Method of forming a security feature for a security document

Abstract

A plurality of optically variable inks (2, 3, 5, 8) are used to print a multiplexed image (18) that can be observed in at least a first observation mode and a second observation mode on a substrate (1) of a security document (10). The optically variable inks (2, 3, 5, 8) are selected such that, in the first observation mode, first and second optically variable inks have a first color and third and fourth optically variable inks have a second color and, in the second observation mode, the first ink and the third ink have a third color and the second ink and the fourth ink have a fourth color. This allows for displaying different bicolor images in the first and second observation modes, where the appropriately configured inks can be selected from a set of available optically variable inks, or obtained by mixing such inks with one or more additional inks.

Description

Technical Field

[0001] The present disclosure generally relates to printing techniques, in particular, to a method of forming a security feature for a security document using a printing apparatus.Background

[0002] Printing apparatuses are widely used to form images as part of a security document. Generally, the color palette with which images can be printed, for example, using an inkjet printer, is constrained by the inks that are available (for example, cyan, magenta, yellow and black).

[0003] In applications such as physical identification documents, it is common to use printing techniques such as laser marking, because this may allow for obtaining image features inside a polycarbonate substrate rather on a surface of the substrate. When a laser is used to produce colors, it may only be possible to reproduce a limited range of colors, and the link between the laser parameters and the produced colors is not trivial.

[0004] In some applications, it is desirable to build a security image, for example, for a security structure of a security document, by multiplexing a plurality of images. In this manner, a multiplexed image that can be observed in a plurality of observation conditions or modes, and that has different appearances in the respective observation modes can be obtained. For example, such a multiplexed image may have a first appearance when viewed in transmission, and a second, different (and possibly uncorrelated) appearance when viewed under backside reflection.

[0005] WO 2014 / 075994 A1 discloses a method for laser-structuring a nanoparticle network to generate colored patterns on a support.

[0006] US 2021 / 0039422 A1 discloses a method for building a security image by multiplexing color images.

[0007] US 5,070,413 A discloses a halftoning method for creating a color binary image from a continuous tone color image.

[0008] US 2007 / 0097389 A1 discloses a method of adapting a color gamut of an image to be produced onto a set of colors achievable with a printing technique (gamut mapping).

[0009] US 10,491,784 B2 relates to generating prints with multiple appearances.

[0010] EP 4 210 314 A1 discloses a method of generating a multiplexed image for printing that can be observed in a plurality of modes.

[0011] The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.Summary of the Disclosure

[0012] According to one aspect of the present disclosure, a method of forming a security feature for a security document, the security feature including a multiplexed image that can be observed in at least a first observation mode and a second observation mode and has different appearances in the at least first and second observation modes, includes the steps of providing a first optically variable ink having a first color in the first observation mode and a second color different from the first color in the second observation mode, and providing a second optically variable ink having a first color in the first observation mode and a third color different from the first and second colors in the second observation mode. The method further includes applying the first optically variable ink to a first portion of an image region of the multiplexed image on a substrate of the security document, for example, using a printing apparatus, and applying the second optically variable ink to a second portion of the image region of the multiplexed image different from the first portion, for example, using the printing apparatus.

[0013] In another aspect, the present disclosure relates to a printing apparatus having a reservoir including a plurality of optically variable inks, each having a first color in a first observation mode and a second color different from the first color in a second observation mode, a printing unit configured to apply the plurality of optically variable inks to a substrate, and a control unit configured to perform the method of the above aspect.

[0014] Other features and aspects of the present disclosure will be apparent from the following description and the accompanying drawings.Brief Description of the Drawings

[0015] Fig. 1 shows a schematic plan view of an exemplary security document in accordance with the present disclosure, Fig. 2 shows two optically variable inks in accordance with the present disclosure, Fig. 3 shows an exemplary multiplexed image in accordance with the present disclosure when viewed in different observation modes; Fig. 4 shows four optically variable inks in accordance with the present disclosure; Fig. 5 shows an image region of an exemplary multiplexed image in accordance with the present disclosure including the four inks in Fig. 4; Fig. 6 shows a schematic cross-sectional view of an exemplary security document in accordance with the present disclosure; Fig. 7 shows a diagram illustrating a finite shape of a set of optically variable inks in a combined color space; Fig. 8 shows another example for a finite shape of a set of optically variable inks in a combined color space; Fig. 9 shows a schematic overview of an exemplary printing apparatus in accordance with the present disclosure; and Fig. 10 shows an exemplary flow diagram of a method in accordance with the present disclosure. Detailed Description

[0016] The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described herein are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as, a limiting description of the scope of protection. Rather, the scope of protection shall be defined by the appended claims.

