Abstract
A method and system for forming a holographic structure in a material. The holographic structure is configured to project a selected target image in the far field under illumination of the holographic structure by a laser. The method calculates a modified design for the holographic structure that encodes a unique identifier within the holographic structure for projecting the target image. The method modifies the material by mapping features corresponding to the modified design into the material so as to form the holographic structure. A basic check of the authenticity of the material is performed by checking whether a projected replica of the target image is as expected. A more detailed check of the authenticity of the material is performed by directly inspecting the features in the holographic structure.
Claims
1-42. (canceled)
43. A method for forming a holographic structure in a material, the holographic structure being configured to project a target image in the far field under illumination of the holographic structure, the method comprising: calculating a design for the holographic structure for projecting the target image; modifying the design to encode an identifier within the holographic structure for projecting the target image; and modifying the material by mapping features corresponding to the modified design into the material.
44. The method of claim 43, wherein the identifier encoded by the modified design is such that the identifier is substantially hidden, masked or otherwise substantially unidentifiable in the projected target image.
45. The method of claim 43, wherein at least one of calculating and/or modifying the design comprises using the identifier as part of an algorithm to calculate the modified design.
46. The method of claim 43, wherein calculating the design comprises calculating a map of phase and/or amplitude values for projecting the target image, wherein each feature of the design corresponds to one of the phase and/or amplitude values.
47. The method of claim 46, wherein modifying the design comprises calculating a map comprising at least one different phase and/or amplitude value such that the target image projected by the holographic structure based on the modified design is indistinguishable from the target image projected before modification of the design.
48. The method of claim 46, wherein calculating the map of phase and/or amplitude values for projecting the target image comprises: calculating a set or array of features that have associated phase and/or amplitude values for projecting the target image, wherein each feature is at a different position in the map, and each position in the map has one of at least two phase and/or amplitude values.
49. The method of claim 43, wherein at least one of: calculating; and modifying the design comprises: obtaining an initial design of the holographic structure for projecting the target image; and selecting a different phase and/or amplitude value for at least one of the features to modify the design, the modified design projecting a target image that is indistinguishable from, or at least similar to, the target image projectable by the holographic structure corresponding to the initial design.
50. The method of claim 49, wherein the initial design (a) comprises a map of phase and/or amplitude values comprising at least one feature corresponding to the identifier; or (b) is a map of phase and/or amplitude values comprising at least one of: constant phase and/or amplitude values across at least part of the map; random phase and/or amplitude values across at least part of the map; and a defined pattern of phase and/or amplitude values across at least part of the map.
51. The method of claim 43, wherein the identifier comprises an initial seed for deterministically generating a map of the features for the design.
52. The method of claim 43, comprising selecting at least one portion of the design and swapping the selected portion(s) with at least one other portion of the design.
53. The method of claim 52, comprising verifying whether the modified design of the holographic structure is configured to project a target image that is indistinguishable from, or at least similar to, the target image projected by a holographic structure corresponding to the unmodified design.
54. The method of claim 52, wherein the selected portion and the at least one other portion are portions that, individually project substantially the same or similar target image but that comprise or represent maps that are at least partially different.
55. The method of claim 54, comprising generating the map comprised in or represented by the selected portion using a first seed and/or first number of iterations and generating the map comprised in or represented by said at least one other portion using a second, different seed and/or a second, different number of iterations.
56. The method of claim 43, comprising modifying the material by at least one of: (a) changing a level of a surface of the material; (b) modifying a refractive index of the material; and (c) using radiation to at least one of: melt, ablate, move, deposit, or otherwise distribute the material; and change a chemical property of the material.
57. A product comprising a holographic structure for projecting a target image in the far field under illumination of the holographic structure, the holographic structure comprising: features based on a modified design that encodes an identifier within the holographic structure for projecting the target image, wherein the identifier encoded by the modified design is preferably such that the identifier is hidden, masked or otherwise unidentifiable in the projected target image.
58. A computer program product that when executed by a processing system or control unit causes the processing system or control unit to at least partially implement the method of claim 43.
59. A system for forming a holographic structure in a material, comprising: a control system for carrying out the method of claim 43 to modify a design for the holographic structure; and a laser system for modifying the material according to the design.
60. A method for determining an authenticity of a material comprising a holographic structure formed by the method of claim 43, the method comprising: inspecting at least part of the holographic structure to determine a design of the holographic structure; and comparing the inspected design with an expected design.
