MATTER OBTAINED

TECHNICAL FIELD

The present invention relates to a method of printing a fluorescent full colour image comprising a red, green and blue colour components, the printed fluorescent full colour image emitting visible fluorescent light when illuminated with an UV-light source and use of the printed fluorescent full colour image in packages, identification documents, passports, bank notes, in product authentication documents or bar codes or other optically readable data codes comprising product data. BACKGROUND ART

Security printing has become more and more important during recent years to prevent counterfeits of security documents. During recent years it has also become more and more important to add hidden information for example in packages relating to e.g. batch information, etc. Thus, there is a need for improved methods which extend the current knowledge and applications to create practical, cost effective technological solutions for security purposes. Since high-quality and low-priced colour photocopiers and desk-top publishing systems are available, counterfeiting of documents is becoming now, more than ever, a serious problem. The same is also true for other valuable products such as cosmetics, software packages, medical drugs, watches, etc., that are often marketed in easy to falsify packages. Hence security protocols must evolve to detect and prevent the increasingly more sophisticated and technologically based methods of counterfeiting and product diversion.

The use of fluorescent materials such as ultraviolet (UV) fluorescent materials as inks to create security features by printing is well known. For example, fluorescent inks are extensively used in the Euro bank notes, where both the stars and the silhouette of Europe are highlighted under UV light. The fact that the fluorescent features or marks are visible under irradiation with suitable UV light source is advantageous as one can easily and quickly authenticate the security marks. Document US 20040233465 discloses a method and ink compositions for producing full image indicia with three fluorescent inks, red, green, and blue. They advocate performing the colour separation from classical cyan, magenta and yellow to red, green and blue fluorescent inks by converting the image colours to their negative forms using commercially available computer software such as Adobe Photoshop. That disclosure converts an image colour to its negative form by starting with an input cyan (c), magenta (m) and yellow (y) image, and deducing the corresponding surface coverage of red (r), green (g) and blue (b) by simple negation, i.e. r=l-c, g=l-m, b=l-y. By replacing cyan, magenta and yellow cartridge inks with red, green and blue fluorescent ink cartridges and by printing the three ink layers in mutual registration, a colour fluorescent image is obtained.

One further document, EP2158090 discloses the use of three ink primaries, UV fluorescent inks red, green and blue, and non-fluorescent paper substrates, wherein the printed colour images are visible under UV-light. The printed images are used for example on security documents and valuable articles. While fluorescent security features have been widely used in bank notes, currency and in passports and other identification documents, most of such applications have used single fluorescent colours or simple combinations of individual colours. However, since fluorescent inks are available on the market, to duplicate this type of security features is less technically demanding compared to full colour images using UV fluorescent inks. Hence single fluorescent colours may not provide the level of security desired for certain applications.

These known prior art techniques are not directly applicable to many of the daily used papers such as office papers and extra-white packaging boards, which are frequently used in many high-end packages. In these products optical brightening agents (OBAs) also known as fluorescent whitening agents (FWAs) are included. These agents fluoresce blue when subjected to UV irradiation, thus distorting or drowning out any fluorescent images. There therefore arises a need to develop a printing method applicable to commonly used office papers and packaging boards which contain fluorescent optical brightening agents. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a printing method that enables the use of substrates comprising an optical whitening agent as a printing substrate for printing a fluorescent colour image. It is another object of the invention to provide an economical way of printing fluorescent colour images with fluorescent inks.

It is still another object of the invention to provide a printing method that provides colour images with high quality sufficient for example for authentication of documents or packages.

It is also an object of the present invention to provide an efficient security printing method. The objects above are obtained by the present method of printing a fluorescent full colour image comprising a red, green and blue colour components that emit detectable fluorescent light when illuminated with an UV-light source, wherein the colours are visible under illumination with the UV-light source, the method comprising steps of: i) Providing a printing substrate comprising an optical whitening agent, wherein the whitening agent constitutes the blue component (B) in an additive RGB-colour model;

ii) Determining a value for the blue component (B) in the substrate, which the blue component (B) has an emission wave length from about 400 to about 500 nm;

iii) Separating colour components of the fluorescent colour image into red, green and blue colour components in the additive RGB-colour model; iv) Calculating an intensity of the blue colour component (B), having an emission wave length from about 400 to about 500 nm in the image, by utilizing the constant value of the blue colour component (B) in the substrate to obtain a desired intensity for the blue component (B) in the image to be printed;

v) Calculating an intensity of a red colour component (R), having an emission wave length from about 600 to about 700 nm in the image, to obtain a desired intensity for the red component (R) in the image to be printed;

Calculating an intensity of a green colour component (G), having an emission wave length from about 500 to about 600 nm in the image, to obtain a desired intensity for the green component (G) in the image to be printed;

vii) Printing the red colour component (R) of the image to the substrate with an ink comprising a red fluorescent colour component;

viii) Printing the green component (G) of the image to the substrate with an ink comprising a green fluorescent colour component.

