Light-guide-based protective optical element for security documents and a security document

Abstract

A protective optical element for security documents comprises a lightguide (12), an incoupler area (14) arranged on a first part of the lightguide (12), and an outcoupler area (18) arranged on a second part of the lightguide (12). Diffrac- tive grating structures (24) are arranged in the incoupler area (14). The incoupler area, has an extension (Ax) of at least 10 mm along the direction of the grating structure and an extension (Ay) of at least 10 mm along the direction perpendicular to the grat- ing structure. The region of the grating structure has a forward edge (26) facing the outcoupler area. The geometry of the forward edge (26) is selected such that any point on the incoupler area (14) is no more than L = 2 mm from at least one point on the forward edge (26). This design is particularly suited to couple in light from typical smartphone light sources while having low losses by avoiding undesired re-outcou- pling at the grating structures (24).

Description

[0001] LIGHT-GUIDE-BASED PROTECTIVE OPTICAL ELEMENT FOR SECURITY DOCUMENTS AND A SECURITY DOCUMENT

[0002] Technical Field

[0003] The invention relates to a protective optical element for security documents as well as to a security document with such an optical element.

[0004] Background Art

[0005] WO 2021 / 151460 and WO 2021 / 151699 describe light-guide-based protective optical elements for security documents. Such elements are typically applied to the security document and exhibit specific optical effects. They are hard to counterfeit and, if tampered with, their performance is affected.

[0006] The known optical elements comprise a lightguide, an incoupler area, and an outcoupler area. Light coupled into the lightguide at the incoupler area travels to the outcoupler area, where it is coupled out to generate the visual effects.

[0007] The light to be coupled in may, e.g., be generated by the flashlight LEDs of a smartphone.

[0008] Disclosure of the Invention

[0009] The problem to be solved by the present invention is to provide an optical element and security document of the type mentioned above that is particularly suited to be illuminated by the light of a smart phone or by a similar light source, in particular an LED-based light source.

[0010] This problem is solved by the optical element and security document of the independent claims.

[0011] Accordingly, the protective optical element for security documents comprises at least the following features:

[0012] - A lightguide: This lightguide is provided to guide light between the incoupler area and outcoupler area as described in the following.

[0013] - An incoupler area arranged on a first part of the lightguide: This area comprises structures adapted to couple light into the lightguide. The incoupler area extends along a direction X and a direction Y, with direction Y being perpendicular to direction X.

[0014] - An outcoupler area arranged on a second part of the lightguide: This area comprises structures adapted to couple light out of the lightguide, which light has been coupled into the lightguide at the incoupler area. The coupled-out light generates the distinctive optical effects of the protective optical element. The direction Y extends between the incoupler area and the outcoupler area, i.e., at least some of the guided light propagating along Y from the incoupler area will arrive at the outcoupler area.

[0015] - Light coupling structures formed on a surface of or inside the lightguide in a “incoupler region” of the incoupler area: Such light coupling structures are advantageously diffractive grating structures, but they may, e.g., also be formed by scattering structures, reflecting structures, refractive structures or non-diffractive grating structures. The light coupling structures couple at least part of the incoming light along direction Y.

[0016] The “incoupler region” is the region of the incoupler area that is filled with grating structures.

[0017] The incoupler area, has an extension of at least 8 mm, in particular at least 10 mm, along the direction X and of at least 6 mm, in particular at least 10 mm, along the direction Y. This allows to easily position the light of a smartphone even if the light faces away from the user and is therefore difficult to position accurately.

[0018] It must be noted that the optical element may comprise further areas with incouplers, but there must be at least one area, the “incoupler area” that fulfills the conditions described here.

[0019] The incoupler region in the incoupler area has a forward edge facing the outcoupler area. There are no grating structures arranged on the lightguide between the forward edge and the outcoupler area, such that the light traveling from the forward edge to the outcoupler area is not coupled out before it reaches the outcoupler area. In this context, “no” grating structures is to be understood such that the outcou- pling due to any grating structures between the forward edge and the outcoupler area is negligible for practical purposes, e.g. smaller than 5%, in particular smaller than 1%. As is clear from the wording, the “forward” edge is the edge of the incoupler area facing towards the outcoupler area, i.e., “forward” is defined as provided in the section “Definitions” below.

[0020] The coordinate E (i.e., the position) of the forward edge along the direction Y is a function E(x) that varies with the coordinate x along the direction X. This function E(x) is non-constant and selected such that any point on the incoupler area is no more than L = 3 mm from at least one point on the forward edge. In other words, when drawing a circle with a radius L = 3 mm around any point on the incoupler area, it intersects the forward edge.

[0021] This design is based on the combination of the following thoughts:

[0022] 1) It is hard to position the light of a smartphone accurately in the incoupler area because the light is facing away from the user and is hidden by the smartphone. Hence, the incoupler area should have a certain minimum size as mentioned above

[0023] 2) The incoupling efficiency of light is largest close to the forward edge of the incoupler region. As explained in more detail below, this is due to the fact that light coupled into the lightguide is reflected back and forth between the surfaces of the lightguide. If it hits, along its path, a grating structure of the incoupler, part of it will be coupled out and is therefore lost.

[0024] 3) Most LED-based light sources of smartphones or similar devices generate, at their exit, a light field with a typical full-width-half-maximum diameter of at least 4 mm, and for almost all of them this diameter is at least 3mm. Therefore, if a point on the incoupler area is no more than L = 3 mm, in particular no more than 2 mm, in particular no more than 1 mm, from at least one point on the forward edge, at least part of the brightest part of the light field will impinge on the forward edge of the grating structure where it can be coupled in most efficiently. If this holds true for any point on the incoupler area, a good part of the light will always be coupled in efficiently.

