HK1161862A - Paired optically variable security element having specific reflection wavelengths - Google Patents

Description

Paired optically variable security element with specific reflection wavelength

Technical Field

The present invention relates to the field of security documents. It relates to a paired optically variable security element comprising first and second optically variable interference patterns (device) in the form of optically variable foils or in printed form made from an ink containing an optically variable interference pigment, wherein the first and second interference patterns exhibit a spectral match at a determined angle of incidence. Also disclosed are sets of optically variable patterns and sets of ink or coating compositions containing optically variable pigments for making the paired optically variable security elements, and the use of the security elements for protecting documents and articles, and security documents and articles bearing the security elements.

Background

Optically variable foils, Optically Variable Pigments (OVPs), coating compositions containing OVPs, Optically Variable Inks (OVIs) are known in the field of security printing). Such optically variable elements exhibit viewing angle or angle of incidence dependent colors and are the preferred choice for protecting banknotes and other security documents from illegal reproduction in office equipment by commonly available color scanning, printing and copying.

In order to enhance the counterfeit resistance and the ease of visual authentication of documents protected by optically variable security elements, it has been proposed to combine more than one optically variable feature on the same document. WO 2005/044583 discloses the use of the same optically variable security element in more than one constituent part of a security document. WO96/39307 discloses pairs of optically variable patterns comprising first and second optically variable patterns in separate locations on the same surface, with first and second optically variable pigments disposed in the first and second optically variable patterns, respectively, wherein the optically variable pigments have the same color at a certain angle of incidence and different colors at all other angles of incidence.

The pattern of WO96/39307 is characterized in that the design of the paired optically variable pigments is chosen to be in (CIELAB) a^*b^*In the figure, the color of the pigment is represented according to the viewing angle or incident angle, wherein there is a cross point of the viewing angle or incident angle corresponding to the two optically variable pigments having the same hue (hue). The first and second optically variable pigments of WO96/39307 are realized as different quarter-wave designs with approximately the same design wavelength in the case of all dielectric interference pigments. In the case of metal-dielectric interference pigments, the first and second pigments are designed for different half-waves having approximately the same wavelength.

The main disadvantage of the WO96/39307 pattern is that the "same hue" required for the first and second color elements at a certain angle of incidence must be produced by different spectral characteristics, since it is not possible to achieve the same spectral characteristics with different quarter-wave or half-wave designs. The observed hue represents only the projection of the spectral reflectance characteristic of the pigment (i.e., the intensity of the reflectance as a function of wavelength) onto the human color perception three-dimensional space, and as is known to those of ordinary skill, different spectral characteristics may have the same projection (color metamerism) onto the human color perception space.

As a result of this fact, the perceived colors of the first and second optically variable pigments used in WO96/39307 depend on the illumination source spectral characteristics in a different way, so that one said intersection point (where both said optically variable pigments have the same hue) may only be observable under certain types of light sources (e.g. incandescent light) and not appear under different types of light sources (e.g. fluorescent light).

The object of the present invention is to overcome this drawback of the prior art and to disclose a precise (true) paired optically variable security element which always presents a color match at a determined angle of incidence regardless of the illumination source used.

Disclosure of Invention

The invention will now be explained with reference to the following disclosure and claims.

According to the invention, the stated technical problem is solved by a paired optically variable security element having a first and a second optically variable interference pattern realized, for example, in a first and a second optically variable foil, or a first and a second optically variable pigment in an ink or coating composition, the patterns of which are arranged such that they can be seen simultaneously and which exhibit an exact spectral match at a certain angle of incidence while having a different spectrum at all other angles of incidence.

The paired optically variable security element of the present invention thus comprises at least one first and one second optically variable interference pattern having different color shifts (color tracks) and realized as an all dielectric multilayer stack, or a metal-dielectric multilayer stack, or a cholesteric (i.e. chiral nematic) liquid crystal film, or a combination thereof. The interference patterns are further characterized in that they have the same (e.g., quarter-wave or half-wave) interference design at spectrally matched angles of incidence (cross-over points), and in that at least one of their constituent electrolyte layers has a different refractive index.

The invention relies on the fact that the colour track of the interference pattern depends on the refractive index of the dielectric material contained in the pattern. This dependence is a physical law, related to the difference in the speed of light propagation inside and outside the different layers of the interference pattern, and therefore applies to all types of color-shifting optical interference patterns, whether all dielectric multilayer films, metal-dielectric multilayer films, or cholesteric liquid crystal types.

"discoloration" and "color shift" in this disclosure mean the observed color change when an optically variable interference pattern is converted from normal incidence to tangential incidence. "discoloration" more preciselyIndicating the color of the pattern as (CIELAB) a^*b^*The viewing angle or angle of incidence in the figure is a function, whereas "color shift" merely indicates a change in the visual appearance of the pattern. In the context of the present disclosure "optically variable" indicates a color dependent characteristic of viewing angle or angle of incidence.

