Optical security device

Abstract

The present invention relates to an optical security device (1) comprising a metallization layer (4) having an optically active microstructure defining an active observation condition and an inactive observation condition for an observer, wherein the metallization layer (4) comprises a micro-patterned mesh of metallized sections (6) and de-metallized sections (5), wherein said metallized sections (6) provide a first optical feature. The optical security device (1) further comprises an underlying layer (3) providing a second optical feature, wherein the mesh constant of the micro-patterned mesh is smaller than 200 micrometres, so that (i) under the active observation condition said micro-patterned mesh makes at least the first optical feature provided by the metallized sections (6) of the metallization layer (4) perceivable by the observer and under the inactive observation condition said micro-patterned mesh makes at least the second optical feature provided by the underlying layer (3) perceivable by the observer.

Description

Optical security deviceSTATEMENT REGARDING FEDERALLY SPONSORED RESEARCH ORDEVELOPEMENT

[0001] This invention was made with government support under Contract Number 2031ZA19D00015, awarded by the Bureau of Engraving and Printing. The government has certain rights in the invention.TECHNICAL FIELD

[0002] The present invention relates to an optical security device comprising a metallization layer.BACKGROUND OF THE INVENTION

[0003] Metallized security elements with overt light-diffracting, refracting or scattering feature effects are known in the art. However, oftentimes they are perceived as dull, continuous metallic stickers under observation conditions where the feature cannot be perceived, i. e. under inactive observation conditions. Although in their active state these security features might present a highly appealing feature effect, this dullness under inactive observation conditions creates the impression of an unsophisticated and clumsy piece of technology that many potential customers are not willing to place on their products.

[0004] To avoid this, optical security devices have been provided in which predefined parts of the optical security device have been de-metallized to avoid a continuous metallic sticker impression under inactive observation conditions.SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide an optical security device that has different optical features under different observation condition.

[0006] This objective is solved by an optical security device as defined in claim 1 . Advantageous configurations and developments are apparent from the dependent claims.

[0007] The optical security device according to the invention comprises: a metallization layer having an optically active microstructure defining an active observation condition and an inactive observation condition for an observer. The metallization layer comprises a micro-patterned mesh of metallized sections and demetallized sections, wherein said metallized sections provide a first optical feature. Furthermore, the optical security device comprises an underlying layer providing a second optical feature. According to the invention the mesh constant of the micropatterned mesh is smaller than 200 micrometres, so that under the active observation condition said micro-patterned mesh makes at least the first optical feature provided by the metallized sections of the metallization layer perceivable by the observer and under the inactive observation condition said micro-patterned mesh makes at least the second optical feature provided by the underlying layer perceivable by the observer.

[0008] In an embodiment of the invention, under the active observation condition said micro-patterned mesh makes only the first optical feature provided by the metallized sections of the metallization layer perceivable by the observer and under the inactive observation condition said micro-patterned mesh makes only the second optical feature provided by the underlying layer perceivable by the observer.

[0009] In another embodiment of the invention, under the active and the inactive observation condition said micro-patterned mesh makes the first optical feature as well as the second optical feature perceivable by the observer. However, the ratio of the perceived first optical feature to the perceived second optical feature is different under the active and inactive observation condition.

[0010] In particular, the ratio of the perceived first optical feature to the perceived second optical feature under the active observation condition is larger than the ratio of the perceived first optical feature to the perceived second optical feature under the inactive observation condition.

[0011] In an embodiment of the invention, under the active observation condition said micro-patterned mesh makes the metallization layer non-transparent to the observer so that the first optical feature provided by the metallized sections of the metallization layer is mainly perceived by the observer and under the inactive observation condition said micro-patterned mesh makes the metallization layer at least semitransparent to the observer so that the second optical feature provided by the underlying layer is mainly perceived by the observer.

[0012] In another embodiment of the invention, under the active observation condition said micro-patterned mesh makes the metallization layer semi-transparent to the observer so that the first optical feature provided by the metallized sections of the metallization layer and the second optical feature are perceived by the observer at the same time and under the inactive observation condition said micro-patterned mesh makes the metallization layer transparent to the observer so that the second optical feature provided by the underlying layer is mainly or only perceived by the observer. Therefore, intentional mixing of the second optical feature with the first optical feature in the active observation condition is realized. This feature may generate a colour shift effect.

[0013] According to the invention, in a viewing direction of an observer the underlying layer is arranged below the metallization layer meaning that light coming from the underlying layer needs to pass the metallization layer on top of the underlying layer in order to reach the observer.

[0014] The optical security device of the present invention utilizes an advantageous micro-patterned de-metallization scheme of metallized and de-metallized sections in combination with an optically active microstructure to create for an observer the impression of a transparency switch for the metallization layer. In consequence, the optical security device delivers a high-contrast and potentially coloured scattering image of a first optical feature under the active observation condition, but is perceived as semi-transparent layer under the inactive observation condition where the optically active microstructure, e. g. the scattering structure of the optically active microstructure, is not active. Hence, the perceived semi-transparency under the inactive observation condition allows visibility of any opaque underlying layer andallows interaction between the metallization layer on top and the underlying layer. The optical security device of the present invention provides interaction with an underlying layer in a meaningful way. The result is an optical appearance that may provide a security feature with toggled transparency that works across multiple functional layers and hence constitutes a highly secure and sophisticated device for securing valuable items or documents.

[0015] The phrase that the first optical feature “is mainly perceived by the observer” shall mean in the context of this document that the luminance of light coming from the microstructure of the metallization layer, i.e., coming from the metallized sections of the metallization layer to an observer under the active observation condition is many times higher than the luminance of light coming from the underlying layer. The result is that although light coming from both the metallization layer as well as the underlying layer reaches the observer, the observer will essentially or mainly or only perceive the first optical feature generated by light coming from the metallized sections of the metallization layer and light coming from the underlying layer is not perceived by the observer. In particular, the luminance of the metallization layer under the active observation condition is x times higher than the luminance of the underlying layer under such active observation condition, wherein x is larger than 3, in particular larger than 5 and preferably larger than 10.

[0016] Furthermore, the value of the parameter x may depend on the contrast, in particular the Weber contrast, of the second optical feature of the underlying layer. The greater the Weber contrast in the underlying layer, the greater the value of the parameter x must be. For a low contrast appearance, characterized by very similar values for hue and brightness, a factor of 3 would typically conceal the underlying layer. For intermediate contrast appearances, characterized by a larger variety of hues or brightness values, a factor of 5 is expected to successfully conceal the underlying layer. For high contrast appearances such as contour lines with maximum contrast between white and black or similarly strong differences in hue or brightness, a factor of 10 might be necessary to fully conceal the underlying layer.

[0017] Likewise, the phrase that the second optical feature “is mainly perceived by the observer” shall mean in the context of this document that the luminance of lightcoming from the underlying layer to an observer under the inactive observation condition is noticeably higher than the luminance of light coming from the metallized sections of the metallization layer. The result is that although light coming from both the metallization layer as well as the underlying layer reaches the observer, the observer will predominately perceive the second optical feature generated by light coming from the underlying layer and light coming from the metallized sections of the metallization layer is not perceived by the observer. In this case, the metallization layer is perceived as transparent or at least semi-transparent. If substantially higher luminance values compared to the metallization layer are not present, for example in case the underlying layer only diffusely reflects light back to the observer, the inactive metallization layer can remain noticeable as gray haze.

[0018] The micro-patterned mesh of metallized sections and de-metallized sections consists of one or more unit cells that may be regular and / or irregular. The dimensions of the unit cells are specified by the mesh constant or several mesh constants. The mesh constant is defined by the largest distance of the center of one de-metallized section to the nearest adjacent de-metallized section wherein a metallized section is between the one and the other de-metallized sections.

[0019] According to the invention, the mesh constant is chosen to be small enough so that the average observer can no longer visually resolve the micro-patterned mesh of metallized sections and de-metallized sections with the unaided eye. In particular, the mesh constant of the micro-patterned mesh is smaller than 200 micrometres, in particular smaller than 120 micrometres and preferably smaller than 70 micrometres. If the mesh comprises unit cells having more than one mesh constant the term “mesh constant” shall refer to the largest mesh constant of the mesh constants.

