Security elements comprising microrelief structures

Abstract

A security element (20) comprising a novel form of microrelief structure (25), e.g. for a document, banknote, passport, visa, ID card or document, driving licence, membership card, ticket, certificate, packaging, work of art, antique or other valuable item, comprises a layer (22) of optical material and at least two microrelief structures (25) formed on or in a surface thereof, wherein the surface of the layer (22) is partitioned into - or it may comprise (either in a real or a notional sense) - a plurality of sub-areas or sub-structures or sub-elements, wherein mutual boundary regions of neighbouring or adjacent said sub-areas / sub-structures / sub-elements are constructed such that, or define an optical function such that, the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially the same or such boundary regions redirect or transform incident light in substantially the same manner, whereby the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements are substantially the same within and passing across their mutual boundary regions with substantially no variation in that optical function occurring in those mutual boundary regions originating from or caused by the mutual boundaries themselves between the said neighbouring or adjacent said sub-areas / sub-structures / sub-elements. In some embodiments, the microrelief structured surface of the element (20) may be constructed such that, or defines an optical function such that, for the or each one of one or more (especially the or each one of a plurality of different) input light directions of light incident on the basic microrelief structure (25), the said incident light in the or each respective said input light direction is redirected or transformed into a respective plurality of - especially into a number that is an integer of from 3 to 10 - output light directions exiting the basic microrelief structure, or a cone defined by these directions.

