Optical security device

Abstract

An optical security device for a valuable document, such as a banknote, has several in-coupling structures, such as gratings, each for coupling incident light into a respective waveguide. The waveguid

Description

[0001] OPTICAL SECURITY DEVICE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of an optical security device for a document, such as a banknote, passport, ticket, bank card or driving licence.

[0004] BACKGROUND OF THE INVENTION

[0005] It is known to provide valuable documents with visible security features such as fine printed patterns, holograms, transparent or semi-transparent regions and so forth. However, these are becoming easier to replicate.

[0006] There are a number of problems with implementing new security devices for valuable documents including that: the security device should make it easy for the user to authenticate that it is a genuine document, and distinguish from a counterfeit document; the security device should be compatible with existing security features on documents, as well as being flexible and robust; the security device should be relatively simple and inexpensive to produce by legitimate manufacturers; and the security device should be hard to replicate by a counterfeiter.

[0007] The present invention aims to alleviate, at least partially, some or any of the above problems.

[0008] SUMMARY OF THE INVENTION

[0009] According to a first aspect of the invention there is provided an optical security device comprising: a plurality of in-coupling structures for receiving light incident on the device; an out-coupling structure for emitting light from the device; and at least one waveguide for each in-coupling structure for conveying light from the respective in-coupling structure to said out-coupling structure, wherein each in-coupling structure is configured to couple light incident on the device into said at least one waveguide, and the out-coupling structure is configured to couple light out of said at least one waveguide to be emitted from the device, wherein the in-coupling structures are spatially separate from each other and from the out-coupling structure, and wherein masking at least one of said in-coupling structures from incident light produces an optical effect in the light emitted from the device at the out-coupling structure . An optical security device according to one preferred embodiment of the invention includes interactive multi-point micro- and nanoscale optical features with the embedded ability to transport light signals across a flexible light-guiding waveguide. All the optical components such as in-coupling and out-coupling structures and waveguides are able to be located on a flexible polymer substrate, suitable for applications such as on a polymer banknote. These optical elements generate a new family of security devices with different colourful visual effects. The security enabling optical components, such as letters, numbers, or images with subwavelength grating features, are spatially separated from the light source stimulating them. Embodiments of the invention can comprise a decentralized design architecture consisting of three functional surface optical elements.

[0010] Preferably the optical effect is distinctive and colourful.

[0011] Preferably the in-coupling structures are designed in the form of onedirectional blazed gratings of specific periodicities such that they can limit the incoupling incident light wavelength range, and in conjugation with a high-index polymer waveguide material the in-coupling structure becomes a colour selective window.

[0012] Preferably ambient light incident on the in-coupling structures is filtered to produce visually identifiable distinctive colours which are conveyed through the waveguides to the out-coupling structure(s) to produce a colourful visual effect.

[0013] Preferably the out-coupling structures comprise gratings designed to direct maximum conveyed light to a user’s eyes, for example a one-dimensional binary grating can be used as an out-coupling structure.

[0014] Preferably the out-coupling structure is pixelated, and the pixelated structure can be used to pattern one or multiple numbers or images, wherein each pixelated out- coupling structure diffracts the light received from the in-coupling structures through connected waveguides.

[0015] Preferably the in-coupling structures, out-coupling structure, and waveguides comprise a flexible polymer, wherein both in- and out-coupling structures are patterned on a low refractive index polymer substrate, and the waveguide comprises a high refractive index polymer suitable for guiding light from in-coupling to out- coupling structures.

[0016] Preferably a metal film of high reflectivity is provided between the substrate polymer and waveguide polymer. The thin metal film (or layer), preferably of aluminum, covers the gratings and the waveguide floor. The metal film is used to produce a reflective mirror effect. The coupled light is reflected during guiding through a waveguide.

[0017] Preferably at least one in-coupling structure comprises a one-dimensional grating, preferably a blazed grating of a particular period depending on the desired colour of light to be selected.

[0018] Further optional features of the invention are defined in the dependent claims.

[0019] DESCRIPTION OF THE DRAWINGS

[0020] Embodiments of the invention will now be described, by way of non-limiting example, with reference to the accompanying drawings. The invention may further comprise, in any combination, any features of the embodiments which will now be described.