By "normal incidence" is meant viewing at an angle of 80 ° to 90 ° relative to the plane of the interference pattern. "Grazing incidence (Grazing incidence)" means viewing at an angle of 0 ° to 10 ° with respect to the plane of the interference pattern. Illumination under spectral conditions is generally assumed.

The "intersection point" is the point where the first and second optically variable patterns are at (CIELAB) a^*b^*The viewing angle or angle of incidence is shown with the same hue.

"spectral matching" in this disclosure means that spectral reflection or propagation characteristics as a function of wavelength are qualitatively similar; i.e. the spectral characteristics of the first and second optically variable interference patterns exhibit the same spectral band at the same wavelength and have the same width. Hereinafter "spectral matching" does not mean that the spectral bands of the first and second optically variable patterns have the same absolute intensity. In fact, different absolute reflection or propagation intensities of the first and second patterns, such as may originate from different material usage or different pigment usage (loading), are acceptable in the context of the present invention.

The reflected color, the observed color shift with angle, and the origin of the interference pattern considered herein are explained below in the example of a half-wave designed metal-dielectric thin film interference pattern. Similar reasoning with necessary changes applies to the propagating color of interference patterns used in translucency, as well as to other types of interference patterns, i.e., quarter wave designs, all dielectric thin film patterns, cholesteric liquid crystal films, and possible combinations of such patterns and designs.

A feature of a half-wave engineered metal-dielectric thin film interference pattern is that it includes an "absorbing/dielectric/reflecting" layer structure, where the "absorbing" layer partially propagates and partially reflects incident light, the dielectric layer propagates incident light, and the reflecting layer reflects incident light. Illustrative embodiments of this pattern are given by the following sequence and thickness of layers: "chromium (5 nm)/magnesium fluoride (400 nm)/aluminum (40 nm)".

Referring to fig. 1, the apparent optical thickness ("optical retardation" OL) of the dielectric layer (D) having a refractive index of n > 1 is equal to OL ═ n × D × sin (θ '), where θ' denotes the angle of incidence of light inside the layer (D) with respect to the plane of the layer. The optical retardation is at a maximum (n d) at normal incidence (θ '90 °), and decreases with decreasing angle of incidence to zero at a minimum value at tangential incidence (θ' 0 °). The light wave propagates forward through the dielectric (D), reflects at the reflective layer (R), and propagates back through the dielectric (D), thus lagging behind the light wave reflected at the topmost absorbing layer (a), by an amount 2 x OL-2 x n-D-sin (θ'), as seen from the interior of the layer (D).

Sin (θ ') can be expressed as a function of the angle of incidence θ, which is the angle of the outside of the layer relative to the plane of the layer, by snell's theorem (Snellius ' law). Assuming an external refractive index of 1 (air), the optical retardation as a function of θ is OL ═ d √ n (n)²-cos²(theta)). The optical retardation of the dielectric layer (D) is at maximum (n x D) at normal incidence (θ) 90 ° and decreases with decreasing incidence angle to a minimum value D x (n x) at tangential incidence (θ) 0 ° as viewed from outside the layer²-1). The light wave propagates forward through the dielectric (D), reflects at the reflective layer (R) and propagates back through the dielectric (D), thus lagging behind the light wave reflected at the topmost absorbing layer (a), by an amount 2 x OL-2 x D (n) as seen from outside the layer (D)²-cos²(theta)). The root symbol √ herein denotes the square root of an argument (argument) in parentheses after it.

The total amount of light intensity (R) reflected by the interference pattern is approximately represented by R (λ) ═ I as a function of the incident wavelength (λ)_{Maximum of}*cos²(2. x. OL. pi.)/λ) wherein I_{Maximum of}Is the maximum intensity of the reflection. In addition to the reflection (reflection of the long-wave radiation) for the maximum value λ, the pattern has been defined in OL ═ λ/2 (first order), λ (second order), 3 λ/2 (third order)Order), 2 λ (fourth order), 5 λ/2 (fifth order),. k λ/2 (kth order), i.e. reflection maxima appear at multiples for all "half waves".

It is readily apparent from this sequence that a pattern originating from the 660nm half-wave design (i.e., having its first reflection maximum at the 660nm wavelength (red light)) will have its second reflection maximum at the 330nm wavelength (ultraviolet light), while a pattern originating from the 1320nm half-wave design (i.e., having its second reflection maximum at the 660nm wavelength) will have its third reflection maximum at the 440nm wavelength (blue), and a pattern originating from the 1980nm half-wave design (i.e., having its third reflection maximum at the 660nm wavelength) will have its fourth reflection maximum at the 495nm wavelength (green). It is therefore evident that, obviously, the same reflected colors of interference patterns originating from different half-wave designs must necessarily be metameric, i.e. their matching or mismatching always depends on the illumination conditions.

