EP3423287A1

EP3423287A1 - Security elements and security documents

Info

Publication number: EP3423287A1
Application number: EP17708336.7A
Authority: EP
Inventors: Brian William Holmes; James Peter Snelling; Frederic Fournier
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2016-02-29
Filing date: 2017-02-27
Publication date: 2019-01-09
Also published as: GB201603484D0; WO2017149284A1; GB2547717A; GB2547717B

Abstract

A security element, comprising: a colourshifting film which, when viewed in reflected or transmitted light, exhibits different colours in dependence on the viewing angle and which, when viewed at any one angle, exhibits first and second colours when viewed in reflected and transmitted white light respectively, the first and second colours being complementary, the colourshifting film comprising a plurality of polymer layers arranged in a periodic stack, including respective layers of at least first and second polymer materials having different refractive indices from one another; and a light absorbing material layer underlying the colourshifting film, the light absorbing material layer having one or more gaps therein so as to define a pattern. At any one viewing angle θ, the percentage of incident light reflected by the colourshifting film varies in dependence on its wavelength according to a function Rθ(λ) having a maximum at a peak wavelength λΒ,θ=0, and the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the percentage RΒ,θ=0 of incident light at the peak wavelength AΒ,θ=0 that is reflected by the colourshifting film is at least 60%, and the maximum of the function Rθ(λ) has a bandwidth (Δλθ=ο) of 150 nm or less. The security element is substantially transparent or translucent to at least some wavelengths of visible light at the locations corresponding to the one or more gaps in the light absorbing material layer, such that in the locations corresponding to the one or more gaps in the light absorbing material layer, the security element exhibits different respective colours when viewed in reflected white light and when viewed in transmitted white light at the same viewing angle.

Description

SECURITY ELEMENTS AND SECURITY DOCUMENTS

The present invention relates to security elements suitable for use in determining the authenticity of security documents, such as banknotes, passports and the like, and other objects of value, as well as to security documents themselves.

It is well known to provide security documents such as banknotes with security elements which exhibit optical effects which cannot be reproduced by standard means such as photocopying or scanning. Typical examples of such elements include holograms and other diffractive devices, which exhibit different appearances, e.g. diffractive colours and holographic replays, at different viewing angles. Similarly, reflective elements can be configured to display different intensities (i.e. brightnesses) at different viewing angles. Photocopies of such elements will not exhibit the same optically variable effects. The term "optically variable effect" means that the device has an appearance which is different at different viewing angles.

Another known class of optically variable security devices are so-called iridescent amplitude interference materials, which display different colours at different viewing angles. Examples include thin-film interference structures, interference pigments, pearlescent pigments, liquid crystal film and pigments, photonic crystals and the like. Thin film interference structures comprise repeating layers of different refractive indices; examples can include purely dielectric stacks (metal oxide or polymer) or those composed of alternate dielectric and metallic layers. Thin film interference structures are also known as Bragg stacks or 1 D photonic crystals. What all of the above examples have in common is the provision of two or more closely spaced interfaces, at least one of which partially reflects and partially transmits incident light, i.e. the amplitude of the incident light is split. The transmitted portion is reflected at the second or subsequent interfaces and interferes with the portion reflected from the first or earlier interfaces, leading to constructive interference of some wavelengths and destructive interference of others, and hence a characteristic colour which varies with viewing angle. Whilst this colourshifting effect does by itself provide a useful security feature, due to its optically variable nature, such materials are becoming more readily available to would-be counterfeiters and so more sophisticated security effects are desirable.

An example of a security element which makes use of iridescent amplitude interference materials is disclosed in WO-A-201 1/051682. Here, two colourshifting materials are provided overlapping one another, with a first light absorbing layer between them arranged in partial areas so as to define a pattern. A second light absorbing layer is provided to the free surface of the second colourshifting material. The colour of the first light absorbing layer is selected so as to match the reflected colour of the combination of the second colourshifting layer and the second light absorbing layer when the security element is viewed at normal incidence. The result is that when the element is viewed in reflected light, at the normal viewing angle the pattern is substantially hidden since the same colour is exhibited to the viewer at all locations across the device, and when the device is tilted (i.e. viewed away from the normal), the pattern is revealed since the colours no longer match. Whilst the security element disclosed in WO-A-201 1/051682 does achieve an improved level of security, there is a constant need to stay ahead of would-be counterfeiters by developing new security elements with alternative security effects which are difficult to replicate and possess a high visual impact to ensure they are recognised by the user.

In accordance with the present invention, a security element comprises:

a colourshifting film which, when viewed in reflected or transmitted light, exhibits different colours in dependence on the viewing angle, and which, when viewed at any one angle, exhibits first and second colours when viewed in reflected and transmitted white light respectively, the first and second colours being complementary, the colourshifting film comprising a plurality of polymer layers arranged in a periodic stack, including respective layers of at least first and second polymer materials having different refractive indices from one another; and

a light absorbing material layer underlying the colourshifting film, the light absorbing material layer having one or more gaps therein so as to define a pattern;

wherein at any one viewing angle Θ, the percentage of incident light reflected by the colourshifting film varies in dependence on its wavelength according to a function R_e(A) having a maximum at a peak wavelength λ_Β,β, and the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the percentage R_B,e=o of incident light at the peak wavelength A_B,e=o that is reflected by the colourshifting film is at least 60%, and the maximum of the function R_e(A) has a bandwidth (Δλ_θ=₀) of 150 nm or less; and

wherein the security element is substantially transparent or translucent to at least some wavelengths of visible light at the locations corresponding to the one or more gaps in the light absorbing material layer, such that in the locations corresponding to the one or more gaps in the light absorbing material layer, the security element exhibits a different respective colours when viewed in reflected white light and when viewed in transmitted white light at the same viewing angle.

The resulting security element exhibits a particularly strong contrast between its appearance in reflected light versus its appearance in transmitted light. This can be readily checked by a user without the need for precise viewing angles and is also straightforward to describe, all of which results in a high security level. This switching effect between the reflective and transmissive colours is in addition to the inherent colourshifting effect which may be observed upon changing the viewing angle, depending on how the security element is deployed. It should be noted that in some cases, the different respective colours exhibited by the security element as a whole in the locations corresponding to the gaps in the light absorbing material layer in reflection and transmission will be the same as the first and second colours exhibited by the colourshifting film alone, e.g. if there are no other layers contributing a visible colour to the element in those locations. However in other cases, one or more such layers may be present (as discussed further below) in which case these will influence the appearance of the security element at least in transmitted light and so the colours exhibited by the element as a whole may differ from those exhibited by the colourshifting film by itself. Nonetheless, the contrasting, complementary colours produced by the colourshifting film in reflected white light compared to transmitted white light will cause the corresponding colours exhibited by the element as a whole under those viewing conditions to also differ from one another (although they may not be complementary). The term "colour" is used here to refer to any visible colours including achromatic hues such as white (or near white), black and grey, as well as chromatic colours such as red, green, blue, yellow, orange, purple etc. However it is preferred that the first and second colours are each chromatic (e.g. not white, black or grey), as discussed further below.

The strong security effect is achieved by a combination of features. Firstly, the security element is configured to be transparent or translucent in accordance with a pattern defined by gaps in a light absorbing material layer - that is, at those locations corresponding to the gaps, the security element as a whole will allow at least some visible light to be transmitted therethrough, although it may be optically scattered if the security element includes one or more layers which are translucent rather than transparent, such as the optional diffusive layer described below. Preferably, the transparent or translucent locations (i.e. the gaps) will have an optical density no greater than 1 , more preferably no greater than 0.6, still preferably less than or equal to 0.3 (optical density is a dimensionless quantity on a logarithmic scale; an optical density of 1 corresponds to 10% transmission). Secondly, the colourshifting layer is specifically engineered to exhibit a large difference between its reflected and transmitted colour at any one angle - i.e. to make them as "complementary" to one another as possible. The present inventors have found that this can be achieved by configuring the colourshifting film to have (in combination): (i) a high peak wavelength reflectivity, which can be characterised with reference to the film's behaviour at normal incidence as reflecting at least 60% of the incident light at the peak wavelength; and

(ii) a narrow reflected bandwidth (i.e. the range of wavelengths around the peak wavelength that are strongly reflected), which can be characterised with reference to the film's behaviour at normal incidence as being no wider than 150nm.

This combination of film characteristics has been found by the present inventors to give rise to a particularly strong contrast between the different colours exhibited by the colourshifting film in reflection as compared with transmitted light at the same viewing angle (and hence also between the different colours exhibited by the security element as a whole in the gap regions in reflected and transmitted light, which may or may not be the same as the first and second colours as discussed above). Without wishing to be bound to theory, it is believed that this may result from the high reflectivity of the film substantially preventing the transmission of the wavelengths falling within the relevant waveband (thus substantially removing the contributions of those wavelengths from the transmitted colour) whilst the narrow width of the waveband ensures that this effect is strictly confined to that waveband. Thus, wavelengths outside the reflected waveband are transmitted largely unhindered. As a result there are few, if any, wavelengths which contribute to both the reflected and transmitted colours at any one viewing angle, leading to a clear visual separation between them.

The above-stated properties of the colourshifting film are achieved through design of its structure and selection of its materials. The use of a stack of polymer layers, as opposed to conventional ceramic dielectric materials, assists in attaining a narrow reflected bandwidth. This is because the difference in refractive index ΔΝ between any two different polymers is typically significantly smaller than the refractive index difference between adjacent ceramic dielectrics. For instance, in a polymer colourshifting film as used in the present invention, the difference in refractive index between the first and second polymer layers may preferably be of the order of 0.02 to 0.4 (more preferably between 0.02 and 0.25), whereas in a conventional ceramic dielectric stack, the refractive index difference is typically around 0.5 to 1.0. As discussed below, the reflected bandwidth increases as the refractive index difference increases. Therefore narrower reflected bandwidths can be achieved with polymer stacks than are typically practicable in ceramic-based films, which must be manufactured by vacuum deposition of each layer. The bandwidth and peak wavelength reflectivity will also be influenced by the thickness of the polymer layers and the number of polymer layers in the stack, in addition to the choice of polymers, and the skilled man will be able to adjust both parameters through control of these variables in order to achieve the above requirements. Guidelines illustrating the dependency of the film's characteristics on these variables are provided below. More information as to how to form multilayer polymer colourshifting films which can be made suitable for use in the present invention through appropriate control of their construction can be found in US-A-2005/0161840.

The use of a multilayer polymer colourshifting film also provides additional benefits in terms of ease of manufacturing, cost and resilience. The polymer stack can be manufactured, for example, by co-extrusion of the various polymer layers which allows for fast and relatively inexpensive manufacture of long lengths of the film as compared with the manufacture of ceramic dielectric stacks, which are formed by depositing the layers one at a time in a vapour deposition chamber. Due to the nature of the polymeric materials the resulting stack is generally more flexible and robust than is the case for ceramic stacks, and depending on the polymers selected (discussed further below), the film can be made resistant to heat, solvents and other potentially damaging environments as necessary for use in currency and other security documents.

Throughout this disclosure, the polymer materials forming the colourshifting layer are each described as having a "refractive index" which differs from one another. One or more (or all) of the polymer materials may be isotropic in which case its refractive index will be the same in all directions within the material. However in preferred embodiments one or more (or all) of the polymer materials may be anisotropic, i.e. having a refractive index which varies with direction within the polymer. Typically the refractive index values lying within the plane of the polymer layer (denoted N_x and N_y) may be substantially the same as one another whilst the refractive index value in the direction normal to the layer (N_z) will be different. In this case it is the refractive index values in the plane of the layer N_x and N_y which should differ from one polymer material layer to the next to produce the desired effect, since these are the directions of electromagnetic oscillation (i.e. the polarisation direction) of an incident light beam along the normal direction. In preferred examples the refractive index values in the normal direction (N_z) may be substantially the same as one another from one polymer material to the next, although this is not essential.

The term "complementary colours" has its common meaning, i.e. a pair of different colours which, when combined, cancel each other out to produce substantially white light. It will be appreciated that the particular first and second (i.e. reflective and transmissive) colours exhibited by the colourshifting film at any one viewing angle will depend not only on the construction of the colourshifting film but also on the spectral characteristics of the incident illumination. Where the incident light is white light, i.e. comprising substantially all wavelengths of the visible spectrum (which is defined here as meaning all light with wavelengths between 390 nm and 700 nm, inclusive), the first and second colours will be complementary as described above. However, in practice the security element can of course also be viewed under other types of incident illumination including coloured light, e.g. not containing all visible wavelengths. In such scenarios, the first and second colours would also each lack certain visible wavelengths and would therefore not combine to form white. If the incident light were monochromatic, this wavelength would be substantially wholly reflected at some viewing angles and substantially wholly transmitted at others such that there would be only a first colour (and not a second) at some viewing angles, and vice versa at others.

