AU2017101291A4

AU2017101291A4 - Optically variable three dimensional moiré device

Info

Publication number: AU2017101291A4
Application number: AU2017101291A
Authority: AU
Inventors: Robert Lee; Darren Phillips; Ben Paul STEVENS
Original assignee: CCL Security Pty Ltd
Current assignee: CCL Security Pty Ltd
Priority date: 2017-09-21
Filing date: 2017-09-21
Publication date: 2017-10-26
Anticipated expiration: 2025-09-21
Also published as: AU2017101291B4; WO2019056066A1; FR3071323A1

Abstract

Abstract An optically variable device for producing a three dimensional visual effect is provided. The optically variable device includes at least two layers, a first layer including a first pattern and a second layer including a second pattern, wherein the second layer is separated from the first layer by a distance and the second pattern includes at least one region that is a scaled version of a corresponding region of the first pattern. t Figure 1 100' 110' 120' Figure 2 140'

Description

OPTICALLY VARIABLE THREE DIMENSIONAL MOIRE DEVICE Technical Field [1 ] The invention relates generally to optically variable devices and more particularly to the configuration of such optically variable devices. Such optically variable devices may have application in a number of fields including as an anticounterfeiting measure on security documents such as banknotes, government documents, tickets or security labels.

Background of Invention [2] Security devices are applied to security documents or similar articles, such as identity cards, passports, credit cards, bank notes, cheques and the like and may take the form of diffraction gratings and similar optically detectable microstructures. Such security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempts to tamper with the document. Often security devices are designed to be overt features of the document, such that they are observable with the naked eye. This type of public or primary security device enables members of the public to perform some degree of authentication of the document, without the use of any additional viewing apparatus.

[3] The ever increasing sophistication of counterfeiting operations requires continuous improvement in the design of security devices for protecting documents against forgery. For example, the ready availability of high resolution digital scanners and coloured photocopiers makes it increasingly plausible for counterfeiters to copy security documents issued using conventional security printing technologies. Accordingly, there is a constant need for alternative and improved security devices.

[4] One such group of improved optical devices, known as optically variable devices, produces images which vary with the angle of view such that they cannot be readily copied or imaged. For this reason, optically variable devices have been very successful at thwarting would be counterfeiters. However, continuous improvement by counterfeiters has resulted in adoption of holographic approaches to enable simulation of optically variable effects produced by genuine security document printing technologies.

[5] Accordingly, there is now proposed an alternative optically variable device based on a moire effect. Moire effects produce a visual perception that occurs when viewing a pattern comprising a series of lines or dots is superimposed on another pattern, where the series of lines or dots differ in relative size, angle, or spacing between the superimposed patterns. Superimposing two similar patterns gives rise to a third pattern referred to as the “moire pattern” which when observed from different angles will vary thereby generating an optical effect that is perceived as an animation effect.

[6] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention [7] According to an aspect of the present invention, there is provided an optically variable device for producing a three dimensional visual effect, the optically variable device including at least two layers, a first layer including a first pattern and a second layer including a second pattern, wherein the second layer is separated from the first layer by a distance and the second pattern includes at least one region that is a scaled version of a corresponding region of the first pattern.

[8] The three dimensional visual effect perceived when viewing the optically variable device is observed due to moire interference. It will be generally understood that moire effects are produced when viewing a pattern that superimposed on another pattern, where the series of lines or dots comprising the pattern differ in relative size, angle, or spacing. In accordance with the invention, the scaling, up or down, of region of the second pattern that corresponds to an unsealed region in the first pattern, causes the moire effect.

[9] In a preferred for of the invention, the first layer and the second layer are parallel and the distance separating the first layer from the second layer lies normal to the first and second layers.

[10] According to an embodiment, the scaled version of a region of the first pattern is scaled by a factor of between 0.5 and 1.5 but excluding a scale factor of 1.0. This magnitude of scaling factor is applicable to a range of large scale applications such as a billboard, through to a smaller scale application such as a security document or bank note. In some embodiments, the scaled version of a region of the first pattern is scaled by a factor of between 0.9 and 1.1 but excluding a scale factor of 1.0. This magnitude of scaling factor is applicable to smaller scale application such as a security document or bank note. For either a large scale application or a smaller scale application, the scale factor cannot equal unity or 1.0 since in this case, no three dimensional visual effect would be produced at all.

[11] In one particular embodiment, the three dimensional image is observed to lie either above or below the first layer of the optically variable device by a distance that is greater than the distance separating the first layer of the device from the second layer of the device. This provides an interesting visual effect. Moreover, the height at which the three dimensional image is observed to lie above or below the first layer may be calculated as H = t/( 1 - a), wherein H is the observed height, t is the distance between the first and second layers and a is the scaling factor between the second pattern that is a scaled version of a corresponding region of the first pattern.

