular, the invention relates to a security document having a substrate and a microperforation, wherein said substrate comprises several laminated substrate layers. The invention also relates to a method for manufacturing such a security document.

Background Art

WO 2004/076198 describes a security document having a substrate with several layers. The document further proposes to provide the substrate with a microperforation, as it is known in security document technology.

Disclosure of the Invention

It is an object of the present invention to provide a security document of this type having improved security properties, as well as a method for manufacturing such a security document.

This object is achieved by the security document of claim 1. Accordingly, the microperforation comprises holes extending through only a part of said substrate layers, i.e. there are at least some holes that extend through at least one, but not through all, the substrate layers.

In the present text, the term "microperforation" designates a perforation comprising holes that have, at least in one direction parallel to the substrate's surface, a diameter of 500 μηι or less. Such microperforations have visual properties that differ strongly from macroscopic windows. In particular, they are hard to see and, in order to be visually perceptible, they are often applied in groups. Further, even if they can be seen, the human eye is typically unable to distinguish any substructure within the hole. In contrast to this, a macroscopic window, which e.g. has a diameter of 1 cm or more, can contain a substructure (e.g. a printed pattern on a transparent foil spanning the opening) that can be resolved by the human eye.

Advantageously, at least some of the holes have, at least in one dimension parallel to the substrate's surface, a diameter of 150 μπι or less. This type of holes is hard to see in reflection, but still well visible in transmission.

Typically, the security document can have at least one region (which may extend over part of the document or over all of it) where it has at least a first and a second substrate layer. In this region, the microperforation comprises holes extending through said first substrate layer but not through said second substrate layer, i.e. the holes stop at the second substrate layer.

Advantageously, the second substrate layer is transparent in at least part of said region. In this case, the perforations can be viewed through the second substrate layer.

The second substrate layer can comprise non-transparent structures in part of said region. This allows to check the alignment of the second substrate layer in respect to the first holes. Advantageously, the non-transparent structures of the second substrate layer can block light transmission through part of said holes in said first substrate layer. In this case, the authenticity of the document can be checked by examining if and how certain of said holes are blocked, in particular when viewing the document in transmission.

If the non-transparent structures of the second substrate layer have, in at least one direction parallel to a surface of said substrate, a diameter of 1 mm or less, in particular of 500 μιη or less, an exact alignment of the second substrate layer and the holes becomes very important in order to achieve a given effect. Such exact alignment is technically difficult to achieve and therefore makes an imitation of the document harder.

The non-transparent structures of the second substrate layer can be formed, at least in part, by a dye layer that is part of the second substrate layer.

The microperforation can further comprise, in said region, second holes extending through the second substrate layer but not through the first substrate layer. Hence, the alignment of the first and second holes can be checked. The second holes can form, between them, at least part of the non-transparent structures of the second substrate.

If the first substrate layer comprises first holes and the second substrate layer comprises second holes, a first part of the first holes can be aligned with a part of said second holes, while a second part of the first holes is not aligned with any of the second holes. In this case, some of the first holes are not blocked, and some are, thereby generating a characteristic pattern which can be used to check the alignment of the first and second holes.

The first and second substrate layers can be adjacent to each other, or they can e.g. be separated by a third substrate layer arranged between them.

In general, the substrate can comprise, in said region, at least a third substrate layer.

The security document can be any kind of document that should be hard to counterfeit. In particular, it can be a banknote, an identification document such as a passport, a cheque, a voucher or any other document of value.

A document of this type is advantageously manufactured by either or both of the following steps:

(a) perforating the first substrate layer for generating at least part of said first holes and then laminating the first and the second substrate layer, and/or

(b) laminating the first and said second substrate layer and then perforating the first substrate layer for generating at least part of the first holes by means of laser irradiation at a given wavelength, wherein the optical absorption coefficient of the first substrate layer at said wavelength is at least twice as high as the optical absorption coefficient of the second substrate layer at said wavelength.

Step (a) takes advantage of the fact that it is easy to manufacture the holes while the substrate layers are still apart.

Step (b), on the other hand, uses difference of absorption between the two layers, which allows to selectively perforate one but not the other layer. For such a scheme to work, the difference of the absorption coefficients between the layers should be sufficiently large, i.e. the absorption coefficient of the first substrate layer should be at least twice, in particular ten times, as large as the absorption coefficient of the second substrate layer at the wavelength of the laser radiation.

