Field of Invention

The present invention concerns security devices, methods for making security devices and methods of authenticating products. In particular, but not exclusively, the

invention relates to the inkjet printing of chiral nematic liquid crystal materials for the creation of security devices.

Background

High value goods and security documents may be marked by security devices using materials exhibiting particular physical or chemical properties in order to distinguish between genuine items and counterfeit versions. Such security devices are typically added to many products, packaging, labels, items of value and documentation to permit validation and to confirm authenticity. A common way to provide a security device is to apply to the surface of goods or a security document a motif which exhibits unusual visual or spectroscopic properties which can either be verified by the unaided eye or by machine.

If security devices are to be attached to mass-produced articles, there is a need for security devices which exhibit such effects and yet can be produced rapidly and cost-effectively. An effective way to do this can be by printing for example by inkjet printing.

An inkjet printer creates printed pictures by firing multiple droplets of ink onto a substrate. The inkjet printer receives its commands from a microprocessor which handles a computer file. The computer file is a theoretical plan of the design which will be printed. The computer file can be in the form of a bitmap, where a pixel on the bitmap image is converted by the printing equipment into a drop of ink on the substrate. Inkjet printing equipment generates printed images as a pattern of discrete droplets of ink. These drops of ink may be printed to merge seamlessly with one another, or not, as the image file dictates.

Liquid crystal materials are a class of functional photonic materials. Liquid crystal materials contain molecules which have a tendency to self-organise along an optical axis. The way in which the molecules in liquid crystal materials self-organise and then macroscopically align dictates the optical properties of the liquid crystal material. For example, chiral liquid crystal molecules have a tendency to self-organize into a

helicoidal arrangement around an optical axis in the material. Due to the difference in refractive index of the liquid crystal molecules parallel and perpendicular to the molecular optical axis, or birefringence, this helicoidal arrangement results in a periodic variation of the refractive index along the optical axis of the material. For suitable periodicities, this gives rise to a photonic band-gap or reflection band for visible wavelengths of circularly polarized light, which is well- known in the art. When viewed at different angles with respect to the helicoidal axis, the apparent reflection band changes according to the viewing direction.

The optical properties of chiral liquid crystal materials make them suitable for use in security devices for authentication by both untrained and trained personnel. Security devices where a printed liquid crystal image changes colour with viewing angle, or is revealed when viewed under particular polarisation conditions, are known. For example, US2011/0097557 discloses the

manufacture of security features, e.g. for bank notes, in which a polymerisable liquid crystal material is printed onto a solid PVA layer. EP2285587 and US8481146 discuss inkjet printing of chiral nematic liquid crystals to give devices exhibiting optical variability with viewing angle. Effects such as colour shifts, wherein a security device exhibits a viewing angle dependent colour, are useful for printed security devices as they cannot be easily replicated with conventional inks.

It may be desirable for security devices to exhibit optical effects that are readily apparent to the

untrained eye and yet difficult to reproduce with

conventional means.

It may also be beneficial for a security device to include different levels of authentication to improve overall deterrence and resistance to counterfeiting.

Overt features allow authentication by untrained

personnel or members of the public and typically involve an easily recognisable optical effect or change upon viewing the feature in a certain way (for example, a colour shift on moving or rotating the feature) . Covert features typically comprise a hidden feature that is revealed or shown by use of a viewing aid or instrument (e.g. ultraviolet activated visible fluorescence). So- called forensic features use a sophisticated, laboratory- based test to provide unequivocal evidence regarding the authenticity of an item (e.g. DNA amplification, GC-MS analysis of a dissolved taggant molecule) . It is particularly desirable that security devices can be changed on an item-level basis if so desired, for example by including a unique code or serial number, to permit additional tracking or serialisation of individual items.

A known approach to allow authentication of articles is to use a holographic security device, typically applied in the form of a pre-prepared label. By virtue of their production process, such holograms cannot be varied on an item level basis and each one is essentially the same. Such labels also need to be produced by a separate process and may be restricted in terms of surfaces or products to which they may be applied. Provision of a separate label may add extra expense to incorporation of the security device. It is therefore further desirable that security devices be added directly to items without the use of a pre-prepared label to both enhance security and reduce cost of the device.

The present invention seeks to provide improved security devices and methods.

Summary of the Invention

According to a first aspect of the invention there is provided a method of producing a security device, the method comprising: providing a substrate; defining an image area on the substrate as a two-dimensional array of printing regions; applying a first image to the image area by applying a first liquid crystal material to a first component image area in the image area comprising every alternate printing region of the two-dimensional array of printing regions in the image area; applying a second image which may be the same as or different from the first image to the image area by applying a second liquid crystal material to a second component image area in the image area comprising each remaining alternate printing region of the two-dimensional array of printing regions in the image area; wherein the first and second liquid crystal materials have opposite chirality.

The product of the invention in this aspect thus

constitutes a composite image made up of two component images. The component images are interpositioned but do not substantially overlap, and are preferably entirely discretely formed.

It is characterised in that the first and second liquid crystal materials used for the respective component images have opposite chirality. Of particular interest is the class of liquid crystal materials which reflect circularly polarised light. These are known either as chiral nematic liquid crystals or as cholesteric liquid crystals .

In a preferred case, the first and second liquid crystal materials are selected to exhibit the same colour

response in unpolarised viewing conditions.

In a typical case, the first and second liquid crystal materials may be respectively a first and second

composition comprising at least one chiral liquid crystal component. In such a case, the first and second

compositions comprise at least one chiral component of opposite chirality. Preferably the first and second compositions comprise enantiomers of the same chiral component. Conveniently the other components of the first and second compositions may be identical. Each

composition is for example an ink for printing and for example inkjet printing. Thus, conditions are created in which the first and second component images may be made less distinguishable in normal viewing conditions but more distinguishable in special viewing conditions. One of the component images of the composite image has been created using a liquid crystal material, and for example printed using a liquid crystal ink, which reflects left handed circularly polarised light and the other component image has been created using a liquid crystal material, and for example printed using a liquid crystal ink, which reflects right handed circularly polarised light. The use of left and right handed circular polarizing filters creates viewing conditions to render the two component pictures

sequentially visible. Moreover, if the first and second liquid crystal materials are selected to exhibit the same colour response in unpolarised viewing conditions a composite image may be created where the two component images appear essentially the same in normal viewing, and the component parts of the composite image cannot readily be resolved, but where one component image can be

preferentially viewed under one viewing condition and the other ink and hence the other component image can be preferentially viewed under another viewing condition with circular polarizing filters. The security device of the invention can be created to exhibit optical effects that are not apparent to the unaided eye but become more apparent in appropriate specialised viewing conditions.

The first and second liquid crystal materials may be applied to the image area by printing and for example by inkjet printing. Thus the image area in this example case constitutes a print area in which a composite image in accordance with the principles of the invention is printed. In this case the ink for printing and for example inkjet printing comprises one of the first or second liquid crystal materials as the case may be.

Unless the context demands otherwise, discussion herein by way of example to methods of inkjet printing will be understood by the skilled person to apply generally to methods of application of an image to a substrate as a distribution of plural picture elements across plural printing regions of an image area in accordance with the principles of the invention.

The first image is applied and is in the preferred case printed and for example inkjet printed as a distribution of a first liquid crystal material across a first

component image area in the image area comprising every second or alternate printing region. Thus, the set of printing regions comprises a two-dimensional array of areas into which a picture element of the first image is applied. That is not to say that the liquid crystal material is applied to every such printing region, as the image may include printing regions that are left without liquid crystal material as part of the image, but rather that the image is created by a distribution across every second printing region of the print area.

The second image is applied and is in the preferred case printed and for example inkjet printed as a distribution of a second liquid crystal material across a second component image area in the image area comprising the printing regions which were not part of the first

component image area. That is to say, the second image is applied as a distribution of a second liquid crystal material in the printing regions which were not part of the first component image area, but which are the

adjacent printing regions to the printing regions of the first component image area and correspond to gaps defined between the printing regions of the first component image area. Thus, the set of printing regions comprises a two- dimensional array of areas into which picture elements of the second image may be created interpositioned between but offset from the image elements of the first image. Again that is not to say that the liquid crystal material is applied to every such printing region, as again the image may include printing regions that are left without liquid crystal material as part of the image.

The method produces a security device comprising a substrate carrying a composite image comprising a first distribution of a first liquid crystal material and a second distribution of a second liquid crystal material; wherein the first distribution of the first liquid crystal material comprises a two-dimensional array of picture elements corresponding to a first image wherein the distribution is applied across every alternate printing region, and the second distribution of the second liquid crystal material comprises a two- dimensional array of picture elements corresponding to a second image wherein the distribution is applied across the other set of every alternate printing region.

The basic principles by which each image is applied to the image area are commonplace to established principles. In accordance with such principles, an image area on a substrate is subdivided into a two-dimensional array of printing regions and material for example comprising a printed ink is applied as required to the printing region to make up an image. To print a single image

conventionally, a drop of ink is applied appropriately or not applied to each of the printing regions separately in accordance with the instructions provided by the image data file. Altering the density of the drops of ink per unit area creates an impression of shade or variation in shade from a single ink colour. This principle of printing is traditionally called half tone printing.

In accordance with the invention, every other printing region is omitted from the process by which each image is generated. For a single image the result would merely be a picture with weaker colour, with the natural colour of the substrate showing through the printed region. The key to the invention is that the second image is printed on the substrate in the same manner, but such that again every other printing region is omitted from the image application process but on an exactly alternate basis to the pattern of omission for the first image. Then regions corresponding to the application of ink in the second image are never coincident with regions corresponding to the application of ink in the first image.

The result is a composite image with the two elements of the composite image respectively printed using first and second liquid crystal materials that constitute the inks in the printing process of the invention. The first and second liquid crystal materials have opposite chirality and for example comprise enantiomers of the same chiral component. The inks are preferably selected to appear the same colour in unaided viewing conditions. However, one of the components of the composite image has been created using a liquid crystal material, and for example printed using a liquid crystal ink, which reflects left handed light and the other component has been created using a liquid crystal material, and for example printed using a liquid crystal ink, which reflects right handed light. The use of left and right handed circular polarizing filters creates viewing conditions to render the two component pictures sequentially visible. The first and second liquid crystal materials of the invention thus comprise inks which exhibit a property where they appear essentially the same in normal viewing, and the component parts of the composite image cannot readily be resolved, but where one ink and hence one component image can be preferentially viewed under one viewing condition and the other ink and hence the other component image can be preferentially viewed under another viewing condition.

An admirable method is offered by inkjet printing or other suitable method to enable the creation of a

security device that exhibits optical effects that are not readily apparent to the unaided eye but become readily apparent in appropriate specialised viewing conditions and that are difficult to reproduce with conventional means.

The effect may be advantageously enhanced if the second image is not the same as the first.

It will be apparent that for the invention to work there must be an accurate offset between the first and second images. Most conveniently this is achieved if each printing region is identically sized and shaped, for example comprising a rectangular or square printing region, each printing region thus being offset from its neighbour by a uniform pitch in each of the x and y directions .

A first step of the process of a more complete embodiment of the method of the invention is to convert each image to a set of machine readable instructions, and for example printer readable instructions, for applying each image to the image area, for example by printing, as a two-dimensional array of picture elements.

In accordance with the invention, two, optionally

different, pictures are used to generate two image data files each comprising a set of machine readable

instructions for applying the respective images as a two- dimensional array of picture elements.

In accordance with the invention, each component image to be created is first converted into an image data file comprising a set of machine readable instructions for applying the image as a two-dimensional array of picture elements. Each image is thus notionally subdivided into a two-dimensional array of picture elements. It will be apparent that for the invention to work there must be an exact spatial correspondence between each picture element used to generate the first image data file and an

equivalent picture element used to generate the second image data file. Most conveniently this is achieved if each picture element is identically sized and shaped, for example comprising a rectangular or square picture element, each picture element thus being offset from its neighbour by a uniform pitch in each of the x and y directions .

Each image data file thus comprises a two-dimensional array of data items, each data item comprising machine readable instructions for applying a corresponding picture element to the substrate. A data item might include information over whether the liquid crystal material should be applied and for example printed into the region of the substrate corresponding to its

respective picture element and information of a quantity of liquid crystal material which should be applied and for example printed into the region of the substrate corresponding to its respective picture element.

Alternatively, each data item may be a simple one-bit data item indicating only that a picture element is either present or not, the image data file thus

comprising a simple bitmap.

Conveniently, a method of achieving the objective of the invention whereby every second picture element is left unprinted for each image data file comprises: converting an image into a raw image data file comprising a two- dimensional array of data items, each data item

corresponding to a picture element in the image;

generating a modified image data file in which every second data item is set to an instruction not to apply material for the corresponding picture element to the substrate and for example not to print the corresponding picture element; using the modified image data file to create the image. For example, in the case of a simple bitmap, every second data item in the modified image data file is set to zero.

The image data file would still have been used to

generate the image but only every other picture element of the original image data file corresponding to a region where liquid crystal material should be applied would have been used.

A more complete embodiment of the method comprises:

converting a first image into a first image data file comprising a set of machine readable instructions for applying the first image as a two-dimensional array of picture elements; converting a second image which may be the same as or different from the first image into a second image data file comprising a set of machine readable instructions for applying the second image as a two-dimensional array of picture elements, each picture element corresponding directly to a picture element of the first image data file; applying and for example inkjet printing a first liquid crystal material onto a substrate in accordance with the first image data file in such manner that every second picture element is not applied; applying and for example inkjet printing a second liquid crystal material onto a substrate in accordance with the second image data file in such manner that every second picture element is not applied, the picture elements not applied thereby corresponding to the picture elements which were applied in accordance with the first image data file; wherein the first and second liquid crystal materials have opposite chirality.

Thus to create both component image elements of the composite image the method comprises: converting a first image into a first raw image data file comprising a two- dimensional array of data items, each data item

comprising image data for a corresponding picture element of the image; converting a second image into a second raw image data file comprising a two-dimensional array of data items, each data item comprising image data for a corresponding picture element of the image; generating a first modified image data file comprising a set of machine readable instructions for application of liquid crystal material to a substrate in which every second data item is set to an instruction not to apply liquid crystal material in respect of the corresponding picture element; generating a second modified image data file in which every data item corresponding to a picture element which was not set to an instruction not to apply material in the first modified image data file is set to an instruction not to apply material in the second modified image data file; using the modified image data files to create the composite image in accordance with the

foregoing .

Thus to print both elements of the composite image the method comprises: converting a first image into a first raw image data file comprising a two-dimensional array of data items, each data item comprising image data for a corresponding picture element of the image; converting a second image into a second raw image data file comprising a two-dimensional array of data items, each data item comprising image data for a corresponding picture element of the image; generating a first modified image data file in which every second data item is set to an instruction not to print the corresponding picture element;

generating a second modified image data file in which every data item corresponding to a picture element not set to an instruction not to print in the first modified image data file is set to an instruction not to print the corresponding picture element; using the modified image data files to print the image in accordance with the foregoing .

Preferably, each picture element is created discretely in the sense that the liquid crystal material applied in each printing region of the substrate corresponding to a picture element does not substantially overlap with its neighbour .

Preferably, a quantity of liquid crystal material applied in each printing region of the substrate corresponding to a picture element and a size of each picture element are together selected such that each picture element is so created discretely.

For example, during an inkjet printing process, drops of liquid crystal material ink are printed at sufficient distance such that consecutive drops only just touch one another. When every other drop of ink is omitted from the printing process of each component image it becomes possible to print on the substrate such that the ink drops of ink making up the second image are only ever placed between the drops of ink in the first image. The second image should be created such that if aligned on a specific point of the first image, then regions

corresponding to the application of ink in the second image are never coincident with regions corresponding to the application of ink in the first image.

In a possible variant of the method, each printing region of the substrate corresponding to a picture element is either supplied with a fixed volume of the liquid crystal material printed per unit area of the substrate or is not supplied with any liquid crystal material at all. In an alternative variant of the method, printing regions of the substrate corresponding to different picture elements may be supplied with different volumes of the liquid crystal material per unit area of the substrate.

Inkjet printers generally operate by the jetting of drops of ink on to the substrate. The drops are jetted from a print head either individually or in groups. In the case of inkjet printing the volume of liquid crystal material printed per unit area may be varied by varying the volume of the drops, by varying the spacing of the drops or of the groups of drops, by varying the number of drops within each group or by other methods. It can be desirable for a security device to exhibit both attractive and recognisable overt features, which can therefore contribute to the quality, look and feel of products or packaging, and covert features, which can provide high levels of certainty of authenticity under forensic examination. Liquid crystal materials can offer excellent covert security features, for example based on the polarisation property of light, and the present invention now permits liquid crystal materials to offer relatively overt and/ or covert features using two liquid crystal materials of opposite chirality. The combination of different levels of security features, in one printed security device, gives significantly enhanced protection against counterfeiting, and diversion, of items to which the security device is added.

The first and second liquid crystal materials in this invention are typically first and second compositions comprising blends of components where the overall handedness or chirality of each blend is opposite to the chirality of the other blend. The first and second compositions may comprise the two enantiomers of the same chiral component. Conveniently the other components of the first and second compositions may be identical.

Preparing two liquid crystal materials using essentially the same liquid crystal formulation but with the two enantiomers of the appropriate component may reduce cost and/or increase the speed at which devices may be

produced. The latter may be particularly important when the liquid crystal material is printed directly onto products or packaging as part of a production line.

Inkjet printing is a preferred method for such

applications, and the availability of applying security features by inkjet printing is therefore advantageous. Preferably each liquid crystal material is therefore a liquid crystal ink formulated for inkjet printing. The liquid crystal materials may be printed in a single pass. The liquid crystal materials may be printed in multiple passes. The multiple passes may be multi-pass printing with a single print head, in which a single print head makes multiple passes across the substrate, or multi-pass printing with multiple print heads in which each print head makes one or more passes across the substrate.

Particularly advantageously the liquid crystal materials are printed in a single pass using two print heads, one carrying the first material and one carrying the second material .

The preferred method of inkjet printing generally

comprises the jetting of drops of ink onto the substrate. The drops may for example be 10 pL each. The printer may operate to a print resolution, expressed in dots per inch (dpi), of between 508 and 5080 dpi in both x and y directions (i.e. parallel to the movement of the print head and perpendicular to the movement of the print head) .

Preferably the volume of the liquid crystal material printed per unit area of the substrate in a region of the substrate corresponding to a printed picture element is within the range from 0.4 to 40 pL/cm ² . More preferably the volume of the liquid crystal material printed per unit area of the substrate corresponding to a printed picture element is within the range from 0.1 to 15 pL/cm ² . Most preferably the volume of the liquid crystal material printed per unit area of the substrate corresponding to a printed picture element is within the range from 0.1 to 10 pL/cm ² . If the volume of liquid crystal material printed per unit area is too low, there may be insufficient liquid crystal material present on the substrate to generate a colour perceivable by the human eye. If the volume of liquid crystal material printed per unit area is too high, the cost of the security device may become excessive.

Preferably the liquid crystal material is of the class of liquid crystal materials which reflect circularly

polarised light. These are known either as chiral nematic liquid crystals or as cholesteric liquid crystals Such liquid crystal materials may be particularly suited to the present invention and may show a particularly

striking visual effect.

The first and second images may in a possible case each be part of an insignia, marking or code wherein different parts of the insignia, marking or code are printed as the first and second images, the entire insignia, marking or code being the composite image.

Alternatively, the first and second images may in a possible case each be a separate insignia, marking or code, the two insignias, markings or codes, thereby being superimposed in the composite image.

For example, the insignia, marking or code may be a bar code. The bar code may be one or two dimensional. Two dimensional barcodes are commonly referred to as QR codes. Bar codes are commonly used to record variable data on products and packaging. An advantage of the present invention is that it uses inkjet printing, which can be used to print variable information, thus allowing the creation of a security device containing variable information. Preferably the security device includes variable information specific to that device, such as information representing a serial number or other product code. The barcodes are typically formed of discrete elements, or bars.

Preferably the liquid crystal material exhibits a

variation in colour with viewing angle. Thus there may be a colour shift in the first and second images with viewing angle. Advantageously this may produce additional effects .

The substrate may be a label, a carton, a packaging container, a surface of a product, a document, a paper substrate, a metallic substrate, a tamper evident

substrate, a polymer substrate, a glass substrate or a PET substrate. It is a particular advantage of the invention that the security device can be formed on a wide variety of substrates. Preferably the substrate is the surface of a product. It will be understood that this is preferably an end product, such as a consumer product or industrial product, that is sold and whose

authenticity may therefore require verification at a later date. By printing directly onto the product, for example as a step on a production line, the invention permits the creation of security devices on the products without disrupting the rate of production of the

products. The security device preferably includes

variable data relating to the product, such as a serial number or time of manufacture. The data may be included as plain text or may be encoded, for example in a

machine-readable format, such as a bar code.

Preferably the substrate is a dark substrate. The dark substrate may be light absorbing and/or non- or

minimally-reflective . It may be a black substrate. The dark substrate may be a layer of dark, preferably black, ink printed or coated onto a surface. The visual features of the security device are advantageously more readily discernible when printed on such a substrate. For

example, the colours may be more vibrant against a dark substrate .

According to a second aspect of the invention, there is provided a security device obtainable by a method

according to the invention, for example according to the first aspect. Preferably there is provided a security device obtained by a method according to the invention, for example according to the first aspect.

According to a third aspect of the invention, there is provided a security device comprising a substrate

carrying a composite image on an image area thereon, the image area divided into a two-dimensional array of printing regions and the composite image comprising: a first image formed by applying a first liquid material onto a first component image area in the image area comprising every alternate printing region; a second image which may be the same as or different from the first image formed by applying a second liquid material onto a second component image area in the image area comprising each remaining alternate printing region;

wherein the first and second liquid crystal materials have opposite chirality.

The security device in this aspect thus constitutes a composite image made up of two component images.

The first and second liquid crystal materials may be formed by printing and for example by inkjet printing. According to a fourth aspect of the invention, there is provided a security device comprising a substrate

carrying a composite image on an image area thereon, the image area divided into a two-dimensional array of printing regions, the composite image comprising a first distribution of a first liquid crystal material and a second distribution of a second liquid crystal material; wherein the first distribution of the first liquid crystal material comprises a two-dimensional array of picture elements corresponding to a first image wherein the distribution is applied across every alternate printing region of an image area on the substrate, and the second distribution of the second liquid crystal material comprises a two-dimensional array of picture elements corresponding to a second image wherein the distribution is applied across each other alternate printing region of an image area on the substrate;

wherein the first and second liquid crystal materials have opposite chirality.

More completely the composite image comprises a first distribution of a first liquid crystal material and a second distribution of a second liquid crystal material; wherein the first distribution of the first liquid crystal material comprises a two-dimensional array of picture elements corresponding to a first image wherein the distribution omits liquid crystal material from every second picture element, the second distribution of the second liquid crystal material comprises a two- dimensional array of picture elements corresponding to a second image wherein the distribution omits liquid crystal material from every second picture element, the picture elements not omitted from the second distribution corresponding to the picture elements omitted from the first distribution; wherein the first and second liquid crystal materials have opposite chirality.

The first and second liquid crystal materials may be printed and for example printed by inkjet printing.

As discussed above, such a device may produce an overt visual effect that is only detectable in specific viewing conditions, whilst maintaining the covert features of the liquid crystal printing and being produced in a cost- effective manner by using two liquid crystal materials or compositions of opposite chirality and for example two enantiomers of opposite chirality of the same liquid crystal .

The first and second liquid crystal materials may be applied as respective inks comprising one of the first or second liquid crystal materials as the case may be. The liquid crystal materials may be in the form of discrete drops, or may be a continuous coating formed, for

example, by the coalescence of a plurality of drops.

Preferably the liquid crystal materials are printed onto the security device by inkjet printing. Preferably the security device is produced in accordance with the invention, for example in accordance with the first aspect of the invention.

The security device of one of the second to fourth aspects of the invention may comprise the substrate, for example when the security device is formed on a label, or the security device may exist on the substrate, for example when the substrate is a packaging container such as a carton or when the substrate is the surface of an industrial or consumer product. The security device may be formed on a label, a carton, a packaging container, a surface of a product, a document, currency, a paper substrate, a metallic substrate, a tamper evident substrate, a polymer substrate, a glass substrate or a PET substrate. The security device may be formed on a surface of a consumer or industrial product. The security device is preferably formed on a dark, absorbing, or non-reflective substrate, for example comprising or being applied to any of the foregoing.

According to a fifth aspect of the invention, there is provided a method of authenticating a product, the method comprising providing on the product a security device according to the invention, for example in accordance with the first, second, third or fourth aspects;

inspecting the security device in such conditions as to separately view images generated by the first and second liquid crystal materials; and, based on the inspection, verifying the authenticity of the product.

In particular, inspecting the security device in such conditions as to separately view images generated by the first and second liquid crystal materials comprises inspecting the security device through a circularly polarising filter.

Cholesteric liquid crystals such as may be used to implement the invention are known to reflect circularly polarised light which can be selectively transmitted or extinguished by a circular polarising filter. The

invention provides a composite image that has been printed with a liquid crystal ink which reflects left handed light and the other component has been printed with a liquid crystal ink which reflects right handed light. The viewing conditions to render the two component pictures sequentially visible is the sequential use of left and right handed circular polarising filters.

Thus, the method may comprise viewing the device and the composite image thereon through a circular polarising filter, and optionally sequentially through left and right handed circular polarising filters, wherein the verifying further comprises identifying the first and second component images thereby. Such an effect may form the basis of an authentication device, which comprises the polarising filter or filters. The method may comprise viewing the security device with left and right handed circular polarising filters in turn. Preferably, the authenticating is carried out using an authentication device having a camera, left and right handed circular polarising filters, wherein the authentication device captures an image of the security device using the camera with each filter in use and optionally additionally without the polarising filters. The authentication device may further comprise image recognition software to compare the images and identify the extent to which the view is changed by application of the left and right handed circular polarising filters.

The authentication device preferably stores expected images, colours or values in a database either on the authentication device or in a cloud location to which the authentication device has access.

It will be appreciated that features described in

relation to one aspect of the invention may be equally applicable to other aspects of the invention. For

example, features described in relation to a method of producing a security device of the invention may be equally applicable to a security device of the invention or a method of authenticating a product of the invention and vice versa. It will also be appreciated that optional features may not apply, and may be excluded from, certain aspects of the invention.

Description of the Drawings

The invention will be further described by way of example only with reference to the following figures, of which:

Figure 1 is a raw print pattern, or bitmap, for each of two constituent images suitable for use in a method according to the invention to create security devices according to the invention;

Figure 2 is a modified print pattern, or bitmap, for each of the constituent images of figure 1;

Figure 3 is a print pattern, or bitmap, for the

combination of the two images of figure 2;

Figure 4 is a printed image printed in accordance with a method according to an embodiment of the invention to create a security devices according to an embodiment of the invention;

Figure 5 is the image of figure 4 viewed through a left- handed circular polarising filter;

Figure 6 is the image of figure 4 viewed through a right- handed circular polarising filter;

Figure 7 is the image of figure 4 viewed at a 45 degree angle ;

Figure 8 is a set of print patterns, or bitmaps, for a further example composite image for use in a method according to the invention to create security devices according to the invention;

Figure 9 is a printed image created from the print patterns of figure 8 and viewed through no, left handed polarising and right handed polarising filters

respectively

Detailed Description

The invention is described by way of example in the context of a method of creating a security article in accordance with the principles of the invention by inkjet printing .

As a first stage of the method, illustrated with

reference to figure 1, each of a first component image and a second component image which are intended to make up the composite printed image of the security device is converted into an image data file by converting the image into a two-dimensional array of picture elements, and creating an image data file consisting of a set of print instructions for each picture element in the array. In the embodiment, two such print patterns are created as bitmaps, respectively for an image of a crown 1, and an image of a rose 2.

In the eventual composite image, the component images are interposed with each other over a single area on the substrate, and accordingly the images are correspondingly sized and shaped for this. It is generally the intention that when the two images are printed together, and in normal viewing conditions, since the two liquid crystal materials constitute inks of essentially identical colour in normal viewing conditions, neither image should be readily visible. To give effect to this, the images should be selected to have broadly similar intensities, and to have patterns such that neither substantially dominates. The two images should be manually adjusted to be approximately the same size, to within 1-2 mm. The two images should be chosen so that the outline of each image is at least in part coincident. This will make each component image harder to recognize. It is probably useful to choose component images with a similar aspect ratio and to choose images with the same sort of

symmetry .

Each bitmap of the images 1, 2 comprises in the case of the embodiment a set of digital instructions for printing the image as a two-dimensional array of pixels in which each pixel is either printed or not. The two bitmaps constitute two-dimensional arrays of pixel by pixel instructions with each array being exactly the same size so that the two arrays may be made exactly co-incident in area .

The raw bitmaps generated at figure 1 are converted to modified bitmaps suitable for co-printing of the two component images as a composite image by selective removal from each of alternate pixels or alternate blocks of pixels. This produces the two modified bitmaps 1', 2 shown in figure 2.

The pixels or blocks of pixels removed from the first image 1 correspond to the pixels or blocks of pixels retained in the second image 2 and vice versa. To ensure consistency of alignment when printing each bitmap also includes an identically positioned check mark 3 in the form of a cross. Consistent positioning of this mark ensures that the images can be printed with the required offset .

A useful print resolution when handling liquid crystal inks is 1270 dpi. This equates to a droplet of ink every 20 micrometres. A typical ink droplet for such a

resolution might be 10 picolitres. This means in the image manipulation software that any detail added to the image is added in increments of 20 micrometres.

Similarly, if any part of the image is erased it is erased in increments of 20 micrometres.

The two raw bitmap images are made exactly the same size, to the very pixel. Both images are set for printing at the same resolution or dots per inch (dpi) . The first image is viewed in suitable image manipulation software under high magnification. In a possible case every other pixel of the image is deleted. In an alternative case pixels are grouped into blocks, for example of n x n pixels, and every other pixel block of the image is deleted. As the printer used in the example printing described below only operates in one-bit mode (i.e. ink or no ink) the deleted pixels of the image appear white. Under high magnification the first image has chequerboard regions. When zoomed out, the image just appears grey.

The second image is then also viewed in the image

manipulation software under high magnification. Every other pixel or every other pixel block of the image is then deleted, however, the proviso is that the pixels/ blocks deleted in the second image are the opposite to the pixels/ blocks that were deleted in the first image. So in the first image, starting at the top left corner, the pixels or blocks may be treated: leave, delete, leave, delete and so on, whereas in the second image starting at the top left corner the pixels or blocks would then be treated: delete, leave, delete, leave and so on .

A simple approach might be to remove alternate individual pixels. In practice, this approach does not always work as the ink flows or 'bleeds' from the region where it is theoretically deposited. The result is that a droplet of ink, measuring 10 picolitres in volume, and supposedly occupying a 20x20 micrometre space on the substrate will spread beyond its theoretical confines. Typically, the ink droplets will spread or 'bleed' by 10-20 micrometres. Thus when the second picture is printed there may be no spaces left in the first picture for the drops of the second ink to fill.

The problem of ink bleed may be reduced by dividing each component picture into larger blocks of pixels. In the example embodiment blocks are 6x6 pixels in size. If printing is carried out at 1270 dpi this means that the picture is printed as blocks of pixels 120x120 micrometre square. There should be no ink in the most part of the space left between the blocks of pixels. It should be noted that where a block of 6x6 pixels overlays the outline of the image if the block of pixels is to be left, then the original outline of the image is still left with the original resolution (corresponding to 1270 dots per inch) but if the block of pixels is to be deleted, then the resolution of the outline drops to 211 dots per inch.

As has been noted previously the ink droplets

constituting the blocks of pixels applied in the first image will tend to bleed and encroach on the area nominally available for the second image. A possible refinement to the method would be to divide the second image into blocks of pixels having the same size and corresponding to the same location as the blocks of pixels left vacant in the first image but modifying the print instruction such that those blocks of pixels in the second image corresponding to where ink is actually to be applied have ink applied to an area smaller in dimension than the blocks of pixels deliberately left vacant in the first image. The raw bitmaps generated at figure 1 were thus in the example converted to be suitable for co printing of the two component images as a composite image by dividing each image into blocks of 6 x 6 pixels and removing alternate blocks of pixels.

A notional composite image produced by the combined bit maps is presented in figure 3, the coincident alignment mark represented by the small cross 3 being used for aligning the printer when printing the second image so that it is aligned with the appropriate offset relative to the first to produce the composite image print pattern 4.

The composite image print pattern was used to print security devices according to the invention using liquid crystal materials by inkjet printing. Examples of liquid crystal materials suitable for inkjet printing are disclosed in W02008/110342 and W02008/110317. Such formulations typically contain a non-reactive liquid crystal, mono-acrylate liquid crystal, diacrylate liquid crystal, chiral dopant, photo initiator and inhibitor. Other such formations typically contain mono-acrylate liquid crystal, diacrylate liquid crystal, chiral dopant, photo initiator and inhibitor. The security devices were printed in a multi-pass manner using a Fujifilm Dimatix DMP2831 printer (Fujifilm, Lebanon NH, United States of America) . With the DMP2831 printing occurs on the outbound motion of the print head away from the blotter, which when complete, is followed by incremental advance of the platen in a manner

perpendicular to the direction of printing. The procedure then repeats until the image has been completed. The controlling software regards the print resolution as being defined by the angle at which the print head is positioned relative to the direction of printing.

However, if the data file, or bitmap, for the image has regions of varying pixel density, then even though the sabre angle of the printer does not vary whilst printing is underway, images where the print resolution varies throughout the image can still be achieved.

Figure 4 shows the security feature represented by the composite picture created using the bitmaps of figure 2 to print onto a suitable substrate. Figure 4 is a photograph of the printed security feature taken without the use of a polarising filter, and thus represents the view of the composite image as it would appear with an unaided eye.

The printer used to generate the example of figure 4 was one that can only print one ink at a time. In the example shown it was necessary for the first picture to be printed, the ink cartridge to be changed and the second picture to be printed. Given that the pictures must be carefully aligned so that the drops of ink of the second picture only fall on the deliberate spaces between the drops of ink of the first picture, the registration of the printhead needs to be as good as possible. In order to achieve a good registration, the check cross was provided as above described. Alternatively, a feature on the first image could be picked out and used as a

reference point when the next image is printed. However, as the print cartridge in this process has changed from one picture to the next, the printer must also undergo a "drop off-set correction" in order to correct for any discrepancy in how the second ink cartridge was inserted.

Although such a printer was not used to generate the illustrative examples, a preferred printer might

constitute a single pass composite printer including two separate print heads to carry each ink.

The invention requires the preparation of two liquid crystal inks capable of being jetted. The two inks constitute identical inks, except that the chiral dopant in the liquid crystal component of each ink has the opposite handedness. This means that one of the inks polarises light in a right-handed direction and the other in a left-handed direction, but on viewing in unpolarised conditions they appear the same colour. Although liquid crystal inks of different handedness are used to print the two component images, the optical variability shown by each of the component pictures should be the same. As a result, the colour of both printed pictures should shift by the same amount on tilting of the substrate. However, the two printed images should reflect circularly polarised light differently.

Figures 5 and 6 are photographs of the image of figure 4 respectively viewed through a left handed circular polarising filter and a right handed circular polarising filter. The rose motif is clearly visible when the composite is viewed through the left handed circular polarising filter and the crown motif is clearly visible when the composite image is viewed through the right handed circular polarising filter. Importantly, it can be seen that the rose and crown motifs are essentially free of interference from the other motif. The motifs in the example were chosen to be immediately recognisable, everyday symbols. As a result, under the viewing

conditions represented by the use of one or other filter, the component pictures of the composite image are readily visible in a manner in which they are not in the

composite picture viewed by the unaided eye as shown in figure 4.

Figure 7 shows the image of figure 4 viewed at a tilt angle of viewing of 45°, but otherwise without optical aides, and in particular without polarising filters.

Both the component pictures in the printed feature exhibit optical variability in that they both change colour on angular viewing. The colour of the printed image in Figure 7 is pale-mid green; by contrast the colour of the images in Figures 2,3 and 4 which are viewed perpendicularly are mid-orange.

It will be appreciated that the embodiments set out above are examples of the invention and that the skilled person would appreciate that variations are possible within the scope of the claimed invention. For example, many

different patterns of the first and second "image" are possible .

In particular, in the above example embodiment, each component image making up the composite image is an image in the narrow sense, in that it is in itself a

recognisable motif of an object. A security device produced according to such principles will be preferred in many circumstances, since it makes it inherently likely that each component image will be recognisable under the appropriate viewing conditions of polarised light .

However, the invention is not limited to the printing of "images" in the narrow sense in which a component "image" is a recognisable picture, logo or motif. In alternative examples, one of the "images" may be a recognisable picture, logo or motif and the other may be a systematic or random pattern, for example to serve as a mask.

Additionally, or alternatively, one or both images may constitute an insignia, marking or code, such as a one or two-dimensional code, and might for example be a QR or barcode. The principles described in relation to the above embodiment are applicable to all such combinations of first image and second image and any other

combination .

By way of example Figure 8 is a set of print patterns, or bitmaps, for a further example composite image

incorporating a QR code. Illustrated from the top are a composite image 21 and the two respective component images 22, 23. The composite image is shown printed in Figure 9, respectively from the top in a normal viewing condition and with left and right polarised filters. It can be seen that the QR code is difficult to distinguish in normal viewing conditions but readily distinguished with the appropriate polarising filter.