FR3071323A1 - OPTICALLY VARIABLE THREE-DIMENSIONAL MOIRE DEVICE - Google Patents

Abstract

The present invention relates to 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, the second layer being separated from the first layer from a distance and the second pattern including at least one region which is a scaled version of a corresponding region of the first pattern. More particularly, the perceived three-dimensional visual effect when looking at the optically variable device is observed due to moiré interference.

Description

OPTICALLY VARIABLE THREE-DIMENSIONAL MOTION DEVICE

Technical area

The present invention relates generally to optically variable devices and, more particularly, to the configuration of such optically variable devices. These optically variable devices can have application in many areas, including use as an anti-counterfeiting measure on security documents such as banknotes, government documents, tickets and security labels.

Invention background

Security devices are applied to security documents or the like, such as identity cards, passports, credit cards, banknotes, checks and the like, and can take the form of diffraction gratings and microstructures optically. similar detectable. These security devices are difficult to falsify or modify, and are easily damaged or destroyed by any attempt to alter the document. Often, security features are designed to be obvious features of the document, so that they can be seen with the naked eye. This type of public or primary security device allows a certain degree of authentication of the document by members of the public, without the aid of any additional observation device.

The ever-increasing sophistication of counterfeiting operations requires continuous improvement in the design of security features to protect documents from counterfeiting. For example, easy access to digital scanners and high-resolution color photocopiers makes it increasingly plausible that counterfeiters can copy security documents issued using conventional security printing technologies. There is therefore a constant need for alternative and improved safety devices.

Such a group of improved optical devices, known as optically variable devices, produces images which vary according to the angle of view, so that they cannot be easily copied or photographed.

For this reason, optically variable devices have met with great success as regards the keeping in check of possible counterfeiters. However, continuous improvement on the counterfeiter side has resulted in the adoption of holographic approaches to allow simulation of optically variable effects produced by authentic security document printing technologies.

Consequently, another optically variable device is now proposed, based on a moiré effect. Moire effects produce a visual perception that appears when looking at a pattern comprising a series of lines or dots superimposed on another pattern, which series of lines or dots differing in size, angle or spacing between the superimposed patterns . The superimposition of two similar patterns creates a third pattern called moiré pattern which, when viewed from different angles, varies and thus generates an optical effect which is perceived as an animation effect.

Any reference in this document to a patent document or other text presented as prior art should not be taken as an admission that the document or text was known or that the information it contained was part of the knowledge general municipalities on the priority date of any one of the claims.

Summary of the invention

According to one 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. , the second layer being separated from the first layer by a certain distance and the second pattern including at least one region which is a modified scale version of a corresponding region of the first pattern.

The three-dimensional visual effect perceived when looking at the optically variable device is observed due to moiré interference. It will generally be understood that moiré effects are produced when looking at a pattern which is superimposed on another pattern, where the series of lines or dots constituting the pattern differ in relative size, angle and spacing. According to the invention, the change of scale, in enlargement or reduction, of a region of the second pattern which corresponds to a region without change of scale of the first pattern causes the moiré effect.

In a preferred embodiment of the invention, the first layer and the second layer are parallel and the distance separating the first layer from the second layer is perpendicular to the first and second layers.

According to one embodiment, the modified scale version of a region of the first pattern is subjected to a scale change according to a factor between 0.5 and 1.5, excluding a scale factor equal to 1.0. This range of scale factor values is applicable to a range of applications from a large scale application such as a display panel to a smaller scale application such as a security document or a bank biilet. In some embodiments, the modified scaled version of a region of the first pattern is subject to a change of scale by a factor between 0.9 and 1.1, excluding a scale factor equal to 1.0. This range of scale factor values is applicable to a smaller scale application such as a security document or a bank note. For large scale applications as well as small scale applications, the scale factor cannot be equal to the unit or 1.0, since in this case no three dimensional visual effect would be produced.

According to a particular embodiment, the three-dimensional image is observed as being above or below the first layer of the optically variable device by a distance which is greater than the distance separating the first layer from the device from the second layer of the device. This produces an interesting visual effect. In addition, the height at which the three-dimensional image is observed as being above or below the first layer can be calculated by H - t / (1 - a), where H is the observed height, t is the distance between the first and second layers and a is the scale factor between the second pattern which is a modified scale version of a corresponding region of the first pattern.

According to one form of the invention, the second motif includes one or more regions directly corresponding to the first motif and at least one other region which is a modified scale version of a corresponding region of the first motif. The regions of the second pattern directly corresponding to the first pattern can be superimposed on the first pattern and the at least one other region of the second pattern, which is a modified scale version, is also superimposed on the first pattern.

In different embodiments, the first and / or second patterns are printed or embossed on a transparent substrate. For example, the first and second patterns are printed or embossed on opposite sides of a transparent substrate.

One or more regions without changing the scale of the first and second patterns can be in perfect alignment. For example, this alignment can be obtained by printing the first and second patterns using a Simultan printing press.

In different embodiments, the first and / or second patterns are formed by a plurality of lenses. For example, the grid lines are made up of refractive lenticular lenses.

In the context of the present description, refractive lenticular lenses generally have a semi-circular cross section, the cross section being generally constant over the length of the lenticular lens. A lenticular lens does not have to be a straight line and can follow any linear pattern. Other types of lenses can also be used, such as diffractive lenses.

In different embodiments, the first and / or second patterns are formed by a plurality of grid lines. For example, the grid lines are made up of diffraction gratings.

The optically variable device can be configured to be used on a security document, namely as an anti-counterfeiting measure.