The term "bandwidth" is used herein to refer to the full width of the maximum in the reflectivity function R_e(A), i.e. the spectral distance between the wavelength at which the reflectivity value starts to increase from its baseline, and the wavelength at which the reflectivity value rejoins the baseline. Examples will be illustrated below. As already noted, by arranging the reflected bandwidth to have a width of 150 nm or less, a strong contrast between the reflected and transmitted colours is exhibited. The present inventors have found that this contrast can be enhanced still further if the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the maximum of the function R_e(A) has a bandwidth (Δλ_θ=ο) of 100 nm or less. It is believed this may be because the reflected bandwidth then occupies approximately one third of the visible spectrum, with the remaining two thirds being transmitted. This has the result of making the first and second colours each more "pure" and hence having a greater contrast between them. For instance, if the reflected waveband is centred on green, by having it extend no more than 50 nm in each direction (i.e. a bandwidth of 100 nm), substantially all the red and blue wavelengths will be transmitted so as to form a strong magenta transmitted colour. In preferred examples, the colourshifting film is configured such that the peak reflected wavelength is either at substantially the centre of the visible red waveband (e.g. 630nm), or at substantially the centre of the visible green waveband (e.g. 530nm), at substantially the centre of the visible blue waveband (e.g. 465nm).

The reflected waveband could be narrow, e.g. reflecting essentially monochromatic light. In this case, the transmitted light would contain a large proportion of the wavelengths of the visible spectrum and so will appear to the naked eye substantially white (or near white). For instance this may be the case if the bandwidth is less than about 30 nm. Nonetheless there will still be a strong contrast between the reflected and transmitted colours (e.g. red vs. white) and hence a strong security effect. However it is more preferred that the reflected waveband is not overly narrow, in order to obtain chromatic colours in both reflection and transmission (i.e. not white, off-white, black or grey) and also to ensure a sufficiently intense reflected appearance. Hence, preferably, the reflected bandwidth is at least 30nm wide and more preferably at least 50nm wide. In particularly preferred cases the reflected bandwidth is between 30 and 150 nm, more preferably between 50 and 100 nm.

The stated peak reflectivity of at least 60% has been found to produce good results. Nonetheless, the contrast can be still further enhanced where the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film is at least 70%, preferably at least 80%. In a particularly preferred implementation the reflectivity R_B,e=o is configured to be at least 72%.

If the reflectivity of the colourshifting film is sufficiently high, in preferred embodiments the security element exhibits a supplementary secure visual effect in the form of an ability to conceal, or at least reduce the visibility of, the pattern formed by the gaps in the light absorbing material layer. In particular, at at least one viewing angle, the intensity of the reflected light may be sufficiently high so as to overwhelm the contrast between the colour of the locations of the security elements defined by the gaps, and that of their surroundings. As the viewing angle is changed, the apparent intensity of light reflected to the viewer will also change as a result of the peak reflected wavelength changing: typically the incident illumination will contain a greater intensity of some wavelengths than others (even if it is "white"), which will in turn limit the reflected intensity at each wavelength, and the human eye is more sensitive to some wavelengths (particularly yellow/green) than others. Therefore the intensity of reflected light will appear to change as the device is tilted which may lead to the pattern being "hidden" by the reflected light at at least one viewing angle, becoming visible again at others. However it should be noted that this is an optional additional security effect and, whilst preferred, may not be observed in all embodiments of the invention.

The reflectivity R_B,e=o can be measured for any particular colourshifting film by arranging a light source and a light detector such that light is incident on the colourshifting film along the normal (Θ = 0 degrees) and the light detector receives light reflected back along the normal. In practice this can be achieved using an arrangement as shown in Figure 1 hereto, in which a light source 100 is positioned to illuminate a colourshifting film 2 along its normal (here, the z-axis) through a beamsplitter 102 such as a semi-silvered mirror. The light beam Bi incident on the film 2 has a known intensity at each of its constituent wavelengths (which can be measured if necessary by inserting a detector between the beamsplitter 102 and film 2). The reflected beam B_R also lies on the normal (shown slightly offset for clarity) and is redirected by the beamsplitter 102 to a detector 104. If the peak wavelength for the film at normal illumination is already known, the detector may be configured to detect only that wavelength, e.g. using an optical filter or through selection of the detector's spectral sensitivity. The reflectivity R_B,e=o can then be calculated as the ratio of the light intensity measured by detector 104 at the peak wavelength to the intensity of the incident light beam Bi at the same wavelength.

If the peak wavelength is not yet known this can be determined by using detector 104 to measure the intensity of the reflected beam B_R over a range of wavelengths (e.g. the whole visible spectrum), for instance by using a variable optical filter. The output of this scan across different wavelengths will be the reflectivity function R_e(A) for Θ = 0 degrees. The peak wavelength will be that for which the reflected beam intensity is greatest. The reflected bandwidth Δλ_θ=ο can be measured from the same output. The same set-up can be used to measure the intensity of the transmitted light beam B_T by providing a light detector 106 on the opposite side of the film 2 on the normal, in alignment with the light source. The transmission T_B,e=o of the film at the peak wavelength can then be calculated by working out the ratio of the light intensity received at detector 106 at the peak wavelength (utilising a filter if necessary) to the intensity of the incident light beam Bi at the same wavelength. The following mathematical model is provided to illustrate the dependence of the colourshifting film's properties on its construction and composition. However as with all mathematical models it will be appreciated that this is an approximation and in practice the values of the film's parameters measured using a method such as that described above may not match the values predicted by the model exactly. Nonetheless, the inventors have found the model to accurately predict the observed trends in the parameters upon changing each of the variables mentioned below and therefore to provide useful guidance as to how to construct a colourshifting film with any particular desired characteristics. The actual reflectivity, bandwidth and any other relevant parameters can then be measured from the resulting film using a method as described above, and the construction of the film adjusted if necessary to refine its characteristics, based on the following principles. More details of the theory on which the mathematical model is based can be found in "Giant Birefringent Optics In Multilayer Polymer Mirrors" by M.F. Weber et al, Science, Vol. 287, 31 March 2000, p. 2451 , and in "Filter Design using Multi-Bragg Reflectors" by F.D. Ismail et al, World Journal of Modelling and Simulation, Vol. 8 (2012) No. 3, pp. 205-210. Thus, the peak wavelength (which may also be referred to as the Bragg wavelength) λ_Β,θ is preferably given by:

A_Bi9 = 2 [N_h. L_h + N_l. L_l]cose (Eq. 1 ) where: N_h and N| are the refractive indices of the first and second polymer materials respectively, N_h being greater than N,; L_h and are the thicknesses of the first and second polymer material layers respectively in the direction normal to the security element; and Θ is the viewing angle relative to the normal for the first order reflection of the peak wavelength. If the polymer materials are anisotropic then the relevant refractive indices are those lying in the plane of the respective layers, i.e. N_x (or N_y), as discussed above.

The percentage R_B,e=o (i.e. the peak wavelength reflectivity at normal incidence) is preferably given by:

1- ^2P(

(Eq. 2)

1 + ^'W

^2P(

where: p is the number of pairs of first and second polymer material layers in the colourshifting film; and N_a and N_b are the refractive indices of the materials a and b adjoining the two outermost layers of the colourshifting film, respectively. In practice, the materials a and b adjoining the outermost layers of the colourshifting film will typically each be a polymer-based material such as an adhesive, or an ink - for instance forming the light absorbing material layer as discussed further below. Therefore, the refractive indices of materials a and b will typically be very similar to one another (N_a ^¾ N_b) and moreover will also be very similar to N_h (N_a ^¾ N_b ~ N_h). As a result the fraction (N_b ²/ N_aN_b) can be approximated to 1 , and the normal peak reflectivity R_B,e=o approximates to:

It is therefore preferred that the above approximation for R_B,e=o of the colourshifting film given by Eq. 3 meets the desired criteria (i.e. at least 60%) irrespective of the nature of the adjoining materials a, b.

According to the model, at a viewing position lying on the normal to the security element (Θ = 0 degrees), the bandwidth Δλ_θ=ο is preferably given by:

Where: λ_Β,θ=ο is the peak wavelength at the normal viewing angle; acos(p) is the arc cosine of the parameter p; p is the Fresnel reflection coefficient which for normal incidence corresponds to (N_h - N|)/(N_h + N|); and E is a percentage manufacturing tolerance in the thicknesses L_h and of the first and second polymer material layers respectively. The error term Ελ_Β,θ=ο is included since variation in the polymer layer thickness has been found to have a significant effect on the spread of reflected wavelengths and hence the bandwidth Δλ_θ=ο- For instance, if the polymer layers are known to be manufactured to within 10% of their desired thickness (which is typical), and the peak wavelength is 500nm, the error term contributes 50nm to the bandwidth Δλ_θ=ο- Of course, if the manufacturing error can be reduced, the observed bandwidth Δλ_θ=ο will be closer to that predicted by the mathematical model alone.

To further improve the strong colour contrast observed in reflected versus transmitted light, it is also preferred that the colourshifting film has very low absorption of visible wavelengths. This can be characterised by the colourshifting film being configured such that, in preferred embodiments, at least at a viewing position lying on the normal to the security element, the sum of the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film and the percentage T_B,e=o of incident light at the peak wavelength that is transmitted by the colourshifting film is at least 80%, more preferably at least 90%, still preferably at least 95%, most preferably substantially 100%. The difference between the sum of the reflected and transmitted light intensity as compared with the incident light intensity (which is preferably less than 10% of the incident intensity) represents not only absorption which might result from the film having a slight body colour (although it preferably has no body colour) but also any internal "trapping" of light which might result from internal reflections. The strong colour contrast can be still further improved by configuring the reflectivity function R_e(A) to have as low a baseline as possible - i.e. to have a high transmittance of visible wavelengths outside the reflected waveband. This can be characterised by the colourshifting film being configured such that, in preferred embodiments, at least at a viewing position lying on the normal to the security element, the percentage T_N,e=o of incident light at wavelengths (λ_Ν,β=ο) at least 50nm away from the peak wavelength (λ_Β,β=ο) that is transmitted by the colourshifting film is at least 75%, preferably at least 80%, more preferably at least 85%. In other words, all (visible) wavelengths spaced from the peak wavelength by 50nm or more should preferably be at least 75% transmitted by the colourshifting film. In still preferred embodiments, this is the case for all (visible) wavelengths at least 25nm away from the peak wavelength, more preferably at least 15nm away from the peak wavelength. This condition should hold at least across the visible spectrum. Since non-visible wavelengths will not affect the visible colours, the behaviour of the film outside the visible spectrum is typically not of importance. However, typically the same behaviour will continue at least into the UV and/or IR spectral ranges. As noted above, the refractive index difference between the different polymer materials will generally be less than that found in conventional ceramic dielectric stacks, and this will have a corresponding reduction on the reflection bandwidth, as is desirable. In preferred examples, the difference between the refractive indices of the first and second polymer materials is less than or equal to 0.4, preferably less than or equal to 0.25, more preferably less than or equal to 0.2. Typically, the difference between the refractive indices of the first and second polymer materials is at least 0.02. In preferred embodiments, the refractive indices of the first and second polymer materials are each between 1.45 and 1.95. Again, if the polymer materials are anisotropic it is the refractive index in the plane of the layers that is relevant.

The sequence of polymer layers will be periodic in the direction normal to the plane of the colourshifting film. For example, if only two different polymer materials H, L are provided in the colourshifting film (as is preferred) then the sequence of the layers will typically be alternating (i.e. H, L, H, L, H, L... ), each pair Ή, L" representing a unit cell of the periodic pattern. If more than two different materials are provided the repeat pattern may be more complex. Preferably, the periodicity of the polymer layers is constant throughout at least a first portion of the thickness of the stack. This first portion may include the whole of the stack (i.e. the periodicity may be constant throughout). However in other cases the periodicity of the polymer layers may be different in respective first and second portions of the thickness of the stack, whilst remaining constant within each respective portion. Varying the periodicity from portion to portion of the stack can be used to help "tune" the characteristics of the colourshifting film such as its reflectivity and reflection bandwidth.

In preferred examples, the plurality of polymer layers have individual thicknesses in the range 0.05 microns to 0.2 microns, preferably 50 nm to 150 nm, more preferably 60 nm to 120 nm. The total thickness of the colourshifting film will depend on the individual layer thickness(es) and the number of layers, but preferably the plurality of polymer layers has a total thickness in the range 5 to 50 microns, preferably 10 to 20 microns. Film thicknesses of this sort are well suited for use in security articles and documents. In preferred implementations, the colourshifting film comprises between 100 and 400 polymer layers, preferably between 120 and 170 polymer layers. As will be apparent from the model above, the greater the number of layers, the greater the reflectivity R_B,e=o since the value of p (the number of pairs of high/low refractive index layers) increases.

