Abstract:
An optically variable device may be manufactured by aligning magnetic flakes on a surface of an adhesive layer by applying the flakes onto the adhesive layer surface in presence of a magnetic field, and curing the adhesive layer having magnetic flakes adhered to the adhesive layer. When cured, the adhesive layer holds the magnetic flakes oriented, enabling subsequent encapsulation of the oriented magnetic flakes in a coating layer on the adhesive layer, without a substantial loss of orientation of the magnetic flakes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Patent Application No. 61/992,093 filed May 12, 2014, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to optically variable devices, and in particular to optically variable devices including magnetically alignable flakes. 
     BACKGROUND 
     Optically variable devices are optical devices whose optical performance depends on angle of incidence of illuminating light or angle of observation. A common example of an optically variable device is an iridescent security feature used as an anti-counterfeiting measure on banknotes, credit cards, stock certificates, government-issued identification documents, etc. An optically variable device may provide a visually varying image, for example an illusory three-dimensional (3D) image, a color-shifting image, or both. Such an image is difficult to counterfeit without knowledge of a specific recipe used to manufacture the optical variable device providing the image. 
     Optically variable devices may be made by coating a surface with an ink or paint including flat platelet-like reflective and, or color-shifting particles. Such surfaces show higher reflectance and brighter colors than surfaces coated with a paint or ink containing conventional pigments. Substrates painted or printed with color-shifting flaked pigments may show change of color when viewed at different angles. 
     Flaked pigments may contain a material that is magnetically sensitive, so as to be alignable or orientable in an applied magnetic field. Such flakes may be manufactured from a combination of magnetic and non-magnetic materials and mixed with a paint or ink vehicle in the production of magnetic paints or inks. A feature of these products is the ability of the flakes to become oriented along the lines of an applied field inside of a layer of liquid paint or ink, while substantially remaining in this position after drying or curing of the paint or ink vehicle. Relative orientation of the flake and its major dimension with respect to the coated surface determines the level of reflectance or its direction and, or may determine angle-dependent color or brightness of the paint or ink. 
     By way of example, Phillips et al. in U.S. Pat. No. 6,808,806 disclose methods and devices for producing color-shifting images on coated articles using magnetically alignable flakes including color-shifting coatings. The color-shifting images are defined by the magnetic field applied to the coatings as the coatings are dried or cured. For example, a sheet magnet shaped as a symbol, a letter, or another indicia may be brought in close proximity to the coating during cure. After the coating is cured, the sheet magnet is removed, and the indicia may be seen as a color-shifting image on the coating. The magnetic field application may be adapted for modern printing environments; for example, Raksha et al. in US Patent Application Publication 2005/0106367 disclose a method and apparatus for orienting magnetic flakes in high-speed, linear printing operation. 
     A 3D illusive image may also be formed on the painted product by applying a spatially varying magnetic field to the surface of the product while the paint still is in the liquid state. When the paint is cured and the magnetic field is removed, the 3D illusive image remains visible on the surface of the painted product. The 3D illusive image appears because light rays incident on the paint layer are influenced differently by differently oriented magnetic particles. Raksha et al. in U.S. Pat. No. 7,934,451 disclose a method and apparatus to orient magnetic flakes in desired 3D patterns in a high-speed linear printing apparatus. 
     Despite interesting and often intriguing optical effects produced by solidified suspensions of magnetic flakes, their application in optical security devices has been somewhat limited, in particular for banknotes. The application of magnetically alignable flake suspensions in banknotes and other valuable documents may be hindered by a poor compatibility of two main printing processes mostly used in manufacturing of banknotes—offset printing and Intaglio printing—with magnetically alignable particle suspensions. An offset printing process typically produces a very thin ink film thickness, and as such, cannot transfer large magnetic particles, for example particles that are 30 micrometers in size. An Intaglio printing process typically uses a highly viscous ink, which does not allow efficient alignment of magnetic particles suspended in the ink, at least without taking special measures to lessen the viscosity of the ink while applying a magnetic field, as is disclosed by Raksha et al. in U.S. Pat. No. 8,211,509. 