The first and the second substrate layer can be laminated directly to each other, or with one or more intermediate substrate layer(s) between them.

The present invention also relates to a method for checking the authenticity of said document by examining an optical transmission of the holes. For this purpose, the document can e.g. be held against a light source and be viewed in transmission, and then it can be verified if an expected motif appears.

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,

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

Fig. 3 shows a sectional view of a second embodiment,

Fig. 4 shows an example of a pattern of non-transparent structures,

Fig. 5 shows an example of a perforation pattern,

Fig. 6 shows the patterns of Figs. 4 and 5 in overlay,

Fig. 7 shows the motif generated by the overlay of Fig. 6,

Fig. 8 shows the dimensions of a hole of the perforation,

Fig. 9 shows the dimensions of a part of the symbol pattern,

Fig. 10 shows the distances between holes,

Fig. 11 shows examples of hole patterns,

Figs. 12 -15 show further examples of hole patterns,

Fig. 16 shows a sectional view of a third embodiment,

Fig. 17 shows a sectional view of a fourth embodiment,

Fig. 18 shows a sectional view of a fifth embodiment and

Fig. 19 shows a sectional view of a sixth embodiment.

Modes for Carrying Out the Invention Definitions:

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

The term "adjacent" in respect to substrate layers, such as "a first substrate layer is adjacent to a second substrate layer" designates that two substrate layers are adjacent in a direction perpendicular to the substrate. However, adjacent layers may be separated by a thin intermediate layer (such as an adhesive or a dye layer) having a thickness that is much smaller (in particular at least 10 times smaller, in particular at least 50 times smaller) than each of the substrate layers.

The term "transparent" designates a substrate layer 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 μιη, in particular in the visible spectral range between 400 nm and 800 nm The term "non-transparent" designates a substrate layer, dye layer 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 μπι, in particular in the visible spectral range between 400 nm and 800 nm

The term "at least partially transparent" designates a substrate layer that has a relative transmission of at least 20%, in particular at least 50%, for at least one wavelength in the UV-VIS-NIR spectral range between 300 nm and 10 μπι, in particular in the visible spectral range between 400 nm and 800 nm

The term "aligned" in respect to holes, such as "two holes are aligned", designates that the two holes are coaxial, i.e. they have a common central axis. If the holes are offset by a small amount, such that their cross sections overlap to at least 50%, in particular at least 80%, the holes are still considered to be aligned.

Embodiments:

Fig. 1 shows a banknote as an example of a security document. The document comprises a substrate 1 carrying a plurality of markings, as known to the skilled person, such as security features 2, artwork 3 and textual information 4.

In addition, in at least one region 5, substrate 1 carries a microperfo- ration 6.

Fig. 2 shows a sectional view illustrating the structure of the security document in region 5. As can be seen, substrate 1 comprises at least two substrate layers 7, 8. Microperforation 6 comprises a plurality of first holes 9, which extend through a first substrate layer 7, but not through the adjacent second substrate layer 8.

In the embodiment of Fig. 2, first substrate layer 7 is non- transparent. It may e.g. be a non-transparent paper layer, and/or it may comprise a non-transparent dye layer 10, with the first holes 9 extending through dye layer 10. Such a dye layer 10, in particular of dark colour, can enhance the contrast of the holes 9.

The holes 9 are arranged in a pattern. In the embodiment of Fig. 2, this pattern is, at least along a direction x (the "first direction"), periodic.

Second substrate layer 8 is, in the shown embodiment, transparent. It may e.g. be of a plastic material. It carries, at one surface, a structured, non- transparent dye layer 11 forming non-transparent structures 12 in at least part of region 5. The non-transparent structures 12 block light transmission through part of the first holes 9, thereby generating, in transmission, a motif that can be verified. In the embodiment of Fig. 2, the non-transparent structures 12 are periodic at least in the direction x. The spacing of the structures 12 can be equal to or non-equal to the spacing of the holes, as explained in more detail below.

Fig. 3 shows an alternative embodiment. Here, second substrate 8 carries second holes 14, which also form part of the microperforation. Advantageously, second substrate layer 8 is non-transparent, with the second holes 14 forming transparent parts therein. The remaining parts of second substrate layer 8 form non-transparent structures 12, functionally similar to the non-transparent structures of the embodiment of Fig. 2.

