Abstract
A method of manufacturing an image pattern for a security device includes providing a metallised substrate; applying a first photosensitive resist layer to a substrate first metal layer exposing the resist layer to radiation; exposing the resist layer to a first reactant substance; activating a cross linking agent in the resist layer; exposing first and second pattern elements of the resist layer to radiation of a wavelength to which the resist layer is responsive whereupon newly-exposed first pattern elements of the first photosensitive resist layer react, resulting in increased solubility by the second etchant substance, the second pattern elements remaining relatively insoluble by the second etchant substance; and applying first and second etchant substances to the substrate whereupon the first pattern elements of both the first resist layer and the first metal layer are dissolved, the remaining second pattern elements of the first metal layer forming an image pattern.
Claims
1. A method of manufacturing an image pattern for a security device, comprising: (a) providing a metallised substrate comprising a substrate material having a first metal layer thereon on a first surface of the substrate material, the first metal layer being soluble in a first etchant substance; (b) applying a first photosensitive resist layer to the first metal layer, the first photosensitive resist layer comprising a thermally-activatable cross-linking agent which, is operable to preferentially cross-link functional groups of a selected class, which functional groups are not present in the first photosensitive resist layer upon application to the first metal layer; (c) exposing the first photosensitive resist layer to radiation of a wavelength to which the resist layer is responsive through a patterned mask, wherein the patterned mask comprises first pattern elements in which the mask is substantially opaque to the radiation and second pattern elements in which the mask is substantially transparent to the radiation, whereupon the exposed second pattern elements of the first photosensitive resist layer react resulting in increased solubility in a second etchant substance, the non-exposed first pattern elements remaining relatively insoluble by the second etchant substance; (d) exposing the first photosensitive resist layer to a first reactant substance, the first reactant substance reacting with the exposed second pattern elements of the first photosensitive resist layer to produce at least one functional group of the selected class, the first reactant substance substantially not reacting with the unexposed first pattern elements of the first photosensitive resist layer: (e) activating the cross-linking agent in the first photosensitive resist layer such that cross-links are formed between the at least one functional group of the selected class in the exposed second pattern elements, whereby the solubility of the exposed second pattern elements of the first photosensitive resist layer in the second etchant substance is decreased; (f) exposing the first and second pattern elements of the first photosensitive resist layer to radiation of a wavelength to which the resist layer is responsive whereupon the newly-exposed first pattern elements of the first photosensitive resist layer react, resulting in increased solubility by the second etchant substance, the second pattern elements remaining relatively insoluble by the second etchant substance; and (g) applying the first and second etchant substances to the substrate whereupon the first pattern elements of both the first resist layer and the first metal layer are dissolved, the remaining second pattern elements of the first metal layer forming an image pattern; wherein in step (g) the first pattern elements of both the first resist layer and the first metal layer are soluble in the same first etchant substance, the first pattern elements of the first metal layer and of the first resist layer are dissolved in a single etching procedure.
2. A method according to claim 1, wherein in step (d) the first photosensitive resist layer is exposed to the first reactant substance by applying the first reactant substance to the first photosensitive resist layer or by passing the substrate through a chamber containing the first reactant substance.
3. A method according to claim 1, wherein in step (e), the thermally-activatable cross-linking agent in the first photosensitive resist layer is activated by heating the first photosensitive resist layer.
4. A method according to claim 1, wherein in step (e), the thermally-activatable cross-linking agent in the first photosensitive resist layer is activated by maintaining the temperature of the first photosensitive resist layer at a level above an activation temperature of the thermally-activatable cross-linking agent for a predetermined period of time.
5. A method according to claim 1, wherein at the end of step (e), the solubility of the exposed second pattern elements of the first photosensitive resist layer in the second etchant substance is less than that of the unexposed first photosensitive resist layer in step (b).
6. A method according to claim 1, wherein the substrate is a substrate web and, in step (c), the first photosensitive resist layer is exposed to the radiation by conveying the substrate web along a transport path and, during the exposure, moving the patterned mask alongside the substrate web along at least a portion of the transport path at substantially the same speed as the substrate web, such that there is substantially no relative movement between the mask and the substrate web.
7. A method according to claim 6, further comprising, after step (d): (d1) drying the substrate web, and (d2) winding up the substrate web and removing from the transport path; whereby step (e) is performed offline.
8. A method according to claim 7, further comprising, after step (e): (d3) unwinding the substrate web back onto the transport path; whereby step (f) is performed by conveying the substrate web along the same transport path as in step (c) during which the first photosensitive resist layer is exposed to the radiation in the absence of the patterned mask.
9. A method according to claim 1, further comprising, after step (g): (h) applying a further etchant substance to the substrate to dissolve the remaining second pattern elements of the first photosensitive resist layer.
10. A method according to claim 1, further comprising providing a colour layer on the first or second surface of the substrate material, the colour layer comprising at least one optically detectable substance provided across the first and second pattern elements in at least one zone of the pattern, such that when viewed from one side of the substrate web, the colour layer is exposed in the first pattern elements between the second pattern elements of the first metal layer.
11. A method according to claim 1, wherein in step (a), the metallised substrate has an optically variable effect generating relief structure in its first surface, the first metal layer conforming to the contours of the relief structure on one or both of its sides.
12. A method according to claim 1, wherein the pattern of first and second pattern elements includes pattern elements with a minimum dimension of 50 microns or less.
13. A method according to claim 1, wherein the pattern of first and second pattern elements is periodic in at least a first dimension and either the first pattern elements are substantially identical to one another and/or the second pattern elements are substantially identical to one another.
14. A method according to claim 1, wherein the pattern of first and second pattern elements defines sections of at least two images interleaved with one another periodically in at least a first dimension.
15. A method according to claim 1, wherein in step (a) the metallised substrate further comprises a second metal layer on the second surface of the substrate material, and the method further comprises manufacturing a second image pattern by applying a second photosensitive resist layer to the second metal layer, the second photosensitive resist layer comprising a composition as defined in step (b), and performing steps (c) to (g) on the second photosensitive resist layer.
16. A method of manufacturing a security device, comprising: (i) manufacturing a first image pattern using the method of claim 1; and (ii) providing a viewing component overlapping the first image pattern; wherein the first image pattern and the viewing component are configured to co-operate to generate an optically variable effect.
17. A method according to claim 16, wherein the viewing component comprises one of: a focusing element array, a masking grid or a second image element array.
Description
(1) Examples of security devices, image element arrays therefor and their methods of manufacture in accordance with the present invention will now be described and contrasted with conventional examples, with reference to the accompanying drawings, in which:
(2) FIG. 1(a) schematically illustrates a step of exposing a resist through an exemplary patterned mask in an embodiment of the invention, FIG. 1(b) showing the resulting image pattern;
(3) FIG. 2 depicts a chemical reaction undergone by an exemplary resist material on exposure to radiation of an appropriate wavelength;
(4) FIG. 3 is a flow chart depicting steps in an embodiment of a method in accordance with the present invention;
(5) FIGS. 4(a) to (g) illustrate the steps of the method of FIG. 3;
(6) FIG. 5 depicts changes undergone by an exemplary resist material suitable for use in the method of FIGS. 3 and 4, at selected stages (i) to (iv) of the method, (a) in portions of the resist corresponding to radiation transparent elements of the patterned mask, and (b) in portions of the resist corresponding to radiation opaque elements of the patterned mask;
(7) FIGS. 6(a) and (b) schematically depict two exemplary apparatus for carrying out selected steps of the method of FIG. 3
(8) FIG. 7 is a flow chart depicting optional additional steps in another embodiment of a method in accordance with the present invention;
(9) FIGS. 8(a) to (c) illustrate selected steps of FIG. 7, FIG. 8(c) showing an embodiment of a security device made in accordance with the method;
(10) FIGS. 9(a) to (d) illustrate steps of the method of FIG. 7 in another embodiment, FIG. 9(e) showing a further embodiment of a security device made in accordance with the method, and FIGS. 9(i) and (ii) showing two further examples of security devices made in accordance with variants of the method;
(11) FIGS. 10(a) to (c) illustrate selected steps of another embodiment of a method in accordance with the present invention;
(12) FIGS. 11(a) and (b) depict two embodiments of image patterns in accordance with the present invention, in cross-section;
(13) FIG. 12 is a photograph showing an enlarged portion of an embodiment of a security device comprising an exemplary image pattern manufactured in accordance with an embodiment of the invention;
(14) FIG. 13(a) illustrates in plan view an exemplary image pattern in accordance with an embodiment of the present invention, FIG. 13(b) showing in plan view the appearance of a security device in accordance with an embodiment of the present invention incorporating the image element array of FIG. 13(a), at one viewing angle;
(15) FIG. 14(a) illustrates an exemplary image pattern in accordance with an embodiment of the invention, and FIG. 14(b) shows the appearance of a security device incorporating the image pattern of FIG. 14(a);
(16) FIG. 15(a) schematically depicts a security device in accordance with a further embodiment of the present invention, FIG. 15(b) showing a cross-section through the security device, and FIGS. 15(c) and (d) showing two exemplary images which may be displayed by the device at different viewing angles;
(17) FIGS. 16(a), (b) and (c) show three further embodiments of security devices in accordance with embodiments of the invention;
(18) FIGS. 17(a) and (b) show two further examples of apparatus suitable for carrying out selected steps of methods in accordance with embodiments of the invention;
(19) FIG. 18 shows a cross section through a security device in accordance with another embodiment of the invention;
(20) FIGS. 19, 20 and 21 show three exemplary articles carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b) in cross-section; and
(21) FIG. 22 illustrates a further embodiment of an article carrying a security device in accordance with the present invention, (a) in front view, (b) in back view and (c) in cross-section.