Description

[0001]JL / 40188 1 SECURITY ELEMENTS COMPRISING MICRORELIEF STRUCTURESTECHNICAL FIELDThis invention relates to security elements, and in particular to security elements comprisingmicrorelief (i.e. optical surface relief) structures. Such security elements are useful forsecurity or authentication purposes in various end applications such as brand protection,document or banknote protection, passports, visas and ID cards or documents, drivinglicences, membership cards, tickets, packaging, works of art, antiques or other valuableitems, etc. More particularly (though not exclusively) the invention relates to securityelements that comprise microrelief structures of new forms or structures that provide anenhanced degree of security or authentication capability, especially owing to their enhanceddegree of resistance to counterfeiting. The invention also relates to such novel microreliefstructures per se, as well as to methods for making such security elements comprising them,and to an article or product having applied or affixed thereto or incorporated therein such asecurity element.BACKGROUND AND PRIOR ARTAs used herein, the term “optical security element” is to be construed broadly and meansany physical or functional device, apparatus, object, item or thing that relies on optics or themanipulation or effects of electromagnetic radiation for, and is designed and intended for usein connection with, one or more security purposes, e.g. for identification, authentication orprotection purposes of an item of property or some other object, item or thing. Such a“device” may be a physical thing per se which may be incorporated into or included in or onthe structure of the item, object or thing whose security is to be so effected, or it may be aninherent part, portion or feature of the item, object or thing itself whose security is to be soeffected.As used herein the term “microrelief” is to be construed broadly as not being limited to opticalsurface relief being of any particular size, scale or dimensions (especially not necessarily ofmicron(s)-depth), unless expressly stated otherwise. Thus, as the term is used herein, the“microrelief” may be or comprise - unless expressly stated otherwise - optical surface reliefof depth of the order of several or many nanometres as well as of the order of a few up toseveral or even many microns (micrometres). JL / 40188 2As used herein the term “optical material” is to be construed broadly as meaning any materialthat is able to interact with incident light in some way, especially by either transmission orreflection (or a combination thereof) of that light, and is capable of having a microreliefstructure formed on or in, or applied to, a surface or face thereof.Considered in their very broadest terms, security elements for various kinds of documents orvaluable items, for which security, authentication or anti-counterfeiting measures need to beincorporated therein, are typically based on optical surface relief for providing the requiredsecurity property or properties. Such optical surface relief - which can be generally (althoughsomewhat loosely) termed “microrelief” - is basically just a roughened surface of a suitablesubstrate (e.g. a sheet, film, foil, strip or plate of a suitable material in whose surface therelief is formed). Although the depth or height of the bumps or ridges / valleys that create theroughness may typically be of the order of around 1 μm or so, the microrelief is engineeredto reflect light in a peculiar way in order to provide an interesting visual illusion, which servesthe security, authentication or anti-counterfeiting purpose. There are two simple types ofmicrorelief frequently used in known security elements: the first one is split into zones wheretheir boundaries resemble contour lines of a map or terrain - as shown by way of schematicexample in FIG.2(a) of the accompanying drawings, where 5 represents some of the contourlines - and the second one is based on a relief surface that is split into small squares (i.e.“small relief elements”) but whose individual relief surface features within each square (orrelief element) may be or are still split into zones, where their boundaries still somewhatresemble directions of similar contour lines - as shown by way of schematic example in FIG.2(b) of the accompanying drawings, where 3 represents some of the small squares. Notethat in these FIGS. 2(a) & (b) the grey tone level indicates the microrelief depth (or height, ifconsidered that way). Also note that this invention may be primarily related to the secondtype of microrelief, i.e. the one split into small relief elements.Looked at more basically, microrelief based security elements, which in common parlanceare often termed simply “holograms” but may generally also be termed “kinegrams” or simply“OVDs” (i.e. optically variable devices), can provide various visual effects useful for security,authentication or anti-counterfeiting purposes, such as colourful or achromatic kinetic effects,3D illusions, flips, animations, colourful or achromatic bas-relief illusions or the like. Despitevarious physical approaches behind the microrelief design, the goal is always the same:namely, to redirect the light reflected off the surface (or transmitted through the surface) tothe eyes of the observer. If, for instance, a small area of the OVD redirects red light to onedirection and green light to another direction, the observer perceives a colour shift when JL / 40188 3viewing the OVD from different directions. This is illustrated in FIG.1(a) of the accompanyingdrawings. This variation of look with the viewing direction makes OVDs impossible to mimicusing conventional printing technologies, thus making them comparatively difficult tocounterfeit.The microrelief, or more precisely microrelief structure, responsible for such effects is oftenvery simple in its nature. There are two or three basic types of such microrelief (i.e. two orthree types of basic microrelief structure) that are commonly used:The first common type of basic microrelief structure resembles mirror facets: a lightray hitting the facet is redirected according to the geometric law of reflection into a particulardirection depending on the facet slope and orientation. This is illustrated in FIG. 1(b) of theaccompanying drawings. (For transmissive security elements, facets are considered assurfaces of microprisms that redirect light according to the geometric law of refraction.) Forinstance, to make an illusion of a spherical mirror, one creates facets of the same slope andorientation as the corresponding part of the spherical surface, this process being well-knownfrom the art of making Fresnel lenses, as illustrated in FIG. 1(c) of the accompanyingdrawings. The depth of such a microrelief is usually in the range of from about 0.5 µm toabout 10 µm or even up to about 100 µm.The second common type of basic microrelief structure employs diffraction gratingsto redirect light, i.e. gratings being sequences of a few substantially equidistant strips ofvarying depth. A diffraction grating redirects light depending on the strips’ spacing (i.e. thegrating period) and orientation. Moreover, the strip depth profile and maximum depth affectthe amount of redirected light. For instance, to make an area that looks red at the top andgreen at the bottom, one makes a microrelief with strips that are (for instance) sparser nearthe top (as illustrated in FIG.1(d) of the accompanying drawings, where 2 is the strip material)and denser near the bottom (as illustrated in FIG.1(e) of the accompanying drawings, where2 is the strip material). The depth of such a microrelief is usually in the range of from about50 nm to about 500 nm.The third common type of basic microrelief structure will be mentioned a little furtherbelow.Since the OVD should generally be big enough in order to be visible by the naked humaneye, and should provide a distinctive, easy to remember optical illusion, the properties of themicrorelief often vary over the surface of the OVD. For instance, the period and orientationof the diffraction grating may change from one point to another. Another example has beenalready mentioned for the making of a bas-relief illusion: namely, a slope of a mirror facet JL / 40188 4may change smoothly across it, and such facets follow substantially smooth curves 5’ similarto the contour lines of a map or terrain - which is illustrated in FIG.2(c) of the accompanyingdrawings. In a completely different approach, one may instead split the area of the OVD intosmall elements 3’ (often called pixels or hogels or dots 3’) and fill each small element with abasic microrelief structure - which is illustrated in FIG. 2(d) of the accompanying drawings.In this case, for the first type of basic microrelief structure the diffraction gratings in the smallelements are often strictly linear, and for the second type of basic microrelief structure thefacets in the small elements are often strictly planar. It can thus be said in a mathematicalway that the properties (such as the diffraction grating period and orientation, or facet slopeand orientation) of such a basic microrelief structure are then piecewise substantiallyconstant across the OVD. One can also conclude that optical properties of such a basicmicrorelief structure are substantially constant within the small elements.Since a primary goal in designing microrelief-based security elements is to make themdifficult to counterfeit, it should usually be the case that such security elements contain avariety of different optical effects. The easiest way to accomplish this is again to partition theOVD area into small elements and to designate some of them to a first optical effect, othersto a second effect, and so on. If, for instance, one splits the OVD area into small squareelements and use, say, 50% of them for a simple colour effect and the other 50% for ananimation (e.g. in a checkerboard pattern or even a random pattern), then the observer willperceive the semi-transparent animation on the colour background. An example of this ideais illustrated in FIG.2(e) of the accompanying drawings.These known approaches share a common weakness. Since the above-discussed commontypes of basic microrelief structures are simple and their organization over the OVD surfaceis often simple as well, it is easy for a counterfeiter to inspect the microrelief (e.g. under amicroscope) and to use the gained knowledge for the purpose of counterfeiting it. Acounterfeiter skilled in the art can reverse engineer the principle of a particular optical effectby examining and observing the mirror facet slopes, the grating periods, and other visiblecharacteristics of the microrelief. It is worth emphasizing that a counterfeiter usually doesnot try to actually replicate the microrelief itself; the goal is generally to provide just a verysimilar looking optical effect, and by whatever means makes that possible cheaply andquickly.The third common type of basic microrelief structure yet to be mentioned is somewhat saferin this respect. Using tools well-known in the field of digital holography, one can calculate a JL / 40188 5synthetic digital hologram and render it in the form of a microrelief. Such a digital hologramusually looks like an unordered tangle of elevated and depressed fringes or spots, and it iscomparatively difficult to recognize its optical function from mere observation. Two examplesof such synthetic digital holograms are illustrated in FIGS. 3(a) & (b) of the accompanyingdrawings. While such a synthetic hologram works reasonably well in some instances, e.g.for 3D illusions, other effects, e.g. colourful kinetic effects, are usually better accomplishedusing the aforementioned other approaches. Even with the synthetic hologram approach,however, it is still often advantageous to split the area of the OVD into small elements andto fill them with small synthetic holograms (e.g. to make a holographic stereogram) oralternative other microreliefs.Thus, it is a primary object of the present invention to address the above shortcomings in theknown art of microrelief-based security elements. This object focuses in particular on tryingto obfuscate the structure of the microrelief in order to make its reverse-engineering moredifficult.SUMMARY OF THE INVENTIONAccordingly, the present invention aims to increase security of OVDs by obfuscating thestructure of the microrelief, in order to make its reverse-engineering more difficult, primarilyby making