GB2539389A

GB2539389A - Image arrays for security devices and methods of manufacture thereof

Info

Publication number: GB2539389A
Application number: GB1510072.0A
Authority: GB
Inventors: Lister Adam
Original assignee: De la Rue International Ltd
Current assignee: De la Rue International Ltd
Priority date: 2015-06-10
Filing date: 2015-06-10
Publication date: 2016-12-21
Anticipated expiration: 2035-06-10
Also published as: EP3307552B1; WO2016198875A1; EP3307552A1; GB2539389B; GB201510072D0

Abstract

A method of manufacturing an array of image elements for a security device 1 is disclosed. The method comprises: (a) applying a release substance 18 across an array area on a first substrate 19 in accordance with a pattern comprising regions 12 in which the release substance is operative, spaced by elements 14 in which the release substance is absent or non-operative; then (b) printing a first image (I1, Fig.1c) continuously across the array area over the patterned release substance; and then (c) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern. Elements of the first image are retained in accordance with the pattern so as to form an array 10 of first image elements 12. The pattern is periodic in at least a first dimension x and the elements defined by the pattern are substantially identical to one another. There is also provided a method where a second image (I2, Fig. 3e) is disposed continuously across the array area over or under the first image. There is further provided a security device having the first and second image.

Description

IMAGE ARRAYS FOR SECURITY DEVICES AND METHODS OF MANUFACTURE THEREOF

This invention relates to image arrays for use in security devices, as well as to security devices themselves. Security devices are used for example on documents of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents, in order to confirm their authenticity. Methods of manufacturing image arrays and security devices are also disclosed.

Articles of value, and particularly documents of value such as banknotes, cheques, passports, identification documents, certificates and licences, are frequently the target of counterfeiters and persons wishing to make fraudulent copies thereof and/or changes to any data contained therein. Typically such objects are provided with a number of visible security devices for checking the authenticity of the object. Examples include features based on one or more patterns such as microtext, fine line patterns, latent images, venetian blind devices, lenticular devices, moire interference devices and moire magnification devices, each of which generates a secure visual effect. Other known security devices include holograms, watermarks, embossings, perforations and the use of colour-shifting or luminescent / fluorescent inks. Common to all such devices is that the visual effect exhibited by the device is extremely difficult, or impossible, to copy using available reproduction techniques such as photocopying. Security devices exhibiting non-visible effects such as magnetic materials may also be employed.

One class of security devices are those which produce an optically variable effect, meaning that the appearance of the device is different at different angles of view. Such devices are particularly effective since direct copies (e.g. photocopies) will not produce the optically variable effect and hence can be readily distinguished from genuine devices. Optically variable effects can be generated based on various different mechanisms, including holograms and other diffractive devices, and also devices which make use of focusing elements such as lenses, including moire magnifier devices, integral imaging devices and so-called lenticular devices.

Moire magnifier devices (examples of which are described in EP-A-1695121, WO-A-94/27254, WO-A-2011/107782 and W02011/107783) make use of an array of focusing elements (such as lenses or mirrors) and a corresponding array of microimages, wherein the pitches of the focusing elements and the array of microimages and/or their relative locations are mismatched with the array of micro-focusing elements such that a magnified version of the microimages is generated due to the moire effect. Each microimage is a complete, miniature version of the image which is ultimately observed, and the array of focusing elements acts to select and magnify a small portion of each underlying microimage, which portions are combined by the human eye such that the whole, magnified image is visualised. This mechanism is sometimes referred to as "synthetic magnification". The magnified array appears to move relative to the device upon tilting and can be configured to appear above or below the surface of the device itself.

Integral imaging devices are similar to moire magnifier devices in that an array of microimages is provided under a corresponding array of lenses, each microimage being a miniature version of the image to be displayed. However here there is no mismatch between the lenses and the microimages. Instead a visual effect is created by arranging for each microimage to be a view of the same object but from a different viewpoint. When the device is tilted, different ones of the images are magnified by the lenses such that the impression of a three-dimensional image is given.

Lenticular devices on the other hand do not rely upon magnification, synthetic or otherwise. An array of focusing elements, typically cylindrical lenses, overlies a corresponding array of image elements, or "slices", each of which depicts only a portion of an image which is to be displayed. Image slices from two or more different images are interleaved and, when viewed through the focusing elements, at each viewing angle, only selected image slices will be directed towards the viewer. In this way, different composite images can be viewed at different angles. However it should be appreciated that no magnification typically takes place and the resulting image which is observed will be of substantially the same size as that to which the underlying image slices are formed. Some examples of lenticular devices are described in US-A-4892336, WO-A-2011/051669, WO-A-2011051670, WO-A2012/027779 and US-B-6856462. More recently, two-dimensional lenticular devices have also been developed and examples of these are disclosed in British patent application numbers 1313362.4 and 1313363.2. Lenticular devices have the advantage that different images can be displayed at different viewing angles, giving rise to the possibility of animation and other striking visual effects which are not possible using the moire magnifier or integral imaging techniques.

Security devices such as moire magnifiers, integral imaging devices and lenticular devices, as well as others such as venetian blind type devices (which utilise a masking grid in place of focusing elements) depend for their success significantly on the resolution with which the image array (comprising either microimages or image elements) can be formed. Since the security device must be thin in order to be incorporated into a document such as a banknote, any focusing elements required must also be thin, which by their nature also limits their lateral dimensions. For example, lenses used in such security elements preferably have a width or diameter of 50 microns or less, e.g. 30 microns. In a lenticular device this leads to the requirement that each image element must have a width which is at most half the lens width. For example, in a "two channel" lenticular switch device which displays only two images (one across a first range of viewing angles and the other across the remaining viewing angles), where the lenses are of 30 micron width, each image element must have a width of 15 microns or less. More complicated lenticular effects such as animation, motion or 3D effects usually require more than two interlaced images and hence each element needs to be even finer in order to fit all of the image elements into the optical footprint of each lens. For instance, in a "six channel" device with six interlaced images, where the lenses are of 30 micron width, each image element must have a width of 5 microns or less.

Similarly high-resolution image elements are also required in moire magnifiers and integral imaging devices since approximately one microimage must be provided for each focusing element and again this means in effect that each microimage must be formed within a small area of e.g. 30 by 30 microns. In order for the microimage to carry any detail, fine linewidths of 5 microns or less are therefore highly desirable.

The same is true for many security devices which do not make use of focusing elements, e.g. venetian blind devices which rely on the parallax effect caused when two sets of elements on different planes are viewed in combination from different angles. In order to perceive a change in visual appearance upon tilting over acceptable angles, the aspect ratio of the spacing between the planes (which is limited by the thickness of the device) to the spacing between image elements must be high. This in practice requires the image elements to be formed at high resolution to avoid the need for an overly thick device.

Typical processes used to manufacture image elements for security devices are based on printing and include intaglio, gravure, wet lithographic printing as well as dry lithographic printing. The achievable resolution is limited by several factors, including the viscosity, wettability and chemistry of the ink, as well as the surface energy, unevenness and wicking ability of the substrate, all of which lead to ink spreading. With careful design and implementation, such techniques can be used to print pattern elements with a line width of between 25 pm and 50 pm.

For example, with gravure or wet lithographic printing it is possible to achieve line widths down to about 15 pm.

Methods such as these are limited to the formation of single-colour image elements, since it is not possible to achieve the high registration required between different workings of a multi-coloured print. In the case of a lenticular device for example, the various interlaced image elements must all be defined on a single print master (e.g. a gravure or lithographic cylinder) and transferred to the substrate in a single working, hence in a single colour. The various images displayed by the resulting security device will therefore be monotone, or at most duotone if the so-formed image elements are placed against a background of a different colour.

One approach which has been put forward as an alternative to the printing techniques mentioned above is used in the so-called Unison MotionTM product by Nanoventions Holdings LLC, as mentioned for example in WO-A-2005052650. This involves creating pattern elements ("icon elements") as recesses in a substrate surface before spreading ink over the surface and then scraping off excess ink with a doctor blade. The resulting inked recesses can be produced with line widths of the order of 2 pm to 3 pm. This high resolution produces a very good visual effect, but the process is complex and expensive. Further, limits are placed on the minimum substrate thickness by the requirement to carry recesses in its surface. Again, this technique is only suitable for producing image elements of a single colour.

In accordance with the present invention, a method of manufacturing an array of image elements for a security device comprises: (a) applying a release substance across an array area on a first substrate in accordance with a pattern comprising regions in which the release substance is operative, spaced by elements in which the release substance is absent or non-operative; then (b) printing a first image continuously across the array area over the patterned release substance; and then (c) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern, elements of the first image being retained in accordance with the pattern so as to form an array of first image elements; wherein the pattern is periodic in at least a first dimension and the elements defined by the pattern are substantially identical to one another.

By defining the pattern elements in a separate step from the application of the first image, the present method removes any constraints on technique by which the first image itself is printed. The size and position of each image element is determined by the pattern according to which the release substance is applied in step (a) and is independent of the image application step. As such, the first image, which will preferably be formed of one or more inks (i.e. a binder carrying a visible colourant such as a dye, pigment, reflective particles or the like), can be printed using any convenient printing technique (e.g. laser printing, inkjet printinglithographic printing, gravure printing, dye diffusion thermal transfer ("D2T2") printing or letterpress printing) and high print resolution of the sort required in previous image array manufacturing methods is not a necessity (although sufficiently high resolution to achieve an image of acceptable appearance to the human eye is of course still desirable). Moreover, the first image can be multi-coloured, i.e. comprising at least two colours, more preferably at least three colours, but most preferably being a "full colour" image such as a RGB, RGBK or CMYK print. The first image can be formed of multiple print workings which need only be registered to one another to the extent necessary to form an acceptable multi-coloured image to the human eye (techniques for which are well established). The first image can be as complex or as basic as desired: the method will produce equally good quality results whether the first image is a full-colour, multi-tonal photographic image such as a portrait or, at the other end of the scale, a uniform block of a single colour, for example. (Preferably the first image will contain at least one item of information; in the case of a uniform block of colour this may be defined by the shape of its periphery.) Removal of the intervening regions of the image results in image elements defined in accordance with the pattern, and depending on the selected image, individual ones of the retained image elements themselves may be multicoloured.

It will be appreciated from the above that whilst the pattern elements (according to which the release substance is applied) are substantially identical to one another, the first image elements resulting from the process will in many cases be different from one another in terms of the image content each displays (i.e. in terms of the colour or colours carried by each and/or the arrangement thereof).

However, the size and shape of the first image elements will be substantially identical to one another since these factors are determined by the pattern elements.

By arranging the pattern elements to be substantially identical to one another (in terms of their size and shape) and periodic in a first dimension, at least in local regions of the array but preferably across the whole array, the resulting image array is configured for forming part of a security device such as a moire magnifier, lenticular device or integral imaging device when combined with an appropriate focusing element array, or potentially for use in other types of security device such as venetian blind devices. It should be noted that when intended for use in a moire magnifier or lenticular device, the pattern elements will preferably be identical to one another (in terms of size and shape), whilst in an integral imaging device there will be variation between the elements in order to display the object from multiple viewpoints, but overall the elements will still be substantially identical to one another.

