RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 61/515,422, filed August 5, 2011, which is incorporated herein in its entirety by reference.

TECH N ICAL FI ELD

[0002] The present invention generally relates to a method for producing complex color shifting effects, and more particularly relates to a method for altering the angle of light reflected by a thin-layer element with color shift effect.

BACKGROU ND AND SU M MARY OF TH E INVENTION

[0003] Security devices (e.g., security threads, strips, patches, overlays and transfer layers) are used widely in security documents such as banknotes, passports and other high value documents. Typically, they are incorporated into the security document during manufacture although in some cases they are adhered onto a surface of the document after manufacture.

[0004] Efforts to increase the security of these devices have included the use of multi-layer thin-layer elements, the color effect of which changes with the viewing angle of the viewer. The color effect is based on interference effects that result from multiple reflections in the various partial layers of the thin-layer element (see, e.g., EP 0 395 410 Bl).

[0005] Attempts have been made to alter the surface angle and thus the light reflected by these thin-layer elements, but such attempts either produce relatively coarse design transitions, or require modifications to the method used to manufacture the thin-layer element.

[0006] The present inventor has developed a method for altering the angle of light reflected by the above-referenced multi-layer thin-layer elements as well as other thin-layer elements with color shift effect that produces improved design transitions and does not require a change in the way these thin-layer elements are made.

[0007] In particular, the present invention provides a method for producing complex color shifting effects, the method comprising applying a plurality of micro optic structures to at least a portion of a surface of a thin-layer element with color shift effect. [0008] The present invention further provides a security device that comprises a thin-layer element with color shift effect and a plurality of micro optic structures applied to at least a portion of a surface thereof, those portions of the element's surface with applied micro optic structures displaying colors different from colors displayed by those portions of the element's surface with no applied micro optic structures.

[0009] Thin-layer elements with color shift effect (i.e., various colors with varying angle of observer) suitable for use in the present invention include elements made up of one or more thin layers having at least one region that exhibits a color shift effect. The layer(s) may be at least partially coated with, imprinted or embossed, or formed from a color shifting pigment, ink, foil, or bulk material. These thin-layer elements can be used for anti-counterfeiting because the color shift effect cannot be reproduced by color copiers and color printers.

[0010] In an exemplary embodiment, the thin-layer element with color shift effect is a color shift film (CSF). CSFs are multilayer structures having the intrinsic property of color shift effect. In particular, CSFs are made by the deposition of thin dielectric coatings over a reflective surface, which causes the film to exhibit at least two distinct colors to the observer.

[0011] Micro optic structures used in the present invention cause refraction or diffraction of the light coming from the surface of the thin-layer element, altering its perceived color at a given angle.

[0012] Suitable micro optic structures are structures that effect controlled refraction and/or diffraction of light emanating from an underlying planar surface (i.e., the thin-layer element). These structures may adopt both uniform and non-uniform shapes (e.g., squares, circles, rectangles, pentagons, triangles, hexagons, ellipses, elongated shapes, curved shapes, etc.) and may have one or more flat faces that are non-parallel relative to the underlying planar surface.

[0013] Examples of suitable structures that effect controlled refraction of light include, but are not limited to, symmetrical and non-symmetrical prisms including prisms having a uniform or non- uniform profile along an axis, curved surface prisms, curved axis prisms, curved face prisms, stepped face prisms, and combinations thereof. Specific examples of suitable structures include multi-faceted prisms such as prisms having triangular cross-sectional shapes (e.g., right angle prisms having one 90° angle and two other angles of the same or different measure), Porro prisms (i.e., a right angle prism with roof), four-sided pyramids, five-sided or pentaprisms, and hexagonal prisms (i.e., a prism with an hexagonal base), as well as cones. [0014] Examples of suitable structures that effect controlled diffraction of light include, but are not limited to, diffractive arrays (e.g., linear, curved and circular diffractive lenticular arrays, blazed gratings, blazed binary gratings, phase gratings, one-dimensional and two-dimensional diffractive arrays, holographic diffractive arrays), diffraction gratings (e.g., Fresnel diffraction structures (zone plates or phase plates)), doublets, and combinations thereof. Specific examples of suitable structures include linear diffractive optical elements (DOEs) that combine smooth or stepped surfaces with specific layout spacing and heights that interfere with the wavelength of light to bend additional light, and diffractive elements with both binary and analog phase profiles.