[0021] Fig. 1 is a top view of bank note including a security device according to one example, wherein the security device is covered by a protective layer or cover, and interactive in-coupling and out-coupling gratings are placed inside square or rectangular windows;

[0022] Fig. 2 is a top view of the banknote of Fig. 1 with the protective layer removed to reveal the optical circuit of the security device;

[0023] Fig. 3 is an enlarged view of the security device of Figs. 1 and 2;

[0024] Fig. 4 is an enlarged cross-sectional view of a metal-coated blazed grating profile used in one example;

[0025] Fig. 4A is an enlarged cross-sectional view of an optical waveguide used in one example;

[0026] Fig. 5 shows the colour production of grating periodicities ranging from 300 nm to 600 nm, increasing in 50 nm steps, in the CIE 1931 colour space;

[0027] Fig. 5A is a plot of normalised reflection power versus reflection angle of light for a blazed grating of period 320 nm at three distinct light wavelengths of 450 nm, 550 nm, and 600 nm;

[0028] Fig. 5B is a plot of normalised reflection power versus reflection angle of light for a blazed grating of period 480 nm at three distinct light wavelengths of 450 nm, 550 nm, and 600 nm;

[0029] Fig. 5C is a plot of normalised reflection power versus reflection angle of light for a blazed grating of period 580 nm at three distinct light wavelengths of 450 nm, 550 nm, and 600 nm; Fig. 6 is a sectional side view along the line marked by I in Fig. 3, showing an example of input and output light-coupling processes;

[0030] Fig. 7 is a sectional side view along the line marked by I in Fig. 3, showing another example of input and output light-coupling processes incorporating a microlens array;

[0031] Fig. 8A illustrates a grating coupled to an optical multimode waveguide through a tapered waveguide section according to an example;

[0032] Fig. 8B is an enlarged cross-sectional view of the multimode polymer waveguide along line I in Fig. 8A;

[0033] Fig. 8C is an enlarged cross-sectional view of the multimode polymer waveguide along line II in Fig. 8A;

[0034] Figs. 9A-9C illustrate schematically the operation of a first example in which different respective colour gratings are blocked by a user’s finger;

[0035] Fig. 10 illustrates a security device according to a second example in which each different colour in-coupling blazed grating is optically connected to a respective out-coupling grating that is embossed in a letter pattern;

[0036] Fig. 11 illustrates a security device according to a third example in which multiple different colour in-coupling blazed gratings are optically connected to portions of an out-coupling grating that is embossed in a single letter pattern;

[0037] Fig. 12 illustrates a security device according to a fourth example in which each different colour in-coupling blazed grating is optically connected to a respective out-coupling pixelated grating;

[0038] Fig. 13 illustrates a security device according to a fifth example in which multiple different colour in-coupling blazed gratings are optically connected to a pixelated out-coupling grating in a mixed pattern; and

[0039] Figs. 14A and 14B are nanoscale grating images obtained through atomic force microscopy (AFM) in which Fig. 14A is a top view of a fabricated blazed grating pattern, and Fig. 14B is a 3D profile of a fabricated blazed grating pattern, on a silicon substrate.

[0040] In the drawings, like parts are in some instances indicated with like reference numerals, and, for conciseness, description thereof will not be repeated. Terms such as “optical” and “light” used herein do not limit the subject matter in any way to visible light, but encompass any suitable region of the electromagnetic spectrum, including at least infra-red (IR), visible, and ultra-violet (UV), and can be considered equivalent to a term such as “photonic”.

[0041] DETAILED DESCRIPTION OF THE INVENTION

[0042] The three primary components of an example of the optical security device are: in-coupling structures; at least one waveguide; and an out-coupling structure. Each of these components will be briefly explained before turning to more detailed specific examples.

[0043] Component 1 : In-coupling structures.

[0044] The role of an in-coupling structure is to divert light coming from sources such as sunlight, desk lamp, fluorescent tubes, or any form of ambient light, and capture it in a localised area. Examples of designs of in-coupling structures include, but are not limited to, ID blazed or binary gratings. The grating structures are designed to increase the light coupling efficiency for distinctive light wavelengths. Different colours can be generated through light coupling and filtering by the light-guiding condition. Thus, a colour palette can be formed by mixing three primary colours predominantly red, green, and blue. Despite being highly selective to distinctive visible wavelengths, the gratings show an ability to accumulate light energy from other adjacent wavelengths. The accumulated light is then coupled to optical waveguides, termed Component 2.

[0045] Component 2: Waveguide.