Hence the present method enables the formation of multi-coloured image elements, thereby allowing the creation of security devices such as lenticular devices which exhibit at least one multi-coloured image, which has not previously been achievable. However, whilst it is preferred that the first image should be multi-coloured, this is not essential and the present method can also be used to form image arrays with a single-colour first image if desired. For instance, where the image array is for use in a moire magnifier or an integral imaging device, it will generally be preferred to utilise a single-colour first image (or a multi-coloured first image in which the spatial frequency of colour variation is much slower than that of the pattern elements which will form the microimages), since otherwise the synthetic magnification mechanism described earlier will visually combine the multiple colours resulting in the magnified version appearing as some in-between hue which may or may not be desirable.

As discussed further below, the first substrate on which the method is performed may be implemented in various different ways and in particular it should be noted that the method need not be carried out directly on the surface of the substrate.

There may instead be one or more pre-existing layers located on the substrate surface, on top of which the method is performed. The first substrate may also be monolithic or could be multi-layered. In some embodiments, the first substrate will be at least semi-transparent (i.e. visually clear, with low optical scattering, and preferably colourless but may carry a tint), but in other cases this is not necessary and the substrate could be translucent or even opaque.

If the optical density of the first image elements which remain after step (c) is sufficiently high, the image array can be placed over a background without diminishing the appearance of the first image elements. However in practice the first image elements may transmit light to such a degree that an underlying background may be visible therethrough, or otherwise affect the appearance of the first image elements in an undesirable manner. Therefore, in particularly preferred embodiments, step (a) further comprises applying a masking layer continuously across the array area before or after applying the release substance, and step (c) further comprises removing the masking layer in the regions of the pattern such that the masking layer is retained only under the first image elements.

The masking layer comprises a material which is preferably substantially opaque to visible light (e.g. advantageously having an optical density in the range of 2 to 3 (optical density is a logarithmic ratio and hence dimensionless), such as a metal or metal alloy layer, or a pigmented masking coat such as a binder containing substantially opaque particles e.g. aluminium oxide particles. The optical density values given refer to optical density when measured on a transmission densitometer, with an aperture area equivalent to that of a circle with a I mm diameter. A suitable transmission densitometer is the MacBeth TD932. Providing the masking layer under each first image element helps to block any light from an underlying surface so that the appearance of the first image elements is not influenced by what lies beneath. Preferably the masking layer is of uniform appearance (e.g. colour) across the array area, and most preferably reflects substantially while light (as would be the case from a white material or silver-coloured metal) so as not to change the appearance of the first image elements itself. Use of a metal or metal alloy as the masking layer also provides the advantage that its surface will be reflective and so enhance the visibility of the overlying first image elements in reflected light.

The masking layer can be applied before or after the release substance. Thus, in one preferred implementation, step (a) comprises: (al) applying the release substance in accordance with the pattern; and then (a2) applying the masking layer continuously over the release substance across the array area; and in step (c), the masking layer is removed in the regions of the pattern by the removal of the release substance thereunder. This approach is advantageous since the release substance and masking layer (and regions of the first image overlying them) can be removed in a single processing step.

In other preferred implementations, step (a) comprises: (al') applying the masking layer continuously across the array area; and then (a2') applying the release substance in accordance with the pattern over the masking layer; and step (c) comprises: (cl) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern; and then (c2) using the retained first image elements as a resist, processing the array area to remove the masking layer in the regions between the first image elements. This approach has the advantage that the masking layer cannot impede the removal of the release substance, thereby improving the reliability with which the image elements are formed. Step (c2) can be performed using any technique appropriate for removing the material from which the masking layer is formed. Preferably, and particularly where the masking layer comprises metal or a metal alloy, step (c2) is performed by etching. The first image elements are therefore formed of a substance (e.g. ink) which will resist the etchant used.

The patterned release substance can be formed via various techniques. In some preferred implementations, in step (a) the release substance is applied selectively, only to the regions of the patterns, preferably by printing. Particularly preferred printing techniques include lithographic, flexographic and gravure printing. This approach has the advantage that existing printing equipment can be used to perform the step, but does impose the same requirements for high print resolution on the step as exist in conventional image array manufacturing methods (though still not on the later step of printing the first image). The release substance can be applied thinly (with a low coat weight) since it plays no part in the strength of the image ultimately visualised. This is advantageous since higher resolution pattern elements (e.g. smaller line widths) can be achieved with lower coat weights since less material is applied to the surface, thereby limiting the amount of spreading that can take place. Preferred coat weights for the release substance are in the range 0.5 grams per square metre (gsm) to 2 gsm, most preferably around 1 gsm.

In other preferred implementations, in step (a), the release substance is applied continuously across the array area and then exposed to radiation of a wavelength to which the release substance is responsive in accordance with the pattern, as a result of which the release substance is rendered non-operative in the pattern elements and operative in the regions therebetween. It should be noted that the release substance could be initially operative or non-operative prior to irradiation, and hence could be configured to react to the radiation in an manner which either prevents (or reduces) or promotes its subsequent removal during the processing of step (c).

That is, one on hand the release substance could be of a type which will remain operative until irradiated, whereupon the irradiated elements become non-operative, in which case only the pattern elements will be exposed to the radiation. In a preferred example, the release substance becomes cross-linked in response to the radiation. An example of a release substance of this sort is polyacrylic acid to which potassium dichromate has been added, which will form cross-links upon exposure to UV radiation and hence will no longer be soluble.

On the other hand, the release substance could be of a type which is not operative until irradiated, in which case only the regions between the pattern elements will be exposed to the radiation. An example of a release substance of this sort is an ortho quinone diazide as disclosed in US-A-4217407, which becomes soluble in alkali only where it has been exposed to appropriate radiation. An example of this material is V215 supplied by Varichem.

These approaches have the advantage of removing the need for any high-resolution print process in the image array manufacturing method. The pattern elements are defined by the irradiation, which does not suffer from ink spreading or wetting issues and thus can achieve finer image element dimensions. The release substance could be exposed to the radiation through a patterned mask or by a radiation beam (such as a laser beam) directed in accordance with the pattern. The release substance could be responsive to any waveband of radiation, but in a preferred example, the release substance is responsive to ultra-violet radiation and the radiation to which the release substance is exposed in accordance with the pattern includes ultra-violet wavelength(s).

The first image will typically be formed of one or more inks and in a preferred implementation, radiation-curable ink(s) may be used. Thus, preferably, in step (b), the first image is formed of one or more radiation-curable inks and after step (c), the retained first image elements are cured by exposure to radiation. It is desirable that curing should take place after the step of removing the release substance (and overlying layers) since otherwise the cured ink(s) may resist removal. The use of radiation-curable (e.g. UV-curable) inks has been found to achieve particularly good results since clean edges between the pattern elements and the intervening regions can be more reliably achieved if removal takes place while the ink(s) forming the first image layer are still relatively fluid.

The transition from fluid to solid can be difficult to reliably predict or detect in a thermally-drying ink with the result that the removal step may inadvertently be performed once the first image is undesirably dry, leading to flashing of the pattern element edges. Also, different areas of the first image may dry at different rates. By using a radiation-curable ink, its state can be reliably controlled by not performing curing until after the removal step has been completed. Radiation-curable inks are also particularly preferred where etching of an underlying metal layer (e.g. masking layer) is to be performed since the cured inks will have good resist properties.

The manner in which the construction is processed in step (c) to remove the release substance will depend on the nature of the release substance used. In some preferred examples, in step (c), processing the array area to remove the release substance comprises: washing the array area with a solvent fluid (preferably water); heating the array area; directing a jet of gas (e.g. air) onto the array area; brushing or wiping the array area; agitating the array area; or any combination thereof (simultaneously or sequentially). For instance, in particularly preferred examples the array area is washed with a solvent at an elevated temperature to promote removal of the release substance. The solvent may either dissolve the release substance or cause the release substance to detach from the underlying surface, or a mixture of the two mechanisms. The release substance could take various different forms and in particularly preferred examples comprises a soluble material, preferably a water-soluble material, most preferably any of: polyacrylic acid, polyvinyl alcohol, starch, carboxymethyl cellulose, polyethylene oxide, polyvinyl pyrolidinone, gelatine, pectin, guar gum, or gum Arabic. Of these, polyacrylic acid has been found to produce particularly good results. In other preferred implementations, the release substance comprises an oil, preferably a low molecular weight oil (such as linseed oil, castor oil or flaxseed oil), or wax (such as paraffin wax or beeswax). The oil or wax may be removed by washing or could prevent the adhering of layers deposited thereon by causing reticulation of deposited substance, or degassing, possibly when heated. In further advantageous alternatives, the release substance could comprise a frangible layer such as an ink with a weak binder and/or a high pigment load which fractures readily upon mechanical action such as brushing or agitation.

In general, for all forms of release layer it has been found advantageous if the release substance further comprises a filler such as a pigment and/or a wetting agent such as ethanol. The filler results in a roughened surface to the layer which assists in preventing subsequent layers forming a contiguous film over the release substance, which could otherwise hinder the access of solvent and/or lead to "flashing" of the pattern elements. The wetting agent improves the coating of the release substance over the desired regions of the array area.

The first image may be half-toned or screened so as to avoid the formation of a contiguous film over the release substance which again could otherwise hinder the access of solvent and/or lead to "flashing" of the pattern elements.

Preferably, prior to step (a) the surface of the substrate is treated to enhance retention of the first image elements thereto, advantageously by application of a primer and/or by corona treatment. As already noted this surface may not be the raw surface of the substrate itself but could carry a pre-existing layer under the release substance.

The pattern will be configured in accordance with the visual effect desired to be generated by the security device of which the image array is to form part. In some preferred embodiments, the proportion of the pattern corresponding to the regions in which the release substance is operative is between 40% and 60%, preferably between 45% and 55%, most preferably around 50%. This is particularly desirable where the image array is to form part of a lenticular device since then the first image will be displayed over a corresponding proportion of viewing angles (i.e. around half), and not at others, resulting in an even switching effect.

In a first preferred implementation the pattern is a line pattern, periodic in the first dimension which is perpendicular to the direction of the lines, the line pattern preferably being of straight parallel lines, and the width of the lines preferably being substantially equal to the spacing between the lines. A line pattern such as this would be particularly appropriate where the image array is to be used in a one-dimensional lenticular device for example, or in a venetian blind device.

In a second preferred implementation the pattern is a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid, the grid pattern preferably being of dots arranged according to the grid, most preferably square, rectangular, circular or polygonal dots. A grid pattern such as this would be particularly appropriate where the image array is to be used in a two-dimensional lenticular device for example, or two-dimensional venetian blind device. In a particularly preferred example, the grid pattern is a checkerboard pattern.

In a third preferred implementation, each region or each element of the pattern defines a microimage, preferably one or more letters, numbers, logos or other symbols, the microimages being substantially identical to one another. This results in a microimage array which would be particularly appropriate where the image array is to be used in a moire magnification or integral imaging device (noting that whilst in the latter case the microimages will vary slightly from one another, they are still considered substantially identical in the present context). The array of microimages could be one dimensional, but preferably the microimages are arranged in a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid. Such an arrangement is suitable for use in a two-dimensional moire magnification or integral imaging device.

As already noted, high resolution of the first image elements is desirable in order to achieve an acceptably thin security device and hence in preferred embodiments, the elements and/or the regions of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less. This at least one dimension may correspond to the line width of an elongate element, the (narrowest) side-to-side dimension of a circular or polygonal element, or the line weight of a microimage such as a letter or number, for example.

