WO2015184556A1

WO2015184556A1 - Manufacturing method for security device

Info

Publication number: WO2015184556A1
Application number: PCT/CH2014/000078
Authority: WO
Inventors: Sylvain CHOOSON
Original assignee: Orell Füssli Sicherheitsdruck Ag
Priority date: 2014-06-06
Filing date: 2014-06-06
Publication date: 2015-12-10

Abstract

A method for manufacturing a security device (1) comprises steps of providing a non-uniform first seed pattern (10'), a non-uniform second seed pattern (20'), and a non-uniform third seed pattern (30'). Then, a first pattern (10) is derived using the first seed pattern (10') such that the first pattern (10) is modulated with the first seed pattern (10'). Furthermore, a second pattern (20) is derived using the second seed pattern (20') such that the second pattern (20) is modulated with the second seed pattern (20'). Additionally, a third pattern (30) is derived using the third seed pattern (30') and using the first seed pattern (10') such that the third pattern (30) is modulated with the third seed pattern (30') and with the first seed pattern (10'). Then, an at least partially transparent substrate (2) is provided and, for yielding the security device (1), the first pattern (10), the second pattern (20), and the third pattern (30) are arranged on the substrate (2) such that the first pattern (10), the second pattern (20), and the third pattern (30) at least partially overlap. At least one but not all of the first pattern (10), the second pattern (20), and the third pattern (30) comprises a photoluminescent material which is photoexcitable using at least one optical excitation wavelength (λ_exc). Because a modulation in the first pattern (10) from the first seed pattern (10') is spatially aligned with a modulation in the third pattern (30) from the first seed pattern (10'), specific visual effects are obtained and security is enhanced.

Description

Manufacturing method for security device

Technical Field

The invention relates to a method for manufacturing a security device, to a security device obtained by such a method, to a security document comprising such a security device, and to a method for verifying an authenticity of such a se- curity document.

Background Art

US 2006/0197990 Al discloses a superposition of two tally images, thus revealing a hidden image. The hidden image cannot be reconstructed from a single tally image.

US 2013/0181435 Al, which is incorporated by reference in its en- tirety, inter alia discloses a model for computing surface coverage in order to obtain a desired color, this model being applicable to security devices based on a pattern comprising photoluminescent material.

Disclosure of the Invention

It is an object of the present invention to provide a method for manufacturing a security device. Another object of the invention is to provide a security device obtained by such a method. Yet another object of the invention is to provide a security document comprising such a security device. Yet another object of the invention is to provide a method for verifying an authenticity of such a security document.

These objects are achieved by the devices and the methods of the independent claims.

Accordingly, a method for manufacturing a security device comprises steps of

- providing a non-uniform first seed pattern,

- providing a non-uniform second seed pattern, and

- providing a non-uniform third seed pattern.

These seed patterns are advantageously halftone, grayscale, or color images each with a number of uniformly colored areas, i.e., pixels. The first to third seed patterns are advantageously different from each other. Thus, the information content is increased and security is improved.

Then, using the first seed pattern, a first pattern is derived such that the first pattern is modulated with the first seed pattern. As an example, the first pattern can be derived from the first seed pattern by setting/compressing a brightness and/or a contrast of the first seed pattern and thus yielding the first pattern. Thus, e.g., only certain pixel values/ value ranges are populated in the first pattern whereas all pixel values are populated in the first seed pattern.

As another step, using the second seed pattern, a second pattern is derived such that the second pattern is modulated with the second seed pattern. As an example, the second pattern can be derived from the second seed pattern by setting/compressing a brightness and/or a contrast of the second seed pattern and thus yielding the second pattern. Thus, e.g., only certain pixel values/ value ranges are populated in the second pattern whereas all pixel values are populated in the second seed pattern.

As yet another step, using the third seed pattern and using the first seed pattern, a third pattern is derived such that the third pattern is modulated with the third seed pattern and with the first seed pattern (advantageously an inversion thereof). Thus, information content in the third pattern depends both on the first seed pattern and on the third seed pattern and security is enhanced because very specific visual effects can be achieved.

Herein, the terms "inversion", "inverted" and similar relate to a transmittance/reflectivity/photoluminescence value (e.g., of a pattern or a specific region of a pattern) which is "inverted" with respect to an ideal 100% transmission/re- flection/photoluminescence at one or more wavelength(s) (in particular in the visible regime between 380 nm and 780 nm) and with respect to another transmittance/re- flectivity/photoluminescence value (e.g., that of another pattern or region). As examples, for a 90% transmittance of a specific region of the first seed pattern, an inverted transmittance would be 10%. As another example, a 20% reflectivity of a specific region is inverted with respect to an 80% reflectivity.

As another step of the manufacturing method, an at least partially transparent substrate is provided (advantageously with a thickness smaller than 500 μη , in particular smaller than 120 μηι) and, for yielding the security device, the first pattern, the second pattern, and the third pattern are arranged (e.g., printed) on the substrate. This is done in such a way that the first pattern, the second pattern, and the third pattern at least partially (in particular fully) overlap. Due to this overlap and the interaction between the patterns, very specific visual effects can be achieved and security of the security device is enhanced.

Herein, the term "at least partially transparent" relates to an optical property of a nonzero transmission of light at at least one wavelength, in particular in the visible regime between 380 nm and 780 nm. Thus, a nonzero amount of light can be shone through the substrate. The substrate can be scattering to a certain degree, but is advantageously substantially non-scattering at the detection wavelength(s) (e.g., in the visible regime between 380 nm and 780 nm). Advantageously, a transmittance of the substrate is higher than 50%, at least for one transmitted wavelength in the visible regime between 380 nm and 780 nm.

Furthermore, at least one of a group of but not all of the group of the first pattern, the second pattern, and the third pattern comprises (in particular consists of) a photoluminescent material (e.g., a fluorescent or phosphorescent material) which is photoexcitable using at least one optical excitation wavelength λ_εχα Thus, the pattern comprising the photoluminescent material can be photoexcited by shining light at the optical excitation wavelength onto the security device and very specific visual effects can thereby be achieved. This enhances the security of the security device.

A modulation in the first pattern stemming from the first seed pattern is spatially aligned with a modulation in the third pattern stemming from the first seed pattern. Thus, very specific visual effects can be achieved because the first pattern - depending on a viewing mode of the security device - interacts with the third pattern. This enhances the security of the security device.

In an advantageous embodiment, the method comprises a step of selecting transmittances, reflectivities, and photoluminescent properties of the first pattern, of the second pattern, of the third pattern, and of the substrate such

* that in a first viewing mode, an acquirable first image of the security device represents the first seed pattern. In other words, a spectator of the first image (e.g., a naked eye of a viewer with or without visual aids or a viewing device such as a camera-equipped cellphone) can discern at least some different regions (e.g., pixels) in the first image such that he can reproduce at least some of the information content of the first seed pattern. As an example for "visibility", i.e., for a discernibility of different regions in the pattern, e.g., AE94-values for the different regions are above 1.8. The first image is acquirable at at least a first optical detection wavelength _det\ , advantageously in the visible regime between 380 nm and 780 nm. Usually, a first optical detection wavelength regime is used.