[0046] The purpose of the waveguide is to transport the light information from one point to another point on the banknote. The light-guiding mechanism depends on the total internal reflection phenomenon where a high-index polymer layer, for example SU8 (an epoxy-based polymer) or others, can be used on top of a low-index substrate, such as a polypropylene banknote. This introduces a high contrast in refractive index to guide light without significant loss from the guiding structure. The waveguides can be straight, tapered, or bent depending on the requirements of the design. A tapered waveguide can be used to match the dimension of the in-coupling structure’s footprint with the waveguide and with the out-coupling structure’s footprint. To prevent light leaking into the substrate, a layer of reflective metal, preferably aluminum, preferably of thickness more than 20 nm, is deposited in between the substrate and the high index polymer waveguide. Component 3: Out-coupling structure.

[0047] The out-coupling structure (which can comprise multiple separate out-coupling structures) is designed to extract the transported light signal and direct it to be seen by a viewer’s eyes. This component can comprise different sub-wavelength optical features for example: reciprocal grating structures to the in-coupling structures, pyramidal-shaped surfaces, simple binary gratings, and / or computer-generated holograms, depending on the application requirement. These sub-wavelength optical features can be used within out-coupling structures, and the out-coupling structure(s) can be patterned, such as specific letters, numbers, or signs.

[0048] Figure 1 shows a top view of a flat polymer banknote 101, which can include conventional images / patterns / security features 102 and readable text 103. An optical security device is provided in or on the polymer substrate, and the zone 104 indicates the location of the security device wherein the complete optical security device is embedded. Three square-shaped windows 105, 106, 107 define three in-coupling areas that respectively couple three distinctive colours predominantly red, green, and blue. A comparative bigger window 108 signifies an out-coupling area that directs either an individual or mixed coloured light to the viewer’s eyes. The optical security device is covered and protected under a protective layer 109, preferably of polymer and which is a physical part of the banknote. Preferably the protective layer 109 is nontransparent in regions other than the windows mentioned above.

[0049] Referring to Figures 2 and 3, Figure 2 shows a top view (x-y plane) of a polymer banknote, wherein the optical security device is embedded into a security zone 201. In this figure, the non-transparent protective polymer layer is not shown, therefore the embedded optical security device is visible within the security zone 201. Figure 3 shows the security device of Figure 2 in more detail, comprising three spatially separated windows 105, 106, 107 containing three distinctive colourgenerating in-coupling gratings 202, 203, 204 with three different periodicities. The in-coupling gratings 202, 203, 204 are designed respectively to couple three different colours predominantly red, predominantly green, and predominantly blue to the optical waveguides 206. The waveguides 206 from the three in-coupling gratings 202, 203, 204 meet at the out-coupling structure 205 located inside the output window 108. The waveguides 206 are used to transport the in-coupled light signals to the out- coupling structure 205. Tapered and bent waveguide portions 207, 208, 206 are used for matching the in-coupling and out-coupling areas and directionalities, respectively. Figure 4 is an expanded side view (x-z plane) of nanoscale blazed grating features. All the in-coupling structures 202, 203, 204 comprise blazed grating features of different periods. The triangular-shaped blazed features are fabricated on a polymer banknote substrate 409, or any polymer substrate coated on top of the polymer banknote substrate. The grating periodicity is defined by the distance A, which in these examples can vary from 300 nm to 600 nm. The light coupling efficiency depends on the blazed grating duty cycle. The fabrication of blaze gratings with 100% duty cycle is quite complex. Besides, it does not couple maximum light to the optical waveguide. Thus, the in-coupling structures in these examples consists of blazed gratings with an 80% duty cycle, but could, for example be in the range of from 50% to 95%. The parameters weand w define the etched and filled section of blazed gratings to give the duty cycle (also known as mark to space ratio). The height of the grating is hg. A thin metal (preferably aluminum) layer 406 preferably of thickness tm> 20 nm covers the surface of the grating. The metal layer 406 helps to enhance the grating’s reflectivity as well as coupling efficiency to a polymer waveguide 408. A relatively high refractive index polymer, in this case, SU8, is used for the lightguiding waveguide 408. Light rays 401 of ambient light are incident on the grating. The -1 diffraction order from the grating is stronger, i.e. contains maximum light power compared to other diffraction orders such as Oth and +1 orders. The diffracted -1 order is highly directional, shown by a light ray 402. The diffracted wavelengths that obey the total internal reflection (TIR) condition, i.e. ac> 0c, where f)cis the critical angle for the high index polymer 408 and air interface, are reflected (ray 404) back into the high index polymer waveguide 408. The diffracted wavelengths that do not follow the TIR condition, leak into the air domain (ray 403), which denotes a cutoff condition. If the metal layer thickness is lower than 20 nm, shown by a dotted line 407, the aluminum becomes partially transmissive, thus a portion of light (rays 405) penetrates into the polymer substrate 409, therefore a colour pattern would be visible from the back sides of every in-coupling and out-coupling grating areas.

