Optical device and manufacturing method thereof

Abstract

An optical device is provided that exhibits a variable optical effect upon illumination. The optical device has a diffractive structure. The diffractive structure comprises a portion of a first kinoform diffractive structure arranged to form a first diffractive region 11 of the diffractive structure, the portion encoding a first image, and a portion of a second kinoform diffractive structure arranged to form a second diffractive region 13 of the diffractive structure, the portion encoding a second image. The first and second diffractive regions are laterally separated, and each of the first and second diffractive regions is distinguishable by the naked human eye. Additionally, a method of manufacturing such an optical device is disclosed.

Description

[Technical field]

[0001] The present invention is directed to an optical device and a method for its manufacture. The invention is particularly applicable in the field of security devices, where such optical devices may be used as a means of determining the authenticity of security documents such as banknotes or passports. [Background technology]

[0002] Objects of value, particularly valuable documents such as banknotes, checks, passports, identification documents, certificates and licenses, are often targets of counterfeiters and those seeking to make unauthorized copies and / or alter the data contained therein. Typically such documents are equipped with a number of visible optical devices that act as security devices to check the authenticity of the object. By "security device" we mean features that cannot be accurately reproduced by making a visible light copy, for example, via standard available copying or scanning equipment. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, Venetian blind devices, lenticular devices, Moire interference devices, Moire magnification devices, etc. Each of the patterns produces a secure visual effect. Other known security devices include holograms, watermarks, embossing, perforations and the use of color shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult or impossible to reproduce using available reproduction techniques such as photocopying. Other security devices may be employed that exhibit invisible effects, such as magnetic materials.

[0003] One class of optical devices includes devices that produce optically varying effects. This means that the appearance of the device is different at different viewing and / or lighting angles. Such devices are particularly effective as security devices because they can be easily distinguished from genuine devices, as direct copying (e.g., photocopying) would not produce the optically varying effect. Optically variable effects can be produced based on a variety of different mechanisms, including diffractive devices such as holograms, mechanisms such as moire interference that rely on parallax, such as venetian blind devices, as well as devices that utilize focusing elements such as lenses, including moire magnifying devices, integrated imaging devices, and so-called lenticular devices.

[0004] As counterfeiting techniques become increasingly sophisticated, there is an increasing need to improve the security level of security devices. Summary of the Invention

[0005] According to a first aspect of the invention there is provided an optical device exhibiting a variable optical effect upon illumination, the optical device having a diffractive structure comprising a portion of a first kinoform diffractive structure arranged to form a first diffractive region of the diffractive structure and encoding a first image, and a portion of a second kinoform diffractive structure arranged to form a second diffractive region of the diffractive structure and encoding a second image, the first diffractive region and the second diffractive region being laterally separated, each of the first diffractive region and the second diffractive region being distinguishable by the naked human eye.

[0006] The invention has the advantage that it provides an optical device with a high level of security by exploiting the diffractive properties of kinoform diffractive structures to generate variable optical effects that are easy to authenticate but difficult to counterfeit.

[0007] A "kinoform" can be described as a relief structure that modifies an incident electric field according to a calculated phase φ(x,y) of the electric field propagating from an object to an image on a "hologram plane". In a kinoform diffraction structure, the field phase φ(x,y) is recorded in such a way that when illuminated, it shows a reconstructed image of the object. In this way, the kinoform diffraction structure "encodes" the respective (diffraction) image. In this specification, the terms "kinoform" and "kinoform diffraction structure" may be used interchangeably.

[0008] Computer-generated holography (CGH) techniques are suitable for generating such kinoform diffraction structures, and the diffraction structures are preferably generated by a computer. When using computational techniques to synthesize kinoform diffraction structures, the object may be approached by multiple point sources, and the kinoforms of each point source may be superimposed to form the kinoform diffraction structure of the object. The kinoform of the point source has the form of a Fresnel lens and can be approximated by a Fresnel zone plate (or a "binary kinoform") to distribute the reconstructed light evenly in the positive and negative diffraction orders. The kinoform of the point source in the form of a Fresnel lens is a multi-level structure with multiple structure depths. Such a multi-level structure can be approximated to a binary structure using thresholding of the structure depth. In the present invention, a binary structure is preferred because the reconstructed light is uniformly distributed in the positive and negative diffraction orders, and typically each of the first kinoform diffraction structure and the second kinoform diffraction structure (and any additional kinoform diffraction structures used) is a binary kinoform diffraction structure. However, both multi-level and binary structures may be utilized within a device to provide different reproduction effects (eg, depth and brightness) with changing viewing angle.

[0009] Although CGH techniques are computationally intensive, they have the advantage of not requiring the presence of physical objects or complex optical arrangements to record the interfering light patterns and generate the diffractive structures.

[0010] Additional background on kinoforms is found in "The Kinoform: A New Wavefront Reconstruction Device," LBLesem, PM Hirsch and JA Jordan Jr. IBM J. Res. Develop (1969).

[0011] When illuminated, a kinoform diffractive structure encoding a particular image reproduces an image that is perceived to exhibit full parallax motion; that is, the reproduced image is perceived to move depending on both the "vertical" (north-south) and "horizontal" (east-west) tilt of the diffractive structure. Furthermore, the reproduced image is perceived to be positioned in a plane in front of or behind the plane of the device depending on the viewing angle and the particular part of the kinoform diffractive structure to which the incident light is diffracted. If the diffracted light rays from the device converge and are focused at the observer's eye, the reproduced image will appear to "float" in front of the plane of the device. In contrast, if the light rays diffracted by the optical device diverge towards the observer, the reproduced image will appear to be behind the plane of the device as a virtual image.

[0012] In general, each portion of such a kinoform diffractive structure will reproduce the encoded image, but different reproduction effects will occur depending on the particular portion.

[0013] The optical device of the present invention has a diffractive structure that includes a portion of each of the first and second kinoform diffractive structures. In other words, the portion of the optical device is a portion of each "complete" kinoform diffractive structure that encodes a respective image. For example, the portion of the first kinoform diffractive structure may be half (e.g., the "top half") of the first kinoform diffractive structure, and the portion of the second kinoform diffractive structure may be the complementary half (e.g., the "bottom half") of the second kinoform diffractive structure. In this way, the portion of the kinoform diffractive structure of the present invention can be considered to be a portion of each "complete" or "full" kinoform diffractive structure. A "complete" kinoform can be described as such because it includes the complete phase function of the electric field propagating from the image object.

[0014] Thus, in the present invention, the relative motion and depth effects of kinoform diffractive structures are advantageously combined with the specific reproduction effects of their particular parts to produce striking visual effects that are difficult to replicate.

[0015] Portions of the first and second kinoform diffractive structures are arranged to form respective first and second (e.g., continuous) diffractive regions of the diffractive structures. Thus, the first image is generated almost entirely by diffraction of incident light from the first diffractive region, and the second image is generated almost entirely by diffraction of incident light from the second diffractive region. The first and second diffractive regions may be described as substantially continuous in the sense that they are uninterrupted across their respective domains. In other words, the first and second diffractive regions do not intersect with each other.

[0016] Each of the first and second diffractive regions is distinguishable by the naked human eye (and therefore each part of the kinoform diffractive structure is distinguishable by the naked human eye). In other words, both the first and second diffractive regions have dimensions such that they are visible to the naked human eye, i.e. without the use of magnifying instruments. For this reason, both the first and second diffractive regions can be described as "macroscopic". In this way, when viewing the device, the observer can perceive the full size of each image reproduced by each arranged diffractive portion, together with the specific reproduction characteristics of that portion. In particular, each of the first and second diffractive regions has dimensions at least the size of the reproduced image, so that the complete image can be observed at at least one viewing angle, depending on the reproduction characteristics of the portion of the "complete" kinoform diffractive structure used. Furthermore, by using such "macroscopic" first and second diffractive regions, the image reproduced by the device has increased brightness, for example, compared to a crossed arrangement.

