LABEL AND SYSTEM FOR VERIFYING THE AUTHENTICITY OF ITEMS AND METHOD FOR VERIFYING AUTHENTICITY OF ITEMS

Abstract

The present invention relates to a label (1, 1, 1, 1) for verifying authenticity of items, the label (1, 1, 1, 1) being configured to be attached to items and comprising a plurality of focusing micro-optical elements (3) and a micropartide layer (5) comprising a plurality of luminescent and/or scattering microparticles (7), wherein the plurality of focusing micro-optical elements (3) and the microparticle layer (5) are arranged so that at least a part of light emitted and/or scattered from the plurality of luminescent and/or scattering microparticles (7) reaches the plurality of focusing micro-optical elements (3).

Claims

1. An authenticity verification label, comprising: a plurality of focusing micro-optical elements; and a microparticle layer comprising a plurality of luminescent and/or scattering microparticles.

2. The label according to claim 1, wherein the focusing micro-optical elements are arranged in a micro-optical element plane, wherein the microparticle layer has a first surface facing the micro-optical element plane, wherein a normal direction of the first surface may be substantially parallel to a normal direction of the micro-optical element plane.

3. The label according to claim 1, wherein the focusing micro-optical elements are converging lenses having positive optical power, each of the focusing micro-optical elements having a focal length and being configured to focus light substantially travelling along the optical axis of the micro-optical element to a focus.

4. The label according to claim 1, wherein at least some of the focusing micro-optical elements are arranged such that foci of the focusing micro-optical elements are located in the microparticle layer.

5. The label according to claim 2, wherein the microparticle layer has a second surface facing away from the micro-optical element plane, wherein a distance between the second surface and a principal plane of the micro-optical elements is larger than the focal length of each of the focusing micro-optical elements.

6. The label according to claim 3, wherein the plurality of micro-optical elements and the microparticle layer are configured such that a mean probability p.sub.mean for a microparticle being located in a focal volume V.sub.focal of a given micro-optical element is lower than 0.9, and is preferably in the range from 0.01 to 0.8, more preferably in the range from 0.01 to 0.6.

7. The label according to claim 6, wherein the plurality of focusing micro-optical elements and the microparticle layer are configured such that a microparticle number density N.sub.p and the probability p of a given focal volume V.sub.focal to be occupied by a microparticle satisfy the equations $N_p=\frac{p}{V_{\mathrm{focal}}}=p\frac{D^4}{8{?}^{\text{?}}{F}^4}\mathrm{if}{V}_p<V_{\mathrm{focal}}$ $\mathrm{and}$ $N_p=\frac{p}{V_p}=\frac{3p}{2?d_v^{\text{?}}}\mathrm{if}{V}_p>V_{\mathrm{focal}},$ $\text{?}\text{indicates text missing or illegible when filed}$ wherein V.sub.p is a microparticle volume, D is a diameter of a micro-optical element, F is the focal length of the micro-optical element, d.sub.p is a diameter of the microparticle, and ?.sup.? is a wavelength of light of an illumination device.

8. The label according to claim 1, wherein the plurality of focusing micro-optical elements is arranged in a regular lattice pattern or with arbitrarily placed positions or in relation to fiducial markers.

9. The label according to claim 1, wherein the microparticles are inorganic or organic microparticles, for example, up-conversion microparticles and/or downshifting microparticles and/or persistent phosphorescence microparticles, and/or wherein the microparticles are nanocrystal microparticles.

10. An item authenticity verification system, the system comprising: at least one label according to claim 1; an image capturing device configured to capture light being emitted and/or scattered from the luminescent and/or scattering microparticles to acquire at least one test optical image of the label; and an authenticity verification device configured to verify authenticity of the items by comparing the at least one test optical image with at least one reference optical image.

11. The system according to claim 10, the system further comprising: an illumination device configured to illuminate the label so as to excite and/or illuminate the luminescent and/or scattering microparticles.

12. The system according to claim 10, wherein the image capturing device is a camera of a mobile device such as a smartphone or tablet computer.

13. The system according to claim 10, wherein the image capturing device is configured to acquire a plurality of test optical images of the label for several illumination angles and/or several positions of the illumination device with respect to the label, and wherein the authenticity verification device is configured to verify authenticity of the items by comparing the plurality of test optical images with a plurality of reference optical images.