It is evident from the above-mentioned fact that the two reflection spectra of the paired optically variable security element disclosed in WO96/39307, which is based on first and second optically variable pigments emanating from different quarter-wave or half-wave designs, cannot match each other (compare, for example, fig. 2a and 2c, which show the spectra of a third order green and a second order green, respectively). The color matching performed in this case only solves the problem of projecting these reflection spectra onto the human color-perceived three-dimensional space, while the perceived color is still intrinsically dependent on the illumination conditions employed.

According to the invention, the paired optically variable security element is based on first and second optically variable pigments originating from the same interference design (e.g. the same quarter-wave or the same half-wave design) at a spectrally matched angle of incidence (cross-over). To be at (CIELAB) a^*b^*Where the cross-over point is provided (where the first and second optically variable pigments have the same hue), the reflectivity of at least one of the constituent dielectric layers of the first and second optically variable pigments must be chosen to be different, thus resulting in different color shifts with angle of incidence, while having the same spectral reflection or transmission characteristics, i.e. at the cross-over pointThe true same color.

This corresponds to viewing or incidence angles θ, d satisfying the following relationship₁*√(n₁ ²-cos²(θ))＝d₂*√(n₂ ²-cos²(theta)), wherein d₁、n₁And d₂、n₂The thickness of the first and second interference patterns and the refractive index of the dielectric layer, respectively.

Thus, the color matching in the present invention is precise, an exact match of spectral reflection or transmission characteristics at a determined angle of incidence, and authentication of a document or article by color matching of the two parts of the paired optically variable security element disclosed herein at a determined angle of incidence is not dependent on the chosen illumination conditions.

The amount of angular "color shift" exhibited by an optically variable interference pattern depends significantly on the refractive index of its dielectric layer or layers, and the envisaged metal-dielectric half-wave design can be estimated from the apparent optical thickness ("optical hysteresis") ratio (r (n)) at normal incidence and at tangential incidence, r (n) ═ n/√ n (n)²-1) which is a function of the refractive index of the dielectric layer and which corresponds to the ratio of the peak reflection wavelength at orthogonal and tangential viewing (√ denotes the square root of the following argument in parentheses).

Table 1 shows r (n) ═ λ as a function of the refractive index n_{Orthogonal (orthogonall)}/λ_{Tangential direction (grazing)}The calculated value of (a).

TABLE 1

n	r(n)	n	r(n)	n	r(n)	n	r(n)
								1.00	Infinite number of elements	1.25	1.67	1.50	1.34	1.75	1.22
1.05	3.28	1.30	1.57	1.55	1.31	1.80	1.20
								1.10	2.40	1.35	1.49	1.60	1.28	1.85	1.19
1.15	2.03	1.40	1.43	1.65	1.26	1.90	1.18
								1.20	1.81	1.45	1.38	1.70	1.24	Infinite number of elements	1.00

The paired optically variable security element of the present invention therefore comprises: having a lower refractive index (n)_{Is low in}) And its k-th order reflection maximum (k) shows a first reflection wavelength (λ) from normal incidence₁) To the second shorter wavelength (lambda) at tangential incidence₂) The offset of (a); and having a higher refractive index (n)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from orthogonal incidence₃) To the fourth shorter wavelength (lambda) at tangential incidence₄) The offset of (a); the security element is characterized in that the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern. What is needed isThe latter aspect is a requirement for the presence of an angle of incidence under which the k-th reflection maxima of the first and second interference patterns coincide.

The first and second optically variable interference patterns must have the same half-wave or quarter-wave design in order to produce spectral matching and can be realized by a design selected from the group consisting of a fully dielectric multilayer stack, a metal-dielectric multilayer stack, a cholesteric liquid crystal film, and combinations thereof.

In a particular embodiment of the security element of the invention, the interference pattern is realized by an optically variable foil. In the case of a metal-dielectric pattern, the optically variable foil may comprise a sequence of "absorbing/dielectric/reflecting" layers, wherein the reflecting layer may be followed by further layers, and the foil is intended to be adhered to a substrate, such that the "absorbing" layer is on the outside.

In another particular embodiment of the security element of the invention, the interference pattern is realized by an optically variable pigment contained in an ink or coating composition and applied to the document or article to be protected. In the case of a metal-dielectric interference pattern, the optically variable pigment may comprise a layer sequence of "absorbing/dielectric/reflecting/dielectric/absorbing", wherein the reflecting layer may also comprise internal layers.