[0025] As explained in more detail below, the extension of the incoupler region in a direction backward from the forward edge is advantageously at least twice the thickness (t) of the lightguide and / or at least 0.25 mm because light coupled in so close to the forward edge is unlikely to suffer great losses from undesired outcou- pling.

[0026] On the other hand, the extension of the incoupler region in the direction backward from the forward edge is advantageously limited in order to have losses that are substantially independent of the incoupling location, or for design integration with other security features or for aesthetics purpose. Hence, advantageously, the extension of the incoupler region in the direction backward from the forward edge is no more than 3 mm, in particular no more than 2 mm, in particular no more than 1 mm, and / or it is no more than 10 times larger than the thickness of the lightguide. The incoupler region may consist of a plurality of disjoint subregions. In this context, a “disjoint” subregion is a region that does not border any other subregion. This design allows to distribute the incoupler region more evenly.

[0027] In this case, advantageously, for any given x-coordinate along direction X, there is no more than one subregion, i.e. the subregions do not overlap when seen along direction Y. This allows unhindered passage of the guided light originating from a given subregion along direction Y towards the outcoupler area, without it hitting another subregion. There may also be x-coordinates where there is no subregion.

[0028] In order to distribute the subregions over the incoupler area, the forward edge has — at a majority of, in particular at all, the pairs of subregions located next to each other along direction X — different locations along direction Y.

[0029] The function E(x) of the coordinate E of the forward edge may also be a continuous, non-constant function of x over at least 50%, in particular over at least 80%, of the extension of the incoupler area long the direction X. In this case, when the light source is displaced along x, the amount of coupled-in light varies more smoothly.

[0030] For further optimization when using the protective optical element with a mobile phone or a similar device, the element further may comprise an “spacer area” arranged between the incoupler area and the outcoupler area. The lightguide crosses the spacer area without any grating structures and outcoupler structures being arranged in the spacer area, such that less than 5%, in particular less than 1% of the light is coupled out along the spacer area. The extension of the spacer area along direction Y is at least 10 mm, in particular at least 20 mm. This design takes into account that, on most smartphones, the distance between the light source and the edge of the housing can be around 10 mm, even around 20 mm. By providing the spacer area, the light can be guided away from the smartphone housing before it hits the outcoupler structures, thereby making sure that the smartphone does not cover the outcoupler structures and the visual effects that they generate.

[0031] Advantageously, the light coupling structures are directional light coupling structures "preferentially" coupling in light along direction Y. This means that the coupling efficiency of light incoming along a direction perpendicular to the lightguide into light guided along direction Y is larger than the coupling efficiency of the same incoming light into light guided along direction X, in particular twice as large. Examples of such coupling structures include diffractive or non-dif- fractive grating structures extending along direction X, reflective and refractive structures whose structures are oriented to reflect or refract light preferentially along direction Y. In the case of diffraction gratings, grating structures extend along a direction X, i.e., the grating lines extend along said direction, and, therefore, diffraction for perpendicularly incident light occurs in a direction Y perpendicular to direction X.

[0032] In particular, the light coupling structures are diffractive grating structures. Such diffractive grating structures have a grating period in the order of the wavelength of the used light, e.g., in the range between 0.1 and 1 pm, such that they generate diffraction of the incoming light of at least one diffraction order, with the diffracted light coupling into the lightguide for at least a part of the incoming light spectrum and for a part of the incoming light angular range.

[0033] In another example, reflective or refractive structures are positioned at one or both of the surfaces of the lightguide, for example cylindrical or elongated microlenses, microprisms or anisotropic diffusers that will redirect a portion of the incoming light in the lightguide, coupled in the lightguide, and preferentially along direction Y.

[0034] The invention also relates to a security document with such a protective optical element, in particular to a banknote or identification document.

[0035] Brief Description of the Drawings

[0036] 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:

[0037] Fig. 1 is a top view of a security document with a protective optical element,

[0038] Fig. 2 is a top view of the incoupler area and part of the spacer area of a protective optical element,

[0039] Fig. 3 is a sectional view along line III-III of Fig. 2,

[0040] Fig. 4 is a qualitative illustration of total outcoupling losses T as a function of the distance D between the forward edge and the incoupling location,

[0041] Fig. 5 is an illustration of calculating the overlap between the forward edge and a circle around a given point of the incoupler area,

[0042] Fig. 6 is a second embodiment of the incoupler area, Fig. 7 is a third embodiment of the incoupler area,

[0043] Fig. 8 is an embodiment of the incoupler area with a continuous forward edge,

[0044] Fig. 9 is another embodiment of the incoupler area with a continues forward edge,

[0045] Fig. 10 is an embodiment of the incoupler area with large backward extension,

[0046] Fig. 11 is an embodiment of a protective optical element where the directions X and Y form a non-Cartesian coordinate system,

[0047] Fig. 12 is an embodiment with an incoupler area between two or more outcoupler areas,

[0048] Fig. 13 is an embodiment with an outcoupler area between two or more incoupler areas,

[0049] Fig. 14 illustrates the light distribution from a non-ideal light source, and

[0050] Fig. 15 illustrates design considerations for divergent light sources.

[0051] Modes for Carrying Out the Invention

[0052] Definitions

[0053] The terms “forward” and “backward” are defined as directions parallel to Y. “Forward” is the direction, along Y, from the incoupler area towards the outcoupler area. “Backward” is the direction, along Y, from the outcoupler area towards the incoupler area.

