Technical Field

The invention relates to a security device for verifying an authenticity of a security document as well as to a security document, e.g., a banknote, a passport, a document of value, a certificate, or a credit card which comprises such a security device. Furthermore, the invention relates to a method for verifying the au ^¬ thenticity of such a security 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 .

Disclosure of the Invention

It is an object of the present invention to provide a security device for verifying an authenticity of a security document. 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 the authenticity of such a security document.

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

Accordingly, a security device for verifying an authenticity of a security document (such as a banknote, a passport, a document of value, a certificate, or a credit card) comprises an at least partially transparent substrate with a first surface and a second surface.

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, in a transmission viewing mode, a nonzero amount of light can be shone through said substrate. Advantageously, a transmittance of the substrate is higher than 50%, at least for one transmitted wavelength (which is in particular in the visible regime between 380 nm and 780 nm) .

Advantageously, the substrate is flat and/or flexible (e.g., its thickness is smaller than 500 μπι, in particular smaller than 120 μπι) and the second surface can be on the opposite side of a flat substrate than the first surface. This simplifies the application in security documents which are usually flat and/or flexible to some degree.

Furthermore, the security device comprises a first pattern (e.g., a halftone, grayscale, or a color image) which is arranged on said first surface of said substrate. This first pattern comprises at least a first region and a second region which second region is non- overlapping with said first region. In other words, the first and second regions are distinct regions of the first pattern.

Herein, the term "region" relates to an area of the respective pattern with a uniform visual appear ^¬ ance within said area, e.g., due to

* a uniform dithering, e.g., in a halftone pattern,

* a uniform grayscale value in a grayscale pattern, or

* a uniform color in a color pattern.

If the pattern is pixelated, i.e., comprises a plurality of pixels, a specific region can comprise one or more contiguous or discontiguous pixels of the respective pattern.

Furthermore, the security device comprises a second pattern (e.g., a halftone, grayscale, or a color image) which is arranged on said second surface of said substrate, e.g., opposite said first surface (see above) . The second pattern comprises at least a third region and a fourth region which fourth region is non-overlapping with said third region. In other words, the third and fourth regions of the second pattern are distinct regions .

As an effect, at least two - usually more than two - regions are comprised in the first and second patterns each which enhances the information content of the patterns .

Furthermore, the first region of the first pattern fully coincides with the third region of the second pattern and the second region of the first pattern fully coincides with the fourth region of the second pattern .

In an example,

* the first pattern is arranged on an opposite side of a flat substrate with respect to the second pattern,

* the first region has the same shape, dimensions, orientation, and position as the third region, and

* the second region has the same shape, dimensions, orientation, and position as the fourth region.

According to the invention, transmittances and reflectivities of said first region, of said second region, of said third region, and of said fourth region are selected such

* that in a transmission viewing mode, for at least one transmitted wavelength (in particular in the visible regime between 380 nm and 780 nm) through said security device, said first region is indiscernible from said second region and said third region is indiscernible from said fourth region. E.g., AE94-values are below 1.8 or the first and second regions and for the third and fourth regions, respectively, in the transmission viewing mode .

As an effect, a transmission-mode-viewer (e.g., with a naked eye without visual aids) cannot discern the first region from the second region of the first pattern and he can also not discern the third region from the fourth region of the second pattern. Thus, information contained in the first and second patterns (i.e., in the first/second and third/fourth regions, respectively) is not reconstructable in the transmission viewing mode .

However, in a reflection viewing mode, for at least one reflected wavelength (which is advantageously the same wavelength than the transmitted wavelength discussed above) from said security device, said first region is discernible from said second region.

As an effect, a reflection-mode-viewer can discern the first region from the second region of the first pattern and thus information contained in the first pattern (i.e., the first/second regions) is reconstructable in the reflection viewing mode.

As an effect, according to the invention, the visual appearance and reconstructable information content of the security device depends on the viewing mode and security is thus enhanced considerably.

Advantageously, the substrate comprises multiple layers with the same or different optical properties (such as transmission spectra). Thus, more specific effects can be realized and security is enhanced.

Advantageously, the first and/or the second pattern can be covered with one or more additional layer (s), e.g., for reducing specular reflections from the first and/or second substrate surface. In an advantageous embodiment of the security device, the first pattern is applied, in particular printed (e.g., via offset printing, screen printing, or sublimation printing) , onto said first surface of said substrate and/or the second pattern is applied, in particular printed (e.g., via offset printing or screen printing, or sublimation printing) , onto said second surface of said substrate. Thus, the security device can be manufactured more easily.