It is thus obtained a method readily applicable to many commonly used substrates such as office papers and extra-white packaging boards. The method limits the number of primary inks needed to two, thus reducing the quantities of fluorescent inks used. Any reduction in ink quantity used is advantageous from an environmental and economical perspective, especially since the potential uses of this method include many high volume applications such as consumer products packaging, e.g. food packaging and bank notes. The amount of inking units, such as the number of print heads used in connection with ink-jet printing can be reduced, leading to reduced risks for clogging and a lesser requirement for maintenance. Other printing methods can also be used, such as conventional printing methods including but not limited to offset, flexograpich printing or gravure printing.

According to one aspect, the intensity of the red and green colour components is adjusted such as to provide a full colour image by using only the red and green colour inks. This allows full colour printing in an economical manner using only two primary inks, thus expanding the range of images that can be reproduced whilst minimizing the usage of fluorescent inks. The ability to produce full-colour images is advantageous from an anti-counterfeiting perspective.

In some instances, the intensities of the red and green colour components are adjusted to mask the blue colour components at the areas of the image where the blue colour component is not desired. This again expands the range of images that can be reproduced using the inventive method and allows more advanced security features to be printed. According to another aspect, the blue colour component can be masked by using a pigment able to mask the optical whitening agent at the areas of the image where the blue colour component is not desired.

Preferably, the separated red and green colour components in the image are halftoned to provide continuous tones in the image. Halftoning methods are well known in the art, and any commercially available method for halftoning could be used, whereby continuous tones can be provided in a common manner.

The red and green colour components may be halftoned using dependent color halftoning. By using dependent colour halftoning, the dot placement is controlled, dot-on-dot printing is avoided as much as possible, image quality is improved and the amount of ink needed is further reduced.

In some instances, the printing substrate is paper or paperboard comprising the optical whitening agent. The printing of these commonly used substrates with a UV-fluorescent image leads to a wide variety of potential applications in security marking of documentation and high-end packaging.

In some instances the printing is performed by using a digital printing method. The digital printing method used can be an inkjet printer. Thus, the method is readily implementable using apparatus that is already in wide use and is economical to purchase.

In some instances the printing is performed by using a printing method chosen from lithography, offset printing, flexography, gravure or combinations thereof. Thus the method can be implemented using a wide variety of printing techniques depending on requirements and existing apparatus.

In some instances, the method further comprises a step of printing an image visible in a day light by using cyan (C), magenta (M), yellow (Y) and black (K) colour components. This image visible in the day light can be used to mask the fluorescent colour image during the day light illumination. Thus the fluorescent features of the image can be hidden from the observer until the printed object is viewed under a UV-lamp. This can be useful when using the method for watermarking purposes for example. The method may further comprise tagging the fluorescent colour image with using DNA, nanoparticles, magnetic or thermochromatic inks or combinations thereof to further improve the security aspects of the printed image.

In a further aspect of the invention, a printed fluorescent colour image obtained by the above- described method is provided. This printed fluorescent colour image emits visible or detectable fluorescent light when illuminated with an UV-light source and the colours are visible under the UV-light source. The printed fluorescent colour image is provided on a printing substrate comprising an optical whitening agent, wherein the whitening agent constitutes the blue component (B) in an additive RGB-colour model, and wherein the red colour component (R) of the image is printed to the substrate with an ink comprising a red fluorescent colour component and the green component (G) of the image is printed to the substrate with an ink comprising a green fluorescent colour component.

In some instances, the fluorescent colour image also comprises colours visible in day light. This combined printing reduces the time required for printing and the quantities of ink required to produce both daylight visible and UV-visible images.

In some instances, the fluorescent colour image comprises security features. This makes the printed fluorescent colour image readily applicable to uses in packages, identification documents, and passports, bank notes, in product authentication documents or bar codes or other optically readable data codes comprising product data. In a further aspect of the invention, uses of the printed fluorescent colour image described above are provided. These uses are in packages, identification documents, and passports, bank notes, in product authentication documents or bar codes or other optically readable data codes comprising product data.