61. A computer program product that when executed by a processing system or control unit causes the processing system or control unit to at least partially implement the method of claim 60.
62. A system for determining an authenticity of a material comprising a holographic structure formed by the method of claim 43, comprising: an inspection system for inspecting a design of the holographic structure; and a comparison system for comparing the inspected design with an expected design.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0116] These and other examples of the present disclosure will now be described by way of example only, and with reference to the accompanying drawings, in which:
[0117] FIG. 1 is a schematic view of an arrangement for indicating the authenticity of a material;
[0118] FIG. 2 is a schematic view of a system for modifying a material by mapping features corresponding to a target image into the material according to an example of the present disclosure;
[0119] FIG. 3 is a schematic view of a laser system for modifying the material according to an example of the present disclosure;
[0120] FIGS. 4a to 4d are perspective view images of craters formed in a material at various laser pulse energy levels;
[0121] FIGS. 5a to 5c respectively illustrate an elevated view image of a map of craters formed in a material; a perspective view image of a map of craters formed in a material; and a graph indicating the level of the craters as a function of position in a material;
[0122] FIG. 6 is a perspective view image of features formed on the surface of a material that can be used as the basis of an amplitude hologram;
[0123] FIG. 7 is a schematic side view of a crater formed in a material as part of a holographic structure contributing to a relative phase delay in radiation reflected from the surface and the crater;
[0124] FIG. 8 is a schematic illustration of a method for marking a material and determining the authenticity of the material;
[0125] FIGS. 9 and 10 respectively show schematic illustrations of methods of forming a holographic structure in a material according to examples of the present disclosure;
[0126] FIG. 11 is a schematic illustration of a plurality of computer generated holograms (CGHs) and corresponding diffractive images projected by some of the CGHs;
[0127] FIG. 12 is a schematic illustration of a method of forming a holographic structure in a material according to an example of the present disclosure;
[0128] FIGS. 13a to 13c respectively illustrate different examples of CGHs and corresponding diffractive images projected by the CGHs;
[0129] FIGS. 14 and 15 respectively show schematic illustrations of methods of forming a holographic structure in a material according to examples of the present disclosure;
[0130] FIGS. 16a and 16b are images of a projected replica of a target image in the far field for two-level and three-level holographic structures, respectively;
[0131] FIG. 17 depicts a hologram design including a plurality of tiled CGHs organised into a pattern;
[0132] FIG. 18 depicts a design for a patterned holographic structure produced by combining a CGH with a QR code design;
[0133] FIG. 19 is a schematic illustration of an example of a system for forming a holographic structure according to an example of the present disclosure; and
[0134] FIG. 20 is a schematic illustration of an example of a system for determining an authenticity of a material including a holographic structure according to an example of the present disclosure.
DETAILED DESCRIPTION OF THE DRAWINGS
[0135] FIG. 1 illustrates an arrangement 10 for providing a basic check of the authenticity of a product, which in this example is illustrated as being in the form of a material 12. The material 12 has been modified to include a holographic structure 14. A laser 16 is used to illuminate at least part of the holographic structure 14 such that reflected radiation 18 projects a replica 20 of a target image 22 in the far field (e.g. for imaging on a screen 24). Providing the replica 20 imaged on the screen 24 is as expected, a user may assume that the material 12 is genuine. As explained herein, it is possible to hide information in the holographic structure 14 that is indicative of the authenticity of the material 12 without the information being revealed in the replica 20. Only an authorised user such as a manufacturer may be able to determine whether the material 12 is authentic by inspecting the holographic structure 14 directly and checking whether the holographic structure 14 is as expected.
[0136] FIG. 2 illustrates part of a method 26 for forming a holographic structure 14 in the material 12. Initially, a target image 22 is selected and used to calculate a design 28 of the holographic structure 14. In this example, the calculation of the design 28 is performed according to an iterative Fourier-transform algorithm (IFTA) and tiling of the calculated phase distributions 30. This calculation is described in Wyrowski et al., Iterative Fourier-transform algorithm applied to computer holography, J. Optical Society of America A, Vol. 5, pp. 1058-1065, 1988, and Wyrowski, Iterative quantization of digital amplitude holograms, Applied Optics, Vol. 28, pp. 3864-3870, 1989, the contents of which is hereby incorporated by reference in its entirety. A laser system 32, only part of which is shown in FIG. 2, is used to modify the material 12 by mapping features 34 corresponding to a phase and/or amplitude value (e.g. each feature may correspond to a pixel of a certain phase and/or amplitude value) in the design 28 into the material 12. The mapped features define a map of phase values in the material 12 that forms the holographic structure 14.