(22) The ensuing description will focus initially on examples of methods of manufacturing image patterns with high resolution, fine detail in the form of image element arrays as required for use in security devices such as moir magnifiers, integral imaging devices and lenticular devices (amongst others). Preferred embodiments of such security devices making use of image element arrays made in accordance with the described method will then be described below. However it should be appreciated that the disclosed methods of manufacturing image patterns can be used to form any high resolution image pattern, as may be suitable for use in other security devices such as microtext or other micro-graphics.
(23) As summarised previously, in embodiments of the invention, image elements are formed by demetallising a metal layer 11 carried on a substrate material 10, in accordance with a desired pattern. As shown in FIG. 1(a), the metal layer 11 is coated with a resist material 2 which is responsive to radiation of a particular wavelength, typically one or more ultraviolet wavelengths, e.g. in the range 350 to 415 nm. The resist 2 is exposed to the radiation R through a patterned mask 1 which in this case carries a mask pattern MP in the form of a radiation-transparent portion defining the letter A surrounded by a background which is substantially opaque to the radiation. The resist 2 is a positive resist meaning that the material reacts on exposure to the radiation to become soluble (or more soluble) in a selected etchant. For example, the photochemical reaction may cause a reduction in cross-linking within the resist material (photo disassociation), resulting in the increased solubility. Thus, in the example shown in FIG. 1(a), the exposed portion 2a of the resist corresponding to the letter A in the mask pattern MP initially reacts so as to become more soluble to the etchant relative to the remaining parts 2b of the resist. However, the resist layer 2 is then further treated as will be described below such that the solubility of the resist portion 2a is reduced and that of portion 2b increased whereby, when etching occurs it is the originally-exposed portion 2a which remains in place, protecting the underlying metal, whilst the surrounding area 2b is dissolved. This is shown in FIG. 1(b). The resulting image pattern IP carried by the metal layer 11 is therefore a negative version of the original mask pattern MP, i.e. a metallised region in the shape of the letter A defined against a background in which the metal layer 11 is absent.
(24) Preferred examples of suitable positive resist materials, used in embodiments of the present invention, include Diazonaphthoquinone-based resists (DNQ), also known as ortho quinine diazides (OQDs), such as 1, 2-Naphthoquinone Diazide. The material is substantially non-soluble in alkali in its initial state. Upon exposure to UV light (e.g. utilising a mercury halide lamp), a reaction occurs as depicted in FIG. 2, resulting in the formation of a carboxylic acid group (Wolff rearrangement). The reacted material is soluble in alkali conditions. In particularly preferred embodiments, by utilising a metal layer 11 which is also soluble in alkali, such as aluminium, aluminium alloy (at least 50% Al), chromium or chromium alloy (at least 50% Cr), application of an alkaline etchant such as sodium hydroxide will remove not only the exposed regions of the resist material but also the underlying portions of the metal layer, allowing for both layers to be removed in a single processing step. For chromium and its alloys, an addition of potassium hexacyanoferrate to the etchant may be required. One suitable commercially available positive resist material is V215 by Varichem Co. Ltd., which comprises a DNQ with a novolak ballast group. Further examples include sulfonyl compounds of the diazo, such as 1,2-Naphthoquinone-2-Diazide-5-sulfonyl chloride. The DNQ substance may be applied as a solution in a suitable solvent such as cyclopentanone, e.g. a 10% (w/w) solution. The solvent and any other volatile components will be dried off after application of the resist to the metal layer, leaving the DNQ and any other solid components to form the resist layer. If necessary a heater or other drying module may be provided to assist this.
(25) Importantly, and in contrast to conventional resist compositions, in embodiments of the present invention the resist layer also comprises a thermally-activatable cross-linking agent which is operable to cross-link certain functional groups (Q) preferentially (relative to other functional groups). Most advantageously the agent is operable to cross-link those functional groups (Q) only. In particular, the cross-linking agent should not be capable of cross-linking (to any significant degree) any functional groups which are present in the resist before it is exposed to the radiation, but only those formed as a result of such exposure (whether directly or indirectly). The cross-linking agent is activated by temperature and not, for instance, by radiation. The agent may or may not have a defined activation temperature above which cross-linking will occur and below which it will not; rather the efficiency with which the agent promotes cross-linking may increase with temperature such that it is relatively low (but not necessarily zero) at low temperatures and relatively high at higher temperatures. In preferred embodiments, the functional groups (Q) which the agent is operable to cross-link are carboxylic acid groups of the type CO.sub.2H, which it will be noted from FIG. 2 are formed when a diazo-containing resist such as DNQ is exposed to suitable radiation (e.g. UV) in the presence of water, e.g. atmospheric water vapour. A preferred class of thermally-activated cross-linking agents which is specific to carboxylic acid groups is carbodiimide. Polyaziridines such as CX-100 from DSM Coatings are not specific to CO.sub.2H but have been found to preferentially cure at this acid group which the present inventor has found also works well. The selected cross-linking agent may be added to the DNQ material at a concentration of at least 15% w/w/, preferably around 30% w/w, for example.
(26) Five further examples of suitable positive resist compositions which can be utilised in embodiments of the present invention are as follows (g=gram):
(27) 1) 0.7 g V215 by Varichem Co. Ltd. or 1, 2-Naphthoquinone-2-Diazide-5-sulfonyl chloride; 0.3 g Permutex XR5580; 10 g PGMEA; 1 g MEK; and 0.03 g Surfynol 61 (from Air Products).
(28) 2) 0.7 g V215 by Varichem Co. Ltd. or 1, 2-Naphthoquinone-2-Diazide-5-sulfonyl chloride; 0.3 g Permutex XR5580; 10 g Cyclopentanone; 1 g MEK; and 0.01 g Byk-055 (from Byk Chemie).
(29) 3) 0.7 g V215 by Varichem Co. Ltd. or 1, 2-Naphthoquinone-2-Diazide-5-sulfonyl chloride; 0.3 g CX-100; 10 g PGMEA; 1 g MEK; and 0.01 g Byk-022 (from Byk Chemie).
(30) 4) 0.625 g V215 by Varichem Co. Ltd. or 1, 2-Naphthoquinone-2-Diazide-5-sulfonyl chloride; 0.375 g CX-100; 10 g PGMEA; 1 g MEK; and 0.2 g Isopropyl alcohol.
(31) 5) 0.7 g V215 by Varichem Co. Ltd. or 1, 2-Naphthoquinone-2-Diazide-5-sulfonyl chloride; 0.08 g Novolak resin; 0.292 g CX-100, 10 g PGMEA.
(32) It will be appreciated that each of the example compositions above describes the wet composition of the resist as applied to the metal layer. Upon drying (which may or may not involve an active drying step but may occur automatically during the time between process steps), the solvent and any other volatile components will evaporate leaving only the solid components. Hence in example composition (1), the DNQ makes up 70% of the dry resist formulation but only approximately 7% of the wet resist composition. In example (5), the Novolak resin is an example of a binder which is a solid component and hence remains in the dry resist formulation.
(33) Surfynol 61, used in composition 1 above, is an example of a surfactant. Resist compositions containing a surfactant such as this have been found by the present inventors to produce particularly good results in the presently disclosed method. The benefit of the surfactant is to assist in forming a more even resist coating. Without the surfactant the coating thickness was found to vary more widely across the substrate. This can lead to difficulties in controlling the downstream processing steps of irradiation and etching, because the thicker sections of the resist require a longer processing time. With the surfactant the resist coating was found to be of much more uniform thickness, meaning that the amount of time under the exposure and through the etchant is the same for the whole coating.
(34) The use of a volatile surfactant (of which Surfynol 61 is an example) is particularly preferred since upon drying of the resist layer, the surfactant substance transitions to a gaseous state and exits the system so as not to interfere with downstream processing. However, non-volatile surfactants have also been found to achieve the above mentioned benefits to some degree.
(35) FIG. 3 is a flow diagram illustrating steps in a method of manufacturing an image pattern in accordance with an embodiment of the invention. FIG. 4 schematically illustrates the steps in an exemplary implementation of the method, and FIG. 5 shows the changes undergone by the resist material at selected stages of the method for an exemplary process chemistry.
(36) First, a metallised substrate is provided (step S101), which comprises a (preferably transparent) substrate material 10 carrying a metal layer 11 on one of its surfaces, as shown in FIG. 4(a). The substrate material 10 typically comprises at least one transparent polymeric material, such as BOPP, and may be monolithic or multi-layered. The substrate may be of a type suitable for forming the basis of a security article such as a security thread, strip, patch or transfer foil, or of a type suitable for forming the basis of a security document itself, such as a polymer banknote. The substrate may include additional layers, such as a filter layer (described below) and/or a primer layer underlying the metal layer 11. The substrate could also carry additional security features such as an optically variable relief structure, e.g. a diffraction structure such as a hologram, kinegram or diffraction grating, which the metal layer 11 follows, over all or part of the substrate surface. The substrate may be supplied pre-metallised, or the metal layer 11 (and any optional underlying layer(s)) could be applied as part of the presently disclosed method, e.g. by vapour deposition, sputtering or the like. The metal layer 11 may cover the whole area of the substrate material 10 (as shown) or could be provided only across selected portion(s) of the substrate within which the demetallised pattern is to be formed. Suitable metals include aluminium, copper, chromium and alloys of each (including in particular aluminium-copper alloys). The metal layer 11 is preferably substantially opaque to visible light and is desirably as thin as possible whilst achieving this opacity. The thinner the layer, the more accurately it can be etched since it will be less susceptible to lateral spread of the etched region. In preferred examples, the metal layer 11 may have a thickness of between 10 and 100 nm, more preferably between 10 and 50 nm, most preferably between 10 and 25 microns.