[0020] The de-metallized sections are transparent whereas adjacent metallized sections are non-transparent or opaque. Therefore the phrase “making the metallization layer semi-transparent” means that for an observer under inactive observation condition the de-metallized sections are not visible because of its size so that in total the metallization layer appears semi-transparent with a gray haze.

[0021] For example, the micro-patterned mesh may be a hexagonal or checkerboard mesh. The mesh constant of such mesh may be chosen as base for shape- adapted dot matrices that may be small enough so that the unaided eye of a human observer will not be able to distinguish between opaque and transparent areas and rather perceive the area as semi-transparent. As in the optical security device of the present invention the metallization layer comprises such micro-patterned mesh, the metallization layer is perceived as semi-transparent. The transparency can be controlled by the observation condition, for example by the orientation of scattering structures of the metallization layer.

[0022] For observation conditions under which the scattering structures are active, the observer mainly perceives the scattered light, whose brightness predominates the perception. Any layer underneath the scattering layer, i.e. , the underlying layer that is arranged under the metallization layer with respect to the direction of observation, will appear dark compared to the active scattering structures and will therefore not be perceived by the observer, effectively rendering the active scattering layer opaque. On the other hand, for observation conditions under which the scattering structures are inactive, the metallization scheme provided by the micropatterned mesh is perceived as gray haze. That in principle allows visibility of the underlying layer, i.e., making the second optical feature of the underlying layer visible. Under these conditions, an observer perceives the overt security feature of the optically active microstructure of the metallization layer as transparent or at least semi-transparent and the observer can see any opaque layer that is located underneath the scattering layer, i.e., the underlying layer located underneath the metallization layer.

[0023] Consequently, the perceived transparency of the first optical feature of the metallization layer that may be a security feature can be tailored by controlling the observation condition, for example the orientation of the optically active microstructure. The resulting transparency that is dependent on the observation condition of the de-metallized optically active microstructure is used in the optical security device of the present invention as a main feature effect in security elements to reveal or conceal an image and / or pattern forming the second optical featurelocated underneath the metallization layer. This main feature effect may for example may be activated by rotation of the optical security device.

[0024] In an embodiment of the invention, the observation condition is the observation angle so that if an observer observes the optical security device under a first viewing angle the active observation condition is present and the first optical feature provided by the metallized sections of the metallization layer is mainly perceived by the observer and if the observer observes the optical security device under a second viewing angle being different from the first viewing angle the inactive observation condition is present so that the micro-patterned mesh makes the metallization layer at least semi-transparent so that the second optical feature of the underlying layer is mainly perceived by the observer and the first optical feature becomes mainly invisible.

[0025] In addition or alternatively the observation condition is the illumination angle so that if the optical security device is illuminated under a first illumination angle the active observation condition is present and the first optical feature provided by the metallized sections of the metallization layer is mainly perceived by the observer and if the optical security device is illuminated under a second illumination angle being different from the first illumination angle the inactive observation condition is present so that the micro-patterned mesh makes the metallization layer at least semitransparent so that the second optical feature of the underlying layer is mainly perceived by the observer and the first optical feature becomes mainly invisible.

[0026] In an embodiment of the invention, the optically active microstructure is an optically effective surface relief microstructure. In particular, the optically active microstructure is a patterned anisotropically scattering structure.

[0027] In particular, as described in WO 2007 / 131375 A1 the surface relief microstructure has a surface modulation of transitions from bottom regions to top regions and from top regions to bottom regions. In a first lateral direction of the surface area there may be in average at least one transition from a top region to a bottom region or vice versa within every 20 micrometres, and preferably additionally in a second lateral direction of the surface area, which is perpendicular to the firstdirection, there may be in average at least one transition from a top region to a bottom region or vice versa within every 200 micrometres. For example, in the first direction the lateral arrangement of the transitions is non-periodic, and the top regions substantially lie in the same top relief plateau and the bottom regions substantially lie in the same bottom relief plateau, such that the relief modulation depth is substantially equal over the surface area, such that the relief modulation depth is substantially equal over the surface area of a homogeneous structural colour impression. Distinct surface relief modulation depths can be utilized to obtain distinct structural colour impressions. Accordingly, the surface relief microstructure may be designated as an optically effective surface relief microstructure.

[0028] In particular, an optically effective surface relief microstructure may be used as described in WO 2007 / 131375 A1 . The surface relief microstructure may be geometrically characterized by suitably chosen transversal and depth properties of the surface relief. One property is the fact that the surface relief is strongly uncorrelated and thus is characterized by a short autocorrelation length. A helpful parameter to characterize non-periodic or non-determ inistic surface profiles is the autocorrelation function and a related autocorrelation length. The one-dimensional or two-dimensional autocorrelation function of a surface profile can be understood as a measure for the predictability of the surface profile for two spatially separated points by a distance in the plane.

[0029] The surface relief microstructure defines an active observation condition and an inactive observation condition for an observer. In case of the active observation condition, the first optical feature is mainly perceived by the observer meaning that the first optical feature is visible. In case of the inactive observation condition, the first optical feature is not mainly perceived by the observer meaning that the first optical feature is invisible. Instead, the second optical feature of the underlying layer is mainly perceived by the observer meaning that the second optical feature is visible. By changing the observation conditions, it can be switched between visibility of the first and second optical feature.

[0030] In an embodiment of the invention, a local metallization density of the metallization layer is in the range of 30% to 75%, in particular in the range of 50% to 60%.

[0031] As the micro-patterned mesh of metallized sections and de-metallized sections consists of one or more unit cells, the dimensions of which are specified by the mesh constant(s), the local metallization density is defined as the area ratio within each of these unit cells that remains metallized.

[0032] Metallization densities may be between essentially 0% and 100% metallization coverage. Furthermore, local metallization density gradients along short length-scales can be achieved. Depending on the local metallization density, the metallization layer of the optical security device of the present invention may appear partially transparent and render the underlying layer partially visible. The degree of transparency that is perceived by human observers is linked to two main properties of the optically active microstructure of the metallization layer: (i) The local metallization density underneath scattering structures and (ii) the dependency of the observation condition of the optically active microstructure, for example the angle dependency of this microstructure.

[0033] A metallization density of 100% renders the layer fully opaque and a metallization density of 0% leaves the layer fully transparent. In particular, according to the invention, a metallization layer is utilized that is effectively non-transparent in the visible range, i.e. has a transmission of typically lower than 5%. In contrast to this, a thin metallization or high-refractive index layer could be utilized with a transmission of about 50% to achieve a possibly similar effect. Yet, this traditional approach may require more expensive materials or strict process control resulting possibly in reduced yield. Furthermore, the approach of the present invention offers more flexibility and the possibility to pattern different feature effects on one security feature (e.g. areas with 100% metallization, areas without metallization, and areas with micro-patterned metallization). The binary nature of the patterning scheme that is provided by the micro-patterned mesh of metallized sections and de-metallized sections leads to a co-existence of opaque and transparent areas for any metallization density in-between the two maximum boundaries. It has been foundthat a metallization density in the above-mentioned ranges achieves advantageously an optimized balance between brightness of scattering structures and transparency of the metallic gray haze.

[0034] In fact, the local metallization density of the scattering structures determines both the brightness of the scattering structures and the density of the remaining gray haze for observation conditions under which the scattering structures do not scatter light, i.e. , under inactive observation conditions, and in this case the metallization layer is supposed to appear as transparent. A balance between optimum brightness that is achieved by a higher metallization density and a minimized gray haze that is achieved by a lower metallization density can be established individually for each optical feature of the metallization layer and the underlying layer, in particular for each graphical layout. It has been found that a metallization density in the above- mentioned ranges may achieve such optimum balance.

[0035] In an embodiment of the invention, the de-metallized sections of the micropatterned mesh are arranged on a regular mesh. In this case, manufacturing of the micro-patterned mesh may be facilitated.

[0036] In an embodiment of the invention, the de-metallized sections of the micropatterned mesh are arranged arbitrary as far as local accumulations of metallized sections are smaller than 200 micrometres, in particular, smaller than 120 micrometres, preferably smaller than 70 micrometres.