In another particular embodiment, the optically variable pigment is incorporated in a plastic foil, resulting in another type of optically variable foil. The optically variable pigments herein may be incorporated into the plastic substance used to cast the foil and oriented by controlled stretching of the foil (e.g., by calendaring). Alternatively, the optically variable pigment may be laminated between two plastic foils to make a single optically variable foil.

Combinations of inks or coatings comprising optically variable pigments, foils comprising optically variable pigments, and optically variable foils may also be used to implement the security element of the present invention, provided that the required spectral matching conditions are met.

The security element may also be implemented in or on a transparent or translucent substrate for viewing in translucency, or on an opaque substrate for viewing in reflection.

More specifically, the security element of the invention may be realized in the form of an optically variable print made with ink on a substrate, in the form of an optically variable foil adhered to a substrate, in the form of a security thread incorporated into a substrate, or in the form of a transparent window substrate.

Also disclosed is a process for making a paired optically variable security element, the process comprising the steps of:

-applying a coating having a lower refractive index (n) to the substrate (S)_{Is low in}) And its k-th order reflection maximum (k) shows a first reflection wavelength (λ) from normal incidence₁) To a second shorter wavelength (lambda) at tangential incidence₂) The offset of (a);

-applying a material having a higher refractive index (n) to the substrate (S)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from orthogonal incidence₃) To the fourth shorter wavelength (lambda) at tangential incidence₄) The offset of (a);

wherein the first and second optically variable patterns are selected such that the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern.

The first and second optically variable interference patterns have the same half-wave or quarter-wave design and are preferably arranged so that they are visible simultaneously.

The paired optically variable security elements according to the invention can be used for the protection against forgery of documents such as banknotes, value documents, identity documents, access documents, labels or tax stamps, and also for the marking of articles.

Also disclosed is a security document, such as a banknote, a value document, an identity document, an access document, or a tax excise stamp, comprising a pair of optically variable security elements according to the invention.

The invention also comprises a set of first and second optically variable interference patterns for implementing a paired optically variable security element, the first optically variable interference pattern having a low refractive index (n)_{Is low in}) And its k-th order reflection maximum (k) shows a first wavelength (λ) from normal incidence₁) To the second shorter wavelength (lambda) at tangential incidence₂) The offset of (a); and the second optically variable interference pattern has a higher refractive index (n)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from orthogonal incidence₃) To the fourth shorter wavelength (lambda) at tangential incidence₄) Wherein the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern.

In particular, the first and second optically variable interference patterns may be selected from the group consisting of optically variable foils, optically variable lines, and optically variable windows.

The invention also comprises a set of first and second optically variable coating compositions, in particular inks, for realising a paired optically variable security element, the first coating composition comprising a polymer having a low refractive index (n)_{Is low in}) And its k-th order reflection maximum (k) shows a first wavelength (λ) from normal incidence₁) To the second shorter wavelength (lambda) at tangential incidence₂) The offset of (a); and the second coating composition comprises a material having a relatively high refractive index (n)_{Height of}) And the same k-th order reflection maximum (k) thereof shows a third reflection wavelength (λ) from the normal incidence₃) To the fourth shorter wavelength (lambda) at tangential incidence₄) Wherein the range covered by the third and fourth wavelengths of the second pigment is within the range covered by the first and second wavelengths of the first pigment.

In particular, the first and second optically variable coating compositions may be selected from the group consisting of screen printing inks, copperplate-intaglio inks and gravure inks.

Brief Description of Drawings

Fig. 1 illustrates the origin of perceived color and color shift of an absorbing/dielectric/reflective/thin film multilayer stack, such as may be used in the present invention.

Fig. 2 shows the physical working principle of the security element according to the invention:

a)Cr/MgF₂orthogonal incidence spectra of/Al metal-dielectric interference stacks;

b)Cr/Y₂O₃orthogonal incidence spectra of/Al metal-dielectric interference stacks;

c)Cr/Y₂O₃a grazing incidence spectrum of the/Al metal-dielectric interference stack;

d)Cr/MgF₂grazing incidence spectra of/Al metal-dielectric interference stacks.

Fig. 3 schematically shows a pair of optically variable security elements of the invention, which are realized by the markings of a first (right part of the image) and a second (left part of the image) interference pattern.

a) A pair of security elements seen at normal incidence (90 °), the left and right parts having different colours;

b) the left and right portions of the paired security elements viewed at a 45 ° angle of incidence have the same color;

c) the left and right portions of the paired security elements, seen at an angle of incidence of 30 deg., have different colours.

Detailed Description

The invention will now be explained with the aid of figures and exemplary embodiments.