In other embodiments, the first layer of the optically variable device comprises a screen of a computer generated display and the second layer comprises a transparent substrate which is applied over the screen of the computer device. For example, such an optically variable computer generated device can be used to verify the authenticity of a website. For example, to assure a user that they are not the victim of a phishing scam if they enter personal information on a website.

According to yet another embodiment, the optically variable device is intended to be used on an advertising medium, including display panels and other printed materials.

Definitions

Security document or Token

As used herein, the term security document includes all types of documents and tokens of valuable and identifying documents including, but not limited to, the following documents: coins such as banknotes and coins, credit cards, checks, passports, identity cards, stock certificates and certificates, driving licenses, property deeds, transport documents such as plane or train tickets, entry cards and tickets, travel certificates birth, death and marriage, and transcripts.

The invention applies in particular, but not exclusively, to security documents such as banknotes or identification documents such as identity cards or passports formed from a substrate on which are applied one or more printing layers. The diffraction gratings and optically variable devices described in this document may also have application in other products, such as packaging.

substratum

As used herein, the term substrate refers to the base material from which the security document or token is formed. The base material can be paper or another 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 paper laminate and at least one plastic material, or two or more polymeric materials.

The use of plastic or polymeric materials for the manufacture of security documents experienced in Australia has been a great success as polymeric banknotes are more durable than their paper counterparts and they can also incorporate new devices and security features. A particularly successful security feature in polymer banknotes produced for Australia and other countries has been a transparent area or window.

Transparent windows and half-windows

As used herein, the term window refers to a transparent or translucent area in the security document as compared to the essentially opaque region on which printing is applied. The window can be completely transparent so that it allows light transmission practically without modification, or it can be partly transparent or partially translucent allowing light transmission but without allowing objects to be seen clearly through the window. window area.

A window area can 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 at least an opacifying layer in the region forming the window area. If opaque layers are applied to both sides of a transparent substrate, a completely transparent window can be formed by omitting your opaque layers on both sides of the transparent substrate in the window area.

A partially transparent or translucent area, hereinafter referred to as a half-window, can be formed in a polymeric security document which has opacifying layers on both sides by omitting the opacifying layers on one side of the security document in the window area, so that the half window is not completely transparent, but allows a certain amount of light to pass through without allowing objects to be seen clearly through the half window.

Alternatively, the substrates may be formed from an essentially opaque material, such as paper or fibrous material, with an insert of transparent plastic material inserted into a cutout or recess formed in the substrate paper or fibrous to form a transparent window area or a translucent half-window.

Opacifying layers

One or more opacifying layers can be applied to a transparent substrate to increase the opacity of the security document. An opacifying layer is such that LT <LO, 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 can include any one or more of various opacifying coatings. For example, your opacifying coatings may comprise a pigment, such as titanium dioxide, dispersed in a binder or a support of crosslinkable polymer material activated by the heat. Alternatively, a substrate of transparent plastic material could be sandwiched between opacifying layers of paper or other partially or essentially opaque material on which marks can then be printed or otherwise applied.

Safety device or feature

As used herein, the term security feature or feature includes any one of a large number of safety features, elements or features intended to protect the security document or token from counterfeiting, copying, alteration or a forgery. Security devices or features can be made in or on the substrate of the security document or in or on one or more layers applied to the base substrate, and can take a wide variety of forms, such as security wires 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 (OVD) such as diffractive devices including diffraction gratings, holograms, diffractive optical elements (DOE).

Radiation curable ink, embossable

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 may be embossed in the soft state to form a relief structure and cured by radiation to fix the embossed relief structure. The curing process does not take place until the radiation-curable ink has been embossed, but it is possible that the curing process takes place either after the embossing, or essentially at the same time as the embossing step. The radiation curable ink is preferably curable by ultraviolet (UV) radiation. Alternatively, the radiation curable ink can be cured by other forms of radiation, such as electron beams or X-rays.

The radiation curable ink is preferably a transparent or translucent ink made of a clear resin material. Such transparent or translucent ink is particularly suitable for the printing of light transmitting security elements such as sub-wavelength gratings, transmissive diffraction gratings and lens structures.

In a particularly preferred embodiment, the transparent or translucent ink preferably comprises a clear embossing lacquer or coating, UV curable, based on acrylic.

Such UV-curable lacquers can be obtained from various manufacturers, including Kingfisher Ink Limited, an ultraviolet product type UVF203 or similar. Alternatively, the radiation curable waffle coatings may be based on other compounds, for example nitrocellulose free.

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

The ink is preferably embossed and cured by ultraviolet (UV) radiation essentially at the same time. In a particularly preferred embodiment, the radiation curable ink is applied and embossed essentially at the same time in a gravure process.

Preferably, to be suitable for gravure printing, the radiation-curable ink has a viscosity essentially between about 20 and about 175 centipoise, and more particularly between about 30 and about 150 centipoise. The viscosity can be determined by measuring the time required to empty the lacquer from a Zahn Cup No. 2. A sample which empties in 20 seconds has a viscosity of 30 centipoise, and a sample which empties in 63 seconds has a viscosity of 150 centipoise.

With certain polymeric substrates, it may be necessary to apply an intermediate layer on the substrate before applying the radiation-curable ink, to improve the adhesion of the embossed structure formed by the ink on the substrate. The intermediate layer preferably comprises a primary layer, and more preferably the primary layer comprises a polyethylenimine. The primer may also include a crosslinking agent, for example a multifunctional isocyanate. Free from other primary layers suitable for use in the invention include: hydroxyl-terminated polymers; copolymers based on a hydroxyl-terminated polyester; crosslinked or uncrosslinked hydroxylated acrylates; your polyurethanes; and anionic or cationic UV curing acrylates. Free from suitable crosslinking agents include: Its isocyanates;

polyaziridines; zirconium complexes; aluminum acetylacetone; your melamines; and carbodiimides.