[12] According to one form of the invention, the second pattern includes one or more regions corresponding directly to the first pattern and at least one other region that is a scaled version of a corresponding region of the first pattern. The regions of the second pattern corresponding directly to the first pattern may be superimposed over the first pattern and the at least one other region of the second pattern that is a scaled version is also superimposed over the first pattern.

[13] In various embodiments, the first and second patterns are printed or embossed onto a transparent substrate. For example, the first and second patterns are printed or embossed on opposing sides of a transparent substrate.

[14] One or more unsealed regions of first and second patterns may be in perfect registration. For example, this may be achieved by printing the first and second patterns using a Simultan printing press.

[15] In various embodiments, the first and second patterns are formed by a plurality of grid lines. For example, the grid lines are comprised of diffraction gratings.

[16] The optically variable device may be configured for use on a security document, i.e. as an anti-counterfeiting measure.

[17] In other embodiments, the first layer of the optically variable device comprises a screen of a computing generated display and the second layer comprises a transparent substrate which is laid over the screen of the computing device. For example, such a computer generated optically variable device may be used to verify the authenticity of a website. For example, to provide confidence to a user that they will not be subject to a phishing scam if they enter personal information in a website.

[18] In yet another embodiment, the optically variable device is for use on advertising media including billboards and other printed materials.

Definitions

Security Document or Token [19] As used herein the term security document includes all types of documents and tokens of value and identification documents including, but not limited to the following: items of currency such as banknotes and coins, credit cards, cheques, passports, identity cards, securities and share certificates, driver's licenses, deeds of title, travel documents such as airline and train tickets, entrance cards and tickets, birth, death and marriage certificates, and academic transcripts.

[20] The invention is particularly, but not exclusively, applicable to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate to which one or more layers of printing are applied. The diffraction gratings and optically variable devices described herein may also have application in other products, such as packaging.

Substrate [21] As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material may be paper or other fibrous material such as cellulose; a plastic or polymeric material including but not limited to polypropylene (PP), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET); or a composite material of two or more materials, such as a laminate of paper and at least one plastic material, or of two or more polymeric materials.

[22] The use of plastic or polymeric materials in the manufacture of security documents pioneered in Australia has been very successful because polymeric banknotes are more durable than their paper counterparts and can also incorporate new security devices and features. One particularly successful security feature in polymeric banknotes produced for Australia and other countries has been a transparent area or “window”.

Transparent Windows and Half Windows [23] As used herein the term window refers to a transparent or translucent area in the security document compared to the substantially opaque region to which printing is applied. The window may be fully transparent so that it allows the transmission of light substantially unaffected, or it may be partly transparent or translucent partially allowing the transmission of light but without allowing objects to be seen clearly through the window area.

[24] A window area may be formed in a polymeric security document which has at least one layer of transparent polymeric material and one or more opacifying layers applied to at least one side of a transparent polymeric substrate, by omitting least one opacifying layer in the region forming the window area. If opacifying layers are applied to both sides of a transparent substrate a fully transparent window may be formed by omitting the opacifying layers on both sides of the transparent substrate in the window area.

[25] A partly transparent or translucent area, hereinafter referred to as a “halfwindow,” may be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side only of the security document in the window area so that the “half-window” is not fully transparent, but allows some light to pass through without allowing objects to be viewed clearly through the half-window.

[26] Alternatively, it is possible for the substrates to be formed from an substantially opaque material, such as paper or fibrous material, with an insert of transparent plastics material inserted into a cut-out, or recess in the paper or fibrous substrate to form a transparent window or a translucent half-window area.

Opacifying Layers [27] One or more opacifying layers may be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LTcLO where LO is the amount of light incident on the document, and LT is the amount of light transmitted through the document. An opacifying layer may comprise any one or more of a variety of opacifying coatings. For example, the opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed within a binder or carrier of heat-activated cross-linkable polymeric material. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or substantially opaque material to which indicia may be subsequently printed or otherwise applied.

Security Device or Feature [28] As used herein the term security device or feature includes any one of a large number of security devices, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or tampering. Security devices or features may be provided in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and may take a wide variety of forms, such as security threads embedded in layers of the security document; security inks such as fluorescent, luminescent and phosphorescent inks, metallic inks, iridescent inks, photochromic, thermochromic, hydrochromic or piezochromic inks; printed and embossed features, including relief structures; interference layers; liquid crystal devices; lenses and lenticular structures; optically variable devices (OVDs) such as diffractive devices including diffraction gratings, holograms and diffractive optical elements (DOEs).

Embossable Radiation Curable Ink [29] The term embossable radiation curable ink used herein refers to any ink, lacquer or other coating which may be applied to the substrate in a printing process, and which can be embossed while soft to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place before the radiation curable ink is embossed, but it is possible for the curing process to take place either after embossing or at substantially the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink maybe cured by other forms of radiation, such as electron beams or X-rays.

[30] The radiation curable ink is preferably a transparent or translucent ink formed from a clear resin material. Such a transparent or translucent ink is particularly suitable for printing light-transmissive security elements such as sub-wavelength gratings, transmissive diffractive gratings and lens structures.