[0017] It is generally understood that the naked human eye cannot resolve dimensions less than about 150 microns at a typical viewing distance of about 30 cm. For this reason, it is preferred that each of the first and second diffractive regions have a size greater than 150 microns (e.g., 150 microns by 150 microns). However, more preferably, each of the first and second diffractive regions (and thus the dimensions of each portion) is less than 1 by 1 millimeter ("1 mm 2 "), and preferably each of the first and second diffraction regions is greater than 10 mm 2 (e.g. 2 x 5 mm) to 300 mm 2 (e.g., 30×10 mm). For this reason, it is preferable that a typical single dimension (e.g., “length” or “width”) of the diffractive region is in the range between 1 mm and 30 mm. In this way, the image reproduced by the first and second diffractive regions is easily perceived by the observer.

[0018] In this way, when the device is illuminated, the first diffractive region typically shows a first image that is perceived to be in front of the plane of the device or behind the plane of the device depending on the viewing angle, and that is perceived to move within a first image area corresponding to the first diffractive region as the viewing angle changes, and the second diffractive region typically shows a second image that is perceived to be in front of the plane of the device or behind the plane of the device depending on the viewing angle, and that is perceived to move within a second image area corresponding to the second diffractive region as the viewing angle changes. It will be understood that each of the diffractive regions will show a complete image at at least one viewing angle. At other viewing angles, for example when looking at the periphery of the diffractive region, only a portion of the image may be displayed, in which case the image may appear cropped relative to the edge of the diffractive region.

[0019] The first and second diffractive regions are laterally separated such that the first and second image areas, and therefore the first and second images, do not overlap, i.e., the reconstructed images do not overlap when the device is viewed.

[0020] In some embodiments, the first and second diffractive regions may be laterally spaced apart such that there is a "gap region" between the first and second diffractive regions. Preferably, however, the first and second diffractive regions abut one another. In other words, in preferred embodiments, the diffractive regions share a common boundary (and thus may be described as adjacent).

[0021] As discussed, typically the diffractive regions are adjacent, but in some alternative embodiments the first and second diffractive regions may be laterally spaced apart. In such embodiments, the optical device preferably further comprises a non-diffractive component located between the first and second diffractive regions. Here, preferably the non-diffractive component is adjacent to each of the first and second diffractive regions. Thus, the non-diffractive component preferably "fills" the gap region between the first and second diffractive regions. Such a non-diffractive component may be, for example, a planar metal or inkwork. The non-diffractive component is typically optically invariant and exhibits substantially the same optical effect at all angles of view (in contrast to, for example, diffractive structures).

[0022] Such "gap regions" and, optionally, the use of non-diffractive elements can be advantageously used to control the perceived optical effect at the boundary between the first and second diffractive regions, for example, to increase the security level of the device. The use of such non-diffractive elements may also be used to enhance the optical effect of the kinoform diffractive structure.

[0023] In embodiments in which the first and second diffractive regions are laterally spaced apart, the distance between the first and second diffractive regions is typically greater than 2 μm and less than 100 μm, typically between 5 μm and 50 μm.

[0024] In some embodiments, parts of the first and second kinoform diffraction structures may be corresponding parts of the respective "complete" kinoform diffraction structures. For example, the first part may be the top half of the first "complete" kinoform diffraction structure, and the second part may similarly be the top half of the second "complete" kinoform diffraction structure. Such an arrangement would mean that the displayed images would exhibit corresponding reproduction characteristics. For example, if corresponding parts of each complete kinoform are used, at a given viewing angle, the reproduced images will be perceived to be on the same side of the device (e.g. appearing to be behind the plane of the device or "floating" in front of the plane of the device) and will move in substantially the same direction when changing the viewing angle, for example by tilting the device.

[0025] However, in a particularly preferred embodiment of the invention, a portion of the first kinoform diffraction structure and a portion of the second kinoform diffraction structure are complementary. In other words, the diffraction properties of the respective portions are such that the attributes of the reproduced image are complementary. This may be achieved by using substantially opposing portions of the respective complete kinoform diffraction structures. For example, in a particularly preferred embodiment, a portion of the first kinoform diffraction structure may be the top half of the "complete" structure and a portion of the second kinoform diffraction structure may be the bottom half of the "complete" structure. In another example, a portion of the first kinoform diffraction structure may be the left half of the "complete" structure and a portion of the second kinoform diffraction structure may be the right half of the "complete" structure. However, other portions of the respective "complete" kinoform diffraction structures may be used to achieve different reproduction effects.

[0026] Preferably, a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure (e.g. the diffractive properties of such portions) are such that, at at least one viewing angle, the first image is perceived to be on one side of the plane of the optical device and the second image is perceived to be on the opposite side of the plane of the optical device. This complementary effect can produce a particularly striking effect, where one of the images appears to be "floating" in an image plane in front of the device plane and the other image appears to be in an image plane behind the device plane.

[0027] The perceived depth of each image reproduced relative to the plane of the optical device will be reversed as the viewing angle moves from one side of the direction of specular reflection (0th order) to the other. When the optical device is viewed along a viewing angle "above" the direction of specular reflection, "positive" (e.g., +1) order diffraction effects are observed and the reproduced image is perceived to have a particular depth relationship to the device plane. If the optical device is then tilted so that it is viewed "below" the direction of specular reflection, "negative" (e.g., -1) order diffraction effects are observed and the perceived depth relationship of the image is reversed.

[0028] Higher diffraction orders (e.g., second, third) will also be reproduced, but typically such diffractions are significantly reduced in intensity compared to the first order effect, so that it is the first order diffraction effect that is primarily perceived. Typically, images are visible within the first diffraction order over a viewing angle range of about 40 to 50 degrees.

[0029] Preferably, the portion of the first kinoform diffractive structure and the portion of the second kinoform diffractive structure (e.g. the diffractive properties of such portions) are such that when the device is tilted, the first and second images are perceived to move in substantially opposite directions when the device is tilted about at least one axis. This complementary kinematic effect provides a striking variable optical effect to the observer. For example, when the device is tilted vertically, the first and second images may be perceived to move in opposite directions along an axis substantially orthogonal to the tilt axis (i.e. the images are perceived to move in opposite directions along a north-south axis). Particularly preferably, when the device is tilted over two independent, preferably orthogonal, axes, the first and second images are perceived to move in opposite directions. For example, not only do the images move in opposite directions along a north-south axis when tilted vertically, but they may also be perceived to move in opposite directions along an east-west axis when the device is tilted horizontally. When the device is tilted with more complex tilts that include both vertical and horizontal tilt components, the images may be observed to move in opposite curvilinear trajectories. Thus, the device of the present invention can be described as showing a complete parallax image.

[0030] However, as discussed above, the perceived movement of each image when the device is tilted is constrained to stay within the respective image area.

[0031] Generally, the portion of the "complete" kinoform used to form each diffractive region defines the relative motion and depth of the reconstructed image, and the arrangement of the diffractive regions defines the (discrete) areas through which the reconstructed image is perceived to move as the viewing angle is changed. The diffractive regions of a device typically have substantially the same dimensions. The diffractive regions of a device may be symmetrical, especially in devices containing exactly two diffractive regions.

[0032] In an embodiment, the diffractive structure may further comprise a portion of a third kinoform diffractive structure arranged to form a third diffractive region of the diffractive structure and encoding a third image. Here, the third diffractive region is laterally separated with respect to the first diffractive region and the second diffractive region, and the third diffractive region is discernible by the naked human eye. In this way, the device may exhibit more complex depth and kinematic effects upon tilt. Typically, at a given viewing angle of the optical device, two of the reproduced images are perceived as being on opposite sides of the optical device plane.

[0033] The device of the invention may comprise portions of four or more kinoform diffractive structures arranged in respective diffractive regions.

[0034] Typically, the first and second images (and third and subsequent images, if present) are in the form of indicia such as symbols and alphanumeric characters. Typically, the first and second images (and third and subsequent images, if present) are different from one another.