Furthermore, the above discussed transmittances, reflectivities, and photoluminescent properties are selected such

* that in a second viewing mode, an acquirable second image of the security device represents the second seed pattern (see above for definition of the term "represents"). The second image is acquirable at at least a second optical detection wavelength _det2, advantageously in the visible regime between 380 nm and 780 nm. Again, usually, a second optical detection wavelength regime is used which advantageously coincides with the first optical detection wavelength regime.

* that in a third viewing mode, an acquirable third image of the security device represents the third seed pattern (see above for definition of the term "represents"). The third image is acquirable at at least a third optical detection wavelength X_det3, advantageously in the visible regime between 380 nm and 780 nm. Again, usually, a third optical detection wavelength regime is used which advantageously coincides with the first and second optical detection wavelength regimes.

The acquirable third image represents the third seed pattern but advantageously not the first seed pattern. Thus, very specific visual effects can be achieved because in the third viewing mode, e.g., the contribution of the first seed pattern in the third pattern cancels out the first pattern almost completely. Thus, the first seed pattern cannot be discerned in the third image. This enhances the security of the security device.

At least one of a group of the first optical detection wavelength _detl , the second optical detection wavelength X_det2, and the third optical detection wavelength λ_det3 is an optical emission wavelength λ_εηι of the photolumines- cent material. Thus, at least one of the first image, the second image, and the third image can be based on the optical emission wavelength λ_εη of the photoluminescent material. Thus, very specific visual effects can be achieved which enhance the security.

At least one of a group of but not all of the group of the first viewing mode, the second viewing mode, and the third viewing mode comprises a photo- excitation of the photoluminescent material at the at least one optical excitation wavelength _QXC of the photoluminescent material. E.g., the third image can be a fluorescence image viewing in a transmission configuration wherein the first and second images are acquired in a reflection configuration in the visible regime. This, in combination with the features discussed above, enhances the security of the security device.

Advantageously, the first image in the first viewing mode is acquirable in a reflection configuration. Herein, a definition for the term "reflection configuration" is that the overall (i.e., spatially integrated over the whole security device) reflected light intensity from the security device outshines an overall (i.e., spatially integrated over the whole security device) transmitted light intensity through the security device at least by a factor of 5. One option which helps to achieve such a reflection configuration is to arrange a light source on the same side of the substrate as a viewing position in the respective viewing mode. Thus, in a reflection configuration, the first image representing the first seed pattern is easier acquirable in the first viewing mode.

Advantageously, the second image in the second viewing mode is acquirable in a reflection configuration. See above for a definition of the term "reflection configuration". Thus, in a reflection configuration, the second image representing the second seed pattern is easier acquirable in the second viewing mode.

Advantageously, the third image in the third viewing mode is acquirable in a transmission configuration or in a reflection configuration. Herein, a definition for the term "transmission configuration" is that the overall (i.e., spatially integrated over the whole security device) transmitted light intensity through the security device outshines an overall (i.e., spatially integrated over the whole security device) reflected light intensity from the security device at least by a factor of 5. One option to achieve such a transmission configuration is to arrange a light source on an opposing side of the substrate as a viewing position in the respective viewing mode. Thus, in a transmission or reflection configuration, the third image representing the third seed pattern (but advantageously not the first seed pattern, see above) is easier acquirable in the third viewing mode.

Advantageously, the third pattern comprises photoluminescent material. Thus, very specific visual effects can be achieved because the third pattern which comprises information from the first seed pattern and from the third seed pattern can be photoexcited. Particularly, the third pattern is substantially transparent for the first optical detection wavelength λ_detl , the second optical detection wavelength _det2, and the third optical detection wavelength _det3. Thus, detection/image acquisition is not hampered by the third pattern which is useful for creating very specific visual effects which increase the security of the security device.

Advantageously, the first pattern is arranged on a first surface of the substrate and the second pattern is arranged on a second surface of the substrate. This second surface is opposite the first surface. Thus, patterns can be arranged on both sides of the substrate of the security device and the information content is enhanced.

Then, the third pattern can be arranged on the first surface or on the second surface of the substrate without interfering with the first pattern or the second pattern, respectively. This simplifies the manufacturing of the security device.

Advantageously, the substrate comprises at least a first substrate layer and a second substrate layer. Then, in particular, at least one of the first to third patterns (advantageously the third pattern) can be arranged between the first substrate layer and the second substrate layer. This enhances security of the security device because putative counterfeiters would need to reproduce the multilayer substrate to achieve the same visual effects as the authentic security device.

In an advantageous embodiment, the first optical detection wavelength _detl , the second optical detection wavelength X_det2, and the third optical detection wavelength ?._det3 (or the regimes, respectively) are between 380 nm and 780 nm. More advantageously, they are the same. Thus, authenticity checking of the security device is simplified.

Advantageously, the optical excitation wavelength _exc is shorter than the first optical detection wavelength X_det\ , the second optical detection wavelength _det2, and the third optical detection wavelength _det3 (or their regimes, respectively). More advantageously, the optical excitation wavelength X_exc is between 200 nm and 380 nm. Thus, authenticity checking of the security device is simplified due to the availability of suitable light sources and detectors.

In another advantageous embodiment, the first pattern, the second pattern, and the third pattern are arranged in a window of the substrate or - if the substrate comprises a plurality of substrate layers - in a window of at least one substrate layer of the substrate. Thus, even more visual effects can be achieved which improves the security of the security device.

Advantageously, at least one of the group of but not all of the group of the first pattern, the second pattern, and the third pattern is arranged on the substrate using visible ink (i.e., ink absorbing/ reflecting at specific wavelengths in the visible regime, but substantially non-photoluminescing), e.g., via printing techniques known to the skilled person. Thus, the manufacturing of the security device becomes easier and more cost-effective.

In another advantageous embodiment, at least one of the group of, in particular all of the group of, the first pattern, the second pattern, and the third pattern is arranged on the substrate using offset printing, screen printing, or sublimation printing. Thus, the manufacturing of the security device becomes easier and more cost-effective.

In yet another advantageous embodiment, at least one of the group of the first pattern, the second pattern, and the third pattern is arranged on the substrate using laser ablation and/or laser darkening of the substrate or parts/ surfaces/ layers thereof. This technique locally changes the optical properties of the substrate and even more visual effects can be achieved which improves the security of the security device. More advantageously, laser ablation/ laser darkening is combined with any one or more of the above-mentioned printing techniques. Thus, security is enhanced considerably. In another advantageous embodiment, at least one of the group of the first pattern, the second pattern, and the third pattern is arranged on the security device using foil application techniques. Thus, security is enhanced, because this manufacturing technique is not readily available to potential counterfeiters. This can also be combined with laser ablation/ laser darkening and/or printing as discussed above.

In another advantageous embodiment, at least one of the group of the first pattern, the second pattern, and the third pattern comprises a color filter. This makes it easier to select one or more wavelength(s) for excitation and/or detection. Note that a color filter can - in addition or as an alternative to the patterns - also be comprised in the substrate at any position, e.g., above or below any of the substrate layers.