An image array made using any of the methods outlined above can be formed into a security device, e.g. by combining the image array with a corresponding focusing element array (as discussed further below) or another viewing element such as a viewing screen, to generate an optically variable effect. For example if the image array is formed into a lenticular device with an appropriate focusing element array, at some viewing angles the first image will be displayed whereas at other viewing angles the device will appear blank or transparent (since the empty "regions" between the first image elements are being directed to the viewer). If the first substrate is opaque, or the first substrate is transparent and the array is placed over a coloured or patterned background, the same device will appear to switch between the first image and a second "image", formed by the first substrate itself or the background, which is visible in the intervening pattern regions.

Thus, the image array resulting from steps (a) to (c), optionally plus any of the preferred features mentioned above, carrying only first image elements, may be supplied as-is (or in combination with a focusing element array) for later combination with a second image, by the same or a different entity. For example, a so-formed image array could be supplied on the first substrate in the form of a thread or patch which is then (possibly in a separate manufacturing line and by a different company) applied to or incorporated into an article such as a document of value, the surface of which may act as a background to the image array and hence forms a second image, which in the case of a lenticular device will be visible at the viewing angles at which the first image is not.

In other, preferred, cases such a second image may be provided as part of the same image array manufacturing method. Hence, preferably the method further comprises: (d) before, during or after steps (a), (b) and (c), providing a second image continuously across the array area over or under the first image such that elements of the second image are exposed through the regions between the retained elements of the first image, whereby the elements of both images can be viewed from the same side of the image array.

It will be noted that the second image could be provided at any point during the aforementioned method, provided that its resulting location is as specified. The second image should preferably be different from the first image (at least in some noticeable attribute, e.g. content, colour, pattern of colours, size or a change in position/orientation) so that the first image elements can be visually distinguished from the second image elements. Importantly, there is no need to register the second image to the first image elements.

In a particularly preferred implementation, in step (d) the second image is provided on a first surface of the first substrate and steps (a), (b) and (c) are performed subsequently on top of the second image on the first surface of the first substrate. Most preferably the release substance is applied directly onto the second image so that the different planes in which the first and second image lie are as closely coincident as possible.

In another preferred embodiment, steps (a), (b) and (c) are performed on a first surface of the first substrate and in step (d) the second image is provided on a second surface of the first substrate, the first substrate being at least semitransparent. In this case the first substrate is desirably as thin as possible so that the two image planes are closely adjacent one another. The second image can be applied to the substrate before during or after the formation of the first image elements.

In other preferred implementations, the first image elements and the second image may be formed on different respective substrates. Hence preferably in step (d) the second image is provided on a second substrate, to which the first substrate is affixed, the first and/or second substrate being at least semi-transparent. The substrates may be affixed by adhesive and/or lamination for example, and the resulting bond may be temporary or permanent. Whilst at least one of the substrates must be at least semi-transparent (as defined previously), the other may be translucent or opaque. The second substrate could for example by a document substrate forming the basis of a security document such as a banknote, e.g. paper, polymer or a hybrid thereof In many security devices it will be desirable to generate substantially focussed versions of both images via the same focusing element array and hence the two image planes should preferably be as close to one another as possible so that both can lie in or close to the focal plane of the focussing element array. In preferred embodiments, the second image either contacts the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.

Where a masking layer is provided, the masking layer should be located between the first and second images, so that the portions of the second image lying under the first image elements are concealed by the masking layer and do not affect the appearance of the first image elements.

Like the first image, the second image is preferably multi-coloured but this is not essential. Where the image array is to be used in a lenticular device, providing a multi-coloured second image enables the device to appear multi-coloured from all viewing angles (if the first image is also multi-coloured). Preferably the second image includes one or more colours not included in the first image so that there is an apparent change in colour from one image to the other. Where the image array is to be used in a moire magnification or integral imaging device it is preferable to utilise a single-colour second image (or a multi-coloured second image in which the spatial frequency of colour variation is much slower than that of the first image elements forming the microimages), since otherwise the synthetic magnification mechanism described earlier will visually combine the multiple colours resulting in the magnified version appearing as some in-between hue which may or may not be desirable.

The second image can be formed by any convenient technique but most preferably step (d) comprises printing the second image, advantageously in more than one print working. As in the case of the first image, the present method avoids the need for the second image to be printed with a particularly high resolution technique. Any print method including inkjet, laser printing, lithographic printing, gravure printing, flexographic printing, D2T2, or letterpress can be used. The second image can also be screened or half-toned.

As noted above the first and second images can be formed entirely independently of one another and combined later, potentially all in separate manufacturing processes. However, in a preferred implementation the present invention provides a method of manufacturing an image array for a security device, comprising: (a) applying a release substance across an array area in accordance with a pattern comprising regions in which the release substance is operative spaced by elements in which the release substance is absent or non-operative; then (b) printing a first image continuously across the array area over the patterned release substance; then (c) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern, elements of the first image being retained in accordance with the pattern; and (d) before, during or after steps (a), (b) and (c), providing a second image continuously across the array area over or under the first image such that elements of the second image are exposed through the regions between the retained elements of the first image, resulting in an image array, whereby the elements of both images can be viewed from the same side of the image array; steps (a), (b) and (c) being performed on a substrate to which the second image has been or will be applied, or adjacent to which the second image has been or will be arranged.

Preferably the pattern is periodic in at least a first dimension and the elements defined by the pattern are substantially identical to one another in the same manner as described above.

The present invention further provides a method of manufacturing a security device, comprising: (i) manufacturing an image array using any of the methods set out above; and (ii) providing a focussing element array overlapping the array area; wherein the image array and focussing element array are configured to co-operate to generate an optically variable effect.

It will be appreciated that the image array may or may not include a second image at the point at which the focussing element array is provided, or such a second image may be provided in a later step. The manufacture of the security device may take place as part of the same process as manufacturing the image array, or could be performed separately, e.g. by a different entity. The focussing element array could be provided before or after the image array is formed. The focussing element array may be applied onto the first substrate, either on the same surface as that on which the first image elements are formed, or on the opposite surface (if no masking layer is provided). Alternatively the focussing element array could be provided on another (at least semi-transparent) substrate to which the image array is affixed.

Preferably the periodicity of the focusing element array is substantially equal to or a multiple of that of the pattern, at least in the first direction.

In a first preferred example, the security device is a lenticular device. Thus, preferably, the focusing element array is configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of the regions therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the regions therebetween, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and not at a second range of viewing angles. Advantageously, step (i) comprises manufacturing an image array to include a second image as described above, whereby the second image elements are exposed in the regions between the first image elements, such that as the device is tilted the first image is displayed by the first image elements in combination at the first range of viewing angles and the second image is displayed by the second image elements in combination at the second range of viewing angles.

In such lenticular devices (with or without a second image), the focusing element array is preferably registered to the image array at least in terms of orientation and optionally also in terms of translation. The latter is not required unless it is desired to ensure a particular one of the images is displayed at particular viewing angles.

In a second preferred example, the security device is a moire magnifier. Hence preferably, each region or each element of the pattern defines a microimage, such that the image array comprises an array of substantially identical microimages, and the pitches of the focusing element array and of the array of microimages and their relative orientations are such that the focusing element array co-operates with the array of microimages to generate a magnified version of the microimages due to the moire effect. Whilst it is preferable that all the microimages in the array would be identical in terms of colour, shape and size, in some cases they may vary in terms of colour or even shape/size from one area of the device to another. However, locally within each area all of the microimages will be substantially identical. In this case, the central portion of each area would display a clear magnified version of the underlying microimages whilst in the vicinity of the boundaries between areas the magnified images would be a mixture of the microimages in each adjacent area, thereby giving rise to a gradual "morphing" effect between areas.

In a third preferred example, the security device is an integral imaging device.

Thus, preferably, each region or each element of the pattern defines a microimage all depicting the same object from a different viewpoint, such that the image array comprises an array of substantially identical microimages, and the pitches and orientation of the focusing element array and of the array of microimages are the same, such that the focusing element array co-operates with the array of microimages to generate a magnified, optically-variable version of the object.

The optically variable effect exhibited by the security device may be exhibited upon tilting the device just one direction (i.e. a one-dimensional optically variable effect), or in other preferred implementations may be exhibited upon tilting the device in either of two orthogonal directions (i.e. a two-dimensional optically variable effect). Hence preferably the focussing element array comprises focusing elements adapted to focus light in one dimension, preferably cylindrical focusing elements, or adapted to focus light in at least two orthogonal directions, preferably spherical or aspherical focussing elements. Advantageously, the focussing element array comprises lenses or mirrors. In preferred examples, the focusing element array has a one-or two-dimensional periodicity in the range 5200 microns, preferably 10-70 microns, most preferably 20-40 microns. The focusing elements may been formed for example by a process of thermal embossing or cast-cure replication.

In order for the security device to generate a focused image, preferably at least the first image elements are located approximately in the focal plane of the focusing element array, and if a second image is provided, the second image elements are preferably also located approximately in the focal plane of the focusing element array. It is desirable that the focal length of each focussing element should substantially the same, preferably to within +/-10 microns, more preferably +/-5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.

The present invention further provides an image array for a security device, comprising: an array of elements of a first image arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions; a second image underlying the array of elements of the first image, the second image extending continuously across the array area; and wherein elements of the second image are exposed through the regions between the first image elements, such that the elements of both images can be viewed from the same side of the image array.

By arranging the array of first image elements over a continuous second image, no registration is required between the processes for forming the two images and hence each can be a multi-coloured image if desired, as discussed previously.

Preferably, a masking layer is provided between the first image and the second image, the masking layer being present only under the first image elements and being absent in the intervening regions. The use of a masking layer between the two images prevents the underlying second image affecting the appearance of the first image elements.

Preferably the masking layer is substantially opaque to visible light (e.g. having a preferred optical density in the range 2 to 3), and advantageously the masking layer is a metal or metal alloy layer or a pigmented masking coat such as a binder containing substantially opaque particles e.g. aluminium oxide particles as discussed above.

Advantageously the first image and/or the second image is/are multi-coloured.

The first image and/or the second image may be a screened or half-toned image. Preferably, both images are printed images.

The pattern elements are preferably substantially identical to one another (e.g. in size and shape). The pattern can preferably have any of the features attributed to the pattern described previously.

Advantageously, the second image is either in contact with the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.

The present invention further provides a security device, comprising: an image array as described above; and a focussing element array overlapping the array area; wherein the image array and focussing element array are configured to co-operate to generate an optically variable effect.

The security device can have any of the features described above with respect to the described method of manufacture. The security device is preferably a lenticular device, a moire magnifier or an integral imaging device.

The invention further provides a security article comprising a security device as described above, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label or patch.

Also provided is a security document comprising a security device according to or a security article, each as described above, wherein the security document is preferably a banknote, cheque, passport, identity card, driver's licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity. The security device could be manufactured directly on the substrate of the security document or on one or more other substrates which are applied to or incorporated into the document. For example, in a document with a transparent (e.g. polymer) substrate, such as a polymer banknote, the image array could be formed on one side of the document substrate, or on another substrate which is then laminated to it, and the focusing element array could be applied to the other side of the document substrate. In a document with a conventional paper substrate the security element could be formed on a thread, stripe or patch and incorporated into or onto the document, e.g. as a windowed thread or via hot stamping or adhesive.