[0017] The present disclosure is based at least in part on the realization that constructing a multiplexed image using colors that are obtainable with a laser marking apparatus can be very complex. In particular, as previously mentioned, such a laser marking apparatus can usually only produce a limited range of colors, and the link between the laser parameters and the produced colors is not trivial. Therefore, in order to print a multiplexed image using a laser marking apparatus, it is first necessary to generate a multiplexed palette by varying the laser parameters, and then use a complicated algorithm to determine those obtainable colors that can be used to create a desired multiplexed image. Here, the skilled person understands that the expression "multiplexed image" as used herein refers to an image that has different appearances (different optical properties) when viewed in different observation modes. For example, such a multiplexed image may have a first appearance in a first observation mode, such as backlight reflection, and a second, different appearance in a second observation mode, such as transmission (i.e., when viewed against white light).

[0018] It has been realized that a use of optically variable inks, i.e., ink materials having different appearances when viewed under the above-mentioned different observation modes, makes it possible to obtain the colors that are needed to form a multiplexed image in a more efficient manner. Namely, the composition of the respective inks can be modified gradually in order to obtain the desired colors in the respective observation modes. Accordingly, even if the required colors are not presently available, for example, as commercially available inks, by an appropriate mixture of inks, or an appropriate modification of the composition of the available inks, the desired colors can be easily obtained.

[0019] The present disclosure is further based on the realization that, using optically variable inks, a dynamic feature for a security document that can be easily authenticated can be obtained, for example, by shifting between two easily distinguishable images that are visible in the respective observation modes, or by selectively hiding and displaying an image in the respective observation modes. It has been realized that such a dynamic feature can be advantageously realized in a particularly simple manner by using a combination of two or four optically variable inks. For example, it may only be necessary to identify or generate four optically variable inks that match two by two in each observation mode. Said optically variable inks can then be printed in correspondingly selected parts of the multiplexed image, resulting in two easily recognizable bicolor images (i.e., images having only two colors) that can be viewed in two different observation modes.

[0020] It has also been realized that forming a multiplexed image by printing results in a complex visual effect relying on color matching between, for example, at least four colored materials. Further, the two images in the respective observation modes are encoded together, and if one of the images is modified, the other image will also be modified. This further increases the security of the security feature.

[0021] Additionally, it has been realized that it is advantageous to provide the optically variable inks on separate layers of a substrate of the associated security document. This may allow for a better registration of the different portions of the multiplexed image formed by the different optically variable inks. Further, it has been realized that it is advantageous to perform a curing of the optically variable inks separately for each layer (for example, by using ultraviolet (UV) light). Especially in case of UV curing, if all inks are provided on the same layer, multiple curing steps may be required, and this may damage the substrate layer and result in problems during the subsequent lamination with the other substrate layers. This can be avoided by providing the inks on separate layers.

[0022] Fig. 1 shows a schematic plan view of an exemplary security document 10 in accordance with the present disclosure. As shown in Fig. 1, security document 10 includes a substrate 1, for example, having a substantially rectangular shape, but not limited to such a rectangular shape. Other shapes, for example, circular, elliptical, or polygonal shapes can also be used, depending on the application. In particular, as shown in Fig. 1, security document 10 may be a personalized or personalizable security document, i.e., may include an image 11 of a holder of the security document, which may be a passport, a driver's license, or any other personalized security document. Image 11 may be formed in an image region provided on substrate 1 in a known manner, for example, using a laser marking apparatus. It should be appreciated, however, that personalizable security document 10 shown in Fig. 1 is only an example, and security document 10 may be any other security document, such as a banknote or the like. In other words, security document 10 does not necessarily have to be a personalized or personalizable security document.

[0023] As shown in Fig. 1, security document 10 includes a security feature 20 formed in substrate 1. In the exemplary embodiment, security feature 20 is a window having, for example, a rectangular shape, in which one or more security features may be provided. For example, as shown in Fig. 1, security feature 20 may include a multiplexed image 18 that can be observed in at least a first observation mode and a second observation mode and has different appearances in the at least first and second observation modes. This will be described in more detail in the following.

[0024] In accordance with the present disclosure, multiplexed image 18 is formed by printing multiplexed image 18 in an image region 14 on substrate 1 of security document 10. In the example shown in Fig. 1, image region 14 may be defined by the size of the window forming security feature 20. However, the present disclosure is not limited to this. As will be described in more detail below, image region 14 may be formed on a surface of substrate 1, i.e., an outer surface of substrate 1, or may be formed inside substrate 1 and may be covered by one or more transparent layers allowing for the observation of multiplexed image 18.