14. The system according to claim 10, wherein the authenticity verification device is further configured to generate an authentication code based on the at least one test optical image and is configured to verify authenticity of the items by comparing the authentication code with a reference code generated based on the at least one reference optical image.

15. A method of verifying authenticity of items, the method comprising: attaching at least one label according to claim 1 to an item; capturing, by means of an image capture device, scattering and/or emission of light coming from the luminescent and/or scattering microparticles and acquiring at least one test optical image of the label; and verifying, by means of an authenticity verifying device, authenticity of the item by comparing the at least one test optical image with at least one reference optical image.

Description

[0153] FIGS. 1A and 1B show a side view and a top view of an embodiment of a label for verifying authenticity of items,

[0154] FIGS. 2A to 2C show the label of FIGS. 1A and 1B under a first illumination condition of collimated light and three different incidence angles (0? in FIG. 2A, ?5? in FIG. 2B, and +5? in FIG. 2C),

[0155] FIGS. 3A and 3B show the label of FIGS. 1A and 1B under a second illumination condition of a distant point source above the center micro-optical element (FIG. 3A) and above the right micro-optical lens (FIG. 3B),

[0156] FIGS. 4A to 4C show a side view of a further embodiment of a label for verifying authenticity of items and its respective illumination and emission behavior,

[0157] FIGS. 5A to 5C show a side view of yet a further embodiment of a label for verifying authenticity of items and its respective illumination emission behavior,

[0158] FIGS. 6A to 6C shows a schematic of light incident onto the microparticle layer and scattered by the microparticles for the embodiments shown in FIGS. 1A to 5C,

[0159] FIGS. 7A and 7B show a measurement apparatus comprising a holding element holding a label for verifying authenticity of items and its respective emission pattern from the microparticles varying with the incidence angle (0? in FIG. 7A and 10? in FIG. 7B) of the incoming light,

[0160] FIGS. 8A and 8B show a method for comparing test and reference images without any need for label registration. FIG. 8A shows two examples of maximum number of votes achieved when comparing two different lists of points, and FIG. 8B shows a comparison for the maximum number of votes achieved,

[0161] FIG. 9 shows an illustration of relevant parameters of a label for verifying authenticity of items that are relevant for the relationship between the micro-optical element design and microparticle number density, and

[0162] FIG. 10 shows an illustration of the estimated microparticle number density N.sub.p from the focal volume V.sub.focal and the volume of a microparticle.

[0163] FIGS. 1A and 1B show an embodiment of a label 1 for verifying authenticity of items. This label 1 is configured to be attached to items (not shown). The label 1 comprises a plurality of focusing micro-optical elements 3 and a microparticle layer 5 comprising a plurality of luminescent and/or scattering microparticles 7.

[0164] Each of the focusing micro-optical elements 3 has an optical axis 9. The focusing micro-optical elements 3 are arranged in a micro-optical element plane 11 such that angles between the optical axis 9 of each of the focusing micro-optical elements 3 and a normal direction {right arrow over (nMOE)} of the micro-optical element plane 11 is below 60?. In the embodiment shown in FIG. 1A, the angle between the optical axis of the micro-optical element 3 and normal direction {right arrow over (nMOE)} of the micro-optical element plane 11 is 0?.

[0165] The focusing micro-optical elements 9 are converging lenses having positive optical power, wherein each of the focusing micro-optical elements 9 has a focal length 13 and being configured to focus parallel beams incident light travelling along the optical axis 9 of the micro-optical element 3 to a focus 15 (see FIG. 2A). The focal length 13 is the distance along the optical axis 9 from the focus 15 to a principal plane of the micro-optical element 3 on the side of the focus 15, when the optical characteristics of micro-optical element 3 are approximated by the thick lens approximation and taking the refractive index of the material into account.

[0166] The plurality of focusing micro-optical elements 3 may be arranged in a regular lattice pattern. FIG. 1B shows nine micro-optical elements 3 being equidistantly arranged in a regular lattice pattern being matrix-like and having three columns and three lines.

[0167] The microparticle layer 5 has a first surface 17 facing the micro-optical element plane 11 and a second surface 19 facing away from the micro-optical element plane 11. A normal direction {right arrow over (n1)} of the first surface is substantially parallel to the normal direction {right arrow over (nMOE)} of the micro-optical element plane 11. In areas between the micro-optical elements 3, the first surface 17 and second surface 19 are substantially flat, and the normal directions of the first and second surfaces 17, 19 are substantially parallel. The distance 21 between the first and second surfaces 17, 19 in the normal direction thereof is the thickness of the microparticle layer 5. The focusing micro-optical elements 3 may be arranged in a micro-optical element layer facing the first surface 17 of the microparticle layer 5.