partitioning of the security element’s surface less obvious and thus more difficultto discern.In certain embodiments of the invention, in particular embodiments which employ an optionalfeature - of constructing the basic microrelief structure such that it defines an optical functionsuch that, for the or each one of one or more (especially the or each one of a plurality ofdifferent) input light directions of light incident on the basic microrelief structure, the saidincident light in the or each respective said input light direction is redirected or transformedinto a respective plurality of - especially into a number that is an integer of from 3 to 10 -output light directions exiting the basic microrelief structure - such embodiments may go evenfurther and make the basic microrelief structures employed in a security element moredifficult to understand.In a first aspect, the present invention provides a security element comprising a layer ofoptical material and at least two basic microrelief structures formed on or in a surface thereof,wherein the surface of the layer is partitioned into - or it may comprise (either in a real or a JL / 40188 6notional sense) - a plurality of sub-areas or sub-structures or sub-elements,wherein mutual boundary regions of neighbouring or adjacent said sub-areas / sub-structures / sub-elements are constructed such that, or define an optical function such that,the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially the same or suchboundary regions redirect or transform incident light in substantially the same manner,whereby the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements are substantially the same within and passing across their mutualboundary regions with substantially no variation in that optical function occurring in thosemutual boundary regions originating from or caused by the mutual boundaries themselvesbetween the said neighbouring or adjacent said sub-areas / sub-structures / sub-elements.As used herein, by the term “basic microrelief structure” is meant a type of a microreliefstructure (such as, for example, a diffraction grating structure, a micro-mirror relief structure,e.g. in a continuous or piece-wise continuous form, a holographic structure, e.g. a Fourierhologram structure, etc) or a basic sub-area / sub-element with such a microrelief structure (inparticular in its central or major portion outside its mutual boundary regions), which isimplemented in a layer of optical material of the security element, and where differentcharacteristics (or properties) of the given basic microrelief structure (such as, for example,grating period, maximum microrelief depth, optical function, microrelief profile or form,Fourier hologram image etc) vary between different sub-areas / sub-elements of the layeraccording to the designer's intentions. Different portions (or areas) of the layer of the opticalmaterial may then contain different types of basic microrelief structures - i.e. a portion of thelayer is, for example, formed or composed of grating structures, another portion of Fourierhologram structures, yet another portion of a micro-mirror, and / or other combinations ofvarious types of structures well known to a person skilled in the art. The sub-areas / sub-structures / sub-elements are distributed across the layer of the optical material, e.g. typicallyin a juxtaposed manner (i.e. having mutual boundaries), and are arranged in a regular,irregular or semi-regular array.As used herein, the term “neighbouring or adjacent” - as referring to any pair of, or to anytwo or more, sub-areas / sub-structures / sub-elements of the surface of the layer - means suchplural sub-areas / sub-structures / sub-elements whose boundaries share at least one commonpoint, or which have a shared boundary that is defined by at least one common point. Thus,“neighbouring or adjacent” sub-areas / sub-structures / sub-elements include not only pairs of,or any two or more (i.e. any plural number of, e.g. possibly even more than two), sub- JL / 40188 7areas / sub-structures / sub-elements which share a common elongate boundary or arecontiguous with one another or are arranged in a side-by-side or end-to-end conjoinedmanner along one or more elongate sides or edges of each thereof, but also includearrangements of any plural number of sub-areas / sub-structures / sub-elements which meet orare conjoined at a corner of each thereof, i.e. which meet or are conjoined in a corner-to-corner manner.In some embodiments of the invention, each said sub-area / sub-structure / sub-element mayalternatively be termed a “pixel”.In some embodiments of the invention, each said sub-area / sub-structure / sub-element maytypically have a size, especially a width, in a range of from about 5 µm up to about 1000 µm,e.g. more desirably or more typically in a range of from about 10 µm up to about 100 µm.The sub-areas / sub-structures / sub-elements may be of any suitable or desired shape, e.g.square, rectangular, triangular, polygonal, or even other regular or irregular shapes or forms.In some embodiments of the invention, each boundary region of a given neighbouring oradjacent sub-area / sub-structure / sub-element may consist of, or may be defined by, arespective border or boundary or edge region or portion of the respective sub-area / sub-structure / sub-element of a width that does not substantially or deleteriously affect the mainoptical function of the basic microrelief structure in the respective sub-area / sub-structure / sub-element. Typically, therefore, the width of each respective border or boundaryor edge region or portion of the respective sub-area / sub-structure / sub-element may forexample be from about 0.1 or 0.5 or 1 or 2 or 3 or 5% up to about 10 or 15 or 20% of therespective overall size / width (as defined above) of the respective sub-area / sub-structure / sub-element. Alternatively, for some practical embodiments, the width of eachrespective border or boundary or edge region or portion of the respective sub-area / sub-structure / sub-element may instead by defined numerically, such as for instance it being in arange of from about 50 or 100 or 500 or 1000 nm up to about 10 or 20 or 50 µm.In some embodiments of the invention, the provision of the feature in which the opticalfunctions of the neighbouring or adjacent sub-areas / sub-structures / sub-elements within theirrespective mutual boundary regions are substantially the same or such boundary regionsredirect or transform incident light in substantially the same manner may be effected byappropriate design and construction of the basic microrelief structure portions themselves ineach respective neighbouring or adjacent sub-area / sub-structure / sub-element. In some JL / 40188 8practical forms, such provision of this feature may be effected for instance by effectivelyoverlapping the basic microrelief structure portions in the relevant neighbouring or adjacentsub-areas / sub-structures / sub-elements, or by blending them in to one another, where theblending is done within their respective mutual boundary regions.In some such embodiments of the invention, in addition to the provision of the feature thatthe optical functions of the neighbouring or adjacent sub-areas / sub-structures / sub-elementswithin their respective mutual boundary regions are substantially the same, or such boundaryregions redirect or transform incident light in substantially the same manner, by virtue ofeffectively overlapping the basic microrelief structure portions in the relevant neighbouring oradjacent sub-areas / sub-structures / sub-elements or by blending them in to one another,where the blending is done within their respective mutual boundary regions, it may be afurther provided feature that there is (within the area(s) of the respective boundary region(s))additionally overlapping or blending-in of or between respective boundary region(s) and therespective central or major portion(s) of at least one or more of, or at least some of, theneighbouring or adjacent sub-areas / sub-structures / sub-elements. In other words, it may bethat such boundary region(s) of a given respective neighbouring or adjacent sub-area / sub-structure / sub-element is / are not constructed independently of the optical function(s) of sucha respective central or major portion(s) thereof.As used herein, by the expressions “substantially same optical function(s)” and / or“redirecting or transforming incident light in substantially the same manner” within the mutualboundary regions of any two or more neighbouring or adjacent sub-areas / sub-structures / sub-elements is meant that their mutual boundary regions provide optical function(s) which arepractically substantially the same on both sides of the mutual boundary, and which forinstance may even deviate (or deviate more) from one another with increasing distance (toeither side) from such a boundary. In other words, “substantially same optical functions(s)”and / or “redirecting or transforming incident light in substantially the same manner” may meanthat the optical function(s) within the mutual boundary regions of two (or any two)neighbouring or adjacent sub-areas / sub-structures / sub-elements do not exhibit discontinuitywhen crossing the boundary between the said neighbouring or adjacent sub-areas / sub-structures / sub-elements, even if the optical functions of the said neighbouring or adjacentsub-areas / sub-structures / sub-elements are different outside of their mutual boundary region(i.e. in their respective central or major portions). It also may mean that the microreliefstructures within the mutual boundary regions of any pair of or plurality of neighbouring oradjacent sub-areas / sub-structures / sub-elements do not exhibit discontinuity (i.e. any obvious JL / 40188 9and / or substantial discontinuity) on, or when crossing, a majority part or portion (i.e. a part orportion being >50% thereof or of the length thereof) of the respective boundary between thesaid pair or plurality of neighbouring or adjacent sub-areas / sub-structures / sub-elements.Alternatively the discontinuities on the microrelief structure may appear on the boundary onlyin a few (e.g. 1 or more, or an integer from 1 up to about 5 or even from 1 up to about 10)singular points (typically, at points of maximum or minimum height of the microrelief structuremodulation depth). That may even be the case even if the optical functions of the saidneighbouring or adjacent sub-areas / sub-structures / sub-elements are different outside oftheir mutual boundary regions (i.e. in their respective central or major portions).In many practical embodiments of the invention the preceding paragraph may apply to anynotional boundary drawn within the microstructure comprised in the layer of the securityelement, especially within the area(s) of the layer which comprise(s) the same type of thebasic microrelief structure.Thus, in accordance with such embodiments, any variations - especially any sharpvariation(s) of the nature of discontinuity(ies) (or aberrations or anomalies or deviations ordisparities) originating from or caused by sharp (i.e. discontinuity-defining) boundaries orborders themselves between neighbouring or adjacent sub-areas / sub-structures / sub-elements may be substantially eliminated, thereby making the partitioning of the securityelement’s basic microrelief-structured surface less obvious and thus more difficult to discern,and thus more difficult to counterfeit.Thus, in such embodiments, whilst the main optical function of the major proportion of eachrespective sub-area / sub-structure / sub-element is still substantially as it is originally intendedfrom the construction of the overall microrelief structure itself (i.e. formed on or provided bythe sub-areas / sub-structures / sub-elements collectively), it is only in the mutual boundaryregions that separate neighbouring or adjacent sub-areas / sub-structures / sub-elements thatthe optical function(s) of the portions of the basic microrelief structure therein is / are tailoredor adjusted or modified such that there is substantially no variation in optical functionoccurring actually in those mutual boundary regions originating from or caused by the mutualboundaries themselves. In other words, the main optical functions or optical properties ofthe basic microrelief structure portions in the respective major proportions of the respectivesub-areas / sub-structures / sub-elements are substantially unchanged or unaffected, or atleast