[0025] In some embodiments, multiplexed image 18 includes a first optically variable ink 2 having a first color 4 in the first observation mode and a second color 6 different from first color 4 in the second observation mode. Further, multiplexed image 18 includes a second optically variable ink 8 having first color 4 in the first observation mode and a third color 7 different from first and second colors 4, 6 in the second observation mode. This is schematically illustrated in Fig. 2.

[0026] As shown in Fig, 3, in some embodiments, first optically variable ink 2 is applied to a first portion 12 of image region 14 of multiplexed image 18 on substrate 1 of security document 10 using a printing apparatus 200 (see Fig. 9). Further, second optically variable ink 8 is applied to a second portion 16 of image region 14 of multiplexed image 18 different from first portion 12 using printing apparatus 200.

[0027] With the above configuration of first optically variable ink 2 and second optically variable ink 8, it is evident that, when multiplexed image 18 is observed in the first observation mode, first portion 12 and second portion 16 have the same color, i.e., first color 4. Advantageously, first portion 12 of image region 14 and second portion 16 of image region 14 are complementary, i.e., together form image region 14. In other words, image region 14 consists of first portion 12 and second portion 16. In this case, image region 14 appears as a uniform color image having the shape of image region 14, for example, the rectangular shape that is shown in Fig. 3, where the second portion 16 is indicated by dashed lines to illustrate that the second portion 16 cannot be distinguished from first portion 12. It will be appreciated, however, that it is not necessary that image region 14 consists of first portion 12 and second portion 16, i.e., does not include any other portions. For example, the effect of "hiding" second portion 16 can already be obtained when second portion 16 is surrounded by first portion 12 on two or more sides, or completely surrounded by the same.

[0028] As can be seen in Fig. 3, when multiplexed image 18 is observed in the second observation mode, first portion 12 has second color 6, while second portion 16 has third color 7, which is different from second color 6. Accordingly, second portion 16 can be distinguished from first portion 12, and, in this manner, image features of multiplexed image 18 may become visible. In particular, in the second observation mode, multiplexed image 18 may be a bicolor image, for example, showing a shape that corresponds to second portion 16 as a structure that is visible against a background formed by first portion 12. As a result, when the observation mode is changed from the first observation mode to the second observation mode, the structure corresponding to second portion 16 becomes visible. This may result in a dynamic feature that can be observed by a person that checks the validity of security document 10.

[0029] In the above manner, a dynamic security feature 20 can be obtained using two different optically variable inks. Here, it will be appreciated that, in particular, second portion 16 may have any appropriate shape, which can be easily identified by an observer. Likewise, optically variable inks 2 and 8 may have any appropriate colors in the different modes, as long as the above relationship, according to which optically variable inks 2, 8 have the same color in the first observation mode, and different colors in the second observation mode, is present.

[0030] Here, appropriate inks can be easily obtained. For example, inks 2, 8 may be selected from a plurality of available inks, for example, commercially available inks, which have the desired relationship. All that is necessary is to identify two optically variable inks, which have the same color in a first observation mode, and different colors in a second observation mode.

[0031] If no available inks have the desired properties, the process may start by selecting a first optically variable ink, and the second optically variable ink may be obtained in a straightforward manner by mixing two or more inks, or by appropriately modifying the properties of a second optically variable ink. In particular, it will be readily appreciated that, if the colors c 1 1< and c 1 2< of the first ink in observation mode 1 and observation mode 2, respectively, are known, and a second optically variable ink has colors c 2 1< and c 2 2< in observation mode 1 and observation mode 2, respectively, where c 1 1< and c 2 1< are not the same color, an optically variable ink having the desired properties can be obtained by mixing the second ink with a third ink.

[0032] Let α 2 be a volume of the second ink, and α 3 be a volume of a third ink (where α 2 and α 3 indicate a ratio of the respective inks, i.e., a number between 0 and 1), then a color c 4 i< of a mixture of the second ink and the third ink in observation mode i can be expressed by c 4 i< = α 2 c 2 i< + α 3 c 3 i< . In particular, when the third ink that is used for the mixing is not an optically variable ink, i.e., has the same color in both observation modes (c 3 1< = c 3 2< ), the color and the amount α 3 of the third ink that results in the fourth ink having the same color as the first ink in the first observation mode (c 4 1< = c 1 1< ) can be determined by straightforward calculation. As such, the second ink that has the desired properties can be easily obtained by an appropriate mixing of an optically variable ink with an optically non-variable ink. Here, the skilled person will recognize how such an optically non-variable ink having a desired color can be obtained, in particular, by providing appropriate pigments, or by mixing different color inks in an appropriate ratio.