[31 ] In one particularly preferred embodiment, the transparent or translucent ink preferably comprises an acrylic based UV curable clear embossable lacquer or coating, [32] Such UV curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, product ultraviolet type UVF-203 or similar. Alternatively, the radiation curable embossable coatings maybe based on other compounds, e.g. nitro-cellulose.

[33] The radiation curable inks and lacquers used herein have been found to be particularly suitable for embossing microstructures, including diffractive structures such as diffraction gratings and holograms, and microlenses and lens arrays. However, they may also be embossed with larger relief structures, such as non-diffractive optically variable devices.

[34] The ink is preferably embossed and cured by ultraviolet (UV) radiation at substantially the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed at substantially the same time in a Gravure printing process.

[35] Preferably, in order to be suitable for Gravure printing, the radiation curable ink has a viscosity falling substantially in the range from about 20 to about 175 centipoise, and more preferably from about 30 to about 150 centipoise. The viscosity may be determined by measuring the time to drain the lacquer from a Zahn Cup #2. A sample which drains in 20 seconds has a viscosity of 30 centipoise, and a sample which drains in 63 seconds has a viscosity of 150 centipoise.

[36] With some polymeric substrates, it may be necessary to apply an intermediate layer to the substrate before the radiation curable ink is applied to improve the adhesion of the embossed structure formed by the ink to the substrate. The intermediate layer preferably comprises a primer layer, and more preferably the primer layer includes a polyethylene imine. The primer layer may also include a crosslinker, for example a multi-functional isocyanate. Examples of other primers suitable for use in the invention include: hydroxyl terminated polymers; hydroxyl terminated polyester based co-polymers; cross-linked or uncross-linked hydroxylated acrylates; polyurethanes; and UV curing anionic or cationic acrylates. Examples of suitable cross-linkers include: isocyanates; polyaziridines; zirconium complexes; aluminium acetyl acetone; melamines; and carbodi-imides.

Comprise, Comprises, Comprised or Comprising

Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

Brief Description of Drawings [37] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be understood that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings: [38] Figure 1 is a schematic cross section view of a security device according to an embodiment of the present invention.

[39] Figure 2 is a schematic cross section view of a security device according to another embodiment of the present invention.

[40] Figures 3A and 3B show two examples of an optically variable device including two layers wherein the first layer and the second layer each include a pattern according to an embodiment of the present invention.

[41] Figures 4A, 4B and 4C show variations of a pattern that could be applied to the layers of an optically variable device according to an embodiment of the present invention and Figure 4D show an enlarged view of scaled regions of the pattern.

[42] Figures 5A and 5B show a detailed cross sectional view of an optically variable device showing the light ray paths through pattern grid lines in different layers of the optically variable device according to an embodiment.

[43] Figure 6 is a schematic view of an optically variable device showing the blocking light ray paths of the first and second order according to another embodiment.

[44] Figures 7A to 7D show computer generated plots of different first and second order ray paths.

[45] Figure 8 is a schematic showing the arrangement of a first or second layer of the device wherein the layer is divided into an array of pixel areas, pixel area having the same or a different scaling to the corresponding region of the other of the first or second layer.

[46] Figure 9A show a first layer grid pattern, Figure 9B shows a second layer grid pattern scaled across the grid lines and Figure 9C shows a resulting three dimensional moire pattern.

[47] Figure 10 is a schematic showing a two layer optically variable device.

[48] Figure 11 shows the geometric basis for the observation of three dimensional moire fringes by the left and right eye of an observer.

[49] Figure 12 shows the geometric basis for the calculation of moire fringes.

Detailed Description [50] Referring firstly to Figure 1, there is shown a cross section of the optically variable device 100 in accordance with an embodiment. The optically variable device 100 includes two layers. A first layer 110 and a second layer 120. The first layer 110 includes a first pattern 130 and the second layer 120 includes a second pattern 140. The first pattern 130 is separated from the second pattern 140 by a distance t. The first and second layers are arranged such that they are substantially parallel.

[51] The second pattern 140 includes a number of regions. The pattern in some of those regions may correspond directly to a corresponding region, i.e. if the first layer 110 and second layer 120 are overlaid such that the first pattern 130 and the second pattern are superimposed, a region of the second pattern that corresponds to a region of the first pattern, will directly superimpose that region of the first pattern. In other regions, the second pattern 140 includes a scaled version of the corresponding region of the first pattern 130. This gives rise to a three dimensional visual effect when the first layer 110 and the second layer 120 are superimposed due to moire interference.