[0037] In an embodiment of the invention, the size and / or shape of the de-metallized sections of the micro-patterned mesh alters. Both differences in the metallization density and local metallization gradients may be achieved via altering the size of shape-adapted dot matrices (e.g. stars, squares, hexagons, circles, letters, numbers) of the micro-patterned mesh that may be a regular hexagonal or checkerboard mesh.

[0038] In an embodiment of the invention, one of the first and second optical features comprises a bold center element and the other of the first and second optical features comprises a background area surrounding the bold center element.According to this embodiment, a pronounced shape adaptation effect is achieved as the transparency switch of the optical security device of the present invention is used to alter the apparent outer shape of the optical feature that is used as security feature. A generic layer description of such shape adaptation effect comprises such a bold center element and a background area. The center element may contain a detailed gray scale or colour icon with iconic outer shape and a uniform patterning direction. Under active observation conditions of the scattering structures, an observer perceives the detailed grayscale or colour icon, which then transforms into its monotone dark silhouette upon transition to inactive observation conditions, for example upon sample rotation by 90°. The micro-corrugations in the background area may be orientated under an angle of 90° with respect to the corrugations of the center element and exhibit a metallization density that in combination with an opaque underlying layer induces the perception of transparency under inactive observation conditions of the scattering structures. Upon sample rotation, the background area gradually changes from being perceived as fully transparent into a bright and high visibility scattering state that appears opaque. With the background area becoming fully visible, this transition noticeably changes the apparent outer shape of the optical security device.

[0039] Moreover, a print reveal effect may be achieved if the transparency switch of the optical security device of the present invention is mainly located within the graphical layout of the first optical feature of the optical security device and in register to a print icon of the second optical feature of the underlying layer. The metallization density of the area designated for such a print reveal effect is chosen in a way that in combination with the second optical feature of the underlying layer the scattering structures are perceived as transparent under inactive observation conditions. As a result, the second optical feature of the underlying layer is fully visible and would appear to be an integrated active part of the first optical feature of the metallization layer. In this case, the metallization density is chosen to ensure that under active observation conditions of the metallization layer the brightness of the scattered light is sufficient to fully conceal the print icon of the second optical feature and render the optically active microstructure of the metallization layer as perceived opaque. Upon change of the observation condition from active to inactive, for example upon rotation of the optical security device, the print icon of the secondoptical feature is either revealed or concealed for human observation, which constitutes the so-called print reveal effect. For graphical layouts that contain an area with a print reveal effect towards the outer boundaries of the layout of the optical features, i.e. , an area that is not fully enclosed by scattering structures with other optical effects, a combination of a print reveal effect and a shape adaptation effect may be obtained since depending on the second optical feature, for example an underlying print pattern, the print reveal effect in the outer parts of the layout also changes the apparent outer shape of the first optical feature of the optical security element.

[0040] In an embodiment of the invention, the brightness of the underlying layer essentially matches the brightness of a gray haze of the metallization layer. The gray haze as used herein refers to the reduced colourfulness of the underlying layer when superimposed with the optically active microstructure of the metallization layer under inactive observation conditions. In terms of colour metric, this reduced colourfulness can be expressed by the saturation as defined in common HSL (hue, saturation, lightness) or HSV (hue, saturation, value) colour models.

[0041] The brightness and tonality of the underlying layer underneath the microstructure of the metallization layer strongly affects the visibility of the gray haze of the metallization layer under inactive observation conditions of the optically active microstructure. If this underlying layer is darker than the metallization layer, the later remains visible under inactive observation conditions as light metallic shine against the background of the underlying layer. On the other hand, in cases where the underlying layer is brighter than the metallization layer, the later remains visible under inactive observation conditions as dark shadow. In both cases, the remaining visibility of the metallization scheme of the metallized sections reveals already the shape of the first optical feature provided by the metallized sections of the metallization layer after activation of the scattering structures under active observation conditions, effectively reducing the boldness of the intended visual effect. Ideally, the brightness of the underlying layer matches the brightness of the gray haze of the metallization layer, so that the later essentially becomes invisible. Hence, for bright underlying layers a low metallization density is preferable to conceal the gray haze of the scattering structures under inactive observationconditions. Darker underlying layers allow for larger metallization densities while still allowing for an effective hiding of the remaining gray haze. Additional opaque layers with dark colours might be introduced into a stack for the purpose of hiding the remaining gray haze and to allow for higher metallization densities on the scattering structures of the metallization layer.

[0042] In an embodiment of the invention, the underlying layer has a modulation of colour, brightness and / or tonality. Such adaptation of the underlying layer supports the perceived disappearance of the remaining gray haze better than uniform backgrounds provided by the underlying layer. Furthermore, the perceived Weber contrast of scattering structures under active observation conditions is influenced by the brightness of the underlying layer. The same physical brightness of scattering structures is perceived much stronger by human observers if the brightness of the underlying layer is lower. Hence, bright scattering structures of the metallization layer exhibit a much higher Weber contrast and thus boldness against a black background provided by the underlying layer compared to a bright underlying layer.

[0043] In an embodiment of the invention, the brightness of the first optical feature of the metallization layer is higher than the brightness of the second optical feature of the underlying layer. Icons or shapes of the second optical feature having a high contrast on the underlying layer require a higher metallization density to provide sufficient brightness to fully conceal the underlying layer so that preferably the brightness of the first optical feature is higher than the brightness of the second optical feature. For high contrast icons of the second optical feature, a local metallization density of the metallization layer is preferably in the range of 55% to 60%.

[0044] Moreover, a planarization layer may be arranged between the metallization layer and the underlying layer. In addition, further layers may be applied in a stack, either with their own optical contribution to the visual appearance of the optical security element. If such layers are not fully concealed, they do not constitute an underlying layer as used herein. Furthermore, materials may be applied that may serve for the overall patch integrity, such as a protection layer or the like.

[0045] For example, a print layer may be placed adjacent to a planarization layer for the micro-patterned mesh in order to de-couple the light scattering properties of the overt first optical feature from any interaction with the print layer.

[0046] In an embodiment of the invention, the underlying layer comprises (i) a pigment ink print material or (ii) metallic ink print material, for example containing metallic nanoparticles with plasmonic activity or (iii) a dichroic dye doped LCP with additional polarization image information.

[0047] For example, metallic nanoparticles with plasmonic activity as described in WO 2011 / 064162 A2, WO2020 / 224982 or WO 2022 / 167377 may be used. Furthermore, dichroic dye doped LCP with additional polarization image information as described in WO2015 / 177062 or WO0237147 A1 may be used.

[0048] A further class of functional material suitable for use in a coloured print of the underlying layer is given by dichroic dye doped liquid crystal polymer (d-LCP) compounds. Such d-LCP materials may be used in the optical security device of the present invention in combination with the patterned surface scattering layouts of the metallization layer and optical axis patterning layers provided by the dichroic dye doped liquid crystal polymer.

[0049] In an embodiment of the invention, the optical security device further comprises a substrate to be secured by the optical security device and a protection layer, wherein the protection layer is located on said substrate and the underlying layer is located on said protection layer.

[0050] In another embodiment of the invention, the optical security device further comprises a substrate to be secured by the optical security device, wherein the underlying layer is located directly on said substrate.

[0051] According to the invention, a first optical feature, whose graphical appearance purposefully interacts with a print image or a print pattern that is located directly on an underlying substrate may be provided. Placing a print layer on a substrate itself brings the benefit of inextricable linking the appearance of the security patch with theappearance of the underlying substrate, which significantly increases harvesting resistance of the security feature.

[0052] In an embodiment of the invention, the optical security device further comprises a liquid crystal polymer layer located on top of the metallization layer. In particular, the liquid crystal polymer layer may be an optical axis patterning layer as described in WO9953349A1 .

[0053] In particular, the patterning is constituted by a controlled variation of the optical axis orientation of a uniaxial liquid crystal within the liquid crystal polymer layer. The sections constituting the patterned area may have identical optical properties throughout, in particular with respect to birefringence. However, the sections of the patterned area may also have different optical properties, in particular with respect to birefringence. In particular, the patterned area is image-wise patterned. The image-wise patterning may provide a further security feature for the optical security device.