Advantageously, the step of deriving the third pattern additionally to the above discussed steps comprises using the second seed pattern, in particular using an inversion of the second seed pattern. Then, the third pattern is - in addition to the first seed pattern and to the third seed pattern - modulated with the second seed pattern. In this embodiment, a modulation in the second pattern which stems from the second seed pattern is spatially aligned with a modulation in the third pattern stemming from the second seed pattern.

Then, the information content in the third pattern depends not only on the first seed pattern and on the third seed pattern, but also on the second seed pattern. This enhances security even further because even more specific visual effects can be created.

Advantageously, more than one pattern comprises photolumines- cent material which is photoexcitable at the same or a different excitation wavelength. By the interaction, e.g., between different fluorescence emissions and visible reflection, even more visual effects can be achieved which improves the security of the security device.

In another advantageous embodiment, the method comprises further steps of

- providing a non-uniform fourth seed pattern,

- deriving a fourth pattern using * the fourth seed pattern and

* using at least one of the first seed pattern and the second seed pattern (or, advantageously, their inversion(s)), and

- arranging the fourth pattern on the substrate such that the fourth pattern at least partially overlaps with the first pattern, the second pattern, and the third pattern. Thus, more information is comprised in the security device, counterfeiting is aggravated and security is enhanced.

In an advantageous embodiment, the substrate is fiat and/or flexible and/or its thickness is smaller than 500 μιη, in particular smaller than 120 μιη. In the case of a multilayer/sandwiched substrate, the thickness of one or more or all substrate layers is advantageously smaller than 250 μπι, in particular smaller than 60 μηι. This simplifies the application in security documents which are usually flat and/or flexible to some degree.

As another aspect of the invention, a security device obtained by a method as described above is disclosed. Due to the interaction between the light from the patterns comprising photoluminescent material and light from the other patterns not comprising photoluminescent material, security of such a security device is enhanced over prior art security devices and counterfeiting is aggravated.

As yet another aspect of the invention, a security document (such as a banknote, a passport, a document of value, a certificate, or a credit card) comprises a security device as described above for verifying an authenticity of the security document. Thus, the security document is harder to counterfeit.

Advantageously, such a security document further comprises a light absorber, in particular arranged at a distance to the security device. Then, for example by folding the security document along an applied, in particular printed and/or perforated, folding line, the light absorber can be brought into an overlap with the security device. As an effect, the amount of transmitted and/or reflected light is reduced by the light absorber and thus a reflection configuration is reached more easily. As an effect, handling is improved when the authenticity of the security document is to be checked.

Advantageously, the light absorber has a reflectivity of less than 50% at least for the at least one reflected wavelength from the security device and/or the light absorber has a transmittance of less than 50% at least for the at least one transmitted wavelength through the security device. The light absorber can, e.g., comprise a region of the security document which is covered by a dark color, e.g., 100 % black. As an effect, a reflection configuration in a specific viewing mode is reached more easily and handling is improved when the authenticity of the security document is to be checked.

As another aspect of the invention, a method for verifying an authenticity of a security document as described above comprises steps of

- in a first viewing mode from a first viewing position acquiring a first image of a security device of the security document at at least a first optical detection wavelength X_det\ which is advantageously in the visible regime between 380 nm and 780 nm. Usually, a first optical detection wavelength regime will be used. The first image is advantageously acquired in a reflection configuration, e.g., with a light source being located on the same side of the substrate as the first viewing position.

The method comprises a further step of

- in a second viewing mode from a second viewing position acquiring a second image of the security device at at least a second optical detection wavelength _det2 which is advantageously in the visible regime between 380 nm and 780 nm. Usually, a first optical detection wavelength regime will be used. The second image is advantageously acquired in a reflection configuration, e.g., with a light source being located on the same side of the substrate as the second viewing position, e.g., opposite the first viewing position.

The method comprises a further step of

- in a third viewing mode from a third viewing position acquiring a third image of the security device at at least a third optical detection wavelength _det3, which is advantageously in the visible regime between 380 nm and 780 nm. Usually, a first optical detection wavelength regime will be used. The third image is advantageously acquired in a transmission configuration, e.g., with a light source being located on an opposite side of the substrate as the third viewing position.

Furthermore, the method comprises a step of

- in at least one of the group of but not in all of the group of the first viewing mode, the second viewing mode, and the third viewing mode photoexciting a photoluminescent material of at least one pattern of the security device. This is achieved using at least one optical excitation wavelength X_exc, advantageously in the near-UV range between between 200 nm and 380 nm. Usually, an optical excitation wavelength regime will be used. At least one of a group of the first optical detection wavelength _detl , the second optical detection wavelength λ_det2, and the third optical detection wavelength X_det3 is an optical emission wavelength λ_ειη of the photolumines- cent material such that the respective acquired image (i.e., the first image, the second image, and/or the third image) is based on emission light from the photoluminescent material. Thus, very specific visual effects are realized and security is enhanced.

Then, as another step, the authenticity of the security document is derived using the acquired first image, using the acquired second image, and using the acquired third image. This is, e.g., achieved by pattern matching the first image to the first seed pattern, the second image to the second seed pattern, and the third image to the third seed pattern and classifying the security document as "authentic" if these pattern matching steps return similarities above a certain threshold.

Remarks:

The invention is not limited to halftone or grayscale patterns. Although the description and figures herein mainly focus on grayscale patterns for the sake of clarity, analogous considerations can be made for each color channel of color patterns which renders the subject-matter of the invention feasible for color patterns.

The described embodiments similarly pertain to the devices and the methods. Synergetic effects may arise from different combinations of the embodiments although they might not be described in detail.

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 - as a technological background - a first pattern 10 and a second pattern 20 as well as a combination 200 of this first partem 10 with this second pattern 20 in a transmission configuration,

fig. 2 shows - as a technological background - a generation of a first pattern 10 and of a second pattern 20 using a first seed pattern 10' and a second seed pattern 20',

fig. 3 shows a first seed pattern 10', a first pattern 10 for use in a security device 1 according to the invention, a second seed pattern 20', a second pattern 20 for use in a security device 1 according to the invention, a third seed pattern 30', an intermediate pattern 30", and a third pattern 30 for use in a security device 1 according to the invention,

fig. 4a shows a security device 1 according to a first embodiment of the invention, the security device 1 comprising a multilayer substrate 2 as well as a first pattern 10, a second pattern 20, and a third pattern 30,

fig. 4b shows a security device 1 according to a second embodiment of the invention, the security device 1 comprising a first pattern 10, a second pattern 20, and a third pattern 30,

fig. 5 schematically shows the security device 1 of fig. 4a in a first viewing mode in a reflection configuration,

fig. 6 schematically shows the security device 1 of fig. 4a in a second viewing mode in a reflection configuration,

fig. 7 schematically shows the security device 1 of fig. 4a in a third viewing mode in which the photoluminescent material of the third pattern is photoex- cited,

fig. 8 shows a security device 1 according to a third embodiment of the invention, wherein a first substrate layer 2a of a multilayer substrate 2 of the security device 1 comprises a transparent window,

fig. 9 schematically shows a security document 100 comprising the security device 1 of fig. 4a, a light absorber 5, and a folding line 500, Fig. 10 schematically shows the security device 1 of fig. 4a in a third viewing mode in which the photoluminescent material of the third pattern is photoexcited,

fig. 1 1 schematically shows the security device 1 of fig. 4a in a first viewing mode with specular reflection,

fig. 12 schematically shows the security device 1 of fig. 4a in a first reflection viewing mode with specular reflection and pattern attenuation by a light absorber 5, and

fig. 13 schematically shows a security device 1 according to a fourth embodiment of the invention, the security device 1 comprising a multilayer substrate 2 as well as a first pattern 10, a second pattern 20, a third pattern 30, a fourth pattern 40, a fifth pattern 50, and a sixth pattern 60.