     SUMMARY 
     In accordance with an aspect of the disclosure, a thickness of a layer including oriented magnetic flakes may be reduced by applying magnetic flakes absent any liquid binder or carrier to an adhesive surface in the presence of magnetic field, which orients the magnetic flakes. For example, magnetic particles may be dusted or blown onto an adhesive surface in the presence of the magnetic field, causing the magnetic flakes to adhere to the adhesive surface in an oriented manner. Then, a thin coating layer may be applied to the oriented magnetic particles adhered to the adhesive surface. The coating layer is cured to maintain the orientation of the magnetic flakes. 
     In accordance with an aspect of the disclosure, there is provided a method of manufacturing an optically variable device, the method comprising: 
     providing a substrate with an first adhesive layer thereon; 
     applying a first magnetic field to the first adhesive layer and providing magnetic flakes absent a liquid carrier or binder onto the first adhesive layer in the presence of the first magnetic field so that the magnetic flakes oriented by the first magnetic field adhere to the first adhesive layer; 
     coating the first adhesive layer and the magnetic flakes adhered thereto with a coating layer; and 
     curing the coating layer, so as to substantially maintain orientation of the magnetic flakes. 
     The first adhesive layer may be only partially cured during depositing the magnetic flakes thereon. The substrate may include a release layer, in which case the coating layer may be adhered to a second substrate, and the release layer may be removed, to obtain a “flipped” orientation pattern of the magnetic flakes. The method may be adaptable to high printing speeds. 
     In one embodiment, a second adhesive layer may be provided on top of the first adhesive layer or beside the first adhesive layer. A second magnetic field may be applied to the second adhesive layer, and second magnetic flakes absent a liquid carrier or binder may be provided onto the second adhesive layer in the presence of the second magnetic field, so that the second magnetic flakes oriented by the second magnetic field adhere to the second adhesive layer. After this, the second adhesive layer may be cured. 
     In accordance with the disclosure, there is further provided a method of manufacturing an optically variable device, the method comprising: 
     providing a substrate with an adhesive layer thereon; 
     applying a magnetic field to the adhesive layer; 
     separately applying magnetic flakes and a coating to the adhesive layer, by initially applying the magnetic flakes absent a liquid carrier, causing the magnetic flakes to adhere to the adhesive layer, wherein the magnetic flakes adhered to the adhesive layer are oriented by the magnetic field; and, after the magnetic flakes have been applied to the adhesive layer, applying the coating to the adhesive layer so as to form a coating layer on the adhesive layer, wherein the coating layer encapsulates the magnetic flakes; and 
     curing the coating layer, so as to substantially maintain orientation of the magnetic flakes. 
     In accordance with the disclosure, there is further provided an optically variable device comprising a substrate; an adhesive layer over the substrate; a plurality of oriented magnetic flakes supported by the adhesive layer; and a coating layer over the substrate adjacent the adhesive layer. The coating may encapsulate the magnetic flakes extending from the adhesive layer, so that a portion of each one of the plurality of oriented magnetic flakes is adhesively attached to the adhesive layer, and a remaining portion of the same magnetic flake extends out of the adhesive layer into the coating layer. 
     In one embodiment, the adhesive layer is disposed on the substrate, and the coating layer is disposed on the adhesive layer. The coating covering the flakes on the adhesive layer may be of the same material as the adhesive layer, or may be a different material. The magnetic flakes may be partially disposed in the adhesive layer. In one embodiment, the magnetic flakes are reflective, and may include color-shifting multilayer coatings. By carefully selecting magnets to generate the magnetic fields, the magnetic flakes may be oriented so as to create a visual appearance of a 3D object such as a hemisphere, a cone, a funnel, a combination of different images obtained at separated stations, etc. The magnetic alignment may be repeated to create other images on top or aside a first image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings, in which: 
         FIG. 1  is a flow chart of a method of manufacturing an optically varying device according to the present disclosure; 
         FIGS. 2A-2H  are side cross-sectional views of an optically variable device of the present disclosure at different progressive stages of manufacturing; 