Understand, understand, understand or understand

As used in this description (including your claims), your terms include, understand, understand or include should be interpreted as specifying the presence of the features, entities, steps or components mentioned, but does not exclude the presence of '' one or more other characteristics, entities, stages or components, or groups thereof.

Brief description of the drawings

We will now describe embodiments of the invention with reference to the accompanying drawings. It will be understood that your embodiments are given by way of illustration only and that the invention is not limited to this illustration. In your drawings:

- Figure 1 is a schematic sectional view of a security device according to an embodiment of the present invention;

- Figure 2 is a schematic sectional view of a safety device according to another embodiment of the present invention;

- Figures 3A and 3B show two free of an optically variable device including two layers, the first layer and the second layer each including a pattern according to an embodiment of the present invention;

- Figures 4A, 4B and 4C show variants of a pattern that could be applied to your layers of an optically variable device according to an embodiment of the present invention and Figure 4D shows an enlarged view of regions on a modified scale the pattern;

- Figures 5A and SB show a detailed cross-sectional view of an optically variable device showing your paths of light rays through your grid lines of the pattern in different layers of the optically variable device according to one embodiment;

- Figure 6 is a schematic view of an optically variable device showing the paths of the light rays blocking the first and second order according to another embodiment;

- Figures 7A to 7D show computer generated graphs of different ray paths of the first and second order;

- Figure 8 is a diagram showing the arrangement of a first or a second layer of the device, in which the layer is subdivided into a network of pixel areas, a pixel area having the same scale or another scale than that of the corresponding region of the other of the first or second layer;

- Figure 9A shows a first layer grid pattern, Figure 9B presents a second layer grid pattern in modified scale across the grid lines, and Figure 9C presents a resulting three-dimensional moire pattern

- Figure 10 is a diagram showing an optically variable device with two layers;

- Figure 11 shows the geometric basis for the observation of three-dimensional moire fringes by the left and right eyes of an observer;

- Figure 12 shows the geometric basis for the calculation of moiré fringes.

detailed description

Referring firstly to Figure 1, there is shown a cross section of the optically variable device 100 according to one 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 d 'a distance t. The first and second layers are arranged so that they are essentially parallel.

The second pattern 140 includes a number of regions. The pattern in some of these regions can correspond directly to a corresponding region, i.e. if the first layer 110 and the second layer

120 overlap so that the first pattern 130 and the second pattern are superimposed, a region of the second pattern which corresponds to a region of the first pattern will be directly superimposed on this region of the first pattern. In other regions, the second pattern 140 includes a modified scale version of the corresponding region of the first pattern 130. This produces a three-dimensional visual effect when the first layer 110 and the second layer 120 are superimposed, due to interference from moire.

By modified scale version, it is understood that one of the first and second patterns 130, 140 is subject to change of scale according to a factor which differs very slightly from the unit (i.e., 1.0) compared to the other of the first or second pattern, which moves the first and second patterns relative to each other by a small distance t to produce a three-dimensional visual effect. For example, if the first pattern has overall dimensions of 20 mm x 20 mm, then, to produce a three-dimensional moiré effect of the type described, the second pattern could be subjected to a change of scale to have dimensions (for example) 19.8mm x 19.8mm to provide a 1% difference in scale. it is a global change of scale of the pattern, which means that the change of scale applies both to the elements that constitute the image, that is to say to the size of the points and or the width of all grid lines, than the spacing between these elements. The second modified scale pattern 140 is then positioned at a small distance t from the first pattern 130. The visual effect or three-dimensional moiré pattern will then appear at a distance of t / 0.01 = 100t from the first pattern 130.

The change in scale of the first pattern relative to the second pattern differs from conventional moiré effects in that, when overlapping two patterns to produce a classic moiré effect, the two overlapping patterns will usually be moved laterally or in rotation. one over the other. If the lines or points forming said pattern differ in relative size, angle or spacing between the two overlapping patterns, this generally means that the sizes and / or angles vary in the local vicinity of each point or line forming the pattern in context for producing a classic moiré effect. It is conceptually different from the overall scale change applied to produce the three-dimensional visual effects of the present invention.

Referring now to Figure 3A, an example of an optically variable device 100 is presented comprising a first layer 110 having a first pattern 130 covered by a second layer 120 having a second pattern 140, which is a modified scale version of the first pattern of the first layer. FIG. 3B presents an example in which the second pattern 140 has widths and / or spacings of grid lines which correspond directly to the widths and / or spacings of grid lines in certain regions, and which are subject to a change of scale , increasing or decreasing, in other discrete regions, for example 150. In the two Figures 3A and 3B, it is this aspect of change of scale of the design of the optically variable device which produces the moiré effect three-dimensional observed.

Referring now to Figure 2, there is shown another embodiment, in which, rather than the first layer 110 and the second layer 120 (as shown in Figure 1) are made of discrete films or discrete substrates of 'a distance t, the first layer 110' and the second layer 120 'consist of opposite faces of the same film or substrate. In this case, the distance t separating the first and second layers 110 ′ is the thickness of the substrate.

The distance t separating the first and second layers 110 and 120, and 110 'and 120', is transparent to light. This is because there is an air gap between the first and second layers, or because at least the second layer is formed of a transparent substrate. This transparent region produces an optically variable three-dimensional visual effect due to the moiré interference which 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 120 or 120 '. More specifically, for an observer, a moire pattern is formed and will vary depending on the angle of observation.