Any at least two different polymeric materials which are optically clear (i.e. can be seen through) could be used to form the multiple layers of the colourshifting film, provided the necessary refractive indices are achieved. Examples of suitable polymer materials include any of those mentioned in US-A- 2005/0161840]. For instance, the first polymer material could be PET and the second polymer material could be PMMA. However, in particularly preferred embodiments, the first polymer material is polyethylene terephthalate (PET) and the second polymer material is polyethylene terephthalate glycol-modified (PETg). As mentioned above, it is particularly preferred that the colourshifting film is a co-extruded multilayer polymer stack, e.g. manufactured using techniques such as those described in US-A-2005/0161840. The light absorbing material acts to define those locations of the security element in which the transmissive colour of the colourshifting film will be exhibited in transmitted light (i.e. the gaps), substantially preventing areas outside those locations displaying a colour in transmitted light. The light absorbing material may additionally enhance the reflected colour of the areas outside the gaps by supressing "stray" or "background" light which might otherwise overwhelm the light reflected by the colourshifting film. Advantageously, the light absorbing material absorbs at least 70% of incident visible light, preferably at least 80%, more preferably at least 90%. Advantageously the light absorbing material is additionally non-transparent and preferably transmits less than 30% of incident visible light in a single pass, more preferably less than 20%, still preferably less than 10%, most preferably is substantially opaque. Desirably, the light absorbing material is dark in colour, preferably black, although alternatives such as dark blue or dark green are also envisaged. For example, the light absorbing material may comprise an ink containing a dark pigment such as carbon black. The light absorbing material may also comprise a magnetic or electrically conductive substance, which may or may not be the same pigment as that which gives the material its colour. Additional optional but preferred characteristics of magnetic layers and/or conductive layers such as this are discussed below. In preferred examples, the light absorbing material is an ink (i.e. a binder containing a suitable pigment or dye), and/or resist material (i.e. a material which is resistant to a solvent or etchant which may be used for instance to dissolve parts of a metal layer).

The light absorbing material layer will be arranged underneath the colourshifting film - that is, on the opposite side of the colourshifting film from that on which an observer viewing the film is typically situated, so that the observer can view portions of the colourshifting film in the locations defined by the gaps as well as areas of the colourshifting film outside the gaps. Preferably, the light absorbing material layer will either be in direct contact with the colourshifting film (i.e. with no intervening layers), or if there are one or more layers between the light absorbing material and the colourshifting film, then these intermediate layer(s) will either also be light-absorbing in nature, or will be optically inactive (that is, having no effect on incident light, e.g. transparent, clear and preferably colourless materials) under all illumination conditions, so that the area(s) in which the light absorbing material is arranged maintain the same light-absorbing functionality. Thus, in some preferred embodiments, the light absorbing material layer directly contacts an outermost one of the plurality of polymer layers of the colourshifting film. For example, the light absorbing material layer could be printed, or otherwise applied in a patternwise manner, directly on to one side of the colourshifting film. This arrangement has the most significant reduction on stray or background light and so helps to achieve a strong reflected colour. Further, this approach improves ease of manufacture as compared with lamination techniques for instance. However this is not essential and in other cases the security element further comprises one or more intermediate layers between the colourshifting film and the light absorbing material layer. These may be layers providing additional functionality and/or security, or could be provided to improve the mechanical integrity of the structure (although as noted above will be configured not to detract from the light-absorbing nature of the region). In all cases, the or each intermediate layer will be either transparent and/or arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern. The intermediate layer(s) preferably comprise any of: an adhesive layer; a barrier layer; a magnetic layer; and an electrically conductive layer. Preferred characteristics of magnetic layers and electrically conductive layers are discussed below. In preferred embodiments, the security effect displayed by the security element can be made more complex by providing at least one optical diffusion layer overlapping the colourshifting film and/or the light absorbing material layer at least in partial areas of the security element. An optical diffusion layer is a layer which converts incident directional light (i.e. light which appears to emanate from a defined source) into light which propagates in multiple directions (i.e. as if from an extended array of light sources). Thus for example the optical diffusion layer will convert collimated light into non-collimated light, i.e. substantially without directionality. For instance, the optical diffusion layer(s) may be optically scattering (and hence translucent rather than transparent, i.e. not optically clear) and/or may contain a photoluminescent substance which emits visible light (typically in all directions) upon excitation by a suitable wavelength (which may or may not be visible). The specific effect of the optical diffusion layer(s) on the appearance of the security element depends on the position of that layer or layers in the element structure as will be described further below, but in all cases the behaviour of the element will be different in the areas where an optical diffusion layer is present as compared with those regions where it is absent (if any), and will also be different if such a layer is provided on both sides of the colourshifting film as compared with just one or the other side. Therefore, preferably, at least one (or all) of the optical diffusion layer(s) is preferably provided only in partial areas of the element (i.e. patternwise). Advantageously, at least one (or all) of the optical diffusion layer(s) extends across some or all of the locations corresponding to gaps in the light absorbing material layer so that the transmitted colour is influenced by the optical diffusion layer(s) as described below. In preferred examples at least one of the optical diffusion layer(s) extends across substantially the whole area of the security element. However if such layers are provided on both sides of the colourshifting film it is preferred that only the optical diffusion layer on one side and not the other extends across the whole element area.

A first preferred implementation provides that at least one of the optical diffusion layers is located on the same side of the colourshifting film as the light absorbing material layer, the light absorbing material layer preferably being between the optical diffusion layer and the colourshifting film, the optical diffusion layer being provided at least in partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security element in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the viewing angle but is substantially independent of the illumination angle.

So far, the colourshifting properties of the multilayer polymer film have been described with reference to changes in the viewing (i.e. observation) angle. However, in practice similar effects will also be observed if the film is illuminated with collimated light and the illumination angle is changed. Including an optical diffusion layer in the security element structure suppresses one of these responses depending on the position of the layer. By positioning the optical diffusion layer on the same side as the light absorbing material layer, when the security element is viewed from the opposite side, the optical diffusion layer has no effect on the appearance of the security element in reflected light. However, when viewed in transmitted light, those locations in which the gaps in the light absorbing material layer are overlapped by the optical diffusion layer will now only display a colourshifting effect when the viewing angle is changed, and not when the illumination angle of incident collimated light is varied. This is because the optical diffusion layer removes the directionality of the incident collimated light so that the light incident on the colourshifting film has substantially the same overall angle irrespective of the position of the light source. As a result the colour of these portions of the pattern defined by the gaps appears static from any one viewing angle while the illumination direction is changed, whereas other portions in which the optical diffusion layer is absent will appear to change in colour. Providing an optical diffusion layer in this position also assists in achieving more uniform illumination of the different parts of the security element, and hence a more uniform colour response, since the incident light is "spread" across an area of the element by the diffusion layer.

In another preferred implementation, at least one of the optical diffusion layers is located on the opposite side of the colourshifting film from the light absorbing material layer, the optical diffusion layer being provided at least in partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security element in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the illumination angle but is substantially independent of the viewing angle. Here the effect on the transmissive appearance of the element is opposite to that in the previous implementation: those locations in which the gaps in the light absorbing material layer are overlapped by the optical diffusion layer will now only display a colourshifting effect when the illumination angle of incident collimated light is changed, and not when the viewing angle is varied (although it will be noted that the inherent viewing angle dependency of the colourshifting film itself remains present, it is merely concealed). This is because the optical diffusion layer redirects the light that has been transmitted through the colourshifting film in all directions so that the colour seen by the viewer is substantially the same irrespective of the observation angle. As a result the colour of these portions of the pattern defined by the gaps appears static for any one illumination angle while the viewing angle is changed, whereas other portions in which the optical diffusion layer is absent will appear to change in colour. The provision of an optical diffusion layer on this side of the colourshifting layer will also render the reflected colour of the element static where the optical diffusion layer is present. This can also be utilised to increase the complexity of the element although in order to preserve the reflected colourshift effect it is preferred that any optical diffusion layer on this side of the colourshifting film does not extend over the whole area of the element but rather is provided in a pattern.

Complex patterns of static and non-static areas can therefore be achieved through careful placement of the one or more optical diffusion layers on either side of the colourshifting layer. If such layers are provided on both sides of the colourshifting film in the same area, the colour of that area will appear static both in transmission and reflection irrespective of illumination and viewing angles (although the core security effect of contrasting colours in reflected and transmitted light will still be present). As such it is preferred that optical diffusion layers are not provided on both sides of the colourshifting film across the whole area of the security element, but only in partial areas if at all.

As noted above the optical diffusion layer(s) can take various different forms but preferably comprise a binder containing a dispersion of optically scattering particles and/or a photoluminescent substance which emits light in the visible spectrum upon excitation. Where luminescent materials are used, preferably the photoluminescent substance emits light in the visible spectrum in response to incident radiation in the ultra-violet spectrum. Photoluminescent optical diffusion layers are particularly preferred, especially if located on the same side of the colourshifting film as the light absorbing material layer, since upon excitation they act as a light source in extremely close proximity to the colourshifting film, with good spatial uniformity, and thereby increase the apparent brightness of the resulting colour exhibited by the security element.

In general, the or each optical diffusion layer may be a non-fibrous material and could be a resist material and/or an ink for example. Where the optical diffusion layer(s) are optically scattering they will be translucent and preferably have an optical density less than or equal to 1 , more preferably less than or equal to 0.6, more preferably less than or equal to 0.3. If there is more than one overlapping optical diffusion layer in the locations corresponding to the gaps in the light absorbing material layer, the combined optical density of all of the optical diffusion layers is preferably less than or equal to 1 , more preferably less than or equal to 0.6, still preferably no greater than 0.3. In particularly preferred embodiments, the or each optical diffusion layer is white or has a visibly coloured tint. The visible colour(s) of the optical diffusion layer(s) (if any) will contribute to the visible colours of the security element in the locations corresponding to the gaps in the light absorbing material layer where the optical diffusion layer(s) extend across such gaps.

To increase the complexity of the security effect still further, the or each optical diffusion layer could be made up of more than one material. Hence, preferably at least one of the layer(s) comprises at least two optically diffusing materials having different optical characteristics, arranged in respective laterally offset regions of the element. For instance, the at least two optically diffusing materials could have different visible colours and/or different luminescent substances in them. In this way an additional pattern can be introduced to the security element, which may be independent of or linked with the pattern defined by the gaps in the light absorbing material layer.

The pattern defined by the gaps in the light absorbing material layer could take any desirable form. For instance, in preferred embodiments the pattern may comprise positive or negative indicia (i.e. indicia defined by the presence or absence of the light absorbing material, respectively), preferably one or more alphanumeric characters, symbols, currency identifiers, logos or the like. The pattern may comprise microtext or other elements which are sufficiently small that they can only be resolved by the human eye under close inspection and/or with a magnification aid.

The security element may be provided with further authentication features. For instance, as mentioned above, either the light absorbing material layer or an intermediate layer between that and the colourshifting layer could be magnetic. Alternatively or in addition, another magnetic layer could be provided and hence advantageously the security element further comprises at least one magnetic layer, preferably on the same side of the colourshifting film as the light absorbing material layer, the or each magnetic layer being either transparent and/or being arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern. Any of these magnetic layer(s) may be arranged spatially so as to form a magnetic coding. If the magnetic material (e.g. magnetic ink) is non-transparent, it is preferably concealed by the reflected colour of the colourshifting film. The magnetic material may have substantially the same appearance as the light absorbing material (assuming this is non-magnetic) so that its presence is camouflaged by the light absorbing material lying above, beneath or around it. Alternatively the security element could comprise a transparent magnetic layer as is known from EP1497141 or WO2009053673A1.

By a magnetic "coding" it is meant a system for communication of hidden information, preferably secret information, in which the meaning of said information is conveyed using machine readable elements said configuration of elements being chosen so as to render the information unintelligible to casual interrogation. More preferably we are referring to a spatial code: i.e. it is the relative position of the individual elements that provides the information rather than the appearance of the elements.

In one example the magnetic regions are formed from a magnetic ink, such as iron oxide, or another iron, nickel or cobalt based material. Ferrites, such as barium ferrite, and alloys, such as AINiCo or NdFeCo, would also be suitable. Hard or soft magnetic materials may also be used, or materials with high or low coercivity. Transparent magnetic inks such as those described in GB-A-2387812 and GB-A-2387813 are also suitable.