Optionally, a primer layer can be applied be ^¬ low the first and/or second pattern in order to ensure the stability of the printed inks.

In another advantageous embodiment of the security device, the first region of the first pattern has an inverted transmittance and an inverted reflectivity with respect to the third region of said second pattern. Furthermore, in this embodiment, the second region of the first pattern has an inverted transmittance and an inverted reflectivity with respect to the fourth region of said second pattern.

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

Thus, it is easier to select the transnait- tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects occur in the transmission and reflection viewing modes.

More advantageously, the whole first and second patterns (i.e., all regions in the respective patterns) have inverted transmittances and reflectivities with respect to each other.

In another advantageous embodiment of the security device, the transmittances and reflectivities of the first to fourth regions are each selected such that for said at least one transmitted wavelength through said security device, a transmitted light intensity both through said first region and through said third region is indiscernible from a transmitted light intensity through said second region and through said fourth region. This is at least then the case when an overall (i.e., spatially integrated over the whole security device) transmitted light intensity through the security device (in the transmission viewing mode) 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. In other words, in this embodiment, a definition for "transmission viewing mode" is that the overall transmitted light intensity through the security device outshines an overall reflected light intensity from the security device at least by the above- mentioned factor.

Thus, it is easier to select the transmittances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects occur in the transmission viewing mode.

In another advantageous embodiment of the se ^¬ curity device, the transmittances and reflectivities of said first to fourth regions are each selected such that for said at least one reflected wavelength from said security device, a reflected light intensity from said first region is discernible from a reflected light intensity from said second region. This is at least then the case when an overall (i.e., spatially integrated over the whole security device) reflected light intensity from the security device outshines an overall (i.e., spatially in ^¬ tegrated over the whole security device) transmitted light intensity through said security device at least by a factor of 5. In other words, in this embodiment, a definition for "reflection viewing mode" is that the overall reflected light intensity from the security device outshines an overall transmitted light intensity through the security device at least by the above-mentioned factor.

Thus, it is easier to select the transmit- tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects occur in the reflection viewing mode.

In another advantageous embodiment, the security device further comprises a third pattern (e.g., a halftone, grayscale, or a color image) arranged on or in said substrate (or, in case of a multilayered substrate, e.g., between different substrate layers). The third pattern comprises at least a fifth region and a sixth region which is non-overlapping with the fifth region. In other words, the fifth and sixth regions are distinct regions of the third pattern. Transmittances and reflectivities of the first to fourth regions and of the fifth region and of the sixth region, respectively, are selected such that the fifth region is discernible from the sixth region in said transmission viewing mode as well as in said reflection viewing mode.

As an effect, a transmission-mode-viewer as well as a reflection-mode-viewer can discern the fifth region from the sixth region of the third pattern and reconstruct information contained in the third pattern (i.e., the fifth/sixth regions) . This is possible in the transmission as well as in the reflection viewing modes. In yet another advantageous embodiment of the security device, a transmitted light intensity through the first region and through the third region differs by less than 5% from a transmitted light intensity through the second region and through the fourth region, at least for said at least one transmitted wavelength through said security device.

This makes it easier to select the transmit- tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects (e.g., the indiscernibility in the transmission viewing mode) occur in the transmission viewing mode. In another advantageous embodiment of the security device, a reflected light intensity from said first region differs by more than 5% from a reflected light intensity from said second region, at least for said at least one reflected wavelength from said security device.

This makes it easier to select the transmit- tances and reflectivities of the first to fourth regions such that the above-discussed visual appearance effects (e.g., the discernibility in the reflection viewing mode) occur in the reflection viewing mode.

In another advantageous embodiment of the security device, the first pattern and/or the second pat ^¬ tern and/or the substrate comprises a color filter. This makes it easier to select one or more transmitted and/or reflected wavelength ( s ) .

In yet another advantageous embodiment of the security device, the first pattern comprises at least one region which does not fully coincide with a region of said second pattern. Thus, additional information can be comprised in the first pattern which enhances security. As another aspect of the invention, a security document (e.g., a banknote, a passport, a document of value, a certificate, or a credit card) comprises a security device as described above. The security device is advantageously arranged in a window (i.e., a transparent region) of (the substrate of) the security document. As an effect, the visual appearance and reconstructable information content of the security document can be more easily made dependent on the viewing mode. Thus, security is enhanced and counterfeiting is considerably aggravat ^¬ ed .