[0033] In the previously described exemplary embodiments, multiplexed image 18 includes first optically variable ink 2 and second optically variable ink 8. In other embodiments, however, multiplexed image 18 may further include a third optically variable ink 3 having a fourth color 9 different from the first to third colors 4, 6, 7 in the first observation mode and second color 6 in the second observation mode, as well as a fourth optically variable ink 5 having fourth color 9 in the first observation mode and third color 7 in the second observation mode. This is illustrated in Fig. 4.

[0034] As shown in Fig. 5, third optically variable ink 3 may be applied to a third portion 13 of image region 14 of multiplexed image 18 different from first and second portions 12, 16 using printing apparatus 200. Further, fourth optically variable ink 5 may be applied to a fourth portion 17 of image region 14 of multiplexed image 18 different from first to third portions 12, 13, 16 using printing apparatus 200.

[0035] Using four optically variable inks having the above configuration, it is possible to display two distinct bicolor images in the first observation mode and the second observation mode, respectively. For example, a first bicolor image 22 to be observed in the first observation mode may be provided as a first input image, and a second bicolor image 24 to be observed in the second observation mode may be provided as a second input image. The first to fourth portions 12, 13, 16, 17 may then be determined in a straightforward manner on the basis of first and second bicolor images 22, 24.

[0036] For example, first color 4 and fourth color 9 may be associated with first bicolor image 22, and second color 6 and third color 7 may be associated with second bicolor image 24. Here, it will be appreciated that the colors in first bicolor image 22 and second bicolor image 24, which serve as input images, do not necessarily have to be identical to the colors of optically variable inks 2, 3, 5, 8. As such, the colors that are observed on security document 10 in the first observation mode and the second observation mode, respectively, may be different from the colors in the input images. However, the image features of said input images can be recognized in the respective observation modes.

[0037] Again, it is advantageous when first to fourth portions 12, 13, 16, 17 are complementary, i.e., form a continuous portion, for example, the entirety of image region 14, as shown in Fig. 5. Of course, if multiplexed image 18 is to be observed in more than 2 observation modes, additional portions for additional inks to be used have to be provided.

[0038] In order to obtain a first bicolor image corresponding to image 22 being visible in the first mode, and a second bicolor image corresponding to image 24 being visible in the second mode, first bicolor image 22 and second bicolor image 24 are brought in registration with each other. After associating first to fourth colors 4, 6, 7 and 9 to images 22, 24, for each image portion, for example, a pixel or another image unit, the combination of two colors that are to be displayed in the first observation mode and the second observation mode can be determined. This combination of colors corresponds to one of the four inks 2, 3, 5, 8. In this manner, portions 12, 13, 16, 17 can be built successively, by identifying the ink that is to be used for each image unit or pixel.

[0039] Here, while exemplary bicolor images 22, 24 are bicolor images displaying relatively simple symbols, it will be appreciated that bicolor images 22, 24 may be any appropriate bicolor image having much more detailed structures, in particular, different shades or lightness values associated with the respective colors. A skilled person will recognize that such different shades or lightness values can be obtained by known image forming techniques such as dithering or the like. Regardless of which image forming technique is used, however, each image unit or pixel will have one of two colors in the respective bicolor images. In this manner, the above-described multiplexing can also be performed for more complex images, by using appropriate inks and printing methods.

[0040] Also for the exemplary embodiments including four optically variable inks, it will be appreciated that the inks to be used can be obtained in a straightforward manner, either by selecting appropriate inks from a plurality of available inks, or starting with one or more available inks, and creating the remaining optically variable inks by mixing two or more inks. For example, if two optically variable inks having the same color in the first observation mode, but different colors in the second observation mode are available, it is straightforward to find a third ink that, for example, has the same color as the first ink in the second observation mode, but has a different color than the first ink in the first observation mode. Based on this, all that is left to do is identify or generate a fourth ink that has the same color as the third ink in the first observation mode, and the same color as the second ink in the second observation mode. For example, if the first to third optically variable inks are known, fourth optically variable ink 5 may be obtained by mixing a fifth optically variable ink with a sixth ink that has a same color in the at least first and second observation modes to obtain fourth optically variable ink 5.