[0054] Any functional layer stacks may be complemented by a device-assisted optical axis patterning security feature. Due to the technical requirement of an optical axis patterning layer to have a highly reflective layer underneath the liquid crystal polymer (LCP) in the stack, the device-assisted optical axis patterning security feature would typically be placed on top of the optically active microstructure of the metallization layer.

[0055] The optical security device according to the invention may be used for governmental document protection, banknotes, brand protection, brand enhancement, and packaging.BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 shows a sectional view of a first embodiment of the optical security device of the present invention,Figure 2 shows a sectional view of a second embodiment of the optical security device of the present invention,Figure 3 shows a sectional view of a third embodiment of the optical security device of the present invention,Figure 4 shows a sectional view of a forth embodiment of the optical security device of the present invention,Figure 5 shows a sectional view of a fifth embodiment of the optical security device of the present inventionFigures 6A and 6B show micrographs of an example of the micro-patterned mesh of the optical security device of the invention,Figures 7A and 7B show photographs of examples of the optical security device of the invention according to a first concept,Figure 8 shows photographs of examples of the optical security device of the invention having different local metallization density of the metallization layer, Figures 9A to 9D show photographs of examples of the optical security device of the invention with different underlying layers having a modulation of colour, brightness and / or tonality,Figures 10A to 10D show photographs of further examples of the optical security device of the invention with different underlying layers having a modulation of colour, brightness and / or tonality,Figure 11 shows photographs of examples of the optical security device of the invention showing a combination concept with an optical axis patterning deviceassisted feature,Figures'! 2A and 12B show photographs of examples of the optical security device of the invention using plasmonic nanoparticle inksFigures 13A-13D show photographs of examples of the optical security device of the invention using dichroic dye doped LCP,Figures 14A-14D show photographs of examples of the optical security device of the invention with the bare dichroic dye doped LCP, andFigures 15A-15D show photographs of examples of the optical security device of the invention using the concept of 1 transparency switch and shape adaptation.DETAILED DESCRIPTION OF THE INVENTION

[0057] A first embodiment of the optical security device 1 of the present invention is described with reference to Figure 1 :

[0058] The optical security device 1 comprises a substrate 2 that is coated with an underlying layer 3. On top of the underlying layer 3, a metallization layer 4 is arranged. The metallization layer 4 comprises an optically active microstructure that is described in further detail below. The optically active microstructure defines at least one active observation condition and at least one inactive observation condition for an observer that perceives the optical security device from the top as depicted in Fig. 1.

[0059] The metallization layer 4 comprises a micro-pattern mesh of metallized sections 6 and de-metallized sections 5. The metallized sections 6 are opaque whereas the de-metallized sections 5 are transparent. The mesh constant of the micro-patterned mesh is chosen to be small enough so that an average human observer cannot visually resolve the micro-patterned mesh of metallized sections 6 and de-metallized sections 5 with the unaided eye. To achieve this, the mesh constant is typically smaller than 200 pm, in particular smaller than 120 pm and preferably smaller than 60 pm. In the present embodiment, the mesh constant is chosen to be 70 pm.

[0060] Figures 6A and 6B show micrographs of examples of a micro-patterned mesh of a de-metallized solid imprint replication of an optical security device based on a hexagonal mesh. Star- and circle-shaped dot matrices were used and their size was modified to obtain locally varying metallization densities as it will be explained below. Fig. 6A shows a micrograph acquired under transmission lighting conditions and Fig. 6B shows a micrograph acquired under darkfield illumination conditions.

[0061] In the example of Fig. 6A, the star-shaped metallized sections 6 are arranged on a regular mesh. Likewise, the de-metallized sections 5 are arranged on a regular mesh. Furthermore, the shape of the metallized sections 6 in each unit cell of the micro-patterned mesh is identical, namely star-shaped.

[0062] In the example of Fig. 6B, a size and shape of the metallized sections 6 and likewise the size and shape of the de-metallized sections 5 of the micro-patterned mesh alters. The shape of the metallized sections 6 is star-shaped and circleshaped. However, local accumulations of metallized sections 6 are small enough sothat the unaided eye of a human observer cannot resolve the structure of the micropatterned mesh.

[0063] Another parameter of the micro-patterned mesh is the local metallization density that is defined as the area ratio within each unit cell of the micro-patterned mesh that remains metallized. Therefore, the local metallization density describes how much of the metallization layer 4 remains metallized after de-metallizing this layer to form de-metallized sections 5. The metallization density is analysed locally on the basis of the unit cells of the mesh. This means that the observer cannot resolve between metallized sections 6 and de-metalized sections 5 and in total the observer perceives a gray haze viewing the metallization layer 4 as light coming from the surface of the metallized sections 6 as well as light coming through the transparent de-metallized sections 5 reaches the eye of the observer. This effect is exploited by the optical security device 1 , as it will be explained in further detail below.

[0064] Referring back to Fig. 1 , the metallization layer 4 has an optically active microstructure that is formed on the surface of the metallized sections 6. In the present embodiment, the optically effective microstructure is a patterned anisotropically scattering structure as described in WO 2007 / 131375 A1 . In this case, the observation condition is the observation angle under which an observer observes the optical security device 1 . Under a first viewing angle, the active observation condition is present and under a second viewing angle the inactive observation condition is present. A further influence to the observation condition is the illumination angle by which the optical security device 1 is illuminated.

[0065] Under the active observation condition, the user perceives the metallized sections 6. These metallized sections 6 provide a first optical feature that is the appearance of a first image. Therefore, under the active observation condition, the first optical feature provided by the metallized section 6 is perceived by the observer. On the other hand, under the inactive observation condition the micro-patterned mesh makes the metallization layer 4 at least semi-transparent, in particular essentially transparent, so that the observer perceives the underlying layer 3. The underlying layer 3 provides a second optical feature that is the appearance of asecond image. The change from the first image into a second image upon change of observation or illumination conditions serves as a security feature.

[0066] As the metallization layer 4 is not fully transparent if the metallization density is not 0% and as the observer cannot resolve between metallized section 6 and demetallized sections 5, the metallization layer 4 appears semi-transparent with a gray haze. The perceived transparency of the metallization layer 4 depends on the viewing and / or illumination angle on the one hand and local metallization density on the other hand. Assuming the optical security device 1 is viewed under a viewing angle that defines the inactive observation condition, the transparency of the metallization layer 4 only depends on the local metallization density. This local metallization density is chosen to be in the range of 54% to 57% in this embodiment. The metallization density has been chosen in consideration of the brightness and luminance of the first optical feature provided by the metallized sections 6 as well as the Weber contrast of the second optical feature provided by the underlying layer 3. This metallization density achieves an optimum so that the observer mainly perceives the first optical feature under the active observation condition and the second optical feature under the inactive observation condition. Examples of this effect will be described below.

[0067] In the following, a second embodiment of the optical security device 1 will be described with reference to Fig. 2:

[0068] The optical security device 1 of the second embodiment differs from the optical security device 1 of the first embodiment in the stack of functional layers for the security features provided by the first and second optical feature. However, it also incorporates both, a patterned surface scattering layer provided by the metallization layer 4 and a print pattern provided by the underlying layer 3.

[0069] In this case, an adhesive layer 10 is formed on the substrate 2 that may for example be a banknote. In this embodiment, the adhesive layer 10 is a hot stamping adhesive. On top of the adhesive layer 10, a protection layer 9 is formed. On top of the protection layer 9, a coloured print layer is formed that is the underlying layer 3. Furthermore, the stack of the optical security device 1 comprises a metallizationlayer 4 that again comprises a micro-patterned mesh of metallized sections 6 and de-metallized sections 5. Between the metallization layer 4 and the underlying layer 3, a planarization layer 12 for the de-metallized patterned surface-scattering structure of the metallization layer 4 is arranged in order to decouple the lightscattering properties of the first optical feature provided by the metallized sections 6 of the metallization layer 4 from any interaction with the underlying layer 3 providing the second optical feature. Therefore, in the stack of the optical security device 1 of the second embodiment, the second optical feature of the underlying layer 3 is built in a coloured print layer.

[0070] Different classes of printing materials can be used as coloured print layer. It comprises (i) a pigment ink print material or (ii) ink containing metallic nanoparticles with plasmonic activity or (iii) a dichroic dye doped LCP with additional polarization image information.