[0035] According to a second aspect of the invention there is provided a method of forming an optical device exhibiting a variable optical effect upon illumination, the method comprising the steps of providing a portion of a first kinoform representation encoding a first image, providing a portion of a second kinoform representation encoding a second image, and forming a diffractive structure in a carrier layer, the diffractive structure comprising a first diffractive region corresponding to the portion of the first kinoform representation and a second diffractive region corresponding to the portion of the second kinoform representation, the first diffractive region and the second diffractive region being laterally separated and each of the first diffractive region and the second diffractive region being distinguishable by the naked human eye.

[0036] The resulting device offers all of the advantages already discussed above.

[0037] The first and second kinoform representations are typically computer generated and in the form of respective computer files encoding the kinoforms. Each kinoform representation is preferably generated by superimposing a kinoform for each point source of the object to be imaged. Typically, the kinoform representations are converted to a binary representation by a thresholding process. Typically, each of the first and second kinoform representations is a binary kinoform representation (so that each resulting kinoform diffraction structure is a binary kinoform diffraction structure), although in some embodiments one kinoform representation may be a binary representation and one kinoform representation may be a multi-level representation.

[0038] The length or width of a "complete" kinoform representation (when formed as a kinoform diffractive structure) is typically between 2 and 30 millimeters. Portions of the first and second kinoform representations are typically combined into a print file to form the diffractive structure.

[0039] The diffraction structures are typically produced in a single writing step. The diffraction structures are preferably formed in the carrier layer using maskless lithography such as electron beam lithography or direct laser writing. Such techniques can produce diffraction structures with grating spacings of less than 0.5 microns. Typically, the maximum grating spacing (or "fineness") of the kinoform diffraction structures is 3 microns or more, preferably 5 microns or more. Typically, the minimum grating spacing (or "fineness") of the kinoform diffraction structures is 0.6 microns or less, preferably 0.5 microns or less. The fineness, or "pitch" (e.g., structure scale and structure distribution) of the kinoform diffraction structures is determined as a result of the computational geometry of the desired image reproduction. For example, the object to be imaged, its dimensions, its distance from the hologram plane, the dimensions of the hologram area to be exposed are typically preselected, and the "pitch" of the kinoform diffraction structures results from the preselected geometry. In general, for a given hologram and object size, the closer the object is to the hologram plane, the finest structure of the kinoform diffractive structure will be.

[0040] The diffractive structures may be written directly into a carrier layer provided on a device substrate to directly form an optical device, but more typically the diffractive structures may be used to form replication tools for mass production of optical devices.

[0041] Preferably, the reflection enhancing layer is applied to follow the contour of the diffractive structure of the device. The surface relief of the diffractive structure may have different geometries, such as sinusoidal, rectangular or triangular geometries. The type of surface relief geometry may affect the diffraction efficiency. Typically, the geometry depth of the surface relief forming the grating elements is 1 micron or less, preferably 0.5 micron or less.

[0042] A portion of each kinoform representation is typically selected from a "full" kinoform representation of the respective image, which may be preformed or generated on demand. Selection of each portion from a provided "full" kinoform representation advantageously increases the flexibility of the formation of the optical device, since a particular portion of each kinoform representation may be selected according to the desired optical effect to be exhibited by the device. Selection of the portions of the first and second kinoform representations may be aided using exposure software capable of decoding a computer file encoding the "full" kinoform representation in the form of an image.

[0043] Thus, in an embodiment, providing a portion of a first kinoform representation comprises providing a first kinoform representation for encoding a first image and selecting a portion of the first kinoform representation, and providing a portion of a second kinoform representation comprises providing a second kinoform representation for encoding a second image and selecting a portion of said second kinoform representation. However, in some embodiments it is envisaged that portions of the first and second kinoform representations may be generated directly (i.e. without first forming a "complete" representation).

[0044] The portion of the first kinoform diffractive structure and the portion of the second kinoform diffractive structure are preferably such that (e.g. selected to be perceived) at at least one viewing angle the first image is perceived to be on one side of the plane of the optical device and the second image is perceived to be on the opposite side of the plane of the optical device. The portion of the first kinoform diffractive structure and the portion of the second kinoform diffractive structure are preferably such that (e.g. selected to be perceived) when the device is tilted the first image and the second image are perceived to move in substantially opposite directions when the device is tilted about at least one axis.

[0045] The first and second kinoform representations are typically in the form of computer files as discussed above, but may alternatively be in the form of physical diffractive structures. The diffractive structures may then be formed by providing (e.g., selecting) a portion of the physical diffractive structure.

[0046] Preferably, the first and second diffractive regions abut each other. In some alternative embodiments, the first and second diffractive regions may be laterally spaced apart. In such embodiments, the method may further include providing a non-diffractive component located between the first and second diffractive regions. Here, preferably, the non-diffractive component is adjacent to each of the first and second diffractive regions.

[0047] Preferably, each of the first and second diffractive regions has a size greater than 150 microns. More typically, each of the first and second diffractive regions has dimensions greater than 1 millimeter by 1 millimeter. Here, preferably, each of the first and second diffractive regions has a size greater than 10 mm. 2 From 300mm 2 The dimensions are in the range of

[0048] As discussed above, the present invention is suitable for use with computer-generated holography ("CGH") techniques. Typically, a portion of the first kinoform representation and a portion of the second kinoform representation are computer-generated. However, it is envisioned that classical holography techniques may be employed to generate the optical device according to the present invention. The first kinoform representation may be holographically generated by exposing a recording medium layer to an optical interference pattern from a first image object, and the second kinoform representation may be holographically generated by exposing a recording medium layer to an optical interference pattern from a second image object. A portion of the generated "classical" kinoform diffraction structure may then be selected to form a diffraction structure in the manner discussed above.

[0049] In an embodiment, one or more additional portions of each additional kinoform representation may be provided such that the formed optical device includes three or more diffractive regions corresponding to respective portions of the "complete" kinoform representation.

[0050] Particularly preferably, the optical device of the present invention is a security device. In other words, the security device may be used to prevent copying or counterfeiting of an article or document to which the security device is applied. Typically, a "security device" cannot be reproduced exactly, for example by making a visible light copy using available standard copying or scanning equipment.

[0051] According to a third aspect of the present invention there is provided a security article comprising a security device according to the present invention, preferably formed as a security thread, strip, foil, insert, patch or substrate for a security document.

[0052] In a further aspect of the invention there is provided a security document comprising a security device or security article according to the invention. The security document preferably comprises a banknote, a cheque, a passport, an identity card, a certificate of authenticity, a revenue stamp or other document guaranteeing value or the identity of an individual. Typically the security document comprises a substrate having a transparent window portion and the security device or security article is at least partially positioned within the transparent window portion. [Brief description of the drawings]