[0168] The luminescent and/or scattering microparticles 7 can, for example, be up-conversion microparticles made from a phosphor or, alternatively, from a gadolinium oxysulphide host and doped with a near-infrared sensitizer such as ytterbium and a visible emitter such as erbium.

[0169] FIGS. 2A to 2C schematically show the label 1 under a first illumination condition of collimated light 25 and three different incidence angles (0? in FIG. 2A, ?5? in FIG. 2B, and +5? in FIG. 2C). FIGS. 3A and 3B show the label 1 under a second illumination condition of a distant point source above the center micro-optical element (FIG. 3A) and above the right micro-optical lens (FIG. 3B). All focusing micro-optical elements 3 are designed and arranged such that, for the given illumination condition, the focus 15 of each focusing micro-optical element 3 is located in the microparticle layer 5.

[0170] The plurality of focusing micro-optical elements 3 and the microparticle layer 5 are arranged so that at least a part of light emitted and/or scattered from the plurality of luminescent and/or scattering microparticles 7 reaches the plurality of focusing micro-optical elements 9 (see, e.g., FIGS. 4C and 5C).

[0171] As shown in FIGS. 2A to 2C, depending on the incidence angle, the foci of the focusing micro-optical elements 3 shift and coincide with an entirely new subset of microparticles 7. The incidence angle of the incident illumination light 25 is the angle between the normal direction direction {right arrow over (n1)} of the first surface 17 and the propagating direction of the parallel incident illumination light 25. The normal direction {right arrow over (n1)} of the first surface 17 corresponds to the direction of the optical axis of the micro-optical elements 3. Only those microparticles 7 located at positions substantially or fully coinciding with the foci 15 of the focusing micro-optical elements 3 will contribute to the optical pattern of bright microparticles.

[0172] Due to the micron-scale randomness of the positions of the microparticles 7 in the microparticle layer 5, light detected by an image capturing device (not shown) originating from some of the microparticles 7 will have light intensities being above the detection threshold of a sensor of the image capturing device, while light originating from misaligned microparticles 7 may have light intensities below the detection threshold. Accordingly, due to the random distribution of the microparticles 7, only some of the focusing micro-optical elements 3 will contribute to an image captured by the image capturing device, whereas other focusing micro-optical elements 3 will not contribute to the detected pattern in the image. Accordingly, the detected pattern of light is characteristic for the label 1 and is unique.

[0173] Thus, a label 1 is provided which is virtually unclonable due to random micron-scale random distribution of the microparticles 7 during the manufacturing process of the label 1, which results in a 3D alignment of microparticles 7 and focusing micro-optical elements 3 that cannot be reproduced purposefully in the manufacturing process and is unique for each label 1. This unique 3D alignment gives rise a unique detectable light signal (i.e., light pattern of bright microparticles) that can be detected on the macroscopic scale.

[0174] The label 1 shown in FIGS. 1A to 3B might be produced by embossing or imprinting the micro-optical elements 3 on the microparticle layer 5 (not shown). In particular, the micro-optical elements 3 may be directly embossed into the microparticle layer 5 by means of a forming tool and/or stamping tool. Alternatively, the micro-optical elements 3 can be printed directly, e.g., by means of a 3D-printer, on the microparticle layer 5.

[0175] The micro-optical elements 3 and the microparticle layer 5 may also be laminated together.

[0176] FIGS. 4A to 4C show a label 1 for verifying authenticity of items and its respective illumination and emission behavior further comprising random surface roughness 27. FIGS. 5A to 5C show a label 1 for verifying authenticity of items and its respective illumination emission behavior comprising extended focusing micro-optical elements 3 such as a Fresnel lens 29. The labels 1, 1 are illuminated by an illumination device 31 so that the micro-optical elements 3 are illuminated with collimated light 25 that is focused to a focus by the micro-optical element 3 (see FIG. 4B), such as by the Fresnel lens 29 (see FIG. 5B). The random surface roughness 27 and Fresnel lens 29 increase the collection of emitted light 26, i.e., the light emitted by a bright microparticle 7 by an image capturing device 33 (see FIGS. 4C and 5C). Light rays scattered or emitted from a bright microparticle 7 can reach the image capturing device more efficiently due to the random surface roughness 27 and micro-optical elements 3, 29.