[0054] The x-coordinate is the coordinate along direction X, and the Y -co- ordinate is the coordinate along direction Y, i.e., the directions X and Y form a coordinate system. This coordinate system is advantageously Cartesian, but it may also be another type of an orthogonal coordinate system. For more details, reference is made to the section “Curved incoupler area” below.

[0055] The directions X and Y both extend parallel to the lightguide, i.e., they extend parallel to the surfaces of the lightguide.

[0056] Overview

[0057] Fig. 1 shows a security document 2 in the form of a banknote. The invention may, however, also be applied to other types of security documents, such as passports, vouchers, certificates, etc. that should be tamper-proof. Security document 2 has a substrate 4, e.g., of paper or plastics, that may carry various features, such as printed text and symbols 6a, 6b, conventional security features 8, etc. In addition, it comprises a protective optical element 10.

[0058] Protective optical element 10 is advantageously arranged on a surface of substrate 4, e.g. by lamination and / or hot-stamping, and it may optionally span an opening in the substrate, thereby forming a window. But it may also be embedded in substrate 4 to be accessible through a window, or it may form part or all of substrate 4 or integrated in a multilayer body, for example a polycarbonate document.

[0059] Element 10 comprises a lightguide 12 extending at least over three areas:

[0060] - An incoupler area 14 is arranged on a first part of lightguide 12 and comprises incoupler structures 15 to couple light into the lightguide.

[0061] - An outcoupler area 16 is arranged on a second part of lightguide 12 and comprises outcoupler structures 17 to couple light out from the lightguide.

[0062] - A spacer area 18 is arranged on a third part of lightguide 12 between incoupler area 14 and outcoupler area 16.

[0063] Fig. 1 illustrates how a user holds a smartphone 20 over the security document, placing the light source 22 of the smartphone over incoupler area 14. Light from light source 22 hits the incoupler structures 15. A portion of the light is guided through lightguide 12 to travel through spacer area 18 to outcoupler area 16, where it leaves lightguide 12 through the outcoupler structures 17. The characteristic appearance of the light leaving the lightguide can be used to authenticate security document 2. This appearance, when controlled using a light source, provides a distinctive luminous appearance from the light exiting the lightguide and propagating towards the observer (eye or photodetector / imager).

[0064] Lightguide

[0065] Lightguide 12, as e.g. shown in Figs. 2, 3, is a transparent body of material adapted to guide light from incoupler area 14 to outcoupler area 16. As known to the skilled person, it advantageously comprises a core layer arranged between cladding layers, with the cladding layers having lower refractive index than the core layer at the wavelength use to authenticate the security document, for example in the visible range, between 400nm and 800nm. (Fig. 3 does not show the cladding layers for simplicity.) Lightguide 12 may also comprise further layers, such as a high-re- fractive-index layer covering the diffraction gratings for improved diffraction efficiency, adhesion enhancement layers, protective layers etc. Lightguide 12 extends over incoupler area 14, spacer area 18, and outcoupler area 16, but it may also expand to further areas. It may cover only a portion of a security document, such as a security patch, a security stripe or a security thread on a banknote, or on the opposite cover a large portion of the security document, such as in a security overlay on a paper passport datapage or a security DOVID (Diffractive Optically Variable Image Device) foil embedded in a polycarbonate passport datapage, an identification card, a driving license and the likes.

[0066] For reasons that will become more apparent below, lightguide 12 is advantageously a multi-mode lightguide with a thickness t much larger than the wavelength of visible light. Advantageously, thickness t is at least 10 pm, in particular at least 20 pm. Except otherwise specified, the thickness t is defined as the lightguide core physical thickness.

[0067] Lightguide 12 is a sheet lightguide that can be attached to the surface of objects or be integrated into other sheet-like objects. Hence, its thickness t is advantageously at least 100 times smaller than its extensions along both the directions X and Y.

[0068] Advantageously, lightguide 12 is flexible for easier mounting and improved robustness. Advantageously, it is flexible in the sense that it can be reversibly bent to a curvature radius smaller than the larger of its extensions along X and Y, or to smaller curvature radius such as less than 2 mm or less than 1 mm.

[0069] Incoupler area

[0070] Incoupler area 14 comprises incoupler structures 15 adapted to couple light into lightguide 12.

[0071] The incoupler structures 15 are advantageously formed by directional optical couplers arranged to couple light from out of plane incidence to the lightguide mode, preferably modes of a highly multimode lightguide. The directionality of the coupling is along axis Y, such that an illumination beam impeding from out of plane incidence will be coupled preferably along axis Y or with a small angle with respect to the axis Y, for example less than 30° from the Y axis in the XY plane, more preferably less than 20° from the Y axis. The main propagation direction of the coupled-in light will depend on the illumination beam orientation and angular distribution, which is discussed with respect to Fig. 15 below. Preferably, the directional coupler may be asymmetric to couple more light along the Y axis towards the outcoupler, as illustrated in Fig. 15, than away from it. The incoupler structures 15 are advantageously formed by diffractive grating structures 24 as illustrated in Figs. 2, 3 arranged on a surface of lightguide 12 (more accurately, on the surface of the core layer of lightguide 12 if lightguide 12 has low-refractive cladding layers with a high-refractive core layer between them).

[0072] The grating structures 24 are periodic structures extending along a direction X or having a small angle with respect to the direction X (in particular less than 20° with respect to the direction X) and varying periodically along a direction Y. The grating period is selected to couple perpendicularly incident light having a wavelength spectrum covering at least part of the visible spectral range (approximately 400 nm to 800 nm) into lightguide 12. Such structures are known to the skilled person and need not be explained in detail herein. By way of example, reference is made to WO 2014 / 016343 A2.