         FIG. 2I  is a magnified view of a single flake of the optically variable device of  FIG. 2H , attached to an adhesive layer; 
         FIG. 3A  is a schematic side cross-sectional view of a magnet underneath a substrate, showing lines of magnetic field of the magnet; 
         FIG. 3B  is a schematic side cross-sectional view of a substrate supporting magnetic flakes aligned along the magnetic field lines of the magnet shown in  FIG. 3A ; 
         FIGS. 4A to 4C  are side cross-sectional views of an inverted optically variable device of the present disclosure at different progressive stages of manufacturing; 
         FIG. 5  is a schematic plan view of an optically varying device, in which the adhesive layer is patterned to form a banknote denomination “100”; 
         FIG. 6A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been aligned on a layer of adhesive ink with a spherical-cylindrical magnet pair including a cylindrical magnet and a spherical magnet centered on top of the cylindrical magnet; 
         FIG. 6B  is an oblique-view photograph of the optically variable device of  FIG. 6A ; 
         FIG. 7A  is a side cross-sectional view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 6A and 6B , showing a viewing direction of  FIG. 6A ; 
         FIG. 7B  is a side cross-sectional view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 6A and 6B , showing a viewing direction of  FIG. 6B ; 
         FIG. 7C  is a top view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 6A and 6B ; 
         FIG. 8A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been aligned on a layer of adhesive ink with a spherical-cylindrical magnet pair including a spherical magnet atop of and near an edge of a cylindrical magnet; 
         FIG. 8B  is an oblique-view photograph of the optically variable device of  FIG. 8A ; 
         FIG. 9A  is a side cross-sectional view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 8A and 8B , showing a viewing direction of  FIG. 8A ; 
         FIG. 9B  is a side cross-sectional view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 8A and 8B , showing a viewing direction of  FIG. 8B ; 
         FIG. 9C  is a plan view of the spherical-cylindrical magnet pair used to align magnetic flakes of the prototypes of  FIGS. 8A and 8B ; 
         FIG. 10A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been aligned on a layer of adhesive ink with a cylindrical-rectangular magnet pair including a cylindrical magnet on top of a rectangular magnet; 
         FIG. 10B  is an oblique-view photograph of the optically variable device of  FIG. 10A ; 
         FIG. 11A  is a side cross-sectional view of the cylindrical-rectangular magnet pair used to align magnetic flakes of the prototypes of  FIGS. 10A and 10B , showing a viewing direction of  FIG. 10A ; 
         FIG. 11B  is a side cross-sectional view of the cylindrical-rectangular magnet pair used to align magnetic flakes of the prototypes of  FIGS. 10A and 10B , showing a viewing direction of  FIG. 10B ; 
         FIG. 11C  is a plan view of the cylindrical-rectangular magnet pair used to align magnetic flakes of the prototypes of  FIGS. 10A and 10B ; 
         FIG. 12A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been aligned on a layer of varnish with the cylindrical-rectangular magnet pair of  FIGS. 11A-11C ; 
         FIG. 12B  is an oblique-view photograph of the optically variable device of  FIG. 12A ; 
         FIG. 13A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been applied to a layer of a UV-cured adhesive ink with the cylindrical-rectangular magnet pair of  FIGS. 11A-11C ; 
         FIG. 13B  is an oblique-view photograph of the optically variable device of  FIG. 13A ; 
         FIG. 14A  is a plan-view photograph of an optically variable device, in which magnetic flakes have been applied to a layer of an uncured adhesive ink with the rectangular-cylindrical magnet pair of  FIGS. 11A-11C ; and 
         FIG. 14B  is an oblique-view photograph of the optically variable device of  FIG. 14A . 
     
    
    
     DETAILED DESCRIPTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 1  with further reference to  FIGS. 2A-2H, 3A, and 3B , a method  10  ( FIG. 1 ) of manufacturing an optically variable device  20  (the manufactured device is shown in  FIG. 2H ) may include a step  11  of providing a substrate  21  with an adhesive layer  22  ( FIG. 2B ) on the substrate  21  ( FIGS. 2A, 2B ), which may be deposited, for example, by coating or printing. The substrate  21  may also be provided with the adhesive layer  22  already present on the substrate  21 , and the adhesive  22  may require only activation, for example by heating. In a magnetic field application step  12 , a magnetic field  31  ( FIG. 3A ) is applied, for example by providing a permanent magnet  30  ( FIGS. 2C and 3A ) under the substrate  21  ( FIGS. 2C and 3A ). An electromagnet may also be used. The magnetic field  31  generated by the magnet  30  extends through and over the adhesive layer  22  ( FIG. 3A ). 