Fig. 1 illustrates the origin of perceived color and color shift of an absorbing/dielectric/reflective/thin film multilayer stack such as may be used in the present invention: the reflective layer (R), which may have an internal layer structure, carries at least one dielectric layer (D), which in turn carries an absorption layer (A) at its outer surface. Incident light (I) falling on the pattern at an incident angle theta₀) Splitting at the absorption layer (A) into primary reflected beams (I)₁) And a primary propagation beam (I)₂) (ii) a The latter propagates forward through the dielectric layer (D) at an angle of incidence θ' changed by refraction, reflects at the reflective layer (R), propagates back through the dielectric layer (D) and the absorbing layer (a), and finally as a secondary reflected beam (I) at the angle of incidence θ₃) Away from the pattern. The primary reflected beam (I)₁) And a secondary reflected beam (I)₃) Interfere with each other causing some wavelengths to partially or completely disappear while others do not disappear (destructive and constructive interference), thereby causing a color display by selective reflection of specific portions of the white light spectrum.

Convenient dielectric materials for implementing all-dielectric or metal-dielectric designs are known to those of ordinary skill and may be found in the specialized literature, such as "thin film transistor optical filters" by h.angus Macleod, third edition, chapter 15; table 2 ", below, optical parameters of exemplary dielectric materials that can be used to implement the present invention are given.

TABLE 2

Data were taken from m.ohring, thin film materials Science (The Material Science of ThinFilms), Academic Press (Academic Press, Inc.), boston, 1992.

Composition of matter	Packing density	Propagation Range (μm)	Refractive index
				NaF	1	0.15-14	1.3
LiF	1	0.10-8	1.3
				CaF2	0.57-1.00	0.15-12	1.23-1.46
AlF3	0.64	0.2-14	1.23
				MgF2	0.72-0.80	0.11-4	1.32-1.39
LaF3	0.80	0.25-2	1.55
				CeF3	0.80	0.3-5	1.63
SiO2	0.9	0.2-9	1.45
				Al2O3	1	0.2-7	1.54
MgO	1	0.2-8	1.7
				Y2O3	1	0.3-12	1.89
La2O3	1	0.3-12	1.98
				CeO2	1	0.4-12	2.2
ZrO2	0.67	0.34-12	1.97
				ZnO	1	0.4-3	2.1
TiO2	1	0.4-3	1.9

Optically variable thin film interference patterns having all-dielectric or metal-dielectric designs can be produced by sequential Physical Vapor Deposition (PVD) of different materials making up the thin film pattern on a suitable carrier substrate, as known to those of ordinary skill, such as described in US4,705,356; US4,838,648; US4,930,866; US 5,084,351; US 5,214,530; US 5,278,590; EP-B-0227423; and EP-B-1366380 and documents related thereto.

The support is preferably a flexible foil (web), for example a release-coated polyethylene terephthalate (PET) foil. The vapor deposition can be carried out in a high vacuum coater in a roll-to-roll process. The material is evaporated using appropriate evaporation sources and processes known to those of ordinary skill that are specific to the material, such as sputtering, reactive sputtering, magnetron sputtering, thermal evaporation, electron beam or laser beam assisted evaporation.

Other ways of depositing the thin film pattern include Chemical Vapor Deposition (CVD), wet coating, and especially sol-gel coating processes. However, in Physical Vapor Deposition (PVD), the material to be deposited is merely evaporated from the source and onto the linerCondensation on the substrate, Chemical Vapor Deposition (CVD), implies that one or more precursor compounds at the surface of the substrate (typically heated, otherwise or excited) undergo a chemical reaction. Critical case of reactive sputtering in which a precursor material (e.g., Ti) is sputtered from a source and reacted with an existing gas phase (e.g., O) at reduced pressure₂) Reaction, depositing reaction products (e.g. TiO) on the substrate₂) Is herein counted as a physical vapor deposition process because it occurs under PVD-like process conditions and results in PVD-like deposition.

Cholesteric liquid crystal films are known from WO 9409086a1, EP 0601483a1, US 5502206, EP 0661287B1, EP0686674B1, US 5683622, EP 0709445B1, EP 0712013a2, WO 9729399a1, EP 0875525a1, EP 0885945a1 and related documents known to the skilled person. Such foils are obtained by coating a carrier foil with a polymerizable cholesteric liquid crystal precursor mixture, followed by aligning the liquid crystal in the cholesteric phase at a suitable temperature and fixing it by polymerization, e.g. by UV curing. Corresponding Cholesteric Liquid Crystal Polymer (CLCP) pigments were obtained by comminuting such foils to the desired particle size. Coating compositions comprising such pigments are disclosed in US 5807497, EP 0758362a1, WO 9532247a1, EP 0887398a1 and related documents known to the skilled person.