[52] By a scaled version, it is intended that one of the first or second patterns 130, 140 is scaled by a factor which differs very slightly from unity (i.e. 1.0) with respect to the other of the first or second pattern, thereby displacing the first and second patterns from each other by a small distance t in order to produce a three dimensional visual effect. By way of example, if the first pattern had overall dimensions of 20 mm x 20 mm, then to produce a three dimensional moire effect of the type described, the second pattern could be scaled to dimensions of (for example,) 19.8 mm x 19.8 mm to provide a 1% difference in scale. That is a global scaling of the pattern which means that the scaling applies both to the elements that comprise the image, i.e. the size of the dots and/or the width of any grid lines, as well as the spacing between these elements. The scaled second pattern 140 is then positioned a small distance t away from the first pattern 130. The three dimensional visual effect or moire pattern will then appear a distance t/0.01 = 10Ot away from the first pattern 130.

[53] Scaling of the first pattern with respect to the second pattern differs from conventional moire effects, in that when superimposing two patterns to produce a conventional moire effect, usually the two superimposed patterns will be either laterally or rotationally displaced with respect to the other. If the lines or dots forming the one pattern differ in relative size, angle, or spacing between the two superimposed patterns, this generally means that the sizes and or angles vary in the local neighbourhood of each dot or line forming the pattern in the context of producing a conventional moire effect. This is conceptually different to the global scaling applied to produce the three dimensional visual effects of the present invention.

[54] Referring now to Figure 3A, there is shown an example of an optically variable device 100 comprising a first layer 110 having a first pattern 130 overlaid by a second layer 120 having a second pattern 140, which is a scaled version of the first pattern on the first layer. Figure 3B shows an example where the second pattern 140, has grid line widths and/or spacings that correspond directly to grid line widths and/or spacings in some regions, and are scaled, either up or down, in other discrete regions, e.g. 150. In both Figures 3A and 3B, it is this scaling aspect of the optically variable device design which gives rise to the observed three dimensional moire effect.

[55] Referring now to Figure 2, there is shown an alternative embodiment, wherein, rather than the first layer 110 and the second layer 120 (as shown in Figure 1), being comprised of discrete films or substrates separated by distance t, the first layer 110' and the second layer 120' comprise opposing sides of the same film or substrate. In this case, the distance t separating the first and second layers 110' and 120', is the thickness of the substrate.

[56] The distance t separating the first and second layers 110 and 120, and 110' and 120', is transparent to light. That is since an air gap exists between the first and second layers, or because at least the second layer is formed from a transparent substrate. This transparent region produces an optically variable three dimensional visual effect due to the moire interference that occurs between the first pattern 130 or 130' in the first layer 110 or 110', and the second pattern 140 or 140' in the second layer 140 or 140'. That is, to an observer, a moire pattern is formed and will vary with the angle of observation.

[57] It will be understood that the first and second patterns may be formed in the substrate, e.g. by embossing the pattern into the substrate (as shown in Figures 1 and 2), or on the substrate, e.g. by printing the pattern with ink. The ink may, for example, be an optically variable ink, or a reflective ink.

[58] In one embodiment, the first and second patterns are printed using a commercially available Simultan printing press. Simultan prints on the first and second layers simultaneously which enables the unsealed regions of the first and second patterns to be printed in perfect registration.

[59] The reference to “first layer” and “second layer” is to be understood in the context of the orientation of the optically variable device when viewed by an observer. That is the first layer 110 or 110' bearing the first pattern 130 or 130' must be on the underside so that light is transmitted through the second layer 120 or 120' and interference with the second pattern 140 or 140' causes a moire effect to be observed.

[60] The distance t between the first layer and the second layer may be determined by the thickness of the substrate itself. For example, where the optically variable device constitutes a security device for a security document, e.g. a polymer bank note, the thickness of the substrate will typically be around 70 pm. This thickness t will dictate the thickness of any lines, dots or polygons forming the pattern and the periodicity of the lines or dots that is required to produce the optical moire effect.

[61] Referring now to Figure 4A, there is shown an example of a first pattern 130 applied to the first layer 110 of the optically variable device 100. In this case, the first pattern 130 comprises a simple grid line pattern. Figure 4B, on the other hand is an example of a second pattern 140 applied to the second layer 120 of the optically variable device 100. In the second pattern 140, it can be seen that discrete regions 150 have been scaled up or down, when compared with the corresponding regions in the first pattern 130. Figure 4C shows an example moire effect 160 that is generated when the first pattern 130 of Figure 4A is superimposed with the second pattern 140 of Figure 4B. Figure 4D, shows enlarged examples of the discrete regions 150 that have been scaled up or down with respect to the corresponding regions in the first pattern 130. The scaled regions may be selected from a predefined palette of scaling factors a.

[62] Referring now to Figures 5A and 5B, there is shown by way of a particular example how a three dimensional moire effect is generated by the optically variable device 100 of the present invention. In this case, there is shown a cross section of the optically variable device 100, showing grid lines comprising the first pattern 130 on the first layer 110 of the device and grid lines that are scaled down when compared with the grid lines comprising the first pattern, comprising the second pattern 140 on the second layer 120 of the device. It will be appreciate that the grid lines themselves may form a linear or a circular pattern, see for example Figures 3A and 3B.