[0177] FIG. 6A shows a schematic representation of light incident onto the microparticle layer 5 and scattered by one of the microparticles 7 for the embodiment of a label 1 not including surface textures as shown, e.g., in FIGS. 1A to 3B. FIG. 6B shows a schematic representation of light incident onto the microparticle layer 5 and scattered by one of the microparticles 7 for the embodiment of a label 1 including a Fresnel lens 29 as shown in FIGS. 4A to 4C. FIG. 6C shows a schematic of light incident onto the microparticle layer 5 and scattered by one of the microparticles 7 for the embodiment of a label 1 including random surface roughness 27 as shown in FIGS. 5A to 5C.

[0178] FIGS. 6A to 6C show that, when the illumination device 31 and the image capturing device 33 are arranged on the same side of the label 1, 1, 1, for example when using a smartphone for illuminating and detection, the focusing optics influence the detection of the luminescent light. In the case for a smartphone, the image capturing device 33 is positioned near the illumination device 31 so that there is only a small distance between the illumination device 31, i.e., the smartphone flashlight, and the image capturing device 33, i.e., the smartphone camera. FIG. 6A shows that three paths of the luminescent light may be distinguished:

[0179] 1. The luminescent and/or scattering microparticle 7 lies in the focus of the focusing micro-optical element 3 for a predetermined illumination condition, in particular incidence angle of the illumination light. Thus, incident light 25 onto the microparticle layer 5, coming from inside the microparticle layer 25, is collimated towards the illumination source 31. It does not reach the imaging capturing device 33.

[0180] 2. Light incident upon a planar surface 43 of the microparticle layer 5 with incident angles below the critical angle ?.sub.c, is refracted to larger angles in air and does not reach the image capturing device 33 close to the illumination device 31. The planar surface 43 might be that part of the first surface 17 of the microparticle layer 5 that does not face the focusing micro-particle elements 3 in the micro-optical element layer.

[0181] 3. Light incident upon the planar surface, with incident angles larger than the critical angle ?.sub.c, is trapped by total internal reflection and does not reach the image capturing device 33.

[0182] FIGS. 6B and 6C show that a surface texture, e.g., a Fresnel lens 29 or random surface roughness 27, may enable a reliable observation of the emitted light 26 in front side detection. The purpose of this surface texture is to redirect luminescent light towards the image capturing device 33.

[0183] The surface texture might be configured as a second micro-optical element, preferably in the form of a Fresnel lens 29, that is positioned around a first central micro-optical element 3 (see FIG. 6B). This Fresnel lens 29 may have a long focal length. As a consequence, it does not focus light inside the microparticle layer 5, thus, illumination of luminescent particles 7 is only achieved with the central micro-optical element 3. Since the luminescent particles 7 do not lie in the focus of the Fresnel lens 29, emitted light is not perfectly collimated, but diverges. This allows detection of the emitted light 26 by the image capturing device 33.

[0184] As an alternative to the ring micro-optical element, random surface roughness 27 surrounding the central micro-optical element 3 might be used. The random surface roughness 27 might comprise rough surfaces which scatter light in all directions. The emitted light 26 emitted by the luminescent particle 7 is scattered at the surface. A fraction of the emitted light 26 will be incident on the image capturing device 33, independent of its position.

[0185] Albeit not shown in FIGS. 1A to 5C, it is conceivable that the microparticle layer 5 is created as a first layer, wherein a second, transparent layer without any microparticles 7 is created above the microparticle layer 5. In particular, the second, transparent layer may be arranged on the microparticle layer 5 facing its first surface 17. Then, a third layer, e.g., the micro-optical element layer and/or the surface textures 27, 29 may be created on the second, transparent layer by laminating or embossing.

[0186] A preferred embodiment of a label might be implemented as follows:

[0187] A hexagonal close-packed array of 240 focusing micro-optical elements forming a lens array may be fabricated by two-photon lithography. This array can be written into a commercial photo-resin (for example, IP-S from nanoscribe GmbH) on a 1 mm-thick glass substrate. The micro-optical elements may have a radius of curvature of 625 ?m and a base diameter of 250 ?m. Such a lens array can then be laminated onto a 2 mm thick slab of polydimethylsiloxane (PDMS) loaded with 500 ppm of up-conversion (UC) microparticles. The microparticle may be made from a gadolinium oxysulphide host and doped with an ytterbium (NIR sensitizer) and erbium (visible emitter): Gd.sub.2O.sub.2S:Yb,Er. The distribution of diameters of the microparticles may be centred around 10 ?m. The micro-optical element array may be placed onto the PDMS layer.