Modes for Carrying Out the Invention

Fig. 1 shows a first pattern 10 and a second pattern 20. In this figure, the first pattern 10 is a grayscale image with a gradient from 100% white (i.e., 0% black) to 100% black (from left to right in a normal viewing position). The second pattern 20 is an inverted pattern with regard to the first pattern 10, i.e., it is a grayscale image with a gradient from 100% black to 0% black (from left to right).

When the first pattern 10 is overlaid with the second pattern 20 (i.e., when a first region 1 1 (or "pixel") of the first pattern 10 fully coincides (i.e., is spatially aligned) with a third region 23 (or "pixel") of the second pattern 20 and a second region 12 (or "pixel") of the first pattern 10 fully coincides (i.e., is spatially aligned) with fourth region 24 (or "pixel") of the second pattern 20) and viewed in a transmission configuration ("transmission viewing mode"), a grayscale image 200 as depicted in the lower part of figure 1 is observed. Specifically, a grayscale image going from 100 % black to 75 % black back to 100 % black is yielded.

The upper part of figure 1 shows the black levels of the single patterns 10 and 20 as well as of the combined grayscale image 200 (in transmission viewing mode) as functions of position.

What can be seen from the diagram is that in the transmission viewing mode (i.e., with transmissions through the first and through the second pattern being combined), the first region 11 is indiscernible from the second region 12 of the first pattern 10, because both the first region 11 and the second region 12 show the same gray levels of 84% black (see the points labeled 12+24 and 1 1+23 of the curve labeled 200 in the diagram).

This is, because the first region 11 of the first pattern 10 fully coincides with the third region 23 of the second pattern 20 (see vertical line). Similarly, the second region 12 of the first pattern 10 fully coincides with the fourth region 24 of the second pattern (see vertical line). Furthermore, the first pattern 10 (i.e., all regions) is inverted with respect to the second pattern 20, i.e., the third region 23 is inverted with respect to the first region 11 and the fourth region 24 is inverted with respect to the second region 12.

One possible theoretical approach to explain this is the so-called Demichel equation. For 2 colors, the Demichel equation shows that for the superposition of a layer of color CI with a density dl and of a layer of color C2 with a density d2 (both layers having a random or independent halftoning), a

surface coverage of white w=(l-dl) * ( l -d2),

a perceived color CI = dl * (l-d2), and

a perceived color C2 = d2 * (1 -dl )

Furthermore, a perceived color C12 = dl * d2

where CI 2 is a color resulting of a superposition of color CI on C2.

If both colors CI and C2 are black and if

d2 = 1— dl (inverted patterns!), the density of black b (i.e., b = 1 - w) for the superposed image equals to b = 1 - dl + dl². This corresponds to the curve labelled 200 in the diagram of figure 1.

As an example, the first region 11 of the first pattern 10 and the fourth region 24 of the second pattern 20 are both 80% black. The second region 12 of the first pattern 10 and the third region 23 of the second pattern 20 are both 20%» black, i.e., inverted. Hence, the first region 1 1 has a different transmittance and reflectivity than the second region 12 and the third region 23 has a different transmittance and reflectivity than the fourth region 24. The superposition of the first region 11 with the third region 23 yields b = 1 - 0.8 + 0.8², i.e., b = 84% black. This is the same value as for the superposition of the second region 12 with the fourth region 24, namely b = 1 - 0.2 + 0.2² = 84%> black. Note that a 100% transmittance of the substrate is assumed here (substrate not shown!).

Thus, in a transmission viewing mode (i.e., in a superposition of the first pattern 10 with the second pattern 20), the first region 11 is indiscernible from the second region 12 and the third region 23 is indiscernible from the fourth region 24.

As can be further seen from the Demichel equation: * With the full range of grayscales (see range 1), the perceived black level of the superposed inversed patterns 10, 20 in transmission viewing mode ranges between a black level b = 100% and b = 75%.

* With a smaller range of grayscales (see range 2) such as 0.2 to 0.8 (i.e., the example above), the perceived black level of the superposed inversed images ranges between b = 84% and 75% (horizontal dashed lines).

* With an even smaller range of grayscales (see range 3) such as 0.35 to 0.65, the perceived black level of the superposed inversed images ranges between b = 77.25%) and 75%. This is a range of black levels b where the black levels are not distinguishable by the naked eye of a viewer without visual aids. Thus, in this example, in a transmission viewing mode through first pattern 10 and second pattern 20, a first region 1 1 ' would be indiscernible from a second region 12'. In general, it can be stated that regions with transmitted light intensity-differences below 5% cannot be discerned.

If the first pattern 10 is viewed in a reflection viewing mode (e.g., with an overall reflected light intensity from the first pattern 11 outshining an overall transmitted light intensity at least by a factor of 5), the superposition of the first pattern 10 with the second pattern 20 does not take place anymore and the first region 11 thus becomes discernible from the second region 12 due to their different reflectivities. In general, it can be stated that regions with reflected light intensity-differences above 5% can be discerned.

Please note, that the same principle shown here for absorbing/reflecting first and second patterns 10, 20 in a transmission configuration using visible light equally applies to the case in which of the patterns, e.g., the first pattern 10, consists of a photoluminescent material (such as a fluorescent material, e.g., fluorescein solution printed to the substrate in differing coverages to create the first pattern 10) which is photoexcitable using at least one optical excitation wavelength exc. Then, upon photoexcitation of the photoluminescent material, an optical fluorescence emission gradient in the visible regime is yielded which is differently absorbed by the second pattern 20 when viewed through the second pattern 20. Thus, upon locally uniform photoexcitation of the first pattern 10 consisting of the photoluminescent material, the same grayscale image 200 is yielded due to the locally varying emission intensities from the first pattern 10 which are overlaid by different absorbances of the second pattern 20.

Thus, very specific patterns can be created under different viewing conditions and security is enhanced. While figure 1 explains the technological background, in figure 2, the generation of a first pattern 10 and of a second pattern 20 is explained.

Figure 2 shows a second seed pattern 20' from 100% white to 100% black and it shows a first seed pattern 10' from 100% black to 100% white (both as seen from left to right). So far, the situation is the same as discussed above with regard to figure 1.

Now, here, instead of overlaying these seed patterns 10' and 20' directly, the brightness and contrast of the second seed pattern 20' is set to ensure that all grayscale levels are darker than 50% black. In other words, the histogram is compressed. Thus, an intermediate pattern 20" is yielded. In other words, in a histogram of this intermediate pattern 20", only black levels between 50% black and 100% black are populated while the gray levels between 0% black and 50% black are unpopulated (i.e., only regions with gray values between 50% black and 100% black are present in the intermediate pattern 20").