[0073] As mentioned above, since the light source 22 (see Fig. 1) is usually arranged on a side of the device 20 that faces away from the user. This makes it difficult to position the light source accurately. Hence, the extensions Ax, Ay of incoupler area 15 along directions X and Y (see Fig. 2) should be at least 8 and 6 mm, respectively. Advantageously, the extension Ax along X is larger than the extension Ay along Y, in particular with Ax being at least at least 8 mm, in particular at least 10 mm, and more preferably at least 12mm.

[0074] The grating structures 24 cover a incoupler region 34 of incoupler area 14, i.e., the incoupler region 34 is defined as being the region formed by all grating structures 24 within incoupler area 14.

[0075] In the embodiment of Figs. 1, 2, the incoupler region 34 is formed by a plurality of disjoint subregions 34-1, 34-2..., with each subregion being, e.g., rectangular.

[0076] Incoupler region 34 has a forward edge 26, which is drawn as a thick line in Fig. 2. Forward edge 26 is defined as the edge of incoupler region 34 that faces outcoupler area 16. This edge of the incoupler region 34 is therefore the closest to the outcoupler area. The y-coordinate E of forward edge 26 is a function E(x) that varies with the x-coordinate.

[0077] As mentioned above, function E(x) is selected such that any point Pi on incoupler area 14 is no more than a distance L away from at least one point on forward edge 26, with L being 3 mm, in particular 2 mm, in particular 1 mm. This condition, which is termed “coverage condition” in the following, is illustrated in Fig. 2 with two points Pl, P2 surrounded by circles having a radius L. As can be seen, a section of forward edge 26 falls into each circle. As already explained, this design is based on the understanding that outcoupling losses of light are smallest for light that has been coupled-in close to forward edge 26 of the incoupler region.

[0078] The effect of undesired outcoupling best understood from Fig. 3, which depicts how a monochromatic, collimated, perpendicularly incident light field 28a is diffracted by the grating structures 24 and coupled into a thick waveguide 10. In this case, the light is diffracted (at least in part) into a propagation angle 0 and propagates as a totally reflected light field 28b along lightguide 12. Since the grating structures 24 are arranged at a surface of lightguide 12, namely (in the example of Fig. 3) on surface 30a facing away from substrate 4, light field 28b is first totally reflected on surface 30b of lightguide 12 that faces towards substrate 4. From there, it propagates back to surface 30a, where a second reflection occurs in an area 32.

[0079] If, in area 32 of the second reflection, there is no grating structure, reflection is again total, i.e., all of the lightguided in lightguide 12 remains within lightguide 12. If, however, the grating structures 24 extend all the way to area 32, some of the guided light 28b will be lost because it is coupled out.

[0080] This mechanism is explained in further detail in WO 2014 / 016343 A2.

[0081] Hence, losses of light due to undesired outcoupling are avoided for light incoupled close to forward edge 26.

[0082] Assuming that

[0083] - lightguide 12 has a thickness t, with t being defined as the distance between its total internal reflection surfaces, i.e. as the thickness of its light-guiding core (which advantageously is between 10 - 100 pm, in particular between 20 - 50 pm),

[0084] - light is coupled in at a distance D backward from forward edge 26,

[0085] - the grating structures 24 are arranged at surface 30a or 30b of lightguide 12, and

[0086] - the internal propagation angle is 0 (in respect to the surface normal of lightguide 12), losses due to undesired outcoupling are avoided if

[0087] D < 2 • t • tan(0) (1)

[0088] If, for example, t = 25 pm and 0 = 70°, light coupled in at a distance of less than 137 pm or 0.137 mm from forward edge 26 will not suffer such losses. For security documents needing less flexibility, such as ID documents, t may e.g. also be 50 m.

[0089] As it will be clear to the skilled person, Fig. 3 and Eq. (1) correspond to the situation for perpendicular incidence of fully collimated, monochromatic light. In practical situations, the light from a smartphone’s light source is not collimated and polychromatic, and the median angle of incidence depends on the relative position of light source 22 in respect to forward edge 26. Hence, Eq. (1) is but a rough approximation. However, this rough approximation, reducing outcoupling losses for non-collimated, polychromatic light beams, with a main out of plane incidence not necessarily parallel to the Z axis, can be achieved with the considerations described above. Reducing the probability or outcoupling losses in the incoupler area 14 while maximizing the incoupling efficiency for an unknown illumination source position over the incoupler area allow to optimize the intensity of the lightguide -coupled light propagating towards the outcoupler.

[0090] Fig. 4 qualitatively illustrates the total outcoupling losses as a function of the distance D between the location of incoupling and forward edge 26, assuming that the incoupler grating extends all the way between the location of incoupling and forward edge 26. As can be seen, outcoupling losses for D < t, with t being the thickness of lightguide 12, are small because most of the light will have its second reflection at a location forward from forward edge 26. As D becomes larger, an ever- larger fraction of the light will have its second or even fourth, sixth, etc. reflection at an area where the incoupler grating structures 24 exist, and each time a fraction of the light will be coupled out.

[0091] In other words, it is most efficient to couple-in light close to forward edge 26, with in a distance D comparable to thickness t, and the incoupler region 34 should cover that area. Hence, advantageously, the extension of incoupler region 34 in a direction backward from forward edge 26 is advantageously at least twice the thickness t of the lightguide, in particular at least 5 times the thickness t of the lightguide, and most advantageously at least 10 times the thickness t of the lightguide.

[0092] For the same reason and for typical lightguides, the extension of incoupler region 34 in a direction backward from forward edge 26 is advantageously at least 0.2 mm, preferably at least 0.25 mm.