are not changed to any significant degree, by the said tailoring or adjusting or modifyingof the optical function(s) of the portions of the basic microrelief structure in those boundary JL / 40188 10 regions.Furthermore, in some embodiments of security elements according the invention, it may bethat the surface of the layer comprises (either in a real or a notional sense) one or more, orany number of, sub-areas or sub-structures or sub-elements (e.g. pixels) other than thosewhose mutual boundary regions have the said substantially same optical function (or whichredirect or transform incident light in substantially the same manner). In other words, insimple more practically expressed terms, it may even be that, in some embodiments ofsecurity elements according the invention, the surface of the layer may comprise (either in areal or a notional sense) one or more, or any number of, sub-areas or sub-structures or sub-elements (e.g. pixels) which do not have the blended or smoothly transitioning mutualboundary regions.In a second aspect the present invention provides a microrelief structure per se, forapplication to or when applied to or incorporated into a security element according to the firstaspect or any embodiment thereof, the microrelief structure being as defined in the firstaspect of the invention or any embodiment thereof.In a third aspect the present invention provides a method for making a security elementaccording to the first aspect or any embodiment thereof, the method comprising:(i) providing a layer of optical material; and(ii) forming on or in a surface of the optical material layer at least two basic microreliefstructures, wherein the surface of the layer is partitioned into - or it comprises (either in a realor a notional sense) - a plurality of sub-areas or sub-structures or sub-elements on or inwhich the basic microrelief structures are formed;wherein mutual boundary regions of neighbouring or adjacent said sub-areas / sub-structures / sub-elements are constructed such that, or define an optical functionsuch that, the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially thesame or such boundary regions redirect or transform incident light in substantially the samemanner, whereby the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements are substantially the same within and passing acrosstheir mutual boundary regions with substantially no variation in that optical function occurringin those mutual boundary regions originating from or caused by the mutual boundariesthemselves between the said neighbouring or adjacent said sub-areas / sub-structures / sub- JL / 40188 11 elements; and optionally, especially if desired or necessary, (iii) assembling or incorporating thelayer with the at least two basic microrelief structures formed thereon or therein into the finalsecurity element being produced, by application of the layer onto, or by embedding of thelayer into, or by combining the layer with (optionally by lamination), one or more carrier layersor other structural or optically functional or non-functional layers of the produced securityelement.The mutual boundary regions may be constructed from phasor representations of individualbasic microrelief structures (or their optical functions) of sub-areas / sub-elements extendedinto their neighbouring or adjacent sub-elements / sub-structures / sub-elements expressed ascomplex functions Ui(x) = wi(x) exp[jθi(x)], where j2 = –1, x is a position on the layer of thesecurity element, θi(x) is a real function describing a basic microrelief structure, i is the indexof a given sub-area / sub-structure / sub-element, wi(x) is a continuous window function withvalues between 0 and 1, window function decreasing to zero value at the end of the saidextension and being equal to zero beyond the said extension, wherein the said decrease tozero value is gradual within overlapping extensions (representing mutual boundary regions)of the said neighbouring or adjacent sub-elements / sub-structures / sub-elements. The phasorrepresentation U(x) of the whole OVD microrelief, including the mutual boundary regions, isthen given as a sum of Ui(x) of all sub-areas / sub-structures / sub-elements. Such aconstruction of the mutual boundary regions is an example how the basic microrelief structureportions of the neighbouring or adjacent sub-areas / sub-structures / sub-elements may beoverlapped or blended in to one other.In some embodiments of the invention, the height (or depth) of the microrelief structure ofthe OVD, including the microrelief structure in the mutual boundary regions, may beexpressed, for example, as height(x) = dMAX × phase(U(x)) / 2π, where dMAX is the designmodulation depth. In yet other embodiments of the invention, the height (or depth) may beexpressed as height(x) = dMAX × cos[phase(U(x)) – phase(UR(x))] / 2, where UR(x) is aphasor of a reference wave, as understood, for example, in the field of holography.In a fourth aspect the present invention provides an article or product having applied oraffixed thereto or incorporated therein a security element according to the first aspect or anyembodiment thereof. Such an article or product may for example be any one of the following:a document, a banknote, a passport, a visa, an ID card or document, a driving licence, amembership card, a ticket, a certificate, packaging, a work of art, an antique or other valuable JL / 40188 12 item.In some especially useful embodiments of the invention in its various aspects, the microreliefstructure may additionally comprise a further useful feature, namely that any said sub-areaor sub-structure or sub-element may comprise the said basic microrelief structure, and thebasic microrelief structure may be constructed such that, or it may define an optical functionsuch that, for the or each one of one or more (especially the or each one of a plurality ofdifferent) input light directions of light incident on the basic microrelief structure, the saidincident light in the or each respective said input light direction is redirected or transformedinto a respective plurality of - especially into a number that is an integer of from 3 to 10 -output light directions exiting the basic microrelief structure.In some such optional embodiments as above, the redirected or transformed output lightdirections may be a respective plurality of - especially a number that is an integer of from 3to 10 - different output light directions which exit the basic microrelief structure as different,discrete such output light directions.However, in other such optional embodiments as above, the redirected or transformed outputlight directions may be a respective plurality of output light directions which exit the basicmicrorelief structure as a notional cone of output light whose boundary is defined by orcontains the said respective plurality of output light directions - especially a number of suchoutput light directions that is an integer of from 3 to 10.It may also be useful to mention and appreciate that a basic microrelief structure as definedabove and forming the basis of such optional embodiments as above, i.e. a basic microreliefstructure that redirects or transforms incoming input light into a small number of differentoutput directions or a cone defined by these directions, may also be useful per se. Forinstance, one may be able to fine tune the look of an OVD or other security element by usingseveral output light directions instead of just one, in order to make the illusion created by theoverall output light smoother, or even to mimic anisotropic optical properties, or the like.Thus, using basic microrelief structures and features as disclosed herein as such optionalembodiments may serve other useful purposes in such OVDs or security elements other thanthe mere obfuscating of the microrelief optical function.The above additional feature thus enables such embodiments to go even further and makethe basic types of microreliefs employed in a security element more difficult to understand, JL / 40188 13thereby making it even harder to counterfeit.In some embodiments of the invention, and indeed in other places within this disclosure, thesecurity element itself may alternatively be termed - or may be provided in the form of - anoptically variable device (i.e. an “OVD”).In practical examples of embodiments of the invention being put into practice, the microreliefstructure applied to or incorporated in the security element may comprise a diffractive opticalstructure which is a record of a security image or object in the form of a hologram or otherrecord of any desired image, design, picture, logo, emblem, pattern, artistic work,combination of one or more alphanumeric characters, or other object, especially for exampleone that is suitably unique and / or complex that its composition, upon its viewing or detection,serves to render the security image or object identifiable and confirmable for its intendedsecurity or authentication purpose.In embodiments the security image or object encoded by the microrelief structure, especiallyupon its viewing or detection for its security or authentication purpose, may be, at least inpart, a two-dimensional or a three-dimensional image or object. Alternatively, one or moreselected portions of the security image or object, especially upon its / their viewing or detectionfor its / their security or authentication purpose, may be two-dimensional, and / or one or moreother selected portions of the security image or object, especially upon its / their viewing ordetection for its / their security or authentication purpose, may be three-dimensional.In some embodiments of the invention in its various aspects, the layer of the security element,whose surface has formed therein or thereon the microrelief structure, may be formed of amaterial that is substantially transparent to light, and incident light interacts with themicrorelief structure to form or recreate the security image or object substantially bytransmission. However, in other embodiments of the invention in its various aspects, thelayer of the security element, whose surface has formed therein or thereon the microreliefstructure, may be formed of a material that is substantially opaque to light, and incident lightinteracts with the microrelief structure to form or recreate the security image or objectsubstantially by reflection. Alternatively still, in certain other embodiments a combination oftransmission and reflection behaviour may be used, by appropriate use of appropriatematerial(s) of the layer and / or the precise form or structure of the microrelief structure(s)itself / themselves. JL / 40188 14In practical implementations of embodiments of the invention, the light with which the securityelement may be used may be electromagnetic radiation in any region or portion of theelectromagnetic spectrum, although light within the visible region of the electromagneticspectrum may often be preferred. Such visible light may for example be white light, oralternatively it may be light of a selected range or band of frequencies / wavelengths withinthe visible region of the spectrum. Alternatively still, light which includes at least a portionthat falls outside the visible region of the spectrum, e.g. into the infrared or ultraviolet regions,may be used instead, if desired.In practical embodiments of the invention, suitable optical materials for forming the layer inwhose surface the microrelief structure(s) is / are formed may include, for example, variouspolymer materials. Examples of suitable such polymers, which may typically be provided inthe form of a sheet, film, foil, strip or plate (e.g. with an average or maximum boundingthickness [i.e. the latter meaning the maximum general thickness if the profile of the surfacemicrorelief is taken into account] in the approx. range of from about 1 or 5 or 10 or 20 or 30μm up to about 100 or 200 or 500 or even 1000 μm) are well-known to the skilled person inthe known art of microrelief-based security elements and are widely commercially available.For practising various embodiments of security elements according to the invention, suitablemethods, techniques and apparatuses that may be used for creating or applying themicrorelief structure(s) to the layer that forms the security element will also be well-knownand readily available to the skilled person in the known art of microrelief-based securityelements.In embodiments of the invention, the or each basic microrelief structure formed on or