[0053] Exemplary embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Figure 1] 1 shows a plan view of a security document carrying a number of optical devices according to the invention; [Figure 2a] 1 shows the kinoform of a single point object. [Figure 2b] 1 shows a kinoform of multiple point objects arranged in a square pattern. [Figure 2c] 1 shows the binary kinoform of a single point object. [Figure 2d] 1 shows a binary kinoform of multiple point objects arranged in a square pattern. [Figure 3a] 1 shows a schematic of the variable optical effect perceived when viewing a kinoform diffractive structure in the top parallax field. [Figure 3b] 1 shows a schematic of the variable optical effect perceived when viewing a kinoform diffractive structure in the lower parallax field. [Figure 4a] 1 shows a schematic representation of the variable optical effect perceived when looking at the top and bottom of a perfect kinoform diffractive structure. [Figure 4b] 1 shows a schematic representation of the variable optical effect perceived when looking at the top and bottom of a perfect kinoform diffractive structure. [Diagram 5] 1 illustrates a schematic representation of an optical device according to an embodiment of the invention and the variable optical effect it provides; [Figure 6] FIG. 6 is a schematic plan view of the optical device shown in FIG. [Figure 7] 7 illustrates the variable optical effect that occurs when the optical arrangement shown in FIGS. 5 and 6 is tilted. [Figure 8] 7 illustrates the variable optical effect that occurs when the optical arrangement shown in FIGS. 5 and 6 is tilted. [Figure 9] 1 shows a security article in the form of a security sled carrying a plurality of optical devices according to the present invention. [Figure 10a] 1 is a schematic plan view of a security device according to an embodiment of the present invention; [Figure 10b] 1 is a schematic plan view of a security device according to an embodiment of the present invention; [Figure 11] 1 illustrates an optical device according to an embodiment of the present invention and the variable optical effects exhibited upon vertical and horizontal tilt. [Figure 12] 1 illustrates an optical device according to an embodiment of the present invention and the variable optical effects exhibited upon vertical and horizontal tilt. [Figure 13] 3 is a flow diagram illustrating a preferred method for manufacturing an optical device according to the present invention. [Figure 14] 1 illustrates exemplary geometries that may be used to form optical devices according to the present invention using classical holographic techniques. [Figure 15] 1 shows an example of incorporating a security device according to the invention into a security document. [Figure 16] 1 shows an example of incorporating a security device according to the invention into a security document. [Figure 17] 1 shows an example of incorporating a security device according to the invention into a security document. [Figure 18] 11 is a schematic plan view of an apparatus according to a further embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] 1 shows a plan view of a security document 1000 comprising a number of optical devices 100 according to the invention, where the optical devices 100 are used to increase the security level of a document, here a banknote, and are therefore security devices. The security devices 100 are provided on a security thread 105 integrated into the document substrate and are visible in a windowed area 101 of the document substrate. Each security device 100 is aligned with a corresponding windowed area so as to be visible. The document substrate may be a fibrous material such as paper or in the form of a polymeric substrate.

[0055] In this example, each security device 100 provided on the sled 110 is substantially identical, so that each exhibits substantially the same variable optical effect. Each device displays two images in the form of alphanumeric characters or symbols (here the symbol "£" and the letter "5"). The two images are perceived to move relative to one another as the viewing angle is changed. The devices 100 of the banknote 1000 are typically displayed in a viewing plane that intersects the document (and thus each device) along a line parallel to the y-axis. A change in viewing angle may be achieved by tilting the security document relative to the observer about a tilt axis parallel to the x-axis ("vertical" or "north-south") and / or about a tilt axis parallel to the y-axis ("horizontal" or "east-west" tilt).

[0056] More specifically, when tilted vertically, the two symbols are perceived to move in opposite directions along the vertical (y) axis. Similarly, when tilted horizontally, the two symbols are perceived to move in opposite directions along the horizontal (x) axis. Furthermore, in this exemplary embodiment, the two symbols are perceived to be located in respective image planes on opposite sides of the plane of the optical device. For example, at one viewing angle, the symbol "￡" is perceived to be in an image plane that is behind the plane of the device relative to the observer, and the letter "5" is perceived to be in an image plane that is in front of the plane of the device relative to the observer. At a different second viewing angle, the respective depths of the reproduced images are inverted, such that the symbol "￡" appears in front of the device plane and the letter "5" appears behind the device plane.

[0057] Before describing the security device 100 of the present invention in further detail, we will first consider the principles behind the variable optical effects produced by the device.

[0058] The kinoform of a single point object ("point source") has the form of a Fresnel lens. The kinoform diffraction structure is a multi-layer structure and is shown in FIG. 2(a). Due to the multi-level structure, the image of the kinoform resembles a grayscale image. The binary kinoform diffraction structure of a single point object has the form of a Fresnel zone plate and is shown in FIG. 2(c). FIG. 2(b) shows a multi-level kinoform diffraction structure of four point objects arranged in a square pattern. Although the kinoform of FIG. 2(b) is substantially much more complex than the kinoform diffraction structure of a single point object shown in FIG. 2(a), it can still be recognized that it is a superposition of four single point kinoforms. The binary kinoform diffraction structure of four point objects is shown in FIG. 2(d). Binary kinoforms are typically generated by performing a thresholding process on the multi-level kinoforms. In the present invention, the binary kinoform diffraction structure is preferentially used.

[0059] Figure 3(a) shows diagrammatically the visual effect observed when illuminating a kinoform diffractive structure 1 in the form of a Fresnel zone plate (i.e. a binary kinoform of a single point object). The structure 1 is illuminated along a direction parallel to its plane normal (i.e. along OO´) and observed at a viewing angle in the top parallax field.

[0060] Light incident on the radially outer sections of the diffractive structure is diffracted at a larger angle than light incident at the center due to the difference in grating pitch. Therefore, when the upper half 3 of the kinoform diffractive structure 1 is observed in the upper parallax field (observation position A), the diffracted light diverges towards the observer and forms a virtual image behind the plane of the diffractive structure. This creates the perception of a moving focal point behind the plane of the zone plate 1 due to the trajectory of eye movements. In contrast, when the lower half 5 of the diffractive structure is observed in the upper parallax field (observation position B), the light is focused to form a real image in front of the plane of the structure. This creates the perception of a moving focal point in front of the plane of the diffractive structure 1 when changing the viewing angle in the upper parallax field when viewing the lower half 5 of the diffractive structure.

[0061] Furthermore, in addition to the light spot being perceived to be on either side of the plane of the structure 1, the focal image is also perceived to move in opposite directions as the viewing angle is changed. This is represented by the opposing arrows shown in Fresnel structure 1. The symmetry of the zone plate structure will result in the perceived image moving in opposite directions corresponding to the tilt direction. For example, when tilted vertically, the reproduced light spot will appear to move in opposite directions along the vertical axis (as indicated by the opposing arrows). Similarly, when tilted horizontally, the light spot will appear to move in opposite directions along the horizontal axis. When the structure is tilted in an even more complex manner that includes both vertical and horizontal tilt components, the light spot will appear to move in opposite curved trajectories around the center point of the structure.

[0062] Here, when we refer to an "upper parallax field", we mean that the viewing angle (measured relative to the direction of specular reflection, in this case along OO') is positive, i.e., "above" the specular direction, so that +1st order diffraction effects are observed.

[0063] Figure 3(b) shows diagrammatically the optical effect perceived when viewing the zone plate diffractive structure 1 in the "lower parallax field". Here the viewing angle is negative, i.e. "down" towards the mirror plane, so that a -1st order diffraction effect is observed. When viewing the upper half of the structure 1 (viewing position A), the trajectory of movement results in the perception of a focused point moving in front of the plane of the structure. In position B, when viewing the lower half of the diffractive structure 1, the eye movement produces the optical effect of a virtual image of a moving light point behind the plane of the lens 1.

[0064] Figure 4(a) shows diagrammatically the visual effect produced by viewing the upper half 3 of the diffractive structure in both the upper parallax field (position A) and the lower parallax field (position B) under illumination along the plane normal of the device. In the upper parallax field, a virtual image I produced by +1 order diffraction is visualized behind the plane of the structure. Conversely, in the lower parallax field, a real image R produced by -1 order diffraction appears to be "floating" in front of the plane of the structure.

[0065] Similarly, when looking at the lower half 5 of the diffractive structure 1 (as shown in Figure 4(b)), a virtual image I is perceived behind the plane of the structure when viewed in the lower parallax field (position B), and a real image R is perceived in front of the plane of the structure when viewed in the upper parallax field (position A).

[0066] These principles described above may be used to consider the optical effects produced by the security device of the present invention.

[0067] Figure 5 shows a schematic representation of a device 100 according to an embodiment of the invention, together with the perceived optical effect. The device comprises a diffractive structure having a first diffractive area 11 and a second diffractive area 13. In this example, the two diffractive areas 11, 13 abut each other in the vertical direction (i.e. so that they share a common edge along a direction perpendicular to the vertical direction), as shown in Figure 6, which is a plan view of the device. Thus, the first and second diffractive areas 11, 13 can be described as adjacent.