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

In one embodiment, the first and second patterns are printed using a Simultan printing press available from Commerce. The Simultan press prints on the first and second layers simultaneously, allowing regions without scaling the first and second patterns to be printed in perfect alignment.

The reference to a first layer and a second layer must be understood in the context of the orientation of the optically variable device when it is seen by an observer. More specifically, the first layer 110 or 110 ′ carrying the first pattern 130 or 130 ′ must be on the lower side, so that the light is transmitted through the second layer 120 or 120 ′ and that the interference with the second pattern 140 or 140 'cause a moire effect to be observed.

The distance t between the first layer and the second layer can be determined by the thickness of the substrate itself. For example, when the optically variable device constitutes a security device for a security document, for example a polymer banknote, the thickness of the substrate will typically be around 70 μm. This thickness t will dictate the thickness of all the lines, points or polygons forming the pattern and the periodicity of the lines or points which is required to produce the optical moiré effect.

With reference now to FIG. 4a, an example of a first pattern 130 applied to the first layer 110 of the optically variable device 100 is presented. In this case, the first pattern 130 comprises a simple pattern of grid lines. Figure 4B is also 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 subjected to a change in scale, increasing or decreasing, relative to the corresponding regions in the first pattern 130. Figure 4C shows an example of moiré effect 160 which is generated when the first pattern

130 of Figure 4A is superimposed on the second pattern 140 of Figure 4B. Figure 4D shows enlarged examples of the discrete regions 150 that have been scaled up or down relative to the corresponding regions in the first pattern 130. The scaled regions can be selected from a palette predefined scale factors a.

Referring now to Figures 5A and 5B, it is presented by way of a specific example how a three-dimensional moire effect is generated by the optically variable device 100 of the present invention. In this case, a cross section of the optically variable device 100 is presented, having grid lines constituting the first pattern 130 on the first layer 110 of the device and grid lines which are subjected to a reduction in scale with respect to the grid lines constituting the first pattern, which constitute the second pattern 140 on the second layer 120 of the device. It will be understood that the grid lines themselves can form a linear or circular pattern, see for example Figures 3A and 3B.

With particular reference to FIG. 5A, the central black moiré radius defined by the triangle points with a focusing radius c9c10Y, produces:

1 "T Triangle c9jk: Tan (Θ) - t t (D Triangle c9UY: Tan (Θ) = (a / 2) / H (2) From (1) and (2): From where : (a / 2) / H H = t / (1 = (a / 2) (1 - a) / 1 -at) (3)

Referring now to Figure 5B, the eccentric black moiré ray defined by the triangle points with a focusing radius c4c5Z, produces:

Triangle c4da7: Tan (p) = da7 / 1 = (11 a / 2 - 9aa / 2) / 1 (4) Triangle c4PZ: Tan (p) = PZ / H (5) Triangle c5ea8: Tan (y) = ea8 / 1 = (9a / 2 - 7aa / 2) / 1 (6) Triangle c5QZ: Tan (y) = QZ / H = (PZ - a) / H (7) From (4) and (5): PZ / H = (11a / 2 - 9aa / 2) / 1 (8)

From (6) and (7): (PZ - a) / H = (9a / 2 - 7aa / 2) / 1 (9)

By solving (8) and (9) for PZ and H:

PZ = H (11a / 2 - 9aa / 2) / 1 (10)

By substituting the PZ of equation (10) for the PZ of equation (9), we obtain:

/ H = (9a / 2-7aa / 2) / t

By simplifying the equation, we obtain: H = t / (1 - a); the same result as equation (3).

Consequently, the eccentric black moiré ray is focused at the same distance from the surface as the central black moiré ray. By substituting this result 10 in equation (10), we obtain:

PZ = a [(11 -9a) / (1 - a)] / 2.

According to Figure 5B, the distance between the black moiré fringe and the central point Y is given by the distance YZ. From the same figure, we can deduce that:

YZ = PZ - PY = PZ - 11a / 2

Using the PZ of equation (10), after simplification, we obtain:

YZ = aa / (1-a) (11) as the radius of the first order black moire fringe.

In the general case of an eccentric moiré radius defined by a triangle of radius c _n c _n + 1 Z, where n is the sum of the number of complete lines and grid space between the central point or origin and the space into which the moiré ray enters, we deduce a corresponding set of equations. For example, in the case described with reference to Figure 5B, n = 4.

In a general case, the trigonometry equations produce:

25 Tan (p _n ) = [(2n + 3) a / 2 - (2n + 1) aa / 2] / 1 (12) Tan (p _n ) = PnZ / H (13) Tan (y „) = [(2n + 1) a / 2 - (2n - 1) aa / 2] / 1 (14) Tan ( _Y „) = QnZ / H = (P„ Za) / H (15)

From (12) and (13):

P _n Z / H = [(2n + 3) a / 2 - (2n + 1) aa / 2] / 1

From (14) and (15):

(P _n Z - a) / H = [(2n + 1) a / 2 - (2n - 1) aa / 2] / 1

From (17):

P „Z = H [(2n + 1) a / 2 - (2n - 1) oca / 2] / 1 + a (16) (17) (18)

Finally, from (18) and (16), P _n Z is eliminated to obtain an expression for H. After a certain simplification, we obtain:

H = t / (1 - a)

By substituting this expression for H in equation (18), we obtain:

P _n Z = na + (a / 2) [(3 - a) / (1 - a)] (19)

The radius Z according to which this fringe of general black moiré is focused is determined from equation (19) and the geometry represented in Figure 5B is extrapolated to the general case n, which produces:

(20)

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

P „Y = (2n + 3) a / 2 (21)

From (19), (20) and (21), after simplification, we obtain:

YZ = aa l (1 - a), which is exactly the same expression as equation (11). it is therefore proven that all the rays of first order black moiré of any n are focused in the same fringe of black moiré, according to the same and with radius R1. It is further proved that all your contributions from the black ray to 1 - to the black moiré fringe are focused on the same point, at a distance H = t / (1 - a) above or below the substrate depending on the side of the substrate from which the three-dimensional image is viewed. It is also clear from the geometry shown in Figure 5B that all of your black moiré rays emanating from the triangles of rays generated by an opaque bar or region, i.e. a grid line, on the first layer (i.e., the upper surface in this example) passing through a transparent region on the second layer (i.e., the lower surface in this example) having the scaled-down regions in the second pattern, will focus at the same points as your black moiré radius triangles emanating from a transparent region on the first layer and passing through an opaque region or ring on the surface of the second layer.

In FIG. 5B, the three-dimensional image is observed so that the second layer is superimposed on the first layer, namely with the modified scale 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 viewed from the opposite side, then the three-dimensional image would appear to float below the surface of the optically variable device.

Figure 5B also shows an example of the formation of a second order moiré fringe via the second order ray components on each side of the central point Y. These second order moiré rays are represented in the form of the triangle of rays c7c8X2 in Figure 5B. The focal points and the focal length of these second order fringes can be located in the same manner as in the example above.

For example, for the triangle c7c8X2 in Figure 5B, we get:

Tan (p) = [7aa / 2 - 3a / 2] / 1 (22) Tan (p) = SX2 / H (23) Tan (x) = [9aa / 2 - 5a / 2] / 1 (24) Tan (x) = RX2 / H = (SX2 - a) / H (25)

The equations concerning SX2 and H are solved in a similar way to derivative (10), etc.

Equations (22) and (23) produce:

SX2 / H = (a / t) (7a / 2 - 3/2) (26)

Equations (22) and (23) produce:

(SX2 - a) / H = (a / t) (9a / 2 - 5/2) (27)

The resolution of the simultaneous equations (26) and (27) for SX2 and H produce:

H = t / (1 - a) (28)

SX2 = (a / 2) (7a - 3) / (1 - a) (29)

Figure 5B shows that SX2 = YX2 - 3a / 2. YX2 is the distance between the central point or origin of the second-order moiré fringe, i.e.

YX2 = SX2 + 3a / 2. Consequently, from equation (29), after a certain simplification, we deduce that:

YX2 = 2aa / (1 - a) (30)

By comparing expression (30) to equation (11), we find that the radius of a second order fringe is equal to twice that of a first order fringe, namely R2 = 2aa / (1 - at). This calculation can be repeated for the general case of radius n as for the first order case, however the result will be the same as that given in equations (28) and (29),

Referring now to Figure 6, using the theory defined above, a series of black ray paths has been plotted to illustrate the formation of black moire fringes. In this context, black rays are the paths of absence of light, namely a convenient way of illustrating the directions of observation where the light rays are prevented from passing through the multilayer grid patterns. Figure 6 schematically shows examples of first order spokes 610 and second order spokes 620 of this type.

In general, as can be seen from Figure 5B, Figure 6, the angles of the individual rays contributing to the first and second order three-dimensional moire fringes are given by the following expressions: Tan (θ „) = ( 2a / t) [a (n - 1/4) - n + 3/4)] for first order radii (31)

Tan (θ _η ) = (2a / t) [a (n + 1/4) - n + 3/4)] for the second order rays (32)

It will be noted that the directions of the radii defined by these expressions are given by the central point or origin of the triangles of radii shown in Figure 6. For example, the line dY in te Figure 5B.

Referring now to Figures 7A to 7D, computer graphics of the first and second order ray paths generated using equations (31) and (32) defined above are presented. Figure 7A shows ray paths close to grid lines. Figures 7B and 7C show ray paths at an increasing distance from the multilayer grid plane. Figure 7D shows ray paths having set equal to unity or 1.0, i.e. without change of scale, showing that no focusing or three-dimensional effect is generated in this case and that only the effects of classic two-dimensional moiré are observed. First order ray paths are shown in black and second order ray paths are shown in gray. Figure 7C also shows the binocular geometry of right eye 710 and left eye 720 for the observation of three-dimensional moiré fringes. Only a radial half of the grid pattern is shown corresponding to the left or right side of Figure 5B.

In summary, when two circular or linear grid patterns having transparent and opaque areas spaced in their respective xy planes are superimposed in alignment and separated by a small distance t in the z direction with the second pattern subjected to a change of scale according to an amount a relative to the first pattern, then the moiré fringes observed will appear at a vertical distance or z relative to the surface of the first pattern. This height or depth of the fringes in the z direction will be given by:

H = t / (1 - a) (33)

The distance of the three-dimensional moiré fringes from the central point in the xy plane is given by:

RN = Naa / (1 - a), where N = 1, 2, ... (34)

The angle of view Φ of the moiré fringes is given by:

Tan (O) = R _N / H = Naa / 1 (35)

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

a = 75 x Tan (30 °) / 0.99 = 75.7575 x 0.577 = 43.7 pm

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

Referring now to Figure 8, the following analysis can be applied to pixelated multi-level or bi-level moiré devices by locating the values of the grid width a and of the scale values a across the optically variable device area so that they vary from one pixel region to another across the device. In this case, a is replaced by ay and a is replaced by ay, where the indices i and j denote the position of the particular moiré region in the global zone of the device, as represented in FIGS. 4A to 4D, where the palette elements p1, p2, etc. represent different values of a and a. Figure 8 shows schematically how these pixel values are arranged in the second scaled layer of the device. The grid lines in Figure 8 are shown simply as a guide for distinguishing areas of discrete pixels.