[0137] FIG. 3 illustrates the laser system 32 used to modify the material 12. The laser system 32 includes a laser 34 outputting a laser beam 36. The average power in the laser beam 36 is controllable using a power control arrangement 38 that includes a /2 waveplate 40, polarising beam splitter 42 and beam dump 44. The laser beam 36 from the power control arrangement 38 is expanded using a beam expander 46 and subsequently directed into a beam scanning apparatus 48. The beam scanning apparatus 48 includes a pair of galvo-scanning mirrors 50 for changing the direction of the laser beam 36. A lens 52 (e.g. an F-theta lens or the like) focuses the changing-direction laser beam 36 onto the material 12, which itself is mounted on a work piece 54 (e.g. translation stage). The focused laser beam 36 has a higher intensity at the material 12 and can be used to modify the material. By changing the direction of the laser beam 36, it is possible to map features corresponding to a phase value into the material 12. The modification process depends on at least the material used and the optical parameters of the laser system 32. It will be appreciated that any appropriate laser-based process may be used to modify the material as required, and that the optical parameters selected may depend on the type of material used and/or requirements for the holographic structure 14.
[0138] In the present example, the laser system 32 is used to apply marks to the surface of a metal so as to form a holographic structure. In this example, the laser 34 is configured to produce 35 ns full-width half maximum (FWHM) laser pulses at a wavelength 355 nm. The lens 52 focuses the laser beam 36 to a FWHM beam diameter of 112 m (as measured at 1/e.sup.2 of its maximum intensity) at the material 12. The laser pulses are delivered on demand to a certain location on the material 12 e.g. in the form of a point-and-shoot operation. The time taken to generate a 1 mm1 mm holographic structure including over 15,000 features is 7 seconds.
[0139] FIGS. 4a-4d each illustrate in more detail a feature 56 in a surface 58 of the material 12 that has been modified using the laser system 32. The features 56 in this example takes the form of an approximately circular crater 60, the dimensions of which depend on the pulse energy of the laser beam 36. FIGS. 4a-4d illustrate the crater 60 formed in stainless steel (ST304LD in this example) according to a pulse energy of 2.6 J, 6.2 J, 7.2 J and 13.5 J, respectively. For reference, the dimensions of the sides of the portion of material 12 illustrated by FIGS. 4a-4d is 24 m, 20 m, 20 m, and 30 m, respectively. Craters can also be formed on the surface of other metals such as nickel, brass, nickel-chromium Inconel alloys (e.g. Inconel 625, Inconel 718, and Inconel X750, and the like), and the like. Features such as craters can be formed in other materials such as glass. In the example of glass, a CO.sub.2 laser system can be used to form features in the glass. It will however be appreciated that there are many different laser system and material 12 selections that can be made by selecting appropriate parameters.
[0140] In each of the figures, the crater 60 extends into the surface 58 such that a centre 62 of the crater 60 is at a lower level than the surface 58. An edge 64 of the crater 60 defines a ridge that is at a higher level than the surface 58. The process of formation of the crater depends on a number of parameters but in this example it is thought that the craters 60 are formed by localised melting or a combination of melting and evaporation. It will be appreciated that other types of light-matter interactions (e.g. which may cause melting, ablation, moving, depositing, or any other form of material distribution, redistribution, modification, or the like) may result, or at least be dominant, in the formation of craters 60 and/or other types of features. It will be appreciated that the formation of the craters 60 or other types of features depends on the type of laser system used, the material type, local conditions, type of cover gas, as well as any other relevant parameters.
[0141] Depending on the desired type of feature, it may be possible to selectively form either a raised feature or a lowered feature in the material during formation of the feature, for example as described in WO2012038707. As explained previously, the type of feature produced depends on a number of parameters. Examples of a raised feature includes: a protrusion, bump, projection or any other feature extending at least partially out of the surface (e.g. of the material, or the like). Examples of a lowered feature includes: a cavity, crater, or any other feature extending at least partially into the surface (e.g. of the material, or the like). Modifying the refractive index of the material may modify the optical length of the modified part of the material.