Examples of image arrays and security devices, and methods of manufacture thereof, in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a first embodiment of a security device, in (a) perspective view, (b) cross-section, and (c) plan view from two different viewing angles; Figure 2 shows steps of a first embodiment of a method of manufacturing an image array and optional incorporation into a security device; Figures 3(a) to (e) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 2; Figures 4(a) and (b) show two alternative security devices which may be formed via the method of Figure 2; Figure 5 shows steps of a second embodiment of a method of manufacturing an image array and optional incorporation into a security device; Figures 6(a) to (f) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 5; Figure 7 shows steps of a third embodiment of a method of manufacturing an image array and optional incorporation into a security device; Figures 8(a) to (g) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 7; Figure 9 shows steps of a fourth embodiment of a method of manufacturing an image array and optional incorporation into a security device; Figures 10(a) to (e) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 9; Figure 11 shows an alternative security device which may be formed via a variant of the method of Figure 9; Figure 12 shows steps of a fifth embodiment of a method of manufacturing an image array and optional incorporation into a security device; Figures 13(a) to (f) show stages in the manufacture of an exemplary image array and security device in accordance with the method of Figure 12; Figures 14(a) and (b) show two alternative security devices which may be formed via variants of the method of Figure 12; Figure 15 shows a first embodiment of apparatus for manufacturing an image array; Figures 16(a) and (b) show an exemplary security document with an exemplary security device as may be manufactured by the apparatus of Figure 15, in plan view from two different viewing angles; Figure 17 shows a second embodiment of apparatus for manufacturing an image array; Figure 18 shows an exemplary security document with an exemplary security device as may be manufactured by the apparatus of Figure 17, in cross section; Figures 19(a) to (d) show four exemplary patterns according to which an image array may be formed, in plan view; Figures 20 and 21 are photographs depicting enlarged portions of two exemplary image arrays; Figures 22 and 23 show two further exemplary patterns according to which an 25 image array may be formed; Figures 24, 25 and 26 show three exemplary articles carrying security devices in accordance with embodiments of the present invention (a) in plan view, and (b)/(c) in cross-section; and Figure 27 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.

The description below will concentrate, in the main part, on image arrays for use in lenticular-type security devices, i.e. in which the image array comprises a series of image elements, each carrying a portion of a corresponding image. However, the disclosed methods and structures are equally well suited to the formation of high resolution image arrays for many other types of security device, including moire magnifiers, integral imaging devices and venetian blind devices, some examples of which will be included below.

Figure 1 depicts a first embodiment of a security device 1, which here is a lenticular device. A transparent substrate 2 (which more generally may be at least semi-transparent) is provided on one surface with an array of focussing elements 5, here in the form of cylindrical lenses, and on the other surface with an image array 10. The image array comprises first image elements 12, each of which carries a (different) portion of a corresponding first image I,, whilst the size and shape of each first image element 12 is substantially identical. The first image elements 12 are spaced by regions 14 in which no image element is present in this example, i.e. gaps. The image elements 12 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 5, which here is along the y-axis. The lateral extent of the pattern (including its elements 12 and regions 14) is referred to as the array area.

As shown best in the cross-section of Figure 1(b), the image array 10 and focussing element array have substantially the same periodicity as one another in the x-axis direction, such that one first image element 12 and one region 14 lies under each lens 5. In this case, as is preferred, the width w of each element 12 is approximately half that of the lens pitch p, as is the space s between each adjacent pair of elements 12 (corresponding to the width of the regions 14).

Thus approximately 50% of the array area carries first image elements 12 and the other 50% corresponds to regions 14. In this example, the image array 10 is registered to the lens array 5 in the x-axis direction (i.e. in the arrays' direction of periodicity) such that a first image element 12 lies under the left half of each lens and a region 14 lies under the right half. However, registration between the lens array 5 and the image array 10 in the periodic dimension is not essential.

When the device 1 is viewed by a first observer 01 from a first viewing angle, each lens 5 will direct light from its underlying first image element 12 to the observer, with the result that the device as a whole exhibits the complete first image 1, across the array area, as illustrated in the left diagram of Figure 1(c). In this example, the first image is a multi-coloured sun-shaped symbol on a white background. When the device is tilted so that it is viewed by second observer 02 from a second viewing angle, now each lens 5 directs light from its underlying blank region 14 to the observer. As such the whole array area now appears blank, as shown in the right diagram of Figure 1(c), which effectively constitutes a second image 12. Hence, as the security device 1 is tilted back and forth between the positions of observer 01 and observer 02, the appearance of the device switches between first image 11 and second image 12, which in this case gives the effect of first image!, "flashing" on and off.

In order to achieve an acceptably low thickness (t) of the security device 1 (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 p of the lenses must also be around the same order of magnitude (e.g. 70 microns or 40 microns). Therefore the width w of the first image elements is preferably no more than half such dimensions, e.g. 35 microns or less.

Figure 2 shows steps of a first embodiment of a method by which the image array 10 can be formed. Figures 3(a) to (e) depict the corresponding process stages for an exemplary array. In a first step S100, a release substance 18 is applied to a first surface of a first substrate 19 (optionally after treating the substrate 19 to enhance ink adhesion, e.g. with a primer layer or by corona treatment), which in this example is a transparent substrate (e.g. PET) although this is not essential as explained below. As shown in Figure 3(a), the release substance 18 is selectively applied according to a pattern such that the release substance is present in regions P2 of the pattern and absent in between, defining elements P1.

The pattern according to which the release substance 18 is applied ultimately defines the size, shape and position of the image elements in the image array and so, preferably, an application technique capable of high resolution is utilised. In preferred examples, the elements P1 and or the regions P2 of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less. This at least one dimension may correspond to the line width of an elongate element, the (narrowest) side-to-side dimension of a circular or polygonal element, or the line weight of a microimage such as a letter or number, for example. Printing methods are preferred, such as intaglio, gravure, wet lithographic printing or dry lithographic printing, with which it is possible to achieve line widths down to around 30 microns. For still finer line widths, flexographic or letterpress type printing techniques are preferred, such as those disclosed in British patent application number 1317195.4. It is not necessary to apply the release substance 18 in a heavy coat weight and indeed a low coat weight may be preferred, as long as good coverage of the regions P2 is obtained. This is beneficial since high coat weights can reduce the attainable print resolution, due to increased ink spreading under gravity. Preferred coat weights for the release substance are therefore in the range 0.5 grams per square metre (gsm) to 2 gsm, most preferably around 1 gsm. The nature of the release substance 18 itself will be described further below. A wetting agent such as ethanol may be added to the release substance to promote formation of the desired pattern regions.

Next, in step S110, a first image 11 is printed over the patterned release substance 11, as shown in Figure 3(b). Any printing technique can be used to apply the first image and high resolution is not a requirement (though still desirable). For example, the first image could be applied by a digital printing method such as laser printing or inkjet, or by techniques including lithographic printing, gravure printing, letterpress printing or flexographic printing. The first image may be formed of one or more inks, an "ink" being a substance comprising a binder (typically polymeric) carrying a visible colourant such as a dye, pigment or reflective (e.g. metallic or optically variable) particles. Most preferably the first image may be a multi-coloured image, i.e. including at least two and more preferably at least three colours forming the first image layer itself, all on top of the release substance 18. The term "colour" used here and throughout this disclosure includes achromatic "colours" such as black, grey and white, as well as hues such as red, green, blue etc., and also metallic "colours" such as silver, gold, bronze etc. For example, the first image could be a full colour image such as a RGB or CMYK image and may be highly complex, e.g. a photographic image such as a portrait. The first image 11 may therefore be laid down in more than one print working using conventional multi-pass printing techniques. The different workings need only be registered to one another to the extent necessary to form a multi-coloured image which is acceptable to the human eye and need not take into account the need for accurately registered image elements, formed later.

In the next step S120, the array area is processed so as to remove the patterned release substance 18 from the substrate 19, which will also lift off those portions of the first image 1, overlying the release substance regions. The result is an image array 10 as shown in Figure 3(c). Only those portions of the first image!, located in the pattern elements P1 remain, as first image elements 12. Between the first image elements 12 are regions 14, which here are transparent gaps. The manner in which the area is processed in step S120 to remove the release substance will depend on the nature of the release substance selected. In particularly preferred examples, the release substance comprises a soluble material, most preferably water soluble, and the processing involves washing the array area with an appropriate solvent such as water. In this case the release substance 18 should preferably be highly soluble whilst also preferably having the ability to film-form into a dry, tack-free layer suitable for printing on. (It is not essential for the release substance 18 to be tack free since the first image could be printed directly over the release substance in the next step of an in-line process without any intermediate storage step which might otherwise involve winding the substrate web up on a reel).

The efficiency of removal may be enhanced by also heating the solvent and/or applying mechanical removal means such as brushing or agitation. Examples of suitable soluble release substances include polyacrylic acid (PAA), polyvinyl alcohol (PVA), starch, carboxymethyl cellulose, polyethylene oxide, polyvinyl pyrolidinone, gelatine, pectin, guar gum, or gum Arabic. Alternatively, the release substance could comprise an oil (e.g. a low molecular weight oil) or a wax. Materials such as these may prevent or reduce retention of the overlying layer(s) onto the substrate, e.g. by degassing upon subsequent printing, or changing the surface energy of the substrate so as to promote reticulation and prevent proper adhesion of the print. In such cases the processing step S120 may be substantially passive, requiring little further intervention to remove the release material from the regions 14, e.g. simply wiping any loose material away. In other cases such an oil or wax based release substance may be activated by heating.

Where the release substance 18 is soluble, e.g. water-soluble, the ink(s) from which the first image is formed should of course not be significantly dissolvable by the same solvent as that in which the release substance dissolves. For example, where the release substance 18 is water-soluble, water-soluble inks (such as common ink jet inks) should not be used for the first image. However, inks based on other solvents and also radiation-curable inks (including for ink jet) may still be used.

In all cases it may be preferable for the release substance 18 to include a filler, such as pigments. This roughens the surface of the release substrate and assists in preventing layers deposited on top (such as first image II) from forming a contiguous film, which may hinder removal of the release substrate. Suitable fillers include aerosols or chalk-like pigments. Removal of the release substance may also be promoted by forming the first image as a half-toned or screened print such that it includes discontinuities. It is also desirable that the ink forming the first image layer should break cleanly along the boundaries between elements P1 and regions P2 of the pattern, and this can be assisted by selecting an ink with a high filler (e.g. pigment) loading. Lithographic inks are particularly preferred for this purpose. Whilst the first image should be allowed to dry to an extent before step S120 is performed, it may be desirable not to allow complete drying so to avoid the first image layer forming a contiguous film.

A particularly preferred option for printing the first image 1, is to use radiation-curable (e.g. UV-curable) ink(s). In this case, the one or more inks will be printed in accordance with the first image and then step S120 will be performed to remove the regions overlying release substance 18. Subsequently, the remaining portions of the first image (the first image elements) are cured by exposure to appropriate radiation (UV) to fix the image elements. This has been found to achieve cleaner demarcation between the retained image elements and the intervening regions as compared with thermally drying inks for which the extent of drying at any point is inherently ill-defined and may vary from one location to another.

The resulting structure shown in Figure 3(c) constitutes a complete image array 10 which can be used in a security device in the manner described with respect to Figure 1 above. All subsequent steps are optional.