The aspects of the invention will be below described more in detail with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a flow chart illustrating the present method; Fig. 2 illustrates measured colour intensities for a printed fluorescent colour image; Fig. 3 shows schematically illumination of a fluorescent colour image which emits visible and/or detectable fluorescent light comprising RGB colour components.

DETAILED DESCRIPTION The printing substrate usable in this invention may be any substrate comprising an optical whitening agent (OWA). By optical whitening agent is equally meant optical brighteners, optical brightening agents (OBAs), fluorescent brightening agents (FBAs) or fluorescent whitening agents (FWAs). The optical whitening agents are chemical compounds, commonly stilbene derivatives that increase whiteness or brightness of a substrate by the conversion of ultraviolet radiation or light to visible blue light. Optical whitening agents cause a "whitening" effect by increasing the overall amount of blue light reflected whereby the materials look less yellow.

The substrate used in the present invention may be paper or paperboard. By "paper or paperboard" is equally meant a substance made from cellulosic fibres. "Paper" may be a single layer or a multi-layer product. Commonly used office papers often contain OWA.

"Paperboard" is a cardboard product, i.e. a stiff board made from paper pulps, and can be made of several layers or paper. Paperboard is commonly used as packaging material.

A colour image can be divided into three image channels, also known as colour separations. In the case of additive RGB-colour mixing these colour separations are red (R), green (G), and blue (B). The red (R), green (G), and blue (B) colours are added together in various

combinations to provide a broad gamut of colours. The RGB additive colour model is commonly used in, for example, displays such as TV's and projectors. By full colour RGB image it is meant that the image comprises each of the RGB colour components.

Printing on a paper containing OWAs differs from on a non-fluorescent substrate. The substrate is not black but blue when illuminated under UV illumination. In other words, it acts not only as a support for depositing the other fluorescent inks, but also as source of blue fluorescent light. Normally, this blue light would distort or drown out any fluorescent images printed on a substrate containing OWA. However, the inventors of the present invention have realized that, by printing with appropriate amounts of green and red fluorescent inks on the fluorescent substrate, a "paper white" can be created. Thus, according to the present invention the OWA is used as one colour component, i.e. blue, of a full colour UV fluorescent image. Together with two other UV fluorescent inks that are printed to the substrate, i.e. red and green, it makes three colour primaries of an RGB colour model utilizing "additive colour mixing". After separation and calculation of the intensity of the green or red colour component, the red and green colour components may be halftoned to simulate continuous tones in a printed image. Halftoning of the colour image is basically about placements of halftone dots in each of the separations, so that the halfoned image resembles the original image when viewed by human eyes. This is achieved by simulating the tones of the image using dots having varying size and/or spacing whereby a gradient like effect is created. Halftoning is usually performed digitally whereby each pixel of the image obtains a value for whether it will be receiving ink or not. Halftoning may be performed by using any commercially available software or program product.

According to one variant of the invention a dependent colour separation/half toning method can be used to avoid overlap between blue (unprinted), green and red ink layers. The dependent color halftoning method employs an iterative halftoning process. Placement of a halftone dot in one color separation will impact on dot placements in the other two color separations. In practice, it begins with finding out the position of the global maximum in all of the separations and placing a dot at this position in the corresponding separation. Through a feedback mechanism, the impact of this rendered position is taken into account when obtaining a new set of color separations. Then a new global maximum can be found and a dot will be placed to the corresponding position/separation. This process continues. Hence, the dependent colour halftoning method has the tendency to place the dots in the halftone image as far apart as possible yet the halftone image still has the same appearance as the original. This means that the dot placement is controlled, dot-on-dot printing is avoided as much as possible, image quality is improved and the amount of ink needed is further reduced. For more information on dependent colour half toning see S. Gooran, Dependent colour half toning, better quality and less ink, J. Imaging Sci. Technol. V.48 (2004), pp.354-362.

A colour prediction model can be incorporated to determine the ink coverages of blue area and the other inks. The value for the blue colour in the printing substrate, i.e. paper or board, can be predetermined by measurements with a spectral photometer at the relevant wavelengths and the other two inks are then combined in a desired combination to produce the desired full colour image and to obtain a maximal colour gamut. Thus, by utilizing the fluorescent blue colour of the substrate, a full colour image visible through UV-light source can be obtained by printing with only two fluorescent inks, e.g. with an ink-jet printer. Thus, a simple and cost- saving printing method is achieved.

