RU2700008C1 - Protective element, which contains hidden information, and containing its valuable document - Google Patents

Abstract

FIELD: protective devices.

SUBSTANCE: invention relates, in particular, to protective element, which contains substrate (1), on which there is at least one imprint (2; 2'; 2''; 3), and which comprises first imprint (2), not visible in visible light, but appearing in first wavelength range (λ₁) outside the visible light spectrum, as well as second imprint (3), not visible in visible light, but appearing in second wavelength range (λ₂) outside the visible light spectrum, wherein said first and second imprints (2,3) are located in areas that partially overlap each other, thus forming at least one overlapping area (4). According to the invention, the security element comprises concealed information (IC), which is formed by said at least one overlapping region (4), wherein concealed information (IC) is manifested in stroboscopic illumination, in which alternating lighting in two wavelength ranges corresponding to said first and second wavelength ranges (λ₁ and λ₂).

EFFECT: disclosed is a security element, which contains concealed information, and containing its valuable document.

10 cl, 36 dwg

Description

FIELD OF THE INVENTION

The invention relates to a security element. It also refers to a valuable document that contains at least one such element.

State of the art

The present invention finds its application, in particular, for a security element included in a valuable document, such as a fiduciary or equivalent document.

The expression "fiduciary document" means all documents, such as banknotes, checks, bank cards or other media, used to transfer the amount of money.

The expression “equivalent document” means all documents issued by state administrative authorities to certify a person’s identity or his right to drive a vehicle, such as an identity card, passport or driver’s license.

This expression also denotes a document of any type that allows you to authenticate a valuable object, for example, such as a label on clothes. It also refers to any document that serves to authenticate tax payments, such as fiscal stamps.

In this area and, in particular, for banknotes, as you know, they add one or more different security elements to protect these documents from possible forgery.

Thus, there are different types of security elements that meet the classification by level, and the present invention relates to the so-called second level security element, that is, an element whose authentication requires the use of a tool, for example, an ultraviolet lamp, namely a widespread tool, which is familiar to the general public.

Document EP1567358 is known from the prior art which describes a protective device having at least two regions containing materials, wherein two or more regions have substantially the same color visible in the first observation conditions under visible light, and different colors visible in the second observation conditions, which are combinations of visible light and ultraviolet light with essentially any wavelength in the range of 235-380 nm.

If the security element is easy to use, it is also at risk, given the increasing spread of completely legal inkjet solutions or fluorescent (in the aforementioned range, i.e. 235-380 nm) “toners” (ink in the form of powder) and even illegal solutions that use materials sold, for example, on sites on the Darknet network.

To enhance protection based on fluorescent materials, EP1719637 proposes to use a through transparent window made in this document, a transparent ultraviolet blocking layer in the area formed by the transparent window, and a first fluorescent ink pattern printed between the ultraviolet blocking layer and the transparent window.

Thus, the pattern is invisible under normal conditions, but becomes visible from two sides if it is illuminated with ultraviolet rays from the side of the window. However, it remains invisible when illuminated from the side of the ultraviolet blocking layer.

Of course, with this combination of fluorescent inks of different colors, ideally located on both sides of the ultraviolet blocking layer, complex effects can be obtained that make it difficult to fake.

However, the disadvantage of this solution is the need to include a transparent window in a paper document or use a polymer substrate and add materials to form an ultraviolet blocking layer. This affects the complexity of the manufacture of substrates and their cost.

WO 2014 177375 refers to a protective element consisting of at least three layers, each of which contains a different material and in which the first, second and third materials can interact with the first, second and third electromagnetic radiation, respectively, and all this forms animation drawing, due to the sequential illumination, which can act precisely and separately in each of the individual radiations, exciting, in particular, one of the materials.

In this case, apply the principle of cinema with a pattern that seems animated when a sequential series of lighting means is turned on. The limitations of such a system can be easily understood for economic reasons, where the number of used image images, providing the visibility of movement, should be very small in order to reduce the number of inks used for printing, and accordingly the number of different lighting means in the device used to authenticate the protection.