(37) In step S102, a photosensitive resist material 12 is then applied onto the metal layer 11, as shown in FIG. 4(b). The resist material 12 may be applied all over the substrate web area or could be applied selectively, e.g. to define a large-scale pattern or image (sufficiently large to be visible to the naked eye). Suitable application techniques include printing or coating the resist material onto the metal layer. As described above, the resist material is a positive resist which upon exposure to appropriate radiation becomes soluble (or more soluble) in an etchant substance (i.e. a solvent) in which the selected metal layer 11 is preferably also soluble. For example, where the metal layer 11 comprises aluminium, a resist material 12 containing a DNQ is suitable since both can be etched using an alkali such as sodium hydroxide. The chemical structure of an exemplary DNQ-based resist in the form in which it is applied to the metal later 11 in step S102 is shown in FIG. 5(a)(i) and in FIG. 5(b)(i). The resist layer 12 further comprises a thermally-activatable cross-linking agent which is operable to cross-link functional groups of the type Q, preferentially (ideally, only). Depending on the composition of the resist material, the susbtrate may be passed through a dryer before onward processing. The so-applied resist layer 12 has an initial solubility level in the selected etchant, S, of S.sub.0. Typically this solubility level S.sub.0 will be low, but not zero.
(38) The resist material 12 is then exposed to appropriate radiation R through a patterned mask 1, as shown in FIG. 4(c)step S103. The mask 1 defines the desired pattern in terms of first pattern elements P.sub.1 in which the mask is substantially opaque to the radiation, and second pattern elements P.sub.2 in which the mask is substantially transparent to the radiation. For the reasons noted above, the mask pattern will be implemented as the negative of the image pattern which is ultimately desired to be carried on the substrate. During exposure, the mask preferably contacts the resist layer 12 in order that the regions of the resist layer exposed to the radiation correspond to the transparent elements of the mask 1 as accurately as possible. In preferred embodiments, as will be described in more detail below, exposure takes place during transport of the substrate web and hence the mask is also moved alongside the web at substantially the same speed, in order to enable continuous manufacture. However this is optional and processing could take place batchwise or sheet-by-sheet.
(39) Simultaneously with or subsequent to the radiation exposure, the resist material 12 is exposed to a reactant 16 (step S104, FIG. 4(d)) which reacts with the resist material in the radiation-exposed elements only to form functional groups of the type Q, i.e. those which the thermally-activatable cross-linking agent is operable to cross-link. In preferred embodiments, the reactant 16 comprises water or water vapour. Where the resist 12 comprises a DNQ-based material, the so-generated functional groups Q will be carboxylic acid groups (CO.sub.2H), as depicted in FIG. 5(a)(ii).
(40) Hence, in the second pattern elements P.sub.2, the exposed resist material 12 initially reacts to become soluble in the selected etchant (or more soluble), achieving a solubility level S of S.sub.1, where S.sub.1 is greater than S.sub.0. In contrast, in the first pattern elements P.sub.1, the resist material receives substantially no radiation and therefore remains unchanged and (relatively) insoluble in the etchant (solubility level S.sub.0), as illustrated in FIG. 5(b)(ii).
(41) It should be noted that the step S104 of exposing the resist 12 to the reactant 16 may be an active or a passive step. That is, a positive action may or may not be required in order to introduce the reactant to the resist 12 and thus initiate the desired reaction. For example, where the reactant comprises water or water vapour, sufficient water vapour may be present in the ambient atmosphere for the desired functional groups Q to be generated simply by exposing the resist 12 to the radiation R in the presence of the ambient environment. However, in other cases it is preferred to actively apply the reactant to the resist 12, e.g. by spraying (e.g. using apparatus 17 such as an array of jets as shown in FIG. 4(d)) or coating the reactant onto the resist 12, or by passing the substrate through a chamber containing the reactant, such as a bath. The reactant 16 could be liquid or gaseous.
(42) Next, in step S105 (FIG. 4(e)), the cross-linking agent is activated. Again this could be an active or a passive step depending on the nature of the cross-linking agent and on the ambient conditions. For instance, the ambient temperature may be sufficiently high that the cross-linking agent can initiate cross-linking without actively raising the temperature of the resist 12, in which case step S105 may require only maintaining the temperature of the resist for a sufficient duration to achieve cross-linking. However, in preferred embodiments, step S105 comprises heating the resist layer 12, e.g. by passing the substrate across one or more heating elements or placing the substrate in an oven. In a preferred example, the resist 12 is heated to a temperature of at least 100 degrees C. (more preferably at least 110 degrees C., most preferably about 120 degrees C.). The resist may be held at such temperatures for a predetermined time to achieve the desired degree of cross-linking between functional groups Q. For instance, the elevated temperature may be maintained for at least 60 minutes, more preferably at least 90 minutes.
(43) Where the resist 12 is a DNQ-based resist, the cross-linking agent is carbodiimide and the reactant in step S104 is water or water vapour, good results have been achieved where, in step S105, the resist is heated to between 110 and 130 degrees C. (preferably about 120 degrees C.) for between 1 and 2 hours. The resulting exemplary cross-linked resist is shown in FIG. 5(a)(iii). Preferably the degree of cross-linking achieved is at least 50% but this is not essential. If the resist is cured enough to protect the underlying metal during the subsequent etching step(s) described below, then the degree of cross-linking achieved is sufficient.
(44) Thus, after step S105, in the exposed second pattern elements P.sub.2, as a result of the cross-linking between functional groups Q, the solubility of the resist has been reduced to S.sub.2, where S.sub.2 is less than S.sub.1, preferably significantly so. Most preferably, the solubility level S.sub.2 is also less than the initial solubility level S.sub.0 of the resist 12 as applied in step S102.
(45) It should be appreciated that the resist 12 in the as-yet unexposed first pattern elements P.sub.1 remains unchanged relative to its state when originally applied to the metal layer 12 and hence still has a solubility level of S.sub.0 (FIG. 5(b)(iii)).
(46) Despite activation in step S105, the cross-linking agent is ineffective in these elements of the resist since no functional groups Q are present.
(47) Next, the whole of the resist layer 12 is exposed to radiation of a wavelength to which the resist is photosensitive, i.e. both the first and second pattern elements (step S106, FIG. 4(f)). This is commonly referred to as flood exposure and does not require any patterned mask. Typically the wavelength to which the resist is exposed in step S106 will be substantially the same as that utilised in step S103 (e.g. UV radiation in the range 350 to 415 nm), and preferably the same exposure apparatus may be used. Since the previously-exposed second pattern elements P.sub.2 have been cross-linked, the resist in these elements will not undergo any further changes as a result of the further irradiation, and its solubility level remains at S.sub.2. However, the resist in the first pattern elements P.sub.1 is now exposed to the radiation for the first time, causing the solubility level of the first pattern elements to increase to around the same level as that achieved in the second pattern elements in step S103 (S.sub.1). This is shown in FIG. 5(b)(iv).
(48) Thus, after step S106, the first pattern elements P.sub.1 of the resist 12 have a solubility level greater than that of the second pattern elements P.sub.2, preferably substantially greater. One or more etchant substance(s) are then applied to the substrate (step S107), in order to dissolve the resist 12 and the underlying metal 11 in the first pattern elements P.sub.1. Preferably, a single etchant is used to achieve this. For example, in the case of an aluminium metal layer 11 and a DNQ resist layer 12, the etchant is typically an alkali such as a solution of sodium hydroxide (NaOH). The second pattern elements P.sub.2 of the metal layer 11 remain on the web, as shown in FIG. 4(g). The removal of the resist material 12 and the metal layer 11 by the same etchant is highly advantageous since no further etchant substance is required. Preferably both materials are removed in a single processing step. However, the same etchant substance could be applied multiple times if necessary to achieve the removal. The etchant can be applied by conveying the substrate through a bath of the etchant or by spraying the etchant onto the substrate, for instance. This may be accompanied by one or more mechanical actions such as brushing or agitation to assist in dissolution, if necessary. In other embodiments, if the metal layer 11 is of a type which will not be dissolved by the same etchant as the resist, step S107 may further comprise applying another, different (e.g. acidic) etchant, after the first elements P.sub.1 of the resist have been removed, to dissolve the exposed elements of the metal layer. Suitable acidic etchants includes sulphuric acid and nitric acid. For instance, where the metal layer comprises copper, copper alloy (at least 50% Cu), chromium or chromium alloy (at least 50% Cr), an acidic second etchant may be used.
(49) The above method has been found to produce particularly high resolution image patterns with good edge definition. It is believed this is due to the solubility contrast achieved between the first and second pattern elements of the resist 12 being greater than previously achieved in conventional methods.
(50) Experiments have shown that the achievable resolution, i.e. the minimum dimensions of the pattern elements that can be obtained, depend on many variables including the thickness of resist layer 12, the etchant concentration and the etching time, but initial studies indicate that the resist thickness and etching time appear to be particularly significant important factors. In one example, a sample was manufactured in accordance with the above-described method, having a metal layer 11 of aluminium and a resist layer 12 of V215 as a 10% (w/w) solution in cyclopentanone applied by a kbar drawdown with a thickness of around 0.6 microns. The resist layer was exposed to UV radiation through a mask for approximately a second, using a Primarc unit comprising a mercury halide lamp with a power of around 150 W/cm. The exposed substrate web was immersed in an etchant comprising 15% w/w/NaOH solution at room temperature for 20 seconds, which was found to achieve good line definition in the metal layer, achieving demetallised line widths of the order of 3 microns.