The refractive index of cholesteric liquid crystal polymers can be varied by appropriate selection of the chemistry used. It is clear that a large number of crosslinkable monomers and oligomers are known for forming cholesteric phases under suitable conditions, the phases of which can be "frozen" in a defined state by introducing radiation or otherwise introducing a crosslinking reaction. Monomers and oligomers free of aromatic residues such as benzene, naphthalene, and other conjugated rings result in low refractive index cholesteric liquid crystal polymers. An example of this type is a liquid crystalline polymer derived from cholesteric. On the other hand, monomers and oligomers containing aromatic residues such as benzene, naphthalene and other conjugated rings result in cholesteric liquid crystal polymers of high refractive index. Examples of this type are the polymers described in EP-B0685749 and EP-B0760836.

In a particular embodiment, a previously embossed and release-coated carrier foil is used, for example a PET carrier foil. The embossing is performed with the aid of a heated embossing shim, as is well known to those of ordinary skill in the art of surface hologram manufacturing. The relief pattern in the carrier foil is then reproduced by an optically variable multilayer interference pattern vapor deposited on top of the carrier foil or by a liquid crystal film produced on top of the carrier foil.

The carrier foil coated with the optically variable interference pattern can also be converted into a hot or cold die transfer foil for document copy protection according to known procedures.

More preferably, however, the optically variable interference pattern film is peeled off from the carrier foil and pulverized into a pigment, thereby obtaining pigment powder (flake) having a particle size in the range of 200nm to 3,000nm in thickness, preferably in the range of 400nm to 5,000nm in thickness, and a particle diameter in the range of 5 to 50 μm. The comminution may advantageously be carried out by means of jet milling, and the resulting particles are preferably classified into suitably sized groups (fractions).

The resulting optically variable pigments are preferably formulated into printing inks which comprise Pigment amounts in the range from 1 to 25% by weight and also comprise at least one organic polymer or polymer precursor as binder, and also suitable further types of pigments, in particular coated particles and/or glitter pigments, conventional dyes, inorganic and organic printing pigments such as described in O.L lake Pigment plus F ü llstoff Tabellen, fifth edition, Laatzen, 1994, as well as extenders, rheological additives, solvents, photosensitizers and drying agents. Other security materials may also be present in the ink, such as magnetic pigments, luminescent pigments or dyes, and infrared absorbing pigments or dyes, among others.

The ink composition is preferably formulated for use in a screen printing process, such as having a viscosity in the range of 0.5 to 2Pa at 40 ℃; however, other preferred options include inks for copperplate intaglio printing processes having a viscosity in the range of 2 to 20Pa at 40 ℃ and inks for flexographic printing processes having a viscosity in the range of 0.1 to 0.5Pa at 40 ℃. The formulation of such inks is known to the skilled person.

The resulting optically variable ink can be used to print indicia on an article to be protected (e.g., a security document) that are implemented as paired optically variable patterns so that they can be viewed simultaneously. The optically variable security feature thus obtained is easily detectable by the human eye, for example by comparing two optically variable patterns forming a pair of optically variable features with an angle of incidence determined by inspection at which they have the same spectral reflection or propagation characteristics. This comparison, which is truly independent of ambient lighting conditions, allows the authenticity of the document carrying the paired optically variable security feature of the present invention to be determined by simple visual testing.

In a further embodiment of the invention, magneto-optical variable pigments are used (e.g. according to US4,838,648 or EP-B-1366380) and the magneto-optical pigment powder (flake) in the ink is further oriented during or after the printing process by applying a corresponding magnetic field (e.g. according to EP-B-1641624) and the position of the thus oriented powder is subsequently fixed by a hardening glue. Preferably, a UV curable ink formulation is used for this application; such formulations may be prepared in a manner known to those of ordinary skill.

Examples of the invention

The first and second optically variable interference patterns are prepared by successive physical vapor deposition of different layers, each of a symmetric half-wave metal-dielectric interference design, on a release-coated PET carrier foil.

Chromium (Cr), manganese fluoride (MgF)₂(ii) a n ═ 1.35), yttrium oxide (Y)₂O₃(ii) a n ═ 1.89) and aluminum (Al) were deposited as known to the skilled person and described in the technical citations, noting that electron beam assisted evaporation sources were used in high pressure.

First pattern

The absorption/dielectric/reflection/dielectric/absorption type of symmetrical design has the following layer sequence:

1. an absorption layer: cr, 3.5 nm

2. Dielectric layer: MgF₂490 nanometer (n ═ 1.35)

3. A reflective layer: al, 40nm

4. Dielectric layer: MgF₂490 nanometer (n ═ 1.35)

5. An absorption layer: cr, 3.5 nm

Designed to have a second order reflection maximum at 600nm at normal incidence (k 2); resulting in a second order reflection maximum at 445nm at tangential incidence. The color shift of the first interference pattern is from green (orthogonal) to magenta (tangential).