If the structure shown in Figure 3(c) is used as an image array by itself in a device of the sort shown in Figure 1, the first image 11 will appear to "flash" on and off as the device is tilted, as described in relation to Figure 1(c). Alternatively, a second image 12, which is continuous across the array area, may be provided in an optional step S130. This may be implemented in various different ways. In a preferred approach, the second image 12 is applied (e.g. printed) onto the second surface of the first substrate, as shown in Figure 3(d) (note that this step could equally be performed before step S100 or at any point thereafter). The second image 12 is exposed in the regions 14 between the first image elements 12, such that both images are visible from the same side of the arrangement (the upper side as depicted in the Figures). Thus once a focusing element array 5 is provided on top of the image array 10 (step S140, Figure 3(e)), the device will appear to switch between the first and second images as the assembly is tilted. The focussing element array 2 can be applied for example by laminating the image array 10 to a transparent layer 2 carrying the lenses. In this example the transparent layer 2 is itself a transparent adhesive layer. It should be noted that the various components of the device are not shown to scale in the Figures and in practice the two image layers 11 and 12 will lie close to one another, preferably approximately in the focal plane of the lenses.

The second image 12 can likewise be formed by any convenient technique such as printing, and does not require especially high resolution. Importantly, no registration between the first and second images is required. Again, it is preferred that the second image 12 is multi-coloured and it may be formed in multiple print workings. The security device will therefore display two different multi-coloured images as the device is tilted. The assortment of colours included in each of the two images is also preferably different in order to increase the visual contrast between the two images upon tilting.

Substantially the same result can be arrived at in various different ways. For example, the order of steps S130 and S140 may be reversed as denoted by the different arrows shown in Figure 2. Hence, the image array 10 depicted in Figure 3(c) (carrying only the first image 11) may be provided with a focusing element array 5 directly, forming a security device of the sort shown in Figure 1. If that security device is then placed over another surface carrying a colour, pattern or image (such as a banknote surface), that surface will be visible through the regions 14 and effectively provides a second image 12, the end construction being substantially the same as shown in Figure 3(e), except that the second image 12 has been formed on a second substrate (not shown), and then adjoined to the first substrate 19, rather than being formed directly on the first substrate 19.

Figure 4 shows two alternative constructions of security device which may be formed using the above-described method. In these cases the first substrate 19 is itself used to space the focusing element array from the image array, i.e. acting as substrate 2 shown in Figure 1). In the Figure 4(a) example, after performing steps S100, S110 and S120 on the first surface of substrate 19, resulting in first image elements 12, the second image 12 is then printed directly over the first image elements 12 on the same surface of substrate 19. The second image effectively fills in the regions 14 between the first image elements 12, so as to form second image element. A focusing element array 5 is applied to the second surface of the substrate 19 and both sets of image elements are viewed through the substrate 19 in use.

The Figure 4(b) example is substantially the same except that rather than print the second image 12 over the first image elements 12, it is formed on a second substrate 20 and then laminated over the first image elements (using a transparent adhesive, not shown). Again, the second image 12 is exposed in the regions 14 between the first image elements 12.

The Figure 4 examples are more preferable than those of Figure 3 since in Figure 3 the two image arrays are separated by the substrate 19 and hence are not at the same position with respect to the lenses unlike Figure 4 where effectively they are in the same plane. The thickness of the substrate 19 may be as low as 6 microns but this will have some effect on the device, albeit acceptable in some devices.

In all cases, the focusing element array 5 can be provided before or after the second image is applied.

For best results using the method of Figure 2, the first image 1, should be applied with a high optical density, most preferably being substantially opaque, in order that when a second image 12 is provided underneath it does not affect the appearance of the first image elements. If the first image elements were significantly light-transmissive, the second image would be visible to an extent through the first image elements which would diminish the resulting visual effect. In practice, it can be difficult to achieve sufficiently high print density and this also places limits on the ink compositions and printing techniques that can be used to form the first image h. A second embodiment of a method of manufacturing an image array which addresses this issue is shown in Figure 5, with corresponding manufacturing stages for an exemplary device being shown in Figures 6(a) to (f).

Steps labelled with like numbers are the same as described with respect to the first embodiment. Hence, in the first step S100, a release substance 18 is applied to a first substrate in accordance with a pattern, as shown in Figure 6(a). Next, in step S106, a masking layer 17 is applied continuously across the array area, over the release substance 18. The masking layer is preferably substantially opaque to visible light, and most preferably is a metal or metal alloy layer, such as aluminium, copper, bronze, chromium or nickel, although any other high optical density material could be used such as a binder containing substantially opaque particles, e.g. aluminium oxide or titanium dioxide.

A first image 11 is then printed over the masking layer 17 (step 110), using any convenient printing technique as described previously. Again, most preferably the first image is multi-coloured.

Next, step S120 is performed whereby the array area is processed to remove the release substance 18. Since the masking layer 17 overlies the release substance 18, this also results in removal of the masking layer 17 (and the first image) from the regions 14 of the substrate to which the release substance had been applied. Again the nature of the removal process will depend on the type of release substance 18 used, as described above. The resulting image array is shown in Figure 6(d) and it will be seen that portions of the masking layer 17 are retained only directly under the first image elements 12. The so-produced image array 10 can then be further processed in any of the ways already described in relation to the first embodiment. For example, a second image 12 may be provided (step S130), either by direct application to the other side of substrate 19 as shown in Figure 6(e) or by applying the image array to another surface. The masking layer portions 17 are located between the first and second images such that, even if the first image elements 12 have low optical density, the underlying second image 12 is substantially obscured (and preferably entirely hidden) by the masking layer locally. As such the appearance of the first image elements 12 is not affected by the underlying second image 12. It will be appreciated that to avoid the masking layer itself affecting the appearance of the first image elements, the masking layer is preferably of uniform appearance and most preferably reflects substantially white light. A metal or metal alloy layer such as aluminium or nickel is particularly preferred for this purpose and also provides the additional advantage of high reflectivity so as to enhance the visibility of the first image.

The image array 10 can be combined with a focussing element array 5 (step S140) to form a security device 1 as shown in Figure 6(f). As before this can be performed before or after the second image is provided.

Depending on the type of release substance 18 selected for use in the Figure 5 method, difficulties can be encountered in removing the release substance in step S120. For example, if the masking layer 17 is a metal or metal alloy layer, this can act as a barrier to the ingress of solvent fluid during washing and impede it from reaching the release substance 18. It has been found that such difficulties can be alleviated by providing the release substance with fillers such as pigments which result in a roughened surface and act to prevent the masking material forming a contiguous film over the release substance in regions 14. This enables fluid to permeate more successfully through the masking layer and thereby assists in the removal step.

An alternative method according to a third embodiment which avoids this difficulty is shown in Figure 7, with corresponding manufacturing stages being depicted for an exemplary image array in Figures 8(a) to (g). Again, method steps which are the same as those described previously are denoted using like numbers. In this case, the masking layer 17 is applied to the substrate 19 first (step S106), continuously across the array area as shown in Figure 8(a). Next, step S100 is performed to apply release substance 18 onto the masking layer 17 in accordance with a desired pattern of elements P1 and regions P2, as shown in Figure 8(b). The first image h is then printed over the patterned release substance 18 (step S110) using any technique as discussed previously. Next, the removal step S120 now comprises two stages. First, in step S122, the release substance 18 is removed together with the overlying portions on image 11 in the same manner as previously described, the particulars of which will depend on the type of release substance in use. The resulting structure is shown in Figure 8(d) and it will be seen that the masking layer 17 is now exposed in the regions 14 between the first image elements 12. Next, in step S124, the exposed portions of the masking layer 17 are removed using the first image elements 14 to protect the unexposed portions of the masking layer 17.

For example, where the masking layer 17 comprises a metal or alloy layer, the exposed portions may be removed by etching with the first image elements 15 acting as an etch resist. The resulting image array 10 is shown in Figure 8(e) which it will be seen is the same as in Figure 6(d). A second image 12 and focussing element array 5 can then be provided as before.

The method according to the third embodiment has the advantage that the release substance 18 is not covered by the masking layer 17 and therefore ingress of solvent is not impeded. However, the ink forming the first image does require suitable protective properties, e.g. to act as a resist against etchant.

Lithographic inks and toners used in laser printing have been found suitable in this regard, amongst other examples. The use of a radiation-curable ink (as mentioned earlier) is particularly preferred in this embodiment since the ink can be made highly resistant to etchant upon curing.

In the embodiments so far, the first image elements 12 have been formed on a transparent first substrate 19 with any second image being provided either on the other surface of the substrate or over the top of the first image elements. However, in other preferred embodiments, the second image may be provided before the first image elements are formed. This applies to all of the embodiments described above and an exemplary fourth embodiment will now be described with respect to Figures 9 and 10 in which this is the case. For consistency, although the two images are applied in reverse order, the image which ultimately lies continuously under the spaced elements of the other will be referred to as the "second" image, and that of which only elements remain, the "first" image. Again, steps which are the same as those described previously are denoted with like reference numbers.

Thus, the first step is now to print the second image 12 onto the first surface of the substrate 19 continuously across the array area (step S130). This can be performed using any desired technique as previously explained and high resolution is not essential. The resulting structure is shown in Figure 10(a).

Step S100 is then performed on top of the second image 12, with the patterned release substance 18 being applied to the surface of the second image as shown in Figure 10(b). The next steps are then the same as in the first embodiment: a first image II is printed over the release substance 18 (step 110), as shown in Figure 10(c) and then the release substance 18 and overlying portions of the first image are removed (step 120) using a technique appropriate to the selected release substance. The resulting image array 10 is shown in Figure 10(d) and it will be seen that the second image 12 is exposed in the regions 14 between first image elements 12, effectively forming second image elements. A focusing element array 5 can then be provided over the image array 10 as before to form a security device 1 as shown in Figure 10(e) (step S140).

As before, in order to prevent the underlying second image 12 affecting the appearance of the first image elements 12, it may be desirable to provide a masking layer 17 between the two images. This can be achieved using the method of the second or third embodiment, but forming the second image first and applying the patterned release substance 18 or masking layer 17 to the surface of the second image 12 as necessary. An exemplary security device formed in this way is shown in Figure 11.

It will be appreciated that in this embodiment the substrate 19 need not be transparent but could be translucent or even opaque.

In all of the embodiments so far, the necessarily high-resolution pattern elements 12 are achieved through high-resolution selective application of the release substance 18, typically by carefully controlled print processes. Since it is the pattern carried by the release substance which defines the ultimate size, shape and location of the image elements 12 (and the intervening regions 14), there are no significant constraints on the process by which the images themselves can be printed, thereby enabling the use of multi-coloured images which typically require multiple workings, between which only relatively low registration is generally achievable. Nonetheless it is still necessary to achieve very high resolution in the application of the release substance 18 itself, which carries with it the same attendant difficulties which are present in conventional single-colour fine line print processes.

A fifth embodiment is depicted in Figures 12 and 13 which addresses this difficulty by providing an alternative way to form the patterned release substance layer 18. This can be used to implement step S100 in any of the embodiments described above, but will be described in the context of the first embodiment only for the sake of brevity. In the first step S102, a photo-responsive release substance 18 is applied continuously across the array area on first substrate 19 (or onto a second image if this has already been formed as discussed above), resulting in the arrangement shown in Figure 13(a). The release substance 18 may comprise any of the materials mentioned above in this connection, e.g. water soluble with the addition of a substance which is responsive to one or more wavelengths of radiation, e.g. UV. The substance causes the release substance 18 to become non-operational (i.e. will no longer act as a release substance, e.g. is no longer water soluble) in those areas in which it has been exposed to the radiation. For example, the substance may respond to radiation by forming cross-links. Most preferably, the release substance 18 comprises a water-soluble, UV-crosslinking material. For example, the release substance could comprise polyacrylic acid to which potassium dichromate has been added.