[0068] The first diffraction area 11 is in the form of a part of a "complete" or "full" kinoform diffraction structure encoding a first image, in this case the symbol "£". The second diffraction area 13 is in the form of a macroscopic part of a "complete" kinoform diffraction structure encoding a second image, in this case the letter "5". In other words, each diffraction area is formed from sub-areas of a complete kinoform diffraction structure. Such a part of the "complete" kinoform will reproduce the complete image, but with different viewing conditions depending on the part selected. In the example of FIG. 5, the parts of the respective "complete" kinoform diffraction structure used for the first and second diffraction areas are selected such that for a certain viewing angle the relative depth of the two reproduced images is such that one of the images is in front of the plane of the device and the other image is behind the plane of the device. In this example, a binary kinoform diffraction structure is used.

[0069] For ease of explanation, it will be considered that the first diffractive region 11 is formed from the upper half of a complete kinoform encoding the symbol "£", and the second diffractive region 13 is formed from the "lower" half of a complete kinoform encoding the letter "5". The overall optical effect produced by apparatus 100 can thus be considered with reference to Figures 4(a) and 4(b), discussed above.

[0070] We can now consider the overall optically variable effect exerted by device 100. When illuminated by illumination source 50 and viewed by an observer O above in the direction of specular reflection (denoted by S), the observer perceives an image of a "5" that appears to be floating in front of the plane of device 100, and an image of the symbol "£" that appears behind the plane of device 100 due to +1 order diffraction of the incident light from the respective diffractive regions 11, 13. When device 100 is tilted, the symbol "5" and the symbol "£" are perceived to move in opposite directions to each other, as shown in Figures 7 and 8.

[0071] 7(a) and 7(b) show schematic diagrams of the perceived variable optical effects exhibited by device 100 when tilted vertically ("north-south") and viewed with a viewing plane that intersects the device along the y-axis of the geometry of FIG. 1. FIG. 7(a) shows the case where the viewing angle is above the specular direction as in FIG. 5 (so the effect of the +1 diffraction order is seen), and FIG. 7(b) shows the case where the illumination and tilt are such that the viewing angle is below the specular direction (so the effect of the -1 diffraction order is seen). As shown in FIG. 7(a), when device 100 is tilted away from the specular (0th order) direction, the images of "£" and "5" move in opposite directions along the vertical axis. In this case, the "5" appears in front of the plane of device 100 and the "£" appears behind the plane of device 100.

[0072] If the device is tilted vertically so that the viewing angle is below the 0th order direction, the reconstructed images move in opposite directions along the vertical axis with the same movement speed as in the case of the +1st diffraction order. However, under these viewing conditions, the relative depths of the images are reversed, with the symbol "￡" appearing to float in front of the plane of the device and the letter "5" appearing behind the plane of the device. In other words, the image movement is symmetrical with respect to the specular direction. Only the perceived "depth" of the symbols is reversed as the diffraction order changes from +1 to -1.

[0073] Figures 8(a) and 8(b) show diagrammatically the perceived relative movement of the two reconstructed images when the device is tilted horizontally (tilted "East-West") and viewed again in a viewing plane that intersects the device along the y-axis. As shown in Figure 8, the two symbols move apart in opposite directions when the device is tilted away from the plane in which the light enters the device. In the viewing angles of Figures 8(a) and 8(b), the vertical tilt of the device is such that the +1 order diffraction effect is reproduced, so that the symbol "£" appears behind the plane of the device and the "5" image appears to be floating in front of the plane of the device. The diffraction effect remains +1 order, so the perceived depth of the reconstructed images does not change with East-West tilt. However, if the device is tilted vertically such that the effect of the -1 diffraction order appears, the perceived depth of the symbol "£" and the symbol "5" will be reversed.

[0074] In this way, the device of the present invention exhibits a complete parallax-variable optical effect upon change of viewing angle. If the device is tilted in a more complex manner that includes both vertical and horizontal tilt components, the reproduced image will be perceived to move in an opposite curved (e.g. circular) trajectory.

[0075] As can be seen from Figures 7 and 8, the movement of the reproduced images of the symbol "£" and the symbol "5" is restricted within the respective image areas corresponding to the respective diffraction regions 11, 13 as shown in Figure 6. In other words, in this example, the symbol "£" is reproduced within the image area 11a corresponding to the top half of the device, while the letter "5" is perceived to move only within the image area 13a corresponding to the bottom half of the device. In Figures 7(a) and 7(b), it can be seen that the symbol "£" can only move vertically within the top half of the viewing area, while conversely, the reproduced "5" is restricted vertically within the bottom half of the viewing area. However, when tilted horizontally as shown in Figures 8(a) and 8(b), both the images of "£" and "5" can move across the full width of the viewing area in accordance with the arrangement of the diffraction regions 11, 13 of the device. For ease of understanding, the image areas 11a, 13a are shown diagrammatically in dashed lines in some of the views shown in Figures 7 and 8.

[0076] When an image is perceived to be moving towards the boundary of its respective diffraction area, the reconstructed image is cropped. However, because the image is cropped at the edge of the diffraction area, a portion of the image from the opposite diffraction order is reconstructed and again cropped at the boundary. This can be seen in FIG. 7(a) when viewed closer to the specular direction (view A). When tilted towards the specular direction, the top of the letter "5" (appearing in front of the plane) is cropped as the "5" moves towards the boundary between image area 11a and image area 13a, and the bottom of the letter "5" appears in the other diffraction order (and is therefore perceived to be behind the plane of the device). In the same way, when the symbol "￡" is perceived to be moving towards the centre of the device (appearing behind the plane), the bottom of the "￡" is cropped and the top of the "￡" appears in front of the plane of the device. This may give the visual impression that the images overlap according to their shape, as can be seen in FIG. 7(a) view A. Similarly, in view B of FIG. 7(b), the "£" and "5" appear to overlap due to the reproduction of additional portions of each image, even though they remain within their respective image areas 11a, 13a.

[0077] FIG. 9 shows a security thread 105 with four substantially identical security devices 100. Each security device is viewed at a slightly different viewing angle upon vertical tilt, and therefore reproduces a different relative position of the reproduced image (in this case the character "5" and the character "0"). The reproduction of the four devices 100 in FIG. 9 shows the relative movement of the symbols upon north-south tilt of the security device. The security thread in FIG. 9 is illuminated with diffuse light, and thus multiple areas of the device are reproduced simultaneously at a particular viewing angle, resulting in some "blurring" in the reproduced image. Under illumination with collimated (or "spot") light, the reproduced image would appear clearer, since the multiple areas are not reproduced simultaneously. In the top view of FIG. 9, the "5" and "0" appear to overlap partially, for the reasons discussed above in connection with FIGS. 7(a) and 7(b). In other words, as each character moves toward the edge of its respective image area, a portion of each character's other diffraction orders are reproduced, cutting each character off. However, each image is reproduced within its respective image area.

[0078] 6, each diffractive region 11, 13 is continuous in that there is no interruption across its domain, and thus each portion of each "complete" kinoform diffractive structure used to form the diffractive region of the device is a continuous portion.

[0079] The diffractive portions 11, 13 of the device 100 are macroscopic in that they are visible to the naked eye. Each diffractive portion 11, 13 preferably has a minimum dimension (e.g., "height" h in FIG. 6) of at least 500 microns, more preferably at least 1 mm. A typical minimum dimension h of a diffractive portion is in the range of 5 mm to 10 mm. Thus, the "width" w of the device 100 of FIG. 6 may typically be in the range of 10 mm to 20 mm. By using macroscopic diffractive regions in the method of the present invention, the reconstructed image advantageously has a high brightness. Furthermore, the ease of manufacture of such a device is increased, for example, when compared to combining different diffractive structures.