[0188] A system for verifying authenticity of items comprises at least one label 1, 1, 1, 1, an image capturing device 33 configured to capture light being emitted and/or scattered from the luminescent and/or scattering microparticles 7 to acquire at least one test optical image 35, 36 of the label 1, 1, 1, 1; and an authenticity verification device (not shown) configured to verify authenticity of the items by comparing the at least one test optical image 35, 36 with at least one reference optical image.

[0189] The system may further comprise an illumination device 31 configured to illuminate the label 1, 1, 1, 1 so as to excite and/or illuminate the luminescent and/or scattering microparticles 7. The image capturing device 33 can be a camera of a mobile device such as a smartphone or tablet computer. The image capturing device 33 is configured to acquire a plurality of test optical images 35, 36 (see FIGS. 6A and 6B) of the label 1, 1, 1, 1 for several illumination (incidence) angles and/or several positions of the illumination device 31 with respect to the label 1, 1, 1, 1. The authenticity verification device is configured to verify authenticity of the items by comparing the plurality of test optical images 35, 36 with a plurality of reference optical images. The authenticity verification device is further configured to generate an authentication code based on the at least one test optical image 35, 36 and is configured to verify authenticity of the items by comparing the authentication code with a reference code generated based on the at least one reference optical image.

[0190] FIGS. 7A and 7B show a label 1 placed in a measurement apparatus, wherein the label 1 is held in a holding apparatus 37, e.g., a label holder, respectively. The holding apparatus 37 is configured to control different angles of rotation ?, 0. The holding apparatus may be rotated around a substantially vertical rotation axis 39 with angle of rotation ?, and/or around a substantially horizontal rotation axis 41 with angle of rotation ?. Thus, the holding apparatus 37 is capable of controlling two axes of rotation 39, 41. In particular, FIG. 7A shows the holding apparatus at an angle of rotation ?=0?, and FIG. 7B shows the holding apparatus at an angle of rotation ?=10?. The respective test optical images 35, 36 of the label 1 that are acquired in dependence of the angel of rotations are also shown in FIGS. 7A and 7B. The incident light 25, e.g., near infrared (NIR) light that is a collimated 980 nm beam, impinges on the label 1 through the micro-optical elements 3 at different incidence angles, respectively. The emission from the microparticles 7 may be measured by the image capturing device 33, e.g., a camera, wherein optical filters may be used to block the excitation light (not shown). Since multiple luminescent and/or scattering microparticles 7 are present in the label 1, each having a unique emission wavelength, a test optical image 35, 36 can be captured by the image capturing device 33 in dependence of the angle of rotation ?.

[0191] FIGS. 7A and 7B show how the emission pattern from the microparticles varies with the incidence angle of the illumination light when the entire array with focusing micro-optical elements 3 lens is illuminated with excitation light. However, only a small fraction of the micro-optical elements 3 focus the illumination light 25 on a subset of the microparticles 7. This leads to the dark optical test images 35, 36 with a few bright spots, corresponding to micro-optical elements 3 that focus onto a microparticle 7 for the given illumination condition. The subset of micro-optical elements 3 that focus onto a microparticle 7 is angle dependent. This leads to an angle-dependent emission pattern that is intimately related to the 3D distribution of the microparticles 7 relative to the micro-optical elements 3. This arrangement is randomly created during label formation, and it is not possible to reproduce this arrangement with existing manufacturing methods. This makes the labels 1, 1, 1, 1 virtually unclonable.

[0192] FIGS. 8A and 8B demonstrate an example method for comparing optical test and reference images without any need for label registration. FIG. 8A shows two examples of maximum number of votes achieved when comparing two different lists of points. A close match was achieved in the example on the left, thus leading to a large number of votes. Even in the best case, the point clouds do not match in the example on the right, thus leading to a small number of votes. FIG. 8B shows a comparison for the maximum number of votes achieved, when comparing each of the 27 images to all other 26 images. Every sample, illuminated from the same incidence angle, received at least 11 votes in the other two trials. At most 6 votes were obtained in all other cases.

