Security device for security document

Abstract

A security device for verifying an authenticity of a security document comprises an at least partially transparent substrate with a first surface and a second surface. A first pattern is arranged on the first surface. A second pattern is arranged on said second surface. The first and the second pattern each comprise a plurality of pixels with at least three different gray levels visible from a macroscopic perspective. The first and second pattern cover only gray levels in a range between 20% black and 80% black, in particular between 35% black and 65% black. The first pattern is inverted with respect to the second pattern.

Claims

1. A security device for verifying an authenticity of a security document, in particular of a banknote, a passport, a document of value, a certificate, or a credit card, the security device comprising: an at least partially transparent substrate and with a first surface and a second surface, a first pattern arranged on the first surface of the substrate, a second pattern arranged on the second surface of the substrate, wherein the first pattern on the first surface is arranged fully in register with the second pattern on the second surface, wherein the first pattern and the second pattern each comprise a plurality of pixels with at least three different gray levels visible from a macroscopic perspective, wherein the first pattern and the second pattern cover only gray levels in a range between 20% black and 80% black, wherein the first pattern is inverted with respect to the second pattern, such that a gray level of x % in said first pattern is inverted to a gray level of (100%-x %) in said second pattern.

2. The security device of claim 1, wherein the first pattern and the second pattern are applied by absorbing inks.

3. The security device of claim 1 wherein said substrate comprises multiple layers.

4. The security device of claim 1, wherein the gray levels of the first pattern and of the second pattern are indiscernible at least when an overall transmitted light intensity through said security device outshines an overall reflected light intensity from said security device at least by a factor of 5.

5. The security device of claim 1 further comprising a third pattern arranged on or in said substrate, wherein said third pattern comprises a plurality of pixels visible from a macroscopic perspective, wherein different gray levels of said third pattern are discernible in said a transmission viewing mode and in a reflection viewing mode.

6. The security device of claim 5, wherein said third pattern comprises a plurality of pixels with at least three different gray levels visible from a macroscopic perspective.

7. The security device of claim 1 wherein a transmittance of said substrate is higher than 50%, at least for at least one transmitted wavelength through said security device.

8. A security document, in particular a banknote, a passport, a document of value, a certificate, or a credit card, wherein the security document comprises a security device of claim 1, in particular arranged in a window of said security document.

9. The device of claim 1 wherein said substrate is partially reflecting.

10. The device of claim 1 wherein the first pattern and the second pattern cover only gray levels in a range between 35% black and 65% black.

11. The device of claim 1 wherein said substrate is specularly reflecting.

12. A method for generating a security device for verifying an authenticity of a security document, in particular of a banknote, a passport, a document of value, a certificate, or a credit card, the method comprising steps of providing an at least partially transparent substrate and with a first surface and a second surface, providing a first source image with at least three different gray levels, modifying a contrast of said first source image to generate a first modified image, wherein the first modified image covers only gray levels in a range between 20% black and 80% black, inverting the first modified image for yielding a second modified image, such that a gray level of x % in said first pattern is inverted to a level of (100%-x %) in said second pattern, generating a first pattern from said first modified image, generating a second pattern from said second modified image, applying the first pattern on said first surface of said substrate, applying the second pattern on said second surface of said substrate, wherein the first pattern on the first surface is arranged fully in register with the second pattern on the second surface.

13. The method of claim 12, wherein for generating the first pattern and/or second pattern, a third pattern is mixed with the first modified image to generate the first pattern and/or wherein a third source image is mixed with the second modified image to generate the second pattern such that in a transmission viewing mode only the third pattern is visible while the first pattern and the second pattern cancel each other out.

14. The method of claim 12 wherein the first pattern and the second pattern are printed by absorbing inks.

15. The method of claim 12 comprising the steps of printing the first pattern on said first surface and printing the second pattern on said second surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) 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,

(3) 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 pattern 10 and a second pattern 20 arranged on opposite surfaces 3, 4 of said substrate 2,

(4) FIG. 3 shows a security document 100 comprising a security device 1 according to a second embodiment of the invention,

(5) 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,

(6) FIG. 5 schematically shows a security document 100 comprising the security device 1 of FIG. 2, a light absorber 5, and a folding line 500,

(7) FIG. 6 schematically shows the security device 1 of FIG. 2 in a transmission viewing mode,

(8) FIG. 7 schematically shows the security device 1 of FIG. 2 in a reflection viewing mode with specular reflection, and

(9) FIG. 8 schematically shows the security device 1 of FIG. 2 in a reflection viewing mode with specular reflection and second pattern attenuation by a light absorber 5.

(10) FIG. 9 schematically shows another security device in the transmission viewing mode with an illustration of the light intensity in different steps.