[0041] Another way of identifying or generating the four optically variable inks that have the desired properties is to determine a set of available optically variable inks. Based on the set of available optically variable inks, a finite shape 42, for example, a convex hull of the set of available optically variable inks in a combined color space can be determined on the basis of color values of the set of available optically variable inks in the first and second observation modes. This is illustrated in a simplified manner in Fig. 7, where the horizontal axis indicates the color values of the respective inks in the first mode, and the vertical axis indicates the color values of the respective inks in the second observation mode. Here, empty circles indicate all available primaries, i.e., all color values that are directly obtainable using the set of available optically variable inks. In other words, each empty circle in Fig. 7 indicates one of the available optically variable inks, having a color value c 1 in the first observation mode and a color value c 2 in the second observation mode. Here, it will be appreciated that the color values c 1 and c 2 generally will be a vector including a plurality of components, for example, three components, in case of a color space such as CIE 1931 XYZ or RGB. In other words, generally, the combined color space will be a six-dimensional color space.

[0042] As shown in Fig. 7, the primaries are not the only available colors for the optically variable inks. It will be readily appreciated that additional optically variable inks (ink mixtures) having different colors, referred to as "halftones" in Fig. 7, can be obtained by mixing, for example, two primaries. In other words, based on the primaries, a number of additional ink mixtures (inks) can be generated by an appropriate mixing of the primaries.

[0043] In accordance with the present disclosure, it has been realized that an appropriate selection of inks that allows for performing the multiplexing can be performed by determining a maximum volume hyperrectangle 44 enclosed in finite shape 42 of the set of available optically variable inks, and selecting first to fourth optically variable inks 2, 3, 5, 8 having the desired properties such that the color values of the same are inside the maximum volume hyperrectangle 44. The reason for proceeding in this manner is the realization that the images to be displayed in the two observation modes are not completely independent, and that the desired properties can be assured by selecting colors that lie inside hyperrectangle 44. In particular, it will be readily appreciated that, for example, a further ink that has the same color value c 1 as a first ink, but a different color value c 2 than the first ink can be immediately obtained by identifying two inks having the same color value c 1 in hyperrectangle 44, but with a different color value c 2 . The same can be done for any further inks that are to be used.

[0044] As previously mentioned, the number of available inks can be increased by performing mixing of the inks to generate the additional ink mixtures that are shown in Fig. 7. In other words, one or more of first to fourth optically variable inks 2, 3, 5, 8 that are used to form multiplexed image 18 may be obtained by mixing at least two optically variable inks of the set of available optically variable inks.

[0045] Fig. 8 shows another example indicating hyperrectangle 44 inside finite shape 42, for example, a convex hull of the color values of the set of available optically variable inks. Fig. 8 assumes that, in the first observation mode, the color value of each image in the RGB color space can be described by two values R 1 and G 1 (assuming, for example, that a value B 1 is fixed), and a color value B 2 in the second observation mode (assuming, for example, that R 2 and G 2 are fixed). The different finite shapes 42 shown in Fig. 8 correspond to the different values of the color value B 2 in the second observation mode. Again, it is possible to define hyperrectangle 44 inside finite shape 42 defined by the three-dimensional shape of the convex hull of the color values of the set of all available optically variable inks.

[0046] As initially mentioned, it may be advantageous to apply the optically variable inks onto different layers of substrate 1. Fig. 6 shows a schematic cross-sectional view of security document 10. As shown in Fig. 2, security document 10 has substrate 1, which is formed by stacking a plurality of layers, for example, polycarbonate layers, and combining them in an appropriate manner, for example, by lamination processing or the like. As also shown in Fig. 6, security feature 20 is formed in substrate 1 and extends through at least part of substrate 1 along a thickness direction d. In the example shown in Fig. 6, substrate 1 includes substrate layers 31, 33, 35, 36, 37 stacked on top of each other. In some embodiments, at least a portion of each substrate layer in which security feature 20 is formed is substantially transparent or at least semitransparent for visible light such that multiplexed image 18 is visible at least from one side of security document 10, in particular, in reflection, for example, under white light.

[0047] As shown in Fig. 6, first optically variable ink 2 is applied onto first substrate layer 31 of the plurality of substrate layers, and second optically variable ink 8 is applied onto second substrate layer 33 of the plurality of substrate layers. Here, it will be appreciated that the view in Fig. 6 is schematic, and it is not necessary that optically variable inks 2 and 8 overlap each other when viewed along thickness direction d. In other words, as previously mentioned, it may be advantageous when optically variable inks 2 and 8 are applied such that corresponding portions 12 and 16 are complementary to each other, for example, form the entirety of image region 14. In other embodiments, however, first optically variable ink 2 and second optically variable ink 8 may also overlap at least in part, provided that this does not negatively affect the visibility of the respective images in the different observation modes. At least in the case where only two optically variable inks 2 and 8 are used, however, it may be possible to provide, for example, second optically variable ink 8 over the entirety of image region 14 on substrate layer 33, and provide second optically variable ink 2 only in a portion of image region 14 on substrate layer 31.