[63] Referring more specifically to Figure 5A, the central dark moire ray defined by the focussing ray triangle points c9c10Y, provides:

Triangle c9jk:

(1)

Triangle c9UY:

(2)

From (1) and (2):

Therefore:

(3) [64] Referring now to Figure 5B, the off-centre dark moire ray defined by the focussing ray triangle points c4c5Z, provides:

Triangle c4da7 :

(4)

Triangle c4PZ:

(5)

Triangle c5ea8:

(6)

Triangle c5QZ:

(7)

From (4) and (5):

(8)

From (6) and (7):

(9)

Solving (8) and (9) for PZ and H:

(10)

Substituting PZ in equation (10) for PZ in equation (9) provides:

Simplifying provides: H = t /(1 — a); the same result as equation (3).

Accordingly, the off-centre dark moire ray focuses at the same distance from the surface as the central dark moire ray. Substituting this result into equation (10) provides:

According to Figure 5B, the distance of the dark moire fringe from centre point Y is given by the distance YZ. From the same figure it can be derived that:

Using PZ from equation (10), after simplifying, provides:

(11) as the radius of the first order dark moire fringe.

[65] In the general off-centre case of a moire ray defined by ray triangle cn cn + 1 Z, where n is the sum of the number of complete grid lines and spaces between the centre point or origin and the space where the moire ray enters, a set of corresponding equations are derived. For example, in the case described with reference to Figure 5B, n = 4.

[66] In a general case, the trigonometric equations provide:

(12)

(13)

(14)

(15)

From (12) and (13):

(16)

From (14) and (15):

(17)

From (17):

(18)

Finally, from (18) and (16), PnZ is eliminated to obtain an expression for H. After some simplification, this provides:

Substituting this expression for H in equation (18) provides:

(19)

The radius Z at which this general dark moire fringe is focussed is determined from equation (19) and the geometry shown in Figure 5B is extrapolated to the case of general n, to provide:

(20)

The case illustrated in Figure 5B extrapolated to the case of general n provides:

(21)

From (19), (20) and (21), after simplification, there is provided: YZ = aa/(l - a) which is exactly the same expression as equation (11). It is therefore proven that all first order dark moire rays of any n, focus into the same dark moire fringe, at the same radius R1 = . It is further proven that all dark ray contributions to a dark moire fringe are focussed at the same point a distance H = t/(l - a) above or below the substrate depending on which side of the substrate the three dimensional image is viewed from. It is also clear from the geometry in Figure 5B, that all dark moire rays arising from ray triangles generated by an opaque bar or region, i.e. a grid line, on the first layer (i.e. upper surface in this example) passing through a transparent region on the second layer (i.e. lower surface in this example) having the scaled down regions within the second pattern, will focus at the same points as the dark moire ray triangles arising from a transparent region on the first layer and passing through an opaque region or ring on the second layer surface.

[67] In Figure 5B, the three dimensional image is observed such that the second layer is superimposed on the first layer, i.e. with the scaled version of the pattern on top. Therefore the three dimensional image will appear to float above the surface of the substrate. If the image were to be observed from the opposite side, then the three dimensional image would appear below the surface of the optically variable device.

[68] Figure 5B also shows an example of second order moire fringe formation via the second order ray components on each side of the centre point Y. These second order moire rays are shown as the ray triangle c7c8X2 in Figure 5B. The focal points and focal distance of these second order fringes can be located in the same manner as the previous example.

For instance, for the triangle c7c8X2 in Figure 5B, there is provided:

(22)

(23)

(24)

(25)

The equations for SX2 and H are solved in a similar fashion to derivation (10), etc. Equations (22) and (23) provide:

(26)

Equations (22) and (23) provide:

(27)

Solving the simultaneous equations (26) and (27) for SX2 and H provides:

(28)

(29)

Figure 5B provides that SX2 = YX2 - 3a/2. YX2 is the distance from the centre point or origin of the second order moire fringe, \.e.YX2 = SX2 + 3a/2. Therefore from equation (29), after some simplification, it is derived that:

(30)

Comparing expression (30) with equation (11) establishes that the radius of a second order fringe is twice that of a first order fringe, i.e. R2 = 2aa/(l - a). This calculation can be repeated for the general n ray case as for the first order case, however the result will be the same as given in equations (28) and (29).

[69] Referring now to Figure 6, using the above defined theory a series of black ray paths can have been plotted to illustrate the formation of dark moire fringes. In this context, “dark rays” are the “absence of light” paths, i.e. one convenient way of illustrating the viewing directions where light rays are blocked from passing through the multiple layer grid patterns. Figure 6 below shows schematically examples of first order rays 610 and second order rays 620 of this type.

[70] In general, as can be seen from Figure 5B ,Figure 6, the angles of the individual rays contributing to the first and seconder order three dimensional moire fringes are provided by the following expressions:

for first order rays (31)

for second order rays (32)

Note that the ray directions defined by these expressions are given by the centre point or origin of the ray triangles shown in Figure 6. For example the line dY in Figure 5B.