[0015] The micro optic structures are arranged on at least a portion of a surface of the thin- layer element in either a regular or irregular format, with the structures positioned contiguous to, or spaced apart from adjacent structures. In one exemplary embodiment, the micro optic structures are arranged in clusters or groups on a surface of the thin-layer element.

[0016] The geometry of the micro optic structures (including the facet geometry for multi- faceted micro optic structures) and the way in which these structures are arranged on the thin-layer element are used to craft different optical effects. As will be readily appreciated by those skilled in the art, variations in the geometries of these structures, the facet angle (i.e., the angle between a facet or face of a multi-faceted micro optic structure and the underlying planar surface) and the angle of orientation (i.e., the angle between a chosen axis of a micro optic structure and a chosen axis on the underlying planar surface) allow for precise control over the visual effects perceived by a viewer under various tilting behaviors. Contemplated visual effects include switching, morphing, moving, rolling, and other types of changes to the perceived regions of colors.

[0017] The present invention also provides sheet materials that employ the inventive security device, as well as documents made from these materials. The term "documents", as used herein designates documents of any kind having financial value, such as banknotes or currency, and the like, or identity documents, such as passports, ID cards, driving licenses, and the like, or non-secure documents, such as labels. The inventive device is also contemplated for use with consumer goods as well as bags or packaging used with consumer goods.

[0018] Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and accompanying drawings.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present disclosure may be better understood with reference to the following drawings. Matching reference numerals designate corresponding parts throughout the drawings, and components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. While exemplary embodiments are disclosed in connection with the drawings, there is no intent to limit the present disclosure to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents.

[0021] Particular features of the disclosed invention are illustrated by reference to the accompanying drawings in which:

FIG. 1 is a schematic side view representation of a thin-layer element with color shift effect showing two different colors of light reflected from the thin-layer element at three different viewing angles;

FIG. 2 is a schematic top plan view representation of an exemplary embodiment of the security device of the present invention in which a micro-optic structure in the form of an elongate right angle prism is shown on a top surface of the thin-layer element depicted in FIG. 1;

FIG. 3 is a schematic side view representation of the security device depicted in FIG. 2, showing that portion of the surface of the thin-layer element with applied elongate right angle prism refracting a color different from the color reflected by that portion of the element's surface with no applied prism;

FIG. 4 is a schematic side view representation of another exemplary embodiment of the inventive security device with an array of elongate right angle prisms applied to a portion of the surface of a thin-layer element, the array of prisms refracting a color different from the color reflected by the portion of the element's surface with no applied prisms. FIG. 4a represents a portion of FIG. 4 showing the colors as reversed when tilting the device;

FIG. 5 is a schematic side view representation of yet another exemplary embodiment of the inventive device with two clusters or groups of elongate right angle prisms with changing facet angles on a surface of a thin-layer element, each prism in each cluster or group refracting a different color with corresponding prisms in each cluster or group refracting the same color, the colors refracted by these prisms being different from the color reflected by the portion of the element's surface with no applied prisms;

FIG. 6 is a schematic top plan view representation of groups or clusters of elongate right angle prisms of various lengths and at various angles, positioned at different locations on a surface of a thin- layer element;

FIG. 7 is a schematic top plan view representation of an exemplary embodiment of the inventive security device in which groups or clusters of right angle prisms having different angles of orientation are used to form different color combinations and moving effects as the device is tilted in four different directions in the x and y planes;

FIGS. 8 to 10 are schematic side view representations at three different viewing angles of yet another exemplary embodiment of the inventive device in which two micro-optic structures in the form of elongate Porro prisms are located on a top surface of a thin-layer element, the device exhibiting one color when viewed perpendicular to the planar surface of the thin-layer element, and exhibiting the same two colors when the device is tilted either left or right;

FIG. 11 is a schematic side view representation of another exemplary embodiment of the inventive security device in which three elongate non-symmetrical prisms are applied to a top surface of a thin-layer element; and

FIG. 12 is a schematic side view representation of yet another exemplary embodiment of the inventive device in which elongate curved face prisms are located on a top surface of a thin-layer element.