[0048] In any case, according to the present disclosure, it is contemplated to apply, in particular, first optically variable ink 2 and second optically variable ink 8 onto the respective substrate layers prior to laminating the same. Only after first optically variable ink 2 and second optically variable ink 8 have been applied to the corresponding substrate layers, the plurality of substrate layers 31, 33, 35, 36, 37 is laminated to form substrate 1 of security document 10.

[0049] The above can also be generalized to the case where, for example, four optically variable inks 2, 3, 5, 8 are present. In such a case, at least two of the first to fourth optically variable inks 2, 3, 5, 8 can be applied onto different substrate layers of the plurality of substrate layers 31, 33, 35, 36, 37, and the plurality of substrate layers 31, 33, 35, 36, 37 may be laminated to form substrate 1 of security document 10 after application of the respective optically variable inks. In some embodiments, the first to fourth optically variable inks 2, 3, 5, 8 are applied to four different substrate layers of the plurality of substrate layers 31, 33, 35, 36, 37 prior to laminating the same.

[0050] It may be advantageous to cure the applied optically variable inks 2, 3, 5, 8 prior to laminating the plurality of substrate layers 31, 33, 35, 36, 37, for example, by thermal or UV curing.

[0051] Further, it may be advantageous to perform a registration of printing apparatus 200 with respect to each of substrate layers 31, 33, 35, 36, 37 prior to applying optically variable inks 2, 3, 5, 8. For example, each substrate layer 31, 33, 35, 36, 37 may include a registration mark 49 to which printing apparatus 200 is registered. For example, a camera or another image acquisition device may be used to detect registration mark 49, and control printing apparatus 200 accordingly to apply the respective optically variable inks at the positions corresponding to first to fourth portions 12, 13, 16, 17. In other embodiments, registration may also be performed mechanically, for example, using one or more registration holes in each substrate layer.

[0052] Optically variable inks 2, 3, 5, 8 may be applied by printing apparatus 200 in any appropriate manner, for example, by screen printing, offset printing, inkjet printing, gravure printing, rotogravure or flexography. In other embodiments, inks 2, 3, 5, 8 may be formed by hot stamping of optically variable colored foils.

[0053] Fig. 9 shows a schematic overview of printing apparatus 200, which comprises a reservoir 202 including a plurality of optically variable inks, for example, first to fourth optically variable inks 2, 3, 5, 7, each having a first color in a first observation mode and a second color different from the first color in a second observation mode, a printing unit 203 configured to apply the plurality of optically variable inks to substrate 1, and a control unit 206 configured to perform the printing that was described above (for example, by executing computer-executable instructions that result in the realization of the described methods). Printing apparatuses that can be used to perform such a printing are known to the skilled person, such that a detailed description will be omitted herein.Industrial applicability

[0054] With the above-described teachings, a security document 10 including a multiplexed image 18 as part of a security feature 20 can be obtained using appropriately configured optically variable inks.

[0055] An exemplary method of forming security feature 20 for security document 10 is described in the following with respect to Fig. 10.

[0056] In a first step 110, a first optically variable ink 2 having a first color 4 in the first observation mode and a second color 6 different from the first color 4 in the second observation mode is provided.

[0057] In a second step 120, a second optically variable ink 8 having the first color 4 in the first observation mode and a third color 7 different from the first and second colors 4, 6 in the second observation mode is provided.

[0058] In a step 150, first optically variable ink 2 is applied to a first portion 12 of an image region 14 of multiplexed image 18 on substrate 1 of security document 10 using printing apparatus 200.

[0059] In step 160, second optically variable ink 8 is applied to a second portion 16 of image region 14 of multiplexed image 18 different from first portion 12 using printing apparatus 200.

[0060] In some embodiments, the method further includes a step 130 of providing a third optically variable ink 3 having a fourth color 9 different from the first to third colors 4, 6, 7 in the first observation mode and second color 6 in the second observation mode, and a step 140 of providing a fourth optically variable ink 5 having fourth color 9 in the first observation mode and third color 7 in the second observation mode. The method further includes a step 170 of applying third optically variable ink 3 to a third portion 13 of image region 14 of multiplexed image 18 different from first and second portions 12, 16 using printing apparatus 200, and a step 180 of applying fourth optically variable ink 5 to a fourth portion 17 of image region 14 of multiplexed image 18 different from first to third portions 12, 13, 16 using printing apparatus 200.