The code may be a block magnetic code. Block magnetic coding describes the arrangement of regions containing magnetic material separated by blank spaces. More advanced magnetic codes digitise the code. IMT is an example of spatial coding, and is described in EP-A-407550 and another type of coding is intensity coding. Magnetic materials with a low coercivity can be used to form the code. The magnetic signal detected from a low coercivity material can differ in polarity from an iron oxide type material depending on the geometry of the detector. Such low coercivity materials have a lower coercivity than conventional iron oxide materials which means that they can be reversed in polarity by weaker bias magnetic fields, whilst they are still magnetically hard so that they retain the induced magnetism which can then be detected when the article is in a region no longer affected by the bias magnetic field. This is known as a reversed edge magnetic signature. Suitable low coercivity magnetic materials preferably have a coercivity in the range 50-150 Oe, most preferably 70-100 Oe. The upper limit of 150 Oe could increase with higher biasing fields. A number of examples of suitable materials include iron, nickel, cobalt and alloys of these. In this context, the term "alloy" includes materials such as Nickel:Cobalt, Aluminium:Nickel:Cobalt and the like. Flake nickel materials can be used. In addition, iron flakes are also suitable. Typical iron flakes have lateral dimensions in the range 10-30pm and a thickness less than 2 m. The preferred materials include metallic iron, nickel and cobalt based materials (and alloys thereof) which have the highest inherent magnetisations and so benefit from the requirement for least material in a product to ensure detectability. Iron is the best of the three with the highest magnetisation, but nickel has been shown to work well from other considerations. EP1770657A2 discloses a method of detecting such low coercivity materials. If both nickel based and iron based magnetic inks are used at set positions, then a more complex code can be achieved. It is important that the code can be detected and related to the physical dimension of the security element. One method for achieving this is to have a binary code with a recognisable start and end bit to a detection trace. The presence of start and end bits enables the detector to "clock" or recognise the detection trace independent of the note speed in the detector and so enable a measurement of the complete length of the security element and thus determine where the other code elements should be. Suggestions to enable a self-clocking code would be a known length of start magnetic block (as described in EP407550), a reversed edge magnetic signature (as described in EP1770657), or the presence of materials with different magnetic properties: for example the material used for the start bit could have a different (e.g. higher or lower) magnetic remanence then the rest of the bits. The security element may preferably further comprise at least one electrically conductive layer, preferably on the same side of the colourshifting film as the light absorbing material layer, the or each electrically conductive layer being arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern. Advantageously, the or each electrically conductive layer is a metal layer, the light absorbing material layer preferably being located between the colourshifting film and the metal layer(s). For instance, the light absorbing material layer could be used first as a resist to pattern the metal layer before incorporation with the colourshifting film. This results in automatic alignment between the gaps in the light absorbing material layer and the corresponding gaps which will be required in the metal layer to ensure transparency/translucency. Preferably, the or each electrically conductive layer is arranged so as to form a continuous conductive path from one edge of the security element to another. This provides an additional feature for authenticity checking.

The colourshifting film may itself be self-supporting, in which case no additional substrate layer need be provided. However, in preferred examples, the security element further comprises a transparent polymer support substrate, preferably comprising polypropylene (PP), bi-axially oriented PP (BOPP), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

The security element may preferably further comprise an adhesive layer on one or both outer surfaces of the security element. For example if the security element is formed as a security thread or other insert for incorporation at least partially inside a document substrate, it is typically preferred to provide an adhesive layer on both sides of the element to ensure good retention. Alternatively if the security element takes the form of a strip or patch to be affixed to an outer surface of a document substrate, an adhesive layer may be provided on one side only; preferably on the same side of the colourshifting film as the light absorbing material layer. The adhesive(s) could be contact pressure adhesives or heat activated adhesives, for example. The element could be formed as a transfer foil for release from a carrier substrate onto the security document.

In preferred implementations, the security element is a security thread, strip, foil or patch. Elongate security elements are particularly preferred.

The present invention also provides a security document comprising a security element as described above, wherein the security document is preferably a banknote, a polymer banknote, a passport, a licence, an identification document, a visa, a cheque or a certificate.

The benefits of including one or more optical diffusion layers in the security element structure have been discussed above. However as an alternative or in addition, similar benefits can be achieved by providing one or more optically diffusive layers as part of the structure of the security document (hereinafter "optically diffusive document layers"). Like the optical diffusion layers described above, an optically diffusive document layer acts to convert incident directional light into light which propagates in multiple directions. As such the layer will redirect incoming collimated light and so change the apparent behaviour of the security element in much the same way as described above, which again will depend on the position of the optically diffusive document layer(s) relative to the colourshifting film (and hence to the security element as a whole). Therefore, in preferred embodiments, the security document comprises at least one optically diffusive document layer, the security element being incorporated into the security document such that the or each optically diffusive document layer overlaps the colourshifting film and/or the light absorbing material layer at least in partial areas of the security element. In some preferred embodiments, at least one of the optically diffusive document layers is located on the light absorbing material layer side of the security element, the optically diffusive document layer being provided at least in regions corresponding to partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security document in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the viewing angle but is substantially independent of the illumination angle. The mechanism behind this effect is the same as described above for optical diffusion layers forming part of the security element itself on the same side of the colourshifting film as the light absorbing material layer. It is advantageous for diffusion layers in this position (whether forming part of the security element or the security document) to extend across substantially the whole area of the security element so as to make the transmitted colour invariant to illumination angle and to increase the uniformity of illumination across the whole element.

In a particularly preferred implementation, the security element itself includes an optical diffusion layer located on the same side of the colourshifting film as the light absorbing material layer and the security document in which the element is incorporated also includes an optically diffusive document layer on the same side of the security element, so that light passes through (and is redirected by) both the optically diffusive document layer and the optical diffusion layer before being transmitted through the colourshifting film to an observer. In this case the optically diffusive document layer may optically scatter the light, and the optical diffusion layer may be photoluminescent in order to further redistribute the incoming light, improving its uniformity, and to achieve a brighter effect.

In further preferred embodiments, at least one of the optically diffusive document layers is located on the colourshifting film side of the security element, the optically diffusive document layer being provided at least in regions corresponding to partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security document in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the illumination angle but is substantially independent of the viewing angle. As described in relation to optical diffusion layers forming part of the security element itself, a diffusive layer in this position will affect both the reflective appearance and the transmissive appearance of the security element rendering the colours invariant to changes in viewing angle. It is particularly preferred that such layers in this position do not cover the whole of the security element. However by providing such layers over partial areas of the element, complex effects can be achieved since the behaviour of the element in areas covered by the diffusive layer will be different to that in other regions. Therefore, preferably, at least one of the optically diffusive document layers overlaps only some of the gaps in the light absorbing material layer and not others. An optically diffusive document layer can take various different forms. For instance, in some preferred embodiments, the security document comprises a document substrate which forms at least one of the optically diffusive document layers, the document substrate preferably comprising a fibrous substrate such as paper. It should be noted that the security element could be embedded within such a document substrate in which case only a partial thickness of the document substrate may act as the optically diffusive layer. For instance, in preferred embodiments, the security element is at least partially embedded in the document substrate, preferably in a windowed fashion. If the security element is configured such that it emerges on the surface of the substrate on both sides of the document in an alternating fashion, the substrate will act as a diffusion layer with different effects in each alternating region due to its changing position from one side of the security element to the other. If on the other hand the security element is configured such that it emerges only on one surface of the substrate, at spaced intervals, those regions in which the element is covered on both sides by the document substrate (the "bridge" regions) will appear optically static (i.e. invariant to changes in viewing and illumination angle, although the contrasting colours in reflected versus transmitted light will still be observed) whilst the intervening regions will exhibit a colourshifting effect upon changing the viewing angle in reflected and transmitted light (although in transmitted light, changes in the illumination angle will not affect the colour).

Similar effects can be achieved in non-fibrous documents, such as polymer banknotes or hybrid paper/polymer banknotes. For example, in other preferred embodiments, the security document comprises a transparent polymer document substrate and at least one opacifying layer thereon which forms the optically diffusive document layer. The security element could be applied to the transparent polymer substrate and then covered on one or both sides by the at least one opacifying layer at least in partial areas, e.g. to imitate a windowed appearance. The opacifying layers could be fibrous (e.g. paper) and laminated to the substrate but preferably the at least one opacifying layer comprises a non- fibrous polymeric material comprising a dispersion of light-scattering particles. For instance, the opacifying layer(s) could be a light coloured ink such as white ink. Such non-fibrous layers can be applied by any suitable printing or coating technique making it possible to achieve particularly complex designs.

In still further preferred examples, the security element may be incorporated into or onto the security document such that at least a portion of the security element extends across a transparent window portion of the security document. In this way the security element can be viewed from both sides of the document in the window region and, if no diffusion layer is included in the security element structure itself, the colourshifting effects of the element will be unimpeded in this region. Transparent window regions such as this can be formed in both fibrous (e.g. paper) and non-fibrous security documents. For instance, where the security document substrate is non-transparent, e.g. paper, a transparent window can be formed either by aperturing the substrate and affixing the security element across the aperture on one surface of the substrate, or by incorporating the element during papermaking and configuring the windowing process such that the paper fibres are absent on both sides of the element at the same location. Examples of security elements and security documents in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 schematically shows an exemplary arrangement for measuring the reflectivity and/or transmissivity of a colourshifting film at normal incidence;

Figure 2 illustrates a first embodiment of a security element in accordance with the present invention, (a) in plan view and (b) in cross-section along the line X-

X';

Figure 3 schematically represents a portion of an exemplary colourshifting film suitable for use in embodiments of the present invention, and the behaviour of light incident thereon;

Figure 4 is a schematic plot representing the reflectivity R at normal incidence of an exemplary colourshifting film suitable for use in embodiments of the invention, across a range of visible wavelengths;

Figure 5 shows a second embodiment of a security element in accordance with the present invention in cross-section;

Figures 6(a), (b) and (c) show three further embodiments of security elements in accordance with the present invention in cross-section;

Figures 7(a), (b) and (c) show three more embodiments of security elements in accordance with the present invention in cross-section;

Figure 8 depicts an embodiment of a security document in accordance with the present invention, in plan view;

Figures 9(a) and (b) illustrate two alternative constructions of the security document of Figure 8, in cross section along the line Q-Q';

Figures 10(a) and (b) illustrate two alternative constructions of the security document of Figure 8, in cross section along the line R-R';

Figure 1 1 illustrates a further alternative construction of the security document of

Figure 8, in cross section along the line R-R';

Figure 12 shows another embodiment of a security element in accordance with the present invention in cross-section; and Figures 13(a) and (b) show two exemplary security elements in accordance with the present invention plan view, Figure 13(c) showing an enlarged detail of Figure 13(b). As already discussed above, Figure 1 shows apparatus suitable for measuring the reflectivity, reflected bandwidth and other characteristics of a colourshifting film 2 at normal incidence. An arrangement such as this can be used to measure the characteristics of any of the security elements described below using the same techniques as already discussed.

Figure 2 shows a first embodiment of a security element 1 in (a) plan view and (b) cross section. Referring first to the cross section, the security element 1 comprises a colourshifting film 2 and a light absorbing material layer 5 arranged on the side of the colourshifting film 2 opposite from that on which the element is to be viewed by an observer O in use. In this case the light absorbing material layer 5 is directly in contact with the surface of colourshifting film 2 but as described below in relation to other embodiments, this is preferred but not essential. The colourshifting film 2 comprises a plurality of stacked polymer layers 3a, 3b of which an arbitrary number are illustrated. In practice, the number of layers may be much greater than that depicted, e.g. in the hundreds. The plurality of polymer layers comprises respective layers of at least two different polymer materials with different refractive indices from one another, arranged periodically in the direction of the stack thickness (z-axis). For instance, where only two different polymer layers are provided (as is preferred), they may be arranged to alternate with one another throughout the thickness of the stack. If more than two different polymer layers are provided, the repeating pattern may be more complex. The colourshifting film 2 is substantially transparent (i.e. optically clear) to at least some wavelengths of visible light and preferably has little or no body colour, and hence low or negligible absorption of visible wavelengths. However as discussed further below the film 2 will exhibit a colour in reflected white light (from light source 100) which depends on the viewing angle Θ and also a (different) colour in transmitted white light (from light source 101 ), due to interference effects at the interfaces between the different polymer layers. The mechanism on which the colourshifting film operates and more details of its characteristics will be discussed with reference to Figures 3 and 4.

The light absorbing material layer 5 typically comprises a dark coloured material such as an ink or a resist, e.g. black, dark blue or dark green. The light absorbing material layer 5 is provided only in partial areas 7 of the security element, leaving gaps 6 in which no light absorbing material is present, so as to define a pattern. In this case, as seen in the plan view of Figure 2(a), the pattern comprises negative indicia in the form of the letters "D" and "L" corresponding to the gaps 6 and surrounded by areas 7 which provide a background to the letters. In other cases, the pattern could comprise positive indicia where the letters (or other symbols) correspond to the areas 7 where the light absorbing material is present and the gaps 6 provide the background. In general the pattern could take any form including alphanumerical characters, currency identifiers, symbols, logos, line patterns, guilloches etc. In this embodiment, the pattern is formed by applying the light absorbing material in a spatially selective manner to the colourshifting film 2, e.g. by any suitable printing method such as gravure. Alternative methods include lamination as discussed below.