Second pattern

The absorption/dielectric/reflection/dielectric/absorption type of symmetrical design has the following layer sequence:

1. an absorption layer: cr, 3.5 nm

2. Dielectric layer: y is₂O₃315 nanometer (n ═ 1.89)

3. A reflective layer: al, 40nm

4. Dielectric layer: y is₂O₃315 nanometer (n ═ 1.89)

5. An absorption layer: cr, 3.5 nm

Designed to have a second order reflection maximum at 600nm at normal incidence (k 2); resulting in a second order reflection maximum at 510nm at tangential incidence. The color shift of the second interference pattern is from violet (orthogonal) to green (tangential).

The physical working principle of the paired optically variable security element according to the invention is now explained with reference to fig. 2 and 3.

At normal incidence (FIG. 3a, right), Cr/MgF of the first pattern₂the/Al metal-dielectric interference stack shows a reflection spectrum as shown in FIG. 2a, which is at 500With a third order reflection maximum in the blue-green spectrum at nm. The second order reflection maximum (k) is in the red spectrum at 660nm (first wavelength, λ)₁). Upon tilting the first pattern to tangential incidence (fig. 3c, right), i.e. to the spectrum in fig. 2d, the second order reflection maximum moves to the blue spectrum at 445nm (second wavelength, λ₂)。

At normal incidence (FIG. 3a, left), Cr/Y of the second pattern₂O₃the/Al metal-dielectric interference stack shows a reflection spectrum as shown in FIG. 2b at 600nm (third wavelength, λ)₃) With a second order reflection maximum in the orange spectrum. Upon tilting the second pattern to tangential incidence (fig. 3c, left), i.e. to the spectrum in fig. 2c, the second order reflection maximum shifts to the blue-green spectrum at 510nm (fourth wavelength, λ₄)。

The range covered by the third and fourth wavelengths of the second pattern is thus within the range covered by the first and second wavelengths of the first pattern. As a result, there is an angle of incidence or viewing at which the discolorations of the first and second patterns must intersect; the spectra at this intersection are equal and therefore the colors of the two patterns are equal, regardless of the illumination conditions.

In this example, the intersection point is at a viewing or incident angle θ at 40 ℃, where the two optical interference paths through the dielectric layers of the first and second interference patterns are equal. The second order reflection maximum is at 545nm and both patterns show the same turquoise interference color (fig. 3 b).

The interference pattern thus obtained may be converted into a stamp foil, together forming a set of first and second optically variable interference patterns for realizing a pair of optically variable security elements according to the invention.

Alternatively, the resulting interference pattern may be removed from the carrier foil, comminuted into color elements and converted into printing ink according to methods known to the person skilled in the art and described in the article, together forming a set of first and second optically variable coating compositions for realizing the paired optically variable security element according to the invention.

An exemplary printing ink may be formulated as follows:

ink for copper plate intaglio printing process:

inks for screen printing process (UV drying):

epoxy acrylate (Epoxyacrylate) oligomers	40％
		Trimethylolpropane triacrylate (trimethyolpropane triacrylate) monomer	10％
Trimethylene glycol diacrylate (Tripropylate) monomer	10％
		Genorad 16(Rahn)	1％
Aerosil 200(Degussa-Huels)	1％
		Irgacure 500(CIBA)	6％
Genocure EPD(Rahn)	2％
		The optically variable pigment according to the present invention	20％
Dowanol PMA	10％

Inks for flexographic (flexographic) printing process (UV drying):

using a corresponding set of such inks, the paired optically variable security element of the present invention may be printed in the form of a mark on a security document such as a banknote, value document, identity document, access document, indicia, or tax excise stamp, or on an article of commerce.

The person of ordinary skill will be able to easily derive other embodiments of the invention based on his technical knowledge, the cited prior art, and the disclosure given herein. The present invention is clearly not limited to the illustrated absorptive, reflective, and dielectric materials, nor to the illustrated interferometric design, and may be practiced with other materials and interferometric designs, so long as the principles outlined herein are followed.

Claims (15)

1. A paired optically variable security element comprising: having a lower refractive index (n)_{Is low in}) And its k-th order reflection maximum (k) shows a first reflection wavelength (λ) at normal incidence₁) A second shorter wavelength (λ) at tangential incidence₂) The offset of (a); and having a higher refractive index (n)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from normal incidence₃) Cutting towardsA fourth shorter wavelength (λ) at down incidence₄) Wherein the first and second optically variable interference patterns are arranged such that they are visible simultaneously, characterized in that,

the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern,

and the first and second optically variable interference patterns have the same quarter-wave or half-wave interference design at spectrally matched angles of incidence.