[0050] Figure 4A illustrates an expanded side view (x-z plane) of the light-guiding waveguide, such as waveguide 206 in Fig. 3. The waveguides are predominantly multimodal in nature because of their robustness and better coupling efficiency. The complete waveguide structure consists of three material layers. A high-index polymer, in this case, SU8, is used as a waveguide 408 on a low-refractive index polymer substrate 409. A thin metal layer 406 (preferably aluminum) preferably of thickness tm> 20 nm is in between low-index polymer substrate 409 and high-index polymer waveguide 408. The bottom metal layer 406 of the waveguide is used as a reflective surface to prevent the light rays from penetrating the polymer substrate 409, and instead reflects light back into the waveguide.

[0051] Figure 5 is a plot in the CIE 1931 colour space, showing the colour production of different blazed gratings depending on the period (A). The dashed line with circular markers and the dashed-dotted line with rhombus markers show how the colour changes with the variation of the grating periodicity for both 0° and 90° polarisations of incident white light, respectively. Varying the grating period from 300 nm to 600 nm changes the in-coupled generated colour in the range from predominantly blue (450 nm wavelength, bottom left of the plot) to red (620 nm wavelength, righthand side of the plot). Intermediate colours can be generated by selecting the grating period.

[0052] Figures 5A to 5C show simulation results of reflection power for three specific grating periods of 320 nm, 480 nm, and 580 nm, respectively, for blazed gratings with duty cycle of 80%, and grating height hgof 100 nm. The simulated results show the normalised reflection powers of grating periods of 320 nm, 480 nm, and 580 nm at three wavelengths 450 nm, 550 nm, and 600 nm that signify three primary colours, predominantly blue, green, and red, respectively, and for both 0° and 90° polarisations of incident light. The Oth order diffraction data are at an angle of 0 degrees in the middle of each graph; the negative diffraction orders are on the left; and the positive diffraction orders are on the right. It is clear that the -1 diffracted order contains maximum reflection power compared to the Oth and +1 diffraction orders.

[0053] A grating period of 320 nm only produces blue light with almost 78% coupling efficiency at the -1 diffraction order for 0° polarization, as shown in Figure 5 A.

[0054] As shown in Figure 5B, a grating period of 480 nm produces a combination of red and green lights, whereas the blue light does not obey the condition of TIR, therefore falls in the cut-off region. The cut-off region is between the vertical dashed lines at approximately ±40 degrees, which is the critical angle for the SU8-Air interface. Light reflected / diffracted at an angle in the cut-off region is not totally internally reflected, so does not propagate in the waveguide, and is not in-coupled to the device. In this case, the reflected green light energy is stronger than the red light.

[0055] As shown in Figure 5C, a grating period of 580 nm produces only red light, as the other two (green and blue) lights fall inside the cut-off region. Thus, only red light propagates through the waveguide. Figure 6 illustrates a cross-sectional side view (x-z plane) of the optical security device along the line I shown in Figure 3. The structure comprises three layers, a low refractive index polymer substrate 409, a reflective metal layer 406, and a high refractive index polymer as a waveguide 408. The in-coupling structure consist of blazed grating features 602 of grating 203 of a specific period (adjacent to, but spatially separate from, the in-coupling structures 202 and 204 of different periods not visible in this cross-section). Incident light ray 401 from ambient light 601 gets incoupled by the blazed grating and propagates (ray 603) through the medium of the waveguide 408 to an out-coupling structure. The grating features 604 of the out- coupling structure 205 diffract the propagated incoming light signals to the viewer’s eyes 606 at multiple angles (rays 605).