[0193] Based on FIGS. 8A and 8B, it will be shown that a method based on the concept of geometrical hashing can accurately compare measured with true images. This merely establishes that existing comparison algorithms are sufficient and present no limitation to allow the above described labels 1, 1, 1, 1 for verifying authenticity of items which are a combination of the micro-optical elements 3 and a microparticle layer 5 to be used for the intended anti-counterfeiting application. Three labels were fabricated. Then, three angles were measured for a label corresponding to a first trial. The label was then removed and replaced in system and the same angles measured again corresponding to the second trial. Then the label was again removed, replaced, and re-measured corresponding to the third trial. As a result, 27 images (three angles, three samples, three trials) are compared using an exemplary algorithm described in the following steps:

[0194] 1. Pre-processing of images. Firstly, a background image is subtracted from all images, and standard procedures for reducing image noise was applied.

[0195] 2. Identification of intense emission regions. A threshold (determined with reference to the dark noise of the acquisition system) is used to transform the greyscale into binary images, with regions of 1 representing regions of intense emission. The centroid and area of each bright region is found. The sum of the pixel values in each of these areas in the original image is found, and the centroids sorted from that corresponding to the brightest total emission to the weakest. The sorted centroid positions obtained from the 27 images are stored in separate lists and the lists are compared to one another. Only the brightest 24 points (10% of the number of micro lenses) are considered.

[0196] 3. Geometric transformation of the reference image. In order to compare the pattern of 24 bright points obtained in the previous step, new lists of point locations are generated that are invariant to affine transformations. This means that bright point patterns could be compared irrespective of camera rotation and position. The new point location lists are made by pairwise selecting each possible combination of 2 points in the image as a new basis. This is illustrated in FIG. 8A. Based on the new basis, the remaining 22 points are each assigned a new location. This results in 552 lists of the locations of 22 points that define the reference point cloud.

[0197] 4. Image comparison. The 27 images are compared one to one. In this comparison one of the images is defined as the test image, and a single list of 22 points is generated by taking the brightest 2 points to form the basis in the test image. The locations of these 22 points in the test image are then compared to each of the 552 lists of locations for the other image, the reference image. In each case the minimum distance between points in the reference and test image are calculated. If this distance is smaller than a tolerance factor, the points are considered to match and a vote is cast that the images are the same. The number of votes for the one of the 552 lists with the most votes is recorded. If this number of votes is over a threshold the images are defined as matching. The threshold can be determined for a given label design by (in the initial calibration) comparing the number of votes cast between matching images (same label, same angle) and non-matching images (different label, or same label but different angle). Such a distribution is shown in FIG. 8B. Whereas less than 5 votes are cast for nonmatching images, more than 10 are cast for matching images. In this instance a threshold of 8 votes is appropriate for assigning same and different images.

[0198] Steps 1 to 4 describe a preferred method on how the images could be compared. Alternatively, it is possible to determine whether the emission under each micro-optical element 3 was on or off. This would then give a binary sequence with a length of the total number of micro-optical elements in the array. Comparison of the distance between two such binary sequences would be easily possible with standard algorithms. This approach of assigning an on or off state to each micro-optical element 3 in the array is likely the most robust implementation.

[0199] FIG. 9 shows a schematic illustration of a focal volume V.sub.focal in the microparticle layer 5 of the label 1. D is the maximum diameter of the micro-optical element 3 perpendicular to the optical axis of the micro-optical element, i.e, the maximum diameter of the micro-optical element in the XY-plane. F is the focal length of the micro-optical element. d is the lateral minimum focal point diameter resulting from diffraction in the XY-plane. L is the axial focal length along the optical axis (Z-axis) over which the minimal lateral focal point diameter d is maintained. As shown above, the minimal focal volume V.sub.focal resulting from diffraction can be approximated by

[00017]$V_{\mathrm{focal}}={?\left(\frac{d}{2}\right)}^{2}L=\frac{16{?}^3{F}^4}{n^3{D}^4}$

[0200] Finally, FIG. 10 shows an example of the estimated microparticle number density N.sub.p in the microparticle layer 5 for a fixed microparticle volume V.sub.p of 4000 ?m.sup.3 and a varying focal volume V.sub.p from 100 to 8000 ?m.sup.3. The graph labelled V_focal illustrates equation (i). The graph labelled V_p illustrates equation (ii).