2. The security element according to claim 1, characterized in that said first and second optically variable interference patterns are realized by an interference design selected from the group comprising: all dielectric multilayer stacks, metal-dielectric multilayer stacks, cholesteric liquid crystal films, and any combination thereof.

3. The security element according to any one of claims 1 to 2, wherein said first and second optically variable interference patterns are selected from the group comprising: optically variable foils, foils comprising optically variable pigments, optically variable pigments comprised in coating compositions, and combinations of optically variable foils and optically variable pigments.

4. The security element according to any one of claims 1 to 2, wherein said first and second optically variable interference patterns are contained on a substrate selected from the group consisting of: a transparent substrate, a translucent substrate, and a non-transparent substrate.

5. A security element as claimed in any one of claims 1 to 2, wherein the security element is implementable in a form selected from the group comprising: optically variable printing made with ink on a substrate, optically variable foil adhered to a substrate, security thread incorporated into a substrate, and a transparent window substrate.

6. A process for making a paired optically variable security element comprising the steps of:

a) applying a material having a lower refractive index (n) to a substrate (S)_{Is low in}) And its k-th order reflection maximum (k) shows a first reflection wavelength (λ) at normal incidence₁) A second shorter wavelength (λ) at tangential incidence₂) The offset of (a);

b) applying a high refractive index (n) to the substrate (S)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from normal incidence₃) A fourth shorter wavelength (λ) at tangential incidence₄) The offset of (a);

wherein the first and second optically variable interference patterns are arranged such that they are visible simultaneously;

it is characterized in that the preparation method is characterized in that,

the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern,

and the first and second optically variable interference patterns have the same quarter-wave or half-wave interference design at spectrally matched angles of incidence.

7. The process of claim 6, wherein the first and second optically variable interference patterns are realized by an interference design selected from the group consisting of: all dielectric multilayer stacks, metal-dielectric multilayer stacks, cholesteric liquid crystal films, and combinations thereof.

8. The process of any one of claims 6 to 7, wherein the first and second optically variable interference patterns are selected from the group consisting of: optically variable foils, foils comprising optically variable pigments, optically variable pigments comprised in coating compositions, and combinations of optically variable foils and optically variable pigments.

9. A security document or a marked article, characterized in that it comprises a security element according to any of claims 1 to 5.

10. A security document or marked article according to claim 9, wherein the security document is a banknote, a value document, an identity document, an access document, a mark, or a tax excise stamp.

11. A set of first and second optically variable interference patterns for implementing a pair of optically variable security elements as claimed in any one of claims 1 to 5, the set comprising:

having a lower refractive index (n)_{Is low in}) And its k-th order reflection maximum (k) shows a first reflection wavelength (λ) at normal incidence₁) A second shorter wavelength (λ) at tangential incidence₂) The offset of (a); and

having a higher refractive index (n)_{Height of}) And its same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from normal incidence₃) A fourth shorter wavelength (λ) at tangential incidence₄) The offset of (a) is determined,

it is characterized in that the preparation method is characterized in that,

the range covered by the third and fourth wavelengths of the second pattern is within the range covered by the first and second wavelengths of the first pattern,

and the first and second optically variable interference patterns have the same quarter-wave or half-wave interference design at spectrally matched angles of incidence.

12. The set of claim 11, wherein the first and second optically variable interference patterns are selected from the group consisting of: optically variable foils, optically variable wires, and optically variable windows.

13. A set of first and second optically variable coating compositions for implementing a paired optically variable security element as claimed in any one of claims 1 to 5, the set comprising:

comprising a refractive index (n) of lower_{Is low in}) And a first coating composition of a first optically variable interference pigment of a first dielectric and having a k-th order reflection maximum (k) showing a first reflection wavelength (λ) at normal incidence₁) A second shorter wavelength (λ) at tangential incidence₂) The offset of (a); and

comprising a refractive index (n) of higher_{Height of}) And the same k-th order reflection maximum (k) shows a third reflection wavelength (λ) from normal incidence₃) A fourth shorter wavelength (λ) at tangential incidence₄) The offset of (a);

it is characterized in that the preparation method is characterized in that,

the range covered by the third and fourth wavelengths of the second pigment is within the range covered by the first and second wavelengths of the first pigment,

and the first and second optically variable interference patterns have the same quarter-wave or half-wave interference design at spectrally matched angles of incidence.

14. The set of claim 13, wherein the first and second optically variable coating compositions are first and second optically variable printing inks.

15. The set of claim 14, wherein the first and second optically variable printing inks are selected from the group consisting of: screen printing inks, copper plate-intaglio inks and flexographic gravure inks.