[0056] Figure 7 illustrates a cross-sectional side view (x-z plane) of a different example of an optical security device. The details are identical to the example of Figure 6, except that a set of microstructured lenses 701 is located on the top surface of the polymer waveguide 408 in the region of the out-coupling structure 205. The features of the out-coupling structure diffract light into multiple angles (rays 605) depending on grating parameters. The microstructured lenses 701 focus the diffracted light rays 605 to rays 702 converging on the viewer’s eyes 703.

[0057] Figures 8A to 8C illustrate an enlarged top view (x-y plane) of the junction of any in-coupling or out-coupling structure with a waveguide. The in-coupling and out- coupling structures 801 comprise different gratings features, surrounded by an open window region 802 to protect the optical security device from any interference from surroundings. At least one side of the coupling structure connects with a waveguide 803, 804, 805. To match the different in-coupling and out-coupling areas, the waveguide comprises various features such as a multimodal tapered waveguide section 803, that creates a bridge between a wider in-coupling or out-coupling structure and a narrower multimode waveguide 805. A cross-sectional view (z-y plane) of the tapered waveguide along the line I of Figure 8A is shown in Figure 8B. The multimode tapered waveguide comprises a three-layered strip waveguide structure with a thin metal layer 807 in between a low refractive index polymer substrate 808 and a high refractive index polymer waveguide 806. This strip waveguide is comparatively wider than the light signal transporting multimode waveguide 805. A cross-sectional view (z-y plane) of a multimode waveguide along line II of Figure 8A is shown in Figure 8C. This waveguide has a similar three-layered structure as the tapered waveguide but the only difference is its dimension. The high-index material thickness hwg, metal layer thickness tm, and low-index polymer layer are the same as the tapered waveguide section. But the transport waveguide width (wn) at the line II is comparatively smaller than the width (wi) of the tapered waveguide section. The width of the tapered waveguide 803 smoothly transitions from the width of the region of the in-coupling or out-coupling structure 801 down to the width wn of the transport waveguide 805 at their junction 804.

[0058] Applications

[0059] By combining all three aforementioned components, different examples of security devices can be made, as required, depending on specific applications.

[0060] Figures 9A to 9C illustrate a first application mode where the masking of incoupling gratings with a finger (or other digit) is shown. For example, in Figure 9A, if a finger 901 obscures a first in-coupling area 902, then the light from that area is blocked 903. Light 904, 906 that is incident on second and third in-coupling areas is coupled and transported 905, 907 to an out-coupling area 908, where it is emitted as light 909 visible to a user or viewer. Similarly, in Figure 9B, if a finger 910 obscures a second in-coupling area 911, then the light from that area is blocked 912. Light 913,

[0061] 915 that is incident on first and third in-coupling areas is coupled and transported 914,

[0062] 916 to an out-coupling area, where it is emitted as light 917 visible to a user or viewer. Furthermore, as shown in Figure 9C, if a finger 922 obscures a third incoupling area 923, then the light from that area is blocked 924. Light 918, 920 that is incident on first and second in-coupling areas is coupled and transported 919, 921 to an out-coupling area, where it is emitted as light 925 visible to a user or viewer.

[0063] If the plurality of in-coupling areas have in-coupling structures that are differently wavelength selective from each other, then distinctive colour visual effects can be observed interactively by the user as the user selectively blocks or obscures different ones of the in-coupling structures. In one preferred example, three incoupling area are provided with in-coupling structures arranged to selectively couple predominantly red, green and blue light, respectively. This scheme generates different visual colours from the out-coupling structure(s) that can be observed by a viewer’s eyes. If the user obscures the in-coupling areas (with their thumb for example) predominantly made for green and blue, then the mixing or the out-coupling area changes colour to predominantly red because there is no blue or green light available to be transported. If the viewer blocks the predominantly blue in-coupling area, then predominantly red and green light mix in the out-coupling area, and predominantly yellow light is produced and shone into the eyes of the viewer. Similarly, if the viewer blocks predominantly red and green in-coupling areas, then only predominantly blue light is visible to the viewer’s eyes. Predominantly red, green, and blue colours are considered here as examples. In this way, a large number of distinctive colours can be generated by selectively masking individual (or pairs of) in-coupling areas or all three of them to create a dark visual effect.