[0080] In the exemplary device 100 considered with reference to Figures 5 to 9, the first and second diffractive regions are formed as the top and bottom of the respective complete kinoforms. However, different parts of the respective complete kinoforms may be used to achieve different parallax and kinematic effects. This is shown diagrammatically in Figures 10(a) and 10(b). In the device 100 shown in Figure 10(a), the first diffractive region 11 is formed from the "left" part of the respective complete kinoforms, and the second diffractive region 13 is formed from the "right" part of the respective kinoforms. In the device 100 shown diagrammatically in Figure 10(b), the top left and bottom right parts of the respective complete kinoforms are used for the first and second diffractive regions 11, 13. When illuminated, the devices shown diagrammatically in Figures 10(a) and 10(b) both exhibit a complete parallax effect, in which the symbol "£" and the symbol "5" are perceived to be at different depths relative to the plane of the device at a particular viewing angle. However, the movement of each reconstructed image will be restricted within its respective image area corresponding to the arrangement of its respective diffractive regions. More specifically, when the device 100 of Figure 10(a) is illuminated, both the "£" and the "5" will be perceived to exhibit full vertical movement across the display area, but the "£" will be restricted to the left side of the display area and conversely the "5" will be restricted to moving within the right side of the display area. Similarly, looking at Figure 10(b), the symbol "£" will be restricted to the top left area of ​​the display portion and the movement of the "5" will be restricted to the bottom right area in accordance with the arrangement of the diffractive regions.

[0081] Thus, the selected portion of the "complete" kinoform used to form each diffractive region determines the relative motion and depth of the reconstructed image, and the placement of the diffractive regions determines the area through which the reconstructed image is perceived to move as the viewing angle changes.

[0082] FIG. 18 is a schematic plan view of an apparatus 100 according to a further embodiment of the invention. The apparatus of FIG. 18 is substantially the same as the apparatus shown in FIG. 6. However, here the two diffractive regions 11, 13 are laterally spaced apart. In other words, the two diffractive regions are not adjacent, and the apparatus comprises a "gap region" 90 between the diffractive region 11 and the diffractive region 13. In this embodiment, the gap region 90 comprises a planar polished metal part 92 positioned to substantially fill the gap region. Thus, the metal part 92 is adjacent to each of the diffractive regions 11, 13. The metal part 92 is planar in that it does not include a surface relief structure, and is therefore non-diffractive. In this embodiment, a planar metallized part is located in the gap region, although alternative non-diffractive parts, for example inkwork, may be positioned in the gap region 90. Although the gap region typically comprises a non-diffractive part, in some embodiments, the gap region may be free of such non-diffractive parts.

[0083] In such embodiments, typical dimensions of the gap regions (e.g., the distance d between the diffractive regions) are typically greater than 2 μm (so as not to produce diffractive effects) and less than 100 μm, typically between 5 μm and 50 μm.

[0084] In the examples considered so far, the selected portions of the complete kinoform diffraction structures are complementary in that the perceived relative depth of the reproduced images and the respective relative motions are substantially opposite to each other. For example, in the described embodiment, the device comprises a top of one kinoform diffraction structure and a complementary bottom of a different kinoform structure, or a left and a right part of each complete kinoform structure. However, in alternative embodiments, the selected portions of each kinoform diffraction structure may be corresponding parts. For example, the device may comprise two top halves of each "complete" kinoform diffraction structure or two left halves of each "complete" kinoform diffraction structure. In such an embodiment where corresponding parts are used, the relative kinematic and depth effects of the reproduced images will be substantially similar (e.g. both images will appear to be on the same side of the plane of the device and will move in the same direction when tilted), but the perceived distance between the images and the plane of the device and / or the speed of movement of the images may be different.

[0085] 11 and 12 show an exemplary device 100 according to an embodiment of the present invention comprising three diffractive regions 11, 13 and 15. The first diffractive region 11 is part of a complete kinoform diffractive structure encoding an image of a star, the second diffractive region 13 is part of a complete kinoform diffractive structure encoding an image of the symbol "£", and the third diffractive region 15 is part of a complete kinoform diffractive structure encoding an image of the letter "5".

[0086] Figure 11 shows diagrammatically the optical effects produced when the device is tilted vertically "north-south". When the device is viewed above the zeroth order (+1st diffraction order), the star and the "£" move upwards and appear behind the plane of the device, while the "5" is perceived to move in the opposite downward direction and appears to float in front of the plane of the device. When viewed below the zeroth order (-1st diffraction order), the relative motion of the three symbols is the same as in the +1st order, but their relative depths are reversed, so that the star and the "£" appear in front of the plane of the device and the "5" appears behind the device. As can be seen in Figure 11, each symbol is constrained to move within an area corresponding to the geometry of the diffraction field in which a portion of its respective "complete" kinoform is located.

[0087] FIG. 12 shows diagrammatically the optical effect produced when tilting a device with three diffractive regions horizontally (left to right). When tilted to the left, away from the plane of incidence of the light, both the star and the letter "5" are perceived to be behind the plane of the device to the right. The "￡" becomes fully visible, but the reconstructed image of the star behind the plane of the device disappears. However, due to the geometry of the diffractive regions 11 (and thus the part of the "complete" kinoform diffractive structure of the star), an image of the star that appears to float in front of the plane of the device appears in the lower half of the device, and moves to the left with further tilt to the left. The image of the "5" floating in front of the device moves to the left as a result of the geometry of its diffractive regions 15, and disappears from view with further tilt to the left.

[0088] Conversely, when tilted to the right, the image of the star behind the plane of the device appears to move to the right and the reconstructed image of the letter "£" behind the plane of the device disappears. The geometry of diffractive area 13 (which is symmetrical to diffractive area 11) causes the real image of the letter "£" in front of the plane of the device to appear in the lower half of the device and move to the right. Again, the geometry of diffractive area 15 causes the virtual image of the "5" behind the plane of the device to begin to move just before it disappears with tilt to the right.

[0089] It will be appreciated that in practice, the diffractive regions of the device will typically be formed in the same manufacturing step (e.g., via direct write techniques) such that device 100 comprises a continuous diffractive structure comprising different grating regions. A preferred method of manufacturing an optical device according to the present invention will now be described with reference to the flow chart shown in Figure 13.

[0090] The process begins at step S101. In this step, a first "complete" kinoform representation is provided that encodes a first image (such as a symbol or alphanumeric character). A kinoform representation is a computer-generated representation that encodes the first image and may be preformed or generated as required. In step S102, a second complete kinoform representation is provided that encodes a second image. Like the first kinoform representation, the second kinoform representation is a computer-generated representation that encodes a second image that is different from the first image. The first and second kinoform representations are in the form of respective computer files that encode the respective images. The complete kinoform representation is typically a binary kinoform representation.

[0091] In step S103, a portion of the first kinoform representation is selected. The selected first portion is a macroscopic portion of the first kinoform representation, such that the selected portion reproduces the entire first image when viewed alone (at least under some viewing conditions), where the first image reproduced by the first portion is perceived as being in front of the device plane or behind the device plane depending on the viewing angle.

[0092] In step S104, a portion of the second kinoform representation is selected. As in step S103, the second portion is a macroscopic portion of the second kinoform, since when viewed alone it will reproduce (at least under some viewing conditions) the entire second image. The second portion is typically selected such that the position of the reproduced second image relative to the device will be complementary to the position of the reproduced first image. In other words, the respective portions of the first and second complete kinoform representations are selected such that when viewing the device, the first and second images will be perceived to be on opposite sides of the plane of the device. For example, at one viewing angle, the first image will be perceived to be "floating" in front of the plane of the device, while the second image will appear behind the plane of the device. And at a second, different viewing angle, the first image will be perceived to be behind the plane of the device, while the second image will be perceived to be floating in front of the plane.

[0093] In this way, when the device is tilted, the first and second images appear to move in opposite directions, creating a striking effect that is easy to authenticate but difficult to reproduce.