Advantageously, such a security document fur ^¬ ther 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, folding line, the light absorber can be brought into overlap with the security device. As an effect, the amount of transmitted light is reduced by the light absorber and thus a reflection viewing mode 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 said at least one reflected wavelength from said security device and/or the light absorber has a transmittance of less than 50% at least for said at least one transmitted wavelength through said 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, the reflection viewing mode of the security device 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 comprises steps of

- providing the security document which com- 5 prises a security device as described above,

- from a first viewing position acguiring a first image of said security device in a transmission viewing mode (e.g., against a ceiling lamp),

- from a second viewing position (which can0 be the same or a different position than the first viewing position) acquiring a second image of said security device in a reflection viewing mode. Hereby, the first pattern is oriented towards the second viewing position.

Furthermore, the method comprises a step ofs - deriving said authenticity of said security document using the first (transmission viewing mode) image and using the second (reflection viewing mode) image.

Because of the specific and different visual appearances in transmission viewing mode (first regiono cannot be discerned from third region and/or second region cannot be discerned from fourth region) and reflection viewing mode (first region can be discerned from third region) , the authenticity of the security document is easier to derive.

5

Advantageously, during the step of acquiring said second image, an overall reflected light intensity from said security device outshines an overall transmit ^¬ ted light intensity through said security device at leasto by a factor of 5. Thus, the reflection viewing mode is easier to establish.

In another advantageous embodiment, during said step of acquiring said first image, an overall

5 transmitted light intensity through said security device outshines an overall reflected light intensity from said security device at least by a factor of 5. Thus, the transmission viewing mode is easier to establish.

Advantageously, the method comprises a step of bringing a light absorbing device into an overlap with said security device. Thus, an amount of transmitted light through the security device is reduced and the reflection viewing mode is easier to establish. Then, the step of acquiring said second image of said security de- vice is carried out with said light absorbing device being arranged in said overlap with said security device, e.g., opposite said second viewing position. This simplifies the handling of the security document for acquiring the reflection viewing mode image.

Advantageously, from a third viewing position (which is, again, the same or different from the first and/or the second viewing position ( s )) , a third image of said security device is acquired in a reflection viewing mode, but now with said second pattern being oriented to ^¬ wards said third viewing position. Then, this third image is also used in said step of deriving said authenticity of said security document. Thus, the security can be fur ^¬ ther enhanced.

Remark :

The invention is not limited to halftone or grayscale patterns. Although the description and figures herein mainly focus on halftone and 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 alt ^¬ hough 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 ap ^¬ parent when consideration is given to the following de ^¬ tailed description thereof. Such description makes reference to the annexed drawings, wherein:

Fig. 1 shows a first pattern 10 and a second pattern 20 for use in a security device 1 as well as a combination of this first pattern 10 with this second pattern 20 in a transmission viewing mode,

Fig. 2 shows a security device 1 according to a first embodiment of the invention, the security device 1 comprising a transparent substrate 2 and a first pat ^¬ tern 10 and a second pattern 20 arranged on opposite surfaces 3, 4 of said substrate 2,

Fig. 3 shows a security document 100 compris ^¬ ing a security device 1 according to a second embodiment of the invention,

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

Fig. 5 schematically shows a security docu ^¬ ment 100 comprising the security device 1 of fig. 2, a light absorber 5, and a folding line 500,

Fig. 6 schematically shows the security device 1 of fig. 2 in a transmission viewing mode,

Fig. 7 schematically shows the security device 1 of fig. 2 in a reflection viewing mode with specular reflection, and Fig. 8 schematically shows the security de ^¬ vice 1 of fig. 2 in a reflection viewing mode with specular reflection and second pattern attenuation by a light absorber 5.

Modes for Carrying Out the Invention

Fig. 1 shows a first pattern 10 and a second pattern 20 for use in a security device 1 according to the invention (the security device 1 is not shown here) . In this figure, the first pattern 10 is a grayscale image with a gradient from 100% white (i.e., 0% black) to 100% black. 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.