[0093] Another aspect that goes into the design of the device is the diameter of the light field illuminated by the light source. As mentioned above, the light source of a typical smartphone or a similar device generates, at its exit, a light field with a typical full-width-half-maximum diameter of at least 2 mm, generally at least 3 mm, in most cases of at least 4 mm.

[0094] Therefore, in view of the fact that light coupled in at forward edge 26 has small outcoupling losses, any point in incoupler area 14 is no more than L = 3 mm, in particular no more than L = 2 mm, in particular no more than L = 1 mm, away from at least one point of forward edge 26, as illustrated for points Pl, P2 in Fig. 2.

[0095] Advantageously, the overlap between forward edge 26 and the circle with diameter L around any point in the incoupler area 14 should be substantial in order to couple in a large amount of light at or close to forward edge 26. Hence, advantageously, the length of forward edge 26 within the circle of a diameter L around any point in the incoupler area is at least L / 4, in particular at least L / 2.

[0096] The calculation of this overlap is illustrated in Fig. 5, which shows a circle of diameter L around a point P and part of the grating structures 24 with forward edge 26. The overlapping sections of forward edge 26 with the circle are denoted in dotted lines, and the cumulated length of these dotted lines is the overlap that should be at least L / 4, in particular at least L / 2.

[0097] If any point in incoupler area 14 has to be no more than the distance L = 3 mm away from at least one point of forward edge 26 (the “coverage condition”), and since input area 14 has extensions Ax and Ay of at least 8 x 6 mm, the y- coordinate E(x) of forward edge 26 must vary as a function of coordinate x.

[0098] In order to fulfill the coverage condition, for at least some values of coordinate x in incoupler area 14, the difference between the maximum value and the minimum value of function E(x) should be at least Ay - 2 • L. Further, function E(x) should neither be monotonic nor strictly monotonic with coordinate x.

[0099] In the embodiment of Fig. 2, function E(x) is a non-continuous, non-constant function of x, with values of E(xl), E(x2), E(x3), E(x4)... at the locations of the respective sub-regions 34-1, 34-2, 34-3, 34-4... of the incoupler region.

[0100] The sub-regions 34-1, 34-2... of the embodiment of Fig. 2 are disjoint, which makes the easy to distribute them over incoupler area 14 while fulfilling the coverage condition.

[0101] At least some of the light coupled in at the forward edges 26 travels forward toward outcoupler area 16 as illustrated schematically by light fields 27 in Fig. 2. As can be seen, the sub-regions are positioned such that, along direction Y, there is no sub-region forward from another sub-region.

[0102] Since there are no further grating structures forward from the forward edges, no undesired outcoupling takes place. Put differently, for any given x-coordinate, there is only a single sub-region 34-1, 34-2.... Therefore, once light is coupled into lightguide 12 at the forward edge 26 of a given sub-region, it can propagate towards outcoupler area 16 without loss as illustrated by the light fields 27. For the case of oblique main direction of illumination on the incoupler area 14, a portion of the light coupled in the lightguide may propagate obliquely - not parallel to the Y axis - and may have outcou- pling losses before reaching the outcoupling area 16. However, such losses are lower than for incoupler design known in the prior art.

[0103] Any two neighboring subregions 34-1, 34-2... along coordinate x (i.e., any two sub-regions next to each other along coordinate x) have their forward edges 26 at different y-coordinates E(x). Advantageously, the y-coordinates E(x) of two such neighboring (along x) subregions differ by at least 3 mm for better distribution over incoupler area 14.

[0104] In the embodiment of Fig. 2, the sub-regions 34-1, 34-2... are substantially rectangular. The extension Bx along direction X is of about 2 mm and the extension By along direction Y is about 0.5 mm.

[0105] The distribution of the sub-regions 34-1, 34-2... in the embodiment of Fig. 2 is repetitive along direction X, but it may also be non-repetitive as long as the coverage condition is fulfilled.

[0106] The details of the distribution as well as the size of the sub-regions 34-1, 34-2... can vary, with two further examples shown in Figs. 6 and 7.

[0107] The embodiment of Fig. 6 is similar to the one of Fig. 2, but the variation E(x) along x is invariant under a change of the sign of x.

[0108] The embodiment of Fig. 7 illustrates that the sub-regions 34-1, 34- 2... may also be smaller, and their number may be larger.

[0109] Figs. 8 and 9 show variants where function E(x) is, over at least 50%, in particular over at least 80%, of the extension of the incoupler area along the direction X, a continuous, non-constant function of x.

[0110] Over this range, incoupler region 34 does not form disjoint sub-regions but is rather contiguous.

[0111] E(x) may be periodic in x. In order to fulfill the “coverage condition” mentioned above, the period P of E(x) should be smaller than 2 • L.

[0112] In the example of Fig. 8, function E(x) consists of a series of linear sections, with adjacent pairs of the linear sections forming, e.g., triangles. In the example of Fig. 9, function E(x) is substantially sinusoidal.

[0113] In the examples shown so far, the extension By of the incoupler region in a direction backward from forward edge 26, for all points on the forward edge 26, is much smaller than the extension Ax (see Fig. 2), for all x, with By < Ax / 5, advantageously By < 3 mm, in particular By < 2 mm, in particular By < 1 mm.

[0114] Alternatively, though, and as shown in Fig. 10, a “deep” incoupler region may be used. Here, the extension of incoupler region 34 in the direction backward from forward edge 26 is, at least for some points on forward edge 26, larger than 3 mm. In that case, depending on the position P of the light source, some light may couple in through parts of the incoupler region 34 that are at a distance D from forward edge 26 much larger than the thickness t of lightguide 12, in particular D > 5 • t or even D > 10 • t. This light will suffer stronger losses due to outcoupling at sections of grating structures 24 closer to forward edge 26, but it may still contribute to the light arriving at outcoupler area 16.