[0071] Examples showing the effects using such coloured print layers will be described below.

[0072] In the following, a third embodiment of the optical security device 1 is described with reference to Fig. 3:

[0073] In the third embodiment, another functional layer stack of the optical security device 1 is used. Similar to the first embodiment, the underlying layer 3 that is formed by a coloured print layer is arranged on the substrate 2 that may again be a banknote. On top of the underlying layer 3, an adhesive layer 10 that may also be a hot stamping adhesive is arranged. The underlying layer 3 may not cover the surface of the substrate 2 totally, the edge area are may be omitted so that the adhesive layer 10 directly contacts the substrate 2 in the edge area so that the coloured print layer of the underlying layer 3 is fully enclosed by the adhesive layer 10 on the one hand and the substrate 2 on the other hand.

[0074] On top of the adhesive layer 10, a planarization layer 12 and then the metallization layer 4 is formed.

[0075] In the optical security device 1 of the third embodiment, the first optical feature of the metallization layer 4 purposefully interacts with a print image or print pattern of the second optical feature of the underlying layer 3 that is located directly on the substrate 2. Placing the print layer of the underlying layer 3 on the substrate 2 itself brings the benefit of inextricably linking the appearance of the security patch with the appearance of the underlying substrate 2, which significantly increasing harvesting resistance of the security feature. If the metallization layer 4 is purposely removed, the coloured print layer of the underlying layer 3 will remain on the substrate 2 as it is housed by the adhesive layer 10 and the substrate 2.

[0076] With reference to Fig. 4, a fourth embodiment of the optical security device 1 is described:

[0077] In the optical security device 1 of the fourth embodiment, the functional layer stack used in the second embodiment is complemented to a device-assisted optical axis patterning security feature. The stack on the substrate 2 is similar to the stack of the second embodiment. It comprises an adhesive layer 10 on the substrate 2, a protection layer 9 thereon, and then a coloured print layer of the underlying layer 3, a planarization layer 12, and a metallizing layer 4 on top of the metallization layer 4.

[0078] In the optical security device 1 of the fourth embodiment, a liquid crystal polymer (LCP) layer 13 is arranged on the metallization layer 4. On top of the LCP- layer 13, an imprint layer 14 is arranged. The device-assisted feature provided by the LCP-layer 13 is placed on top of the metallization layer 4 comprising the optically effective microstructure as the optical axis patterning feature of the LCP-layer 13 needs to have a highly reflected layer underneath.

[0079] The LCP layer 13 is formed from a composition comprising polymerizable liquid crystals and one or more photo-orientable substances. The LCP layer 13 can also be formed by a polymerizable liquid crystal compound, whose optical axis is aligned using surface relief microstructures as outlined in WO 2007 / 131375 A1.

[0080] Photo-orientable substances incorporate photo-orientable moieties, which are capable of developing a preferred direction upon exposure to aligning light and thuscreating anisotropic properties. Such photo-orientable moieties preferably have anisotropic absorption properties.

[0081] For example, photo-orientable moieties are substituted or un-substituted azo dyes, anthraquinone, coumarin, mericyanine, 2-phenylazothiazole, 2-phenylazobenzthiazole, stilbene, cyanostilbene, fluorostilbene, cinnamonitrile, chaicone, cinnamate, cyanocinnamate, stilbazolium, 1 ,4-bis(2- phenylethylenyl)benzene, 4,4’-bis(arylazo)stilbenes, perylene, 4,8- diamino-1 ,5- naphthoquinone dyes, aryloxycarboxylic derivatives, arylester, N-arylamide, polyimide, diaryl ketones, having a ketone moiety or ketone derivative in conjugation with two aromatic rings, such as for example substituted benzophenones, benzophenone imines, phenylhydrazones, and semicarbazones.

[0082] Preparation of the anisotropically absorbing materials listed above are well known as shown, e.g. by Hoffman et al., U.S. Patent No. 4,565,424, Jones et al., in U.S. Patent No. 4,401 , 369, Cole, Jr.et al., in U.S. Patent. No. 4,122,027, Etzbach et al., in U.S. Patent No. 4,667,020, and Shannon et al., in U.S. Patent No. 5,389,285.

[0083] Preferably, the photo-orientable moieties comprise arylazo, poly(arylazo), stilbene, cyanostilbene, cinnamate or chaicone.

[0084] A photo-orientable substance may in particular be a monomer, an oligomer or a polymer. The photo-orientable moieties can, for example, be covalently bonded within the main chain or within a side chain of a polymer or oligomer, or they may be part of a monomer or other compounds which are not polymerizable. A photo- orientable substance may further be a copolymer comprising different types of photo- orientable moieties, or it may be a copolymer comprising side chains with and without photo-orientable moieties.

[0085] Polymers denotes for example polyacrylate, polymethacrylate, polyimide, polyurethane, polyamic acids, polymaleinimide, poly-2-chloroacrylate, poly-2- phenylacrylate; unsubstituted or with Ci-Cealkyl substituted poylacrylamide, polymethacyrlamide, poly-2-chloroacrylamide, poly-2-phenylacrylamide, polyether, polyvinylether, polyester, polyvinylester, polystyrene-derivatives, polysiloxane,straight-chain or branched alkyl esters of polyacrylic or polymethacrylic acids; polyphenoxyalkylacrylates, polyphenoxyalkylmethacrylates, polyphenylalkylmethacrylates with alkyl residues of 1-20 carbon atoms; polyacrylnitril, polymethacrylnitril, cycloolephinic polymers, polystyrene, poly-4- methylstyrene or mixtures thereof.

[0086] Moreover, linear photopolymerizable polymers (LPP) as described in the embodiments of US 2016 / 0033698 A1 is used in another embodiment.

[0087] The desired alignment of the LCP layer 13 consisting of a composition comprising polymerizable liquid crystals can also be obtained by utilizing a substrate with patterned surface relief microstructure, that imposes sufficient anchoring strength onto the LCP layer prior cross-linking.

[0088] The above-described combination concept security feature with an LCP-layer 13 providing optical axes patterning introduces additional requirements for optimizing the metallization density of the metallization layer 4: The visibility and recognizability of the device-assisted optical axis patterning feature of the LCP-layer 13 strongly depends on the metallization density used for the first optical feature provided by the metallized sections 6, as it requires a reflective or at least semi-reflective layer underneath the LCP-layer 13 in the stack of the optical security feature 1 . The balance between the optimum metallization density for achieving the transparency switch effect by toggled transparency and a sufficient visibility of the device-assisted security feature provided by the LCP-layer 13 in the stack need to be established.

[0089] In the fourth embodiment, the metallization density of the metallization layer 4 is in the range between 54% and 57%.

[0090] With reference to Fig. 5, a fifth embodiment of the optical security device 1 is described:

[0091] Just as the fourth embodiment, the optical security device 1 of the fifth embodiment comprises a functional layer stack including a device-assisted optical axis patterning security feature. The stack firstly comprises the stack of the opticalsecurity device 1 of the third embodiment as shown in Fig. 3. Further, an imprint layer 14 is arranged on the LCP-layer 13.

[0092] The LCP-layer 13 and the metallization layer 4 of the optical security device 1 , in particular the metallization density of this layer 4, of the fifth embodiment correspond to the LCP-layer 13 and the metallization layer 4 of the optical security device 1 of the fourth embodiment.

[0093] In the following, examples of optical security devices 1 in accordance with the present invention are described by the photographs of Figures 7 to 17. By these examples, parameters of the optical security devices 1 of the embodiments have been evaluated, in particular the mesh constant of the micro-patterned mesh of the metallization layer 4 and the metallization density of the metallization layer 4.

[0094] Figures 7A and 7B show photographs of examples of the optical security device 1 according to the present invention. In both examples, a metallization layer 4 had been used having the following properties: The Example of Figure 7A and Figure 7B both exhibit a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants. The center part of Figure 7A exhibits a mesh constant of 67 pm and a micro-patterned mesh of hexagons, circles and stars. The local metallization density is varied to account for the intended grayscale appearance of the implemented icon. The background area surrounding this icon exhibits a mesh constant of 170 pm and a micro-patterned mesh of stars. The metallization density varies from 100% in the center to 0° at the edges of the security patch.