[71] Referring now to Figures 7A to 7D, there are shown computer plots of the different first and second order ray paths generated using equations (31) and (32) defined above. Figure 7 A shows ray paths close to grid lines. Figures 7B and 7C show ray paths at increasing distance from the multilayered grid plane. Figure 7D shows ray paths with a set equal to unity or 1.0, i.e. no scaling, showing that no focussing or three dimensional effect is generated in this case and only the conventional two dimensional moire effects are observed. The first order ray paths are shown in black and the second order ray paths are shown in grey. Figure 7C also shows the right eye 710 and left eye 720 binocular geometry for the observation of three dimensional moire fringes. Only one radial half of the grid pattern is shown corresponding to the left or right hand side of Figure 5B.

[72] In summary, when two circular or linear grid patterns having equally spaced transparent and opaque areas in their respective xy planes are superimposed in register and separated by a small distance t in the z direction with the second pattern scaled by an amount a with respect to the first pattern, then the moire fringes will be observed to appear at a vertical or z distance away from the surface of the first pattern. This height or depth of the fringes in the z direction will be given by:

(33)

The distance of the three dimensional moire fringes from the centre point in the xy plane is given by;

, where N = 1,2, ... (34)

The angle of view φ of the moire fringes is given by:

(35) [73] For example, for patterns embossed or printed on each side of a 75 pm thick transparent substrate and a scale ratio of a =0.99 between the two patterns the three dimensional moire fringes will appear at a vertical distance of 75/0.01 pm = 7.5mm above or below the substrate depending on which side of the substrate the image is observed from. For a first order three dimensional moire fringe to be observed within an angle of 30° from this substrate then:

For this value of “a” the distance of this first order fringe from the registration point of the two grids will be 0.99 x 43.7/0.01 = 4.327mm.

[74] Referring now to Figure 8, the foregoing analysis can be applied to pixelated multilevel or bi-level moire devices by localising the values of the grid width “a” and scaling values a across the area of the optically variable device so that they vary from pixel region to pixel region across the device. In this case “a” is replaced by ay and a is replaced by ay where the subscripts i and j denote the position of the particular moire region within the overall area of the device, as shown in Figures 4A to 4D, where the palette elements p1, p2, etc, represent different values of a and “a”. Figure 8 below shows schematically how these pixel values are arranged within the second “scaled” layer of the device. The grid lines of Figure 8 are shown merely as a guide to distinguish the discrete pixel areas.

[75] In the illustrated case, since the scale factor ay and the grid width/spacing factor ay are each discretely varied throughout the optically variable device, the image depth and position will also vary throughout the device according to equations (33), (34) and (35) above. Hence, within each bi-level moire pixel the following relationships exist:

(36)

, where N = 1,2,... (37)

(38)

The above expressions may be used to design multilayer moire device with predetermined three dimensional image properties within a pixelated design format.

[76] Another implementation of the invention may be generated using the continuous limit of expressions (36) to (38). In this case, a = a(x,y) and a = a(x,y) and the fundamental image conditions provide that:

(39)

, where N = 1,2,... (40)

(41) [77] Referring now to Figures 9A to 9C, there is shown a particular example of where the scaling factor a varies across the grid plane. In this example, the unsealed first layer grid pattern shown in Figure 9A consists of a series of equally spaced lines of equal width according to the equation x = na, where a is fixed and n ranges from 0 to rimax· referring to Figure 9B, the second layer grid pattern is scaled in the x direction across the grid lines by a variable scaling factor. In this case the scaling factor a has been modulated across the grid lines (i.e. horizontally in the x direction) according to the function a = 1 + 0.005Cos(jtx/xmax), where x ranges from 0 to xmax, where xma=a rimax· The scaling factor a therefore ranges in value from 0.995 to 1.005 and therefore the height/depth of the three dimensional moire fringe when the two grids are separated by a distance t ranges from t/0.005 to -t/0.005.

[78] The foregoing description relating to three dimensional moire effects generated by multi-layer grid patterns was illustrated by reference to patterns of equally spaced opaque grid lines with transparent spaces. The following description is applicable to an optically variable device comprising any bi-level grid pattern structure with a second pattern being a scaled version of the first pattern.

[79] Referring now to Figure 10, the general situation is illustrated schematically by shows a side view of a two layer structure wherein a first layer, i.e. the top layer, includes a grid pattern defined by the equation f(x) = n and a second layer, i.e. the bottom layer, includes a scaled version of the same grid function defined by f(ax) = m. Both n and m are both integers. This only considers moire fringes occurring in one direction, e.g. in the x direction, to simulate three dimensional moire effects resulting from binocular vision, i.e. where the eyes of the observer lie in a single direction.