[0142] FIGS. 5a-5b respectively illustrate an elevated view image of a map 66 of craters 60 formed in a material 12; and a perspective view image of another map 66 of craters 60 formed in a material 12. FIG. 5c illustrates a graph 68 indicating the level (e.g. the height in m) of the craters 60 as a function of position (e.g. distance in mm) in a material 12. Each crater 60 has an approximate depth (or relative difference in level) of 250 nm and may be considered to be optically smooth. The example of FIG. 5c represents a two-level (or binary) phase hologram in the surface of stainless steel. The difference in the height between the surface 58 and the centre 62 of the crater 60 contributes to a relative phase delay between radiation reflected from the surface 58 and from the centre 62.
[0143] FIG. 6 is a perspective view atomic force microscopy (AFM) image of features 56 formed on the surface 58 of a material that can be used as the basis of an amplitude hologram. In this example, the features 56 are in the form of craters 60. However, the internal surface of the craters 60 is shaped (e.g. by having a non-uniform or rough surface) to cause scattering of incident radiation, hence reducing the amplitude of directly reflected light. The features may be more highly scattering or absorbing than the surface 58 surrounding the craters 60. In this example, the material is 304-grade stainless steel and each feature 56 has been generated by 80 laser pulses, each with a duration of 6 ps at a wavelength of 343 nm, using a 70 mW average power and a 400 kHz pulse repetition frequency.
[0144] FIG. 7 illustrates a crater 60 in a material 12 that is illuminated with radiation, as indicated by arrow 61, for projecting a replica of the target image, as described in relation to FIG. 1. The radiation 61 for illuminating the surface 58 and the crater 60 of the material 12 is in phase as indicated by arrows 63. In this example, the crater 60 has a depth that is equivalent to a quarter wavelength /4 (i.e. /2 radians) such that radiation reflected, as indicated by arrow 65, from the crater 60 is relatively out of phase 65 by /2 (i.e. radians), as indicated by arrows 67, with light reflected from the surface 58. The presence of the crater 60 causes the formation of the replica (see FIG. 1) in the far field. The relationship between the relative phase delay () between radiation reflected from the crater 60 and the surface 58 and the depth (d) of the crater 60 is defined by =4d/. Since the phase change is bounded by 0 and 2, any change in the depth leading to a phase change greater than 2 or less than 0 has the same effect as modulo 2. Therefore, a phase delay of 2 is equivalent to a phase delay of 0, 4, and the like.
[0145] With reference to FIGS. 1 to 7, FIG. 8 is a schematic illustration of a method 70 for modifying a material 12 and determining the authenticity of the material 12 after the material 12 has been marked. In a first part 72 of the method 70, the material 12 is marked with a holographic structure 14. Initially, an identifier 74, which in this example is in the form of a serial number, part ID or number, is used to calculate or modify a design as part of a hologram calculation 76 for the holographic structure 14. The method includes calculating a design for the holographic structure for projecting a target image 22 as part of the hologram calculation 76. The method further includes modifying the design to encode the identifier 74 within the holographic structure 14 for projecting the target image 22, which may also be performed as part of the hologram calculation 76.
[0146] The identifier 74 is hidden or otherwise encoded within the modified design. For example, the identifier 74 can be in the form of a hidden code 78 (such as may be derived from the serial number, part ID or number, or from a one-time pad recorded against the serial number of part ID, or the like) within the modified design in such a manner that the holographic structure 14 projects a replica 20 of the target image 22 that is indistinguishable from, or at least similar to, a replica of the target image where the design does not contain a hidden code 76. Features corresponding to phase values calculated in the modified design are mapped into the material 12 by using the laser system 32 to form a laser marked pattern or holographic structure 14 in the material 12.
[0147] In a second, optional, part 80 of the method 70, a user can perform a quick visual check of the authenticity of the holographic structure 14 in accordance with the procedure outlined in relation to FIG. 1. In the second part 80, a replica 20 of the target image 22 (e.g. in the form of a projected pattern 81) is projected in the far field. The projected pattern 81 may include a serial number, part ID or the like, which is not hidden so that the user (such as a consumer) can perform a quick visual check 82 of the authenticity of the material 12. It will be appreciated that the quick visual check 82 may be hard to copy, but not impossible. However, the consumer or other user may be able to quickly perform a basic check on the authenticity of the material 12.