(51) The thickness of the resist also has an impact on achievable line width, thinner coatings requiring a shorter etching time. As such, it is preferred that the resist be applied to the substrate using a method which achieves a substantially even coat weight across the area of the web such as using a post metered slot die. Thinner resist layers also exhibit less undercutting of the mask, i.e. reduced lateral spread of the reacted region. As such, preferred resist layers have a thickness of less than 1 micron, more preferably between 0.2 and 0.6 microns. Particularly good results have been obtained using a resist coating of approximately 0.35 microns.
(52) At the end of step S107, the result is an image pattern made up of second pattern elements P.sub.2 formed of metal layer 11, spaced by first pattern elements P.sub.1 where the metal is absent. Depending on the configuration of the elements, the image pattern could itself act as a security device, e.g. micro-text. Alternatively the image pattern could take the form of an image element array which can be incorporated into a security device such as a moir magnifier, integral imaging device or lenticular device by combining the so-formed image element array with an overlapping array of focusing elements such as lenses as will be discussed further below, or alternatively combined with some other viewing component such as a viewing grid or another image element array, e.g. to form a venetian blind device or a moir interference device (examples below).
(53) The above-described method could be performed batchwise, i.e. sequentially on individual substrate sheets, but more preferably the method is adapted for continuous production on a substrate web. FIGS. 6(a) and (b) show two examples of apparatus suitable for carrying out step S103 of the above method in a continuous manner. In the embodiment of FIG. 6(a), the substrate web W is conveyed around a roller 5 which incorporates the patterned mask 1, in this case carrying the mask 1 on its surface. A radiation source 6 is disposed inside the roller 5, which is at least partially transparent to the radiation, at least around a portion of the roller's circumference. The roller may be made from quartz, for example. The mask 5 may be formed in the surface of the roller itself or could take the form of an additional layer carried on its surface. For instance, the pattern could be engraved in the surface of the roller 5 and the engravings filled with radiation-opaque material to form the first pattern elements P.sub.1, the gaps therebetween forming second pattern elements P.sub.2. Alternatively the pattern may be formed as apertures defining second pattern elements P.sub.2 in an otherwise opaque sheet such as metal which is affixed to the roller surface.
(54) The substrate web W is arranged to make close contact between the resist layer 12 and the mask 1 as it is conveyed around the roller. This can be achieved by appropriate tensioning rollers 7a, 7b for example. The roller 5 rotates with the substrate web at substantially the same speed so that during exposure there is substantially no relative movement between the resist and the mask. The duration of exposure can be adjusted by changing the speed at which the web is conveyed, although typically a short exposure time of around 1 second is sufficient, depending on the power of the radiation source.
(55) FIG. 6(b) shows an alternative arrangement in which the patterned mask 1 takes the form of a belt which is supported by a number of rollers 5, in this case four. The mask belt 1 is opposed by a roller 9 (or other guide structure) and the substrate web S is conveyed through the nip between the two. The mask belt 1 is driven around the rollers 5 at substantially the same speed as the substrate web W. A radiation source 6 is positioned so as to irradiate the resist layer 12 through the mask 1 as it passes through the nip. As before, the mask is arranged so as to make close contact with the resist layer 12 during exposure. This can be achieved using tensioning rollers 7a, 7b to hold the substrate web closely against the roller 9, and positioning belt rollers 5 so as to wrap the belt 1 around a portion of the circumference of the roller 9 or hold it taught against a point on the roller surface.
(56) FIG. 6(b) also shows reel 8a from which the substrate web may be supplied at the start of the process, and reel 8b onto which the substrate web may be wound after exposure has taken place. In preferred embodiments, the substrate will be exposed to the reactant (step S104), e.g. sprayed with water, after exposure and then dried before being wound up onto reel 8b, e.g. by heating the web W to between 80 and 100 degrees C. The next step of activating the cross-linking agent (step S105) can then be performed offline, e.g. by placing the reel of exposed substrate web into an oven at the appropriate temperature for the predetermined time. In this way the exposure apparatus can be utilised for other processes while cross-linking takes place. The same applies to the alternative apparatus shown in FIG. 6a, where reels 8a and 8b may be provided at each end of the process line.
(57) After cross-linking, step S106 of flood exposing the substrate to the radiation is preferably performed using the same or like apparatus as shown in FIG. 6a or 6b, but in the absence of mask 1. Thus, the cross-linked reel of substrate web W may be loaded back on to unwind reel 8a and conveyed along the same transport path past radiation source 6. Since the mask is now absent, both the first and second pattern elements of the resist will now be exposed, as previously described. The substrate can then be passed through an etchant bath to perform step S107.
(58) FIG. 7 illustrates further optional but preferred steps in the manufacture of the image pattern and subsequent security device. Any one or more of the steps depicted in FIG. 7 could be added to the method already described with respect to FIG. 3. As explained below, step S109 of providing a colour layer could be performed at various different stages in the process and is therefore shown in dashed lines. FIGS. 8(a), (b) and (c) show corresponding examples of so-produced image patterns.
(59) As already discussed, FIG. 4(g) shows an example of a finished image pattern in one embodiment. The remaining portions of resist material 12 may be left in situ since they will not be visible when the image element array is viewed through substrate 10. However, in order to minimise the thickness of the finished security device it is preferred to remove the remaining resist and this can be achieved by applying a further etchant in which the non-exposed resist is soluble, and the metal layer is not (step S108). In the case of a DNQ resist, methyl ethyl ketone (MEK) etchant is suitable. The resulting structure is shown in FIG. 8(a).
(60) In the image patterns produced so far, the second pattern elements P.sub.2 will all have the same appearance (corresponding to that of metal layer 11), and the first pattern elements P.sub.1 will be transparent. This may be desirable in some implementations of security devices. However in many cases it is preferable to modify the optical characteristics of the first pattern elements P.sub.1 and this can be achieved, in one example, by applying a colour layer 13 over the patterned metal layer 11 (step S109), as shown in FIG. 8(b). The colour layer 13 comprises at least one optically detectable substance and is applied over at least a zone of the array. Whilst in preferred cases the colour layer will have a visible colour (i.e. visible to the naked eye), this is not essential since the at least one optically detectable substance could be, for example, a luminescent substance which emits outside the visible spectrum and is only detectable by machine. In general, the colour layer may comprise any of: one or more visible dyes or pigments; luminescent, phosphorescent or fluorescent substances; metallic pigments; interference layer structures or interference layer pigments (e.g. mica, pearlescent pigments, colour-shifting pigments etc.), for example. Substances such as these may be dispersed in a binder to form an ink, for example, suitable for application by printing or coating, or could be applied by other means such as vapour deposition. Most preferably the colour layer is applied by a printing technique such as: laser printing, inkjet printing, lithographic printing, gravure printing, flexographic printing, letterpress or dye diffusion thermal transfer printing. Since the high resolution detail of the image element array is provided by the metal layer 11, the colour layer 13 does not need to be applied using a high resolution process and can if desired be applied in more than one working. In place of printing or coating, the colour layer 13 over the metal layer 11, the colour layer could also be formed on another substrate and then laminated to or transferred onto the metal layer 11. It should be noted that a colour layer 13 can also be applied over remaining elements of the resist layer 12 if step S108 has been omitted.
(61) Meanwhile, step S110 is an example of a process step which may be performed to manufacture a security device 20 comprising an image pattern formed using the method already described (with or without steps S108 and/or S109). Here, the image pattern takes the form of an image element array and a security device such as a moir magnifier, integral imaging device or lenticular device is formed by combining the image element array with an overlapping array of focusing elements such as lenses (step S110), or alternatively with some other viewing component such as a viewing grid or another image element array, e.g. to form a venetian blind device or a moir interference device (examples below). An example of a focussing element array 21 is shown in FIG. 8(c) which therefore depicts an example of a security device 20. In this case the focusing element array is disposed on the opposite surface of the transparent substrate 10, e.g. by lamination or cast curing, although other constructions are also envisaged as described further below. It will also be appreciated that focusing element array 21 could be applied to the substrate web prior to the formation of the image element array, or at any stage during the above manufacturing process.
(62) FIGS. 9(a) to (e) illustrate steps in a further embodiment of the present method in which a colour layer 13 is provided, as mentioned above. FIG. 9(a) shows the substrate as it stands at the end of step S107, and FIG. 9(b) after optional step S108. In step S109, a colour layer 13 is applied as previously described (FIG. 9(c)). Since the colour layer 13 does not need to be applied at high resolution, it can be made relatively thick and therefore may possess sufficiently high optical density to produce a good quality image by itself. However, in some cases it is desirable to increase the optical density by applying a substantially opaque backing layer 14 over the colour layer 13 as depicted in FIG. 9(d). The backing layer 14 most preferably comprises a further metal layer, e.g. of aluminium. The provision of a backing layer 14 reduces the amount of light transmitted through the device which could otherwise confuse the final image, thereby improving the visual appeal and (in the case of a metal backing layer) making the colour of the first pattern elements, provided by colour layer 13, more reflective and therefore more intense.
(63) Finally, FIG. 9(e) shows the resulting image element array formed into a security device 20 by the application of focusing element array 21 (step S110) as previously described.
