Method for additively printing on a medium using fluorescent invisible inks

Abstract

The invention relates to a method for printing an image on a security document, said method using invisible inks that are fluorescent and visible under ultraviolet radiation, characterised in that it comprises a step of additive colour synthesis, consisting in: - decomposing a visible digital image into three (red, green and blue (R, G, B)) digital image masks (13) such that the superposition of the three masks (13) corresponds to the base image; - applying to each R, G, B digital colour mask (13) computer processing (14) consisting in introducing security patterns into each of the R, G, B digital masks; - printing on a (paper, polycarbonate) physical medium, on top of one another, these three red, green and blue masks provided with their security patterns, using invisible luminescent inks so as to form, on the medium, an invisible image () containing the security patterns () previously introduced into the three RGB digital masks.

Description

[0001] DESCRIPTION

[0002] Additive printing process for a substrate using invisible fluorescent inks

[0003] The present invention relates to a new method of printing a security document such as a smart card or a page of an identity document such as a passport or equivalent.

[0004] STATE OF THE ART

[0005] In the field of security or identity documents, processes are already known that involve printing using invisible inks that become fluorescent and therefore visible when exposed to ultraviolet radiation.

[0006] Such processes are described in particular in documents WO 98 / 40223 Al (Polaroid Corp), WO 99 / 14055 Al (Bank of England), DE 10 2021 001018 Al (Giesecke Devrient), DE 10 2022 000101 Al (Giesecke Devrient).

[0007] However, the type of invisible inks used is becoming increasingly available on commercial websites, and document forgers can easily obtain them, which reduces the security level of documents printed with this type of ink.

[0008] In the printing industry, it is well known that to obtain a true color image, a subtractive color mixing process is used for certain colors, such as Cyan, Magenta, and Yellow. This well-known subtractive color mixing process, when applied to security document printing, involves the following steps:

[0009] - An image is printed onto a substrate such as white paper, or transparent polycarbonate supported by a layer of white polycarbonate

[0010] - Next, a series of visible ink dots in Cyan / Magenta / Yellow colors are printed, which will successively make the white background obtained in the previous step opaque;

[0011] - The final color observed corresponds to the white background of the substrate, except for the areas that have been opaque due to the visible ink layers. If the prints were made with invisible fluorescent inks, their observation will require lighting at a suitable wavelength, often 365 nm.

[0012] As explained above, this process is easy to replicate and provides only a low level of security to the document thus printed by a process known as subtractive synthesis.

[0013] To enhance the security of known security pattern printing techniques, it is also known to apply cyan / magenta / yellow inks using iridescence, a gradual transition between two colors within the same printing unit of the printing station. This complicates the printing process and, in principle, makes it accessible only to official security document printers. Despite these measures, law enforcement checks are increasingly uncovering counterfeit documents from illegal printers who are also beginning to master these iridescence techniques.

[0014] It has therefore become necessary for official printers of security documents or sovereign documents to innovate and find radically new printing techniques, capable of greatly strengthening the security of prints using invisible fluorescent inks.

[0015] In addition, more and more governments are requesting tools and processes to create new identity documents that combine the security and functionality of these documents with high-resolution images that use colors close to reality, in order to showcase their heritage or environment, such as fauna, flora, monuments or sites of particular interest.

[0016] PURPOSE OF THE INVENTION

[0017] The invention therefore aims to propose a new method of printing documents using invisible inks, but which become visible by fluorescence when checked under ultraviolet light.

[0018] Another objective of the invention is to propose a method for printing security documents containing high-resolution images, for example greater than 5080 dpi, and exhibiting colors close to those of the real image to be reproduced when printed in visible ink.

[0019] SUBJECT OF THE INVENTION

[0020] In principle, the process according to the invention consists of applying the principle of additive, rather than subtractive, color synthesis to the printing of physical images. This principle is currently used only in digital color displays, such as digital television, video, or computer screens. These displays physically reproduce "light colors" by additive synthesis, using pixels of the three primary colors: Red, Green, and Blue.

[0021] The invention therefore relates to a method for printing an image on a security document, said method using invisible inks that are fluorescent and visible under ultraviolet radiation, characterized in that it comprises an additive color synthesis step, consisting of:

[0022] Decompose a visible digital image into three digital image masks Red, Green and Blue (R, G, B) so that the superposition of the three masks corresponds to the base image; apply to each digital color mask R, G, B a computer processing consisting of introducing security patterns into each of the digital masks R, G, B; during or after the introduction of the security patterns into the color masks, adapt the masks to allow subsequent printing with luminescent inks visible under UV; print on a physical substrate (paper, polycarbonate) these three Red, Green and Blue masks one on top of the other, using invisible luminescent inks so as to form on the substrate an invisible image containing the security patterns previously introduced into the three RGB digital masks.