The release substance 18 is then exposed to radiation of the appropriate wavelength (e.g. UV) in accordance with the desired pattern of elements P1 in which the radiation is incident on the release substance and regions P2 in which it is not. This can be achieved for example by exposing the layer to an appropriate radiation source via a patterned mask, or by utilising a radiation beam such as a laser and directing it across the layer 18 in accordance with the pattern. The result is regions 18a in which the release substance remains operative, and elements 18b in which it has been rendered non-operational, e.g. cross-linked and no longer soluble, as shown in Figure 13(b). The remaining steps are as described in relation to the first embodiment: a first image 11 is printed over the release layer 18 using any convenient technique (step S110, Figure 13(c)). The array area is then processed in step S120 to remove the regions 18a of the release layer which remain operational, resulting in the image array 10 shown in Figure 13(d). Each first image element 12 has underlying the first image 11 a non-operational portion 18b of the release substance 18, spaced by empty regions 14. A second image 12 can then be provided using any of the techniques previously described (step S130, Figure 13(e)), and a focusing element array 5 can be overlaid to form a security device 1 as shown in Figure 13(f).

As before, the second image 12 may in practice be provided on the first surface of the substrate 19 before steps S102, S104 etc are performed on top. The resulting security device structure is shown in Figure 14(a), the non-operational release substance portions 18b being located immediately between the two images.

In a variation of the fifth embodiment, the release substance 18 could be of a type which only becomes operative upon irradiation at a suitable wavelength. For example, US-A-4217407 discloses an ortho quinone diazide which is suitable for this purpose, one available form of which is V215 by Varichem. Upon exposure to appropriate radiation, the material becomes soluble in alkali whereas, where it has been masked, it will resist dissolution. Hence, when using this type of release substance 18, in step S104 it will be exposed to radiation of the appropriate wavelength (e.g. UV) in the regions P2 of the desired pattern and not in the elements P1.

Whichever of the above radiation-patterning approaches is adopted, depending on the composition of the release substance 18, the retained non-operational release substance 18b may be of sufficiently high optical density to perform the function of the masking layer 17 described in previous embodiments. A non-transparent pigment could be added to the release substance to enhance this. However in practice a separate masking layer 17 may still be desirable and this can be achieved using either the method of the second embodiment or that of the third embodiment with steps S102 and S104 used to implement step 100.

The security device structure resulting from the method of the second embodiment modified in this way is shown in Figure 14(b), each first image element 12 having a masking layer portion 17 and a non-operative release layer portion 18b thereunder. Again, in the example structure shown the second image 12 has been provided on the first surface of the substrate before the above-described steps have been performed. A correspondingly modified version of the third embodiment would result in the masking layer portions 17 lying under the non-operative release layer portions 18b, i.e. the order of these two layers will be reversed relative to that shown in Figure 14(b).

In all of the above embodiments it is preferable that the two planes in which the first and second images lie respectively are as close to one another as possible, so that when the image array is combined with a focussing element array or other viewing component in order to form a security device, both images can be located close to the focal point of the array and/or little or no parallax between the two images is apparent. This is achieved automatically in the method of Figure 9 in which the first image elements 12 are formed directly on top of the second image so that the two are in direct contact (or via a thin masking layer 17 in some cases), and also in the Figure 4 configurations, but in other implementations where the two images are formed on different sides of a substrate or on separate substrates it is desirable that the two images are spaced from one another by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less. For example, in the Figure 3 embodiment the substrate 19 is preferably 15 microns thick or less.

Some examples of apparatus suitable for carrying out the above methods will now be described. It will be appreciated that the order of the various stations can be changed in order to implement the different options mentioned above, and just two of many potential variations will be described to illustrate this.

Figure 15 shows a first exemplary manufacturing line suitable for implementing the method as a continuous web-based process (as is strongly preferred). A transparent substrate 19 is provided from a reel (not shown) as a continuous web. At a first print station 30, a release substance 18 is applied to one surface of the substrate 19 according to a desired pattern of regions, spaced by elements in which the release substance is absent. A first image II is then printed over the patterned release substance 18 by a multi-colour print station 32, optionally in multiple passes. The substrate 19 then passes into a processing chamber 34 adapted to remove the release substance 18 and the portions of the first image II thereon, e.g. by passing the substrate through a heated water bath and subsequent brushing. The result is a series of first image elements 12. The so-formed image array is then conveyed through a lens-forming station 36, which here comprises a cast-cure apparatus. A transparent curable resin 22 is applied to the second surface of the substrate 19 by an application roller 36a. A surface relief profile defining a lens array 5 is cast into the surface of the curable resin by an embossing roller 36b equipped with a corresponding profile in its surface. During or after shaping, the resin 22 is cured, e.g. by irradiation or heat, so as to fix the lens structure. The result is a security device with a cross section corresponding to that shown in Figure 1(b).

Figures 16(a) and (b) show an exemplary security document 50, here a paper-based banknote, provided with a security device 1 as formed by the process described with respect to Figure 15. The banknote surface carries graphics such as star indicium 51, which have been printed on the banknote in a separate conventional process, e.g. by intaglio printing. The security device 1 is applied over a portion of the star shaped indicium 51, e.g. in the form of a foil or patch, affixed by way of a transparent adhesive. The resulting cross section through the document at a location across the security device 1 corresponds to that shown in Figure 4(b), where the security document 50 forms substrate 20 and the second image 12 is the star indicium 51 thereon.

From a first viewing angle, as shown in Figure 16(a), the security device 1 directs light from the first image elements 12 to the viewer with the result that a portion of the underlying star-shaped indicium 51 is concealed and instead the observer sees the first image 11. For simplicity this is depicted here as a uniform region but in practice a multi-coloured image is preferred as described above.

When the document is tilted, at a second viewing angle as shown in Figure 16(b), the security device 1 directs light from the regions 14 between the first image elements 12 to the viewer, i.e. exhibiting second image 12 which here is the underlying star graphic 51. Hence the full star shape is visible.

It will be appreciated from the above example that different aspects of the manufacturing process which results in the complete security device 1 can be performed separately from one another, potentially on different manufacturing lines and possibly by different entities. For instance, in this example the process depicted in Figure 15 may be carried out by a first entity and the resulting product supplied as a security article such as a thread, strip, foil or patch, to another entity which has produced the security document 50 (including the graphics thereon), which then applies or otherwise incorporates the security article into or onto the document. It would also be possible for the lens array 5 to be formed in yet another separate process and later combined with the array of image elements 12 at the time of application to the security document 50.

Figure 17 shows another exemplary manufacturing apparatus. In this case, the process begins by printing the continuous second image 12 onto the first substrate 19 (which may or may not be transparent) at a first print station 38, preferably in multiple colours. A patterned release substance 18 is then applied onto to the second image 12 at station 30. A first image!, is then printed over the release pattern at a further print station 32, preferably in multiple colours. The substrate is then processed in chamber 34 to remove the release substance 18 and portions of the first image thereon, resulting in an image array corresponding to that in Figure 10(d). The so-formed image array, containing elements of both first and second images, can then be supplied as a security article such as a foil, thread, strip or patch. Figure 18 shows an exemplary security document 60, here a polymer banknote, into which the security article may be incorporated. The image array 10 is affixed to a transparent document substrate 61 in a window region defined by a gap in opacifying layers 62a, 62b provided on the document substrate. The image array 10 is arranged so that the elements of both the first and second images can be viewed through the document substrate 61, and joined by a transparent adhesive for example. On the opposite side of the document substrate, a focusing element array 5 is provided to complete the security device. The focusing element array 5 and/or the image array 10 may be formed directly on the substrate or on respective additional layer(s) which are adhered to the substrate. The formation of the image array 10, focusing element array 5 and the polymer banknote 60 may each be performed in separate processes, optionally by separate entities, before being assembled to create the final security device. The device 1 may also be formed in a half-window region, for example in Figure 18 by extending the lower opacifying layer 62b across the device 1.

In all of the above embodiments, the pattern of elements 12 and regions 14 can be configured to take any desirable form and this will be dictated by the type of security device in which the array is to be used. In all cases, the elements 12 forming the pattern will be substantially identical to one another in terms of size and shape, and the pattern will be periodic at least in a first dimension. In the case of a one-dimensional lenticular device in which the focusing elements are cylindrical (as shown in Figure 1), the image elements 12 will preferably be straight, parallel lines as shown for example in Figure 19(a). The image array will be registered to the focusing element array in terms of orientation but not necessarily in terms of translational position along the periodic direction (i.e. x-axis, in this case). Preferably the ratio of surface area carrying first image elements 12 to that of the regions 14 therebetween will be around 1:1 so that about 50% of the available area is dedicated to each of the two images I, and 12 (or to h and a blank "image" if no second image is provided). In this way, the first image will be displayed at approximately half of the possible viewing angles and the second image will be displayed over the other half. However this is not essential and the relative proportions of each image could be varied by adjusting the element width relative to the spacing between the elements. Preferably the proportion will lie in the range 40 to 60%, more preferably 45 to 55%. The periodicity of the pattern (i.e. the pitch between one element 12 and the next) must however be related to that of the focusing element array and lie in the same direction. Preferably, the pitch of the image elements 12 is substantially the same as that as the focusing elements 5, in which case the footprint of one focusing element is represented by dashed outline 5a. However in other cases the pitch of the focusing element array may be substantially equal to a multiple of that of the image array. For example, the line 5b represents a focusing element with a pitch twice that of the image element pitch. Such an arrangement will cause the images displayed by the device to switch three times as the device is tilted from one extreme to the other, rather than just once as would be the case for a focusing element 5a of equal pitch.

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 are shown in Figures 19(b) to (d). In each case the image elements 12 are formed as grid patterns of "dots", with periodicity in more than one dimension. In the Figure 19(b) example, the first image elements 12 are square and arranged on an orthogonal grid to form a "checkerboard" pattern with resulting regions 14 in which the first image is absent. 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 be represented by the dashed line 5a. From an off-axis starting position, as the device is tilted left-right, the displayed image will switch as the different elements or regions 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. Again the proportion of image elements 12 to regions 14 is approximately 50%.

In Figure 19(c), the pattern is substantially the same as that of Figure 19(b), but here the patterns elements 12 are circular rather than square. Any other "dot" shape could alternatively be used, e.g. polygonal. The regions 14 between the elements 12 join one another due to the increased spacing of the elements 12 with the result that here the proportion of the array corresponding to the first image is less than 50%.

In Figure 19(d), the elements 12 are once again circular but are arranged on a close-packed hexagonal grid. This may be appropriate for example where the focusing element array is also arranged on a hexagonal grid. Again any other "dot" shape may be adopted and in this case hexagonal regions may be preferred. Once again the proportion of the array corresponding to the first image is less than 50%.

The patterns of Figures 19(c) and (d) could of course be reversed such that it is the first image elements which surround dot regions 14 in which the second image is displayed, such that the proportion of the array corresponding to the first image is more than 50%.

For lenticular devices, it is strongly preferred that either the first image or the second image, or both, be multi-coloured, i.e. made up of at least two different colours and more preferably at least three. In particularly preferred embodiments, the first image may contain at least one colour which is not included in the second image, so that there is a colour contrast upon switching between different images. Either or both images may be printed in multiple print workings and may be screened or half-toned.