in asurface of the layer of optical material may comprise any suitable or appropriate form, shape,structure and configuration of basic microrelief, which of course is able to record or encodethe relevant security image or object for the intended security or authentication purpose forwhich the security element is intended for use. In many embodiments, basic microreliefstructures which are substantially (or at least partially) diffractive in nature, or actpredominantly (or at least partially) by diffraction, may be especially useful or desirable. Insome embodiments such diffractive (or at least partially diffractive) basic microreliefstructures may work in the first diffraction order, whereas in some other embodiments suchdiffractive (or at least partially diffractive) basic microrelief structures may work in the secondor higher diffraction order. Such basic microrelief structures may comprise relief featureswhich typically have depths (or heights, if thought of in that way) of their relief features in a JL / 40188 15range of from about 50 nm up to about 50 µm. Typical “shallow” basic microreliefs may havedepths (or heights) of their relief features in a range of from about 50 up to about 500 nm oreven up to about 1000 nm, and their optical behaviour may typically or mostly be exhibitedat the first order of diffraction. On the other hand, typical “deeper” basic microreliefs mayhave depths (or heights) of their relief features in a range of from about 0.5 µm up to about10 µm or even up to about 50 µm, and their optical behaviour may typically or mostly beexhibited at higher orders of diffraction.Further descriptions of the principles of, and practical features of, microrelief structures andsecurity elements based thereon as employed for and in various embodiments of theinvention will be provided in the further description hereinbelow, which is set out here by wayof non-limiting example disclosure only (unless expressly stated or clearly apparentotherwise).Within the scope of this specification it is envisaged that the various aspects, embodiments,examples, features and alternatives, and in particular the individual constructional oroperational features thereof, set out in the preceding paragraphs, in the claims and / or in thefollowing description and accompanying drawings, may be taken independently or in anycombination of any number of same. For example, individual features described inconnection with one particular embodiment are applicable to all embodiments, unlessexpressly stated otherwise or such features are incompatible.BRIEF DESCRIPTION OF THE DRAWINGSOne or more embodiments of the present invention in its various aspects will now bedescribed in detail, by way of example only, with reference to the accompanying drawings,in which:FIGURES 1(a), (b), (c), (d) and (e) are simplified schematic illustrations showingsome of the basic principles involved in the formation and function of various known basictypes of microrelief structures used in known security elements (which FIGS. have alreadybeen referred to and described);FIGURES 2(a) & (b) are two schematic face-on representations of two basic types ofmicrorelief structure currently used in known security elements (which FIGS. have alreadybeen referred to and described);FIGURES 2(c) & (d) are two further schematic face-on representations of knownbasic types of microrelief structure currently used in known security elements (which FIGS. JL / 40188 16have already been referred to and described);FIGURE 2(e) is a schematic face-on representation of an example of another knownform of microrelief structure used in known security elements, which employs a plurality ofdifferent optical effects (which FIG. has already been referred to and described);FIGURES 3(a) & (b) are schematic face-on representations of two examples of knownsynthetic digital holograms, i.e. another basic microrelief type (which FIGS. have alreadybeen referred to and described);FIGURE 4 is a schematic face-on representation of a microrelief structure-basedsecurity element or OVD according to one embodiment of the present invention;FIGURE 5 is an enlarged view of a portion of an example of a basic microstructurerelief according to an optional feature of some embodiments of the present invention;FIGURE 6 is a schematic face-on representation of a microrelief structure-basedsecurity element or OVD according to one embodiment of the present invention;FIGURES 7(a) & (b) are enlarged views of basic microrelief types without and withblending, according to some particular embodiments of the invention;FIGURES 8(a) & (b) & (c) are face-on representations of steps applied in the makingof an OVD according to one embodiment of this invention.DETAILED DESCRIPTION OF THE INVENTION AND EMBODIMENTS AND FEATURESTHEREOFThe following detailed description is intended to be representative of merely embodimentsand features of the present invention in its broader sense and aspects, and how to put it / theminto practice, and is thus to be interpreted as being non-limiting on the overall scope of theinvention (unless expressly stated to be, or clearly apparent to be, otherwise), which is asdefined in the appended claims.The underlying principles and main features of the invention in its main embodimentsThe essence of present invention relates to the basic microrelief structures, in particular howto make a microrelief less understandable than known basic microrelief structures based onmirror facets (as shown in FIGS.1 (b) & (c)) or a diffraction grating (as shown in FIGS. 1(d)& (e)). These two types of basic microrelief structure, when thinking about them inside asmall area, redirect light incoming from a single direction into a substantially single outgoingdirection, either owing to the geometric law of reflection (or refraction), or owing to the law ofdiffraction on a grating. (It is worth noting that while a diffraction grating redirects white light JL / 40188 17into many directions owing to colour dispersion and redirects monochromatic light into manydirections owing to the existence of diffraction orders, those directions are not independentand the grating is usually designed with one particular direction in mind.)The uniqueness of light redirection is a subtle but very important property. For instance, ifone wants to make an illusion of a spherical mirror, it is possible to partition the area of theOVD (or security element) into small elements (for instance in a regular rectangular grid), tocalculate a normal vector of the sphere in the centre of each small element, and to fill eachsmall element with a basic microrelief structure redirecting light into the same direction as aplanar mirror with a corresponding normal vector. This process is depicted in FIGS. 2(c) &(d), and for other than a spherical mirror it is depicted in FIGS. 2(a) & (b), where smallelements and basic microrelief structures therewithin are denoted 3’ and 3. In a practicalOVD design process, these steps are usually separated. In the first step, one picks thepartition of the OVD into small elements, preferably in a form of a regular rectangular grid.In the second step, one creates an auxiliary image, often called a normal map, where eachpixel encodes a single normal vector corresponding to a particular small element (it can befor instance an RGB image, where R, G, B channels encode X, Y, Z components of thenormal vector). In the third step, one takes the normal map and actually makes basicmicrorelief structures corresponding to the pixels of the normal map. It should be noted thatwhile the terms used in the given examples assume using the mirror based basic microreliefstructure, the process is substantially the same for the diffraction grating based microreliefstructure. For the sake of simplicity, the term “normal vector” and “normal map” will be usedhereinafter, regardless of whether the vector actually means a normal vector of a mirror facet,a direction of outgoing light, a grating vector, or the like.It is obvious that the normal map does not have to correspond to any particular 3-D geometry.One can, for instance, make a normal map corresponding to a spherical mirror andconsequently rotate each normal vector by 90°, which leads to an interesting optical illusionimpossible to mimic by any 3D object with a smooth mirror surface. Another example is totake two independent normal maps corresponding to two optical illusions and to make a newone, where 50% of pixels are taken from the first normal map and the rest from the otherone, as depicted in FIG. 2(e).In summary, a basic microrelief structure redirecting light into one prescribed direction is animportant building block of currently known security elements / OVDs, mostly from practicalreasons. However, its primary weakness has been already mentioned - namely, as the small JL / 40188 18elements are clearly visible, it is easy to inspect such basic microrelief structures inside themand to reverse-engineer the OVD. The aim of embodiments of this invention is to eliminatethe visible boundaries between the small elements, which makes reverse-engineering moredifficult, as there are no distinct small elements to start with. As the boundaries between thesmall elements manifest themselves as an (unintentional) optical function of the OVD, it maybe possible to formulate embodiments of this invention in terms of optical functions.In the present invention, the optical functions of the neighbouring or adjacent pixels / sub-areas / sub-structures / sub-elements within their respective mutual boundary regions aresubstantially the same or such boundary regions redirect or transform incident light insubstantially the same manner, for example by virtue of the basic microrelief structureportions in the relevant neighbouring or adjacent pixels / sub-areas / sub-structures / sub-elements being effectively overlapping or blended in to one another. This feature and how itmay be put into practice will now be described in further detail:This feature relates to a partitioning of the OVD into small elements, for instance squares,rectangles, triangles, polygonal areas, or the like. As was already explained above, the OVDis usually partitioned in order to change the properties of the basic microrelief in a piecewiseconstant manner, or to spatially multiplex several effects in the area of the OVD. When theborders of the small elements are easily visible, it helps in the reverse-engineering of theoptical effects - so that a counterfeiter skilled in the art can guess which elements make aparticular visual effect and further explore how the effect is made. When the small elementssmoothly change from one to another, it is for one thing more difficult to reverse-engineer theOVD, especially when the partition is irregular, and for another thing, it is difficult to recognizewhich features of the microrelief are functional and which are not important for the opticalfunction. It should be emphasized that the smooth change of the small elements must notsubstantially change their optical properties significantly.One way of accomplishing such a task is to imaginarily make the small elements larger thanthey should be so that they overlap, and to blend them using a window function in a processsimilar to the inverse modified discrete cosine transform (IMDCT) well known from the fieldof digital signal processing (DSP). It is also advantageous to perform the blending processin the complex domain in order to better control properties of light, and to use tools of digitalholography to convert the surface defined in the complex domain to a surface simply definedby its height. JL / 40188 19To explain this idea, it is worth reformulating the process of making small elements with sharpborders. Here, one wants to restrict, e.g. a diffraction grating based microrelief A in one smallelement, and a diffraction grating based microrelief B in an adjacent small element. Adiffraction grating based microrelief can be designed by defining an auxiliary complexnumber (phasor):UA(x) = exp[(2πj / λ)(nA · x)]where x is a position on the OVD, j2 = –1, λ is the design wavelength and nA is the directionof outgoing light. Restriction to a small element is simply done by multiplication of UA(x) bythe function rect(x) that is equal to 1 for x within the small element and 0 elsewhere:UAR(x) = UA(x) rect(x)One defines the complex numbers UB(x) and UBR(x) for a microrelief B in an adjacent smallelement in the same way. Note that while this example uses phasors in the form of a