[0095] The center part of Figure 7B exhibits a mesh constant of 60 pm and a micropatterned mesh consisting of regular hexagons. The metallization density is constant across the center of the icon and exhibits 42%.In both examples, the second layer of the Security Feature is printed directly onto the underlying paper substrate, so that an adhesive layer is placed between the metallization layer and the second layer

[0096] The metallized sections 6 comprise an optically active microstructure. In the present case an optically effective surface relief microstructure as described in WO 2007 / 131375 A1 have been used. The depth of the surface relief microstructure for both Examples in Figure 7A and Figure 8A is between 100 nm and 110 nm

[0097] The optically active microstructure used by the metallization layer 4 defines an active observation condition that is a first illumination angle and an inactive observation condition that is a second illumination angle. Rotating the optical security device 1 about an axis parallel to the normal of the device 1 will switch between an active and an inactive observation condition. The examples of Figures 7A and 7B utilized two main pattering directions, namely the illumination angle 0° and 90°, for the scattering structures of the metallized sections 6 of the metallization layer 4. It is mentioned that in principle also a larger number of main scattering angles, e.g., three, four or even more, could also be utilized.

[0098] In Figures 7A and 7B, the inactive observation condition with an illumination angle of 0° is shown in the upper parts of the photographs and the active observation condition with an illumination angle of 90° is shown in the lower parts of the photographs. The photographs of Figures 7A and 7B are collages showing what is seen under active observation condition in the upper part and showing what is perceived under inactive observation condition in the lower part.

[0099] In the example shown in Fig. 7A, the first optical feature of the metallized section 6 of the metallization layer 4 is a detailed gray scale or colour icon with iconic outer shape. In the present example, the first optical feature comprises an image of the statue of liberty. The second optical feature of the underlying layer 3 comprises a monotone dark silhouette of the image of the first optical feature. Therefore, when the scattering structures are visible under active observation condition as shown in the upper part of Fig. 7A, an observer perceives the detailed gray scale or colour icon of the statue of liberty, which then transforms into its monotone dark silhouette upon rotation of the optical security element 1 by 90° as shown in the lower part of Fig. 7A.

[0100] In the example shown in Fig. 7A, the transparency switch of the optical security device 1 is used to alter the apparent outer shape of the first optical feature that may therefore be used as security element. This effect is designated as shape adaptation effect.

[0101] The micro-corrugations in the background area provided by the underlying layer 3 are oriented under an angle of 90° with respect to the corrugations of the center element provided by the metallization layer 4. The metallization density has been chosen so that in combination with an opaque underlying layer 3 the perception of transparency in the inactive state of the scattering structures is induced, namely under inactive observation condition. Upon rotation of the optical security device 1 , the background area gradually changes from being perceived as full transparency into a bright and high visibility scattering state that appears opaque. With the background area becoming fully visible, this transition notably changes the apparent outer shape of the security element provided by the first optical feature of the metallization layer 4.

[0102] With reference to Fig. 7B, a print reveal effect is described:

[0103] The print reveal effect can be achieved if the transparency switch provided by the optical security device 1 is mainly located within the graphical layout of a security element provided by the first optical feature of the metallization layer 4 and in register to a print icon provided by the second optical feature of the underlying layer 3. The metallization density of the area designated for the print reveal effect is chosen in a way that in combination with the underlying print icon of the second optical feature the scattering structures of the metallized section 6 are perceived as transparent under inactive observation condition. As a result, the underlying static print layer of the underlying layer 3 is fully visible and would appear to be an integrated active part of the security feature as it can be seen in Fig. 7B. Under active observation condition, the eagle head is visible within a particular graphical pattern whereas stars are visible instead of the eagle head under inactive observation condition. The chosen metallization density ensures that under active observation condition the brightness of scattered light is sufficient to fully conceal the print icon of the second optical feature and render the metallization layer 4 asperceived opaque. Upon rotation of the optical security device 1 , the print icon is either revealed or concealed for human observation, which constitutes the print reveal effect.

[0104] In a further embodiment (not shown), the graphical layout contains an area with a print reveal effect towards the outer boundaries of the layout, i.e. , not fully enclosed by scattering structures with other optical effects. In this case, a combination of the print reveal effect and the shape adaptation effect is obtained, since depending on the underlying print pattern of the underlying layer 3, the print reveal effect in the outer parts of the layout also changes the apparent outer shape of the security element provided by the first optical feature.

[0105] Fig. 8 shows photographs of test samples that have been used to optimize properties of the optical security device 1 . The upper row shows photographs under active observation condition and the lower row shows photographs under inactive observation condition. From left to right, the photographs of the samples are ordered with decreasing metallization density so that in the photographs on the outer right side no light from scattering structures from the metallized sections 6 are visible any more.

[0106] The Example of Figure 8 exhibits a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes with a constant mesh constant of 50 pm. The metallization density is stepwise decreased from left to right with the following densities: 66% I 51 % 141 % 34% 128% 120% 1 16%. The print icon is directly placed on the underlying paper substrate. No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm, for blue areas between 235 nm and 245 nm.

[0107] The local metallization density on scattering structures in the metallization layer determines both, the brightness of the scattering structures and the density of the remaining gray haze for inactive observation conditions under which the scattering structures of the metallized section 6 do not scatter light and the metallization layer 4 is supposed to be transparent. A balance between optimumbrightness (higher metallization density) and minimized gray haze (lower metallization density) needs to be established individually for each graphical layout.

[0108] The test samples shown in Fig. 8 were fabricated for this purpose. They exhibit seven homogeneously patterned samples with coloured numerals 1 to 7 and a varying metallization density between 66.7% for the example on the left and 16.0% for the example on the right. It is observed that with increasing metallization density, the homogenously oriented patterned surface scattering structures of the metallized sections 6 appear brighter and less transparent and the visibility of the underlying layer 3 for the background image is drastically reduced (see top row of Fig. 8). On the other hand, for test samples for which the metallization density is high, the remaining gray haze for inactive observation conditions under which the scattering structure of the metallized section 6 do not scatter light covers substantially the underlying layer 3 and deteriorate its visibility.

[0109] The result of the test samples shown in Fig. 8 is that the ideal metallization density range for achieving an optimized balance between brightness of scattering structures and transparency of the metallic gray haze is between 54% and 57%.

[0110] Icons or shapes with a high contrast on the underlying layer 3 require a higher metallization density to provide sufficient brightness to fully conceal the underlying layer 3. Therefore, for high contrast icons of the second optical feature, the ideal metallization density range is between 54% and 57%.

[0111] Figures 9A to 9D show photographs of test samples that have been made in order to determine the ideal brightness provided by the metallization layer 4 and the underlying layer 3. Furthermore, the ideal tonality has been determined.

[0112] The Example of Figure 9 exhibits a 40 nm thick layer of aluminium metallization that is patterned into a checkerboard mesh with a constant mesh constant of 100 pm. In the inactive background areas, a micro-patterned mesh of squares and stars is used, the metallization density is set to 37%. The foreground icon “50” exhibits a full metallization. The print layer is directly placed on the underlying paper substrate. No adhesive is used between first layer and secondlayer, a thin gap of air can hence be expected between these two layers. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm

[0113] The brightness and tonality of the underlying layer 3 strongly affects the visibility of the gray haze of the metallization layer 4 on scattering structures under inactive observation conditions. If this background provided by the underlying layer 3 is darker than the metallization layer 4, the latter remains visible also under inactive observation condition as light metallic shine against the background. This effect is shown in Fig. 9A.

[0114] On the other hand, in cases where the background of the underlying layer 3 is brighter than the metallization layer 4, the latter remains visible under inactive observation conditions as dark shadow as it can be seen in Figures 9C and 9D. In both cases, the remaining visibility of the metallization scheme of the micropatterned mesh reveals already the shape of the first optical feature providing a patterned surface scattering image after activation of the scattering structures under active observation conditions, effectively reducing the boldness of the intended visual effect. It has been determined that ideally the brightness of the underlying layer 3 matches the brightness of the gray haze of the metallization layer 4, so that the latter essentially becomes invisible as it is shown in Fig. 9B. Hence, for light background of the underlying layer 3, a low metallization density is preferable to conceal the gray haze of scattering structures under inactive observation conditions. A darker background of the underlying layer 3 allows for larger metallization densities while still allowing for an effective hiding of the remaining gray haze.