[0115] Hence, the embodiment of Fig. 10 has the advantage to potentially feed a larger amount of light to outcoupler area 16 for a given position P of the light source, and it may be useful for other design consideration or appearance in non light- guided optical control but in free-space observation of its reflection / diffraction / refrac- tion / diffusion.

[0116] The incoupler structures 15 can be a combination of continuous and non-continuous / discreet incoupler structures or have more complex shapes / functions E(x).

[0117] Outcoupler area

[0118] As mentioned, outcoupler area 16 comprises outcoupler structures 17 (see e.g. Fig. 1) that couple out at least part of the lightguided in lightguide 12 and coming from incoupler area 14. The outcoupler structures 17 are located in direction Y from the incoupler structures 15 in order to receive the light that was coupled in there.

[0119] The outcoupler structures 17 may again comprise diffractive gratings, e.g. surface gratings on one or both of the surfaces of lightguide 12 or inside the lightguide 12 and / or volume gratings arranged within lightguide 12. They may, however, also comprise scattering structures, fluorescent structures, reflective structures, refractive structures (e.g. macroscopic surface gratings or surface lenses), or any other means suitable to emit light visible to the user.

[0120] The outcoupler structures 17 may be distributed over outcoupler area 16, such as taught by WO 2021 / 151460, and / or they may generate different images depending on the incoupling location as taught by WO 2021 / 151699, Fig. 6. In the embodiment of Fig. 1, for example, the outcoupler structures 17 form a series of items (in the shown embodiment consisting of alphanumeric symbols, namely “TEXT1”, “TEXT2”, etc.) arranged side by side along direction X. Hence, when the user moves the light of their smartphone along direction X in incoupler area 14, the items light up sequentially, thereby creating a distinct optical effect that makes document verification easy.

[0121] Spacer area

[0122] As mentioned above, the present optical element may further comprise a spacer area 18 arranged on a third part of lightguide 12 between incoupler area 14 and outcoupler area 16, see e.g. Fig. 1.

[0123] Spacer area 18 has, along direction Y, an extension Cy of at least 10 mm, in particular of at least 20 mm.

[0124] As mentioned above, are no incoupler or outcoupler structures arranged in spacer area 18 in order to keep light losses in this region low.

[0125] The presence of spacer area 18 allows to position outcoupler area 16 at a distance of at least 10 mm from incoupler area 14. This is advantageous because, for most smartphones 20 or similar devices, the light source 22 is not located at the edge 40 of the device but rather at a distance of, e.g., 10 mm therefrom. If outcoupler area 16 were adjacent or very close to incoupler area 14, the housing of the smartphone would cover part of the outcoupler structures 17, thereby making verification more difficult.

[0126] Curved incoupler area

[0127] In the embodiments discussed so far, the grating direction or (perpendicular thereto) the grating vector of the grating structures 24 in the incoupler area 16 does not change over the whole incoupler area.

[0128] Fig. 11 shows an embodiment where the grating direction changes over the incoupler area 14. Accordingly, the directions X and Y do not for a Cartesian coordinate system. Yet they still form an orthogonal coordinate system since Y is, by definition, perpendicular to the direction X and extends between the incoupler area and the outcoupler area.

[0129] Advantageously, though, the radius of curvature of X is large because, for strong curvatures, the direction of the coupled-in light depends too much on the hard-to-control position of the light source. Advantageously, the curvature of X is, anywhere within incoupler area 14, at least 5 cm, in particular at least 10 cm. Non-diffractive incoupler area

[0130] Alternatively to using diffractive grating structures as light coupling structures, other types of structures may be used to couple incoming light into guided light within the lightguide.

[0131] In one embodiment, non-diffractive grating structures may be used, such gratings having a grating period much larger than the wavelength of visible light, e.g., at least 10 pm, in particular at least 50 pm, where refraction or reflection on oblique facets / interfaces is used to deflect the incoming light.

[0132] In another embodiment, scattering structures may be used as light coupling structures. Such scattering structures may be irregular, non -repetitive structures arranged, e.g., on a surface of the lightguide. Such scattering structures are advantageously anisotropic, e.g., having a larger roughness along direction Y than along direction X to preferentially couple light in along direction Y.

[0133] In yet another embodiment, non-directional light coupling structures may be used, e.g., for simplifying the manufacturing process.

[0134] Multiple incoupler and / or outcoupler areas

[0135] In the examples described so far, the protective optical element has a single incoupler area 14 and a single outcoupler area 16 separated by a single spacer area 18.

[0136] As illustrated in Fig. 12, however, there may be several outcoupler areas 16, with incoupler area 14 arranged between them, advantageously with a spacer area 18 arranged between incoupler area 14 and each outcoupler area 16. All of these components are arranged in a single lightguide 12.

[0137] In the example of Fig. 12, incoupler area 14 is arranged, along direction Y, between the outcoupler areas 16. This is advantageous if the light coupling structures in incoupler area 14 are directional light coupling structures preferentially coupling in light along direction +Y and -Y, e.g., if the light coupler structures in incoupler area 14 are gratings, with the grating lines extending along X.

[0138] On the other hand, and as illustrated in Fig. 13, there may be several incoupler areas 14, at different sides of outcoupler area 16, advantageously with a spacer area 18 arranged between each incoupler area 14 and outcoupler area 16. Again, all of these components are arranged in a single lightguide 12.

[0139] In the example of Fig. 13, outcoupler area 16 is arranged, along direction Y, between the incoupler areas 14. This is advantageous if the light coupling structures in outcoupler area 16 are directional light coupling structures preferentially coupling out light coming from direction +Y and -Y, e.g., if the light coupler structures in outcoupler area 16 are gratings, with the grating lines extending along X.