(64) The colour layer 13 could alternatively be provided by laminating the colour layer 13 over the demetallised layer 11 to achieve substantially the same structure as shown in FIG. 6(e). In still further embodiments the colour layer could be located differently within the device structure, provided that from one side of the structure both the metal pattern elements P.sub.2 and the portions of the colour layer 13 in the first pattern elements P.sub.1 can be seen alongside one another. FIGS. 9(i) and (ii) show two further exemplary security devices with different structures to illustrate this.
(65) In FIG. 9(i) the patterned metal layer 11 has been formed on the first surface of a transparent substrate 10 using the same method as previously described. The colour layer 13 is provided on the second surface of the transparent substrate so that when the structure is viewed from the side of the metal layer 11, the colour layer is visible through the gaps in the first pattern elements. Optionally, a substantially opaque backing layer 14 may be provided over the colour layer 13 on the second surface of the substrate as previously described. The so-formed assembly can then be laminated to a second transparent substrate 22 carrying focusing element assembly 21, the second substrate 22 providing the necessary optical spacing between the focussing elements and the image elements formed by metal layer 11 so as to place them substantially in the focal plane of the focusing elements. Preferably the thickness of the first substrate 10 is kept small so that the colour layer 13 is also close to the focal plane. It will be appreciated however that this structural configuration results in an increased device thickness as compared with that of FIG. 9(e).
(66) In FIG. 9(ii) the colour layer 13 is provided on the first surface of substrate 10 before the patterned metal layer 11 is formed on the same surface. That is, the colour layer 13 is disposed on the metallised substrate web between the substrate and the metal layer 11 provided in step S101 of the above-described method. A substantially opaque backing layer 14 may optionally also be provided under the colour layer 13 on the first surface, or on the second surface of the substrate 10 (not shown). In these embodiments, the substrate 10 need not be transparent since the image element array will not be viewed through it in the finished device. A focusing element array 21 on a second (transparent) substrate 22 can then be laminated to the image element array to form the security device 20. Again, the end thickness of the device will be greater than that achievable in the FIG. 9(e) embodiment.
(67) Embodiments in which the demetallised pattern is formed on a substrate with a pre-existing colour layer 13 (whether located on the first or second surface of the substrate) are better adapted for use in circumstances where no registration is desired between the colour layer 13 and the demetallised pattern, since it is technically more straightforward to register the application of the colour layer 13 to an existing demetallised pattern than vice-versa.
(68) In many implementations, the uniformly metallic appearance of the second pattern elements P.sub.2 will be desirable. However, the specularly reflective nature of the metal layer 11 can have the result that the appearance of the elements will depend significantly on the nature of illumination. As such in some embodiments it is preferred to reduce the degree of specular reflection by providing a filter layer 15 (FIG. 10) in the form of a light diffusing layer which will ultimately sit between the metal layer and the viewer, acting to diffuse the light reflected by the metal pattern elements P.sub.2 and hence improve the light source invariance of the finished device. The light diffusing layer 15 is located between the transparent substrate 10 and the metal layer 11 and may therefore be incorporated already in the metallised substrate web provided at the start of the method. Alternatively, if the metallisation is carried out as part of the method, the light diffusing layer 15 may be applied to the substrate in an earlier step. The light diffusing layer can comprise a scattering pigment dispersed in a binder and may be coloured or colourless. The layer can be applied by coating or printing, preferably flexographic, gravure, lithographic or digital printing, and may optionally be a radiation-curable material, e.g. requiring UV-curing. In some embodiments, the appearance of the light diffusing layer 15 may be uniform across the image element array. However in other cases the light diffusing layer could comprise multiple different materials arranged as a multi-coloured pattern or image. The light-diffusing layer need not be applied with high resolution and so can be formed of multiple workings if desired.
(69) In still further embodiments, the filter layer 15 may not be light-diffusing (i.e. optically scattering), but may comprise a clear, coloured material which can be used to modify the appearance of the metal pattern elements. For example, by providing a filter layer 15 having an orange/brown tint in combination with a metal layer 11 of aluminium, the metal takes on the appearance of copper. The tinted filter layer 15 could be applied to selected regions only (optionally with a clear colourless layer in other areas) to give a bimetallic effect.
(70) The filter layer 15 will typically not be soluble in the etchant used in step S107 and so will typically remain across the whole image array once the metal layer 11 has been patterned, as shown in FIG. 10(a). If the filter layer is sufficiently translucent such that a contrast can still be observed between the first and second pattern elements P.sub.1, P.sub.2, this may be acceptable and the light diffusing layer may remain across both sets of elements in the final array. However, generally it is preferred to remove the filter layer 15 from the first pattern elements and this can be achieved by applying a suitable further etchant in which the filter is soluble. The result is shown in FIG. 10(b). A colour layer 13 can then be applied if desired, followed by an optional backing layer 14 (both as described above), as shown in FIG. 10(c).
(71) Since the filter layer 15 is backed up by metal layer 11, it is not required to be of high optical density, although it should act to diffuse and/or to tint or selectively absorb and reflect different colours. Consequently the filter layer 15 can be made thin and this is preferred in order to minimise undercutting of the filter layer during etching. Preferably, the thickness of the filter layer 15 should be equal to or less than the minimum dimension (e.g. line width) of the pattern elements P.sub.1, P.sub.2, more preferably half that dimension or less. For example, if the pattern elements P.sub.1 or P.sub.2 have a dimension of 1 micron, the filter layer should preferably be no thicker than 1 micron, more preferably no thicker than 0.5 microns.
(72) Like the (optional) filter layer 15, the colour layer 13 may have a uniform appearance across the array, or at least a zone of the array in which it is provided, in which case the finished image element array will be duotone (unless a multi-coloured light diffusing layer is provided). This will be desirable in certain types of security device. However, to increase the complexity and security level of the device, it is preferred that the colour layer 13 comprises multiple zones each comprising different optically detectable substances, e.g. being of different visible colours. The arrangement of different zones may be highly complex, e.g. representing a photograph, or may comprise a simpler arrangement of larger distinct zones. Preferably the colour layer 13 displays an image or indicia (e.g. letters, numbers or symbols) either through the relative arrangement of the zones and/or by the periphery of the whole colour layer (i.e. the combined periphery of the zones). In the ensuing examples, different zones of the colour layer 13 will be described for simplicity as having different colours but as noted above whilst in preferred cases these will be different visible colours, this is not essential as the optically detectable substances could be machine readable only. The term colour is also intended to include achromatic appearances such as black, grey, white, silver etc., as well as red, green, blue, cyan, magenta, yellow etc.
(73) FIG. 11(a) shows an embodiment of an image element array formed using the method described above in relation to FIGS. 9(a) to 9(d) (omitting the provision of a backing layer), in which the colour layer 13 comprises two zones 13a, 13b of different colours. In this case, each zone is significantly larger than the dimensions of the pattern elements P.sub.1, P.sub.2, as is preferred for use in moir magnifier and integral imaging devices to avoid the colours of adjacent zones being averaged together by the synthetic magnification mechanism. In zone 13a, the first pattern elements P.sub.1 possess a first colour, provided by the colour layer 13, and in zone 13b, the first pattern elements P.sub.1 possess a second colour. In preferred examples, the pattern defined by metal layer 11 comprises an array of negative microimages; that is, the first pattern elements P.sub.1 take the form of the microimages and the metallic second pattern elements P.sub.2 provide the surrounding background (in practice this may be a single contiguous area rather than multiple distinct elemental areas). Hence, the microimages have a first colour in zone 13a and a second different colour in zone 13b. Preferably each of the zones is sufficiently large so as to encompass a plurality of the microimagesgenerally at least 10 but in many cases tens or hundreds of the microimages. The individual microimages will be small such that they cannot be resolved by the naked eye (in the absence of focusing elements), whereas the colour layer zones are preferably sufficiently large to be discernible by eye without magnification. For example, each microimage may have an overall lateral dimension of between 15 and 30 microns, whilst each zone may have dimensions of the order of millimetres, e.g. 2 to 3 mm or larger. Examples of visual effects which can be achieved using colour layers of this sort will be described below.
(74) In the example shown in FIG. 11(a) the colour layer 13 extends across the whole image element array. However this is not essential and in other preferred examples the colour layer may not cover the whole array, e.g. so as to define indicia by way of its periphery. First pattern elements P.sub.1 falling outside the colour layer 13 may remain transparent. Alternatively, if a backing layer 14 is provided this may extend beyond the colour layer 13 as shown in FIG. 11(b). The backing layer 14 may have an appearance similar to that of the metal layer 11, particularly if they are both metal layers, in which case the contrast between the first and second pattern elements P.sub.1, P.sub.2 may be diminished or eliminated in areas outside the colour layer. This can be made use of to produce particular visual effects as will be exemplified below.
(75) A portion of an exemplary image pattern is shown in FIG. 12, which is a photograph taken in transmission. Here, a microimage in the form of a digit 4 is formed by a second image element P.sub.2 in which the metal of metal layer 11 remains present and its surroundings are defined by a first image element P.sub.1 where the metal has been removed. The microimage could form part of a security device comprising microtext, or could be part of an image element array, e.g. suitable for use in a moir magnification device. As will be seen from the annotation, the line width of the digit 4 is approximately 5.49 microns. Significantly, it will be noted that the edges of the digit 4 are well-defined and smooth with a low edge roughness.