Furthermore, the brightness and contrast of the first seed pattern 10' is set as to ensure that all grayscale levels are brighter than 50% black. Thus, the first pattern 10 is yielded. In other words, in a histogram of this first pattern 10, only black levels between 0% black and 50%> black are populated while the gray levels between 50% black and 100% black are unpopulated.

Now, as a next step, a second pattern 20 is generated using the first pattern 10 and the intermediate pattern 20". The second pattern 20 in figure 2 is created such that

* in a transmission configuration, in combination with the first pattern 10, the intermediate pattern 20" is yielded when a perfect 100% transmittance of a substrate (not shown) is assumed. This intermediate pattern 20", however, represents the second seed pattern 20' (with the exception of a modified brightness and contrast).

The diagram at the top of figure 2 shows these relations.

This last step of generating the second pattern 20 is carried out by using the Demichel equation as explained above with regard to figure 1. Specifically, the Demichel equation as introduced above for a layer of color CI (black in this case) with a density dl and of a layer of color C2 (black in this case) with a density d2 tells how to do this generation step: It states that

b = l-(l-dl)*(l-d2) = l -(l -d2-dl + d2dl)

b = dl+d2-dl *d2

Here, b is again indicative o the perceived density of black for the transmission-superposed pattern 10+20=20". In other words, the black level in a specific region of the to be generated second pattern 20 can be calculated by

d2 = 1 - [(l -b) / (l-dl)]

For an example, please refer to the dashed vertical line in the diagram on top of fig. 2: In the specific region of the patterns, the first pattern 10 has a gray level of 40%. Now, the task is to find a second pattern 20 (i.e., its gray level in this region) that combines (in transmission) with the first pattern to yield a gray level of 60% (i.e., the gray level of the intermediate pattern 20" in the respective region). So, with b = 0.6 and dl = 0.4, it follows that

d2 = 1 - [(1 -0.6) / (1-0.4)] = 0.33 = 33% black

This corresponds to point 201 on the pattern-20-curve in the diagram of figure 2.

For a pattern generation rule, it needs to be imposed that d2 >= 0.

This leads to

(1 - b) / (l - dl) <= 1 or

dl <= b.

This means, however, that a gray level of any region of the first pattern 10 (i.e., dl) is always brighter than a corresponding gray level of a region of the intermediate pattern 20" at the same position. For this to be taken into account, the step of histogram-compression as described above is used (if necessary).

In the examples herein, two equal ranges for dl (i.e., black levels in the first pattern 10) and b (i.e., black levels in the intermediate pattern 20") such as 0- 50% for dl and 50%- 100% for b are selected. Other ranges are possible as well. Please also note that in general, no complete histogram separation into two parts is necessary: only locally at every position x,y of the patterns, the above pattern generation Rile needs to be fulfilled.

Please note that the same principles apply to photoluminescent material (see discussion above). As an effect, first pattern 10 and second pattern 20 are easier to generate.

Generally speaking, the same principle for pattern generation also applies to an overlay of three patterns (first pattern, second pattern, third pattern, e.g., arranged between first pattern and second pattern):

In an example, for every quadruple of pixels (i.e., corresponding pixels in the first pattern, second pattern, third pattern, and third seed pattern): b = 1 - [(1 - dl) * (1 - dl) * (1- d3)]

Here, b is again indicative of the perceived density of black for the superposition of the patterns in a transmission configuration (i.e., through an overlay of all three patterns), dl is the black density of the first pattern's pixel, d2 is the density of the second pattern's pixel, and d3 is the (to be derived) density of black (or inverse photo luminescence) of the (to be derived) third pattern's pixel, respectively.

Then:

1 - b

d3 - 1 - ₍₁ _ di) (i _ _d2)

Because 0 < = d3 < - 1, we need to have

1 - b

0 < < 1

^" (1 - dl) (l - dl) ^~ and thus a pattern generation rule becomes:

(1 - b) < = (1 - dl) (l - dl)

Note that d2=0 give the same result as for the two patterns as discussed above.

Please also note that the above discussed approach also works in two dimensions (for pixelates 2d images) as well as for color images:

Demichel equation in CMYK:

Ccyan = dcyan x ( 1 -dmagenta) x (1-dyellow) x (1 -dblack)

Cmagenta = dmagenta x (1 -dcyan) x (1-dyellow) x (1-dblack) Cyellow = dyellow x (1 -dcyan) x (1- dmagenta) x (1-dblack)

Ccyanmagenta = dcyan x dmagenta x ( 1 -dyellow) x ( 1 -dblack) Ccyanyellow = dcyan x ( 1 -dmagenta) x dyellow x (1 -dblack) Cmagentayellow = dmagenta x ( 1 -dcyan) x dyellow x (1-dblack)

If cyanmagentayellow = black Cblack = (1-dcyan) x (1-dmagenta) x (1-dyellow) x dblack

+ dcyan x dmagenta x dyellow x (1 -dblack)

+ dcyan x dmagenta x dyellow x dblack

+ dcyan x (1-dmagenta) x (1-dyellow) x dblack

+ dmagenta x (1-dcyan) x (1-dyellow) x dblack

+ dyellow x (1-dcyan) x (1-dmagenta) x dblack

+ dcyan x dmagenta x (1-dyellow) x dblack

+ dcyan x (1-dmagenta) x dyellow x dblack

+ dmagenta x (1-dcyan) x dyellow x dblack

Cwhite = (1-dcyan) x (1-dmagenta) x (1-dyellow) x (1 -dblack)

With regard to fig. 3 the above-described principles are applied to generate a first pattern 10, a second pattern 20, and a third pattern 30 for use in a security device 1 according to the invention.

Fig. 3a shows a non-uniform first seed pattern 10' ("David") and a first pattern 10 which is derived therefrom for use in a security device 1 according to the invention. The first pattern 10 is modulated with the first seed pattern 10'. As it can be seen in the lower part of figure 3a, a histogram H10' of the first seed pattern 10' is compressed such that a histogram H10 of the thus yielded first pattern 10 comprises a first unpopulated region HlOu and a first populated region HI Op. This is achieved by setting a brightness and/or a contrast of the first seed pattern 10', if necessary. Specifically here, the first pattern 10 appears brighter than the first seed pattern 10'. Note, that this "setting" step is unnecessary if the first seed pattern H10' already fulfil the pattern generation rales. The first pattern 10 is later arranged, e.g., printed using visible inks, onto a first surface 3 of a multilayer substrate 2 of a security device 1 according to the invention (see below).

Furthermore, the first pattern 10 is inverted to yield an inverted first pattern lOi which is later used for generating the third pattern 30 for use in the security device 1 according to the invention.

Fig. 3 b shows the same steps as described above with regard to fig. 3 a for a second pattern 20 for use in the security device 1 according to the invention. This second pattern 20 is derived from a non-uniform second seed pattern 20' ("Marilyn", histogram H20') such that a second histogram H20 of the second pattern 20 comprises at least a second unpopulated region H20u and a second populated region H20p. The second pattern 20 is modulated with the second seed pattern 20'. The first unpopulated region HlOu overlaps the second unpopulated region H20u and the first populated region HI Op overlaps the second populated region H20p. The second pattern 20 is later arranged, e.g., printed using visible inks, onto a second surface 4 of a multilayer substrate 2 of a security device 1 according to the invention (see below). A second inverted pattern 20i is later used for generating the third pattern 30.