When the first pattern 10 is overlaid with the second pattern 20 (i.e., when a first region 11 fully coincides with a third region 23 and a second region 12 fully coincides with fourth region 24) and viewed in a 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 shov/s 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 11+23 of the curve labeled 200 in the diagram) . Similarly, a third region 23 is indiscernible from a fourth region 24 of the second pattern 20, because both the third region 23 and the fourth region 24 show the same gray levels of 84% black (see the above-referenced points) .

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.

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 halftoning) , a

surface coverage of white w= ( 1-dl ) x ( l-d2 ) , a perceived color CI = dl x (l-d2), and a perceived color C2 = d2 x (1-dl) .

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 + dl ² . 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 both are 80% black. The second region 12 of the first pattern 10 and the third region 23 of the second pattern 20 both are 20% black. Hence, the first re ^¬ gion 11 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 + 0.8 ² , i.e., b = 84% black. This is the same value as for the superposi- tion of the second region 12 with the fourth region 24, namely b = 1 - 0.2 + 0.2 ² = 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 in transmission viewing mode of the superposed inversed images ranges between b = 100% and 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 or indis ^¬ cernible 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 11' would be indiscernible from a second re- gion 12' and a third region 23' would be indiscernible from a fourth region 24' . 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 re- flection 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 full superposition of the first pattern 10 with the second pattern 20 does not take place any more and the first region 11 is thus 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.

Thus, very specific patterns can be created under different viewing conditions and security in enhanced .

Fig. 2 shows a security device 1 with a transparent flat flexible multilayer polymer substrate 2 with a thickness of 110 μηι. A first pattern 10 (a grayscale image) is applied, in particular printed, onto a first surface 3 of the substrate 2 and a second pattern 20 (a grayscale image) is applied, in particular printed, onto a second opposite surface 4 of the substrate 2. The first pattern 10 comprises a first region 11 ("OFS" and "123" in 80% black) and a second region 12 (background in 20% black) which does not overlap but is adjacent the first region 11. The second pattern 20 comprises a third region 23 ("OFS" and "123" in 20% black) and a fourth region 24 (background in 80% black) which does not overlap but is adjacent to the third region 23. The first region 11 fully coincides with the third region 23 and the second region 12 fully coincides with the fourth region 24. This is e.g. achieved by a high registration printing process of the first and second patterns 10, 20 onto the first and second surfaces 3,4 of the polymer substrate 2.

As shown for the first image II taken from a first viewing position PI in a transmission viewing mode, the first region 11 is indiscernible from the second re ^¬ gion 12, because the whole image appears at a uniform gray level of 84% black. As discussed above with regard to figure 1, other combinations of black levels would be possible as well as long as the first/second regions 11,12 and the third/fourth regions 23,24 remain indiscernible in the transmission viewing mode.

However, in a reflection viewing mode, which is here facilitated by overlaying the security device 1 with a light absorber 5, the first region 11 is discernible from the second region 12. As shown in a second image 12 taken from a second viewing position P2 (with the first pattern 10 being oriented towards said second viewing position P2) in a reflection viewing mode, the first region 11 appears in a darker color than the surrounding second region 12.

A third image 13 taken from a third viewing position P3 (with the second pattern 20 being oriented towards said third viewing position P3) in a reflection viewing mode shows the third region 23 in a lighter color than the surrounding fourth region 24. Thus, the third region 23 is discernible from the fourth region 24.

Figure 3 shows a security document 100 comprising a security device 1 according to a second embodiment of the invention. The security device 1 is very similar to the first embodiment shown in figure 2 with the exception that the first pattern 10 and the second pattern 20 are inverted grayscale images each comprising a plurality of pixels and not only two distinct regions. Thus, the first and second regions 11,12... (and likewise the third and fourth regions 23, 24,...) are in general defined by a single pixel each and not any more by geometrical letters/numbers.

Other than that, as it is schematically shown on the right hand side of figure 3, the security device 1 according to the second embodiment behaves very much like the first embodiment discussed above, i.e., the different regions /pixels in one pattern/image are indiscernible in a transmission viewing mode (first image II from a first viewing position PI), while they are discernible in a reflection viewing mode (second image 12 from a second viewing position P2 with the first pattern 10 being oriented towards the second viewing position and third image 13 from a third viewing position P3 with the second pattern 20 being oriented towards the third viewing position P3) .