(76) Another embodiment of a security device will now be described with reference to FIGS. 13(a) and (b). In this case the security device is a moir magnifier, comprising an image element array formed using the methods described above defining an array of microimages and an overlapping focussing element array with a pitch or rotational mismatch as necessary to achieve the moir effect. FIG. 13(a) depicts part of the image element array as it would appear without the overlapping focusing element array, i.e. the non-magnified microimage array (but shown at a greatly increased scale for clarity). In contrast, FIG. 13(b) depicts the appearance of the same portion of the completed security device, i.e. the magnified microimages, seen when viewed with the overlapping focussing element array, at one viewing angle.
(77) In this example, the microimage array is formed using the methods described above and has a cross section corresponding substantially to that shown in FIG. 11(a). FIG. 13(a) shows the patterned metal layer 11 and underlying colour layer 13 in plan view and it will be seen that the first pattern elements P.sub.1 form a regular array of microimages 31a which here each convey the digit 5. In this case all of the microimages are of identical shape and size. The metallic second pattern elements P.sub.2 form a contiguous, uniform background surrounding the microimages. Since the colour layer 13 has two zones of different colour, the microimages 31a in zone 13a appear in a first colour (here represented as black), whilst those in zone 13b appear in a second colour (here represented as white).
(78) FIG. 13(b) shows the completed security device 30, i.e. the image element array 31 shown in FIG. 13(a) plus an overlapping focusing element array 33, from a first viewing angle which here is approximately normal to the plane of the device 30. It should be noted that the security device is depicted at the same scale as used in FIG. 13(a): the apparent enlargement is the effect of the focusing element array 33 now included. The moir effect acts to magnify the microimage array such that magnified versions of the microimages 31a are displayed. In this example just two of the magnified microimages 34a, 34b are shown. In practice, the size of the enlarged images and their orientation relative to the device will depend on the degree of mismatch between the focussing element array. This will be fixed once the focusing element array is joined to the image element array. In this example, the first magnified microimage 34a is formed from microimages all within zone 13a and hence appears black whilst the second magnified microimage 34b is from microimages all within zone 13b and hence appears white. Upon tilting the magnified microimages 34 may appear to change colour since their position relative to the device will change and they may cross into the other zone of colour layer 13.
(79) In the above examples of security devices, the microimages 31 are all identical to one another, such that the devices can be considered pure moir magnifiers. However, the same principles can be applied to hybrid moir magnifier/integral imaging devices, in which the microimages depict an object or scene from different viewpoints. Such microimages are considered substantially identical to one another for the purposes of the present invention. An example of such a device is shown schematically in FIG. 14, where FIG. 14(a) shows the unmagnified microimage array, without the effect of focusing elements 33, and FIG. 14(b) shows the appearance of the finished device, i.e. the magnified image. As shown in FIG. 14(a), the microimages 31 show an object, here a cube, from different angles. It should be noted that the microimages are formed as demetallised lines corresponding to the black lines of the cubes in the Figure, the remainder of the metal layer being opaque although this is shown in reverse in the Figure for clarity. A colour layer 13 is provided, here in the form of a single hexagonal zone, which provides colour to the demetallised lines and is concealed by the metal layer elsewhere. Outside the colour layer 13, the microimages may in practice not be visible due to a lack of contrast between the metal layer 11 and a backing layer 14 as previously mentioned. In the magnified image (FIG. 14(b)), the moir effect generates magnified, 3D versions of the cube labelled 34. In reality, only those lines of the magnified cubes 34 which coincide with the colour layer 13 will be visible whilst those portions outside the coloured zone will be invisible or only weakly visible. As the device is tilted the magnified cubes 34 will appear to move across the device and so enter or leave the coloured zone 13 depending on their location and the degree of tilt. This gives the visual impression of the magnified images appearing and disappearing as they move across the central portion of the device. This, combined with the 3D appearance of the images, amounts to an effect with significant visual impact.
(80) FIG. 15 depicts a further embodiment of a security device 40, which here is a lenticular device. A transparent substrate 10 is provided on one surface with an array of focussing elements 43, here in the form of cylindrical lenses, and on the other surface with an image element array preferably formed of a patterned metal layer 11 and colour layer 13 as described above. The image array comprises first pattern elements P.sub.1, and second pattern elements P.sub.2. The size and shape of each first pattern element P.sub.1 is substantially identical. The pattern elements in this example are elongate image strips and so the overall pattern of elements is a line pattern, the elongate direction of the lines lying substantially parallel to the axial direction of the focussing elements 43, which here is along the y-axis. The lateral extent of the pattern (including its elements P.sub.1 and P.sub.2) is referred to as the array area.
(81) As shown best in the cross-section of FIG. 15(b), the pattern formed in metal layer 11 and the focussing element array have substantially the same periodicity as one another in the x-axis direction, such that one first pattern element P.sub.1 and one second pattern element P.sub.2 lies under each lens 43. In this case, as is preferred, the width w of each element P.sub.1, P.sub.2 is approximately half that of the lens pitch. Thus approximately 50% of the array area carries first pattern elements P.sub.1 and the other 50% corresponds to second pattern elements P.sub.2. In this example, the image array is registered to the lens array 43 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first pattern element P.sub.1 lies under the left half of each lens and a second pattern element P.sub.2 lies under the right half. However, registration between the lens array 43 and the image array in the periodic dimension is not essential.
(82) The colour layer 13 can take any form, including that of a complex, multi-coloured image such as a photograph.
(83) When the device is viewed by a first observer O.sub.1 from a first viewing angle, each lens 43 will direct light from its underlying first pattern element P.sub.1 to the observer, with the result that the device as a whole appears to display the appearance of the colour layer 13, which in this case carries a star shaped image as shown in FIG. 14(c) which constitutes an image I.sub.1. When the device is tilted so that it is viewed by second observer O.sub.2 from a second viewing angle, now each lens 43 directs light from the second pattern elements P.sub.2 to the observer. As such the whole device will now appear uniformly metallic as shown in FIG. 15(d). This is referred to more generally as image I.sub.2 since in other examples if a patterned light-diffusing layer were provided over the metal layer (as described in previous embodiments), the second pattern elements P.sub.2 may collectively display any image according to that provided by the light-diffusing layer. Hence, as the security device is tilted back and forth between the positions of observer O.sub.1 and observer O.sub.2, the appearance of the device switches between image I.sub.1 and image I.sub.2.
(84) In order to achieve an acceptably low thickness of the security device (e.g. around 70 microns or less where the device is to be formed on a transparent document substrate, such as a polymer banknote, or around 40 microns or less where the device is to be formed on a thread, foil or patch), the pitch of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width of the pattern elements is preferably no more than half such dimensions, e.g. 35 microns or less.
(85) Two-dimensional lenticular devices can also be formed, in which the optically variable effect is displayed as the device is tilted in either of two directions, preferably orthogonal directions. Examples of patterns suitable for forming image arrays for such devices include forming the second pattern elements P.sub.2 as grid patterns of dots, with periodicity in more than one dimension, e.g. arranged on a hexagonal or orthogonal grid. For instance, the second pattern elements P.sub.2 may be square and arranged on an orthogonal grid to form a checkerboard pattern with resulting square first pattern elements P.sub.1 in which the colour layer 13 is visible. The focusing elements in this case will be spherical or aspherical, and arranged on a corresponding orthogonal grid, registered to the image array in terms of orientation but not necessarily in terms of translational position along the x or y-axes. If the pitch of the focussing elements is the same as that of the image array in both the x and y directions, the footprint of one focussing element will contain a 2 by 2 array of pattern elements. From an off-axis starting position, as the device is tilted left-right, the displayed image will switch as the different pattern elements are directed to the viewer, and likewise the same switch will be exhibited as the device is tilted up-down. If the pitch of the focusing elements is twice that of the image array, the image will switch multiple times as the device is tilted in any one direction.
(86) Similar effects can be achieved with other two dimensional arrays of pattern elements, e.g. using second pattern elements P.sub.2 which are circular rather than square. Any other dot shape could alternatively be used, e.g. polygonal. The patterns could of course be reversed such that it is the second pattern elements define the surroundings of negative dots in which the colour layer 13 is visible.
(87) Lenticular devices can also be formed in which the two or more images (or channels) displayed by the device at different angles do not correspond exclusively to the first pattern elements on one hand and the second pattern elements on the other. Rather, both pattern elements are used in combination to define sections of two or more images, interleaved with one another in a periodic manner. Thus, in an example the first pattern elements may correspond to the black portions of a first image and those of a second image, whilst the second pattern elements may provide the white portions of the same images, or vice versa. Of course the images need not be black and white but could be defined by any other pair of colours with sufficient contrast. Sections of the first and second images are interleaved with one another in a manner akin to the pattern of lines shown in FIG. 15. When the device is tilted the two images will be displayed over different ranges of angles giving rise to a switching effect. More than two images could be interleaved in this way in order to achieve a wide range of animation, morphing, zooming effects etc. In embodiments such as these the colour layer 13 preferably has a uniform appearance (e.g. single colour) across the array as does any light-diffusion layer provided resulting in a duo-tone appearance.
(88) In all of the above examples of security devices, a focusing element array is employed to co-operate with the image element array to generate an optically variable effect. However, this is not essential and FIG. 16 shows some examples of security devices with image element arrays made using the above described methods which do not require focussing element arrays. In these examples, two image element arrays are manufactured using the above-described methods, one on each surface of the substrate material 10, as will be described further below with reference to FIG. 17. However in each case it will be appreciate that just one or other of the described image arrays 11a, 11b need be formed using this technique and the other could be formed using any other available method, e.g. printing.