DETAILED DESCRIPTION OF TH E INVENTION

[0022] Exemplary embodiments of the inventive security device will now be disclosed in connection with the drawings. There is no intent, however, to limit the present disclosure to the embodiments disclosed herein. For example, while the micro optic structures are mainly shown and described as multi-faceted refractive structures and specifically as elongate right angle prisms contained in a single layer on a surface of the thin-layer element, these structures are not so limited. These structures may also have curved sides (e.g, circular cones), may assume any of the uniform or non- uniform shapes mentioned above, may constitute diffractive structures or combinations of refractive and diffractive structures, and may be contained in multiple layers. Moreover, these micro optic structures are not limited to raised three-dimensional structures but may also be obtained by forming three-dimensional concavities on or in a surface of the thin-layer element.

[0023] Thin-layer elements with color shift effect suitable for use in the present invention are made up of one or more thin layers having at least one region that exhibits a color shift effect. The region(s) exhibits a spectral shift and hence a visual color shift that varies with the viewing angle. The amount of color shift is dependent on the materials used to form the layer(s) and the thickness of the layer(s). Moreover, color shift components may, at certain wavelengths, exhibit the property of higher reflectance with increased viewing angle. One such thin-layer element is shown in FIG. 1, marked with reference numeral 10. Two different colors of light 12a, 12b are shown as reflected from the thin-layer element 10 at three different viewing angles A, B, C.

[0024] As noted above, the thin-layer element may be at least partially coated with, imprinted or embossed, or formed from a color shifting pigment (e.g., liquid crystal flakes), ink (e.g., liquid crystal color shifting ink), foil, or bulk material, and in an exemplary embodiment, is a CSF.

[0025] Color shifting inks are available from SICPA Securink Corporation, SICPA Product Security LLC, 8000 Research Way, Springfield, VA 22153, while liquid crystal materials are available from BASF Corporation North America, 100 Campus Drive, Florham Park , NJ 07932.

[0026] CSFs are available from JDS Uniphase Corporation, 430 North McCarthy Boulevard,

Milpitas, California 95035, under the trade designation Color Shift Film, and from Giesecke & Devrient GmbH, Prinzregentenstrasse 159, D-81677, M unich, Germany under the trade designation Color A/Color B Color Shift Foil.

[0027] Preferred thicknesses of these thin-layer elements range from about 50 nanometers

(nm) to about 3 microns, while more preferred thicknesses range from about 200 nm to about 1000 nm.

[0028] The micro optic structures used in the present invention offer precise light control, causing controlled refraction or diffraction of light emanating from an underlying thin-layer element with color shift effect. As noted above, specific examples of suitable refractive micro optic structures include multi-faceted structures such as prisms having a triangular cross-sectional shape, Porro prisms, four-sided pyramids, pentaprisms, and hexagonal prisms, as well as cones. In one exemplary embodiment, the angle of light is controlled by prisms only microns away, which may advantageously impact upon the level of brightness exhibited by the inventive security device. As also noted above, specific examples of suitable diffractive micro optic structures include linear DOEs that combine smooth or stepped surfaces with specific layout spacing and heights that interfere with the wavelength of light to bend additional light, and diffractive elements with both binary and analog phase profiles.

[0029] The design or geometry of the micro optic structures can impact upon the amount of light and the wavelength(s) of light that are observed when the inventive security device is viewed at any given angle. The net color perceived is the sum of the light emerging from the thin-layer element with color shift effect as changed by the micro optic structures. This includes the light changed by external as well as internal reflections in addition to the light observed (for multi-faceted micro optic structures) from the intended facet face and other facet faces (e.g., a side facet face), which could range from vertical to parallel to the element's surface. In specific regard to right angle prisms, two colors would be viewed, one color from the intended facet face, which in a preferred embodiment constitutes the dominant color, and one color from the vertical side of the prism, those two colors possibly forming a third "dirty" color. Light scattering also impacts upon the observed color(s). Intentional light scattering can be used to cause white or more neutrally colored regions along a surface of the thin-layer element.