[0064] Figure 10 illustrates spatially separated independent optical devices with one in-coupling structure 1002 that is connected to one out-coupling structure 1010 through an optical waveguide 1007, 1008, 1009. The in-coupling structure efficiently couples selective light colours to the waveguide. The out-coupling structure directs the transported colour light to the viewer’s eyes. Out-coupling structures comprising patterned gratings, patterned in the form of letters or numbers 1010, 1011, 1012, can be used in this optical device configuration. In this case, the purpose of the out- coupling structure cannot filter the transported optical signal. An optical waveguide 1008 with tapered sections 1007, 1009 is used to match the in-coupling structure 1002 and out-coupling structure 1010 dimensions. In this example, three in-coupling areas or windows 1001, 1003, 1005 are provided. By using different in-coupling grating periods different several distinctive colours can be generated and at the output end different letters or numbers can be illuminated by the out-coupling structures.

[0065] Figure 11 shows the design of an optical security device in which three spatially separated windows or areas 1101, 1103, 1105 are provided with respective in-coupling structures 1102, 1104, 1106 which are optically connected by waveguides 1107, 1108, 1109, 1110 (tapered, straight, and / or bent) to an out-coupling structure 1111 in common (though to different segments thereof). The out-coupling structure 1111 is patterned into one or multiple letters, numbers, or symbols. The letter, number, or symbol of the out-coupling structure is divided into segments corresponding to each of the in-coupling structures (so in this example there are three segments). The segments are optically isolated from each by micro-separations 1112. Each of the segments is connected to the corresponding in-coupling structure through bent waveguides 1108 and / or straight waveguides 1109. Waveguide tapered sections 1107, 1110 are used to match the footprint widths of in-coupling and out-coupling structures to the bent or straight waveguides. Ambient light 1113 is incident on an incoupling area or window 1101 and a portion of it is in-coupled, guided and then out- coupled as emitted light 1114 at the out-coupling structure 1111. With this security device configuration, if a user obscures the in-coupling areas 1103, 1105 predominantly made for in-coupling green and blue, then only one section of the letter, number, or symbol illuminates with a colour predominantly red. If the user blocks the predominantly blue in-coupling area 1105, then two other sections of the letter, number, or symbol illuminate with predominantly green and red. If the user obscures all three in-coupling areas, the letter, number, or symbol 1111 shows a dark visual effect with a partial reflection from the out-coupling area.

[0066] Figure 12 illustrates a design of an optical security device, in which three comparatively large in-coupling areas 1201, 1202, 1203 are connected to pixelated out-coupling areas 1206. Each out-coupling area comprises a 2D spatial array of pixels 1205, each pixel 1205 comprising a respective out-coupling structure. The pixels 1205 can be arranged in patterns to define images, shapes, letters, numbers, symbols and so forth. Each in-coupling area 1201, 1202, 1203 contains an in-coupling structure with a different grating period to couple three distinctive light colours, such as predominantly red, green, and blue. These optical signals of different colours are transported to the pixelated out-coupling areas 1206. Multiple optical waveguides connect the in-coupling and out-coupling areas. Multiple waveguides of smaller dimensions (smaller as compared to the aforementioned waveguides in Figures 10 and 11) are used to transport light signals from the in-coupling areas to the pixelated out- coupling areas, typically one waveguide for each pixel. In this example, the pixelated out-coupling areas 1206 are grouped based on colour reproductions. As an example, red colour production out-coupling areas can be grouped at the top, green out-coupling areas at the middle, and blue out-coupling areas at the bottom. By arranging the pixelated out-coupling areas in this manner, a tricolor symbol can be made or illuminated, as desired. By masking an in-coupling area, the corresponding colour can be blocked from being transported to the out-coupling pixels. Obscuring all three incoupling areas stops any coloured light being transported to the out-coupling areas, thus the out-coupling pixels show a dark visual effect.

[0067] Figure 13 illustrates an optical security device design, similar to the design of Figure 12, in having multiple in-coupling areas 1301, 1302, 1303 (each comprising a different in-coupling structure) connected by miniature waveguides 1304 to pixels 1305 (each pixel comprising an out-coupling structure) arranged in an out-coupling area 1306. The difference is that there is a single out-coupling area 1306 and the pixels in that area are not segregated by colour, but are arranged in a mixed form. This mixed arrangement of pixelated out-coupling structures is suitable for designing any image or symbol, as desired. Thus, the symbol or image at the out-coupling area 1306 is illuminated by (emits) three distinctive light colours, such as predominantly red, green, and blue. In this way, full-colour images can be rendered using known RGB mapping. Optionally, if desired, intermediate colours can be produced by introducing new in-coupling areas with a different grating period structures based on the grating periodicity map on the CIE colourmap, as shown in Figure 5. In either case it is possible to generate distinctive visual effects at the out-coupling area 1306 by masking at least one or multiple in-coupling areas 1301, 1302, 1303. A dark visual effect can be seen if the user blocks all three in-coupling areas.

