Additive printing process for a substrate using invisible fluorescent 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 light, characterized in that it comprises an additive color synthesis step, consisting of: decomposing a visible digital image into three digital image masks Red, Green, and Blue (R, G, B) such that the superposition of the three masks corresponds to the base image; applying 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; printing these three Red, Green, and Blue masks one on top of the other on a physical substrate (paper, polycarbonate), using luminescent invisible inks so as to form on the substrate an invisible image containing the security patterns previously introduced into the three RGB digital masks. Figure for the Abstract: Figure 2

Description

Title of the invention: Additive printing process for a substrate using invisible fluorescent inks

[0001] 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. STATE OF THE ART

[0002] In the field of security or identity documents, processes are already known for producing prints using invisible inks that become fluorescent and therefore visible when exposed to ultraviolet radiation. However, this type of invisible ink is becoming increasingly available on commercial websites, making it easy for document counterfeiters to obtain it, which reduces the security level of documents printed with this type of ink.

[0003] 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 known subtractive color mixing process, when applied to security document printing, comprises the following steps:

[0004] - An image is printed on a substrate such as white paper, or polycarbonate transparent material supported by a layer of white polycarbonate

[0005] - A series of visible ink dots of Cyan color are then printed / Magenta / Yellow which will successively opacify the white background obtained in the previous step;

[0006] - We finally observe a color that corresponds to the white background of the support, to with the exception of areas that have been opaque due to visible layers of ink.

[0007] If the prints were made with invisible fluorescent inks, their observation will require lighting at a suitable wavelength, often 365 nm.

[0008] 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 called subtractive synthesis.

[0009] In order to enhance the security of known techniques for printing security patterns, it is also known to apply Cyan / Magenta / Yellow inks in the form of iridescence, namely a gradual transition of two colors within the same printing unit of the printing station. This complicates the printing process and, in principle, makes it accessible only to authorized printers. security documents. Despite these measures, checks carried out by Law Enforcement are increasingly revealing fake documents from illegal offices that are also beginning to master these iridescence techniques.

[0010] 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.

[0011] In addition, more and more governments of countries are requesting tools and processes to create new identity documents that can combine the security and functionality of these documents with images that have high resolution and use colors close to reality, in order to highlight their heritage or environment, such as fauna, flora, monuments or sites of particular interest. PURPOSE OF THE INVENTION

[0012] 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.

[0013] Another object of the invention is to propose a method for printing security documents comprising 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. SUBJECT OF THE INVENTION

[0014] 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.

[0015] 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: - 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 basic 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 safety patterns into the color masks, adapt the masks to allow subsequent printing with luminescent inks visible under UV; - print on a physical medium (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 medium an invisible image containing the security patterns previously introduced in the three RGB digital masks.

[0016] According to one embodiment of the invention, said security patterns introduced in the three RGB digital masks include fine geometric patterns.

[0017] According to one embodiment, the fine geometric patterns are identical on the three RGB digital masks, but alternatively they can also be different on the three RGB digital masks.

[0018] According to one embodiment, the fine geometric patterns are taken, in no way limitingly, from guilloches, hatching, dotted lines, or micro-lettering.

[0019] 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.

[0020] According to one embodiment of the invention, the invisible and fluorescent RGB inks have bi-fluorescence properties, so as to become visible under ultraviolet lighting at two distinct ultraviolet wavelengths.

[0021] 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.

[0022] Preferably, the inks used are chosen to be compatible with a laser personalization step of the variable information on an identity document, in particular a polycarbonate document.

[0023] According to one embodiment of the printing process, 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. This allows verification during UV inspection that the image that appears is the expected one, namely identical to or complementary to the image printed in visible form.

[0024] 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.

[0025] The invention also relates to a printed security document, in particular an identity document, characterized in that it includes a security motif obtained by the process as described above. DETAILED DESCRIPTION

[0026] The invention will be described in more detail with reference to the drawings, in which: - Fig. 1 represents a schematic diagram of the known process of printing a document using the principle of subtractive synthesis; 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).

[0027] In an image printed using the known subtractive synthesis process, the ink pigments absorb part of the light that reaches the object. Only part of the incident light is therefore diffused and visible.

[0028] Reference is made to [Fig.1] which schematically illustrates the steps of a conventional process of printing a color image on a physical medium, using the three primary colors of cyan, yellow and magenta, according to the principle of subtractive synthesis explained above.

[0029] In step 2, a physical image 1 to be digitized is decomposed into four images, one of 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, four halftone images are obtained, one of each color C, M, Y, K. In step 6, these halftone images are used to create printing plates, namely an engraved offset plate for each color. Then, in step 7, successive printings of the four halftone images are carried out in an offset printing block, to obtain in step 8 a four-color printed image, visible in daylight.

[0030] Figure 2 schematically illustrates the printing process according to the invention. In this process, a digital image 1 visible in daylight is used. At 12, this digital image is decomposed into RGB components using software that emulates physical RGB filters, and three digital images are obtained at 13, namely one for each color: R, G, and B. At this stage, instead of dithering the extracted RGB images, they are subjected at 14 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 and capable of increasing the security level of the future printed security document are integrated into each R, G, and B digital image.These digital patterns can be of any kind 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.

[0031] 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. ADVANTAGES OF THE INVENTION

[0032] The invention achieves the stated objectives. The security of the printed documents is enhanced because forgers do not have access to the invisible fluorescent inks, if especially the software for creating security patterns that must be integrated into the digital image before printing.

[0033] The visual effect produced by the printed image illuminated under UV light is significantly better: the image printed on the security document is of high quality and high resolution, depending on the engraving resolution of the offset plates, these high resolutions being reserved for official printers of security documents. The colors obtained on the printed image are luminescent against a light background.

[0034] The new printing process is not restricted to documents having 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 medium.

[0035] The printed image obtained is discreet: its presence is invisible under normal conditions, i.e. under visible light: only a white background is visible.

[0036] 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: - Decomposing a visible digital image into three digital image masks Red, Green and Blue (R, G, B) such that the superposition of the three masks corresponds to the base image; - applying 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; - printing on a physical substrate (paper, polycarbonate) these three Red, Green and Blue masks one on top of the other, using luminescent invisible inks so as to form on the substrate an invisible image 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 include fine geometric patterns.

3. Method according to claim 1, characterized in that said computer processing includes a step 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 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, so as to become visible under ultraviolet light 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. A method for checking a security document obtained by the method according to any 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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