The inventive method will now be described in more detail, with reference to Figure 1, which shows a flow chart illustrating the steps of the inventive method. In step i) a printing substrate containing an OWA is provided. As previously described, this substrate may for example be a paper or paperboard. The OWA already present in the printing substrate will constitute the blue (B) component in the final printed fluorescent RGB image.

In step ii) the intensity of the emission of the blue component (B) by the substrate is determined, for instance using a spectrophotometer, within the wave band from about 400 to about 500 nm (see Figure 2). This intensity henceforth constitutes a "baseline" to which relevant quantities of red (R) and green (G) must be added in order to be able to provide a full colour image.

In step iii) the image that is intended to be fluorescently printed is analysed to separate it into the red (R), green (G) and blue (B) colour components of the RGB colour model in a conventional manner.

In step iv) the intensity of the blue colour component (B) having an emission wavelength from about 400 to about 500 nm in the image is compared to the "baseline" blue intensity of the OWA in order to calculate the intensity of blue component that should be present in the final image. In step v) the intensity of the red colour component (R) having an emission wavelength from about 600 to about 700 nm in the image is calculated (see Figure 2). Depending on the colours present in the final image, red may be used in an additive manner to provide further colours. So, for instance, red in combination with the blue fluorescence from the OWA gives magenta. Further addition of green to this magenta would give white. Thus, taking the "baseline" blue intensity from the substrate into account, excess amounts of red may be required to compensate for, or neutralize, the observed blue colour.

In step vi) the intensity of the green colour component (G) having an emission wavelength from about 500 to about 600 nm in the image is calculated (see Figure 2). Depending on the colours present in the final image, green may be used in an additive manner to provide further colours. So, for instance, green in combination with the blue fluorescence from the OWA gives cyan. Further addition of red to this cyan would give white. Thus, taking the "baseline" blue intensity from the substrate into account, excess amounts of green may be required to compensate for, or neutralize, the observed blue colour. In step vii) the red colour component (R) is printed to the substrate using a red fluorescent ink, using for example an inkjet printer. This results in a printed image component that fluoresces red with the desired intensity from step v).

In step viii) the green colour component (G) is printed to the substrate using a green fluorescent ink, using for example an inkjet printer. This results in a printed image component that fluoresces green with the desired intensity from step vi).

The final image comprising the green and red components printed on a substrate containing blue-fluorescing OWAs will appear as a full colour image when viewed under a UV lamp, see Figure 3. Areas where blue tones are not desired can be masked by printing red and green at appropriate intensities to give the alternative desired colour or, if all three primaries are mixed in similar intensities, white instead. Alternatively, black may be obtained by printing a pigment able to mask the blue colour provided by the OWA. The colour components may be halftoned so that contiuous tones can be provided.

Depending on the photochemical properties of the fluorescent inks used, the printed fluorescent image may be seen weakly even under certain ambient light conditions. This can be undesirable in circumstances, such as where it is preferred to keep the presence of fluorescing security features undisclosed. In order to avoid the fluorescent image being inadvertently discovered, it can be overlaid with an image printed using conventional daylight visible inks, for example an image printed using a substractive CMYK (cyan, magenta, yellow, key = black) colour model. In some applications, daylight visible dyes or pigments may be present in the fluorescent inks used, thus leading to simultaneous printing of fluorescent and daylight visible images.

Further security features may be present in the inks used for printing the fluorescent image. These security features may include tagging using DNA or nanoparticles as well as other commonly used features such as magnetic or thermochromatic inks, or combinations thereof.

The printed images produced using the method of the invention can be used in a broad variety of applications.

Consumer product packaging or food packaging can be printed with batch and production information in order to improve traceability. Products that are targets for counterfeiting, such as pharmaceuticals and luxury goods can be printed with security and anti-counterfeiting methods. Alternatively, authentication documents provided with these goods can be printed with such measures.

Banknotes and passports can also be furnished with such security and anti-counterfeiting measures. The presence of full colour fluorescent images makes counterfeiting significantly more difficult.

The images printed can be intended for ocular inspection by humans at for example airport passport counters. Alternatively, they can be machine readable for use in for example automated passport controls. The images printed can also be a combination of elements, some intended to be machine readable and some intended to be recognized by humans. The examples given above are not to regarded as limiting the scope of the invention. Instead the scope of the invention is defined in the appended claims.