[0148] In a third part 83, an authorised user inspects the holographic structure 14 to determine the authenticity of the material 12. In an inspection step 84, the hidden code 78 is determined by using a microscope, phase contrast microscope, or specialised instrument, or the like so as to provide a definitive check 85 of the authenticity of the hidden code 78 when the hidden code (e.g. which may be in the form of a hidden pattern) corresponds, by the secret relationship, to the identifier 74. The method 70 includes a step 86 of comparing the inspected holographic structure 14 with the expected features for the holographic structure 14.
[0149] FIGS. 9, 10, 12, 14 and 15 respectively show schematic illustrations of methods of forming a holographic structure in a material while making reference to features described in relation to FIGS. 1 to 8.
[0150] FIG. 9 illustrates an example method 90 for forming a holographic structure 14 in a material 12. The method 90 includes a step 92 of selecting a target image 22 such that the holographic structure 14 is configured to project the selected target image 22 in the far field under illumination of the holographic structure 14. The method 90 further includes a step 94 of calculating a design 28 for the holographic structure 14 for projecting the target image 22. The method further includes a step 96 of modifying the design 28 to encode an identifier (e.g. a serial number, unique code, part number, signature, logo, image, photo, name, brand, code, symbol, set of characters, one-time-input, or any form of identification, or the like) within the holographic structure 14. The method 90 further includes a step 98 of modifying the material 12 by mapping features corresponding to the modified design 28 into the material 12.
[0151] FIG. 10 illustrates an example method 100 for forming a holographic structure 14 in a material 12. The method 100 includes a step 102 of selecting a target image 22 such that the holographic structure 14 is configured to project the selected target image 22 in the far field under illumination of the holographic structure 14. The method 100 further includes a step 104 of performing an initial guess for the design 28 of the holographic structure 14 for projecting a replica of the target image 22. Optionally, the method 100 includes a step 106 of entering an initial seed for deterministically generating a map of the features for the design 28. An example of the initial seed includes a serial number, or the like, that is used to define initial conditions to deterministically generate the map, e.g. as part of the step 104. The method 100 further includes a step 108 of performing at least one iteration of an algorithm to modify the design 28 to encode an identifier within the holographic structure 14 for projecting the target image 22. The method 100 further includes a step 110 of modifying the material 12 by mapping features corresponding to the modified design 28 into the material 12.
[0152] FIG. 11 depicts a hologram design 28 constructed from sixteen different computer generated holograms (CGHs) each of which may be calculated, for example, using the method 100 of FIG. 10. For clarity, the CGHs in FIG. 11 are depicted as individual tiles spaced apart from each other. However, in reality the individual tiles may be joined together in a 44 array to form one single hologram design 28 comprising the sixteen CGHs. Each of the different CGHs projects or produces the same or very similar replica 20 (e.g. each CGH projects or produces similar diffractive images). Shuffling and/or swapping of the CGHs may not affect the appearance of the replica 20 since the replicated images are indistinguishable from each other (or at least it may be very difficult for a user to visually identify differences between the replicas 20 produced by each CGH). In the present example, each CGH is designed using a different number of iterations N in the IFTA. As depicted by FIG. 11, the number of iterations N used to produce each CGH is between 100 and 475 iterations (separated by 25 iterations between each subsequent CGH). The initial seed was the same for designing all sixteen CGHs (e.g. the initial phase values contained zeros). FIG. 11 also depicts four replicas 20 produced by the CGHs that represent N=150, 275, 325 & 475 iterations respectively. The CGHs are sufficiently similar to project or produce similar replicas 20. However, there are subtle differences between the sixteen CGHs that, when mapped onto the holographic structure 14 of the material 12, can be inspected to determine whether or not the pattern of CGHs in the hologram design 28 indicates that an associated product is genuine.
[0153] FIG. 12 illustrates an example method 120 for forming a holographic structure 14 in a material 12. The method 120 includes a step 122 of selecting a target image 22 such that the holographic structure 14 is configured to project the selected target image 22 in the far field under illumination of the holographic structure 14. The method 120 further includes a step 124 of calculating a design 28 for the holographic structure 14 for projecting the target image 22. The method 120 further includes a step 126 of selecting at least one portion of the design 28 and swapping the at least one portion with at least one other portion of the design 28 to modify the design 28. The method 120 further includes a step 128 of modifying the material 12 by mapping features corresponding to the modified design 28 into the material 12.