     In a flake application step  13 , magnetic flakes  23  are applied to the adhesive layer  22 , for example, by blowing the magnetic flakes  23  onto the adhesive layer  22  using a stream  27  of gas e.g. air, argon, or nitrogen, having the magnetic flakes  23  suspended in the stream  27  of gas and carried by the stream  27  of gas, as shown schematically in  FIG. 2D . Alternatively, the magnetic flakes  23  may be provided by dusting, or spreading the magnetic flakes  23  with the help of mechanical means, such as a blade, for example. Upon reaching the adhesive layer  22 , the magnetic flakes  23  may adhere to the adhesive layer  22  ( FIG. 2E ). The magnetic field  31  causes the magnetic flakes  23  to become oriented or aligned along field lines  37  of the magnetic field  31  ( FIG. 3B ). 
     Preferably, the magnetic flakes  23  are applied to the adhesive layer  22  in presence of the magnetic field  31 . In case of dusting of deposition with gaseous stream, the magnetic field  31  facilitates orientation of the magnetic flakes  23  during their flight towards the adhesive layer  22 , so that the magnetic flakes  23  may land onto the adhesive layer  22  already oriented along the magnetic field  31  lines. If the magnetic field  31  is not applied in the flake application step  13 , some of the magnetic flakes  23  may land flat on and adhere flat to the adhesive layer  22 , which may make hinder their further orientation of the magnetic flakes  23  by the magnetic field  31 . 
     In an optional adhesive layer curing step  14  of the method  10  ( FIG. 1 ), the adhesive layer  22  may be fully cured e.g. by applying heat  24  ( FIG. 2E ), ultraviolet (UV) light, etc., after application of the magnetic flakes  23  in the flake application step  13 . The adhesive layer  22  may be already partially cured (partially uncured) prior to application of the magnetic flakes  23 . 
     In a coating step  15  of the method  10  ( FIG. 1 ), the adhesive layer  22  having the magnetic flakes  23  adhered to the adhesive layer  22 , or anchored in the adhesive layer  22 , is coated with a coating layer  25  ( FIG. 2F ), for example a transparent adhesive layer or a varnish layer. The coating layer  25  may also include a semi-transparent colored layer in combination with the magnetic flakes  23 , which may be colored or non-colored. In a curing step  16 , the coating layer  25  is cured e.g. by applying heat  26 , UV light, or both ( FIG. 2G ), so as to substantially preserve the orientation of the magnetic flakes  23  after the magnetic field  31  is removed. In this step, the adhesive layer  22  may also be fully cured, from a partially or fully uncured state. 
     A second adhesive layer, not shown, may be provided on top of the adhesive layer  22  or beside the adhesive layer  22 . A second magnetic field, not shown, may be applied to the second adhesive layer, and second magnetic flakes may be provided onto the second adhesive layer in the presence of the second magnetic field so that the second magnetic flakes oriented by the second magnetic field adhere to the second adhesive layer. The second magnetic flakes may also be absent a liquid carrier or binder. The second magnetic field may be different from the magnetic field  31 , for example the second magnetic field may have a different orientation or strength, or field lines pattern. The second magnetic flakes may also be different from the magnetic flakes  23 , for example the second magnetic flakes may have different color, size, material composition, etc. Magnetic fields and different flake types may be applied consecutively to obtain multi-color 3D indicia. 
     The manufactured optically variable device  20  is shown in  FIG. 2H . The optically variable device  20  includes the substrate  21 , the adhesive layer  22  over the substrate  21 , and the magnetic flakes  23  supported by the adhesive layer  22 . The magnetic flakes  23  are adhered to the substrate  21 , and may appear extending from the substrate  32 . The magnetic flakes  23  are oriented by the magnetic field  31  ( FIGS. 3A and 3B ). Herein, the term “oriented” means that the magnetic flakes  23  are aligned, that is, disposed in a non-random, coordinated fashion. The coating layer  25  extends over the substrate  21  adjacent the adhesive layer  22 , encapsulating the magnetic flakes  23 . As seen in  FIG. 2I , a portion  23 A of the magnetic flake  23  is adhesively attached to the adhesive layer  22 , and another portion  23 B of the same magnetic flake  23  extends out of the adhesive layer  22  into the coating layer  25 . In one embodiment, the magnetic field  31  may be configured to have the field lines parallel to the surface of the substrate  21 . Most of the flakes  23  planarized by the magnetic field  31  would have one major side in contact with the adhesive layer  22 , and another major side in contact with the coating layer  25 . 