(89) FIG. 16(a) shows a security device 20 which operates on similar principles to those of the lenticular device described above with respect to FIG. 15, but utilising two demetallised image element arrays 11a, 11b rather than a single image element array combined with a focusing element array. In this case, one image element array 11a formed on a first surface of transparent substrate 10 forms a masking grid of metal lines P.sub.2 spaced by gaps P.sub.1, whilst the other image element array 11b formed on the second surface of transparent substrate 10 exhibits a pattern comprising a sequence of image components, labelled A, B, C, etc. Each of the complete images A, B, C, etc from which the image elements are taken is shown under the cross-section of the device and it will be seen that these comprise a sequence of animation steps depicting a star symbol changing in size. To create the pattern formed in metal layer 11b, the five images A to E are split into elements or slices and interleaved with one another so that a slice of image A is positioned next to a slice of image B, which in turn is positioned next to a slice of image C, and so forth. The resulting pattern is formed on a mask and transferred to a resist layer 12 on metal layer 11b in the manner described above, before etching as appropriate. On the opposite side of transparent substrate 10, a masking grid is formed by patterning metal layer 11a using the same method via a different patterned mask resulting in a spaced array of visually opaque lines P.sub.1 with intervening transparent portions P.sub.2 through which the pattern in metal layer 11b may be viewed.
(90) The device could be designed to be viewed in reflected or transmitted light. Transmitted light is preferred since the contrast in the image can generally be perceived more clearly and in addition the same visual effect can be viewed from both sides of the device. When the device is viewed from above the masking grid 11a, at any one instant, the image slices from only one of the images A to E are visible. For example, in the configuration shown in FIG. 16(a), when the device is viewed straight-on, only the image slices forming image E will be visible, and thus the device as a whole will appear to exhibit a complete reproduction of image E. Provided the dimensions of the device are correctly selected, when the device is observed from different angles, different images will become visible. For example, when the device is viewed from position A, only the image slices forming image A will be visible through the masking grid 11a, the device as a whole whereby exhibiting the complete image A. Similarly, when the device is viewed from position C only the image slices forming image C will be visible. As such, as the device is tilted and the viewer observes it at different angles, different stages of the animation will be seen and, provided the images are printed in the correct sequence, an animation will be perceived. In the present example this will appear as a star symbol increasing or decreasing as the device is tilted. Thus, in this case the animation is perceived as a zooming in and out but in other cases the images could be arranged to depict, for example, perceived motion (e.g. a horse galloping), morphing (e.g. a sun changing into a moon) or perceived 3D depth (by providing multiple images of the same object, but from slightly different angles). Of course, in other examples, fewer images (e.g. 2) could be interleaved resulting in a switch from one image to another at certain tilt angles, rather than an animation effect.
(91) In order to achieve this effect, the width of each image slice, X, must be smaller than the thickness, t, of the transparent support layer 10, preferably several times smaller, such that there is a high aspect ratio of the thickness t to image slice width X. This is necessary in order that a sufficient portion of the pattern on metal layer 11b can be revealed through tilting of the device. If the aspect ratio were too low, it would be necessary to tilt the device to very high angles before any change in image will be perceived. In a preferred example, each image slice has a width X of the order of 5 to 10 m, and the thickness t of the support layer 10 is approximately 25 to 35 m. The use of the above-described demetallisation process to form the pattern 11b is therefore particularly advantageous since the high resolution nature of the process allows the formation of image elements at these small dimensions.
(92) The dimensions of the masking grid 11a are generally larger than those of the pattern elements 11b, requiring opaque stripes of width ((n1)X) where n is the number of images to be revealed (here, five), spaced by transparent regions of approximately the same width as that of the image slices (X). Thus, in this example the opaque regions P.sub.2 of the masking grid 11a have a width of around 20 to 40 m and hence could alternatively be produced using conventional techniques such as printing.
(93) FIG. 16(b) shows a further embodiment of a venetian blind-type security device in cross-section, comprising first and second patterned metal layers 11a and 11b positioned on either surface of a transparent substrate 10. Metal layer 11a has been demetallised according to a first pattern P.sub.a whereas metal layer 11b has been exposed to a second pattern P.sub.b. In this example, the device has two laterally offset regions A and B. In region A, the exposed pattern elements of pattern P.sub.a and pattern P.sub.b are identical and aligned with one another. In area B the patterns P.sub.a and P.sub.b are identical in pitch but 180 out of phase with one another such that the remaining regions of the first metal layer 11a forming pattern P.sub.a align with the removed regions of the second metal layer 11b forming second pattern P.sub.b, and vice versa.
(94) When viewed in transmission from directly above, observer (i) will perceive region A as having a lower optical density than region B where light transmission is blocked by the interplay between the two patterns. In contrast, when viewed from an angle at the position of observer (ii), area A will appear relatively dark compared with area B, since light will now be able to pass through aligned transparent regions of patterns P.sub.a and P.sub.b in area B, whereas it will be blocked by the alignment between pattern elements in area A. This contrast flip between areas A and B provides an easily testable, distinctive effect. In order for the switch to be observable at relatively low tilt angles, the aspect ratio of the support layer thickness relative to the spacing of the pattern elements should again be at least one-to-one. It should be noted that it is not essential to ensure an entirely accurate registration between the two patterns P.sub.a and P.sub.b since provided the sizing of the pattern elements is correct, a switch in contrast between the two regions will still be visible as the device is tilted.
(95) FIG. 16(c) shows a further embodiment of a security device in cross-section which here takes the form of a moir interference device. In this embodiment, two patterned metal layers 11a, 11b are provided as on either side of transparent substrate 10 but as in the previous embodiments, one or other of the patterns provided by the metal layers could be provided by other means such as printing.
(96) To form a moir interference device, each of the metal layers 11a, 11b carries a pattern of elements, mismatches between the two patterns combining to form moir interference fringes. In the example shown, each of the patterns P.sub.a and P.sub.b consists of an array of line elements, with those of one pattern rotated relative to those of the other. In other cases, the mismatch could be provided by a pitch variation rather than a rotation, and/or isolated distortions within one or other of the patterns. When viewed from above such that the two patterns are viewed in combination with one another, moir interference bands are visible and these will appear to move relative to the device depending on the viewing angle. This is due to the precise portions of the two patterns which appear to overlap changing as the viewing angle changes. For instance, in the example of FIG. 25, when viewed directly from above, portion a of pattern P.sub.a will appear to overlap and therefore interfere with portion b of pattern P.sub.b, whereas at a second viewing angle illustrated by observer (ii), the same portion a of pattern P.sub.a will appear to overlap and therefore interfere with a different portion c of the second pattern P.sub.b. In order to achieve significant perceived motion at relatively low viewing angles, a high aspect ratio of the spacing between the two patterns (represented by the thickness t of support layer 10) relative to the spacing s of the line elements in each of the patterns is required. For example, where the line elements have a width and spacing of around 5 m, a thickness t of around 25 m is suitable. No registration between the two patterns P.sub.a and P.sub.b is required.
(97) The security device structures shown in FIGS. 16(a), (b) and (c) are preferably formed by carrying out the above-described demetallisation method on both sides of a transparent substrate. FIG. 17(a) shows a first example of apparatus which may be used to produce both patterned metal layers. As shown, the substrate web W provided in step S101 may include a second metal layer 11b located on the second surface of the substrate 10, which may be of the same composition as the first metal layer 11a, or may be different. Preferably, however, the second metal layer 11b is soluble in the same etchant substance as the first metal layer 11a. A second photosensitive resist layer 12b is applied over the second metal layer 11b and again this may be of the same composition as the first photosensitive resist layer 12a or may be different. In the FIG. 17(a) embodiment, both resist layers 12a, 12b are then exposed simultaneously to radiation through respective patterned masks 5a, 5b each carrying a pattern P.sub.a, P.sub.b of opaque and transparent elements in the manner previously described. The two patterns P.sub.a, P.sub.b may be the same as one another or different, and/or may be laterally offset from one another (in the transport path direction and/or in the orthogonal direction), depending on the desired optical effect. In this example the two masks 5a, 5b are shown supported on respective opposing rollers in a manner correspond to that described above in relation to FIG. 6(a) but alternatively one or both of the masks 5a, 5b could be provided in the form of a belt as shown in FIG. 6(b). For example, roller 9 of FIG. 6(b) could be a patterning mask as shown in FIG. 6(a) carrying pattern P.sub.a whilst pattern P.sub.b is carried on the belt 5 supported on rollers 5 as shown.
(98) Alternative apparatus for patterning metal layers 11a, 11b on both sides of a transparent substrate is shown in FIG. 17(b). Here the two resist layers 12a, 12b are exposed sequentially rather than simultaneously but still preferably in register with one another. In this case, a second patterning roller 5b is positioned downstream of a first patterning roller 5a with the transport path arranged to include a portion of the circumferential surface of both patterning rollers 5a, 5b. Sequential exposure in this way may not achieve the same levels of registration between the two patterns as in the FIG. 17(a) embodiment, but may reduce the risk of slippage occurring between the masks and the substrate web W.
(99) In still further examples, security devices including those discussed above in relation to FIG. 16 could be formed by producing two demetallised image element arrays on separate transparent substrates 10 using the above described method, and then laminating them together such that the two metal layers are spaced apart by the two transparent substrates.