In the illustrated case, since the scaling factor ay and the grid width / spacing factor ay are each discretely varied across the optically variable device, the depth and position of the image will also vary at through the device according to equations (33), (34) and (35) above. So in each bi-level moiré pixel, the following relationships exist:

Hÿ = t / (1 -tty)

R _Nÿ = NaÿSÿ / (1 - a _ÿ ), where N = 1, 2, ...

(36) (37) (38)

The above expressions can be used to design a multilayer moiré device having predetermined three-dimensional image properties in a pixelated design format.

Another implementation of the invention can 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 produce:

H (x, y) = t / (1 - a (x, y))

R _n (x, y) = N (x, y) a (x, y) a (x, y) / (1 - a (x, y)), where N = 1, 2, ...

Tan (<D (x, y)) = R _N (x, y) / H (x, y) = Na (x, y) a (x, y) / 1 (39) (40) (41)

Referring now to Figures 9A to 9C, a particular example is presented of the case where the scale factor a varies across the grid plane. In this example, the first layer grid pattern without scaling shown in Figure 9A consists of a series of lines of equal width spaced uniformly according to the equation x = na, where a is fixed and n is included between 0 and n _max . Referring to Figure 9B, the second layer grid pattern is subjected to a scale change in the x direction across the grid lines, according to a variable scale factor. In this case, the scaling factor has been modulated across the grid lines (i.e., horizontally in the x direction) according to the function a = 1 + 0.005 cos (nx / x _m ax)> where x is between 0 and x _max , where x _max = an _max . The scale factor a is therefore between 0.995 and 1.005 and the height / depth of the three-dimensional moire fringe when the two grids are separated by a distance t is between t / 0.005 and -t / 0.005.

The following description of the three-dimensional moiré effects generated by multilayer grid patterns has been illustrated by reference to opaque grid line patterns uniformly spaced with transparent spaces. The following description is applicable to an optically variable device comprising any two-level grid pattern structure with a second pattern which is a modified scale version of the first pattern.

Referring now to Figure 10, the general situation is illustrated schematically by a side view of a two-layer structure in which a first layer, namely the top layer, includes a grid pattern defined by the equation f (x) = n and a second layer, namely the lower layer, inciut a modified scale version of the same grid function defined by f (ax) = m. Both n and m are integers. It only considers moiré fringes appearing in one direction, for example in the x direction, to simulate three-dimensional moiré effects resulting from binocular vision, that is, when the eyes of the observer are in only one direction.

In Figure 10, a moiré fringe emanating from a two-layer structure is observed at an angle Θ. According to the index equation method of the standard moiré theory, the moiré fringes observed from two superimposed grid patterns f (x) = n and g (x) = m will be given by the expression nm = K, where n, m and K are all integers. In this case, the grid lines of the pattern on the top layer are projected onto the pattern on the bottom layer. The two grid patterns then exist in the same plane and the equation for moiré fringes becomes:

f (x + 1TanO) - f (ax) = K (42)

Expression (42) can be rewritten as f (x + t TanO) - f (x + (a - 1) x) = K. In general, the separation distance t and the difference d 'scale (a - 1) are small.

Therefore, using the approximation f (x + ε) = f (x) + (df (x) / dx) e for a small ε, we can precisely approximate equation (42) above by f ( x) + (df (x) / dx) t TanO - f (x) - (df (x) / dx) (a - 1) x = K. This formula can be written in the following form:

[t TanO + (1-- a) x] (df (x) / dx) = K (43)

Equation (43) is a general expression that can be applied to a range of f (x) functions to explore the properties of their three-dimensional moire fringe patterns. For example, in the case described above having a periodic linear grid pattern having grid lines of equal width and spacings, x = na, where a is the spacing between the grid lines. Therefore, f (x) = x / a. The substitution in equation (43) and the reorganization of the terms produces an expression for the positions of moiré fringes as a function of the angle of observation:

X = Ka / (1 - a) - t TanO / (1 - a) (44)

Referring now to Figure 11, when applied to both the left eye and the right eye, the above expression for the position of the three-dimensional moire fringes produces the following expression for the depth of moire fringes:

H = t / (1 - a) (45)

Expression (45) is again identical to the expression given by equation (33) deduced by purely geometric means. It will be noted that, in the case of linear grid lines spaced equally, the depth / height of the three-dimensional fringes is the same as for all the moiré fringes, because H is independent of the sequence number K. However, this is not the case for the example described below, which concerns your fringes of moiré of three-dimensional zone plate.

In the case of a plate grid pattern of area r = (x ² + y ² ) ¹ ' ² = a (2n + 1) ^1/2 , where n is a positive integer. Consequently, n = (x ² + y ² - a ² ) / (2a ² ) = f (x). Equation (43) then generates the following expression:

(1 - a) x ² +1 TanQ x - Ka ² = 0 (46)

When the second layer is separated from the first layer by a distance t = 0, i.e. there is no separation between your grid planes, equation (46) gives the type of moiré fringes of well-known zone plate defined by x = (Ka ² / (1 - a)) ^1/2 and when a = 1 (i.e., there is no change of scale). In this case, the set of evenly spaced moiré fringes is given by x = Ka ² / (t Tan (0)). In the general case, the quadratic equation (46) admits two general solutions for each value of Θ. These solutions are given by:

x = - [t Tanô / (1 - a)] + - [(Ka ² / (1 - a)) + (t TanQ / (1 - a)) ² ] ^1/2 (47)

The plus and minus signs refer to the two solutions on each side of the center point of the zone plate. For example, for Θ = 0, we get x = + - (Ka ² / (1 - a)) ^1/2 . Thus, it has moire fringes on each side of the central point, at a radius of (Ka ² / (1 - a)) ^1/2 . Due to the square root operation, your fringes form an area plate pattern in themselves. Moire fringes are indeed a beat pattern with a spacing determined by the serial number and vice versa of the difference between your scale factors. The calculation of the depth of moiré fringes can be determined by reference to Figure 12.