Referring now to Figure 3, the characteristics of the colourshifting film 2 will be described in more detail. In Figure 3, the colourshifting film 2 is depicted schematically as comprising four polymer layers but in practical examples there will typically be hundreds of such layers as mentioned above. Here, the film 2 comprises a first polymer material 3a and a second polymer material 3b arranged in alternating layers. Thus, any two adjacent layers form a pair P which can be considered the repeating unit cell of the stack in the z direction. Figure 3 depicts two such pairs Pi and P₂, each of which includes one layer of the first polymer material 3a and one layer of the second polymer material 3b. Here, polymer 3a has a higher refractive index N_h than that of polymer 3b, denoted N|. In the example depicted, all of the layers 3a, 3b have substantially the same thickness as one another in the z direction. However this will not be the case in all embodiments and the respective thicknesses L_h, of the different polymer layers can be varied in order to adjust the characteristics of the film 2. Indeed it should be noted that in practice, layers 3a and 3b will typically not be of the same actual thickness as one another due to the different refractive indices of the two materials. The overall thickness of the film, D, will depend on the number of layers and the individual thicknesses of each. Whilst the example depicted contains an integer number of pairs (two), this is not essential and the stack could include an incomplete "pair" on either surface of the stack, e.g. an additional layer of polymer 3a underlying pair P₂.

Figure 3 shows an incident light beam B| striking the first surface of the film 2 on the same side as observer d, at an incident angle Θ. A portion of the incident light will be reflected and a portion transmitted into the first layer 3a of the film 2, its direction altered as a result of the refractive index difference between the layer 3a and the medium a adjacent the first surface of the film. On reaching the interface between layers 3a and 3b, again a portion of the light beam will be reflected and a portion transmitted into the next layer 3b, redirected by the change in refractive index. Similar reflections will occur at each of the interfaces between layers of different refractive index in the film 2. The reflected light beams from each interface interfere with one another. At any one angle Θ, certain wavelengths will suffer destructive interference and thus be substantially removed from the overall reflected light B_R whilst others will experience constructive interference. The observer Oi will therefore see reflected light B_R of a colour which varies in accordance with the viewing angle Θ. Whilst only one transmitted light beam is shown, for clarity, the transmitted light B_T will also experience interference resulting from the multiple interfaces and hence its colour (seen by observer O₂) will also vary depending on the viewing angle. At any one viewing angle, the colour exhibited in reflected white light will be different from that exhibited in transmitted white light. As described below, the parameters of the film 2 are configured to achieve a strong contrast between the reflected and transmitted colours. In particular, the film 2 is designed to exhibit complementary colours in reflected and transmitted white light respectively. For any one viewing angle, the percentage of each wavelength that is reflected by the film 2 as a whole (i.e. its reflectivity, R) as well as the spectral position (i.e. colour) of the reflected waveband and its size depends primarily on the polymer materials 3a, 3b selected (particularly their refractive indices N_h, N|), the thicknesses of the layers L_h, U the number of pairs (unit cells), p, in the film 2, and to a lesser extent on the refractive indices N_a, N_b, of the materials a, b adjoining each surface of the film 2. By adjusting these parameters, the characteristics of the film 2 can be controlled. Examples of multilayer polymer colourshifting films and more details of their principles of construction which determine the characteristics of the films can be found in US-A-2005/0161840, "Giant Birefringent Optics In Multilayer Polymer Mirrors" by M.F. Weber et al, Science, Vol. 287, 31 March 2000, p. 2451 , and "Filter Design using Multi-Bragg Reflectors" by F.D. Ismail et al, World Journal of Modelling and Simulation, Vol. 8 (2012) No. 3, pp. 205-210.

In embodiments of the present invention, the colourshifting film 2 is configured to exhibit a high reflectivity and a narrow reflected bandwidth, each as characterised further below. This combination of features has been found by the present inventors to achieve a particularly strong contrast between the reflective and transmissive colours, as compared for example with conventional colourshifting films, e.g. formed of a ceramic dielectric stack or liquid crystal material.

Thus, the colourshifting film 2 is configured such that, at least at a viewing position lying on the normal to the security element (i.e. under normal incidence), the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film 2 is at least 60%, preferably at least 70%, and more preferably at least 80%. The peak wavelength is the wavelength that is reflected by the film 2 with the greatest intensity relative to other wavelengths at any particular viewing angle, and may be referred to as the Bragg wavelength. Figure 4 is a schematic plot representing the reflectivity R at normal incidence (i.e. viewing angle Θ = 0 degrees) across the visible wavelength range for an exemplary colourshifting film 2 suitable for use in embodiments of the present invention. It will be seen that the function R_e=o(A) has a maximum M which is approximately centred on the peak wavelength λ_Β,θ=ο, which in this case lies at about 635 nm, i.e. green. In this case the reflectivity at the peak wavelength, R_B,e=o, is approximately 95%. By arranging the peak reflectivity to be high (i.e. at least 60%) in this way, the reflected wavelengths (corresponding to those falling inside maximum M) are substantially wholly removed from the light transmitted through the film 2, thereby increasing the contrast between its reflective and transmitted colours. The spectral width of the maximum M also influences the contrast between the two colours exhibited by the film 2 in reflected and transmitted light respectively at any one viewing angle and in embodiments of the present invention the film 2 is configured such that, at least at a viewing position lying on the normal to the security element (i.e. normal incidence), the maximum M of the function R_e(A) has a bandwidth Δλ_θ=ο of 150 nm or less, preferably 100 nm or less, more preferably between 30 and 150 nm, and most preferably between 50 and 100 nm. The bandwidth is measured as the full width of the maximum M, that is from the wavelength at which the reflectivity R starts to increase from the baseline reflectivity value (which here lies at around 15%) to the wavelength at which the reflectivity R returns to the baseline reflectivity value. In this example the bandwidth is approximately 90 nm. Narrow reflected bandwidths of this sort have been found to improve the contrast between the reflected and transmitted colours since the purity of both colours are increased. For instance in the present example, substantially all the "green" wavelengths are strongly reflected by the film 2 while all the other wavelengths, i.e. "red" wavelengths and "blue" wavelengths are transmitted largely unhindered. This results in a substantially pure green reflected colour and a pure magenta transmitted colour, and hence a strong visual contrast between the two. In other examples, the bandwidth may be designed to be even narrower, e.g. less than 30nm, in which case the reflected colour may appear substantially monochromatic (e.g. red) whilst the transmitted colour may appear substantially achromatic (e.g. white or off white) since it still contains the majority of visible wavelengths. A strong contrast between the two appearances will therefore still be displayed, although this option is less preferred due to the achromatic appearance in transmission and possibly low reflected intensity.

As a result of the strong contrast between the reflected and transmitted colours exhibited by the colourshifting film 2, the security element 1 displays an easily recognisable security effect. Referring back to Figure 2, when the security element 1 is viewed by observer O in reflected white light (from light source 100), both the gaps 6 defining the letters "D", "L" and the surrounding areas 7 will exhibit a colour which changes depending on the viewing angle, e.g. appearing green when viewed on the normal if the film of Figure 4 is employed. Depending on the degree of reflection, the pattern in light absorbing layer 5 defining the letters "D", "L" may be visible at all viewing angles, or not. This is because the light absorbing material suppresses stray light rays and other sources of light and so enhances the brightness of the reflective colour of the film 2 in the regions 7 where the light absorbing material is present, relative to that in the gaps 6. However, if the reflectivity of the film 2 is sufficiently high, at certain viewing angles the intensity of the reflected light may be so high that the presence or absence of the light absorbing material makes a negligible difference to the apparent brightness, such that the reflected light effectively conceals the presence of the pattern. Hence the letters "D", "L" (or other pattern) may become hidden at certain reflective viewing positions. This provides an additional security feature but is optional and may not be provided in all embodiments of the invention. The colourshifting film 2 which is the subject of Figure 4 was formed of two alternating polymer materials 3a, 3b formed of PET and PETg respectively, having respective refractive indices of about 1.63 to 1.8 (= N_h) and 1.57 (= N|). These materials are anisotropic and hence these values relate to the refractive indices of the respective materials in the plane of the layers 3a, 3b (i.e. in the x-y plane). Each layer was approximately 90 to 100 nm thick (= L_h = U) and between 130 and 150 layers (hence p = 65 to 75) were provided, giving a total film thickness D of about 14 to 15 microns. More generally, any transparent polymer materials with different refractive indices could be used to form the layers 3a, 3b ... and suitable examples include all of those mentioned in US-A- 2005/0161840. The thickness of the layers and the number of layers can be varied to adjust the reflectivity and bandwidth to provide the film 2 with the required characteristics as outlined above. The reflectivity and bandwidth of such a film can be tested using the apparatus described above with reference to Figure 1 . Detailed guidance as to how adjusting each of the parameters will affect the characteristics of the resulting film can be found in the above- mentioned reference documents. In general terms however the inventors have found the following mathematical model to provide a reasonable approximation to measured results, and to correctly indicate the trends in the measured characteristics that will result from varying the film's parameters.

The peak (or Bragg) wavelength λ_Β,θ is approximated by:

A_Bi9 = 2 [N_h. L_h + N_l. L_l]cose (Eq. 1 ) where: N_h and N| are the refractive indices of the first and second polymer materials 3a, 3b respectively, N_h being greater than N,; L_h and are the thicknesses of the first and second polymer material layers respectively in the direction normal to the security element (z-axis); and Θ is the viewing angle relative to the normal for the first order reflection of the peak wavelength.

The percentage R_B,e=o (i.e. the peak wavelength reflectivity at normal incidence) is approximated by:

1- ^2P(

(Eq. 2)

1 + ^'W

^2P(

where: p is the number of pairs of first and second polymer material layers in the colourshifting film; and N_a and N_b are the refractive indices of the materials a and b adjoining the two outermost layers of the colourshifting film, respectively.

In practice, the materials a and b adjoining the outermost layers of the colourshifting film will typically each be a polymer-based material such as an adhesive, or an ink - for instance forming the light absorbing material layer 5. Therefore, the refractive indices of materials a and b will typically be very similar to one another (N_a ^¾ N_b) and moreover will also be very similar to N_h (N_a ^¾ N_b ^¾ N_h). As a result the fraction (N_b ²/ N_aN_b) can be approximated to 1 , and the normal peak reflectivity R_B,e=o approximates to:

According to the model, at a viewing position lying on the normal to the security element (Θ = 0 degrees), the bandwidth Δλ_θ=ο is approximated by:

By changing the construction of the film, different colours (in reflection and in transmission) can be exhibited at certain viewing angles and characteristics such as the reflectivity can be adjusted as desired, within the above boundaries. Hence, in preferred examples, the difference between the refractive indices of the first and second polymer materials is less than or equal to 0.4, preferably less than or equal to 0.25, more preferably less than or equal to 0.2. Typically, the difference between the refractive indices of the first and second polymer materials is at least 0.02. In preferred embodiments, the refractive indices of the first and second polymer materials are each between 1.45 and 1.95. The plurality of polymer layers may preferably have individual thicknesses in the range 50 to 150nm and more preferably 60 to 120nm. The total thickness of the colourshifting film will depend on the individual layer thickness(es) and the number of layers, but preferably the plurality of polymer layers has a total thickness in the range 5 to 50 microns, preferably 10 to 20 microns. Film thicknesses of this sort are well suited for use in security articles and documents. In preferred implementations, the colourshifting film comprises between 100 and 400 polymer layers, preferably between 120 and 170 polymer layers. As will be apparent from the model above, the greater the number of layers, the greater the reflectivity R_B,e=o since the value of p (the number of pairs of high/low refractive index layers) increases. As already mentioned the colourshifting film 2 should preferably exhibit very low absorption of visible wavelengths. This can be characterised by the colourshifting film being configured such that, in preferred embodiments, at least at a viewing position lying on the normal to the security element, the sum of the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film and the percentage T_B,e=o of incident light at the peak wavelength that is transmitted by the colourshifting film is at least 80%, more preferably at least 90%, still preferably at least 95%, most preferably substantially 100%. The difference between the sum of the reflected and transmitted light intensity as compared with the incident light intensity (which is preferably less than 10% of the incident intensity) represents not only absorption which might result from the film having a slight body colour (although it preferably has no body colour) but also any internal "trapping" of light which might result from internal reflections. The strong colour contrast can be still further improved by configuring the reflectivity function R_e(A) to have as low a baseline as possible - i.e. to have a high transmittance of visible wavelengths outside the reflected waveband. This can be characterised by the colourshifting film being configured such that, in preferred embodiments, at least at a viewing position lying on the normal to the security element, the percentage T_N,e=o of incident light at wavelengths (λ_Ν,β=ο) at least 50nm away from the peak wavelength (λ_Β,β=ο) that is transmitted by the colourshifting film is at least 75%, preferably at least 80%, more preferably at least 85%. This condition should hold at least across the visible spectrum. Since non-visible wavelengths will not affect the visible colours, the behaviour of the film outside the visible spectrum is typically not of importance. However, typically the same behaviour will continue at least into the UV and/or IR spectral ranges. As mentioned above, in the example of Figure 4, the baseline reflectivity is around 5 to 15% and hence, given that the film 2 absorbs substantially no visible light, the transmittance at wavelengths outside the maximum M is around 85 to 95%.