[0030] Snell's law (a.k.a the law of refraction) allows one to calculate the refraction of light as it goes from a micro optic structure to the air on its path to an observer. As such, the effect of, for example, the refractive index of the prism, the prism angle and the observer angle can be precisely modeled. This coupled with the color curve of the particular thin-layer element with color shift effect allows for predictive modeling of the final effect exhibited by the inventive security device. The Fresnel equations may also be used to model surface and internal reflections. The term "final effect", as used herein, is intended to mean the different optical effects that are observed as the viewing angle of the inventive device changes.

[0031] The micro optic structures may have a length along a major axis ranging from about 1 micron to many hundreds of millimeters. Preferred lengths range from about 10 to about 100 microns (more preferred, from about 15 to about 30 microns). The micro optic structures may have a width along a minor axis that ranges from about 1 to about 100 microns. Preferred widths range from about 3 to about 50 microns (more preferred, from about 5 to about 30 microns). The micro optic structures may have a height along a minor axis ranging from about 1 to about 100 microns. Preferred heights range from about 1 to about 50 microns (more preferred, from about 1 to about 8 microns). As will be readily appreciated, due to the very small size of these micro optic structures, the degree of resolution achieved by these structures may exceed the resolution of the human eye. [0032] In addition to the use of micro optic structures of consistent size and shape across all or part of the surface of the thin-layer element, the present invention also contemplates the use of micro optic structures having random or varying sizes and shapes. By way of example, the multi-faceted micro optic structures may have decreasing or increasing facet angles or apex angles to vary the perceived optical effect. By way of further example, these structures may have random or varying lengths to disrupt light interference patterns that result from consistent size objects. Similar to anti-aliasing methods or techniques, the sizes of these structures may be further manipulated to allow increased image resolution along detail boundaries.

[0033] No specific limitation is placed on the material used to prepare the micro optic structures provided that the material is transparent and has a certain strength or rigidity. Both inorganic materials (e.g., glass, quartz, silicon) and organic materials can be used advantageously. In some applications it may be advantageous to use materials that have substantially no visible light absorption ability. In other applications it may be advantageous to use materials having specific light absorption characteristics such as tinted resins, which aid in further customizing the optical effect by shifting the spectrum and thereby altering observed colors.

[0034] Examples of suitable organic materials include acetate resins such as triacetyl cellulose; polyester resins such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN); polyether sulfone resins, polysulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, polypropylene resins, acrylic resins, polynorbornene resins, cellulose resins, epoxy resins, polyacrylate resins, polystyrene resins, polyurethane resins, polyvinyl alcohol) resins, polyvinyl chloride resins, polyvinylidene chloride resins, and polyacrylic resins. These resins may be used individually or in combinations of two or more thereof.

[0035] In a preferred embodiment, the micro optic structures are prepared using a substantially transparent or clear radiation curable resin that includes, but is not limited to, acrylics, epoxies, polyesters, acrylated polyesters, polypropylenes, urethanes, acrylated urethanes, and the like. In a more preferred embodiment, the micro optic structures are formed using an acrylated urethane, which is available from Lord Corporation, World Headquarters, 111 Lord Drive, Cary, NC 27511-7923 USA, under the product designation U107.

[0036] The arrangement of micro optic structures may be cast directly on a surface of the thin- layer element, or may be cast onto a thin transparent film that is subsequently laminated to a surface of the thin-layer element. [0037] Forensic artifacts may be applied on or within a surface of the micro optic structures.

These artifacts may be in the form of letters, codes, numbers, or other objects, or intentional micro optic structure size variations, observable under high magnification but not visible to the human eye upon observation of the inventive security device.