Referring now to Figure 12, equation (47) gives that the position of the moiré fringes on the right side of the central point, at Θ = 0, at point Pr, is given by:

XR = (Ka ² / (1 - a)) ^1/2 (48)

Expression (48) defines the position of the fringes seen by the right eye of the observer alone. The observer's left eye sees the position of the same fringe from a different position P _L , as represented by the geometry in Figure 12. The position of the fringes is given according to equation (47) by:

XL = - [t TanQ / (1 - a)] + [(Ka ² / (1 - a)) + (t TanQ / (1 - a)) ² ] ^1/2 (49)

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

D = [t Tan0 / (1 - a)] + (Ka ² / (1 - a)) ^V2 - [(Ka ² / (1 - a)) + (t Tan9 / (1 - a)) ² ] ¹ ' ² (50)

The apparent height or depth of the moiré fringe with two open eyes is given by H, which, according to the geometry presented in Figure 12 is given by H = D / Tant). Therefore :

H = t / (1 - a) + Q (K, a) - [Q ² (K, a) + (t / (1 - a)) ² ] ^1/2 (51) where Q (K, a) = [(Ka ² / (1 - a)) ^1/2 / TanO] (52)

It will be understood that the first term of equation (51) is the same as the first term of the example of a periodic linear grid pattern described above. However, te Q (K, a) in the present case shows that the heights of the fringes are also a function of the order number. The angle Θ in the above expression is associated with the distance of observation between the plane of the eye and the plane of the device, d (for example, d = 28 cm), and the distance between your eyes, e (for example, e = 7 cm). Because the distance H is small compared to d, we can write TanQ = e / d = 1/4. In this case, Q (K, a) = (Ka ² / (1 - a)) ^1/2 / 4. An example of typical H values can be calculated for a first order fringe using a film thickness of t = 75 pm, a = 0.99 and K = 1. The use of a zone plate with a radius of 1 cm and 50 black zones interlaced with 50 transparent zones, since the maximum radius R _max = a (2n _max + 1) ^1/2 , where a = 10 mm / (2 x 50 + 1) ^1/2 = 1.05 mm. Therefore, Q (K, a) = (1.05 x 1.05 / 0.01) ^1/2 / 0.25 mm = 42.05 mm. In addition, the first term of equation (51), t / (1 - a) = 75 / 0.01 = 7.5 mm. H can then be calculated by H = 7.5 mm + 42.05 mm - ((7.5) ² + (42.05) ² ) ¹⁷² = 7.5 + 42.05 - 42.7 mm = 6, 85 mm. The dominant term in equation (51) is the term Q (K, a). Consequently, the height / depth of the three-dimensional moiré fringes is given by the first term of equation (51) with a reasonable approximation, namely H = t / (1 - a).

Due to the relationship between the relative viewing angle between the two eyes of an observer Θ and the distance from the device, we can replace Tan0 in the above expressions for Q (K, a) by the ratio e / d. Therefore, Q (K, a) = S (K / (1 - a)) ¹⁷² , where S = ad / e. Consequently, as the distance from the device d increases and the distance between the eyes e remains fixed, the term Q (K, a) increases correspondingly and the height / depth of the moiré fringes approaches asymptotically H = t / (1 - a). For example, if the viewing 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 ) ¹⁷² = 7.16 mm, which is much closer to 7.5 mm than in the previous case and is in agreement with the observation which shows a greater perception depth as the viewing distance increases.

Because the optically variable device proposed by the present invention uses planes of grid patterns made up of opaque and transparent regions, it will be obvious to those skilled in the art that a variety of methods could be used to create the opaque areas. These methods include printing on opaque regions with ink or embossing those regions to create diffuse regions. Alternatively, the opaque regions (for example, the black grid lines shown in Figures 3A and 3B, Figures 4A to 4D, Figures 5A and 5B, Figure 6 and Figures 9A to 9C) may consist in diffraction grating regions, since the light diffracted from these regions will reduce the intensity of the light transmitted directly through the zero order and therefore create a degree of opacity in these regions, which allows a difference in intensity of the light transmitted between these regions and the adjacent transparent regions to create the required three-dimensional moire effect. The end result will be an optically variable device which, in addition to generating three-dimensional moire effects, will generate optically variable diffractive effects in non-harmful orders. These additional diffractive effects can produce an additional level of security, in addition to the main three-dimensional moire effects, for security applications, namely anti-counterfeiting.

In addition, as is well known in the field of moiré enlargement, it is also possible to replace a revealing grid, the first and second patterns of the present invention can both be classified as such, by a corresponding lens array. For example, the grid of opaque and transparent regions shown in Figure 4A can be replaced by an array of lenticular lenses having a corresponding period. A lens array such as this, as is well known, produces the same sampling effect as that produced by the opaque regions, since the incoming light rays are focused on an area of the opposite face, but with greater luminous efficacy, which improves its contrast. If lenses are used to replace one of the patterns, the device ceases to be effective when viewed from both sides, and, instead, produces only three-dimensional effects when viewed through your lenses. However, if lenses are used to replace the two opaque region patterns, in all of the embodiments described above, the device therefore still has optical effects visible from both sides.