Fig. 3c shows a non-uniform third seed pattern 30' ("inventor", histogram H30') from which an intermediate pattern 30" is derived by setting/compressing the histogram H30' of the third seed pattern 30' such that a histogram H30" of the yielded intermediate pattern 30" comprises a third unpopulated region H30"u and a third populated region H30"p. The third unpopulated region H30"u overlaps the first and second populated regions HI Op and H20p. The third populated region H30"p overlaps the first unpopulated region HlOu and the second unpopulated region H20u. These histogram modifications are only one of various possible solutions to fulfil the above discussed pattern generation rules.

Unlike the first pattern 10 and the second pattern 20, the intermediate pattern 30" is not directly applied onto the security device 1, but a third pattern 30 for use in the security device 1 according to the invention is derived using the first seed pattern 10' (specifically, using the inversion lOi of the first pattern 10), using the second seed pattern 20' (specifically, using the inversion 20i of the second pattern 20), and using the third seed pattern 30' (specifically, using the intermediate pattern 30"). Thus, the third pattern 30 is modulated with the inverted first pattern lOi (and thus with the first seed pattern 10'), with the inverted second pattern 20i (and thus with the second seed pattern 20'), and with the intermediate pattern 30" (and thus with the third seed pattern 30'). The third pattern 30 is later arranged, e.g., printed using UV-excitable inks, between the layers 2a and 2b of a multilayer substrate 2 of a security device 1 according to the invention (see below). The third pattern 30 consists of a photocxcitablc photolumincsccnt material with an excitation wavelength λ_εχο in the near-UV regime. The photoluminescent material is substantially transparent in the visible regime.

The above described derivation of the third pattern 30 is done in such a way

* that in a first viewing mode in a reflection configuration, an acquirable first image II of the security device 1 represents the first seed pattern 10' ("David"). This first image I I is acquirable at a first optical detection wavelength _detl in the visible regime and with the first pattern 10 being oriented towards a first viewing position PI (see below). * That in a second viewing mode in a reflection configuration, an acquirable second image 12 of the security device 1 represents the second seed pattern 20' ("Marilyn"). This second image 12 is acquirable at a second optical detection wavelength X_det2 in the visible regime (which is the same as the first optical detection wavelength X_det\) and with the second pattern 20 being oriented towards a second viewing position P2 (see below).

* That in a third viewing mode in a transmission configuration and with a photoexcitation of the photo luminescent material of the third pattern 30, an acquirable third image 13 of the security device 1 represents the third seed pattern 30' ("inventor"), but not the first seed pattern 10' ("David") or the second seed pattern 20' ("Marilyn"). This is achieved because the first pattern 10 and a contribution in the third pattern 30 cancel each other just as the second pattern 20 and another contribution in the third pattern 30. Note that a spatial alignment of the corresponding modulations is necessary (printing in registration). The approach is based on color prediction models such as the Demichel equation discussed above with regard to figs. 1 and 2.

The third image 13 is acquirable at a third optical detection wavelength X_det3 which corresponds to an optical emission wavelength λ_εηι of photolu- minescent material in the third pattern 30. _det3/ X_em are in the visible regime and are the same as the first optical detection wavelength X_detl and the second optical detection wavelength X_det2.

By arranging the thus created first pattern 10, the second pattern 20, and the third pattern 30 on a security device 1, information content in the security device 1 is increased, the perceived information content depends on the specific viewing modes, and counterfeiting attempts are thus aggravated. Thus, security is enhanced considerably.

Note: As it is demonstrated, e.g., in PCT/CH2013/00023 1 (which is hereby incorporated by reference in its entirety), in fig. 6b and the corresponding description on page 23 et seq., lines 28 et seq., the use of halftoning techniques (not shown here) can simplify the manufacturing of the security device 1.

Fig. 4a shows a security device 1 according to a first embodiment of the invention, the security device 1 comprising a multilayer substrate 2 which comprises a first substrate layer 2a and a second substrate layer 2b. The security device 1 further comprises the first pattern 10 of fig. 3 arranged on a first surface 3 of the first substrate layer 2a. The second pattern 20 of fig. 3 is arranged on a second surface 4 of the second substrate layer 2b. The third pattern 30 comprising photoluminescent material of fig. 3 is arranged between the first substrate layer 2a and the second substrate layer 2b of the security device 1. Because of the high registration accuracy necessary for arranging the first to third patterns 10, 20, 30 on the security device 1, counterfeiting attempts of the security device 1 are aggravated. Both substrate layers 2a and 2b have substantially the same optical properties and are substantially non-scattering in the visible and in the near-UV regimes.

Fig. 4b shows a security device 1 according to a second embodiment of the invention. The second embodiment is very similar to the first embodiment with the difference that a single-layer substrate 2 is used and that the third pattern 30 is arranged on the second surface 4 together with the second pattern 20. This is possible because the photoluminescent material (UV-excitable ink) of the third pattern 30 is substantially transparent in the visible regime. Thus, the manufacturing of the security device 1 is simplified.

Fig. 5 schematically shows the security device 1 of fig. 4a in a first viewing mode. In this first viewing mode, the first pattern 10 is oriented towards a first viewing position PI and a light absorber 5 is in overlap with the security device 1 facing the second pattern 20. Thus, an overall reflected light intensity from the security device 1 outshines an overall transmitted light intensity at least by a factor of 5 (reflection configuration). From the first viewing position PI, a first image II is acquired (e.g., by a viewer's naked eye) which represents the first seed pattern 10' ("David").

Fig. 6 schematically shows the security device 1 of fig. 4a in a second viewing mode. In this second viewing mode, the second pattern 20 is oriented towards a second viewing position P2 and a light absorber 5 is in overlap with the security device 1 facing the first pattern 10. Thus, an overall reflected light intensity from the security device 1 outshines an overall transmitted light intensity at least by a factor of 5 (reflection configuration). From the second viewing position P2, a second image 12 is acquired (e.g., by a viewer's naked eye) which relates to the (mirrored) second seed pattern 20' ("Marilyn").

Fig. 7 schematically shows the security device 1 of fig. 4a in a third viewing mode. In this third viewing mode, the first pattern 10 is oriented towards a third viewing position P3 and a UV light source (at an optical excitation wavelength _exc of the photoluminescent material of the third pattern 30) is facing the second pattern 20. Thus, an overall transmitted light intensity through the security device 1 outshines an overall reflected light intensity from the security device 1 at least by a factor of 5. From the third viewing position P3, a third image 13 is acquired (e.g., by a viewer's naked eye) which relates to the third seed pattern 30' ("inventor"). It should be noted here that a viewing mode in which the second pattern 20 faces the third viewing position P3 would obtain the same resulting third image 13 with the exception of a mirroring of the "inventor"-image.

Thus, very specific visual effects are created and the security is enhanced.