[0140] The embodiments of Figs. 12 and 13 may be combined, e.g., by repetitively arranging incoupler areas 14, spacer areas 18, outcoupler areas 16, and spacer areas 18 along direction Y.

[0141] Hence, advantageously, the protective optical element comprises at least two outcoupler areas 16 with an incoupler area 14 between them, in particular with a spacer area 18 arranged between each outcoupler area 16 and incoupler area 14.

[0142] In that case, advantageously, incoupler area 14 is arranged, along direction Y, between the outcoupler areas 16.

[0143] Similarly, advantageously, the protective optical element comprises at least two incoupler areas 14 with an outcoupler area 16 between them, in particular with a spacer area 18 arranged between each incoupler area 14 and outcoupler area 16.

[0144] In that case, advantageously, outcoupler area 16 is arranged, along direction Y, between the incoupler areas 14.

[0145] Non-ideal light sources

[0146] As illustrated in Fig. 14, consumer electronics light sources tend to be divergent and are often polychromatic, for example with the white light source - usually a white LED - used as flash to illuminate the scene when taking images with an adjacent camera, or to be used as pocket lamp. Such light sources have typically broad angle emission, which can be approximated to Lambertian light sources or can have a cone shape having a full width half maximum (FWHM) intensity angle of for example 30°, 40° or 50°. Such an illumination cone can be oblong to match the field of views of cameras or can have even wider angular ranges to be used with (ultra) wide angle cameras.

[0147] Fig. 15 illustrates the use of such light sources for the illumination on the incoupler area 14. In this example, a large smartphone casing 20 is covering a large portion of a secured document 2 and of the lightguide 12 part of protective optical element 10. As the light source is located below the smartphone, it is difficult for a user to place it in a narrow and specific location, and having a large incoupler area 14 allows a fast and efficient control. The illumination spot 22 impending on the lightguide 12 and on the incoupler area 14 will typically have a gaussian shape having a few millimeters, for example 3mm, of full width half maximum. This is represented with the solid line circle for the area of highest intensity and dash circle for the area reaching up to half the maximum intensity. The illumination beam is also not collimated but diverging, and the illumination rays are propagating over a wide range of angles as illustrated in Fig. 14. Whatever the position of this illumination spot 22 in the incoupler area 14, the coverage condition described above allow the illumination of efficient areas of the incoupling structures 24. In this example, two different discreet incoupling structures 24a, 24b are illuminated partially with the high intensity portion of the illumination spot as well as partially with lower intensity portion of the illumination spot. This results in the light coupling by each incoupling structure 24a, 24b etc. in the lightguide 12 of light beams propagating in a wide angular range around the Y axis, for example here + / - 20° around the Y axis at FWHM. In addition to the angular spread around the Y axis, the guided light has different propagation angle with respect to the XY plane (illustrated as angle 0 in Fig. 3).

[0148] The light coupled into the lightguide 12 by a rearward incoupler structure 24b, illustrated in widely-doted lines 23a and propagating obliquely towards another incoupled structure 24c located forwards, will be outcoupled to a large extent and create high propagation losses and a shadowing effect, removing a part of the light beams propagating towards the outcoupler area 16. However, since the discreet incoupler structure 24c is not at the same X coordinate, these losses and shadowing effect do not occur for the light propagating parallel or close to parallel to the Y axis, and which is the most useful to illuminate the outcoupler structures 17. In this example the incoupler structure 24a does not suffer from such shadowing losses and reduction of angular range or lateral spread of the incoupled light, see the closely-dotted lines 23b, and propagating toward the outcoupler area 16.

[0149] With this example, the arrangement of the incoupler structures 15 in the incoupling area 14 can be optimized according to the light source properties (spectral, beam width and angular spread), the lightguide thickness and properties (refractive indexes of the core and cladding, optical losses), the optical protective element overall layout and according to the coupler technologies used. It can be also optimized for other design, fabrication technologies, materials and overall secured documents considerations.

[0150] Incoupler location

[0151] As mentioned in reference to Fig. 3 above, the incoupler structures 24 may be arranged on surface 30a or 30b of lightguide 12, advantageously only on one such surface such that the in-coupled light has to travel through the lightguide, be totally reflected from the other side and travel back before it hits the first surface again. Hence, the arrangement on only one of the surfaces 30a, 30b reduces losses due to undesired outcoupling.

[0152] In another embodiment, though, the incoupler structures may also be located inside lightguide 12, such as illustrated by reference number 24' in Fig. 3. Such embedded incoupler structures may, e.g., be manufactured by assembling lightguide 12 from several high-refractive-index layers and locating the incoupler structures at the interface of one of them as known by the skilled person.

[0153] However, for such embedded incoupler structures 24', the path, along Y, until in-coupled light is totally reflected from one surface and returns back to the layer of the incoupler structures 24' is shorter. Therefore, advantageously, the incoupler structures 24, 24' are located close one of the surfaces 30a, 30b of lightguide 12.

[0154] In particular, the distance between the incoupler structures 24' and the closer one of the surfaces 30a, 30b of lightguide 12 is advantageously less than 20%, in particular less than 10%, of the thickness t of lightguide 12.

[0155] For similar reasons, the incoupler structures 24, 24' are advantageously thin (along direction Z), such that once diffracted light does not have to travel through them for a long way.

[0156] Hence, in particular, the extension of the incoupler structures 24, 24' in the direction Z perpendicular to the light guide is advantageously less than 20%, in particular less than 10%, of the thickness t of lightguide 12.