In the embodiment of Fig. 3, some of the holes 9, 14 are aligned, and some are not, thereby generating a pattern of spatially varying brightness when viewed in transmission.

Both, the embodiments of Figs. 2 and 3, form a combination of a pattern of first holes 9 in first substrate layer 7 overlaid with non-transparent structures 12 of second substrate layer 8. Such combinations can be used to generate a variety of characteristic optical effects that can be used for verifying the authenticity of the document, such as moire patterns, tilt-effect structures, zooming structures, etc., some of which will be illustrated in the following.

Fig. 4 shows an example where the non-transparent structures 12 form a periodic pattern of symbols 15, e.g. in the form of a letter "F" as shown. Advantageously, the symbols 15 are small, e.g. formed by microwriting with letters of a height of less than 2 mm, such that they are hard to discern.

The symbols 15 can e.g. be printed on second substrate layer 8 by means of dye layer 11 of Fig. 2. Alternatively, the symbols 15 can be formed by transparent regions separated by the non-transparent structures 12. In that case, the symbols can be formed by the second holes 14 of the embodiment of Fig. 3, or by the gaps in the non-transparent dye layer 11 in the embodiment of Fig. 2.

In the example of Fig. 4, the symbols 15 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. Alternatively, the spacing along first direction x may be periodic, while the spacing along second direction y may be non- periodic. In further embodiments, both spacings may be non-periodic.

Fig. 5 shows an example of the pattern formed by the first holes 9. The first 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. Again, alternatively, the spacing along first direction x may be periodic, while the spacing along second direction y may be non-periodic. In further embodiments, both spacings may be non-periodic.

Fig. 6 shows the overlay of the patterns of Figs. 4 and 5 as it is present in region 5. When viewing this pattern in transmission, parts of the first holes 9 will be blocked, thereby forming a motif 16.

Assuming that the black parts of Fig. 4 are transparent, the view in transmission will reveal a motif 16 as shown in Fig. 7, with dark parts showing the regions where the structure is transparent.

As can be seen, for the present embodiment, motif 16 shows a zoomed version of symbol 15. This is due to the fact that the spacings sx, ax and sy, ay differ by a small amount. In that case, the first holes 9 "scan" different locations of the symbols 15, thereby generating a lens effect.

To achieve such a zooming, the ratio a/s of the spacing of the non- transparent structures 12 in a first direction x and of the spacing of the holes in said first direction should be in the range of [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, as compared to the size of the non-transparent structures 12, are small. This is illustrated in Figs. 8 and 9 and can generally be described as follows: The non-transparent structures 12 or symbols 15 should, at least in one direction, have a width b larger than the diameter d of the holes 9.

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. 10, 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.

The arrangement of the holes of the microstructure can be varied in numerous ways. Fig. 11 shows several possible arrangements of the holes:

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

(Fig. 11 A)

- 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. 1 IB). 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. 11C). - The holes 9 can form at least two different row patterns, one with a smaller hole spacing than the other (Fig. 1 ID).

- The holes can enclose a non-perforated symbol (Fig. 1 IE) or they can form a perforated symbol (Fig. 11 G).

- The holes can be continuous stripes (Fig. 11H).

The shapes of the holes can vary. In most of the above examples, the holes had circular cross-section. Figs. 12 - 15 show some alternatives and further embodiments:

- The holes can be elongate (Fig. 12).

- The holes can be a mix of elongate and circular holes (Fig. 13).

- The holes can be star-shaped (Fig. 14) or have any other polygonal shape.

- The holes can be formed along rows extending non-perpendicular to the edges of substrate 1 (Fig. 15).

While Figs. 2 and 3 show embodiments of the document having two substrate layers 7, 8, the present technique can also be advantageously employed in documents having a larger number of substrate layers.

Fig. 16 shows an embodiment having a first substrate layer 7, a second substrate layer 8 and a third substrate layer 20, with third substrate layer 20 being arranged between first substrate layer 7 and second substrate layer 8.