[0154] FIGS. 13a to 13c depict different examples of hologram designs 28 (centre image) constructed using sixteen CGHs arranged in a pattern (left image) to form an identical or nearly identical replica 20 (right image). FIG. 13a depicts a hologram design 28 constructed using sixteen identical CGHs (each CGH is referenced as number 16), for example, using the method 100 of FIG. 10. The reference number of each CGH may, for example, refer to the number of iterations used to produce the CGH. FIG. 13b depicts an alternative hologram design 28 constructed using sixteen different CGHs (the CGHs each have a different reference number from 1 to 16), for example, using the method 100 of FIG. 10 with reference to the example of FIG. 11. FIG. 13c depicts an alternative hologram design 28 constructed using the same CGHs as in FIG. 13b. However, the CGHs have been shuffled as depicted in the pattern of the left image, for example, using the method 120 of FIG. 12. The replicas 20 projected using the designs 28 of FIGS. 13a, 13b & 13c are indistinguishable (or at least difficult for a user to visually identify differences between them).
[0155] FIG. 14 illustrates an example method 130 for forming a holographic structure 14 in a material 12. The method 130 includes a step 132 of selecting a target image 22 such that the holographic structure 14 is configured to project the selected target image 22 in the far field under illumination of the holographic structure 14. The method 130 further includes a step 134 of calculating a design 28 for the holographic structure 14 for projecting the target image 22. The method 130 further includes a step 136 of selecting a different phase value for at least one feature of the design that is equivalent to an initial phase value for the at least one feature to modify the design 28. The method 130 further includes a step 138 where the calculation and/or modification of the design 28 results in a design 28 where the relative difference between the initial and different phase values is 2m (2*pi*m) for integer values of m1 (m greater than or equal to plus one) or m1 (m less than or equal to minus one). The method 130 further includes a step 140 of modifying the material 12 by mapping features corresponding to the modified design 28 into the material 12.
[0156] FIG. 15 illustrates an example method 150 for forming a multi-level holographic structure 14 in a material 12. The method 150 includes a step 152 of selecting a target image 22 such that the holographic structure 14 is configured to project the selected target image 22 in the far field under illumination of the holographic structure 14. The method 150 further includes a step 154 of calculating a design 28 for the holographic structure 14 for projecting the target image 22. In step 154, calculating the design 28 includes calculating a map of phase and/or amplitude values for projecting a replica 20 of the target image 22, wherein each feature of the design 28 corresponds to one of the phase and/or amplitude values. The method 150 further includes a step 156 of using an identifier to select at least one of the features of the map and modifying the design by assigning at least one different phase value to the feature(s) that is equivalent to an original phase value of the feature(s) of the design 28. The method 150 further includes a step 158 of modifying the material 12 by mapping features corresponding to the modified design 28 into the material 12. The features may be in the form of levels or phase values or provide a phase response. Additionally or alternatively, the features may be in the form of amplitude values or provide an amplitude response. For example, at least one or all of the features may be used to impart a phase delay onto incident radiation (e.g. at the feature(s)) reflected and/or transmitted by the holographic structure 14. Additionally or alternatively, at least one or all of the features may be used to provide impart an amplitude response (e.g. at the feature(s)) onto incident radiation reflected and/or transmitted by the holographic structure 14. The holographic structure 14 may comprise features providing a phase-only response (which may be referred to as a phase hologram), amplitude-only response (which may be referred to as an amplitude hologram), or a phase-and-amplitude response (which may be referred to as a phase-amplitude hologram).
[0157] In an example, the method 150 modifies the design 28 such that the map includes features that define a phase value or level in the holographic structure 14. With reference to the example illustrated by FIG. 16a, there is shown a replica 160 projected by a holographic structure 14 that includes a two-level holographic structure. The two-level holographic structure 14 results in the projection of a twin image, or 1.sup.st diffraction order image in the far field such that a mirror-like image is projected. With reference also to the example illustrated by FIG. 16b, there is shown a replica 162 projected by a holographic structure 14 that includes a three-level hologram. The three-level holographic structure 14 breaks the symmetry of the holographic structure, thereby suppressing the formation of a twin image, which can be observed by comparing FIGS. 16a & 16b. Alternatively or additionally, it may be possible to define more than one amplitude value or level in the holographic structure 14. In such a holographic structure 14 comprising more than one amplitude value (e.g. in the form of a three-level amplitude hologram or the like), it may be possible to suppress the formation of a twin image.
[0158] The two-level replica 160 includes two possible nominal phase and/or amplitude values for the features of holographic structure 14. By way of example, these two phase values could be 0 radians (e.g. corresponding to a surface level of the material) and radians (e.g. corresponding to a crater 60, or any other appropriate feature), or indeed any other appropriate phase values.