Whilst in the examples shown so far the periodicity of the polymer layer stack is uniform throughout the whole thickness of the film 2, this is not essential. In some cases it may be desirable to form the stack of two or more portions each having a different arrangement of polymer layers, and potentially a different periodicity (albeit uniform within each portion). This approach can be used to further tune the reflection and transmission characteristics of the film 2. Figure 5 schematically depicts an example of a security element 1 , forming a second embodiment of the invention, including such a colourshifting film 2 comprising a first portion 2a and a second portion 2b. Here, the polymer layers making up the first portion 2a are thinner, and hence have a higher periodicity, than those in second portion 2b. All other features of the second embodiment are the same as in the first embodiment. Colourshifting films 2 comprising multiple portions of differing periodicity such as that of Figure 5 could be used in any of the other embodiments disclosed herein.

The security element 2 may be provided with various additional optional features in order to improve its performance and/or to increase its functionality and examples of these will now be discussed with reference to Figures 6 and 7. Figures 6(a), (b) and (c) depict three further embodiments of a security element 1 , each in cross section. Components of the security elements already described above with reference to the first and second embodiments are denoted using like reference numerals and will not be detailed again. In Figure 6(a), the security element 1 is shown to include three additional adhesive layers 8a, 8b and 8c. Outer adhesive layers 8a and 8b are provided on the two outer surfaces of the security element 1 for bonding the security element to a security document or other article in use. In practice, depending on how the security element 1 is to be deployed, only one or the other of adhesive layers 8a, 8b may be provided. For instance, if the security element 1 is formed as a stripe or patch for attachment to an outer surface of a non-transparent security document, only adhesive layer 8b may be provided. Alternatively if the security element 1 is to be affixed across a transparent window in a security document, either one of the adhesive layers 8a, 8b might be provided. Further alternatively if the security element 1 is to be formed as an insert, such as a security thread, which is at least partially embedded inside a security document, the provision of both adhesive layers 8a, 8b is preferred. It should be noted that outer adhesive layers 8a and/or 8b may be provided for the same purpose in the security elements described in any of the other embodiments herein.

The security element 1 of Figure 6(a) further includes an intermediate layer between the colourshifting film 2 and the light absorbing material layer 5 in the form of adhesive layer 8c. This may be provided to improve the retention of the light absorbing material when it is applied to the colourshifting film 2 (e.g. acting as a primer layer) or in order to join a pre-existing light absorbing material layer 5 to the colourshifting film 2, e.g. by lamination. In either case the adhesive 8c is preferably transparent (i.e. optically clear), so as not to detract from the light- absorbing nature of the regions where material 5 is present or obscure the gaps in between, although it may optionally carry a coloured tint.

The security element 1 of Figure 6(b) is similar to that of Figure 6(a), although outer adhesive layers 8a, 8b are not depicted, and includes a further intermediate layer between the adhesive layer 8c and the light absorbing material layer 5 in the form of magnetic layer 1 1. The magnetic layer 1 1 comprises a magnetic material such as magnetic ink, which is preferably provided only in partial areas so as to form a spatial pattern which may constitute a magnetic code. In the example depicted the magnetic material is provided only in two spaced blocks arranged at either edge of the element 1 , and is absent elsewhere. The blocks may be continuous along the length of the element, i.e. forming tracks along each edge, or could be discontinuous so as to form a series of magnetic "bits" spaced by gaps allowing a code to be read by a magnetic read head when the element is conveyed past it. The layer 1 1 could optionally be formed of two or more different magnetic materials having different magnetic properties, e.g. coercivity. For example, the blocks/track on the left hand side of the Figure could be of a first magnetic material and the blocks/track on the right hand side of the Figure could be of a second magnetic material in order to introduce an additional level of coding.

Unless the magnetic material is transparent, it will be arranged so as not to extend across the gaps 6 in the light absorbing material layer 5, so as not to conceal the pattern defined therein. The magnetic material 11 could for instance be arranged to match exactly the extent of the regions 7 in which the light absorbing material is present, and to be absent elsewhere. However in this case the resulting magnetic "code" would correspond to the visible pattern and so it is preferred that the magnetic material takes some other arrangement in order to conceal its code. The magnetic material 1 1 may preferably be of substantially the same optical appearance as the light absorbing material 5 (i.e. both are light absorbing) so that those area(s) carrying magnetic material 1 1 do not appear visually distinct from the remainder of region 7.

In an alternative embodiment, magnetic features such as those described above can instead be incorporated into the security element by forming the light absorbing material layer in its entirety of a material which is additionally magnetic, or to form selected parts of the light absorbing material layer of such a material, whilst other parts remain non-magnetic. Figure 6(c) shows a variant of the security element 1 just described, which further comprises a barrier layer 12 located between the magnetic layer 1 1 and the light absorbing material layer 5. The barrier layer 12 is optically transparent (i.e. clear, but may carry a coloured tint) and therefore can extend across the whole area of the element without impeding viewing of the pattern formed by gaps 6. The barrier layer 12 is formed of an electrically insulating material such as a water based acrylic ink and acts to prevent corrosion which could otherwise be caused by electric contact between magnetic and/or metallic layers. The barrier layer is optional and could be included in any embodiment where such anti-corrosion measures are desirable.

The above-described security elements provide a strong security effect by virtue of the high contrast between the colours displayed in the gaps 6 between reflective and transmissive viewpoints. In addition, both the reflected colour and the transmitted colour will change as the viewing angle Θ is varied, e.g. by tilting the security element. Indeed, the angle of illumination - which can be changed independently from the viewing - will also affect the colour exhibited. In further preferred embodiments, the dependency of the colours on the viewing angle and the illumination angle is exploited through the addition of at least one optical diffusion layer to the security element, to produce more complex visual effects. Examples of this will be described with reference to Figure 7.

An optical diffusion layer is a layer which converts incoming directional light into outgoing non-directional light. That is, the layer removes the light's directionality. This could be achieved for example by optical scattering - e.g. including a substance in the layer which redirects incoming light, such as a pigment - or by absorption and re-emission of the incoming light - e.g. via a photoluminescent substance in the layer. For instance the substance may be responsive to UV or other invisible radiation, but will emit in the visible spectrum. A combination of both scattering and luminescent mechanisms could be employed. Thus, the or each optical diffusion layer could comprise, for example, a non-fibrous material such as a polymeric binder containing a diffusing substance such as a pigment or a luminescent material, e.g. white or visibly coloured ink or resist, or a fluorescent ink or resist. In still further examples described below, similar effects can be achieved using a fibrous diffusing layer such as paper. In all cases the optical diffusion layer(s) should have an optical density no greater than 0.6 (preferably less than 0.3) in order to ensure that the transmissive colour of the film 2 can still be seen when the security element is viewed in transmitted light. If multiple overlapping optical diffusion layers are provided, their total optical density should be no more than the same value. Any visible colour possessed by the optical diffusion layer(s) - whether inherent or caused by excitation under irradiation of certain wavelengths - will contribute to the colours exhibited by the security element as a whole, at least in transmission, in the locations where the optical diffusion layer(s) extends across the gaps in the light absorbing material layer. However the contrasting colours exhibited by the colourshifting film in reflection and transmission will ensure that the colours exhibited by the element as a whole under those conditions will also contrast.

The effect of the optical diffusion layer on the behaviour of the security element depends on the position of the layer in the security element's structure. Figure 7(a) shows an embodiment of a security element 1 which comprises, in addition to the features already discussed in relation to Figure 2, a first optical diffusion layer 15a arranged on the same side of the colourshifting film as the light absorbing material layer 5, and overlapping gaps 6a and 6b in the layer 5. However, the optical diffusion layer 15a is only provided in partial areas of the security element such that in this example it does not extend over gap 6c. It should be noted that whilst the optical diffusion layer is shown as lying under the light absorbing material layer 5, as is preferred, it could be provided between the light absorbing material layer 5 and the colourshifting film 2 provided it does not significantly visually obscure the presence of gaps 6a, 6b.

The optical diffusion layer shown in Figure 7(a) will have no significant effect on the appearance of the security element when viewed in reflected light, which retains its colourshifting effect as described above. However, when viewed in transmitted light, the colour of gap regions 6a and 6b will now be independent of the illumination angle φ between light source 101 and the normal (z-axis). This is because collimated light from source 101 will be redirected by the optical diffusion layer 15a such that the light which enters the colourshifting film 2 has no specific directionality. The interference effects already described act on the incident light such that different wavelengths will emerge at different angles with the result that the observer O will still see a change in the transmitted colour as they change their viewing position Θ. However, at any one viewing position Θ the colour of the gaps 6a and 6b will be invariant to changes in the relative position of the light source 101. Meanwhile, the transmitted colour of gap 6c will appear to vary both upon changes in the viewing angle Θ and in the illumination angle cp, since the optical diffusion layer 15a is not present in this region. As such, viewing the security element in transmitted light at any one viewing position whilst changing the relative location of the light source will reveal the unexpected visual effect that some of the gaps 6 in the pattern will appear to change colour whereas others will remain static. For instance, in this example the gaps 6a and 6b correspond to the letter "D" shown in Figure 2a whilst the gap 6c corresponds to the letter "L". Hence in this scenario, the letter "D" will appear static whilst the letter "L" will appear to change colour.

Figure 7(b) shows an alternative embodiment in which no optical diffusion layer is provided underneath the colourshifting film 2. Instead, a second optical diffusion layer 15b is arranged on the surface of the colourshifting film 2 (the direct contact between these components illustrated is preferred but not essential). The second optical diffusion layer 15b extends over a partial area of the security element 1 which includes gaps 6b and 6c but not 6a. In this location, the optical diffusion layer 15b has the effect of redirecting light emerging from the colourshifting film 2 before it reaches observer O. This has the effect that the colours exhibited by the element, both in reflection and in transmission (which will still be different from one another) do not vary with viewing angle Θ, but will vary upon changing the illumination angle cp. Therefore when the security element of Figure 7(b) is examined in transmitted light against a light source 101 of constant position, upon changing the viewing angle Θ the colour of gaps 6b and 6c will appear static whilst that of gap 6a will change. Whilst in both of the above embodiments the respective optical diffusion layers 15a, 15b are provided only in partial areas of the element 1 , extending across only some of the gaps 6 and not others, in other embodiments it is desirable for at least one of the optical diffusion layers 15a or 15b to extend across the whole area of the element 1. This is particularly advantageous where the optical diffusion layer in question is located on the same side of the colourshifting film 2 as the light absorbing material layer 5, as in the case of layer 15a in Figure 7(a). Whilst (in the absence of any additional optical diffusion layer over parts of the element on the opposite side) this will cause all the gaps to exhibit the same behaviour as one another, this has the benefit of improving the uniformity with which all of the gaps 6 are illuminated in transmitted light thereby improving their visibility and assisting the viewer in perceiving the contrast between the transmitted and reflected colour of the gaps 6. Further, if the optical diffusion layer 15a is luminescent, it acts as a light source in close proximity to the colourshifting film thereby enhancing the brightness of the effect. It is less advantageous to have an optical diffusion layer 15b extend across the whole security element on the opposite side of the film 2 since this will inhibit the reflective colourshifting effect upon changes in viewing angle Θ, but this may be desirable in some embodiments.

Optical diffusion layers can also be provided on both sides of the colourshifting film 2, either overlapping one another or not (in which latter case the two effects described above will be observed adjacent one another along the element). An example of such a security element is shown in Figure 7(c) in which optical diffusion layers 15a and 15b partially overlap one another such that only gap 6b is located in the overlap region. Where the two diffusion layers overlap, the displayed colour will be static and invariant to changes in both the viewing angle Θ and the illumination angle φ (both in reflection and transmission), although the reflected and transmitted colours will still be different from one another. Therefore, when the element of Figure 7(c) is viewed in transmitted light, the colour of gap 6a will appear to change with viewing angle Θ but not illumination angle, the reverse will be true for gap 6c, and gap 6b will appear to stay a constant colour throughout. It will be appreciated that in order to preserve a colourshifting effect it is generally not desirable to provide optical diffusion layers on both sides of film 2 which overlap across the whole area of the element 1.