[0068] Figures 14A and 14B illustrate one example of a fabricated blazed grating that can be used for either in-coupling or out-coupling structures of the optical security device. Blazed gratings provide a high directionality of light, therefore, an efficient light-coupling is possible with this type of grating structure. Figure 14A illustrates a top view of a fabricated blazed grating and Figure 14B illustrates a 3D profile of a fabricated blazed grating, both illustrations being obtained through atomic force microscopy. To fabricate a security device, blazed shaped gratings of different nanoscale periodicities, such as ranging from 300 nm to 600 nm, are patterned on a silicon substrate which is then used as a stamp to transfer the grating and other waveguide patterns on a flexible polymer substrate by using nanoimprinting lithography. A thin layer of aluminum is then deposited on the top through the process of metallisation, followed by adding the higher refractive index polymer layer, and patterning these through lithography techniques.

[0069] The specific examples described above have three in-coupling gratings in three in-coupling areas or windows, but numbers other than three are contemplated, such as two, four or more.

[0070] The selectively in-coupled colours in the above examples are predominantly red, predominantly green, and predominantly blue. However, these three specific colours are not essential, and different colours and colour combinations can be used.

[0071] It is convenient for the security device to work with visible light, so that a human user can see the optical effect of masking one or more of the in-coupling structures without any additional equipment and using ambient light for illumination. However, this is not essential, and other wavelengths of light can be used, such as infrared (IR) and ultraviolet (UV), with one or more of the in-coupling structures suitably arranged (e.g. with a selected grating period) to couple light wavelengths corresponding to such parts of the spectrum. In use, the security device can be illuminated with a specific light source, such as UV lamp, if there is insufficient ambient light at the appropriate wavelength. The out-coupling regions can be viewed with a suitable camera (such as an IR or UV camera) if it is desired to see emitted light outside the visible range. In another version, wavelength-shifting material, such as a fluorescent material, can be incorporated into an out-coupling structure or window, such that emitted UV light is converted to visible light that can be seen by a user’s eyes unaided.

Claims

CLAIMS1. An optical security device comprising: a plurality of in-coupling structures for receiving light incident on the device; an out-coupling structure for emitting light from the device; and at least one waveguide for each in-coupling structure for conveying light from the respective in-coupling structure to said out-coupling structure, wherein each in-coupling structure is configured to couple light incident on the device into said at least one waveguide, and the out-coupling structure is configured to couple light out of said at least one waveguide to be emitted from the device, wherein the in-coupling structures are spatially separate from each other and from the out-coupling structure, and wherein masking at least one of said in-coupling structures from incident light produces an optical effect in the light emitted from the device at the out-coupling structure.

2. A device according to claim 1, wherein the in-coupling structures are wavelength selective.

3. A device according to claim 2, wherein the in-coupling structures are differently wavelength selective from each other.

4. A device according to claim 3, comprising three in-coupling structures, one wavelength selective to couple predominantly red light, one wavelength selective to couple predominantly green light, and one wavelength selective to couple predominantly blue light.

5. A device according to any preceding claim, wherein the optical effect is a change of colour and / or a change of pattern.

6. A device according to any preceding claim, wherein the out-coupling structure is spatially patterned.

7. A device according to any preceding claim, wherein the out-coupling structure comprises a plurality of regions, each region receiving light via a respective waveguide from only one of the in-coupling structures.

8. A device according to any preceding claim, wherein the out-coupling structure is pixelated.

9. A device according to any preceding claim wherein the in-coupling structures, out-coupling structure, and waveguides comprise a flexible polymer.

10. A device according to any preceding claim provided on a flexible polymer substrate.

11. A device according to claim 10, further comprising a metal layer between the in-coupling structures and the substrate, between the out-coupling structure and the substrate, and between the waveguides and the substrate.

12. A device according to any preceding claim, wherein at least one in-coupling structure comprises a grating, preferably a blazed grating.

13. A device according to any preceding claim, wherein the incident light is ambient light.

14. A banknote comprising a device according to any preceding claim.

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