[0094] In step S105, the selected first and second portions are used to form a diffractive structure in the carrier layer. The diffractive structure is typically formed using a maskless lithography technique, such as electron beam lithography or direct laser writing. Using computer software used to generate a print file for lithography, the first and second portions are arranged in corresponding first and second diffractive regions as desired, and the diffractive structure is directly written in a single process step. The arrangement of the first and second diffractive regions is selected according to the desired positioning of the reproduced image.

[0095] The diffractive structure of step S105 may be used to form the device 100 itself, for example by writing the diffractive structure directly into a carrier layer provided on the device substrate. However, if a large number of devices are to be replicated, it is more preferable to use the diffractive structure generated in step S105 to form a replication tool that defines the diffractive structure of the completed device. The replication tool (or "master") may then be used to form a large number of such devices using techniques such as embossing and hot stamping. The device of the invention is designed primarily for viewing in reflection, in which case the diffractive structure of the device will be provided with a thin reflective metal or high refractive index coating. However, in an alternative arrangement, the device of the invention may be designed for viewing in transmission.

[0096] In embodiments in which interstitial regions are provided between the diffractive regions, the diffractive regions are typically formed in the same manufacturing steps as described above, and if non-diffractive components are provided within the interstitial regions, the non-diffractive components are typically provided in a separate manufacturing step from the formation of the diffractive structures, using techniques known in the art.

[0097] In the preferred method shown in Figure 13, the complete kinoform is first generated or obtained and then respective portions of it are selected in steps S103 and S104. This allows great flexibility in the generation of the device since different portions of the "complete" representation may be selected depending on the desired reproduction characteristics without the need to recalculate the diffractive structures on a case by case basis. However, in alternative embodiments it is envisaged that the desired portions of the complete kinoform used to form the first and second diffractive regions may be generated directly (i.e. without selecting portions from the generated "complete" kinoform).

[0098] In the above steps S101 and S102 of FIG. 13, the first and second kinoform representations were obtained using computational techniques. In an alternative embodiment, it is envisaged that such a "perfect" kinoform may be obtained using classical techniques, as will now be discussed with reference to FIG. 14. FIG. 14 is a schematic diagram of a geometry for recording a "perfect" kinoform hologram using classical techniques. The object beam 301 is first directed through a convex lens 309 so that a parallel beam is incident on the object 305 (in this case the alphanumeric character "5"). The phase information of the electric field arising from the object 305 is inherently contained in the object beam. The object beam is then incident on a photosensitive resist (such as a silver halide emulsion) on a hologram plate 311. An interference pattern generated with the reference beam 313 (aligned with the object beam 301) is recorded in the photosensitive resist to form a classical "perfect" kinoform diffraction structure that encodes the image of the object 305. Respective portions of the physical kinoforms may then be selected as discussed above with reference to steps S103 and S104 of FIG. 13 and combined within respective diffractive regions to form the diffractive structure of the optical device.

[0099] Optical devices of the above kind can be incorporated or applied to any product for which authenticity checking is required. In this case, the optical device functions as a security device. In particular, such devices may be applied or incorporated to valuable documents such as banknotes, passports, driver's licenses, cheques, identity cards, etc. The complete security device may be formed directly on the security document or may be supplied as part of a security article such as a security thread or patch. The security article can then be applied or incorporated to such documents.

[0100] Such security articles can be entirely disposed on the surface of the base substrate of a security document, as in the case of stripes or patches, or can be only partially visible on the surface of the document substrate, for example in the form of windowed security threads. Security threads are currently present in many world currencies as well as in documents, including vouchers, passports and traveller's cheques. In many cases, the threads are partially embedded or provided in a windowed format. In the windowed format, the thread appears to be woven in and out of the paper and is visible in a window on one or both sides of the base substrate. One method for producing paper with so-called windowed threads is described in EP 0059056 A1. EP 0860298 A1 and WO 03 / 095188 A1 describe different approaches for embedding partially exposed wide threads in a paper substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful, since the additional exposed thread surface area allows for even better use of optically variable devices as currently disclosed.

[0101] The security article may be incorporated into a paper or polymer-based substrate in at least one window of the document, such that it is visible from both sides of the finished security substrate. Methods for incorporating security elements in such a way are described in EP 1 141 480 A1 and WO 03 / 054297 A1. In the method described in EP 1 141 480 A1, one side of the security element is totally exposed at one surface of the substrate in which the security element is partly embedded, and partly exposed at a window at the other surface of the substrate.

[0102] A base substrate suitable for producing a security substrate for a security document may be formed from any conventional material, including paper and polymers. Techniques for forming substantially transparent areas in each of these types of substrates are known in the art. For example, WO 83 / 00659 describes a polymer banknote formed from a transparent substrate with an opaque coating on both sides of the substrate. The opaque coating is omitted in localized areas on both sides of the substrate to form transparent areas. In this case, the transparent substrate may be an integral part of the security device or a separate security device may be applied to the transparent substrate of the document. WO 00 / 39391 describes a method for producing transparent areas in a paper substrate. Other methods for forming transparent areas in a paper substrate are described in EP 723501, EP 724519, WO 03 / 054297 and EP 13 / 98174.

[0103] The security device may also be applied to one side of the paper substrate, optionally partially located in an aperture formed in the paper substrate. An example of a method for producing such an aperture is described in WO 03054297. An alternative method of incorporating a security element that is visible in an aperture on one side of the paper substrate and entirely exposed on the other side of the paper substrate is described in WO 2000 / 39391.

[0104] Figures 15, 16 and 17 show examples of security documents incorporating a security device of the above kind. Figure 15 shows a first exemplary security document, here a banknote 1000, in (a) plan view and (b) cross-section along line Q-Q'. Here, the banknote 1000 is a polymer banknote, comprising an internal transparent polymer substrate 102, which is coated on each side with opaque layers 103a and 103b in a conventional manner. In some cases, the opaque layer may be provided on only one side of the substrate 102. The opaque layers 103a and 103b are omitted in an area of ​​the document to form a window 101, here having a square shape. Within the window area 101, a security device 100 according to any of the embodiments discussed above is positioned. The security device 100 may be formed by applying a suitable resist material onto the substrate 102 on which the diffractive structure is to be formed. Alternatively, the security device 100 may be formed separately on a security article, such as a transfer patch or a transfer label. In this case, the security device 100 may be attached to the transparent substrate 102 inside the windowed area 101 using a suitable adhesive. Application may be achieved by hot or cold transfer methods, for example hot stamping.

[0105] It should be noted that a similar construction could be achieved using a paper / plastic composite banknote, where the opaque layers 103a and 103b are replaced by a paper layer laminated (with or without adhesive) to the inner transparent polymer layer 102. The paper layer could be omitted from the windowed area from the start, or the paper could be locally removed after lamination. In other constructions, the order of layers could be reversed, with the (windowed) paper layer on the inside and the transparent polymer layer on the outside.

[0106] 15 shows the use of a "full" window where the area where the opaque layer is excluded is aligned. It will be appreciated that the device 100 may be applied to a "half window" where, for example, an opaque layer 103b is present across the windowed area 101.

[0107] In FIG. 16, the banknote 1000 is of conventional construction having a substrate 102 formed, for example, of paper or other relatively opaque or translucent material. A windowed area 101 is formed as an opening through the substrate 102. The security device 100 is applied as a patch overlapping the edge of the window 101, utilizing an adhesive to bond the patch security article to the document substrate 102. Furthermore, application of the security device to the document may be accomplished using a variety of methods, including hot stamping. FIG. 16(c) shows a variation in which the window 101 is omitted and the device 100 is simply applied to a section of the substrate 102 using any convenient application technique, such as hot stamping. In such an arrangement, the device 100 will, of course, only be visible from one side of the security document 100.