By arranging optical diffusion layer(s) on one or both sides of the colourshifting film, complex effects can therefore be achieved in which some parts of the pattern will appear optically variable whilst others do not under any one particular lighting condition. The optical diffusion layer(s) can be applied according to any desirable arrangement, e.g. by applying the optical diffusion material by printing, e.g. gravure printing, or another spatially selective process.

Security elements of the sorts described above can be applied to any article which requires its authenticity to be established. However, the security elements are particularly well suited to use in security documents such as banknotes, passports, identification cards, cheques, certificates etc. The security element can take various different forms depending on how it is to be integrated with the security document or other article. For example, the security element 1 may be a security thread, stripe, patch or foil. The security element could be configured for embedding inside a security document substrate (e.g. during papermaking), or for adhesion to a surface thereof, e.g. as a label or by hot or cold stamping from a transfer foil.

Figure 8 shows an embodiment of a security document 20 in accordance with the invention, which here takes the form of a banknote. The document 20 comprises a substrate 21 which in this example carries two security elements 1 , 1 ', which each may be formed as described in relation to any of the embodiments above. The security element 1 is a security thread type element, whilst the security element 1 ' is a patch type element. The document 20 can be constructed, and the security elements 1 , 1 ' applied, in various different ways of which examples will be described with reference to the cross sections in Figures 9, 10 and 11.

In Figures 9 and 10, the banknote 20 is of a conventional fibrous construction, e.g. having a substrate 21 formed of paper. Figures 9(a) and (b) show cross sections along the line Q-Q' of Figure 8 in two alternative constructions of the banknote 20. In Figure 9(a) the security element 1 is a security thread embedded into the paper substrate 21 during papermaking in a windowed fashion such that portions of the element 1 are exposed at windows 25 on one surface of the banknote, spaced by intervening paper bridge regions 26, whereas the whole of the element 1 is covered by paper on the opposite side. For clarity, the portions of the paper substrate 21 lying over the element 1 are labelled as 21 a whereas the remainder of the paper substrate 21 lying under the element 1 are labelled as 21 b but it will be appreciated that these are typically two parts of the same paper ply rather than two separate plies. The security element 1 has a construction similar to that shown in Figure 7(a), comprising a colourshifting film 2, patterned light absorbing material layer 5 and an optical diffusion layer 15 which here is positioned in the same side of the colourshifting film 2 as the light absorbing material layer 5 and extends across substantially the whole area of the element 1. Adhesive layers (not shown) will typically be provided on both outer surfaces of the element 1 to help secure it within the paper substrate 21.

Since the paper substrate 21 is optically scattering, it acts as a further optically diffusive layer and affects the colour effects exhibited by the element 1 in a similar manner to that described previously in relation to Figure 7. Thus, when the document is viewed by observer O in reflected light, the parts of element 1 exposed in windows 25 will exhibit the aforementioned colourshifting effect upon changing the viewing angle. As described at the outset, if the reflectivity is sufficiently high the effect may be such as to hide and reveal the presence of the pattern defined by gaps 6 in the light absorbing material layer 5 (here in the form of star shaped symbols as seen in Figure 8) upon tilting the element to certain angles. In the areas 26 where the element 1 is covered by the paper bridges, depending on the optical density of the paper, the element 1 may be entirely concealed but if not, its colour will appear static due to the paper bridge removing the directionality of both the incident light and the reflected light. In transmitted light, the underlying paper 21 b and the optical diffusion layer 15 both act to reorientate incident light before it passes through the colourshifting film 2. Preferably the layer 15 is a photoluminescent material so as to increase the brightness of the transmitted colour. When viewed by observer O against a backlight, the gaps 6 in the window regions 25 will therefore appear to change colour upon changing the viewing angle Θ (although not the illumination angle cp), whereas the gaps 6 in the paper bridge regions 26 will appear static due to the overlapping diffusion layers 21 a, 15 and 21 b. In each region, the colour of the gaps 6 will nonetheless appear different in reflected light from its colour in transmitted light. The paper 21 is preferably sufficiently translucent so as to permit viewing of the transmitted colour through the paper at least in the regions 25 and preferably also in regions 26. Figure 9(b) shows an alternative construction in which the security element 1 is once again introduced as a windowed thread into a paper substrate 21 during papermaking. However, in this case the security element 1 emerges on both surfaces of the document substrate 21 at alternate positions along its length so as to appear in windows 25 seen from the first side of the note (observer Oi) and in windows 26' seen from the second side of the note (observer 0₂). In the example shown the windowing is such that there is no overlap between the paper portions 21a and 21 b at any position along the length of the element but in practice there may be such overlapping regions between any two of the adjacent windows 25, 26'. The security element 1 is of a similar construction to that shown in Figure 2 above but with the addition of an adhesive layer 8c on one side. In practice an adhesive layer 8a may also be provided on the other side.

The paper portions 21 a and 21 b act as optically diffusing document layers in the same manner as previously described. Thus, when the device is viewed by observer d in reflection, the parts of element 1 exposed in windows 25 will exhibit the aforementioned colourshifting effect upon changing the viewing angle, and the hide and reveal effect may be seen if the reflectivity is sufficiently high. Those parts under paper bridges in regions 26' will either be hidden or appear optically static. In transmitted light, meanwhile, observer d will see different behaviours of the gaps 6 in regions 25 and 26' respectively. Those gaps 6 in regions 25 will appear to change colour on changing the viewing angle Θ but not the illumination angle cp, and vice versa in regions 26'. In all regions, the colour of the gaps in reflection will be different from that in transmission. When the element is viewed from the opposite side (observer 0₂), the transmissive appearance of the element as a whole will be substantially the same except that the behaviours of regions 25 and 26' will be reversed. In reflected light the colourshifting appearance of the gaps in regions 26' will be visible but their surroundings will appear dark and optically invariable due to the light absorbing material layer 5 masking the colourshifting film 2 in these regions.

Figures 10(a) and (b) show two alternative arrangements of the second security element 1 ', here a patch, on the paper document substrate 21. In both cases, the security element 1 ' is arranged partially across a window region 27 which here is formed as an aperture through the substrate 21 , e.g. by punching, laser cutting, abrasion or during the papermaking process. In both the embodiments shown in Figures 10(a) and (b), a security element 1 ' is affixed over the aperture 27 by an adhesive layer: 8c in Figure 10(a) and 8a in Figure 10(b). The security element could be constructed as a label, which is removed from a supportive backing and then placed on the substrate 21 , or as a transfer foil, where the element is carried on a carrier foil via a release layer such as wax and then transferred off the carrier foil directly onto the substrate 21 , e.g. by hot stamping. In Figure 10(a) the security element 1 ' is constructed in the same way as that in Figure 9(a). When viewed by observer d in reflection, the security element will display the aforementioned colourshifting effect upon changing the viewing angle across its full area. If the reflectivity is sufficiently high, the effect may be such as to hide and reveal the presence of the pattern defined by gaps 6 in the light absorbing material layer 5 (here in the form of the repeated letters "DLR" as seen in Figure 8) upon tilting the element to certain angles. In transmitted light, in the region of window 27 the pattern of gaps 6 will be seen to exhibit a (different) transmitted colour which will change in dependence on the viewing angle but not the illumination angle, due to optical diffusion layer 15. Outside the window 27 the transmitted appearance will be the same unless paper 21 is of so high an optical density that it blocks the transmission of light therethrough.

In Figure 10(b) substantially the same security element V is adhered to the opposite surface of the paper 21 with the result that where the element V extends over the paper outside window 27, the transmitted colour of the gaps 6 will be optically invariable since an optical diffusion layer now exists on both sides of the colourshifting film 2. Further, in this example the complexity of the design has been increased further by forming the optical diffusion layer 15 of two optically diffusing materials 15', 15" in different respective regions of the element 1 ', laterally offset from one another. The two materials 15', 15" will have different optical characteristics, e.g. colours or scattering / luminescent substances so that the gaps 6 in one region will have a different appearance from those in the other region. Any number of such materials could be used to form the or each optical diffusion layer, and could be provided in any desired lateral arrangement so as to form a pattern such as text, numbers, a logo etc.

In other implementations, the security document 20 could be formed of a non- fibrous, transparent substrate 22 such as a polymer, e.g. BOPP, PET, PC or similar. Figures 1 1 (a) and (b) show exemplary cross sections along lines Q-Q' and R-R' of Figure 8 respectively for such a polymer security document. In Figure 1 1 (a) the document 20 is shown to comprise a transparent substrate 22 with opacifying layers 23a, 23b applied to each side in partial areas. The opacifying layers 23a, 23b are typically non-fibrous coatings such as white or other light coloured ink. One surface of substrate 22 is provided with security element 1 which here has a construction similar to shown in Figure 6a, i.e. comprising a colourshifting film 2 and a light absorbing material layer 5 which is patterned with gaps 6, with an adhesive layer 8c between the two. However, in this case outer adhesive layers 8a, 8b are not required since the security element 1 is constructed directly on the document substrate 22. Thus, the light absorbing material layer 5 is first applied to the surface of substrate 22, e.g. by printing in accordance with the desired pattern of gaps 6. Then, the colourshifting film 2 is affixed over the light absorbing material layer 5 via adhesive layer 8c although as shown in Figure 1 1 (a) the two components need not align precisely and registration is not required. Opacifying layer 23a (which may in practice comprise multiple layers) is then applied over parts of the security element 1 leaving gaps so as to form windows 25 spaced by coated regions 26. On the other side of substrate 22, opacifying layer 23b is provided over the whole area of the element 1.

The appearance and behaviour of the construction shown in Figure 1 1 (a) is very similar to that described above in relation to Figure 9(a) since the opacifying layers 23a, 23b will be optically scattering and therefore constitute optically diffusive document layers.

Whilst in the Figure 1 1 (a) embodiment, the security element 1 is constructed in situ on the document substrate, in other cases it may be formed separately, e.g. as a patch or stripe and then affixed to a polymer substrate 22. Figure 1 1 (b) shows an example of this construction in the context of second security element 1 '. Here, the security element V has the same construction as that discussed above in relation to Figure 10(a) and is affixed to the polymer substrate 22 via adhesive layer 8c preferably by cold stamping. Opacifying layers 23a, 23b are then applied to each side of the substrate so as to define transparent window region 27. The appearance of this construction will be very similar to that described above in relation to Figure 10(a) except that in the regions outside window 27, the colourshift effect will be inhibited by the overlapping of opacifying layer 23a, 23b on each side of film 2 and therefore the colours here will not change with viewing angle Θ or illumination angle cp.

As already mentioned, the security element may have additional layers in order to improve its performance and/or functionality. It will be appreciated that any of the intermediate layers such as adhesive layer 8b, magnetic layer 1 1 or barrier layer 12 described with respect to Figure 6 could be provided in combination with any of the optical diffusing layers described in relation to Figure 7 and/or with optically diffusive document layers as illustrated in Figures 9, 10 and 11. For instance, Figure 12 shows a preferred structure of a security element in another embodiment of the invention. Features which have already been described are denoted using like reference numerals and will not be detailed again. In examples so far, the colourshifting film 2 is generally self-supporting such that no additional support substrate is required. However it may be desirable to increase the robustness of the element 1 by provided a support substrate which is shown as item 14 in Figure 12. This will be a transparent (i.e. optically clear, but may have a coloured tint) substrate, typically of a polymer material such as PET, e.g. 9 microns thick. Where the security element is to be provided on a polymer document substrate such as item 22 in Figure 1 1 however, this may fulfil the same function.

The security element may preferably also be provided with an electrically conductive layer such as metal layer 13 shown in Figure 12. This not only provides a further security feature, since the element can be checked for the presence of electrically conductive material, but can also be used to improve the appearance of the element from one or both sides. For instance, in the Figure 12 example, the extent of metal layer 13 matches that of the light absorbing material layer 5. This can be achieved by providing substrate 14 as a metallised polymer layer, i.e. carrying metal 13 over its whole area. A light absorbing material 5 in the form of a resist can then be applied to the metal layer 13 in accordance with the desired pattern of gaps 6 and regions 7. A suitable etchant is then applied to dissolve the metal exposed in the gaps 6 whilst the material 5 in the regions 7 protect and retain the metal 13 here. The result will be substantially exact alignment between the light absorbing material layer 5 and the metal layer 13, as shown in Figure 12. The so-formed demetallised substrate, carrying light absorbing material layer 5 can then be laminated to the colourshifting film 2 via intermediate adhesive layer 8c, magnetic layer 1 1 a and barrier layer 12 all as previously described.