     Application of the magnetic flakes  23  and the coating layer  25  in separate steps may enable resulting optically variable devices  20  to remain quite thin. Essentially, the minimal thickness of the coating layer  25  is limited by size of individual flakes  23 . For instance, for &lt;20 micrometer sized flakes, the coating layer  25  thickness may remain as small as 20-40 micrometers. In the flake application step  13 , the magnetic flakes  23  are applied to the adhesive layer  22  absent the coating layer  25 . The magnetic flakes  23  may extend from the adhesive layer  22  e.g. by 15-20 micrometers. Once the magnetic flakes  23  adhere to the adhesive layer  22 , being oriented along the field lines  37  of the magnetic field  31 , the coating layer  25  may be applied to the adhesive layer  22  in the coating step  15 , to encapsulate the magnetic flakes  23  within the coating layer  25 , which can remain as thin as 100 micrometers. It is preferred that the coating layer  25  be substantially transparent to visible light, being colorless or colored, depending on required optical performance of the optically variable device  20 . Smaller magnetic flakes  23 , for example having an average size of 5 to 10 micrometers, may be preferable, depending on a particular printing application. 
     The magnetic flakes  23  may be reflective, e.g. the magnetic flakes  23  may have an optical reflectivity at visible wavelengths between 380 nm and 750 nm of at least 50%. Reflective magnetic flakes  23 , when oriented, for example by a spherical or conical permanent magnet, may create a visual appearance of a metallic 3D-looking object, due to apparent reflectivity varying with illumination angle and, or observation angle. The magnetic flakes  23  may also include pearlescent or multilayer color-shifting coatings, which change color upon a change of angle of observation or illumination. Flakes which include multilayer color-shifting coatings may create a visual appearance of color-shifting 3D-looking objects, and may be particularly attractive for optical security applications. The magnetic flakes  23  may also have low reflectivity, so as to appear dark or black on a light background. 
     The shape of 3D-looking objects depends on shape and magnetization direction of the magnet  30  placed under the substrate  21  ( FIG. 3A ). The magnet  30  may be shaped and oriented to create the magnetic field  31  of a particular configuration. Furthermore, the resulting 3D looking shape may be inverted by flipping over the structure of the optically variable device  20 . 
     Turning to  FIGS. 4A-4C  with further reference to  FIG. 1 , an optically variable device  40  may be manufactured using the method  10  of  FIG. 1 . A substrate  41  of the optically variable device  40  includes a release layer  41 A. The coating layer  25  may be adhered to a second substrate  42  as shown in  FIG. 4A . The release layer  41 A may be then removed as shown in  FIG. 4B , resulting in the optically variable device  40  being supported upside down by the second substrate  42 , as shown in  FIG. 4C . 
     Referring to  FIG. 5 , the adhesive layer  22  may include voids  22 A in the adhesive layer  22 , e.g. forming visible indicia such as the number “100”, for example. The voids  22 A in the adhesive layer  22  may be formed using any suitable method, such as silk screen printing or other printing methods, lithography, etc. Once the magnetic flakes  23  are applied to the adhesive layer  22  in the flake application step  13  of the method  10 , the magnetic flakes applied to the voids  22 A may be removed, for example, by directing a flow of gas on the voids  22 A or by shaking. Masking may be applied while printing the adhesive, and, or providing the magnetic flakes  23 , and, or providing further coating. For added security, the magnetic flakes  23  may optionally include a diffractive pattern and, or covert identification indicia discernible under magnification. 
     Several prototypes of the optically variable device  20  ( FIG. 2H ) have been manufactured, and optically variable performance of the prototypes has been evaluated. Referring to  FIGS. 6A, 6B, and 7A-7C , with further reference to  FIG. 2H , a plan-view photograph ( FIG. 6A ) of a prototype of the optically variable device  20  ( FIG. 2H ) is shown. The adhesive layer  22  of the prototype of  FIG. 6A  included an adhesive ink layer, the magnetic flakes  23  included a color-shifting magnetic pigment changing color from gold at normal angle of viewing to green color at oblique angles. The coating layer  25  included varnish. The adhesive ink was cured prior to application of the varnish. 