(11) FIG. 10 schematically shows another security device in the reflection viewing mode with an illustration of the light intensity in different steps.

MODES FOR CARRYING OUT THE INVENTION

(12) FIG. 1 shows a first pattern 10 and a second pattern 20 for use in a security device 1 (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 record to the first pattern 10, i.e., it is a grayscale image with a gradient from 100% black to 0% black.

(13) 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 FIG. 1 is observed. Specifically, a grayscale image going from 100% black to 75% black back to 100% black is yielded.

(14) The upper part of FIG. 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.

(15) 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).

(16) 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.

(17) 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 C1 with a density d1 and of a layer of color C2 with a density d2 (both layers having a random halftoning), a

(18) surface coverage of white w=(1?d1)?(1?d2),

(19) a perceived color C1=d1?(1?d2), and

(20) a perceived color C2=d2?(1?d1).

(21) If both colors C1 and C2 are black and if d2=1?d1 (inverted patterns), the density of black b (i.e., b=1?w) for the superposed image equals to b=1?d+d12. This corresponds to the curve labelled 200 in the diagram of FIG. 1.

(22) 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 region 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.sup.2, 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+0.2.sup.2=84% black. Note that a 100% transmittance of the substrate is assumed here (substrate not shown!).

(23) 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.

(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 indiscernible 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 region 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.

(25) 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 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.

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

(27) FIG. 2 shows a security device 1 with a transparent flat flexible multilayer polymer substrate 2 with a thickness of 110 ?m. 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.

(28) As shown for the first image I1 taken from a first viewing position P1 in a transmission viewing mode, the first region 11 is indiscernible from the second region 12, because the whole image appears at a uniform gray level of 84% black. As discussed above with regard to FIG. 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.

(29) 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.

(30) 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.

(31) FIG. 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 FIG. 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.

(32) Other than that, as it is schematically shown on the right hand side of FIG. 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 I1 from a first viewing position P1), while they are discernible in a reflection viewing mode (second image I2 from a second viewing position P2 with the first pattern 10 being oriented towards the second viewing position and third image I3 from a third viewing position P3 with the second pattern 20 being oriented towards the third viewing position P3).

(33) Note that in this embodiment, as in the first embodiment shown in FIG. 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 FIG. 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 FIG. 1). In other words, as an example, here, only gray levels between 35% black and 65% black are covered by the patterns 10,20, thus leading to superposed black levels (in the first image I1) between 77.25% and 75% (see above). As discussed above, this is not discernible by a viewer's naked eye.

(34) 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 FIG. 2 and to the second embodiment shown in FIG. 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 application 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.

(35) Then, as it is shown in the first image I1, 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.

(36) However, in a reflection viewing mode as shown in second image I2 (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).

(37) In a reflection viewing mode as shown in third image I3 (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).

(38) 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).

(39) 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 I1 at a viewer's first viewing position P1), for at least one transmitted wavelength through said security device, said first region is indiscernible from said second region and said third region is indiscernible from said fourth region (only schematically shown).

(40) 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 I2 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 discernible from said second region (only schematically shown).

(41) FIG. 8 schematically shows the security device 1 of FIG. 2 in a reflection viewing mode with specular reflection and second pattern attenuation which is facilitated by a light absorber 5. The situation is essentially 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 propagation of light and the multiple reflections of the light inside the substrate 2.

(42) Yet another embodiment is shown in FIG. 9. It shows a security device in the transmission viewing mode. The first pattern 10 is printed on the first surface 3 and the second pattern 20 is printed on the second surface 4. The first and second patterns e.g. comprise black levels in a range between 20% and 80% according to the color gradients shown in FIG. 9. The black levels on the first pattern are inverted with respect to the black levels on the second pattern. The thicknesses of the arrows show the light intensity in different steps on the way the light goes through the security device. The light, originating from a light source, goes uniformly through the second surface 4 and enters the substrate 2. Light is absorbed by the second pattern 20 in dependence of the black levels of the second pattern. Consequently the light intensity going through the substrate 2, which is shown by the thickness of the arrows in the substrate 2, is the smaller the higher the black level is on the second pattern 20. Since the black level on the first pattern 10 is inverted with respect to the second pattern 20, the light intensity is homogenized after having left the substrate 2 and having passed the first surface 3.

(43) FIG. 10 shows the same security device as in FIG. 9, but in the reflection viewing mode. The substrate can reflect the light specularly or diffusely. The arrows show that the lower the black level on the first surface 3 is, the more light is reflected, because the black ink absorbs the light and does not reflect it. The effect of the second surface 4 is attenuated by the light absorber 5.

(44) Remark:

(45) 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.