[0061] In some embodiments, the method further includes a step 190 of laminating a plurality of substrate layers 31, 33, 35, 36, 37, to which the plurality of optically variable inks have been applied, in order to form substrate 1 of security document 10.

[0062] It will be appreciated that the methods disclosed herein are not limited to the exemplary method that is shown in Fig. 10, and that any of the above-described additional steps can be included in the claimed method.

[0063] For example, the method may include a further step of providing a first bicolor image 22 to be observed in the first observation mode, a step of providing a second bicolor image 24 to be observed in the second observation mode, and a step of determining the first to fourth portions 12, 13, 16, 17 on the basis of the first and second bicolor images 22, 24.

[0064] The method may further include a step of associating the first color 4 and the fourth color 9 to the first bicolor image 22, and a step of associating a second color 6 and a third color 7 to the second bicolor image 24.

[0065] The step of providing a fourth optically variable ink 5 may include a step of mixing a fifth optically variable ink with a sixth ink that has a same color in the at least first and second observation modes to obtain the fourth optically variable ink 5.

[0066] In some embodiments, the method may further include steps of determining a set of available optically variable inks, determining a finite shape of the set of available optically variable inks in a combined color space based on color values of the set of available optically variable inks in the first and second observation modes, determining a maximum volume hyperrectangle enclosed in the finite shape of the set of available optically variable inks, and selecting the first to fourth optically variable inks 2, 3, 5, 8 such that the color values of the same are inside the maximum volume hyperrectangle 44.

[0067] The method may further include a step of mixing at least two optically variable inks of the set of available optically variable inks to obtain one or more of the selected first to fourth optically variable inks 2, 3, 5, 8.

[0068] The method may further include steps of applying the first optically variable ink 2 onto the first substrate layer 31 of the plurality of substrate layers, applying the second optically variable ink 8 onto a second substrate layer 33 of the plurality of substrate layers, and laminating the plurality of substrate layers 31, 33, 35, 36, 37 to form substrate 1 of security document 10.

[0069] In some embodiments, the method may further include applying at least two of the first to fourth optically variable inks 2, 3, 5, 8 onto different substrate layers of the plurality of substrate layers, and laminating the plurality of substrate layers 31, 33, 35, 36, 37 to form substrate 1 of security document 10. In particular, in some embodiments, the first to fourth optically variable inks 2, 3, 5, 8 may be applied to four different substrate layers of the plurality of substrate layers 31, 33, 35, 36, 37 prior to laminating the same.

[0070] In some embodiments, the method may further include a step of curing the applied optically variable inks 2, 3, 5, 8 prior to laminating the plurality of substrate layers 31, 33, 35, 36, 37. Further, the method may include a step of performing a registration of printing apparatus 200 with respect to each of substrate layers 31, 33, 35, 36, 37 prior to applying optically variable inks 2, 3, 5, 8.

[0071] The method may further include a step of registering printing apparatus 200 to a registration mark 49 included in each substrate layer 31, 33, 35, 36, 37 to which an optically variable ink is applied.

[0072] In some embodiments, the method includes applying optically variable inks 2, 3, 5, 8 by inkjet printing, offset printing, screen printing, or flexography.

[0073] It will be appreciated that, although in the above cases two or four optically variable inks are used to form a multiplexed image 18 that can be observed in two observation modes, the present disclosure is not limited to such cases. In particular, the present disclosure can also be applied to cases in which images that are to be viewed in more than two observation modes form multiplexed image 18. Generally, in case of multiplexed image 18 being observed in n observation modes, it will be readily appreciated that the number of different optically variable inks that have to be used in order to display different bicolor images in each mode is 2 n< . Examples for appropriate ink combinations for more than two observation modes, in particular, three observation modes can be found, for example, in US 2021 / 0039422 A1.

[0074] It will be appreciated that the foregoing description provides examples of the disclosed systems and methods. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the general disclosure.

[0075] Recitation of ranges of values herein are merely intended to serve as a shorthand method for referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All method steps described herein can be performed in any suitable order, unless otherwise indicated or clearly contradicted by the context.

[0076] Although the preferred embodiments of the present disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.