(100) Security devices of the sorts described above are suitable for forming on security articles such as threads, stripes, patches, foils and the like which can then be incorporated into or applied onto security documents such as banknotes and examples of this will be provided further below. However the security devices can also be constructed directly on security documents which are formed of a transparent document substrate, such as polymer banknotes. In such cases, the image pattern may be manufactured on a first substrate, using the method discussed above, and then transferred onto or affixed to one surface of the document substrate, optionally using a transparent adhesive. This may be achieved by foil stamping, for example. An exemplary structure is shown in FIG. 18 where substrate 46 is the transparent document substrate, e.g. BOPP, and layer 47 is an adhesive used to join the image array comprising metal layer 11, colour layer 13 and backing layer 14 (all formed previously) to the substrate. Alternatively, the demetallised pattern array could be formed directly on the document substrate 46 by providing a metal layer on the surface of the substrate 46 (optionally across selected portions only), and performing the above-described method on substrate 46 to form an image element array thereon. Focusing element array 48 can be applied to the opposite side of document substrate 46, e.g. by embossing or cast-curing, before or after the image element array is applied.
(101) Security devices of the sorts described above can be incorporated into or applied to any product for which an authenticity check is desirable. In particular, such devices may be applied to or incorporated into documents of value such as banknotes, passports, driving licences, cheques, identification cards etc. The image array and/or the complete security device can either be formed directly on the security document (e.g. on a polymer substrate forming the basis of the security document) or may be supplied as part of a security article, such as a security thread or patch, which can then be applied to or incorporated into such a document.
(102) Such security articles can be arranged either wholly on the surface of the base substrate of the security document, as in the case of a stripe or patch, or can be visible only partly on the surface of the document substrate, e.g. in the form of a windowed security thread. Security threads are now present in many of the world's currencies as well as vouchers, passports, travellers' cheques and other documents. In many cases the thread is provided in a partially embedded or windowed fashion where the thread appears to weave in and out of the paper and is visible in windows in one or both surfaces of the base substrate. One method for producing paper with so-called windowed threads can be found in EP-A-0059056. EP-A-0860298 and WO-A-03095188 describe different approaches for the embedding of wider partially exposed threads into a paper substrate. Wide threads, typically having a width of 2 to 6 mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.
(103) The security article may be incorporated into a paper or polymer base substrate so that it is viewable from both sides of the finished security substrate at at least one window of the document. Methods of incorporating security elements in such a manner are described in EP-A-1141480 and WO-A-03054297. In the method described in EP-A-1141480, one side of the security element is wholly exposed at one surface of the substrate in which it is partially embedded, and partially exposed in windows at the other surface of the substrate.
(104) Base substrates suitable for making security substrates for security documents may be formed from any conventional materials, including paper and polymer. Techniques are known in the art for forming substantially transparent regions in each of these types of substrate. For example, WO-A-8300659 describes a polymer banknote formed from a transparent substrate comprising an opacifying coating on both sides of the substrate. The opacifying coating is omitted in localised regions on both sides of the substrate to form a transparent region. In this case the transparent substrate can be an integral part of the security device or a separate security device can be applied to the transparent substrate of the document. WO-A-0039391 describes a method of making a transparent region in a paper substrate. Other methods for forming transparent regions in paper substrates are described in EP-A-723501, EP-A-724519, WO-A-03054297 and EP-A-1398174.
(105) The security device may also be applied to one side of a paper substrate, optionally so that portions are located in an aperture formed in the paper substrate. An example of a method of producing such an aperture can be found in WO-A-03054297. An alternative method of incorporating a security element which is visible in apertures in one side of a paper substrate and wholly exposed on the other side of the paper substrate can be found in WO-A-2000/39391.
(106) Examples of such documents of value and techniques for incorporating a security device will now be described with reference to FIGS. 19 to 22.
(107) FIG. 19 depicts an exemplary document of value 50, here in the form of a banknote. FIG. 19a shows the banknote in plan view whilst FIG. 19b shows a cross-section of the same banknote along the lines X-X. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 51. Two opacifying layers 53 and 54 are applied to either side of the transparent substrate 51, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 51.
(108) The opacifying layers 53 and 54 are omitted across a selected region 52 forming a window within which a security device is located. In FIG. 19(b), the security device is disposed within window 52, with a focusing element array 48 arranged on one surface of the transparent substrate 51, and image element array 11 on the other (e.g. as in FIG. 18 above). As described in relation to FIG. 18, the image element array 11 could be manufactured on a separate substrate which is then laminated to the document substrate 51 in the window region, or could be manufactured directly on the document substrate 51 by metallising the substrate 51 (at least in the window region 52, optionally all over the substrate) and then forming a demetallised pattern in the metal layer using the above-described method.
(109) It will be appreciated that, if desired, the window 52 could instead be a half-window, in which one of the opacifying layers (e.g. 53 or 54) is continued over all or part of the image array 11. Depending on the opacity of the opacifying layers, the half-window region will tend to appear translucent relative to surrounding areas in which opacifying layers 53 and 54 are provided on both sides.
(110) In FIG. 20 the banknote 50 is a conventional paper-based banknote provided with a security article 55 in the form of a security thread, which is inserted during paper-making such that it is partially embedded into the paper so that portions of the paper 56 lie on either side of the thread. This can be done using the techniques described in EP0059056 where paper is not formed in the window regions during the paper making process thus exposing the security thread 55 in window regions 57 of the banknote. Alternatively the window regions 57 may for example be formed by abrading the surface of the paper in these regions after insertion of the thread. It should be noted that it is not necessary for the window regions 57 to be full thickness windows: the thread 55 need only be exposed on one surface if preferred. The security device is formed on the thread 55, which comprises a transparent substrate a focusing array 21 provided on one side and an image array 11 provided on the other. Windows 57 reveal parts of the device, which may be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, with different or identical images displayed by each.
(111) In FIG. 21, the banknote 50 is again a conventional paper-based banknote, provided with a strip element or insert 58. The strip 58 is based on a transparent substrate and is inserted between two plies of paper 56a and 56b. The security device is formed by a lens array 21 on one side of the strip substrate, and an image array 11 on the other. The paper plies 56a and 56b are apertured across region 59 to reveal the security device, which in this case may be present across the whole of the strip 58 or could be localised within the aperture region 59. It should be noted that the ply 56a need not be apertured and could be continuous across the security device.
(112) A further embodiment is shown in FIG. 22 where FIGS. 22(a) and (b) show the front and rear sides of the document 50 respectively, and FIG. 22(c) is a cross section along line Z-Z. Security article 58 is a strip or band comprising a security device according to any of the embodiments described above. The security article 58 is formed into a security document 50 comprising a fibrous substrate 56, using a method described in EP-A-1141480. The strip is incorporated into the security document such that it is fully exposed on one side of the document (FIG. 22(a)) and exposed in one or more windows 59 on the opposite side of the document (FIG. 22(b)). Again, the security device is formed on the strip 58, which comprises a transparent substrate with a lens array 21 formed on one surface and a co-operating image array 11 as previously described on the other
(113) Alternatively a similar construction can be achieved by providing paper 56 with an aperture 59 and adhering the strip element 58 onto one side of the paper 56 across the aperture 59. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.
(114) In still further embodiments, a complete security device could be formed entirely on one surface of a security document which could be transparent, translucent or opaque, e.g. a paper banknote irrespective of any window region. The image array 11 can be affixed to the surface of the substrate, e.g. by adhesive or hot or cold stamping, either together with a corresponding focusing element array 21 or in a separate procedure with the focusing array 21 being applied subsequently.
(115) In general when applying a security article such as a strip or patch carrying the security device to a document, it is preferable to bond the article to the document substrate in such a manner which avoids contact between those focusing elements, e.g. lenses, which are utilised in generating the desired optical effects and the adhesive, since such contact can render the lenses inoperative. For example, the adhesive could be applied to the lens array(s) as a pattern that leaves an intended windowed zone of the lens array(s) uncoated, with the strip or patch then being applied in register (in the machine direction of the substrate) so the uncoated lens region registers with the substrate hole or window.
(116) The security device of the current invention can be made machine readable by the introduction of detectable materials in any of the layers or by the introduction of separate machine-readable layers. Detectable materials that react to an external stimulus include but are not limited to fluorescent, phosphorescent, infrared absorbing, thermochromic, photochromic, magnetic, electrochromic, conductive and piezochromic materials.
(117) Additional optically variable devices or materials can be included in the security device such as thin film interference elements, liquid crystal material and photonic crystal materials. Such materials may be in the form of filmic layers or as pigmented materials suitable for application by printing. If these materials are transparent they may be included in the same region of the device as the security feature of the current invention or alternatively and if they are opaque may be positioned in a separate laterally spaced region of the device.
(118) The presence of a metallic layer in the security device can be used to conceal the presence of a machine readable dark magnetic layer, or the metal layer itself could be magnetic. When a magnetic material is incorporated into the device the magnetic material can be applied in any design but common examples include the use of magnetic tramlines or the use of magnetic blocks to form a coded structure. Suitable magnetic materials include iron oxide pigments (Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4), barium or strontium ferrites, iron, nickel, cobalt and alloys of these. In this context the term alloy includes materials such as Nickel:Cobalt, Iron:Aluminium:Nickel:Cobalt and the like. Flake Nickel materials can be used; in addition Iron flake materials are suitable. Typical nickel flakes have lateral dimensions in the range 5-50 microns and a thickness less than 2 microns. Typical iron flakes have lateral dimensions in the range 10-30 microns and a thickness less than 2 microns.
(119) In an alternative machine-readable embodiment a transparent magnetic layer can be incorporated at any position within the device structure. Suitable transparent magnetic layers containing a distribution of particles of a magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent are described in WO03091953 and WO03091952.
(120) Negative or positive indicia visible to the naked eye may additionally be created in the metal layer 11 or in any suitable opaque layer, e.g. backing layer 14, either inside or outside the image element array area.