[0157] Notes

[0158] The teaching of this invention enables the design and fabrication of optical protective elements having optimized incoupler areas and structures for incoupling efficiency for unknown light source position in the incoupler area.

[0159] The present technique allows to keep losses due to undesired out- coupling low. It therefore enhances the overall efficiency of the protective optical element. It also allows more flexibility in material section because it works well even without using highly-tuned elements, such as components having a very high refractive index.

[0160] As explained above, the incoupler geometry provides good incoupling with low loss irrespective of the exact placement of typical LED light sources as, e.g., found in smartphones or similar devices.

[0161] As mentioned, the protective optical element may be used on a banknote, an identification document (see, e.g., Figs. 4, 5 of WO 2012 / 151699A1), or any other document that needs to be protected from tampering and / or counterfeiting. Protective optical element 10 is advantageously flexible. It may be pre-fabricated separate from the security document and then be laminated thereon. Alternatively, it may be manufactured, at least in part, together with the security document. The protective optical element preferably contains other optical security elements, such as secured inked, secured prints, other optically variable devices and the like, for enhanced security and resilience against counterfeiting or mimicking attempts.

[0162] While there are shown and described presently preferred embodi- ments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

Claims

Claims1. A protective optical element for security documents comprising a lightguide (12), an incoupler area (14) arranged on a first part of the lightguide (12) extending along a direction X and a direction Y perpendicular to the direction X, an outcoupler area (16) arranged on a second part of the lightguide (12), with the direction Y extending between the incoupler area (14) and the outcoupler area (16), light coupling structures (24) formed on a surface of or inside the lightguide (12) in an incoupler region (34) of the incoupler area (14), wherein the grating structures (24) extend along a direction X perpendicular to a direction Y, with the direction Y extending between the incoupler area (14) and the outcoupler area (16), and outcoupler structures (17) arranged in the outcoupler area (16), wherein the incoupler area (14), has an extension (Ax) of at least 8 mm along the direction X and an extension (Ay) of at least 6 mm along the direction Y, the incoupler region (34) has a forward edge (26) facing the outcoupler area (16), with no grating structures (24) being arranged between the forward edge (26) and the outcoupler area (16), wherein a coordinate E of the forward edge (26) along the direction Y is a function E(x) varying with a coordinate x along the direction X such that any point (Pl, P2) on the incoupler area (14) is no more than L = 3 mm from at least one point on the forward edge (26).

2. The element of claim 1 wherein an extension (By) of the incoupler region (34) in a direction backward from the forward edge (26) is at least twice the thickness (t) of the lightguide (12), in particular at least 5 times the thickness (t) of the lightguide (12).

3. The element of any of the preceding claims wherein an extension (By) of the incoupler region (34) in a direction backward from the forward edge (26) is at least 0.25 mm.

4. The element of any of the preceding claims wherein an extension (By) of the incoupler region (34) in a direction backward from the forward edge (26) is no more than 3 mm, in particular no more than 2 mm, in particular no more than 1 mm.

5. The element of any of the preceding claims wherein an extension (By) of the incoupler region (34) in a direction backward from the forward edge (26) is no more than 10 times larger than a thickness of the lightguide (12).

6. The element of any of the preceding claims wherein, for any point (Pl, P2) in the incoupler area, a length of the forward edge (26) within a circle of a diameter L around said point (Pl, P2) is at least L / 4, in particular at least L / 2.

7. The element of any of the preceding claims wherein the incoupler region (34) consists of a plurality of disjoint subregions (34-1, 34-2...).

8. The element of claim 7 where, at a given coordinate along direction X, there is no more than one subregion.

9. The element of any of the claims 7 or 8 wherein the forward edge (26) has, for a majority of, in particular all, pairs of subregions (34-1, 34-2...) located next to each other along direction X, different locations along direction Y.

10. The element of claim 9 wherein the different locations differ by at least 3 mm.

11. The element of any of the preceding claims wherein the function E(x) is a continuous, non-constant function of x over at least 50%, in particular over at least 80%, of the extension of the incoupler area (14) along the direction X.

12. The element of any of the preceding claims further comprising a spacer area (18) arranged between the incoupler area (14) and the outcoupler area (16), wherein the lightguide (12) crosses the spacer area (18) without any grating structures (24) and outcoupler structures being arranged in the spacer area (18), wherein an extension (Cy) of the spacer area (18) along direction Y is at least 10 mm, in particular at least 20 mm.

13. The element of any of the preceding claims wherein the incoupler area (14), has an extension (Ax) of at least 20 mm along the direction X.

14. The element of any of the preceding claims wherein the light coupling structures (24) are directional light coupling structures preferentially coupling in light along direction Y.

15. The element of claim 14 wherein the light coupling structures (24) are diffractive grating structures.

16. The element of any of the preceding claims wherein L = 2 mm, in particular wherein L = 1 mm.

17. The element of any of the preceding claims comprising at least two outcoupler areas (16) with the incoupler area (14) between them, and in particular with spacer areas (18) arranged between each outcoupler area (16) and the incoupler area 14.

18. The element of claim 17 wherein the incoupler area (14) is arranged, along the direction Y, between the outcoupler areas (16).

19. The element of any of the preceding claims comprising at least two incoupler areas (14) with the outcoupler area (16) between them, in particular with spacer areas (18) arranged between each incoupler area (14) and the outcoupler area (16).

20. The element of claim 19 wherein the outcoupler area (16) is arranged, along the direction Y, between the incoupler areas (14).

21. The element of any of the preceding claims wherein the light coupling structures (24) formed on only one surface (30a, 30b) of the lightguide (12).

22. A security document with an element of any of the preceding claims.

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