Figure 20 is a photograph showing a portion of an exemplary image array 10 made in accordance with the above-described techniques, at a much enlarged scale. In this case the pattern is a line pattern as described in relation to Figure 19(a). The first image has been formed as a multi-coloured halftone print such that multiple colours are exhibited by each of the first image elements 12. The regions 14 between the line elements 12 are transparent but if the structure is placed over a second image, portions of that second image would be visible therethrough. In this case the width w of each image element 12 is 150 microns, the spacing s between them (= width of regions 14) is 150 microns and the pattern pitch is 300 microns (this sample was produced with a relatively coarse resolution for test purposes).

Figure 21 is a photograph showing a portion of another exemplary image array 10 made in accordance with the above-described techniques, again at a much enlarged scale. Again, the pattern is a line pattern of first image elements 12 and transparent intervening regions 14. The first image is multi-coloured, here consisting of two colours, which give rise to the variation in colour seen along certain of the first image elements 12 and also between different ones of the first image elements 12. For example, image element 12' is wholly displayed in a first colour, which here appears light, while another image element 12" is wholly of a second colour, which here appears relatively dark. Other image elements such as 12* include portions of the first colour, e.g. portion 12a, as well as portions of the second colour, e.g. portion 12b. The arrangement of the various colours will depend on the content of the first image. In this example, the first image elements 12 have a width w of approximately 30 microns and the spacing s between them is around 50 microns, the pattern pitch being around 80 microns. In this case the proportion of the image array 10 corresponding to the first image is therefore around 38%.

Whilst the preceding embodiments have been described in the context of lenticular devices, as mentioned at the outset the same manufacturing techniques and image array structures can be used in other types of security device, including moire magnifiers and integral imaging devices. The arrangement of image elements can be varied accordingly by appropriate selection of the pattern in which the release substance 18 is applied in step S100.

Generally, for both moire magnifiers and integral imaging devices, each image element 12 will now define a microimage, i.e. a complete miniature representation of the image to be viewed, rather than a portion thereof For example, each microimage could comprise one or more letters, numbers, logos or other symbols, all of the microimages in the array being substantially identical.

The image elements 12 can correspond to the microimages themselves or the background surrounding them (i.e. the microimages can be positive or negative). The microimages will primarily be defined by the shape and size of the pattern elements as laid down by the release substance, rather than by the first (or second) image, which now only acts to colour the microimages. Figure 22 shows an exemplary image array suitable for use in a moire magnifier device.

Each first image element 12 is a star shaped symbol, and the elements are arranged on a regular orthogonal grid, spaced from one another by region 14 which acts as a background. Each star 12 may have a diameter of around 20 microns for example. A corresponding focussing element array is illustrated by dashed-line circles 5, each of which represents a spherical or aspherical lens.

The lens array is also arranged on an orthogonal grid of similar pitch to that of the microimage array. However, the two arrays are slightly mismatched in terms of pitch and/or orientation, in order to give rise to the moire magnification effect. Registration between the two arrays is not required. The observer viewing the security device will see a magnified version of the array of stars.

Figure 23 shows an exemplary image array suitable for use in an integral imaging device. Here, each first image element 12 defines the outline of a cube, each microimage depicting the cube from a different viewpoint, and again arranged on an orthogonal grid. The line width of each microimage may be around 5 microns for example. The region 14 provides the background to the microimages (both inside and outside the cube outlines). A corresponding focussing array 5 is provided, this time of matching pitch and orientation to the microimage array, and preferably registered to the image array at least in terms of orientation. When the security device is viewed, a three-dimensional image of the cube will be exhibited.

In the case of both moire magnifiers and integral imaging devices, due to the mechanism by which the optically variable effect is generated, which involves sampling multiple microimages and combining them to form the enlarged version, it may be preferable for the first and second images to be single-colour images, i.e. blocks of uniform colour. If the first and second images are multi-coloured, the synthetic magnification mechanism may lead to visual combining of several colours resulting in the magnified image taking on some in-between colour which may or may not be desirable. However, this can be avoided (or at least reduced) by the use of multi-colour images in which the variation in colour is at a spatial frequency greater, and preferably much greater, than that of the focusing element and microimage arrays so that at any one location all of the microimages being sampled are of the same colour. In this way any colour mixing will be restricted to regions adjacent the boundaries between one colour and the next, which can itself act as a useful multi-coloured effect since additional colour(s) may be visually generated over and above those actually used in the formation of the first and/or second image.

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 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.

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 6mm, are particularly useful as the additional exposed thread surface area allows for better use of optically variable devices, such as that presently disclosed.

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.

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.

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.

Examples of such documents of value and techniques for incorporating a security device will now be described with reference to Figures 24 to 27.

Figure 24 depicts an exemplary document of value 70, here in the form of a banknote. Figure 24a shows the banknote in plan view whilst Figure 24b shows a cross-section of the same banknote along the lines X-X' and Figure 24c shows a cross-section through a variation of the banknote. In this case, the banknote is a polymer (or hybrid polymer/paper) banknote, having a transparent substrate 72. Two opacifying layers 73 and 74 are applied to either side of the transparent substrate 72, which may take the form of opacifying coatings such as white ink, or could be paper layers laminated to the substrate 72.

The opacifying layers 73 and 74 are omitted across selected regions 71 (and 71'), each of which forms a window within which a security device 1, 1' is located. In Figure 24(b), a security device 1 is disposed within window 71, with a focusing element array 5 arranged on one surface of the transparent substrate 72, and image array 10 on the other (e.g. as in Figure 18 above). Figure 24(c) shows a variation in which a second security device 10' is also provided on banknote 70, in a second window 71'. The arrangement of the second security device 1' can be reversed so that its optically variable effect is viewable from the opposite side of the security document as that of device 1, if desired.

It will be appreciate that, if desired, any or all of the windows 71, 71' could instead be "half-windows", in which an opacifying layer (e.g. 73 or 74) is continued over all or part of the image array 10. 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 73 and 74 are provided on both sides.

In Figure 25 the banknote 80 is a conventional paper-based banknote provided with a security article 85 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 82 and 83 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 85 in window regions 81 of the banknote. Alternatively the window regions 81 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 81 to be "full thickness" windows: the thread 85 need only be exposed on one surface if preferred. The security device is formed on the thread 85, which comprises a transparent substrate a focusing array 5 provided on one side and an image array 10 provided on the other. Windows 81 reveal parts of the device 1, which may be formed continuously along the thread. (In the illustration, the lens arrays are depicted as being discontinuous between each exposed region of the thread, although in practice typically this will not be the case and the lens arrays (and image arrays) will be formed continuously along the thread. Alternatively several security devices could be spaced from each other along the thread, as in the embodiment depicted, with different or identical images displayed by each).

In Figure 26, the banknote 90 is again a conventional paper-based banknote, provided with a strip element or insert 95. The strip 95 is based on a transparent substrate and is inserted between two plies of paper 92 and 93. The security device 1 is formed by a lens array 5 on one side of the strip substrate 95, and an image arrays 10 on the other. The paper plies 92 and 93 are apertured across region 91 to reveal the security device 1, which in this case may be present across the whole of the strip 95 or could be localised within the aperture region 95. It should be noted that the ply 93 need not be apertured and could be continuous across the security device.

A further embodiment is shown in Figure 27 where Figures 27(a) and (b) show the front and rear sides of the document 100 respectively, and Figure 27(c) is a cross section along line Z-Z'. Security article 105 is a strip or band comprising a security device 1 according to any of the embodiments described above. The security article 105 is formed into a security document 100 comprising a fibrous substrate, 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 (Figure 27(a)) and exposed in one or more windows 101 on the opposite side of the document (Figure 27(b)). Again, the security device 1 is formed on the strip 105, which comprises a transparent substrate with a lens array 5 formed on one surface and a co-operating image array 10 as previously described on the other Alternatively a similar construction can be achieved by providing paper 100 with an aperture 101 and adhering the strip element 105 onto one side of the paper 100 across the aperture 101. The aperture may be formed during papermaking or after papermaking for example by die-cutting or laser cutting.

In still further embodiments, a complete security device 1 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 10 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 5 or in a separate procedure with the focusing array 5 being applied subsequently.

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.

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.

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.

The security device may comprise a metallic layer laterally spaced from the security feature of the current invention. The presence of a metallic layer can be used to conceal the presence of a machine readable dark magnetic layer. 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 (Fe2O3 or Fe304), 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.

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 W003091953 and W003091952.

Negative or positive indicia may be created in the metallic layer or any suitable opaque layer. This includes the masking layer 17 where the optical effect may be locally removed by the presence of "coarse" negative indicia or it can be placed in an adjacent area of the mask layer not involved in the optical effect. The masking layer 17 could itself be a magnetic material. One way to produce partially metallised/demetallised films in which no metal is present in controlled and clearly defined areas, is to selectively demetallise regions using a resist and etch technique such as is described in US-B-4652015. Other techniques for achieving similar effects are for example aluminium can be vacuum deposited through a mask, or aluminium can be selectively removed from a composite strip of a plastic carrier and aluminium using an excimer laser. The metallic regions may be alternatively provided by printing a metal effect ink having a metallic appearance such as Meta!stare inks sold by Eckart.