[0159] The three-level replica 162 includes three possible nominal phase and/or amplitude values for the features of holographic structure 14. By way of example, these three phase values could be 0 radians (e.g. corresponding to a surface level of the material), 2/3 radians (e.g. corresponding to a crater 60, or any other appropriate feature extending into the surface of the material), and 2/3 radians (e.g. corresponding to a bump (not shown), or any other appropriate feature extending out of the surface of the material, or the like). In another example, the three possible phase values are: 0 radians, 2/3 radians (e.g. corresponding to a crater 60 of a certain depth and width), and 4/3 radians (e.g. a crater 60 corresponding to a relatively deeper and/or wider crater). As noted previously, the replica 162 of the three-level holographic structure 14 includes a suppressed twin image in the far field. Fine process control may be required in order to form a three-level (or higher) holographic structure 14, which may be harder to copy. By providing the quick visual check described in relation to FIG. 1, a user may be able to identify an authentic material 12 by visually inspecting the replica 162 projected in the far field for any suppression of twin images, which may indicate that the material 12 is genuine.
[0160] FIG. 17 depicts a hologram design 28 comprising a plurality of tiled CGHs (e.g. a set of tiled CGHs). In this example, there are two subsets of CGHs arranged to form a pattern (in this example the pattern is in the form of HI lettering). Any suitable pattern may be used in alternative embodiments, for example a watermark or the like. A first sub-hologram (or CGH) is denoted #1 and forms a background of the design 28. A second sub-hologram (or CGH) is denoted #2 and forms the lettering. Any appropriate pattern can be provided. The sub-holograms #1 and #2 can produce substantially the same image but may have a different characteristic (e.g. using the same initial seed but each generated using a different number of iterations with the IFTA algorithm, or the like). Additionally or alternatively, the sub-holograms #1 and #2 may be configured to produce the same or similar hologram but with a different orientation (e.g. the projected images may be perpendicular to each other, or the like). Providing a set of CGHs or sub-holograms with different designs and/or patterning may be difficult to replicate consistently.
[0161] FIG. 18 illustrates a hologram design 28a that is combined with another pattern (in this example, a QR code design 29 but it could be a watermark, barcode or other pattern) to generate a design 28b for generating a patterned holographic structure. The combination in this example is performed by multiplying amplitude values (i.e. 1 or 0) at different positions within the QR code design 29 with corresponding positions of the hologram design 28a. The patterned holographic structure can provide the functionality of a QR code, as well as being used to check the authenticity of the product as described herein. Providing a patterned holographic structure may be difficult to replicate consistently.
[0162] FIG. 19 illustrates a system 170 for forming the holographic structure 14 in the material 12. The system 170 includes a control system 172 for carrying out the method according to any example described herein to modify a design 28 for the holographic structure 14. The control system 172 may include or be in the form of a computer program product that when executed by a processing system or control unit 174 of the control system 172 causes the processing system or control unit 174 to at least partially implement a method as claimed or described herein. The system 170 further includes a laser system 176 for modifying the material 12 according to the design 28. The control system 172 is operable to control the laser system 172, which in this example includes the laser system 32 described in relation to FIG. 3, such that the material 12 is modified to include the holographic structure 14 according to the design 28.
[0163] FIG. 20 illustrates a system 180 for determining an authenticity of a material 12 including a holographic structure 14 formed by the system 170 or any other appropriate system. The system 180 includes an inspection system 182 for inspecting a design 28 of the holographic structure 14 in a material 12. The inspection system 182 may include a microscope, phase contrast microscope, white light interferometer, stylus profilometer, atomic force microscope, or the like. The system 180 further includes a comparison system 184 for comparing the inspected design 28 with an expected design. If an inspection reveals that the holographic structure 14 in the material 12 includes features (e.g. craters and/or bumps, or the like) that do not correspond with the expected design (e.g. a design that is known to or calculated by an authorised user such as a manufacturer, dealer or repairer based on the identifier 74), then the material 12 may be identified as being fake, or at least may prompt further investigation. Optionally, the system 180 may include the control system 172 of FIG. 19 for carrying out the method according to any example described herein to modify a design 28 for the holographic structure 14 in order for the authorised user to calculate the expected design.