planewave, one can use a phasor of a spherical wave, phasor of a sum of several plane waves,or the like. Also note that the example definition of UA(x) does not assume any constantphase shift. By introducing it, i.e. by defining UA(x) = exp[(2πj / λ)(nA · x + φA)], one may getan additional degree of freedom allowing better obfuscation, especially when the phase shiftsφA, φB, etc. are made random.The next step is to sum up all “restricted” phasors:U(x) = UAR(x) + UBR(x)(for more small elements, one simply sums more terms). The last step is to transform U(x)to a microrelief height, for instance in the form of kinoform:height(x) = dMAX × phase(U(x)) / 2πwhere dMAX is the design modulation depth, or in the form of bipolar intensity hologram:height(x) = dMAX × cos[phase(U(x)) – phase(UR(x))] / 2where UR(x) is a phasor of a reference wave, as understood, for example, in the field ofholography. There are many other standard ways of converting U(x) to a microrelief height.Note that for a reflection or a transmission geometry, one has generally to include a factoraccounting the geometry, indices of refraction, etc, as will be well-known and well-appreciated by persons skilled in the art.Restriction of the phasor UA(x) to a small element was simply done by multiplication with thefunction rect(x), whose support (i.e. support of f(x) is the domain where f(x) ≠ 0) is exactlythe area of the small element. However, rect(x) is just one example of “a window function”,a term commonly used in signal processing, and it is possible to use any suitable windowfunction. For instance, if the OVD is partitioned to a regular grid of square elements of size JL / 40188 201 × 1, and one small element is located at a plane z = 0 in the area –0.5 < x < 0.5, –0.5 <y < 0.5 for x = (x, y, z), it is possible to use a window functionwCOS(x, y, 0) = cos2(xπ / 2) cos2(yπ / 2) for –1 < x < 1, –1 < y < 1,wcos(x, y, 0) = 0 elsewhere.That is, the window function smoothly spills into adjacent small elements. Note that otherwindowing function(s) can be used as well, see below. Version of UA(x) restricted bywindowing is nowUAW(x) = UA(x) wCOS(x)and similarly for UBW(x). The rest of the procedure is the same, one just uses UAW(x), UBW(x),etc., instead of UAR(x), UBR(x), etc.One can easily recognize a standard process with overlapping windows known from, e.g. theinverse modified discrete cosine transform. An important difference here is that thewindowing and summation is taken in the complex domain rather than in the real domain. Aphysical interpretation of the process is straightforward. As UAR(x) is a light phasor in theOVD plane strictly restricted by the small element shape, one can consider UAW(x) as anapproximation of the light phasor at a plane slightly off the OVD. Smooth falloff of wCOS(x)towards edges of support of wCOS(x) roughly approximates beam broadening due todiffraction; a lateral offset that would be caused by the direction nA is simply ignored.Summation of phasors UAW(x), UBW(x), etc. is just a superposition of contributions from thesmall elements, and the conversion of U(x) to the microrelief height is a standard procedurefrom digital holography to encode an optical field to a kinoform, a conventional hologram, orthe like.Owing to the summation in the complex domain and the phase extraction in the subsequentstep, limitations to the windowing function are not very strict. One should use a function witha simply connected (sometimes called “compact”) support, preferably limited just to theadjacent elements, more preferably not going farther than into the middle of the adjacentsmall elements. The window function should be closer to its maximum value within thecorresponding small element, and closer to 0 in the adjacent small elements, and 0 at thesupport border. The window function is preferably continuous. An example of other windowfunction is:wSHARPER(x) = 3[wCOS(x)]2 – 2[wCOS(x)]3that may be used if one wants to slightly suppress the effect of blending small elementstogether. As already mentioned, many other forms of the window function are possible. JL / 40188 21The window function need not be the same for every small element; for instance, one canuse a window function s × wCOS(x) for a particular small element, where s is “the strength” ofthat small element. One can then suppress or emphasize the particular small element. Thewindow function may also vary if one wants to use small elements of variable size, similarlyto using long and short windows in, e.g. digital signal processing of audio data.Note that the small elements need not make a regular rectangular partition of the OVD. Onecan use a regular triangular or polygonal partition, an irregular partition based on Penrosetiling, Voronoi diagrams, or the like. The window function can be then defined on a circularsupport, an elliptical support, a polygonal support, or the like.The process of avoiding sharp borders between small elements of the security element / OVDhas been explained above with diffraction-based microrelief in mind. The only critical part isto pick an appropriate design wavelength λ, so that the modulation of the basic microrelief(before the blending process) causes phase shift less than 2π. For simple diffraction-basedmicrorelief, λ is usually between 400 and 700 nm, i.e. it is visible light, and the microrelief isusually shallow. For much deeper profiles, where high order diffraction starts to resemblegeometrical optics, one can choose an auxiliary wavelength, much longer than for visiblelight, to overcome the limitation of 2π modulation.It is worth mentioning here that avoiding sharp borders between small elements of the OVDmay improve the optical properties of the security element / OVD, as sharp borders oftencause uncontrolled light scattering. Also, sharp borders between small elements may affectthe manufacturability of the security element / OVD, and thus avoiding them may have otheruseful impact(s). Another reason to avoid sharp borders between small elements of thesecurity element / OVD is that such borders, especially when they make a regular pattern,create strong diffraction orders when illuminated by coherent light. A smooth change fromone element to another may significantly suppress them, i.e. the invention adds a newsecurity feature so as to be able to better and more reliably tell a valid security element / OVDfrom a counterfeit one with sharp edges between small elements.FIG. 4 shows in schematic form a representation of a microrelief structure-based securityelement or OVD 20 according to one embodiment of this invention. A microrelief structure25 is formed on an upper surface (or face) of a base or substrate layer 22 (e.g. of any suitableknown material, such as a polymer), and the surface bearing the microrelief structure 25 isat least notionally (or may be actually in reality, by design) divided up into, or is defined by, JL / 40188 22an array of a plurality of individual pixels or sub-areas / sub-structures / sub-elements (notshown in the FIGS.), each of which contributes a respective portion of the completemicrorelief structure 25 to the OVD / element 20. Here, the surface of the layer 22 bearing themicrorelief structure 25 is thus, or can thus be considered to be, divided up or partitioned intothe discrete pixels or sub-areas / sub-structures / sub-elements, but the boundaries betweenneighbouring or adjacent pixels / sub-areas / sub-structures / sub-elements are substantiallyinvisible. This is achieved by the provision of the key feature of this invention, which is thatthe optical functions of the neighbouring or adjacent pixels / sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially the same or suchboundary regions redirect or transform incident light in substantially the same manner. Thisconfiguration may for instance be achieved by effectively overlapping the basic microreliefstructure portions in the relevant neighbouring or adjacent pixels / sub-areas / sub-structures / sub-elements, or by blending them in to one another. Terms used in the abovediscussion are shown and explained in detail in FIGS.7(a) & (b). FIG.7(a) shows a detail ofa microrelief frequently in use in current OVDs. Here, OVD 40 comprises two pixels / sub-areas / sub-elements 41 bearing basic microrelief structures 16 separated by a distinct border42. FIG 7(b) shows how pixel blending avoids the distinct boundaries. The OVD 40’ stillnotionally comprises two pixels / sub-areas / sub-elements 41’, but now their border 42’ is nolonger visible. (This can also be seen in FIG. 4, as FIG. 7(b) is a detail of microreliefs 16’.)Invisibility of the border is caused by the fact that the microrelief structure in the boundaryregion 43’a of the upper pixel / sub-area / sub-element is very similar to the microrelief structurein the boundary region 43’b of the lower pixel / sub-area / sub-element, which also means thattheir optical functions are substantially the same. Note that the microrelief structure in thecentres of both pixels / sub-areas / sub-elements 41’ is still very similar to the microreliefstructure 16 of OVD 40, which means that the optical functions of OVDs 40 and 40’ are stillsubstantially the same provided that the boundary regions are sufficiently small. Note thatobfuscation of basic microreliefs in FIG 7(b) is not high, as the microrelief shown in FIG.7(b)was chosen for clarity. However, the full OVD shown in FIG 4 shows features such as forks23 or phase singularities 24 which make understanding of the microrelief comparativelydifficult.The underlying principles and main features of embodiments based on the optional featureof constructing the basic microrelief structure such that it defines an optical function suchthat, for the or each one of one or more (especially the or each one of a plurality of different)input light directions of light incident on the basic microrelief structure, the said incident lightin the or each respective said input light direction is redirected or transformed into a JL / 40188 23 - into a number that is an integer of from 3 to 10 - light directions exiting the basic microrelief structure, or a cone defined by these directionsIn some useful embodiments of this invention, the above-defined and above-discussedoptional additional feature may additionally be present or included in the securityelement / OVD, in which the basic microrelief structure is constructed such that it defines anoptical function such that, for the or each one of one or more (especially the or each one ofa plurality of different) input light directions of light incident on the basic microrelief structure,the said incident light in the or each respective said input light direction is redirected ortransformed into a respective plurality of - especially into a number that is an integer of from3 to 10 - output light directions exiting the basic microrelief structure, or a cone defined bythese directions - such embodiments may go even further and make the basic microreliefstructures employed in a security element more difficult to understand. This optionaladditional feature and how it may be put into practice will now be described in further detail:Referring to the embodiment of FIG. 6 in particular by way of explanatory example, withineach notional (or perhaps even real, by design) individual pixel or sub-area / sub-element ofthe surface bearing the microrelief structure 35, the microrelief structure 35 is formed on anupper surface (or face) of a base or substrate layer 32 (e.g. of any suitable known material,such as a polymer), and the surface bearing the microrelief structure 35 is divided upnotionally (or perhaps even really, by design) into, or is defined by, a plurality of individualpixels or sub-areas / sub-elements (not shown in the FIG.), each of which contributes arespective portion of the complete microrelief structure 35 to the OVD / element 30. Thus, inaccordance with this optional additional feature, each pixel or sub-area / sub-element has itsbasic microrelief structure portion formed such that the complete microrelief structure 35defines an optical function such that, for the or each one of one or more (especially the oreach one of a plurality of different) input light directions of light incident on the basicmicrorelief structure, the said incident light in the or each respective said input light directionis redirected or transformed into a respective plurality of - especially into a number that is aninteger of from 3 to 10 - output light directions exiting the basic