[0023] According to one embodiment of the invention, the security patterns introduced in the three RGB digital masks comprise fine geometric patterns. In one embodiment, the fine geometric patterns are identical on all three RGB digital masks, but alternatively, they may also be different on all three RGB digital masks.

[0024] According to one example, the fine geometric patterns are taken, in no way limitingly, from guilloches, hatching, dotted lines, or micro-lettering.

[0025] According to one embodiment, the invisible fluorescent RGB inks incorporate particles of a molecular tracer capable of adding level 3 security. Such a tracer includes a molecule having particular physico-chemical properties paired with a specific detection device.

[0026] According to one embodiment of the invention, the invisible and fluorescent RGB inks exhibit bi-fluorescence properties, so as to become visible under ultraviolet lighting at two distinct ultraviolet wavelengths.

[0027] Preferably, the inks used are chosen to withstand a temperature of around 180 °C for a period of at least 20 minutes, corresponding to the lamination phase of a smart card body.

[0028] Preferably, the inks used are chosen to be compatible with a laser personalization step of the variable information on an identity document, especially a polycarbonate document.

[0029] In one embodiment of the printing process, a visible image is also printed onto the substrate receiving the invisible RGB prints. This image represents the same image as the invisible fluorescent image, or a complementary image to it. This allows verification during UV inspection that the image that appears is the expected one, namely identical or complementary to the image printed in visible form.

[0030] The invention also relates to a method for checking a security document obtained by the method as described above, consisting of observing the document under ultraviolet lighting with a wavelength of approximately 365 nm.

[0031] The invention also relates to a printed security document, particularly an identity document, characterized in that it incorporates a security motif obtained by the process described above. DETAILED DESCRIPTION

[0032] The invention will be described in more detail with the aid of the drawings, in which:

[0033] Figure 1 represents a schematic diagram of the known process of printing a document using the principle of subtractive synthesis;

[0034] Figure 2 shows a schematic diagram of the new process for printing invisible fluorescent inks according to the invention, using an additive color mixing principle. According to this principle, three primary colors are sufficient to create all possible pigment colors. Mixing them using a process called subtractive trichromatic color mixing yields secondary colors used in additive color mixing, also known as fundamental colors. In particular, mixing the three primary colors in subtractive color mixing (cyan, magenta, and yellow) produces black. To facilitate offset printing and avoid excessive color overlap, black is sometimes added during image processing to work in four-color process printing, denoted CMYK. This does not fundamentally change the known printing process. The absence of color produces white. Mixing only two of the primary colors produces a secondary color (blue, green, or red).In an image printed using the subtractive synthesis process, the ink pigments absorb some of the light that reaches the object. Only a portion of the incident light is therefore diffused and visible.

[0035] We refer to Figure 1, which schematically illustrates the steps of a classic color image printing process on a physical medium, using the three primary colors cyan, yellow, and magenta, according to the subtractive synthesis principle explained above.

[0036] In step 2, a physical image 1 to be digitized is broken down into four images, one for each of the basic colors of conventional printing: cyan, magenta, yellow, and black. Then, in step 4, each image C, M, Y, K is "halftone," according to the classic printing terminology. This is a computer processing of the image to make it printable using the conventional printing process. Thus, in step 5, we obtain four halftone images, one for each color C, M, Y, and K. In step 6, these halftone images are used to create printing plates, namely an offset plate engraved for each color. Finally, in step 7, the four halftone images are successively printed in an offset printing block to obtain, in step 8, a four-color process printed image, visible in daylight.