[0038] Optional coatings that serve to coat or either partially or totally embed the micro optic structures are also contemplated by the present invention. For example, the micro optic structures may be coated with, for example, anti-reflection coatings, anti-soiling coatings and/or adhesive coatings. Higher or lower refractive index materials may be used to partially or totally embed the micro optic structures so as to achieve a substantially smooth or continuous outer surface. This filled surface can be used for its optical properties as in a doublet construction where the filled portion acts as a light bending part of the micro optic structure. Patterned adhesive coatings may also be applied to the micro optic structures and a polymer film laminated thereto for the purpose of maintaining an air interface between the micro optic structures and the polymer film while providing a flat outer surface to the security device.

[0039] In FIG. 2, an exemplary embodiment of the security device of the present invention is shown generally at 14. Security device 14 comprises a micro optic structure in the form of an elongate right angle prism 16 located on a top surface of thin-layer element 10 (as depicted in FIG. 1). As noted above, multi-faceted micro optic structures used in the practice of the present invention cause refraction of the light coming from the surface of the thin-layer element, altering its perceived color at a given angle. As best shown in FIG. 3, portions or regions of the surface of the thin-layer element 10 with applied elongate right angle prism 16, when viewed at a perpendicular viewing angle, display a color 18a that is different from the color 18b displayed by those portions or regions of the element's surface with no applied prism(s). It is noted that when viewing at non-perpendicular viewing angles along either the security device's longitudinal or transverse axis, the colors perceived from both regions may be the same or different from each other, but different from the colors displayed by those regions at perpendicular viewing angles.

[0040] The micro optic structures may be contained in one or more regions covering all or part of the surface of the thin-layer element. The micro optic structures in each such region may be configured in one- or two-dimensional arrangements having regular, random or varying sizes and spacing. As noted above, the geometry of the micro optic structures (including the facet geometry of multi-faceted micro optic structures) and the way in which these structures are arranged on the thin- layer element are used to craft different optical effects. Variations in the geometries of these structures (e.g., the facet angle(s)) and their angle of orientation allow for precise control over the visual effects perceived by a viewer under various tilting behaviors.

[0041] In another exemplary embodiment, a plurality of micro optic structures in the form of elongate right angle prisms are arranged on the thin-layer element of the inventive security device in a direction perpendicular to the longitudinal or lengthwise direction of the device. When formed with consistent facet angles relative to a surface of the thin-layer element (see FIG. 4), a single color 18a is observed at a perpendicular viewing angle, while a different color 18b is observed from portions of the element's surface with no applied prisms. As shown in FIG. 4a, those colors reverse when the device is tilted left (i.e., when viewed from a non-perpendicular viewing angle). When the elongate right angle prisms are formed at two different facet angles but at the same angle of orientation on the thin-layer element (not shown), a set of two different colors would be observed at a perpendicular viewing angle, while another set of two different colors would be observed at a non-perpendicular viewing angle. Similarly, when the elongate right angle prisms are arranged at the same angle of orientation in regions on a surface of the thin-layer element with the prisms in each region having a different facet angle (not shown), many zones of colors at different degrees of color change would be observed at a perpendicular viewing angle, while many zones of different colors at different degrees of color change would be observed at non-perpendicular viewing angles.

[0042] The type and relative speed of the color change that is observed is controlled in the exemplary embodiment shown in FIG. 4 by the angle of orientation of the elongate right angle prisms and the direction of tilt of the inventive security device. In particular, by arranging the hypotenuse face of the group of elongate right angle prisms to run perpendicular to the longitudinal direction of the inventive security device (instead of arranging the hypotenuse face of the prisms to run parallel to the longitudinal direction), a faster optical or color change will occur when the device is tilted left-to-right or right-to-left. The change in the observed color presented by the right angle prisms is proportional to the change in the facet or hypotenuse angle (relative to the observer) as the security device is tilted. The observed color is a combination of the degree of refraction, which is dependent upon the observation angle, and the angle of observation of the thin-layer element. When the security device is tilted front- to-back or back-to-front, the change in the observed color is somewhat slower and represents a combination of the color change of the thin-layer element and the fixed amount of refraction effected by the right angle prisms. In other words, when tilted in this direction, the amount of refraction is fixed at different tilt angles. As the shape, size, and orientation of these micro optic structures change, as well as the direction of tilt of the security device, the type and relative speed of perceived optical effects will also change.