The optically variable devices proposed by the present invention can have application in a variety of diverse fields. For example, the optically variable device offers an alternative security device to be used as an anti-counterfeiting measure on security documents including currency, such as banknotes, credit cards and checks, government documents including your passports and licenses, and your tickets. Alternative uses could include packaging, either the packaging itself, or the security labels associated with goods to indicate their authenticity.

Another example is to generate the first pattern on a computer generated display, which forms the first layer of the device, and to place the second pattern, printed on a transparent substrate, above the first pattern displayed on a tablet or smartphone, for example, for authenticating a website. This embodiment of the invention could have a particular application in the authentication of a payment platform, to reduce the frequency of deception of customers by phishing websites (ph / sh / ng).

Another application not associated with anti-counterfeiting measures concerns the use on an advertising medium such as billboards or other printed materials. In this case, the optically variable device could provide a visual point of interest to distinguish the advertisement from other advertisements in the immediate environment and by providing a memorable visual characteristic. In this case, a static printed pattern is printed on the support, thereby defining the reference plane, and a second pattern is printed on a transparent substrate and superimposed on the reference plane with an appropriate separation to obtain the desired visual effect. . The motif could be designed, for example, to be perceived by an observer as a text which leaves the advertising medium.

The optically variable device according to the present invention has been generally described in the context of use as a security device for authenticating a security document. It will however be understood that the device is not limited to any particular size or range of sizes and could in fact go from a very small size to a very large size depending on whether the device is used to authenticate a banknote, or to offer a point of visual interest on an advertising billboard. It will be understood that, depending on the dimensions of the optically variable device, the separation between layers required to generate the desired visual effect could be considerably greater than otherwise envisaged in the present description. In addition, the scale factor a will similarly be chosen from a wider range, for example a factor between 0.5 and 1.5, excluding a scale factor of 1.0, for a large-scale application such as an advertising billboard, and in a narrower range of 0.9 to 1.1, excluding a scale factor of 1.0, for an application on a small scale, such as a bank note. It will be understood that, both for a large-scale application and for a small-scale application, the scale factor cannot be equal to unity or 1.0, since, in this case, It would not be produced no three-dimensional visual effect at all.

In another embodiment, it is envisaged that the patterns 5 could include distinct colors so that various color effects can be generated by the optically variable device. For example, one pattern could be printed in red and another pattern could be printed in blue so that a color mixing effect occurs when the patterns are layered to create a purple three-dimensional moire effect.

Although the invention has been described in conjunction with a limited number of embodiments, the person skilled in the art will understand in the light of the above description that many alternatives, modifications and variants are possible.

Claims (18)

claims 1. 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, the second layer being separated from the first layer by a certain distance and the second pattern including at least one region which is a modified scale 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 moiré interference. 3. Optically variable device according to claim 1 or 2, in which the first layer and the second layer are parallel and the distance separating the first layer from the second layer is perpendicular to the first and second layers. 4. Optically variable device according to any one of claims 1 to 3, in which the modified scale version of a region of the first pattern is subjected to a change of scale according to a factor between 0.5 and 1.5 , excluding a scale factor of 1.0. 5. Optically variable device according to claim 4, in which the modified scale version of a region of the first pattern is subjected to a change of scale according to a factor between 0.9 and 1.1, excluding d 'a scale factor equal to 1.0. 6. Optically variable device according to any one of claims 1 to 5, in which the three-dimensional image is observed as being above or well below the first layer by a distance which is greater than the distance separating the first layer of the second layer. 7. Optically variable device according to any one of claims 1 to 5, in which the three-dimensional image is observed as being above or well below the first layer at an observed height calculated by H = t / (1 - a), where H is the height observed, t is the distance between the first and second layers and a is the scale factor between the second pattern which is a modified scale version of a corresponding region of the first pattern. 8. An optically variable device according to any one of claims 1 to 7, in which the second pattern includes one or more regions directly corresponding to the first pattern and at least one other region which is a modified scale version of a corresponding region of the first motif. 9. The optically variable device according to claim 8, in which the regions of the second pattern corresponding directly to the first pattern are superimposed on the first pattern and the at least one other region of the second pattern which is a modified scale version is also superimposed on te first motif. 10. Optically variable device according to any one of claims 1 to 9, in which the first and second patterns are printed or embossed on a transparent substrate. 11. Optically variable device according to any one of claims 1 to 9, in which the first and second patterns are printed or embossed on opposite faces of a transparent substrate. 12. Optically variable device according to claim 10 or 11, in which one or more regions without change of scale of the first and second patterns are in perfect alignment. 13. An optically variable device according to any one of claims 1 to 12, in which the first and / or the second patterns are formed by a plurality of lenses. 14. Optically variable device according to any one of claims 1 to 12, in which the first and / or the second patterns are formed by a plurality of grid lines. 15. The optically variable device according to claim 14, in which the grid lines consist of diffraction gratings. 16. Optically variable device according to any one of claims 1 to 15, wherein the optically variable device is intended to be used on a security document. 17. The optically variable device according to any of claims 1 to 15, wherein the first layer comprises a screen of a computer generated display and the second layer comprises a transparent substrate which is applied over the screen of the computer device . 18. An optically variable device according to any one of claims 1 to 15, wherein the optically variable device is intended to be used on an advertising medium including display panels and other printed materials.