In this example, additional layers are also provided on the opposite side of substrate 14, namely a further magnetic layer 1 1 b and optical diffusion layer 15 which here is a fluorescent yellow ink provided over substantially the whole area of the security element.

The additional magnetic layer 1 1 b could be of any of the forms discussed above in relation to layer 1 1 of Figure 6(b), however in this example the layer 1 1 b is formed of a substantially transparent magnetic ink and therefore can take any spatial arrangement, preferably a code, since it will not interfere with the colour appearance of the pattern defined by gaps 6. In another preferred example, such a transparent magnetic layer 1 1 b can replace the magnetic layer 1 1 a in between the colourshifting film 1 and light absorbing material layer 5, instead of being provided on the other side of substrate 14 as shown in Figure 12.

Figures 13(a) and (b) show two examples of security elements 1 in accordance with embodiments of the present invention, in plan view so as to illustrate exemplary patterns as may be defined by gaps 6. Both of the exemplary elements take the form of elongate security threads with repeating patterns of negative indicia along their long direction. The threads preferably have widths in the range 0.5 mm to 10 mm. The negative indicia preferably takes the form of microtext which is generally inconspicuous to the human eye and requires close inspection or magnification to observe. In the Figure 13(a) example the negative indicia comprises a series of chevron symbols and the numbers "101" whereas in the Figure (b) example the negative indicia comprises star shaped symbols and the letters "DLR". The stem widths of the letters and numbers are typically in the range 0.1 to 0.3 mm. Figure 13(c) shows an enlarged portion of the thread of Figure 13(b) with exemplary dimensions. Here, the thread width w is 0.85 mm, the repeat distance di of the pattern is 15mm, the length of a pattern unit d₂ is 10mm, the gap at each end of the unit d₃ is 2mm, and a space d₄ of 1 mm is provided between neighbouring blocks. The text stem width is 0.15mm.

Claims

1. A security element, comprising:

a colourshifting film which, when viewed in reflected or transmitted light, exhibits different colours in dependence on the viewing angle and which, when viewed at any one angle, exhibits first and second colours when viewed in reflected and transmitted white light respectively, the first and second colours being complementary, the colourshifting film comprising a plurality of polymer layers arranged in a periodic stack, including respective layers of at least first and second polymer materials having different refractive indices from one another; and

wherein the security element is substantially transparent or translucent to at least some wavelengths of visible light at the locations corresponding to the one or more gaps in the light absorbing material layer, such that in the locations corresponding to the one or more gaps in the light absorbing material layer, the security element exhibits different respective colours when viewed in reflected white light and when viewed in transmitted white light at the same viewing angle.

2. A security element according to claim 1 , wherein the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the maximum of the function R_e(A) has a bandwidth (Δλ_θ=ο) of 100 nm or less, preferably between 30 and 150 nm, more preferably between 50 and 100 nm.

3. A security element according to claim 1 or 2, wherein the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film is at least 70%, preferably at least 80%.

4. A security element according to any of the preceding claims, wherein the peak wavelength λ_Β,θ is given by:

λ_Ββ = 2 [N_h. L_h + N_l. L_l]cose

where: N_h and N| are the refractive indices of the first and second polymer materials respectively, N_h being greater than N,; L_h and are the thicknesses of the first and second polymer material layers respectively in the direction normal to the security element; and Θ is the viewing angle relative to the normal for the first order reflection of the peak wavelength; and

the percentage R_B,e=o is given by:

1- ^2P(

1 + ^'W

^2P(

where: p is the number of pairs of first and second polymer material layers in the colourshifting film; and N_a and N_b are the refractive indices of the materials adjoining the two outermost layers of the colourshifting film, respectively.

5. A security element according to any of the preceding claims, wherein at a viewing position lying on the normal to the security element, the bandwidth Δλ_θ=₀ is given by: ,θ = 0

Where: λ_{Β Θ}=ο is the peak wavelength at the normal viewing angle; acos(p) is the arc cosine of the parameter p; p is the Fresnel reflection coefficient which for normal incidence corresponds to (N_h - N|)/(N_h + N|); and E is a percentage manufacturing tolerance in the thicknesses L_h and of the first and second polymer material layers respectively.

6. A security element according to any of the preceding claims, wherein the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the sum of the percentage R_B,e=o of incident light at the peak wavelength λ_Β,θ=ο that is reflected by the colourshifting film and the percentage T_B,e=o of incident light at the peak wavelength that is transmitted by the colourshifting film is at least 80%, more preferably at least 90%, still preferably at least 95%, most preferably substantially 100%.

7. A security element according to any of the preceding claims, wherein the colourshifting film is configured such that, at least at a viewing position lying on the normal to the security element, the percentage T_N,e=o of incident light at wavelengths (λ_Ν,β=ο) at least 50nm away from the peak wavelength (λ_Β,β=ο) that is transmitted by the colourshifting film is at least 75%, preferably at least 80%, more preferably at least 85%.

8. A security element according to any of the preceding claims, wherein the difference between the refractive indices of the first and second polymer materials is less than or equal to 0.4, preferably less than or equal to 0.25, more preferably less than or equal to 0.2.

9. A security element according to claim 8, wherein the difference between the refractive indices of the first and second polymer materials is at least 0.02.

10. A security element according to any of the preceding claims, wherein the refractive indices of the first and second polymer materials are each between 1.45 and 1.95.

1 1. A security element according to any of the preceding claims, wherein the periodicity of the polymer layers is constant throughout at least a first portion of the thickness of the stack.

12. A security element according to claim 1 1 , wherein the periodicity of the polymer layers is different in respective first and second portions of the thickness of the stack, and is constant within each respective portion.

13. A security element according to any of the preceding claims, wherein the plurality of polymer layers have thicknesses in the range 50 to 150 nm and more preferably 60 to 120 nm.

14. A security element according to any of the preceding claims, wherein the plurality of polymer layers has a total thickness in the range 5 to 50 microns, preferably 10 to 20 microns.

15. A security element according to any of the preceding claims, wherein the colourshifting film comprises between 100 and 400 polymer layers, preferably between 120 and 170 polymer layers.

16. A security element according to any of the preceding claims, wherein the first polymer material is polyethylene terephthalate (PET) and the second polymer material is polyethylene terephthalate glycol-modified (PETg).

17. A security element according to any of the preceding claims, wherein the colourshifting film is a co-extruded multilayer polymer stack.

18. A security element according to any of the preceding claims, wherein the light absorbing material absorbs at least 70% of incident visible light, preferably at least 80%, more preferably at least 90%.

19. A security element according to any of the preceding claims, wherein the light absorbing material is additionally non-transparent and preferably transmits less than 30% of incident visible light in a single pass, more preferably less than 20%, still preferably less than 10%, most preferably substantially opaque.

20. A security element according to any of the preceding claims, wherein the light absorbing material is dark in colour, preferably black.

21. A security element according to any of the preceding claims, wherein the light absorbing material comprises a magnetic or electrically conductive substance.

22. A security element according to any of the preceding claims, wherein the light absorbing material is a resist material and/or an ink.

23. A security element according to any of the preceding claims, wherein the light absorbing material layer directly contacts an outermost one of the plurality of polymer layers of the colourshifting film.

24. A security element according to any of claims 1 to 22, further comprising one or more intermediate layers between the colourshifting film and the light absorbing material layer, the or each intermediate layer being either transparent and/or being arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern.

25. A security element according to claim 24, wherein the intermediate layer(s) are either light-absorbing, or are optically inactive under all illumination conditions.

26. A security element according to claim 24 or 25, wherein the intermediate layer(s) comprise any of:

• an adhesive layer;

• a barrier layer;

• a magnetic layer; and

· an electrically conductive layer.

27. A security element according to any of the preceding claims, further comprising at least one optical diffusion layer overlapping the colourshifting film and/or the light absorbing material layer at least in partial areas of the security element, the optical diffusion layer(s) preferably extending across substantially the whole area of the security element.

28 A security element according to claim 27, wherein at least one of the optical diffusion layers is located on the same side of the colourshifting film as the light absorbing material layer, the light absorbing material layer preferably being between the optical diffusion layer and the colourshifting film, the optical diffusion layer being provided at least in partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security element in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the viewing angle but is substantially independent of the illumination angle.

29. A security element according to claim 27 or 28, wherein at least one of the optical diffusion layers is located on the opposite side of the colourshifting film from the light absorbing material layer, the optical diffusion layer being provided at least in partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security element in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the illumination angle but is substantially independent of the viewing angle.

30. A security element according to any of claims 27 to 29, wherein the or each optical diffusion layer comprises a binder containing a dispersion of optically scattering particles and/or a photoluminescent substance which emits light in the visible spectrum upon excitation.

31. A security element according to claim 30 wherein the photoluminescent substance emits light in the visible spectrum in response to incident radiation in the ultra-violet spectrum.

32. A security element according to any of claims 27 to 31 , wherein the or each optical diffusion layer is a resist material and/or an ink.

33. A security element according to any of claims 27 to 32, wherein the or each optical diffusion layer is translucent and has an optical density less than or equal to 1 , preferably less than or equal to 0.6, more preferably less than or equal to 0.3.

34. A security element according to any of claims 27 to 33, wherein the or each optical diffusion layer is white or has a visibly coloured tint.

35. A security element according to any of claims 27 to 34, wherein the or each optical diffusion layer comprises at least two optically diffusing materials having different optical characteristics, arranged in respective laterally offset regions of the element.

36. A security element according to any of the preceding claims, wherein the pattern defined by the gaps in the light absorbing material layer comprises positive or negative indicia, preferably one or more alphanumeric characters, symbols, currency identifiers, logos or the like.

37. A security element according to any of the preceding claims, further comprising at least one magnetic layer, preferably on the same side of the colourshifting film as the light absorbing material layer, the or each magnetic layer being either transparent and/or being arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern.

38. A security element according to claim 37, wherein at least one of the magnetic layer(s) is arranged spatially so as to form a magnetic coding.

39. A security element according to any of the preceding claims, further comprising at least one electrically conductive layer, preferably on the same side of the colourshifting film as the light absorbing material layer, the or each electrically conductive layer being arranged partially across the security element so as not to obscure the gaps in the light absorbing material layer defining the pattern.

40. A security element according to claim 39 wherein the or each electrically conductive layer is a metal layer, the light absorbing material layer preferably being located between the colourshifting film and the metal layer(s).

41. A security element according to claim 39 or 40 wherein the or each electrically conductive layer is arranged so as to form a continuous conductive path from one edge of the security element to another.

42. A security element according to any of the preceding claims, further comprising a transparent polymer support substrate, preferably comprising polypropylene (PP), bi-axially oriented PP (BOPP), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

43. A security element according to any of the preceding claims, further comprising an adhesive layer on one or both outer surfaces of the security element.

44. A security element according to any of the preceding claims, wherein the security element is a security thread, strip, foil or patch.

45. A security document comprising a security element according to any of the preceding claims, wherein the security document is preferably a banknote, a polymer banknote, a passport, a licence, a identification document, a visa, a cheque or a certificate.

46. A security document according to claim 45, wherein the security document comprises at least one optically diffusive document layer, the security element being incorporated into the security document such that the or each optically diffusive document layer overlaps the colourshifting film and/or the light absorbing material layer at least in partial areas of the security element.

47. A security document according to claim 46, wherein at least one of the optically diffusive document layers is located on the light absorbing material layer side of the security element, the optically diffusive document layer being provided at least in regions corresponding to partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security document in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the viewing angle but is substantially independent of the illumination angle.

48. A security document according to claim 46 or 47, wherein at least one of the optically diffusive document layers is located on the colourshifting film side of the security element, the optically diffusive document layer being provided at least in regions corresponding to partial areas of the security element overlapping at least some of the gaps in the light absorbing material layer, such that the colour exhibited by the security document in transmitted light at at least some of the locations corresponding to the gaps in the light absorbing material layer varies in dependence on the illumination angle but is substantially independent of the viewing angle.

49. A security document according to claim 47 or 48, wherein at least one of the optically diffusive document layer(s) overlaps only some of the gaps in the light absorbing material layer and not others.

50. A security document according to any of claims 46 to 49, comprising a document substrate which forms at least one of the optically diffusive document layers, the document substrate preferably comprising a fibrous substrate such as paper.

51. A security document according to claim 50, wherein the security element is at least partially embedded in the document substrate, preferably in a windowed fashion.

52. A security document according to any of claims 46 to 49, comprising a transparent polymer document substrate and at least one opacifying layer thereon which forms at least one of the optically diffusive document layers, the at least one opacifying layer preferably comprising a non-fibrous polymeric material comprising a dispersion of light-scattering particles.

53. A security document according to any of the preceding claims, wherein the security element is incorporated into or onto the security document such that at least a portion of the security element extends across a transparent window portion of the security document.