[0043] Referring now to the exemplary embodiment shown in FIG. 5, proceeding in a longitudinal direction along thin-layer element 10, elongate right angle prisms 16 have gradually decreasing facet angles (or increasing apex angles) in a first region 20 and gradually increasing facet angles (or decreasing apex angles) in a second region 22. A gradual change in perceived colors (i.e., a rolling effect) is exhibited by this security device, which starts and ends with the same color as the inventive device is tilted from left-to-right and from right-to-left. This gives the observer the perceived effect that the colors are moving from one region to another as the device is tilted. In this exemplary embodiment, the prisms 16 have quite steep facet angles at one end of each region and quite shallow facet angles at the other end of each region. Preferably, these facet angles (or apex angles) fall in the range of from about - 80 to about 80 °, more preferably, in the range of from about -65 to about 65 °.

[0044] Rolling effects similar to that demonstrated by the security device shown in FIG. 5 can also be achieved using stepped face prisms, or by using a series of similar triangular prisms in which a gradual twisting of the facet angle occurs over either a short or long distance to cause a rolling effect perpendicular to the axis of tilting.

[0045] While adjacent regions 20, 22 shown in FIG. 5 have low levels of contrast in terms of color and optical effects (e.g., rolling, moving), these regions may also be made to have high levels of contrast relative to each other. As will be readily appreciated, the level of contrast would depend upon the degree to which facet angles and/or orientation angles change from one region to an adjacent region. High and low levels of contrast are contemplated by the present invention as well as combinations thereof (e.g., fast changing contrast and slow rolling effects).

[0046] In FIG. 6, elongate right angle prisms 16 of various lengths and angles of orientation are shown in groups or clusters on a surface of thin-layer element 10. These prisms, as explained above, achieve various amounts of refraction depending upon the angle the facet or hypotenuse face has relative to the observer. By using prisms with different angles of orientation, the change in facet angles relative to the observer upon tilt will also be different. If, for example, a surface of a thin-layer element is covered with 45 degree prisms at various angles of orientation, they would all have the same color when viewed from a perpendicular viewpoint. However, when you tilt the security device in any direction, each prism would have a different facet angle relative to the observer. As noted above, the different observed colors result from a combination of the degree of refraction, which is dependent upon the facet angle relative to the observer, and the angle of observation of the thin-layer element.

[0047] In FIG. 7, an exemplary embodiment in which groups or clusters of elongate right angle prisms 16 having different angles of orientation are used to form different color combinations and moving effects as the device is tilted in four different directions in the x and y planes. Here, the arrows show the direction the prisms are facing. This so-called wagon wheel arrangement may be used to produce circular fluid movement as the device is tilted rotationally.

[0048] Curved or circular micro optic structure orientations, for example, concentric circles prepared from elongate right angle prisms, or Porro prisms, may also be used to achieve rotational movement and unusual color behavior. Rainbow effects are also possible. By gradually changing the rotational orientation of the micro optic structures within select regions on the thin-layer element, it is possible to have rainbow-like color regions that are both different from each other and resistant to change as the inventive device is tilted in a given direction. Contemplated embodiments include devices in which the foreground and background change upon different tilt angles. For example, the background could be designed to change when the device is tilted up, while the foreground could be designed to change when the device is tilted down. Conversely, the foreground could change when the device is tilted up, while the background could change when the device is tilted down. Such regions could provide design contrast or form part of a more complex optical effect(s).

[0049] Referring now to FIGS. 8 to 10, micro optic structures in the form of Porro prisms 24 are shown. These prisms exhibit one color when viewed from a perpendicular viewpoint (see FIG. 8), and when tilted left (see FIG. 9) or right (see FIG. 10) produce the same optical effect (i.e., exhibit the same two colors), which is different from the optical effect exhibited when viewed perpendicularly. Symmetrical rolling effects are possible with this design, with the rolling moving in the same direction if you tilt the inventive device up or down. As you move past an observation point perpendicular to the plane of the inventive device, the perceived direction or behavior of the effect would reverse. When tilted right-to-left or left-to-right, the effect would not be symmetrical and would not reverse.