Fig. 8 shows a security device 1 according to a third embodiment of the invention. This security device 1 is very similar to the first embodiment described above with regard to fig. 4a with the difference that the first substrate layer 2a comprises a fully transparent window 202 in the area of the first substrate layer 2a in which the first and the third patterns 10, 30 are arranged. This further aggravates counterfeiting attempts. Note that as an alternative or in addition, it is also possible to arrange a fully transparent window on the second substrate layer 2b in the area in which the second and the third patterns 20, 30 are arranged (not shown).

Fig. 9 schematically shows a security document 100 (a banknote with a denomination 501) comprising the security device 1 of fig. 4a. The security device 1 is arranged in a window of the security document 100 and a light absorber 5 consisting of a region with dark color is arranged at a distance to the security device 1. If the security document 100 is folded along a perforated and printed folding line 500, the light absorber 5 can be brought into overlap with the security device 1 facing the first pattern 10 or the second pattern 20, respectively. Thus, a first or second viewing mode in reflection configuration is easier to achieve (also see below for attenuation effects).

Fig. 10 schematically shows the security device 1 of fig. 4a in a third viewing mode in transmission configuration. The security device 1 comprises the multilayer substrate 2 with the first surface 3 and the second surface 4. The first pattern 10 ("David") is arranged on the first surface 3 (only schematically shown). The second pattern 20 ("Marilyn") is arranged on the second surface 4 (only schematically shown). The third pattern 30 ("inventor" + further contributions, generated using the first seed pattern 10', using the second seed pattern 20', and using the third seed pattern 30', see above) is arranged between a first substrate layer 2a and a second substrate layer 2b (only schematically shown). In a third viewing mode in transmission configuration (image 13 at a viewer's third viewing position P3) with a pho- toexcitation of the photoluminescent material, only the third seed pattern 30' ("inventor") is visible because the contributions of "David" and "Marilyn" are invisible in this viewing mode due to the specific generation of the third pattern 30. This is according to the Demichel equation as discussed above. In other words, the first pattern 10 ("David") and the second pattern 20 ("Marilyn") are invisible in the third viewing mode, because combined perceived grayscale differences for the "David" and "Marilyn" pixels are the same or at least below discernible thresholds, just as the regions 11 ' and 12' in figure 1.

Fig. 1 1 schematically shows the security device 1 of fig. 4a in a first viewing mode with specular reflection only. In such a first viewing mode (image II at a viewer's first viewing position PI), for at least one (specularly by the first surface 3) reflected wavelength from the first pattern 10 and/or from the first surface 3, only the first pattern 10 ("David") is visible. This is because, in this model, almost all light is reflected from the first pattern 10 and/or from the first surface 3. Thus, the second pattern 20 does not interact with the light.

Fig. 12 schematically shows the security device 1 of fig. 4a in a first viewing mode with specular reflection and pattern attenuation which is facilitated by a light absorber 5. The situation is essentially the same as in fig. 1 1, but in addition to only specular reflection on the first surface 3 and/or the first pattern 10, a light absorber 5 is arranged facing the second surface 4 and the second pattern 20. This light absorber 5 helps to attenuate the second pattern 20. This is due to the propagation of light and the multiple reflections of the light inside the substrate 2.

Fig. 13 schematically shows a security device 1 according to a fourth embodiment of the invention, the security device 1 comprising a multilayer substrate 2 with a first substrate layer 2a and a second substrate layer 2b. A first pattern 10, a second pattern 20, and a third pattern 30 are arranged as discussed above with regard to figure 4a. Furthermore, the security device 1 comprises a fourth pattern 40 which comprises a photoluminescent material (called "UV" in the figure) and which is arranged on a first surface 3 of the first substrate layer 2a. A fifth pattern 50 not comprising photoluminescent material (called "vis" in the figure) and a sixth pattern 60 comprising photoluminescent material are arranged between the first substrate layer 2a and the second substrate layer 2b. By arranging the patterns 10, 20, 30, 40, 50, and 60 on the substrate 2, in different viewing modes relying on

* a reflection configuration in the visible regime,

* a "reflection" configuration in the UV, i.e., with a photoexcitation of the corresponding pattern from one side and with a detection of emitted light from the photoluminescent material from the same side of the substrate,

* a transmission configuration in the visible regime, and

* a "transmission" configuration in the UV, i.e., with a photoexcitation of patterns comprising photoluminescent material and with a detection of emitted light from the photoluminescent material from the other side of the substrate,

a plurality of different images can be acquired. Depending on how the patterns are generated, these images can reproduce different information content. Thus, a large variety of visual effects is achieved which enhances the security.

Remarks:

Although the above examples rely on near-UV optical excitation wavelengths, also other wavelength regimes such as near-IR can be used. Nonlinear optical effects such as multiphoton absorption can in addition or as an alternative be used to further enhance the security.

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 method for manufacturing a security device (1), the method comprising steps of

- providing a non-uniform first seed pattern (10'),

- providing a non-uniform second seed pattern (20'),

- providing a non-uniform third seed pattern (30'),

- deriving a first pattern (10) using the first seed pattern (10') such that the first pattern (10) is modulated with the first seed pattern (10'),

- deriving a second pattern (20) using the second seed pattern (20') such that the second pattern (20) is modulated with the second seed pattern (20'),

- deriving a third pattern (30) using the third seed pattern (30') and using the first seed pattern (10') such that the third pattern (30) is modulated with the third seed pattern (30') and with the first seed pattern (10'),

- providing an at least partially transparent substrate (2) and, for yielding the security device (1), arranging the first pattern (10), the second pattern (20), and the third pattern (30) on the substrate (2) such that the first pattern (10), the second pattern (20), and the third pattern (30) at least partially overlap,

wherein at least one of a group of but not all of the group of the first pattern (10), the second pattern (20), and the third pattern (30) comprises a photolu- minescent material which is photoexcitable using at least one optical excitation wavelength (kjdxc), and

wherein a modulation in the first pattern (10) from the first seed pattern (10') is spatially aligned with a modulation in the third pattern (30) from the first seed pattern (10').

2. The method of claim 1 comprising a step of

- selecting transmittances, reflectivities, and photolumincscent properties of the first pattern (10), of the second pattern (20), of the third pattern (30), and of the substrate (2) such

* that in a first viewing mode, an acquirable first image (II) of the security device (1) represents the first seed pattern (10'), wherein the first image (II) is acquirable at at least a first optical detection wavelength (>„_detl),

* that in a second viewing mode, an acquirable second image (12) of the security device (1) represents the second seed pattern (20'), wherein the second image (12) is acquirable at at least a second optical detection wavelength _det2), and * that in a third viewing mode, an acquirable third image (13) of the security device (1) represents the third seed pattern (30'), wherein the third image (13) is acquirable at at least a third optical detection wavelength (X_det3),

wherein at least one of a group of the first optical detection wavelength ( _detl), the second optical detection wavelength (λ_άβί2), and the third optical detection wavelength ( _det3) is an optical emission wavelength (λ_βηι) of the photoluminescent material, and

wherein at least one of a group of but not all of the group of the first viewing mode, the second viewing mode, and the third viewing mode comprises a photoexcitation of the photoluminescent material at the at least one optical excitation wavelength _exc) of the photoluminescent material.

3. The method of claim 2

wherein the first image (II) in the first viewing mode is acquirable in a reflection configuration.