[0115] In a further embodiment, additional opaque layers with dark colours are introduced into the stack for the purpose of hiding the remaining gray haze and allow for higher metallization densities on the scattering structures of the metallization layer 4.

[0116] Furthermore, it has been found by the examples of Figures 9A to 9D that patterned backgrounds of the underlying layer 3 with a modulation of the colour, the brightness or the tonality support the perceived disappearance of the remaining gray haze better than uniform backgrounds of the underlying layer 3.

[0117] A second aspect influenced by the brightness of the underlying layer 3 is the perceived Weber contrast of scattering structures under active observation conditions provided by the metallization layer 4. The same physical brightness of such scattering structures is perceived much stronger by human observers if the brightness of the background provided by the underlying layer 3 is lower. Hence, bright scattering structures of the metallization layer 4 exhibit a much higher Weber contrast and thus boldness against a black background of the underlying layer 3 compared to a bright background of the underlying layer 3. This effect is shown by the photographs of Figures 10A to 10D showing photographs of an optical security device 1 on top of various coloured paper backgrounds in which Fig. 10A shows a photograph of an optical security device 1 with black background of the underlying layer 3 and Fig. 10D shows a photograph of an optical security device 1 with a bright background of the underlying layer 3.

[0118] The Example of Figure 10 exhibits a 40 nm thick layer of aluminium metallization that is patterned into a checkerboard mesh with a constant mesh constant of 100 pm. In the active background areas, a micro-patterned mesh of squares and stars is used, the metallization density is set to 37%. The print layer is directly placed on the underlying paper substrate. No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm.

[0119] Fig. 11 shows photographs of examples that illustrate the particular effect of the fourth and fifth embodiment of Figures 4 and 5. The first and second row of Fig.11 show examples of optical security devices 1 showing the print reveal effect under active (first row) and inactive (second row) observation conditions. In the third row, photographs of examples are shown providing the effect of the LCP-layer 13 under linear polarized lighting condition.

[0120] The Example of Figure 11 exhibits a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes with a constant mesh constant of 50 pm. The metallization density is stepwise decreased from left to tight with the following densities: 66% / 51 % / 41 % 34% / 28% / 20% / 16%. The print icon isdirectly placed on the underlying paper substrate. No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm, for blue areas between 235 nm and 245 nm.

[0121] In this case, the examples shown in Fig. 8 have been equipped with an LCP- layer 13 providing optical axes patterning that exhibits a contrast inversion effect of radial stripes in the background of a static grayscale image of the continental USA flag in the center. By assessing both, the toggled transparency properties of the seven test examples against the various inkjet-printed office paper backgrounds and the respective visibility of the feature provided by the LCP-layer 13 under linear polarized lighting conditions, a suitable metallization density for the utilization of the transparency switch effect was established. Columns 3 and 4 of Fig. 11 provide sufficient balance between cover and reveal of the print of the underlying layer 3, while maintaining a good visibility and brightness of the feature effect provided by the metallization layer 4 and the LCP-layer 13 under active observation condition.

[0122] Based on the observations shown by the photographs of the examples of Fig. 11 , the ideal metallization density range for combination concept security features with additional LCP-layer 13 for optical axis patterning is between 54% and 57%. .

[0123] In the following, effects in further embodiment of particular functional materials for a coloured print layer of the underlying layer 3 are described:

[0124] Figures 12A and 12B show photographs of a circular plasmonic ink patch behind a de-metallized patterned surface scattering layer provided by the metallization layer 4 showing the image of the statue of liberty as first optical feature. The photograph of Fig. 12A is taken under 0° illumination direction and the photograph of Fig. 12B is taken under 90° illumination direction.

[0125] The Example of Figure 12 exhibit a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants. The center part of the security feature exhibits a mesh constant of 67 pm and amicro-patterned mesh of hexagons, circles and stars. The local metallization density is varied to account for the intended grayscale appearance of the implemented icon. The background area surrounding this icon exhibits a mesh constant of 170 pm and a micro-patterned mesh of stars. The metallization density varies from 100% in the center to 0° at the edges of the security patch. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm.

[0126] The plasmonic ink for the circular patch shape is a proprietary BASF R&D material, which was applied via spin-coating onto a glass substrate and subsequently selectively crosslinked under UV light within the circular shape. Noncrosslinked ink was then washed off using alcoholic solvents.

[0127] No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers.

[0128] The optical security devices that had been used for the photographs of Figures 12A and 12B use plasmonic nanoparticle inks. Such ink has been used for the second optical feature of the underlying layer 3 in combination with the stack of the fourth embodiment shown in Fig. 4. It provides a secondary metallization layer. The metallic reflection of this secondary metallization layer provided by the underlying layer 3 in combination with the reflection from the metallization layer 4 that is an aluminium layer in the stack ensures a good visibility of the device-assisted optical axis patterning of the LCP-layer 13. The plasmonic nanoparticle inks are available in different plasmonic colours with high colour saturation, which allows high flexibility in combining the artwork of the second optical feature of the underlying layer 3 and the first optical feature of the metallized sections 6 of the metallization layer 4 into a coherent unit. Plasmonic nanoparticle inks also exhibit distinct reflection and transmission colours, so that on otherwise transparent substrates the colour change constitutes an additional feature effect that can be used as authentication method for the genuineness of the optical security device 1 .

[0129] In the photographs taken from optical security devices 1 shown in Figures 13 and 14, another class of a functional material suitable for use in the coloured print of the underlying layer 3 is given by dichroic dye doped LCP (d-LCP).

[0130] Figures 13A to 13D as well as Figures 14A to 14D show photographs of further examples of a patterned underlying layer 3 of dichroic dye doped LCP with additional polarization image information. The examples are based on two different dye systems. The examples of Figures 13A to 13D use a combination of this LCP- layer 13 with a de-metallized patterned surface scattering sample under the two main lightning conditions; Figures 14A and 14B use the LCP-layer 13 as seen with the unaided eye and Figures 14C and 14D show the bare LCP-layer 13 as seen under linearly polarized light. The photographs show a combination of retardation patterning and angle-selected absorbance of a dichroic dye dopant.

[0131] The example of Figure 13 exhibit a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants. The center part of the security feature exhibits a mesh constant of 67 pm and a micro-patterned mesh of hexagons, circles and stars. The local metallization density is varied to account for the intended grayscale appearance of the implemented icon. The background area surrounding this icon exhibits a mesh constant of 170 pm and a micro-patterned mesh of stars. The metallization density varies from 100% in the center to 0° at the edges of the security patch. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm.

[0132] Two dichroic dyes (red I blue) were added to a standard LCP formulation and the d-LCP formulation was processed onto an optically varying device (OVD) varnish with respective alignment structures to obtain the directional patterning of the dye. Selective UV cross-linking through a binary mask is used to obtain the flowerlike shape of the ink layer. Non-crosslinked d-LCP was then washed off using ethylacetate solvent.

[0133] No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers.

[0134] Under normal observation conditions with the unaided eye, the d-LCP appears as coloured ink (see Figures 14A and 14C) that supports the contrast and visibility of patterned surface scattering layouts of the metallization layer 4 as it can be seen in Figures 14A and 14B as well as 14C and 14D, which were acquiredunder linearly polarized lighting conditions. Since the optical axis of d-LCP can be patterned similar to undoped LCP-materials via light controlled molecular orientation (LCMO), observation of the d-LCP under linearly polarized light can reveal additional imagery pattern into the LCP-layer 13 (see Figures 14B and 14D).

[0135] The example of Figure 14 exhibit a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants. The center part of the Security Feature exhibits a mesh constant of 67 pm and a micro-patterned mesh of hexagons, circles and stars. The local metallization density is varied to account for the intended grayscale appearance of the implemented icon. The background area surrounding this icon exhibits a mesh constant of 170 pm and a micro-patterned mesh of stars. The metallization density varies from 100% in the center to 0° at the edges of the security patch. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm.