[0037] Figure 2 schematically illustrates the printing process according to the invention. In this process, we start with a digital image 1 visible in daylight. At 12, this digital image is decomposed into RGB components using software that emulates physical RGB filters, and at 13, we obtain three digital images, namely one for each color: R, G, B. At this stage, instead of dithering the extracted RGB images, at 14 they are subjected to a transformation process comprising a security step and a processing step for subsequent printing with invisible fluorescent inks, visible under UV light. During the image security step, geometric patterns that are difficult to reproduce are integrated into each R, G, B digital image, thus increasing the security level of the future printed security document.These digital patterns can be of any type or shape, such as, but not limited to: guilloches, hatching, dotted lines, or micro-lettering. It should be noted that the security patterns can be identical across the three RGB images, or different from one image to another. Thus, in step 15, three secure RGB images are obtained and processed for UV printing. Next, in step 16, printing plates must be created to allow the three secure RGB images to be printed on top of each other. Specifically, offset printing plates are engraved, one for each of the three images, and the images are printed onto a physical substrate (paper, polycarbonate) in an offset printing block 17, using invisible fluorescent inks.

[0038] The three masks are thus printed one on top of the other using a conventional offset printing process, but with the use of invisible luminescent inks to form an invisible image on the substrate containing and blending the geometric transformations previously introduced into the three digital masks. The printing step requires, as is known, very precise registration (on the order of 0.1 mm) of the positioning of the three images relative to each other, using reference indices or alignment targets positioned in suitable areas of the images. The output 18 of the process according to the invention yields an invisible, secure, three-color printed image that will only become visible during inspection under appropriate UV lighting.

[0039] ADVANTAGES OF THE INVENTION

[0040] The invention achieves its objectives. The security of the printed documents is enhanced because counterfeiters have no access to either the invisible fluorescent inks or, more importantly, the software used to create the security patterns that must be integrated into the digital image before printing.

[0041] The visual effect produced by the printed image illuminated under UV light is significantly better: the image printed on the security document is high-quality and high-resolution, depending on the engraving resolution of the offset plates. These high resolutions are reserved for official printers of security documents. The colors obtained on the printed image are luminescent against a light background.

[0042] The new printing process is not restricted to documents with a particular form factor. It can be used for security documents in smart card format, passport format, data page format, or for any other paper or plastic media.

[0043] The printed image obtained is discreet: its presence is invisible under normal conditions, i.e. under visible light: only a white background is visible.

[0044] The control of security documents printed according to the process of the invention is easy and requires only tools already existing in the Law Enforcement, for example a portable tool capable of projecting UV light at 365 nm onto the document.

Claims

DEMANDS 1. A method for printing an image on a security document, said method using invisible inks that are fluorescent and visible under ultraviolet radiation, characterized in that it comprises an additive color synthesis step, consisting of: Decompose (2) a visible digital image (1) into three digital image masks (13) Red, Green and Blue (R, G, B) so that the superposition of the three masks (13) corresponds to the basic image; apply (14) to each digital color mask (13) R, G, B a computer processing consisting of introducing security patterns into each of the digital masks (13) R, G, B; print (17) on a physical support (paper, polycarbonate) these three Red, Green and Blue masks with their security patterns, one on top of the other, using invisible luminescent inks so as to form on the support an invisible image (18) containing the security patterns previously introduced into the three RGB digital masks.

2. A method according to claim 1, characterized in that said security patterns introduced into the three RGB digital masks (13) comprise fine geometric patterns.

3. Method according to claim 1, characterized in that said computer processing includes a step (16) of transformation into printable masks by invisible luminescent inks becoming visible under UV.

4. Method according to claim 2, characterized in that the fine geometric patterns are identical on the three RGB digital masks, or distinct on the three RGB digital masks.

5. Method according to claim 2, characterized in that the fine geometric patterns are taken from among guilloches, hatching, dotted lines, or micro-lettering.

6. A method according to claim 1, characterized in that the invisible fluorescent RGB inks incorporate particles of a molecular tracer capable of adding level 3 security.

7. A method according to claim 1, characterized in that the invisible and fluorescent RGB inks exhibit bi-fluorescence properties, so as to become visible under ultraviolet lighting at two distinct ultraviolet wavelengths.

8. A printing method according to any one of the preceding claims, characterized in that the ink used is chosen to withstand a temperature of around 180 °C for a period of at least 20 minutes, corresponding to the lamination phase of a smart card body.

9. A printing method according to any one of the preceding claims, characterized in that a visible image representing the same image as the invisible fluorescent image, or a complementary image thereof, is also printed on the substrate receiving the invisible RGB prints.

10. Method for checking a security document obtained by the method according to one of the preceding claims, consisting of observing the document under ultraviolet lighting with a wavelength of about 365 nm.

11. Printed security document, in particular identity document, characterized in that it includes a security motif obtained by the process according to any one of claims 1 to 9.

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