[80] In Figure 10, a moire fringe from the two layer structure is observed at angle Θ. According to the indicial equation method of standard moire theory, the observed moire fringes from two superimposed grid patterns f(x) = n and g(x)= m will be given by the expression n - m = K , where n, m and K are all integers. In the present case, the grid lines of the pattern on the upper layer are projected onto the pattern on the lower layer. Both grid patterns then exist in the same plane and the equation for the moire fringes becomes:

(42)

The expression (42) can be rewritten as f(x + tTan0) - f(x + (a - l)x) = K. Generally speaking, the separation distance t and the scaling difference (a-1) are small. Therefore, using the approximation f(x+ ε) = f(x) +(άί(χ)/άχ)ε for small ε, we can accurately approximate the above equation (42) as f(x) + (df(x)/dx)tTan0 - f(x) -(df(x)/dx)(a - l)x = K. This can be rewritten as:

(43) [81] Equation (43) is a general expression which can be applied to a range of functions f(x) to explore the properties of their three dimensional moire fringe patterns. For example, in the earlier described case having a periodic linear grid pattern with equal width grid lines and spaces, x = na, where a is the grid line spacing. Therefore, f(x) = χ/a. Substituting into equation (43) and re-arranging terms provides an expression for the moire fringe positions as a function of the angle of observation:

(44) [82] Referring now to Figure 11, when applied to both left and right eyes, the above expression for the position of the three dimensional moire fringes provides the following expression for the depth of the moire fringes:

(45)

Expression (45) is again the same as the expression given by equation (33) derived by purely geometric means. Note that in the case of linear equally spaced grid lines the depth/height of the three dimensional fringes is the same for all moire fringes since H is independent of the order number K. Flowever, this is not the case for the following described example, which relates to three dimensional zone plate moire fringes.

[83] In the case of a zone plate grid pattern

, where n is a positive integer. Therefore

Equation (43) then generates the following expression:

(46)

When the second layer is separated from the first layer by a distance t=0, i.e. no separation between the grid planes, equation (46) gives the well-known zone plate type of moire fringes defined by x = (Ka2/(1 - a))1/2 and when a= 1 (i.e. no scaling).

In this case, the set of equally spaced moire fringes given by x = Ka2/(tTan(0)). . In the general case, the quadratic equation (46) admits of two general solutions for each value of Θ. These solutions are given by:

(47)

The plus and minus signs refer to the two solutions either side of the centre point of the zone plate. For example for Θ = 0 gives x =+-(Ka2/(l- a))1/2. So there are moire fringes on each side of the centre point at a radius of (Ka2/(1 - a))1/2. Due to the square root operation, the fringes form a zone plate pattern in themselves. The moire fringes are in effect a beat pattern with a spacing determined by the order number and inversely as the difference in scaling factors. The calculation of the depth of the moire fringes may be determined by reference to Figure 12.

[84] Referring now to Figure 12, equation (47) provides that the moire fringe position on the right hand side of the centre point at Θ = 0 at point Pr is given by:

(48)

Expression (48) defines the fringe position as seen by the right eye of the observer alone. The left eye of the observer sees the position of the same fringe from a different position PL as shown by the geometry in Figure 12. The fringe position is given according to equation (47) as:

(49)

The distance D in Figure 12 is given by XR - XL . Therefore:

(50) [85] The apparent height or depth of the moire fringe with both eyes open is given by H, which according to the geometry shown in Figure 12 is given by H = D/Tan0. Therefore:

(51) where

(52)

It will be noted that the first term in equation (51) is the same as the first term in the periodic linear grid pattern example described earlier. However the Q(K,a) in the present case shows that the fringe heights are also a function of order number. The angle Θ in the above expression is related to the observation distance from the eye plane to the plane of the device, d (e.g. d = 28 cms), and the distance between the eyes, e (e.g. e = 7cms). Since the distance H is small compared to d, we can write Tan0 =e/d = 1/4. In this case, Q(K,a) = (Ka2/(1- α))ν2/4 . An example of typical values of H may be calculated for a first order fringe using a film thickness of t =75 pm, a = 0.99 and K =1. Using a zone plate with a radius of 1 cm and 50 dark zones interlaced with 50 transparent zones, since the maximum radius Rm3x= a^nMax+l)1/2, then a = lOmm/^xSO+l)1^ = 1.05mm. Therefore, Q(K,a) = (Ι,ΟδχΙ.Οδ/Ο.ΟΙ)1^ /0.25 mm = 42.05 mm. Also, the first term in equation (51), 1:/(1- a) = 75/0.01 = 7.5mm. H may then be calculated as H = 7.5mm + 42.05 mm - ((7.5)2 +(42.05)2)1^ = 7.5 + 42.05 - 42.7 mm = 6.85 mm. The dominant term in equation (51) is the Q(K,a) term. Therefore the height/depth of the three dimensional moire fringes is given by the first term in equation (51) to a reasonable approximation, i.e. H = t/(l- a).