     To provide a 3D appearance of a metal ball image  60  seen in the photograph of  FIG. 6A , a spherical-cylindrical magnet pair including a spherical magnet  71  atop a cylindrical magnet  72  ( FIG. 7A ) has been placed under the optically variable device  20 . The direction of viewing of  FIG. 6A  is shown in  FIG. 7A  at  74 A. The direction of viewing  74 A is shown in  FIGS. 7A and 7B  superimposed with the spherical  71  and cylindrical  72  magnets only to illustrate the geometry of the magnets in relation to the geometry of observation. For an actual observation, the spherical  71 -cylindrical  72  magnet pair was removed. In  FIG. 6B , the same prototype is viewed at an oblique angle shown in  FIG. 7B  at  74 B.  FIG. 7C  shows a plan view of the spherical  71 -cylindrical  72  magnet pair. 
     Referring to  FIGS. 8A, 8B, and 9A-9C , with further reference to  FIGS. 2H and 6A , a prototype of  FIG. 8A  has a similar layer structure as the prototype of  FIG. 6A , the only difference being the position of the spherical magnet  71  ( FIG. 9A ) in the magnet pair used to orient the magnetic flakes  23  ( FIG. 2H ). In  FIG. 9A , the direction of viewing is shown at  74 A. In  FIGS. 9A and 9B , the spherical magnet  71  is positioned close to an edge of the cylindrical magnet  72 , resulting in a shifted position of a metal ball image  80  in  FIGS. 8A and 8B . In  FIG. 8B , the prototype of  FIG. 8A  is viewed at an oblique angle, as shown in  FIG. 9B  at  74 B.  FIG. 9C  shows a plan view of the spherical  71 -cylindrical  72  magnet pair. 
     Turning to  FIGS. 10A, 10B, and 11A-11C , with further reference to  FIGS. 2H and 6A , a prototype of  FIG. 10A  has a similar layer structure as the prototype of  FIG. 6A , the only difference being that instead of the spherical  71 -cylindrical  72  magnet pair, a cylindrical  111 —rectangular  112  magnet pair ( FIGS. 11A-11C ) is used to orient the magnetic flakes  23  ( FIG. 2H ) to form an image of a 3D cone  100  within a round-cornered rectangle  101  ( FIGS. 10A, 10B ). The direction of viewing corresponding to  FIG. 10A  is shown at in  FIG. 11A  at  74 A. In  FIG. 10B , the same prototype is viewed at an oblique viewing angle shown in  FIG. 11B  at  74 B.  FIG. 11C  shows a plan view of the cylindrical  111 —rectangular  112  magnet pair. 
     The cylindrical  111 —rectangular  112  magnet pair shown in  FIGS. 11A-11C  has been used to orient the magnetic flakes  23  in prototypes of  FIGS. 12A, 12B, 13A, 13B, 14A , and  14 B described below. These prototypes have been manufactured with different layer materials, using varying layer curing schedules. 
     In a prototype shown in  FIGS. 12A and 12B , the adhesive layer  22  ( FIG. 2H ) included not adhesive ink but a same varnish material as the coating layer  25 . The varnish of the adhesive layer  22  was cured after application of the Go/Gr color-shifting magnetic pigment flakes. The 3D effect was present, as can be seen by comparing  FIGS. 12A and 12B , when the varnish was used in the adhesive layer  22 . 
     In a prototype shown in  FIGS. 13A and 13B , a UV-curable adhesive ink was used to form the adhesive layer  22 . The UV-curable adhesive ink was pre-cured by UV light prior to application of achromatic magnetic flakes  23 , which included 5 layers MgF 2 /Al/magnetic layer/Al/MgF 2 . The 3D cone was not observed. In a prototype shown in  FIGS. 14A and 14B , the UV-curable adhesive ink was not pre-cured prior to application of the achromatic magnetic flakes  23 . Rather, the UV-curable adhesive ink was cured after the application of the achromatic magnetic flakes  23 . 3D cone features  141 A ( FIG. 14A ) and  141 B ( FIG. 14B ) were observed with this prototype. Therefore, it may be preferable to cure the adhesive layer  22  ( FIG. 2E  and step  14  of the method  10  of  FIG. 1 ) after application of the magnetic flakes  23  on the adhesive layer  22  ( FIG. 2D  and step  13  of the method  10  of  FIG. 1 ). 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.