In the embodiment of Fig. 16, third substrate layer 20 is transparent in at least part of region 5, while first and second substrate layer 7, 8 are non- transparent. First substrate layer 7 carries the first holes 9 of the microperforation. In second substrate layer 8, non-transparent structures 12 are formed by means of the second holes 14 (or by means of a dye layer 11 as shown in Fig. 2), In that case, at least when viewed perpendicularly, the motif 16 generated in transmission is similar to the one generated by the arrangement of Fig. 3. However, in contrast to the embodiment of Fig. 3, third substrate layer 20 acting as a spacer gives rise to a tilting effect (as will be described in more detail in respect to Fig. 18 below).

Fig. 17 shows a further embodiment with three substrate layers 7, 8, 20, where first substrate layer 7 carries holes 9 of the microperforation, and where second substrate layer 8 and third layer 20 are at least partially transparent in at least part of region 5. This structure can generate the same effect as described e.g. in EP 0861156, if round holes are used, or as in WO 2004/011274, if at least part of the holes are elongate, but it has the advantage that the perforation is protected by the substrate layers 9 and 20. In particular, this design allows to decrease the lateral distances between the holes while still maintaining a structure of sufficient stability. For example, structures of the types shown in Figs. 1 IB, C, D or H can be created, with very elongated, strip-shaped openings, with the outer layers 9 and 20 protecting the integrity of the perforation pattern. For the holes 9 to be visible, layers 9 and 20 need not be fully transparent. Even if they are only at least partially transparent, the holes 9 can be observed as a slightly brighter region when the document is viewed with sufficient illumination in transmission.

Fig. 18 shows an embodiment that also illustrates the tilt effect that can be obtained when a transparent substrate layer is arranged between the first holes 9 and the non-transparent structures 12. Again, as in Fig. 16, the substrate comprises three substrate layers 7, 8, 20, with third substrate layer 20 being arranged between first substrate layer 7 and second substrate layer 8. Third substrate layer 20 is transparent in at least part of region 5, while first and second substrate layer 7, 8 are non- transparent. First substrate layer 7 carries the first holes 9, while second substrate layer 8 carries the non-transparent structures 12 formed between the second holes 14.

In this embodiment the first holes (9) form a first hole pattern and the second holes (14) form a second hole pattern. These hole patterns are offset such that a maximum transmission of the superposition of the hole patterns is under a direction 22 that is non-perpendicular to the surface of the substrate. As illustrated in Fig. 18, when viewing the substrate of Fig. 18 under a direction 22 that is at a nonzero angle 23 in respect to a direction 24 perpendicular to the surface of substrate 1, transmission is highest. Advantageously, for strong effects, the lateral offset of the two hole patterns is between 0.1 and 1.0 times the thickness of third substrate layer 20. Further, a diameter of the holes 9 and 14, in a direction parallel to the offset, is between 0.5 and 2.0 times the thickness of third substrate layer 20.

As mentioned, a tilting effect can also be achieved with a structure as shown in Fig. 2 because dye layer 11 is arranged on a surface of second substrate layer 8 that is facing away from first substrate layer 7, such that the bulk of second substrate layer 8 forms an spacer between the first holes 9 and the non-transparent structures 12.

The effect shown in Fig. 18 can be enhanced if at least part of the holes 9, 14 extend non-perpendicularly through the first and/or second substrate layer 7, 8, as it is illustrated in Fig. 19. In this case, there are at least some first and second holes that extend coaxially and non-perpendicularly through their respective surface layers.

Non-perpendicular holes, or elongate holes, as described in WO 2004/011274, can also be used for generating tilting effects. The holes 9 are advantageously manufactured using laser perforation, as e.g. described in EP 0861156 . In order to generate holes that extend only through some, but not all, of the surface layers, the following techniques can be used:

- At least part of the substrate layers are perforated prior to their lamination, and, only after perforation, they are laminated to each other.

- The perforation is carried out after lamination of at least the first and the second substrate layer, in a region where first substrate layer 7 is non- transparent for the laser beam and second substrate layer 8 is transparent, in which case the laser beam perforates the first but not the second substrate layer.

Lamination can e.g. place take using an adhesive or using a technique as described in WO 2004/076198.

Notes

The substrate layers are advantageously of a thickness of at least 20 μιη, in particular of at least 40 μηι. Dye layers applied to the substrate by printing are not substrate layers in the sense of the present application.

Advantageously, the first and second (and preferably all other) substrate layers should extend laterally (i.e. in the directions parallel to the surface of substrate 1) over the substantially the whole substrate, or they should at least cover 80% of the area of the substrate.

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.