[0136] Two dichroic dyes (red I blue) were added to a standard LCP formulation and the d-LCP formulation was processed onto an OVD varnish with respective alignment structures to obtain the directional patterning of the dye. Selective UV cross-linking through a binary mask is used to obtain the flower-like shape of the ink layer. Non-crosslinked d-LCP was then washed off using ethylacetate solvent.

[0137] No adhesive is used between first layer and second layer, a thin gap of air can hence be expected between these two layers.

[0138] In the following further examples of optical security devices 1 are shown by photographs wherein several layouts utilize the transparency switch effect.

[0139] Figures 15A to 15D show photographs of examples of an optical security device 1 having an image of the statue of liberty as first optical feature of the metallized sections 6 of the metallization layer 4 on a printed paper coupon. The photographs were acquired under spotlight illumination conditions and the four principal illumination directions. The halo surrounding the statue of liberty is either inactive and concealed or becomes activated and visible, effectively rendering the halo area opaque.

[0140] The example of Figure 15 exhibit a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants. The center part of the Security Feature exhibits a mesh constant of 67 pm and a micro-patterned mesh of hexagons, circles and stars. The local metallization density is varied to account for the intended grayscale appearance of the implemented icon. The background area surrounding this icon exhibits a mesh constant of 170 pm and a micro-patterned mesh of stars. The metallization density varies from 100% in the center to 0° at the edges of the security patch. The depth of the surface relief microstructure in silver areas is between 100 nm and 110 nm.

[0141] The second layer of the Security Feature is printed directly onto the underlying paper substrate, so that an adhesive layer is placed between the metallization layer and the second layer.

[0142] As in Fig. 7A, Figures 12A to 12B show a shape-adaptation effect, as the transparency switch provided by the optical security device 1 induces a perceived change of the outer shape of the first optical feature. For illumination conditions under which the halo remains dark, the metallization scheme of the metallization layer 4 providing the halo blends into the dark background print of the underlying layer 3 and effectively becomes invisible. The outer shape of the first optical feature then appears to resemble the image of the statue of liberty. Upon activation of the scattering structures of the halo, the perceived outer shape then changes into a circular shape, which constitutes a pronounced and bold feature effect that can be used as authentication feature for the optical security device 1 .

[0143] In Fig. 16 as well as Figures 17A to 17D, a print reveal effect with print icon is illustrated. Fig. 16 shows a schematic illustration of the separation of icons into a print icon that is the image of the head of an eagle and an OVD icon of the “seal” on a printed paper coupon or an optically active microstructure. The security patch of the first optical feature is covering the print icon and the visibility of the later is controlled via rotation of the paper coupon.

[0144] The example of Figure 16 exhibits a 40 nm thick layer of aluminium metallization that is patterned into hexagonal meshes of varying mesh constants.The center part of security patch exhibits a mesh constant of 60 pm and a micropatterned mesh consisting of regular hexagons. The metallization density is constant across the center of the icon and exhibits 42%. The second layer of the Security Feature is printed directly onto the underlying paper substrate, so that an adhesive layer is placed between the metallization layer and the second layer.

[0145] Figures 17A to 17D show photographs of examples of the optical security device 1 on a printed paper coupon showing the print icon “eagle” as first optical feature. The photographs were acquired under spotlight illumination conditions and the four principal illumination directions. The print reveal effect is activated via rotation of the optical security device 1 and the print icon is either revealed under the security patch or concealed by the scattering structures of the metallization layer 4.

[0146] It is mentioned that the size and shape of the print icon background area provided by the underlying layer 3 resemble the shape and size of the center plate of the security patch “seal” provided by the metallized sections 6 of the metallization layer 4, which is equipped with the print reveal effect. Depending on the coupon orientation with respect to the dominant light sources, the scattering structures of the center plate of the metallization layer 4 either reveal the underlying print icon or conceal the print icon upon inspection.LIST OF REFERENCE SIGNS1 Optical security device 12 substrate3 underlying layer 3 4 metallization layer 45 de-metallized sections 56 metallized sections7 first interface8 second interface 9 protection layer10 adhesive layer12 planarization layer13 LCP layer14 further imprint layer

Claims

Claims1 . Optical security device (1 ) comprising: a metallization layer (4) having an optically active microstructure defining an active observation condition and an inactive observation condition for an observer, wherein the metallization layer (4) comprises a micro-patterned mesh of metallized sections (6) and de-metallized sections (5), wherein said metallized sections (6) provide a first optical feature, and an underlying layer (3) providing a second optical feature, wherein the mesh constant of the micro-patterned mesh is smaller than 200 micrometres, so that(i) under the active observation condition said micro-patterned mesh makes at least the first optical feature provided by the metallized sections (6) of the metallization layer (4) perceivable by the observer and(ii) under the inactive observation condition said micro-patterned mesh makes at least the second optical feature provided by the underlying layer (3) perceivable by the observer.

2. Optical security device (1 ) of claim 1 , wherein the ratio of the perceived first optical feature to the perceived second optical feature under the active observation condition is larger than the ratio of the perceived first optical feature to the perceived second optical feature under the inactive observation condition.

3. Optical security device (1 ) of claim 1 or 2, wherein(i) under the active observation condition said micro-patterned mesh makes the metallization layer (4) non-transparent to the observer so that the first optical feature provided by the metallized sections (6) of the metallization layer (4) is mainly perceived by the observer and(ii) under the inactive observation condition said micro-patterned mesh makes the metallization layer (4) at least semi-transparent to theobserver so that the second optical feature provided by the underlying layer (3) is mainly perceived by the observer.

4. Optical security device (1 ) of claim 1 or 2, wherein(i) under the active observation condition said micro-patterned mesh makes the metallization layer (4) semi-transparent to the observer so that the first optical feature provided by the metallized sections (6) of the metallization layer (4) and the second optical feature are perceived by the observer at the same time and(ii) under the inactive observation condition said micro-patterned mesh makes the metallization layer (4) transparent to the observer so that the second optical feature provided by the underlying layer (3) is mainly perceived by the observer.

5. Optical security device (1 ) of claim 1 , wherein the optically active microstructure is an optically effective surface relief microstructure.

6. Optical security device (1 ) of claim 2, wherein a local metallization density of the metallization layer (4) is in the range of 30% to 75%, in particular in the range of 50% to 60%.

7. Optical security device (1 ) of any of the preceding claims, wherein the de-metallized sections (5) of the micro-patterned mesh are arranged on a regular mesh.

8. Optical security device (1 ) of any of claims, wherein the de-metallized sections (5) of the micro-patterned mesh are arranged arbitrary as far as local accumulations of metallized sections are smaller than 200 micrometres.

9. Optical security device (1 ) of any of claims 1 to 4, wherein the size and / or shape of the de-metallized sections (5) of the micropatterned mesh alters.

10. Optical security device (1 ) of any of the preceding claims, wherein the first optical feature of the metallization layer (4) is in register with the second optical feature of the underlying layer (3).11 . Optical security device (1 ) of any of the preceding claims, wherein the brightness of the underlying layer (3) essentially matches the brightness of a gray haze of the metallization layer (4).

12. Optical security device (1 ) of any of the preceding claims, wherein the underlying layer (3) has a modulation of colour, brightness and / or tonality.

13. Optical security device (1 ) of any of the preceding claims, wherein the brightness of the first optical feature of the metallization layer (4) is higher than the brightness of the second optical feature of the underlying layer (3).

14. Optical security device (1 ) of any of the preceding claims, wherein a planarization layer is arranged between the metallization layer (4) and the underlying layer (3).

15. Optical security device (1 ) of any of the preceding claims, wherein the underlying layer (3) comprises (i) a pigment ink print material or (ii) metallic ink print material, for example containing metallic nanoparticles with plasmonic activity or (iii) a dichroic dye doped LCP with additional polarization image information.

16. Optical security device (1 ) of any of the preceding claims, further comprising a substrate to be secured by the optical security device (1 ) and a protection layer, wherein the protection layer is located on said substrate and the underlying layer (3) is located on said protection layer.

17. Optical security device (1 ) of any of claims 1 to 14, further comprising a substrate to be secured by the optical security device (1 ), wherein the underlying layer (3) is located directly on said substrate.

18. Optical security device (1 ) of any of the preceding claims, further comprising a liquid crystal polymer layer located on top of the metallization layer (4).

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