[0164] The comparison system 184 may include or be in the form of a computer program product that when executed by a processing system or control unit 186 of the comparison system 184 causes the processing system or control unit 186 to at least partially implement a method as claimed or described herein. The comparison system 184 may be operable to control or interact with the inspection system 182, e.g. via the control unit 186, such that the inspection system 182 can inspect the holographic structure 14 and send information regarding the holographic structure 14 to the comparison system 184.
[0165] It will be appreciated that any combination of phase values, whether positive and/or negative (e.g. corresponding to a lowered or raised feature, or the like) can be used for creating a map of features. It will also be appreciated that there could be any number of levels in the holographic structure, e.g. 2, 3, 4 or more levels, or the like.
[0166] It will be appreciated that features that produce a phase response may additionally or alternatively comprise features that produce an amplitude response. For example, any appropriate example described herein may be implemented, modified or otherwise adapted to be in the form of a phase-only, amplitude-only or phase-and-amplitude hologram. For example, any individual reference to a phase-only hologram may be implemented, modified or adapted to be in the form of an amplitude-only hologram or a phase-and-amplitude hologram. Any reference to amplitude may also refer to intensity. For example, an amplitude hologram may be referred to as an intensity hologram.
[0167] Where appropriate, any reference to a design, for example the design 28 described herein, may refer to a computer generated hologram (CGH) or vice versa. The design and/or CGH may be implemented in the form of the holographic structure 14, for example, using the laser system 176 or any other appropriate system for modifying the material 12. Where appropriate, any reference to a replica 20 may refer to a hologram and/or diffractive image and/or an image formed on the screen 24 or projected in the far-field. Where appropriate, the target image 22 may refer to a computer-generated image or CGH representative of the replica 20 expected to be projected. A person of ordinary skill in the art will appreciate that, where appropriate, references to any these terms may be modified or used in a different context.
[0168] At least one feature of any example of the present disclosure may be modified, combined with any other example, or otherwise adapted in any appropriate way.
[0169] Although examples of the present disclosure refer to a material 12 including the holographic structure 14, it will be appreciated that the material 12 may include or be in the form of any article such as a product, packaging, label, or the like.
[0170] Although examples of the present disclosure illustrate and describe a laser-based process for modifying a surface 58 of the material 12, it will be appreciated that an internal part of the material 12 or a product may be modified using an appropriate laser-based process, which may depend on the transparency of the material 12 (or a surface thereof) used.
[0171] Although examples of the present disclosure describe a simple visual check of the replica projected by the holographic structure by reflecting e.g. a laser beam from the surface of the holographic structure, it will be appreciated that similar principles may apply for a transmission-based visual check of the replica in the far field for e.g. a transparent material including the holographic structure. Thus, radiation transmitted through the holographic structure may form a replica of the target image in the far field, instead of (or as well as) a replica being projected by reflection from the holographic structure.
[0172] Although examples of the present disclosure describe a holographic structure for reflecting radiation to project a replica of the target image, it will be appreciated that the holographic structure may transmit radiation to project a replica of the image. The holographic structure may be at least partially transparent, which may allow incident radiation to be transmitted through the holographic structure to project the replica of the target image. It will be appreciated that a certain amount of radiation may be reflected as well as transmitted.
[0173] Although examples of the present disclosure describe various systems (e.g. systems 170 and 180, and the like), it will be appreciated that such systems may relate to or include at least one of: apparatus including at least one feature or element of any example of the present disclosure; methods including at least one feature or element of any example of the present disclosure; apparatus for implementing at least one method of the present disclosure; and the like.
[0174] Although examples of the present disclosure mainly describe various systems for representing the holographic design as a map of phase values, it will be appreciated that the technique can be extended entirely analogously to representing the holographic design as a map of amplitude values, or even a combination of phase and amplitude values.
[0175] For example, where a phase value or relative phase value may be represented by a refractive index value or difference in refractive index values, or by an optical length or difference in optical length, an amplitude value or relative amplitude value may be represented by a difference in surface scattering or absorption or a combination these. In this way, a similar amplitude hologram could be produced by replacing the phase referred to above with a grey scale, where a phase of corresponds e.g. to black, and a phase of zero corresponds e.g. to white. Such a grey scale map could then be encoded onto the product by generating absorbing or scattering regions.
[0176] An example of an amplitude hologram is described in Wgaard et al, High-resolution computer-generated reflection holograms with three-dimensional effects written directly on a silicon surface by a femtosecond laser, Optics Express Vol. 19 pp. 3434-3439, the contents of which is hereby incorporated by reference in its entirety.