Claims

1. A method of forming a security feature (20) for a security document (10), the security feature (20) including a multiplexed image (18) that can be observed in at least a first observation mode and a second observation mode and has different appearances in the at least first and second observation modes, the method comprising: - providing a first optically variable ink (2) having a first color (4) in the first observation mode and a second color (6) different from the first color (4) in the second observation mode; - providing a second optically variable ink (8) having the first color (4) in the first observation mode and a third color (7) different from the first and second colors (4, 6) in the second observation mode; - applying the first optically variable ink (2) to a first portion (12) of an image region (14) of the multiplexed image (18) on a substrate (1) of the security document (10); and - applying the second optically variable ink (8) to a second portion (16) of the image region (14) of the multiplexed image (18) different from the first portion (12).

2. The method of claim 1, further comprising: - providing a third optically variable ink (3) having a fourth color (9) different from the first to third colors (4, 6, 7) in the first observation mode and the second color (6) in the second observation mode; - providing a fourth optically variable ink (5) having the fourth color (9) in the first observation mode and the third color (7) in the second observation mode; - applying the third optically variable ink (3) to a third portion (13) of the image region (14) of the multiplexed image (18) different from the first and second portions (12, 16); and - applying the fourth optically variable ink (5) to a fourth portion (17) of the image region (14) of the multiplexed image (18) different from the first to third portions (12, 13, 16).

3. The method of claim 2, further comprising: - providing a first bicolor image (22) to be observed in the first observation mode; - providing a second bicolor image (24) to be observed in the second observation mode; and - determining the first to fourth portions (12, 13, 16, 17) on the basis of the first and second bicolor images (22, 24).

4. The method of claim 3, further comprising: - associating the first color (4) and the fourth color (9) with the first bicolor image (22); and - associating the second color (6) and the third color (7) with the second bicolor image (24).

5. The method of any one of claims 2 to 4, wherein the step of providing the fourth optically variable ink (5) includes mixing a fifth optically variable ink with a sixth ink that has a same color in the at least first and second observation modes to obtain the fourth optically variable ink (5).

6. The method of any one of claims 2 to 4, further comprising: - determining a set of available optically variable inks; - determining a finite shape (42) of the set of available optically variable inks in a combined color space based on color values of the set of available optically variable inks in the first and second observation modes; - determining a maximum volume hyperrectangle (44) enclosed in the finite shape (42) of the set of available optically variable inks; and - selecting the first to fourth optically variable inks (2, 3, 5, 8) such that the color values of the same are inside the maximum volume hyperrectangle (44).

7. The method of claim 6, wherein one or more of the selected first to fourth optically variable inks (2, 3, 5, 8) are obtained by mixing at least two optically variable inks of the set of available optically variable inks.

8. The method of claim 1, wherein the substrate (1) includes a plurality of substrate layers (31, 33, 35, 36, 37), the method further comprising: - applying the first optically variable ink (2) onto a first substrate layer (31) of the plurality of substrate layers; - applying the second optically variable ink (8) onto a second substrate layer (33) of the plurality of substrate layers; and - laminating the plurality of substrate layers (31, 33, 35, 36, 37) to form the substrate (1) of the security document (10).

9. The method of any one of claims 2 to 7, wherein the substrate (1) includes a plurality of substrate layers (31, 33, 35, 36, 37), the method further comprising: - applying at least two of the first to fourth optically variable inks (2, 3, 5, 8) onto different substrate layers of the plurality of substrate layers (31, 33, 35, 36, 37); and - laminating the plurality of substrate layers (31, 33, 35, 36, 37) to form the substrate (1) of the security document (10).

10. The method of claim 9, wherein the first to fourth optically variable inks (2, 3, 5, 8) are applied to four different substrate layers of the plurality of substrate layers (31, 33, 35, 36, 37) prior to laminating the same.

11. The method of any one of claims 8 to 10, further comprising curing the applied optically variable inks (2, 3, 5, 8) prior to laminating the plurality of substrate layers (31, 33, 35, 36, 37).

12. The method of any one of claims 8 to 11, further comprising: performing a registration of the printing apparatus (200) with respect to each of the substrate layers (31, 33, 35, 36, 37) prior to applying the optically variable inks (2, 3, 5, 8).

13. The method of claim 12, wherein each substrate layer (31, 33, 35, 36, 37) includes a registration mark (49) to which the printing apparatus (200) is registered.

14. The method of any one of claims 1 to 13, wherein the optically variable inks (2, 3, 5, 8) are applied by at least one of inkjet printing, offset printing, screen printing, or flexography.

15. A printing apparatus (200), comprising: - a reservoir (202) including a plurality of optically variable inks (2, 3, 5, 7), each having a first color in a first observation mode and a second color different from the first color in a second observation mode; - a printing unit (203) configured to apply the plurality of optically variable inks (2, 3, 5, 7) to a substrate (1); and - a control unit (206) configured to perform the method of any one of the preceding claims.

Images