4. The method of any of the claims 2 or 3

wherein the second image (12) in the second viewing mode is acquirable in a reflection configuration.

5. The method of any of the claims 2 to 4

wherein the third image (13) in the third viewing mode is acquirable in a transmission configuration or in a reflection configuration.

6. The method of any of the claims 2 to 5

wherein the third pattern (30) comprises photoluminescent material, and

in particular wherein the third pattern (30) is substantially transparent for the first optical detection wavelength ( _detl ), the second optical detection wavelength ( _det2), and the third optical detection wavelength (l_det3).

7. The method of any of the preceding claims

wherein the first pattern ( 10) is arranged on a first surface (3) of the substrate (2) and wherein the second pattern (20) is arranged on a second surface (4) of the substrate (2), which second surface (4) is opposite the first surface (3).

8. The method of claim 6 and of claim 7 wherein the third pattern (30) is arranged on the first surface (3) or on the second surface (4) of the substrate (2).

9. The method of any of the preceding claims

wherein the substrate (2) comprises at least a first substrate layer (2a) and a second substrate layer (2b), and

in particular wherein at least one of the patterns (10, 20, 30) is arranged between the first substrate layer (2a) and the second substrate layer (2b).

10. The method of any of the claims 2 to 9

wherein the first optical detection wavelength ( _detl), the second optical detection wavelength ( _det2), and the third optical detection wavelength ^__det3) are between 380 nm and 780 nm.

11. The method of any of the claims 2 to 10

wherein the optical excitation wavelength (λ_εχο) is shorter than said first optical detection wavelength f^,_detl), said second optical detection wavelength ^_det2), and said third optical detection wavelength ( _det3).

12. The method of any of the claims 2 to 1 1

wherein the optical excitation wavelength (k_exc) is between 200 nm and 380 nm.

13. The method of any of the preceding claims wherein a thickness of the substrate (2) is smaller than 500 μηι, in particular smaller than 120 μηι.

14. The method of any of the preceding claims wherein the substrate (2) is substantially non-scattering for the first optical detection wavelength ( __detl), for the second optical detection wavelength ( _det2), and for the third optical detection wavelength ( _det3).

15. The method of any of the preceding claims wherein the first pattern (10), the second pattern (20), and the third pattern (30) are arranged in a window (202) of the substrate (2) or in a window (202) of at least one substrate layer (2a, 2b) of the substrate (2).

16. The method of any of the preceding claims wherein at least one of the group of but not all of the group of the first pattern (10), the second pattern (20), and the third pattern (30) is arranged on the substrate (2) using visible ink.

17. The method of any of the preceding claims

wherein at least one of the group of, in particular all of the group of, the first pattern (10), the second pattern (20), and the third pattern (30) is arranged on the substrate (2) using offset printing, screen printing, or sublimation printing.

18. The method of any of the preceding claims

wherein at least one of the group of the first pattern (10), the second pattern (20), and the third pattern (30) is arranged on the substrate (2) using laser ablation and/or laser darkening.

19. The method of any of the preceding claims

wherein the step of deriving the third pattern (30) comprises using an inversion of the first seed pattern (10').

20. The method of any of the preceding claims

wherein the step of deriving the third pattern (30) in addition comprises using the second seed pattern (20'), in particular using an inversion of the second seed pattern (20'),

wherein the third pattern (30) is modulated with the second seed pattern (20'), and

wherein a modulation in the second pattern (20) from the second seed pattern (20') is spatially aligned with a modulation in the third pattern (30) from the second seed partem (20').

21. The method of any of the preceding claims

wherein more than one pattern (10, 20, 30) comprise photolumines- cent material.

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

- providing a non-uniform fourth seed pattern (40'),

- deriving a fourth pattern (40) using * the fourth seed pattern (40') and

* using at least one of the first seed pattern (10') and the second seed pattern (20'), and

- arranging the fourth pattern (40) on the substrate (2) such that the fourth pattern (40) at least partially overlaps with the first pattern (10), the second pattern (20), and the third pattern (30).

23. A security device (1) obtained by a method of any of the preceding claims.

24. A security device (1), in particular the security device (1) of claim 23, comprising

- an at least partially transparent substrate (2),

- a first pattern (10) derived using a non-uniform first seed pattern (10') such that the first pattern (10) is modulated with the first seed pattern (10'), wherein the first pattern (10) is arranged on a first surface (3) of the substrate (2),

- a second pattern (20) derived using a non-uniform second seed pattern (20') such that the second pattern (20) is modulated with the second seed pattern (20'), wherein the second pattern (20) is arranged on a second surface (4) of the substrate (2),

- a third pattern (30) derived using a non-uniform third seed pattern (30'), using the first seed pattern (10'), and using the second seed pattern (20') such that the third pattern (30) is modulated with the third seed pattern (30'), with the first seed pattern (10'), and with the second seed pattern (20'),

wherein the third pattern (30) is arranged on the substrate (2) such that the first pattern (10), the second pattern (20), and the third pattern (30) at least partially overlap, and

wherein the third pattern (30) comprises a photoluminescent material which is photoexcitable using at least one optical excitation wavelength ( _exc), wherein a modulation in the first pattern (10) from the first seed pattern (10') is spatially aligned with a modulation in the third pattern (30) from the first seed pattern (10'), and

wherein a modulation in the second pattern (20) from the second seed pattern (20') is spatially aligned with a modulation in the third pattern (30) from the second seed pattern (20').

25. A security document (100), in particular a banknote, a passport, a document of value, a certificate, or a credit card, comprising a security device (1) of any of the claims 23 or 24 for verifying an authenticity of the security document (100).

26. The security document (100) of claim 25 further comprising a light absorber (5), in particular arranged at a distance to the security device (1).

27. A method for verifying an authenticity of a security document (100) of any of the claims 25 to 26, the method comprising steps of

- in a first viewing mode from a first viewing position (PI) acquiring a first image (II) of a security device (1) of the security document (100) at at least a first optical detection wavelength ^__detl),

- in a second viewing mode from a second viewing position (P2) acquiring a second image (12) of the security device (1) at at least a second optical detection wavelength ( _det2),

- in a third viewing mode from a third viewing position (P3) acquiring a third image (13) of the security device (1) at at least a third optical detection wavelength ( _det3),

- in at least one of the group of but not in all of the group of the first viewing mode, the second viewing mode, and the third viewing mode photoexciting a photoluminescent material of at least one pattern (10, 20, 30) of the security device (1) using at least one optical excitation wavelength (>,_exc),

wherein at least one of a group of the first optical detection wavelength ( jdetl), the second optical detection wavelength ( _det2), and the third optical detection wavelength ( _det3) is an optical emission wavelength (λ_εηι) of the photoluminescent material, and

wherein at least one of a group of but not all of the group of the acquired first image (II), the acquired second image (12), and the acquired third image (13) is based on emission light from said photoluminescent material,

- deriving the authenticity of the security document (100) using the acquired first image (II), using the acquired second image (12), and using the acquired third image (13).

28. The method of claim 27

wherein in at least one of a group of but not in all of the group of the first viewing mode, the second viewing mode, and the third viewing mode, a light source is located on the same side of the substrate (2) as the respective viewing posi' tion (PI , P2. P3) in the respective viewing mode.