[0108] FIG. 17 shows a third example of a security document, again a banknote 1000, provided with a security article 105 in the form of a security thread or security strip. Three security devices 100, each carried on a strip 105, appear through windows 101 arranged in a row on the document 1000. Two alternative structures of the document are shown in cross-section in FIG. 17(b) and FIG. 17(c). FIG. 17(b) shows a security thread or strip 105 incorporated in the security document 1000 between two portions 102a, 102b of the document substrate. For example, the security thread or strip 105 may be incorporated in the structure of the substrate during the papermaking process using known techniques. To form the window 101, the paper may be locally removed after completion of the papermaking process, for example by peeling. Alternatively, the papermaking process may be designed to exclude the paper in the desired windowed areas. Figure 17(c) shows an alternative arrangement in which a security thread or strip 105 carrying the security device 100 is applied to one side of a document substrate 102, for example using an adhesive 107. The window 101 is formed by providing an opening in the substrate 102. The opening may be present before application of the strip 105 or may be formed later, for example again by peeling.

[0109] Many alternative techniques for incorporating security documents of the type discussed above are known and may be used. For example, the device structures described above may be formed on other types of security documents, including identity cards, driver's licenses, bank cards, passports, and other types of security documents, including laminate structures. In that case, the security device may be incorporated directly into the multi-layer structure of the document.

Claims

1. 1. An optical device exhibiting a variable optical effect upon illumination, the optical device having a diffractive structure, the diffractive structure comprising: a portion of a first kinoform diffractive structure arranged to form a first diffractive region of the diffractive structure, the portion encoding a first image; a portion of a second kinoform diffractive structure arranged to form a second diffractive region of the diffractive structure, the portion encoding a second image, the first diffraction region and the second diffraction region are laterally separated; An optical device, wherein each of the first diffractive region and the second diffractive region is discernible by the naked human eye.

2. when the device is illuminated, the first diffractive region presents the first image which, depending on the viewing angle, is perceived to be in front of the plane of the device or behind the plane of the device, and which is perceived to move within a first image area corresponding to the first diffractive region as the viewing angle changes; 2. The optical device of claim 1, wherein the second diffractive region presents the second image that is perceived to be in front of or behind the plane of the device depending on the viewing angle, and that is perceived to move within a second image area corresponding to the second diffractive region as the viewing angle changes.

3. The optical device of claim 2 , wherein the first and second image areas are non-overlapping.

4. 4. The optical device of claim 1, wherein the first image is generated substantially entirely by diffraction of incident light from the first diffraction region, and the second image is generated substantially entirely by diffraction of incident light from the second diffraction region.

5. The optical device according to any one of claims 1 to 4, wherein each of the first and second diffractive regions has a size greater than 150 microns.

6. The optical device according to any one of claims 1 to 5, wherein each of the first and second diffractive regions has a dimension greater than 1 millimeter by 1 millimeter.

7. The optical device of claim 1, wherein each of the first diffraction region and the second diffraction region has a dimension within a range of 10 mm 2 to 300 mm 2 .

8. 8. Apparatus according to any one of claims 1 to 7, wherein a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure are complementary.

9. 9. The optical device of claim 1, wherein a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure are such that, at at least one viewing angle, the first image is perceived to be on one side of a plane of the optical device and the second image is perceived to be on the opposite side of the plane of the optical device.

10. An optical device according to any one of claims 1 to 9, wherein a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure are such that when the device is tilted, the first image and the second image are perceived to move in substantially opposite directions when the device is tilted about at least one axis.

11. The optical device according to any one of claims 1 to 10, wherein the first diffraction region and the second diffraction region abut each other.

12. The optical device according to any one of claims 1 to 10, wherein the first diffractive region and the second diffractive region are laterally spaced apart.

13. The optical device of claim 12 , further comprising a non-diffractive component located between the first diffractive region and the second diffractive region.

14. The optical device of claim 13, wherein the non-diffractive component is adjacent to each of the first diffractive region and the second diffractive region.

15. The optical device according to any one of claims 1 to 14, wherein the first diffraction region and the second diffraction region do not intersect with each other.

16. The optical device according to any one of claims 1 to 15, wherein the first diffractive region and the second diffractive region are substantially adjacent to each other.

17. An optical apparatus according to any preceding claim, wherein the first and second images are in the form of indicia, such as symbols and alphanumeric characters.

18. 18. The optical device according to any one of the preceding claims, wherein each of the first and second kinoform diffractive structures is a binary kinoform diffractive structure.

19. the diffractive structure further comprises a portion of a third kinoform diffractive structure arranged to form a third diffractive region of the diffractive structure, the portion encoding a third image; the third diffractive region is laterally separated from the first diffractive region and the second diffractive region; The optical device according to any one of claims 1 to 18, wherein the third diffractive region is discernible by the naked human eye.

20. 1. A method of forming an optical device that exhibits a variable optical effect when illuminated, the method comprising: providing a portion of a first kinoform representation encoding a first image; providing a portion of a second kinoform representation encoding a second image; forming a diffractive structure in a carrier layer, the diffractive structure comprising a first diffractive region corresponding to a portion of the first kinoform representation and a second diffractive region corresponding to a portion of the second kinoform representation, the first diffractive region and the second diffractive region are laterally separated; Each of the first diffractive region and the second diffractive region is discernible to the naked human eye.

21. The method of claim 20 , wherein the diffractive structures are formed using maskless lithography, such as electron beam lithography or direct laser writing.

22. 22. The method of claim 20 or 21, wherein the step of providing a portion of the first kinoform representation comprises the steps of providing a first kinoform representation for encoding the first image and selecting a portion of the first kinoform representation, and the step of providing a portion of the second kinoform representation comprises the steps of providing a second kinoform representation for encoding the second image and selecting a portion of the second kinoform representation.

23. 23. The method according to claim 20, wherein a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure are such that, at at least one visual angle, the first image is perceived to be on one side of a plane of the optical device and the second image is perceived to be on the opposite side of the plane of the optical device.

24. A method according to any one of claims 20 to 23, wherein a portion of the first kinoform diffractive structure and a portion of the second kinoform diffractive structure are such that, when the device is tilted, the first image and the second image are perceived to move in substantially opposite directions when the device is tilted about at least one axis.

25. The method according to any one of claims 20 to 24, wherein the first diffractive region and the second diffractive region abut each other.

26. The method of any one of claims 20 to 24, wherein the first and second diffractive regions are laterally spaced apart.

27. 27. The method of claim 26, further comprising providing a non-diffractive component located between the first diffractive region and the second diffractive region.

28. The method of claim 27, wherein the non-diffractive component is adjacent to each of the first diffractive region and the second diffractive region.

29. The method of any one of claims 20 to 28, wherein each of the first and second diffractive regions has a size greater than 150 microns.

30. The method of any one of claims 20 to 29, wherein each of the first and second diffractive regions has a dimension greater than 1 millimeter by 1 millimeter.

31. Each of the first diffraction region and the second diffraction region is 10 mm ２ From 300 mm ２ The method according to any one of claims 20 to 30, wherein the diameter of the first and second electrodes is in the range of 1 to 10 mm.

32. A method according to any one of claims 20 to 31, wherein a portion of the first kinoform representation and a portion of the second kinoform representation are computer-generated.

33. A method according to any one of claims 20 to 32, wherein each of the first and second kinoform representations is a binary kinoform representation.

34. An optical device formed by the method of any one of claims 20 to 33.

35. 35. The optical device of any one of claims 1 to 19 or 34, wherein the optical device is a security device.

36. 36. A security article comprising a security device according to claim 35, said security article preferably being formed as a security thread, strip, foil, insert, patch or substrate for a security document.

37. A security document comprising a security device as claimed in claim 35 or a security article as claimed in claim 36, said security document preferably comprising a banknote, a cheque, a passport, an identity card, a certificate of authenticity, a revenue stamp or another document for vouching for value or the identity of an individual.

38. 38. The security document of claim 37, wherein said security document comprises a substrate having a transparent window portion, said security device or security article being positioned at least partially within said transparent window portion.

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