[0201] Two separate regimes and equations (i) and (ii) for the estimation of microparticle number density N.sub.p are necessary. The curves for V_p and V_focal cross when the focal volume V.sub.focal and particle volume V.sub.p are equal. The particle density N.sub.p should be estimated from the lower of the two functions, i.e., when the focal volume V.sub.focal is smaller than the particle volume V.sub.p, the curve based on the particle volume V.sub.p should be used (equation (ii)). When the focal volume V.sub.focal is larger than the particle volume V.sub.p, the particle volume N.sub.p must be reduced, and the curve based on the focal volume V.sub.focal is then used (equation (i)). Here a mean probability of p.sub.mean=0.01 was selected, and the particle volume V.sub.p was held constant at 4000 ?m.sup.3.

[0202] According to an illustrative example, the micro-optical element 3 in FIG. 9 may have a diameter D of 200 ?m and a focal length F of 600 ?m. For an illumination device having a wavelength ?=550 nm and a desired probability p of 0.01 in a 1 mm thick microparticle layer 5 of FIG. 9, the minimum focal point diameter d is 2 ?m, the focal depth L is 50 ?m, and V.sub.focal is 200 ?m.sup.3. The change in the position of the focus in the XY plane with a 2? change in incidence angle from normal (i.e., from the optical axis) is 21 ?m. To keep the pattern stable with 2? repositioning accuracy a particle size of 20 ?m could therefore be selected. The particle volume V.sub.p is calculated as follows:

[00018]$V_p=\frac{4}{3}{?\left(\frac{d_p}{2}\right)}^3=4000\mathrm{.Math.}{m}^3.$

Since V.sub.p is larger than V.sub.focal, equation (ii) is used to calculate the microparticle number density N.sub.p, as follows:

[00019]$N_p=\frac{0.01}{V_p}=2.4?10^{12}{m}^{-3}.$

[0203] The volume fraction of particles in the microparticle layer 5 can be calculated by N.sub.p V.sub.p, where

[00020]$V_p=\frac{4}{3}{?\left(\frac{d_p}{2}\right)}^3.$

In this case the volume fraction of the particles would be 1%. As the density of microparticle is a factor of 10 higher than the density of the polymer host, this would correspond to a 0.1% wt. addition of microparticles to the host polymer.

[0204] Accordingly, a label 1, 1, 1, 1 is provided with additional security due to the dependence of the pattern of bright microparticles 7 on the conditions of the illumination, i.e., incidence angles, polarization, and divergence of the light. In an advantageous manner, the micron-scale randomness of the distribution of the microparticles 7 can be read out on the macroscopic scale using an image capturing device 33 such as a camera of a smartphone. Thus, the label 1, 1, 1, 1 is virtually unclonable due to random micron-scale alignment during a manufacturing process that cannot be reproduced purposefully but produces a signal due to this alignment that is easily read on the macroscopic scale without the need for microscopy.

[0205] The security of the label 1, 1, 1, 1 is gained through the random micron-scale alignment between the microparticles 5 and the micro-optical elements 3. This alignment is unique for each label 1, 1, 1, 1 so that it is virtually impossible to clone or purposefully reproduce. The read out at the macroscopic scale is possible as the light emitted or scattered from a bright particle is possible to be read out as a point pattern with an image capturing device 33 having focal lengths such as those found in smartphone cameras.

[0206] Combining robust microscopic security and macroscopic authentication, labels 1, 1, 1, 1 are easy to create, easy to compare, and prohibitively difficult to counterfeit. Thus, a low-cost label is provided that is easy to read in the field, i.e., via a low-cost illumination source and camera module including those provided by a smartphone flash and camera.

LIST OF REFERENCE NUMERALS

[0207] 1, 1, 1, 1 label for verifying authenticity of items [0208] 3 focusing micro-optical elements [0209] 5 microparticle layer [0210] 7 luminescent and/or scattering microparticles [0211] 9 optical axis [0212] 11 micro-optical element plane [0213] 13 focal length [0214] 15 focus [0215] 17 first surface of the microparticle layer [0216] 19 second surface of the microparticle layer [0217] 21 distance between first and second surfaces [0218] 25 incident light [0219] 26 emitted light [0220] 27 random surface roughness [0221] 29 Fresnel lens [0222] 31 illumination device [0223] 33 image capturing device [0224] 35 test optical image [0225] 36 test optical image [0226] 37 holding apparatus [0227] 39 vertical rotation axis [0228] 41 horizontal rotation axis [0229] 43 planar surface