Claims

CLAIMS1. A method of manufacturing an array of image elements for a security device, comprising: (a) applying a release substance across an array area on a first substrate in accordance with a pattern comprising regions in which the release substance is operative, spaced by elements in which the release substance is absent or non-operative; then (b) printing a first image continuously across the array area over the patterned release substance; and then (c) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern, elements of the first image being retained in accordance with the pattern so as to form an array of first image elements; wherein the pattern is periodic in at least a first dimension and the elements defined by the pattern are substantially identical to one another.
2. A method according to claim 1, wherein step (a) further comprises applying a masking layer continuously across the array area before or after applying the release substance, and step (c) further comprises removing the masking layer in the regions of the pattern such that the masking layer is retained only under the first image elements.
A method according to claim 2, wherein step (a) comprises: (al) applying the release substance in accordance with the pattern; and then (a2) applying the masking layer continuously over the release substance across the array area; and in step (c), the masking layer is removed in the regions of the pattern by the removal of the release substance thereunder.
A method according to claim 2, wherein step (a) comprises: (at) applying the masking layer continuously across the array area; and then (a2') applying the release substance in accordance with the pattern over the masking layer; and step (c) comprises: (c1) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern; and then (c2) using the retained first image elements as a resist, processing the array area to remove the masking layer in the regions between the first image elements.
5. A method according to any of claims 2 to 4, wherein the masking layer is substantially opaque to visible light, preferably having an optical density of 15 between 2 and 3.
6. A method according to any of claims 2 to 5, wherein the masking layer is a metal or metal alloy layer, or a pigmented coating.
7. A method according to claims 4 and 6, wherein step (c2) is performed by etching.
8. A method according to any of the preceding claims, wherein in step (a) the release substance is applied selectively, only to the regions of the patterns, preferably by printing.
9. A method according to any of claims 1 to 7, wherein in step (a), the release substance is applied continuously across the array area and then exposed to radiation of a wavelength to which the release substance is responsive in accordance with the pattern, the radiation rendering the release substance non-operative in the pattern elements.
10. A method according to claim 9, wherein the release substance is exposed to the radiation through a patterned mask or by a radiation beam directed in accordance with the pattern.
11. A method according to claim 9 or claim 10, wherein the release substance becomes cross-linked in response to the radiation.
12. A method according to any of claims 9 to 11, wherein the release substance is responsive to ultra-violet radiation and the radiation to which the release substance is exposed in accordance with the pattern includes ultra-violet wavelength(s).
13. A method according to any of the preceding claims, wherein in step (b), the first image is formed of one or more radiation-curable inks and after step (c), the retained first image elements are cured by exposure to radiation.
14. A method according to any of the preceding claims, wherein in step (c), processing the array area to remove the release substance comprises: * washing the array area with a solvent fluid; * heating the array area; * directing a jet of gas onto the array area; * brushing or wiping the array area; * agitating the array area; or * any combination thereof. 25
15. A method according to any of the preceding claims, wherein the release substance comprises a soluble material, preferably a water-soluble material, most preferably any of: polyacrylic acid, polyvinyl alcohol, starch, carboxymethyl cellulose, polyethylene oxide, polyvinyl pyrolidinone, gelatine, pectin, guar gum, or gum Arabic.
16. A method according to any of claims 1 to 14, wherein the release substance comprises an oil, preferably a low molecular weight oil, or wax.
17. A method according to any of the preceding claims, wherein the release substance further comprises a filler such as a pigment and/or a wetting agent such as ethanol.
18. A method according to any of the preceding claims, wherein prior to step (a) the surface of the substrate is treated to enhance retention of the first image elements thereto, preferably by application of a primer and/or by corona treatment.
19. A method according to any of the preceding claims, wherein the first image is multi-coloured.
20. A method according to any of the preceding claims, wherein the first image is screened or half-toned.
21. A method according to any of the preceding claims, wherein in step (b) the first image is applied in more than one print working.
22. A method according to any of the preceding claims, wherein in step (b) the first image is printed by laser printing, inkjet printing, lithographic printing, gravure printing, flexographic printing, letterpress or dye diffusion thermal transfer printing.
23. A method according to any of the preceding claims, wherein the proportion of the pattern corresponding to the regions in which the release substance is operative is between 40% and 60%, preferably between 45% and 55%, most preferably around 50%.
24. A method according to any of the preceding claims, wherein the pattern is a line pattern, periodic in the first dimension which is perpendicular to the direction of the lines, the line pattern preferably being of straight parallel lines, and the width of the lines preferably being substantially equal to the spacing between the lines.
25. A method according to any of the preceding claims, wherein the pattern is a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid, the grid pattern preferably being of dots arranged according to the grid, most preferably square, rectangular, circular or polygonal dots.
26. A method according to claim 25, wherein the grid pattern is a checkerboard pattern.
27. A method according to any of claims 1 to 23, wherein each region or each element of the pattern defines a microimage, preferably one or more letters, numbers, logos or other symbols, the microimages being substantially identical to one another.
28. A method according to claim 27, wherein the microimages are arranged in a grid pattern, periodic in the first dimension and in a second dimension, wherein the grid pattern is preferably arranged on an orthogonal or hexagonal grid.
29. A method according to any of the preceding claims, wherein the elements and/or the regions of the pattern are 50 microns or less in at least one dimension, preferably 30 microns or less, most preferably 20 microns or less.
30. A method according to any of the preceding claims, further comprising: (d) before, during or after steps (a), (b) and (c), providing a second image continuously across the array area over or under the first image such that elements of the second image are exposed through the regions between the retained elements of the first image, whereby the elements of both images can be viewed from the same side of the image array.
31. A method according to claim 30, wherein in step (d) the second image is provided on a first surface of the first substrate and steps (a), (b) and (c) are performed subsequently on top of the second image on the first surface of the first substrate.
32. A method according to claim 30, wherein steps (a), (b) and (c) are performed on a first surface of the first substrate and in step (d) the second image is provided on a second surface of the first substrate, the first substrate being at least semi-transparent.
33. A method according to claim 30, wherein in step (d) the second image is provided on a second substrate, to which the first substrate is affixed, the first and/or second substrate being at least semi-transparent.
34. A method according to any of claims 30 to 33, wherein the second image either contacts the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.
35. A method according to any of claims 30 to 34 when dependent on any of claims 2 to 7, wherein the masking layer is located between the first and second images.
36. A method according to any of claims 30 to 35, wherein the second image is multi-coloured.
37. A method according to any of claims 30 to 36, wherein the first image is screened or half-toned.
38. A method according to any of claims 30 to 37, wherein step (d) comprises printing the second image, preferably in more than one print working.
39. A method according to claim 38, wherein in step (d) the second image is printed by laser printing, inkjet printing, lithographic printing, gravure printing, flexographic printing, letterpress printing, or dye diffusion thermal transfer printing.
40. A method of manufacturing an image array for a security device, comprising: (a) applying a release substance across an array area in accordance with a pattern comprising regions in which the release substance is operative spaced by elements in which the release substance is absent or non-operative; then (b) printing a first image continuously across the array area over the patterned release substance; then (c) processing the array area so as to remove the release substance and the portions of the first image thereon in the regions of the pattern, elements of the first image being retained in accordance with the pattern; and (d) before, during or after steps (a), (b) and (c), providing a second image continuously across the array area over or under the first image such that elements of the second image are exposed through the regions between the retained elements of the first image, resulting in an image array, whereby the elements of both images can be viewed from the same side of the image array; steps (a), (b) and (c) being performed on a substrate to which the second image has been or will be applied, or adjacent to which the second image has been or will be arranged.
41. A method of manufacturing a security device, comprising: (i) manufacturing an image array using the method of any of claims 1 to 40; and (ii) providing a focussing element array overlapping the array area; 30 wherein the image array and focussing element array are configured to cooperate to generate an optically variable effect.
42. A method according to claim 41, wherein the periodicity of the focusing element array is substantially equal to or a multiple of that of the pattern, at least in the first direction.
43. A method according to claim 41 or 42, wherein the focusing element array is configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of the regions therebetween in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the regions therebetween, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and not at a second range of viewing angles.
44. A method according to claim 43 wherein step (i) comprises manufacturing an image array using the method of any of claims 30 to 38 whereby the second image elements are exposed in the regions between the first image elements, such that as the device is tilted the first image is displayed by the first image elements in combination at the first range of viewing angles and the second image is displayed by the second image elements in combination at the second range of viewing angles.
45. A method according to claim 43 or 44, wherein the focusing element array is registered to the image array at least in terms of orientation and preferably also in terms of translation.
46. A method according to claim 41 or 42, wherein each region or each element of the pattern defines a microimage, such that the image array comprises an array of substantially identical microimages, and the pitches of the focusing element array and of the array of microimages and their relative orientations are such that the focusing element array co-operates with the array of microimages to generate a magnified version of the microimages due to the moire effect.
47. A method according to claim 41 or 42, wherein each region or each element of the pattern defines a microimage all depicting the same object from a different viewpoint, such that the image array comprises an array of substantially identical microimages, and the pitches and orientation of the focusing element array and of the array of microimages are the same, such that the focusing element array co-operates with the array of microimage to generate a magnified, optically-variable version of the object.
48. A method according to any of claims 41 to 47, wherein the focussing element array comprises focusing elements adapted to focus light in one dimension, preferably cylindrical focusing elements, or adapted to focus light in at least two orthogonal directions, preferably spherical or aspherical focussing elements.
49. A method according to any of claims 41 to 48, wherein the focussing element array comprises lenses or mirrors.
50. A method according to any of claims 41 to 49, wherein the focusing element array has a one-or two-dimensional periodicity in the range 5-200 microns, preferably 10-70 microns, most preferably 20-40 microns.
51. A method according to any of claims 41 to 50, wherein the focusing elements have been formed by a process of thermal embossing or cast-cure replication.
52. A method according to any of claims 41 to 51, wherein at least the first image elements are located approximately in the focal plane of the focusing element array, and if a second image is provided, the second image elements are preferably also located approximately in the focal plane of the focusing element array.
53. A method according to any of claims 41 to 52, wherein the focal length of each focussing element is substantially the same, preferably to within +/-10 microns, more preferably +/-5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.
54. An image array for a security device manufactured in accordance with any of claims 1 to 40.
55. A security device manufactured in accordance with any of claims 41 to 10 54.
56. An image array for a security device, comprising: an array of elements of a first image arranged across an array area in accordance with a pattern which is periodic at least in a first dimension, the first image elements being spaced from one another by regions; and a second image underlying the array of elements of the first image, the second image extending continuously across the array area; wherein elements of the second image are exposed through the regions between the first image elements, such that the elements of both images can be 20 viewed from the same side of the image array.
57. An image array according to claim 56 further comprising a masking layer between the first image and the second image, the masking layer being present only under the first image elements and being absent in the intervening regions
58. An image array according to claim 56 or 57, wherein the first image and/or the second image is/are multi-coloured.
59. An image array according to claim 56, 57 or 58, wherein the first image and/or the second image is a screened or half-toned image.
60. An image array according to any of claims 56 to 59, wherein the masking layer is substantially opaque to visible light, preferably having an optical density in the range 2 to 3.
61. An image array according to any of claims 56 to 60, wherein the masking layer is a metal or metal alloy layer, or a pigmented coating.
62. An image array according to any of claims 56 to 61, wherein the pattern has any of the features defined in claims 23 to 29.
63. An image array according to any of claims 56 to 62, wherein the second image is either in contact with the first image elements or is spaced from the first image elements by 15 microns or less, preferably 10 microns or less, still preferably 5 microns or less.
64. A security device, comprising: an image array in accordance with any of claims 56 to 63; and a focussing element array overlapping the array area; wherein the image array and focussing element array are configured to co-operate to generate an optically variable effect.
65. A security device according to claim 64, wherein the periodicity of the focusing element array is substantially equal to or a multiple of that of the pattern, at least in the first dimension.
66. A security device according to claim 64 or 65, wherein the focusing element array is configured such that each focusing element can direct light from a respective one of the first image elements or from a respective one of second image elements in dependence on the viewing angle, whereby depending on the viewing angle the array of focusing elements directs light from either the array of first image elements or from the second image elements, such that as the device is tilted the first image is displayed by the first image elements in combination at a first range of viewing angles and the second image is displayed by the second image elements in combination at a second range of viewing angles.
67. A security device according to claim 64 or 65, wherein each region or each element of the pattern defines a microimage, such that the image array comprises an array of substantially identical microimages, and the pitches of the focusing element array and of the array of microimages and their relative orientations are such that the focusing element array co-operates with the array of microimages to generate a magnified version of the microimages due to the moire effect.
68. A security device according to claim 64 or 65, wherein each region or each element of the pattern defines a microimage all depicting the same object from a different viewpoint, such that the image array comprises an array of substantially identical microimages, and the pitches and orientation of the focusing element array and of the array of microimages are the same, such that the focusing element array co-operates with the array of microimage to generate a magnified, optically-variable version of the object.
69. A security device according to any of claims 64 to 68, wherein at least the first image elements are located approximately in the focal plane of the focusing element array, and if a second image is provided, the second image elements are preferably also located approximately in the focal plane of the focusing element array.
70. A security device according to any of claims 64 to 69, wherein the focal length of each focussing element is substantially the same, preferably to within +/-10 microns, more preferably +/-5 microns, for all viewing angles along the direction(s) in which it is capable of focussing light.
71. A security article comprising a security device according to any of claims 64 to 70, wherein the security article is preferably a security thread, strip, foil, insert, transfer element, label or patch.
72. A security document comprising a security device according to any of claims 64 to 70, or a security article according to claim 71, wherein the security document is preferably a banknote, cheque, passport, identity card, driver's licence, certificate of authenticity, fiscal stamp or other document for securing value or personal identity.