microrelief structure, or intoa cone defined by said output light directions.Thus, in this embodiment version of such security elements / OVDs employing this optionalform of the basic microrelief structure, a basic microrelief type redirects light into a smallnumber of (especially for example into from three up to about ten) independent directions atonce (i.e. simultaneously), either well separated or alternatively into a cone of light defined JL / 40188 24by those plural directions. FIG. 5 shows an example of such a basic microrelief structure35’. Instead of a simple diffraction grating, one can use tools of digital holography and designa synthetic diffractive optical element (DOE) redirecting light in this manner, in the form ofany of a binary relief hologram, a bleached hologram, a kinoform, an amplitude-phasehologram, or the like. Instead of a simple mirror (i.e. microprism) facet, one can exploit thefact that geometric reflection (or refraction) is a manifestation of diffraction in a high diffractionorder. One can thus again design a diffractive optical element operating in a high diffractionorder and redirecting light to several design directions.An example of making a security element / OVD employing this optional form of the basicmicrorelief structure is shown in FIG.8. FIG.8(a) shows an OVD 30a simulating a sphericalmirror, which was constructed in the same conventional way as the one in FIG. 2(d). Theonly difference is that the pixels / sub-areas / sub-elements in OVD 30a are smaller, and thatphases of the basic microreliefs are randomized. FIG. 8(b) shows an OVD 30b preparedmerely using this optional form of the basic microrelief structure; namely, each pixel / sub-area / sub-element of OVD 30b incorporates normal vectors corresponding to 2×2 adjacentpixels / sub-areas / sub-elements of OVD 30a as well as their phases. FIG.8(c) shows the finalOVD where individual pixels / sub-areas / sub-elements were blended together according tomethods of embodiments of this invention.It should be noted that very similar or coplanar output light directions exiting the basicmicrorelief structure tend to make a microrelief with an easily recognizable orientation andshould therefore be avoided for obfuscation reasons. This has been already seen inFIG. 8(b) where most basic microrelief structures are not difficult to understand. It is thusadvantageous to employ at least three non-coplanar output light directions. There are tworeasons to employ three or more output light directions exiting the basic microreliefstructures, in some embodiments of this invention. The first reason is that just one directionleads to simple, easy to understand basic microrelief structures shown e.g. in FIGS. 2(b) or2(d). Two directions such that the normal vector of the OVD is their linear combination (i.e.the said two directions and the said OVD normal vector are coplanar) lead to a basicmicrorelief structure with easy to recognize orientation, i.e. the obfuscation level is low. Twodirections such that the normal vector of the OVD is linearly independent (i.e. the said twodirections and the said OVD normal vector are not coplanar) often lead to a basic microreliefstructure with distinct sub-areas therewithin, which are easy to analyse. It was found thatthree or more directions that are not coplanar (i.e. make a three-dimensional linear vectorspace) and are not “almost coplanar” or “almost collinear” lead to basic microrelief structures JL / 40188 25that are difficult to understand, i.e. obfuscating its optical function.The second reason is that one can fine tune the look of the OVD by using several output lightdirections exiting the basic microrelief structure instead of just one, for instance to make theillusion smoother.One way to make a diffraction-based microrelief type (i.e. a replacement for a simplediffraction grating) is to take N unit vectors n1, n2, …, nN, and to calculate a complex number:UO(x) = A1 exp[(2πj / λ)(n1 · x)] + A2 exp[(2πj / λ)(n2 · x)] + … + AN exp[(2πj / λ)(nN · x)],where A1, A2, …, AN are arbitrary complex constants, j2 = –1, λ is the design wavelength ofvisible light, x is a position vector in the OVD area, and · is the dot product. One canrecognize a sum of plane wave phasors in this formula, which may be understood asintended directions of outgoing light, each with a complex amplitude Ai. There are manyways to convert UO(x) to a microrelief, for instance by extracting its phase (i.e. making akinoform): height(x) = dMAX × phase(UO(x)) / 2πwhere dMAX is the design microrelief depth, usually in a range from about 10 to about 1000nm. Another way uses bipolar intensity approach:height(x) = dMAX × cos[phase(UO(x)) – phase(UR(x))] / 2where UR(x) is a phasor of a reference wave. There are many other standard ways ofconverting UO(x) to a microrelief height. It is important to note that for a reflection or atransmission geometry, one generally must include a factor accounting for the geometry,refractive indices, etc, as will be well-known and well-appreciated by persons skilled in theart.The formulas given in the previous paragraph are indeed well known and well-appreciatedby and amongst persons skilled in the art. They are, for instance, used to make holographicstereograms, where the area of the OVD is split into comparatively large elements, forinstance squares of 100 × 100 µm2, and each element encodes a view (i.e. a conventionalimage) of a 3D scene. However, there are important differences here. First of all, aholographic stereogram must balance angular and spatial resolution of the illusion. As aconventional image usually contains hundreds or thousands of pixels that have to beencoded as directions of outgoing light, the area of the holographic stereogram element mustbe quite big due to space-bandwidth product limitations. It then follows that spatialmultiplexing of several effects may be difficult as the multiplexing pattern (e.g. acheckerboard) may become visible even to the unaided / naked eye. The second difference JL / 40188 26is that one usually does not think about elements of a holographic stereogram as aboutstructures encoding independent outgoing light directions. The idea disclosed here inrespect of the present invention still allows one to use OVDs partitioned into small elements,and still to use the design approach based on normal maps. For instance, instead of usingone normal vector to define a microrelief in one small element, it is possible to use, e.g., 2 × 2normal vectors to define a microrelief in one small element.The given formulas can be also used to make a replacement for a mirror-based microrelieftype. The goal is now to design a kinoform-like structure, whose optical properties are givenmostly by geometric optics rather than diffraction. Such a structure may be still diffraction-based, but operating in a higher diffraction order, as geometric optics is in fact a manifestationof diffraction in a very high diffraction order, see e.g. Sinzinger and Testorf, “Transitionbetween diffractive and refractive micro-optical components”, Applied Optics Vol. 34, Issue26, pp.5970–5976. One way to design such a structure is again to sum phasors of severalplane waves and make a kinoform. However, in this case, one should intentionally use m-times longer wavelength to shift the peak efficiency to the m-th diffraction order. One mayrecognize that this simple reasoning works well just for design of a simple blazed diffractiongrating, but microreliefs encoding more than one outgoing light direction are affected byunwanted diffraction orders. Without any optimization, a microrelief encoding N outgoinglight directions usually results in outgoing light cone or pyramid whose shape is delimited bythe design outgoing light directions. A microrelief may be optimized using conventional toolssuch as gradient descent or a ping-pong iterative algorithm, where the step of lightpropagation simulation is properly implemented using a rigorous method such as RCWA oran approximate method such as BPM (beam propagation method) or WPM (wavepropagation method).It should be noted that in construing or defining or practising embodiments of the presentinvention, the main feature that characterizes this invention - namely the avoiding of orsubstantial absence of sharp boundaries between neighbouring / adjacent pixels / sub-areas / sub-elements - should not be construed overly and unreasonably broadly. Forinstance, a simple diffraction grating with a constant period, orientation and depth over thesecurity element / OVD area could be fictitiously split into small elements (e.g. by the notionaldrawing thereon of an array of regular gridlines). In that situation there may obviously be nosharp boundary between the small elements. The same applies e.g. to an illusion of aspherical mirror created using Fresnel flattening, i.e. smooth curved reflective facets. Suchtrivial cases are excluded from the scope of the present invention. On the other hand, one JL / 40188 27may not take an OVD clearly split into sub-areas / sub-structures / sub-elements with sharpboundaries between them, make an artificial notional partitioning of the OVD through smoothparts of the OVD, and argue that there are no sharp boundaries between these notional sub-areas / sub-structures / sub-elements. Such extensive or overly and unreasonably broadinterpretations as any of those mentioned above are thus to be considered as excluded fromthe scope of the present invention.For the above exclusion purposes, it may therefore be the case - in construing or defining orpractising embodiments of the present invention - that in many practical embodiments of theinvention, or even for defining the invention as a whole, the security element may be definedas being a security element in which either one of or both of the following features is / arepresent or satisfied:(i) the basic microrelief structures within a majority of (i.e. within >50% of) the adjacentsub-areas / sub-elements are not obvious extensions or extrapolations of each other, and / or(ii) the microrelief structure(s) within a majority amount or number of (i.e. within anamount or number of which is >50% of the total thereof) the mutual boundary regions ofadjacent sub-areas / sub-elements each comprise(s) one or more, or a plurality of, forks orphase singularities or vertices (which one or more, or plurality of, forks or phase singularitiesor vertices may of course be additionally defined as being inside the said mutual boundaryregions).(In the above definition (i), the expression “are not obvious extensions or extrapolations ofeach other” means that the basic microrelief structures in question are not continuations ofthe microrelief structure patterns in regions outside the mutual boundary regions of adjacentsub-areas / sub-elements.)The above restriction(s) on the intended scope of construction of the invention or many of itsembodiments may be alternatively - or even perhaps additionally - expressed or defined interms of the security element being a security element in which the basic microreliefstructures within the said sub-areas / sub-elements blend into each other according to acontinuous window function with support that is larger than that within the respectivecorresponding sub-areas / sub-elements themselves, especially wherein such continuouswindow function with support is / are as defined and discussed hereinabove.Throughout the description and claims of this specification, the words “comprise” and“contain” and linguistic variations of those words, for example “comprising” and “comprises”, JL / 40188 28mean “including but not limited to”, and are not intended to (and do not) exclude othermoieties, additives, components, elements, integers or steps.Throughout the description and claims of this specification, the singular encompasses theplural unless expressly stated otherwise or the context otherwise requires. In particular,where the indefinite article is used, the specification is to be understood as contemplatingplurality as well as singularity, unless expressly stated otherwise or the context requiresotherwise.Throughout the description and claims of this specification, features, components, elements,integers, characteristics, properties, compounds, chemical moieties or groups described inconjunction with a particular aspect, embodiment or example of the invention are to beunderstood to be applicable to any other aspect, embodiment or example described hereinunless incompatible therewith or expressly stated otherwise.