[0050] In addition to the uniform multi-faceted micro optic structures mentioned above, nonuniform structures such as the structures 26 shown in FIG. 11 and the curved face prisms 28 shown in FIG. 12 are also contemplated for use in the present invention.

[0051] The micro optic structures may also be configured to form simple designs such as text or solid objects, and also complex designs. [0052] In regard to text designs, and in an exemplary embodiment, the text and background regions are made up of elongate right angle prisms having different facet angles. For example, the hypotenuse face of right angle prisms in the background region could face in one direction (e.g., have a facet angle of 30° left) and the hypotenuse face of right angle prisms in the text region could face in an opposite direction (e.g., have a facet angle of 50° right). This would allow for a distinct color difference between the text and background regions and allow for color swapping between the two regions. A similar approach may be taken when designing simple solid object designs.

[0053] In other exemplary embodiments of the security device of the present invention, more complex photographic or other graphic designs in the form of duotone type designs are utilized. In other words, the design is made up of two colors (color A, color B) ranging from 100 percent of color A to 100 percent of color B. These colors change as the tilt angle of the security device changes, with each color pixel of the design represented by one or more multi-faceted micro optic structures (e.g., prisms) of various facet angles and/or angles of orientation. By coordinating the various facet angles and/or angles of orientation in the duotone type design, the perceived image can be specifically manipulated. For example, the design could be made to change in different ways when tilting in different directions (e.g., tilting left-to-right versus tilting right-to-left). The design could also be made with symmetrical micro optic structures such that the design changes in the same way when tilting the device in different directions. In addition, the use of, for example, four-sided pyramids or circular cone-type micro optic structures would allow the device to exhibit the same design regardless of tilt direction. Moreover, by combining these techniques, designs, portions of designs and/or covert information could be made to appear and disappear. For example, an image of a butterfly could be designed to display a denomination number 50 in the wings when tilted left-to-right but the butterfly image would not display the number 50 when the device is tilted up and down.

[0054] As noted above, the inventive security device may be used in the form of, for example, a security strip, thread, patch, overlay, or transfer layer and mounted to a surface of, or at least partially embedded within a fibrous or non-fibrous sheet material (e.g., banknote, passport, I D card, credit card, label), or commercial product (e.g., optical disks, CDs, DVDs, packages of medical drugs), etc., for authentication purposes. The inventive device may also be used in the form of a standalone product (e.g., substrate for subsequent printing or personalization), or in the form of a non-fibrous sheet material for use in making, for example, banknotes, passports, and the like, or it may adopt a thicker, more robust form for use as, for example, a base platform for an ID card, high value or other security document.

[0055] When used in the form of a security strip, thread, patch, or overlay, the total thickness of the inventive device is preferably less than about 50 microns (more preferably, less than about 45 microns, and most preferably, from about 10 to about 40 microns).

[0056] The security strips, threads, patches, overlays, or transfer layers may be partially embedded within or mounted on, or embedded within, a surface of a document. For partially embedded strips and threads, portions thereof are exposed at the surface of the document at spaced intervals along the length of the strip or thread at windows or apertures in the document.

[0057] The inventive security devices may be at least partially incorporated in security papers during manufacture by techniques commonly employed in the papermaking industry. For example, the inventive security device in the form of a strip or thread may be fed into a cylinder mold papermaking machine, cylinder vat machine, or similar machine of known type, resulting in surface, total or partial embedment of the strip or thread on or within the body of the finished paper.

[0058] The security strips, threads, patches, overlays, or transfer layers may also be adhered or bonded to a surface of a document with or without the use of an adhesive. Bonding without the use of an adhesive may be achieved using, for example, thermal welding techniques such as ultrasonic welding, vibration welding, and laser fusing. Adhesives for adhering the inventive devices to a surface of a document may be one of hot melt adhesives, heat activatable adhesives, pressure sensitive adhesives, and polymeric laminating films. These adhesives are preferably crosslinkable in nature, such as ultraviolet (UV) cured acrylic or epoxy, with crosslinking achieved while the adhesive is in the melt phase.

[0059] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the exemplary embodiments.

[0060] We claim: