WO2019175579A1

WO2019175579A1 - Security device, method of making a security device and method of authenticating a product

Info

Publication number: WO2019175579A1
Application number: PCT/GB2019/050701
Authority: WO
Inventors: Damian James GARDINER; Duncan MCCALLIEN; Felicity Jane ROBERTS
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2018-03-13
Filing date: 2019-03-13
Publication date: 2019-09-19
Also published as: GB2573048A; GB201903428D0; GB201803979D0

Abstract

A method of producing a security device is disclosed. The method comprises ink jet printing a liquid crystal material onto a first region of a substrate in a first printing direction, and inkjet printing the same liquid crystal material onto at least one further region of the substrate in at least one further printing direction at an angle to the first printing direction. Security devices and authentication methods using such security devices are also disclosed.

Description

Security device, method of making a security device and method of authenticating a product.

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 ink-jet printing.

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.

Chiral nematic liquid crystals exhibit unusual

spectroscopic properties such as reflecting circularly polarized light and exhibiting a variation in colour with viewing angle. This can be contrasted with conventional inks, where the colour observed depends upon the

interaction of the ink with the substrate, the colour of the substrate and the print density. If the surface of the substrate is uniform, then common experience would lead us to expect that the colour observed at a

particular angle of viewing with a particular

illumination would be invariant.

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. Chiral nematic liquid crystals can be formulated as inks and deposited onto a substrate by inkjet printing. 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 inkjet printing a liquid crystal material onto a first region of a substrate in a first printing direction, and inkjet printing the same liquid crystal material onto at least one further region of the substrate in at least one further printing direction at an angle to the first printing direction. That is, the method comprises: inkjet printing the same liquid crystal material onto a plurality of regions of a substrate, wherein each region is printed in a printing direction, and wherein each printing direction is at an angle to each other printing direction.

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 as the print head moves across the substrate in a movement direction. Thus, in accordance with a more complete embodiment of the invention a method of producing a security device comprises providing a print head of an ink-jet printer with an ink comprising a liquid crystal material; inkjet printing the ink comprising the liquid crystal material onto a first region of a substrate while moving the print head in a first movement direction, and inkjet printing the same ink onto at least one further region of the substrate while moving the print head in at least one further movement direction at an angle to the first movement direction, and in the case of printing the same ink onto a plurality of further regions while moving the print head in a corresponding plurality of further movement directions each at an angle to each other movement direction.

At least some of the plurality of regions may be formed as discrete regions on the substrate so that the area of liquid crystal applied to each region is discrete and separate from the area of liquid crystal applied to the each other region. Preferably in this case each such discrete region abuts at least one further region. The visual effects of the invention may be more readily perceived by the human eye when the discrete regions abut one another.

Additionally or alternatively, at least some regions may be at least partly coincident so that the area of liquid crystal applied to at least one of the said regions at least in part overlays the area of liquid crystal applied to at least one other of the said regions.

The perceived colour of a printed region depends in particular on the wavelength of peak reflectance of the ink. If a conventional ink is printed by making multiple passes of a print head across a substrate, the wavelength of peak reflectance remains invariant despite the

multiple passes although the intensity of the reflected light changes if the multiple passes produce variation in volume of ink per unit area.

However, it is known that chiral nematic liquid crystals for example exhibit unusual spectroscopic properties and in particular may exhibit a variation in peak reflectance with viewing angle. Surprisingly, in the present

invention, it has been found that varying the direction of printing between each of a plurality of regions creates an effect whereby the variance in wavelength of peak reflectance coupled with the efficiency of

reflection with viewing angle that occurs as a property of the printed liquid crystal material does not

necessarily occur identically as between each of a plurality of regions but instead may vary differentially depending on the angle of viewing relative to the print direction .

Although the invention is not limited by any particular theory, it is considered that this is consequence of the directional nature of the inkjet printing coupled with the multi-pass printing process. The response of a region of printed liquid crystal material is a consequence of the alignment of its component molecules in addition to the material deposited in the preceding printing pass which can overlap the subsequent printing pass. The directional nature of these effects is sufficient for this alignment to vary as between regions printed at different angles. Subject to appropriate selection of material, a discernible and useful differential behaviour may be obtained.

Liquid crystal materials can exhibit optical variability, reflecting shorter wavelengths of light when viewed at more oblique angles. The variation in intensity of reflectance associated with printing the liquid crystal at different orientations on the substrate becomes more pronounced when the angle of viewing is more oblique, varying from good reflectance at certain print

orientations to zero reflectance at other print

orientations .

The varying alignment referred to above means that this effect may occur differentially as between different regions. For a given angle of viewing under a single light source the first region will have a first

wavelength of peak reflectance and a second region will have essentially the same peak reflectance but may have a noticeably different magnitude of reflectance. Since the colour perceived by the eye is a combination of the wavelength of peak reflectance and its magnitude plus any reflectance from the substrate, that means the first region can have a first colour and the second region a second colour. In hue/ saturation/ value colour space terminology under a single light source, the hue of each colour is the same but the brightness and value will differ. The effect of this is to produce a colour change that may vary differentially between each of a plurality of regions as the angle of viewing of a security device printed in accordance with the method of the first aspect of the invention is changed. Such an unusual visual effect is highly advantageous in the production of a security device as it allows the creation of visually attractive and readily recognisable security features with the use of a single liquid crystal material.

It is very 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 striking overt features using a single liquid crystal material. 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 .

Using a single liquid crystal material 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. Ink jet printing is a preferred method for such applications, and the availability of striking overt security features by inkjet printing a single liquid crystal material is therefore advantageous. Preferably the liquid crystal material is therefore a liquid crystal ink formulated for inkjet printing.

Each region of the plurality of printed regions may be printed in a single pass. Each region of the plurality of printed regions may be printed in multiple passes. The plural regions are preferably 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.

Multi-pass printing may be advantageous in producing striking visual effects.

Colour can be quantified in terms of the HSV colour space, where the colour is given three coordinates describing Hue (H) , Saturation (S) and Value (V) . Hue is the shade of the colour, with red around 0 and blue around 240. Saturation is the colour intensity, where 0 is grey and 100 is the pure colour. Value describes lightness, the degree between black and white (0 and 100 respectively) .

The unusual spectroscopic properties of chiral nematic liquid crystals such as without limitation their ability to reflect circularly polarized light and to exhibit variation in intensity and hue of colour with viewing angle can be exploited by appropriate printing of

different regions in accordance with the method of the invention to produce a security device on a substrate exhibiting one, some or all of the following effects.

1) The brightness or in HSV space the Value of a region of the image will vary if the substrate is rotated on the spot whilst the position of the observer and the position of the illumination are kept constant. The relative brightness of plural regions printed in accordance with the principles of the invention at different orientations may therefore be observed to vary differently.

2) The colour or in HSV space the hue of the image will vary if the substrate is rotated on the spot whilst the position of the observer and the position of the illumination are kept constant. The colours of plural regions printed in accordance with the principles of the invention at different

orientations may therefore be observed to vary differently.

3) The effects described in either 1) and 2) may

optionally be made far more visible by printing the image close to a reference image or associated sub image that forms a whole. Preferably the method of the invention includes applying such a reference image or associated sub-image to the substrate.

Preferably, the reference image or associated sub image should also be printed with liquid crystal ink. The ink may be the same as or differ from that used to create the plurality of regions. On rotating the substrate, one image will become easily visible and the other will be less visible and vice-versa. The situation will reverse on further rotation. 4) The visibility of a liquid crystal image at an arbitrary angle of viewing and illumination may be improved by overprinting the image with a second image, where the overprinting is carried out with the substrate in a different orientation to the way the first image was printed.

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. The substrate may be a printed circuit board (PCB) . 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 .

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 liquid crystal material is printed in a first

direction on a first region of the substrate and in at least one further direction on at least one further region 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 both x and y directions (i.e. parallel to the movement of the print head and by virtue of the print head being at an angle to the

substrate and also by moving the substrate perpendicular to the movement of the print head) . The print direction in this context means the direction of movement of the print head.

Preferably all other aspects of the print process, such as for example jetting voltage, jetting frequency, non jetting voltage, waveform, ink temperature, meniscus pressure, platen temperature and curing conditions, remain constant, with only the printing direction being changed. In other words, changing the printing direction may be sufficient to produce the differences exploited by the invention, and in particular the differential variability in peak reflectance at a given angle of viewing for different regions without needing to change other print properties.

In a possible alternative, the liquid crystal material may be printed at a first volume per unit area in a first region of the substrate and a second volume per unit area in a second region of the substrate. 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.

The volume per unit area may be varied in this way to produce further differential effects as between the plurality of regions.

It will be appreciated that each region making up the plurality of regions each having a printing direction at an angle to each other could have any size or shape.

Optionally, at least some of the regions making up the plurality of the printed regions may be similarly and for example identically sized and/or shaped to each other. Optionally, at least some of the regions making up the plurality of the printed regions may be differently sized and/or shaped to each other.

In accordance with the invention each region making up the plurality of regions has printed thereon liquid crystal material having a printing direction at an angle to that of each of the other regions. Preferably an angle between at least two printing directions is at least 60°. Preferably, each such angle between printing directions is at least 60°. Conveniently, each such angle may be about 90°, the printing directions being orthogonal to each other.

In a possible embodiment, the security device comprises at least three of the said regions having a printing direction at an angle to that of each of the other regions, and the method comprises ink jet printing a liquid crystal material onto at least three regions of the substrate in at least three respective printing directions each at an angle to the other.

In a preferred case, four regions are provided, having a printing direction at an angle to that of each of the other regions, and the method comprises ink jet printing a liquid crystal material onto four regions of the substrate in four respective printing directions each at an angle to the other. Preferably, exactly four regions are provided. Preferably, the four regions are printed in generally orthogonal printing directions.

It will be appreciated that a region making up the plurality of regions may be block printed with a

substantially uniform distribution of ink across a print area of the region corresponding to or contained within the region, or may have printed upon it on a print area of the region corresponding to or contained within the region a pattern, design, motif or image as desired, the region in consequence having printed thereon the liquid crystal material in a suitable distribution and density to create the desired pattern, design, motif or image.

Optionally, at least some of the regions making up the plurality of the regions may be printed with identically sized, shaped and conformed printed patterns, designs, motifs or images, differing only in their respective printing directions. Optionally, at least some of the regions making up the plurality of printed regions may be printed with patterns, designs, motifs or images that are differently sized and/ or shaped and/ or conformed from those printed on other regions.

Preferably the plurality of regions each having a

printing direction at an angle to each other printing direction or the printed areas thereon form part of an insignia, marking or code wherein different regions of the insignia are printed with the liquid crystal material at different printing directions. For example, the plurality of regions each having a printing direction at an angle to each other may form part of 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 and exhibiting different colour effects in different regions despite being printed with the same liquid crystal material.

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, and barcodes according to the invention preferably have a first element printed in a first direction and at least one further element printed in at least one further direction such that there is a variance in exhibited effects with angle of viewing between the first element and the at least one further element.

The plurality of regions each having a printing direction at an angle to each other printing direction or the printed areas thereon may be arranged according to a design which aids in the authentication of a product or item. The regions may be arranged according to a design or rule, so as to permit more ready authentication. Such a design could be a radial, linear, non-linear or

geometric arrangement or patterning of the regions with different volumes of liquid crystal material per unit area, for example. There may be further regions printed or coated with another material, such as a conventional ink or a different liquid crystal material. Such regions may add to the visual effect of the invention, and may result in advantageous security devices, but are not essential to the invention.

The wavelength of peak reflectance may be determined by plotting a spectrum of intensity of reflected light against wavelength. The liquid crystal material will typically reflect light across a relatively narrow band of wavelengths. The wavelength of peak reflectance can be determined for example using the peak picking function of a spectrometer. Alternatively the wavelength of peak reflectance could be determined from the midpoint of the reflection band. The skilled person is able to determine the peak in a reflectance spectrum and the precise peak finding method used is not critical to the invention. It is sufficient that there is a potential difference in the wavelength and/ or magnitude of the peak reflectance in different regions at a given viewing angle and that this varies differentially as the viewing angle is changed and/ or the image is rotated. Preferably the difference in the wavelength and/ or magnitude of peak reflectance is such that a first region has a different colour to a second region when viewed by eye at at least some viewing angles or when rotated.

When using one light source, all that varies between printed regions is the magnitude of peak reflectance and not wavelength of the peak reflectance.

When two light sources are used then other effects become possible. For instance, if the light sources are

suitably arranged it is possible for one printed region to reflect one colour principally from one light source and for the adjacent printed region to reflect a

different colour using light principally from the other light source. Thus adjacent regions can appear different colours .

Alternatively, if the light sources are suitably arranged then the perceived colour in one region colour can be created from light from one of the sources, but on slightly altering the angle of the sample the perceived colour can be created principally from the other light source. The effect is a rapid flip from one colour to another colour.

Preferably the liquid crystal material exhibits a

variation in colour with viewing angle. Thus there is a colour shift in each region with viewing angle. Printing the liquid crystal material at different angles as between the different regions advantageously affects the shift in the colour of the liquid crystal material with viewing angle. Preferably the liquid crystal material exhibits a variation in colour with viewing angle and the variation in colour is different in the first region to the second region. Preferably, on changing the viewing angle of the device, for example from a viewing angle of 90° (i.e. perpendicular) to the substrate to a viewing angle of 45°, the degree of reflection at the wavelength of peak reflectance of the first region shifts to an extent that differs from the degree of reflection at the wavelength of peak reflectance of the further region. Preferably additionally or alternatively, on changing the viewing angle of the device for example from a viewing angle of 90° to a viewing angle of 45°, the colour hue in the first region changes by a first amount and the colour hue in the second region changes by a second amount. For example the hue generated by the peak reflectance of the first region shifts by an amount that differs from the hue generated by the peak reflectance of the further region. The result is particularly advantageous as not only is the colour different between the regions, but also, as the device is tilted, the colours in the two regions both change to different colours. The overt security feature may thus be one that is readily

discernible to an inexpert observer and therefore

particularly valuable for marking products to allow their authenticity to be verified. Preferably the change of colour of the first region to the unaided eye is greater than the change of colour of the second region to the unaided eye.

For example, when changing the viewing angle of the security device from a viewing angle of 90° (i.e.

perpendicular) to the substrate to a viewing angle of 45° some regions may start brightly coloured and some almost colourless. The result is that for a given change in angle of viewing some regions will markedly change colour and others not. If there is more than one light source, then it is possible for regions to change to different colours at an arbitrary angle of viewing.

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 first region of a liquid crystal material and at least one further region of the same liquid crystal material, wherein each of the first and at least one further region has been inkjet printed in a printing direction, and wherein each

printing direction is at an angle to each other printing direction .

The invention in the third aspect is thus for example a product of the method of the first aspect. In particular, a security device is provided having a liquid crystal material inkjet printed in a plurality of regions in a corresponding plurality of printing directions at an angle to each other printing direction.

It is known that chiral nematic liquid crystals for example exhibit unusual spectroscopic properties and in particular may exhibit a variation in peak reflectance with viewing angle. Surprisingly, in the present

invention, it has been found that varying the direction of printing between each of a plurality of regions creates an effect whereby the variance in response with angle of viewing that occurs as a property of the printed liquid crystal material does not necessarily occur identically as between each of a plurality of regions but instead may vary differentially depending on the angle of viewing relative to the print direction.

Thus in a security device in accordance with the

invention, identifiably different properties can be produced in each region depending on the printing

direction used to print the liquid crystal material in each region. Identifiably different properties can be produced in each region using the same material for each region and merely changing this fabrication process parameter .

The effect of this is to produce a colour change that may vary differentially between each of a plurality of regions as the angle of viewing of a security device is changed .

As discussed above, such a device may produce a striking overt visual effect, whilst maintaining the covert features of the liquid crystal material and being

produced in a cost-effective manner by using the same liquid crystal material in both regions.

The liquid crystal material may be comprised of discrete drops, or may be a continuous coating formed, for

example, by the coalescence of a plurality of drops.

The liquid crystal material is printed 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. Preferably the security device is printed on a substrate. The security device 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.

According to a fourth aspect of the invention there is provided a method of producing a security device, the method comprising inkjet printing a liquid crystal material onto a first region of a substrate in a first printing direction, and ink jet printing the same liquid crystal material onto at least one further region of the substrate at an angle to the first printing direction such that the visible colour response of the liquid crystal material on the first region is different to the visible colour of the liquid crystal material on the at least one further region.

In particular, the method comprises printing a liquid crystal material that is known to exhibit a visible colour response where the peak reflectance wavelength varies with viewing angle, and the first and at least one further regions are printed in different printing

directions such that the variation of peak reflectance wavelength with viewing angle occurs differentially as between the first and the at least one further region.

The provision of a security device having different visible colours at different viewing angles using a single ink may be advantageous in producing a visually striking security device in a time- and cost- effective manner. Such an effect may be difficult to replicate with conventional inks, whilst easy to recognise without special tools or training, and may thus provide a highly effective security device. 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 at a first viewing angle and identifying a first colour response in the first region and a further colour response in the further region; inspecting the security device at a second viewing angle and identifying a first variation in colour response in the first region and a further variation in colour response in the further region that differs from the first variation in colour response; and, based on the comparison, verifying the authenticity of the product.

Such an inspection may be achieved by tilting the device and observing the colour shift. A particularly

advantageous feature of the device is that the shifts in the first and second regions are different, and that difference may be readily identified, even by an

untrained observer, when the shifts are viewed

simultaneously on tilting of the security device. The shifts may thus provide a striking overt security feature that can be readily examined to provide a check of authenticity. For example, a consumer or retailer could confirm the appearance of the security device compared with another security device.

The comparison may be carried out, for example, by eye or using a digital device, such as a smartphone with a camera or a bespoke authentication reader.

Preferably the authenticating is carried out using an authentication device that identifies the first and further regions and compares colour shifts between the regions. Preferably the authentication device compares the colour shifts by comparing features of measured reflectance spectra with features of expected reflectance spectra. Preferably the authentication device comprises a camera and image recognition software to identify the first and further regions. The authentication device may comprise a smartphone.

For example, the authenticating is carried out using a device comprising a camera and image recognition software that identifies the first and second regions and compares the shifts in those two regions as the security device is tilted. Preferably the image recognition software

calculates viewing angle by comparing relative positions of features of the security device. The authentication device may compare the reflectance spectra, for example the wavelengths of peak reflectance, of the first and second regions at first and second viewing angles with expected reflectance spectra, for example expected wavelengths of peak reflectance, for those regions at those viewing angles. Preferably the authentication device automatically calculates the viewing angle, for example by comparing the relative positions of features of the security device. As the viewing angle of the device changes, the relative positions of the features will change and the authentication device preferably tracks the features and calculates the change in viewing angle. The device preferably records the image of the device at the correct viewing angles and compares the colours of the image with the colours of an expected image at those angles.

An authentication device may compare the variable colour responses displayed by those regions as the viewing angle in changed to an expected colour response from a database stored either on the device or in a cloud location to which the device communicates.

In some embodiments, the inspecting may be carried out using a microscope. That may advantageously permit the inspection of small regions, for example variations printed along a single line, that may not be discernible by eye. In that way, a covert security feature may be provided .

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 security device according to the invention printed with a liquid crystal material illuminated from above and shown in four views respectively at four rotations at 90° to each other; Figure 2 is a further embodiment of security device according to the invention liquid crystal material illuminated from above and shown in four views

respectively at four rotations at 90° to each other;

Figure 3 is the security device of figure 1 in an

alternative view now tilted at 45° to the horizontal and shown in four views respectively at four rotations at 90° to each other;

Figure 4 is the security device of figures 1 and 3 viewed and illuminated at an arbitrary angle;

Figure 5 is the security device of figure 2 in an

Figure 6 is the security device of figures 2 and 5 viewed and illuminated at an arbitrary angle;

Figure 7 is an example of a liquid crystal material printed on a substrate according to the method of the invention and illustrative of some features of the invention;

Figure 8 is a further example of a liquid crystal

material printed on a substrate according to the method of the invention and illustrative of some features of the invention;

Figure 9 is a further example of a liquid crystal

material printed on a substrate according to the method of the invention and comprising an embodiment of the invention . Detailed Description

Referring to Figure 1, a security device 1 in accordance with the principles of the invention is shown which has been inkjet printed by applying liquid crystal material in the form of a chiral nematic liquid crystal based inkjet ink to form a printed pattern 3 on a PCB substrate 2.

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.

In the illustrated embodiment, a square area of the surface of the PCB on which the printed pattern 3 is to be applied is in effect notionally divided into four identically sized square regions, in each of which a printed area is printed with a component part of the printed pattern 3. Each of the four printed areas, respectively designated e, f, g, h, consists of an identically shaped arrow motif, the four printed areas abutting each other in such a way that the arrows abut each other and form the circular printed pattern 3.

The four printed areas, respectively designated e, f, g, h, are printed with a print direction at 90° to each other. That is to say, the inkjet printer has been used to create each printed area in familiar manner in which a print head moves over and in parallel to the substrate surface and deposits ink in a two-dimensional array of dots in x, y directions onto the substrate surface while the head moves progressively in the x direction

corresponding to the print direction.

In the examples, 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. This defines a printing direction.

The printed areas are for example printed in four

successive passes, for example four successive single passes, of the print head, with the x direction of movement of the print head being changed by 90° between each pass. Alternatively, the printed areas are printed in four successive groups of multiple passes with the x direction of the print head being changed by 90° between each group of passes.

Figure 1 shows pictures taken with a static camera with the images illuminated from a single direction of

illumination, in the example both illuminated from and viewed from above, but with the substrate rotated

respectively by 0° in figure 1A, by 90° in figure IB, by 180° in figure 1C, and by 270° in figure ID. For

clarity, the photographs have been rotated so that the PCB is always portrayed in the same orientation. The selected ink has a peak reflectance at this angle of illumination that creates a red/ orange hue of peak reflectance about 620 nm. In broad terms the illustrated effect is that the wavelength of peak reflectance of the sample remains relatively constant but the brightness varies dramatically. This was listed as "effect 1" above, whereby the brightness of a printed image can be found to vary if the image is rotated whilst the position of the observer and the position of the illumination are kept constant. The varying brightness but constant wavelength give rise to a variation in hue. This was listed as "effect 2" , whereby the hue of the image can be found to vary if the image is rotated whilst the position of the observer and the position of the illumination are kept constant.

Figure 2 shows a further embodiment of the invention in which the same effect is illustrated by creating a security device 11 comprising the same patterned area 13 on a PCB substrate 12, again consisting of four otherwise identical arrows printed in a four orthogonal print directions and respectively designated a, b, c, d. A different ink is used.

Figure 2 again shows pictures taken with the images illuminated from a single direction of illumination with a static camera, in the example from above, but with the substrate rotated respectively by 0° in figure 2A, by 90° in figure 2B, by 180° in figure 2C, and by 270 ° in figure ID. The selected ink has a peak reflectance at this angle of illumination that creates a green hue. Again the primary illustrated effect is that the colour of the sample (that is, its hue) remains relatively constant but the brightness varies.

Figure 3 provides representations of the embodiment of figure 1, with the same four views shown in terms of rotation relative to the camera and again designated A,

B, C, D, but in which the substrate is now viewed at an angle of 45° to the surface. The peak reflectance at this angle and the resultant hue exhibited by the arrows shift to a shorter wavelength as a result of the optical variable nature displayed by the chiral nematic liquid crystals. As a result, there is a colour shift relative to figure 1 to about 560 nm. In the view in figure 1, the arrows appear generally red/orange in hue; in the view in figure 3 they have shifted to yellow.

By combining the first effect and the second effect and by using two light sources it is possible to obtain quite different colours simultaneously, as is illustrated in figure 4, wherein the security device of figures 1 and 3 has been photographed when viewed and illuminated at an arbitrary angle. In the resultant view the arrows f and g are relatively dark, the arrow e appears bright orange, and the arrow h appears bright green-yellow. In effect, as a result of this arbitrary angle of viewing and arbitrary direction of illumination, arrow e appears as if viewed at 90°, and arrow h at an oblique angle, so that each of the effects shown in figures 1 and 3 is present simultaneously.

Figures 5 and 6 illustrate the same effect as those illustrated in figures 3 and 4 for the embodiment shown in figure 2.

Figure 5 provides representations of the embodiment of figure 2, with the same four views shown in terms of rotation relative to the camera and again designated A,

B, C, D, but in which the substrate is now viewed at an angle of 45° to the surface. The peak reflectance at this angle and the resultant hue exhibited by the arrows again shift to a shorter wavelength to produce a colour shift relative to figure 2, in this case from generally green in hue to generally blue.

Figure 6, wherein the security device of figures 2 and 5 has been photographed when viewed and illuminated at an arbitrary angle, combines the first effect and the second effect. In the resultant view the arrow c is relatively dark, the arrow b appears bright green, the arrow d appears bright blue, and the arrow a has a somewhat intermediate hue.

Figures 7 to 9 illustrate a further feature which can be exploited in security devices of the invention. Three further examples of printed devices are shown in which a liquid crystal ink is used to print a design onto a substrate. In each case a printed device comprising a QR-style pattern was printed onto a substrate consisting of a tamper evident label.

The device in figure 7, also referred to and labelled as 120117C, consists of a single image printed in a single print direction. The device in figure 8, also labelled as sample 120117D consists of two overlaid images, the two images being printed directly one over the other in a single print direction. The device in figure 9, also referred to as sample 120117E consists of two overlaid images with the substrate being rotated by 180° before printing of the second image. The second image was printed as an upside-down version of the first image. The example in figure 9 thus has two print regions, in this case essentially coincident with one another, in which each of the two regions is printed at a print angle to the other region, and is therefore an example embodiment of the invention. The three examples were photographed under identical conditions. The Hue, Saturation and Value (HSV) for each image at a particular view angle of viewing is given in Table 1. The Value (lightness) for each image is given in bold.

It is seen that the average Value for an overprinted image where the second layer is applied at a different substrate orientation to the first layer (i.e. image 120117E) is greater. In practical terms, this means that sample 120117E is more likely to be brighter when viewed at an arbitrary angle. In other words, the printed motif featured in sample 120117E will be more readily visible.

This is illustrative of what was discussed more generally above as "effect 4" whereby the visibility of a liquid crystal image at an arbitrary angle of viewing and illumination may be improved by overprinting the image with a second image, where the overprinting is carried out with the substrate in a different orientation to the way the first image was printed. 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 invention. For example, many different patterns of the plurality of regions printed at an angle to each other are possible. Moreover, the invention is concerned with the presence of a single liquid crystal material printed at different print angles in different regions of the security device, that can be achieved while also printing or coating further inks or liquid crystal materials in other regions of the security device .

Claims

1. A method of producing a security device, the method comprising inkjet printing a liquid crystal material onto a first region of a substrate in a first printing direction, and inkjet printing the same liquid crystal material onto at least one further region of the substrate in at least one further printing direction at an angle to the first printing direction .

2. A method according to claim 1 comprising providing a print head of an ink-jet printer with an ink

comprising a liquid crystal material; inkjet

printing the ink comprising the liquid crystal material onto a first region of a substrate while moving the print head in a first movement direction, and inkjet printing the same ink onto at least one further region of a substrate while moving the print head in at least one further movement direction at an angle to the first movement direction.

3. A method according to any preceding claim, wherein at least some of the regions are formed as discrete regions on the substrate so that the area of liquid crystal applied to each such region is discrete and separate from the area of liquid crystal applied to the each other region.

4. A method according to any preceding claim, wherein at least some of the regions are at least partly coincident so that the area of liquid crystal applied to at least one of the said regions at least in part overlays the area of liquid crystal applied to at least one other of the said regions.

5. A method according to any preceding claim, wherein each region is printed in a single pass.

6. A method according to any one of claims 1 to 4,

wherein each region is printed in multiple passes.

7. A method according to any preceding claim, wherein the printed liquid crystal material exhibits a variation in colour with viewing angle and wherein the variation in colour is different as between the first region and the further region.

8. A method according to claim 7, wherein, when

changing from a viewing angle of 90° to the substrate to a viewing angle of 45° to the substrate, the hue generated by the peak reflectance of the first region shifts by an amount that differs from the hue generated by the peak reflectance of the further region .

9. A method according to any preceding claim, wherein the substrate comprises 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, a PET substrate or a printed circuit board (PCB) .

10. A method according to claim 9, wherein the substrate is the surface of a consumer or industrial product.

11. A method according to any preceding claim wherein the substrate comprises a dark, absorbing, or non- reflective substrate.

12. A method according to any preceding claim, wherein the liquid crystal material is a chiral nematic liquid crystal material.

13. A method according to any preceding claim wherein an angle between at least two printing directions is at least 60 ° .

14. A method according to claim 13, wherein each angle between printing directions is at least 60°.

15. A method according to claim 14, wherein each angle between printing directions is about 90°.

16. A method according to any preceding claim comprising ink jet printing a liquid crystal material onto at least three regions of the substrate in at least three respective printing directions each at an angle to the other.

17. A method according to claim 16 comprising ink jet printing a liquid crystal material onto four regions of the substrate in four respective generally orthogonal printing directions.

18. A method according to any preceding claim, wherein the first and at least one further region are arranged according to a design which aids in the authentication of a product or item.

19. A method according to any preceding claim, wherein the first and at least one further region are part of an insignia, marking or code wherein different regions of the insignia, marking or code are printed in different print directions.

20. A method according to any preceding claim, wherein the method comprises printing variable information specific to the security device.

21. A security device obtainable by a method according to any of claims 1 to 20.

22. A security device comprising a first region of a

liquid crystal material and at least one further region of the same liquid crystal material, wherein each of the first and at least one further region has been inkjet printed in a printing direction, and wherein each printing direction is at an angle to each other printing direction.

23. A security device according to claim 22, wherein at least some of the regions comprise discrete regions on the substrate in that the area of liquid crystal on each such region is discrete and separate from the area of liquid crystal on each other region.

24. A security device according to claim 22 or 23,

wherein at least some of the regions are at least partly coincident in that the area of liquid crystal on at least one of the said regions at least in part overlays the area of liquid crystal on at least one other of the said regions.

25. A security device according to any one of claims 22 to 24, wherein the printed liquid crystal material exhibits a variation in colour with viewing angle and wherein the variation in colour is different as between the first region and the further region.

26. A security device according to claim 25, wherein, when changing from a viewing angle of 90° to the substrate to a viewing angle of 45° to the substrate, the hue generated by the peak reflectance of the first region shifts by an amount that differs from the hue generated by the peak reflectance of the further region.

27. A security device according to any one of claims 22 to 26, wherein the substrate comprises 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, a PET substrate or a printed circuit board (PCB) .

28. A security device according to claim 27, wherein the substrate is the surface of a consumer or industrial product

29. A security device according to any one of claims 22 to 28, wherein the substrate comprises a dark, absorbing, or non-reflective substrate.

30. A security device according to any one of claims 22 to 29, wherein the liquid crystal material is a chiral nematic liquid crystal material.

31. A security device according to any one of claims 22 to 30, wherein an angle between at least two

printing directions is at least 60°.

32. A security device according to claim 31, wherein

each angle between printing directions is at least 60° .

33. A security device according to claim 32, wherein each angle between printing directions is about 90°.

34. A security device according to any one of claims 22 to 33 comprising at least three regions having a printing direction at an angle to that of each of the other regions .

35. A security device according to claim 34 comprising four regions printed in generally orthogonal

printing directions.

36. A security device according to any one of claims 22 to 35, wherein the first and at least one further region are arranged according to a design which aids in the authentication of a product or item.

37. A security device according to any one of claims 22 to 36 wherein the first and at least one further region are part of an insignia, marking or code wherein different regions of the insignia, marking or code are printed in different print directions.

38. A security device according to any one of claims 22 to 37, wherein the security device comprises

variable information specific to the security device .

39. A method of producing a security device, the method comprising inkjet printing a liquid crystal material onto a first region of a substrate in a first printing direction, and ink jet printing the same liquid crystal material onto at least one further region of the substrate at an angle to the first printing direction such that the visible colour response of the liquid crystal material on the first region is different to the visible colour of the liquid crystal material on the at least one further region .

40. A method according to claim 39 comprising printing a liquid crystal material that is known to exhibit a visible colour response where the peak reflectance wavelength varies with viewing angle, and the first and at least one further regions are printed in different printing directions such that the

variation of reflectance with viewing angle occurs differentially as between the first and the at least one further region.

41. A method of authenticating a product, the method

comprising providing on the product a security device according to any one of claims 22 to 38 or printed according to the method of any one of claims 1 to 20; inspecting the security device at a first viewing angle and identifying a first colour

response in the first region and a further colour response in the further region; inspecting the security device at a second viewing angle and identifying a first variation in colour response in the first region and a further variation in colour response in the further region that differs from the first variation in colour response; and, based on the comparison, verifying the authenticity of the product .

42. A method according to claim 41, wherein the

inspection is carried out by tilting the device and observing the colour shift.

43. A method according to claim 41 or 42, wherein the authenticating is carried out using an

authentication device that identifies the first and further regions and compares colour shifts between the regions .

44. A method according to claim 43 wherein the

authentication device comprises a camera and image recognition software to identify the first and further regions .

45. A method according to claim 44 wherein the image recognition software calculates viewing angle by comparing relative positions of features of the security device.