Note that in this embodiment, as in the first embodiment shown in figure 2, the first pattern 10 is inverted with respect to the second pattern 20. Here, additionally, care should be taken that grayscale values of the first and second patterns 10, 20 (x-values in a histogram, see ranges in figure 1 at the top!) only cover a range of black levels that lead to indiscernible resulting black level differences in transmission viewing mode (see resulting black level differences on the y-axis of the diagram of figure 1) . In other words, 3.S 3.Π example , here, only gray levels between 35% black and 65% black are covered by the patterns 10,20, thus leading to super ^¬ posed black levels (in the first image II) between 77.25% and 75% (see above) . As discussed above, this is not dis- cernible by a viewer's naked eye.

Fig. 4 schematically shows a security device 1 according to a third embodiment of the invention. The security device 1 is very similar to the first embodiment shown in figure 2 and to the second embodiment shown in figure 3 with the exception that the security device 1 additionally to the first pattern 10 (dark ^, OFS"=first region 11, medium background=second region 12, light "123"=additional region of the first pattern 10) on the first surface 3 of the substrate 2 (which is not shown here for clarity) and to the second pattern 20 (inverted with respect to the first pattern 10, i.e., light

"OFS"=third region 23, medium background=fourth region 24, dark " 123"=additional region of the second pattern 20) on the second surface 4 of the substrate 2 comprises a third pattern 30 that is mixed into the first pattern 10 and into the second pattern 20 before the actual ap- plication of the patterns. Another option which is not shown here would be to apply, in particular print, the third pattern 30 between single layers of the multi- layered substrate. The third pattern 30 comprises a fifth region 35, a sixth non-overlapping region 36, ... which are single pixels each.

Then, as it is shown in the first image II, in a transmission viewing mode, only the third pattern 30 is visible because the first pattern 10 and the second pattern 20 cancel each other out as discussed above with regard to the first two embodiments of the invention.

However, in a reflection viewing mode as shown in second image 12 (first pattern 10 is oriented towards the second viewing position P2), both the first pattern 10 and the third pattern 30 are visible (i.e., the first region 11 is discernible from the second region 12 and, respectively, the fifth region 35 is discernible from the sixth region 36) .

In a reflection viewing mode as shown in third image 13 (second pattern 20 is oriented towards the third viewing position P3), both the second pattern 20 and the third pattern 30 are visible (i.e., the third region 23 is discernible from the fourth region 24 and, respectively, the fifth region 35 is discernible from the sixth region 36).

Fig. 5 schematically shows a security document 100 (a banknote with a denomination 501) comprising the security device 1 of fig. 2. The security device 1 is arranged in a window of the security document 100 and a light absorber 5 consisting of a region with 100% black is arranged at a distance to the security device 1. If the security document 100 is folded along a folding line 500, the light absorber 5 can be brought into overlap with the security device 1 and thus a reflection viewing mode is easier to achieve (see below) . Fig. 6 schematically shows the security device 1 of fig. 2 in a transmission viewing mode. The security device 1 comprises the transparent substrate 2 with the first surface 3 and the second surface 4. The first pattern 10 with the first region 11 and the second region 12 is arranged on the first surface 3 (only schematically shown) . The second pattern 20 with the third region 23 and the fourth region 24 is arranged on the second surface 4 (only schematically shown) . In a transmission viewing mode (image II at a viewer's first viewing position PI), for at least one transmitted wavelength through said security device, said first region is indis ^¬ cernible from said second region and said third region is indiscernible from said fourth region (only schematically shown) .

Fig. 7 schematically shows the security device 1 of fig. 2 in a reflection viewing mode with specular reflection. In a reflection viewing mode (image 12 at a viewer's second viewing position P2), for at least one (specularly by the first surface 3) reflected wavelength from said security device, said first region is discerni ^¬ ble from said second region (only schematically shown) .

Fig. 8 schematically shows the security de ^¬ vice 1 of fig. 2 in a reflection viewing mode with specu ^¬ lar reflection and second pattern attenuation which is facilitated by a light absorber 5. The situation is es ^¬ sentially the same as in fig. 7, but in addition to only specular reflection on the first surface 3, a light absorber 5 is arranged at the second surface 4 and helps to attenuate the second pattern 20. This is due to the prop ^¬ agation of light and the multiple reflections of the light inside the substrate 2.

Remark : 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 prac- ticed within the scope of the following claims.