Claims

JL / 40188 29 CLAIMS1. A security element comprising a layer of optical material and at least two basicmicrorelief structures formed on or in a surface thereof, wherein the surface of the layer ispartitioned into - or it may comprise (either in a real or a notional sense) - a plurality of sub-areas or sub-structures or sub-elements,wherein mutual boundary regions of neighbouring or adjacent said sub-areas / sub-structures / sub-elements are constructed such that, or define an optical function such that,the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially the same or suchboundary regions redirect or transform incident light in substantially the same manner,whereby the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements are substantially the same within and passing across their mutualboundary regions with substantially no variation in that optical function occurring in thosemutual boundary regions originating from or caused by the mutual boundaries themselvesbetween the said neighbouring or adjacent said sub-areas / sub-structures / sub-elements.

2. A security element according to claim 1, wherein:(i) the basic microrelief structures within a majority of the adjacent sub-areas / sub-elements are not extensions or extrapolations of each other, and / or(ii) the microrelief structure(s) within a majority amount or number of themutual boundary regions of adjacent sub-areas / sub-elements each comprise(s) one or more,or a plurality of, forks or phase singularities or vertices.

3. A security element according to claim 1 or claim 2, wherein any one or more of thefollowing is satisfied:(i) the optical function(s) within the mutual boundary regions of two, or any two,neighbouring or adjacent sub-areas / sub-structures / sub-elements do not exhibit discontinuitywhen crossing the boundary between the said neighbouring or adjacent sub-areas / sub-structures / sub-elements, even if the optical functions of the said neighbouring or adjacentsub-areas / sub-structures / sub-elements are different outside of their mutual boundary region(optionally in their respective central or major portions);(ii) the microrelief structures within the mutual boundary regions of any pair of orplurality of neighbouring or adjacent sub-areas / sub-structures / sub-elements do not exhibitdiscontinuity on, or when crossing, a majority part or portion of the respective boundarybetween the said pair or plurality of neighbouring or adjacent sub-areas / sub-structures / sub-JL / 40188 30 elements; (iii) any discontinuities on the microrelief structure appear on the boundary betweenthe said neighbouring or adjacent sub-areas / sub-structures / sub-elements only in a few(optionally 1 or more, or an integer from 1 up to 5 or 10) singular points or points of maximumor minimum height of the microrelief structure modulation depth.

4. A security element according to any one of claims 1 to 3, wherein in addition toprovision of the feature that the optical functions of the neighbouring or adjacent sub-areas / sub-structures / sub-elements within their respective mutual boundary regions aresubstantially the same, or such boundary regions redirect or transform incident light insubstantially the same manner, by virtue of effectively overlapping the basic microreliefstructure portions in the relevant neighbouring or adjacent sub-areas / sub-structures / sub-elements or by blending them in to one another, it is a further provided feature that there isadditionally overlapping or blending-in of or between respective boundary region(s) and therespective central or major portion(s) of at least one or more of, or at least some of, theneighbouring or adjacent sub-areas / sub-structures / sub-elements, or it is the case that suchboundary region(s) of a given respective neighbouring or adjacent sub-area / sub-structure / sub-element is / are not constructed independently of the optical function(s) of sucha respective central or major portion(s) thereof.

5. A security element according to any preceding claim, wherein the surface of the layercomprises (especially either in a real or a notional sense) one or more, or any number of,sub-areas or sub-structures or sub-elements other than those whose mutual boundaryregions have the said substantially same optical function (or which redirect or transformincident light in substantially the same manner),whereby the surface of the layer comprises (especially either in a real or a notionalsense) one or more, or any number of, sub-areas or sub-structures or sub-elements whichdo not have blended or smoothly transitioning mutual boundary regions.

6. A security element according to any preceding claim, wherein each said sub-area / sub-structure / sub-element has a size, optionally a width, in a range of from 5 µm up to1000 µm.

7. A security element according to any preceding claim, wherein each boundary regionof a given neighbouring or adjacent sub-area / sub-structure / sub-element consists of, or isdefined by, a respective border or boundary or edge region or portion of the respective sub-JL / 40188 31area / sub-structure / sub-element of a width that does not substantially or deleteriously affectthe main optical function of the basic microrelief structure in the respective sub-area / sub-structure / sub-element, optionally wherein either (i) the width of each respective border or boundary or edgeregion or portion of the respective sub-area / sub-structure / sub-element is from 0.1 or 0.5 or1 or 2 or 3 or 5% up to 10 or 15 or 20% of the respective overall size / width of the respectivesub-area / sub-structure / sub-element, or (ii) the width of each respective border or boundaryor edge region or portion of the respective sub-area / sub-structure / sub-element is in a rangeof from 50 or 100 or 500 or 1000 nm up to 10 or 20 or 50 µm.

8. A security element according to any preceding claim, wherein the microrelief structureadditionally comprises a further feature, namely that any said sub-area or sub-structure orsub-element comprises the said basic microrelief structure, and the basic microreliefstructure is constructed such that, or it defines an optical function such that, for the or eachone of one or more (optionally the or each one of a plurality of different) input light directionsof light incident on the basic microrelief structure, the said incident light in the or eachrespective said input light direction is redirected or transformed into a respective plurality of- especially into a number that is an integer of from 3 to 10 - output light directions exiting thebasic microrelief structure.

9. A security element according to any one of claims 1 to 8, wherein the redirected ortransformed output light directions are a respective plurality of - optionally a number that isan integer of from 3 to 10 - different output light directions which exit the basic microreliefstructure as different, discrete such output light directions.

10. A security element according to any one of claims 1 to 8, wherein the redirected ortransformed output light directions are a respective plurality of output light directions whichexit the basic microrelief structure as a notional cone of output light whose boundary isdefined by or contains the said respective plurality of output light directions - optionally anumber of such output light directions that is an integer of from 3 to 10.

11. A security element according to any preceding claim, wherein the or each basicmicrorelief structure formed on or in a surface of the layer of optical material is at leastpartially diffractive in nature.

12. A security element according to any one of claims 1 to 11, wherein the or each basicJL / 40188 32microrelief structure formed on or in a surface of the layer of optical material is diffractive, orat least partially diffractive, in nature and works in the first diffraction order.

13. A security element according to any one of claims 1 to 11, wherein the or each basicmicrorelief structure formed on or in a surface of the layer of optical material is diffractive, orat least partially diffractive, in nature and works in the second or higher diffraction order.

14. A security element according to any preceding claim, wherein the or each basicmicrorelief structure formed on or in a surface of the layer of optical material comprises relieffeatures with depths (or heights, if thought of in that way) in a range of from 50 nm up to 50µm.

15. A microrelief structure per se, for application to or when applied to or incorporatedinto a security element according to any one of claims 1 to 14, wherein the microreliefstructure is as defined in any respective one of said claims 1 to 14 (as the case may be).

16. A method for making a security element according to any one of claims 1 to 14, themethod comprising:(i) providing a layer of optical material; and(ii) forming on or in a surface of the optical material layer at least two basic microreliefstructures, wherein the surface of the layer is partitioned into - or it comprises (either in a realor a notional sense) - a plurality of sub-areas or sub-structures or sub-elements on or inwhich the basic microrelief structures are formed;wherein mutual boundary regions of neighbouring or adjacent said sub-areas / sub-structures / sub-elements are constructed such that, or define an optical functionsuch that, the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements within their respective mutual boundary regions are substantially thesame or such boundary regions redirect or transform incident light in substantially the samemanner, whereby the optical functions of the neighbouring or adjacent said sub-areas / sub-structures / sub-elements are substantially the same within and passing acrosstheir mutual boundary regions with substantially no variation in that optical function occurringin those mutual boundary regions originating from or caused by the mutual boundariesthemselves between the said neighbouring or adjacent said sub-areas / sub-structures / sub-elements; and optionally (iii) assembling or incorporating the layer with the at least two basicJL / 40188 33microrelief structures formed thereon or therein into the final security element beingproduced, by application of the layer onto, or by embedding of the layer into, or by combiningthe layer with (optionally by lamination), one or more carrier layers or other structural oroptically functional or non-functional layers of the produced security element.

17. An article or product having applied or affixed thereto or incorporated therein asecurity element according to any one of claims 1 to 14,optionally wherein the article or product is any one of the following: a document, abanknote, a passport, a visa, an ID card or document, a driving licence, a membership card,a ticket, a certificate, packaging, a work of art, an antique or other valuable item.

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