[86] Due to the relationship between the relative angle of view between the two eyes of an observer Θ and the distance from the device can replace TanO in the above expressions for Q(K,a) by the ratio e/d. Therefore Q(K,a) = S(K/(1- a))1/2 where S = ad/e. Therefore as the distance from the device d increases and the eye separation distance e remains fixed, the Q(K,a) term correspondingly increases and the height/depth of the moire fringes asymptotically approaches H = t/(l- a). For example if the observation distance in the above example is doubled from 28 cm to 56 cm then Q(K,a) is also doubled and H = 7.5 +2 x 42.05 - ((7.5)2 + 4x(42.05)2)1/2 = 7.16 mm, which is much closer to 7.5 mm than in the previous case and is in accord with observation which shows an increased depth perception as the observation distance increase.

[87] Since the optically variable device proposed by the present invention uses grid pattern planes consisting of opaque and transparent regions, it will be apparent to those skilled in the relevant art that a range of methods could be used to create the opaque areas. These methods include printing on the opaque regions with ink or embossing these regions to create diffusely scattering regions. Alternatively, the opaque regions (e.g. the black grid lines shown in Figures 4A to 4D, Figures 5A and 5B and Figure 6) may consist of diffraction grating regions since the diffracted light from these regions will reduce the intensity of the light transmitted directly through the zero order and therefore create a degree of opaqueness in these regions, thereby permitting a difference in intensity of transmitted light between these regions and the adjacent transparent regions to create the requisite three dimensional moire effect. The end result will be an optically variable device which, besides generating three dimensional moire effects, will generate optically variable diffractive effects in the nonzero orders. These additional diffractive effects can provide an additional level of security over and above the primary three dimensional moire effects for security, i.e. anti-counterfeiting applications.

[88] The optically variable devices proposed by the present invention may have application in a variety of diverse fields. For example, the optically variable device provides an alternative security device for use as an anti-counterfeiting measure on security documents including currency such as such as banknotes, credit cards and cheques, government documents including passports and licences, and tickets. Alternative uses could include on packaging, whether the packaging per se, or security labels associated with goods to indicate their authenticity.

[89] Another example, is generating the first pattern on a computer generated display which forms the first layer of the device, and placing the second pattern, printed on a transparent substrate over the first pattern displayed on a tablet or smart phone, for example, for the purpose of authenticating a website. This embodiment of the invention could have particular application in authenticating a payment gateway, to reduce the incidence of consumers being duped by phishing websites.

[90] An alternate application not associated with anti-counterfeiting measures involves use on advertising media such as billboards or other printed materials. In this case the optically variable device could provide a point of visual interest to distinguish the advertising from other advertising in the immediate vicinity and by providing a memorable visual feature. In this case, the media as printed thereon a static printed pattern, thereby defining the reference plane, and a second pattern is printed on a transparent substrate and superimposed over the reference plane at a suitable separation to achieve the desired visual effect. The pattern could be designed, for example, to be perceived as text to an observer, that “jumps out” of the advertising media.

[91] The optically variable device of the present invention has been described generally in the context of use as a security device to authenticate a security document. However, it is to be understood that the device is not limited to any particular size or range of sizes and could indeed range from very small to very large depending on whether the device is used to authenticate a bank note, or to provide a visual point of interest on an advertising bill board. It will be appreciated that, depending on the dimensions of the optically variable device, the separation between the layers required to generate the desired visual effect could be significantly larger than otherwise contemplated in this disclosure. Moreover, the scaling factor a will be similarly adopted from a broader range, e.g. a factor of between 0.5 and 1.5, but excluding a scale factor of 1.0, for a large scale application such as an advertising billboard, and from a narrower range of 0.9 to 1.1, but excluding a scale factor of 1.0, for a small scale application, such as a bank note. It will be understood that for either a large scale application or a smaller scale application, the scale factor cannot equal unity or 1.0 since in this case, no three dimensional visual effect would be produced at all.

[92] In another embodiment, it is envisaged that the patterns could comprise distinct colours so that varied colour effects can be generated by the optically variable device. For example, one pattern could be printed in red and other pattern printed in blue such that a colour mixing effect occurs when the patterns are superimposed to create a purple three dimensional moire effect.

[93] While the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative, modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed.

[94] The present application may be used as a basis or priority in respect of one or more future applications and the claims of any such future application may be directed to any one feature or combination of features that are described in the present application. Any such future application may include one or more of the following claims, which are given by way of example and are non-limiting in regard to what may be claimed in any future application.

Claims

The claims defining the invention are as follows

1. An optically variable device for producing a three dimensional visual effect, the optically variable device including at least two layers, a first layer including a first pattern and a second layer including a second pattern, wherein the second layer is separated from the first layer by a distance and the second pattern includes at least one region that is a scaled version of a corresponding region of the first pattern.
2. The optically variable device according to claim 1, wherein the three dimensional visual effect is produced due to a moire interference.
3. The optically variable device according to claim 1 or 2, wherein the second pattern includes one or more regions corresponding directly to the first pattern and at least one other region that is a scaled version of a corresponding region of the first pattern.
4. The optically variable device according to any one of claims 1 to 3, wherein the first and second patterns are formed by a plurality of grid lines.
5. The optically variable device according to claim 4, wherein the grid lines are comprised of diffraction gratings.