More classical paints are also known that react to ultraviolet (UV) rays and which, under the influence of a lamp with a wavelength of less than 400 nm, emit a luminescent signal in the visible region (400-780 nm). These paints are a common example of second level protection.

In addition, certain pigments or dyes are known for absorption at specific wavelengths. They require the use of a lamp emitting at a selected wavelength.

The selected wavelength should be understood as the wavelengths of 365 nm, 312 nm and 254 nm.

However, the main disadvantage of luminescent pigments or dyes of these paints is that they absorb ultraviolet radiation in wide spectra with more or less high intensity.

In addition, ultraviolet lamps containing fluorescent tubes emit in broadband spectra. This makes it easier for criminals who, using standard ultraviolet lamps, can scan areas designed to authenticate second-level fluorescent colors. Consequently, they can forge documents using commercially available aforementioned inks.

At the same time, there are luminescent materials, the absorption peak of which is concentrated on a narrower band, as well as ultraviolet illuminants based on LEDs (Light Emitting Device), which emit only in the spectral band around a specific wavelength, and which also provide wider technical possibilities compared to vacuum tubes, commonly called neon tubes, in terms of electricity consumption, on / off cycles, etc.

This feature allows you to limit the actions of fraudsters, as it requires special equipment. However, the cost of this equipment remains acceptable, in particular, due to the use of LEDs, which also have small dimensions, which makes it possible to produce miniature devices.

On the other hand, the so-called “anti-Stokes” luminescent materials are known which, under the influence of a laser calibrated at a specific wavelength, emit in the visible region.

Under a certain length of a wavelength, it is to be understood a wavelength exceeding 780 nm and, in particular, a wavelength of 980 nm.

Such a security element is intrinsically difficult to manufacture and requires special equipment for identification.

Also known document WO 2015/144646, which disclosed some of the features of the restrictive part of paragraph 1 of the claims of the present application. This document proposes to develop the first image recognized by itself under special lighting (shortwave ultraviolet rays), then the second image recognized by itself under special illumination (longwave ultraviolet rays), and thanks to the layer filtering shortwave ultraviolet rays to obtain a clear and a contrast transition between two images when one lighting is changed to another.

Thus, there is a need for security elements that are not visible to the naked eye and in the visible region, the complexity of which is not in the materials themselves (pigments, paints or substrates that affect the manufacturing cost), but in the arrangement of printed drawings and in the use of economical authentication devices , which makes in this way any attempt at fraud unsuccessful.

Disclosure of the invention

In connection with the foregoing invention, a security element is provided that comprises a substrate on which at least one print is present. This element contains a first imprint that is not visible in visible light, but appears in the first wavelength range outside the spectrum of visible light, as well as a second imprint that is not visible in visible light, but appears in the second wavelength range outside the spectrum of visible light. These first and second prints are located in areas that partially overlap each other, thus forming at least one overlapping zone. This element contains hidden information that is formed by the indicated at least one overlapping zone and which appears under stroboscopic illumination, in which the illumination alternates in two wavelength ranges corresponding to the indicated first and second wavelength ranges.

According to other non-limiting and preferred features of the invention:

- the specified second wavelength range is different from the specified first wavelength range and is outside it;

- the element contains at least one other fingerprint, not visible in visible light, but manifested in a wavelength range outside the spectrum of visible light, and this wavelength range is different from other wavelength ranges and is outside them, this at least one the other fingerprint, together with the other fingerprints, has at least one common overlapping zone, and said at least one common overlapping zone forms at least one hidden information that appears when stroboscopic sveschenii in which alternate light in the wavelength ranges corresponding to said first, second and another wavelength range;

- the specified hidden information is a picture, number or number, text, code, embossed imprint created by the brain of the observer, or a combination thereof;

- all prints are present on one side of the specified substrate;

- at least one of these prints is present on one side of the specified position, while at least one second print is present on the opposite side;

- said first and / or second fingerprint (s) is (are) solid (s), that is, it forms (s) a “homogeneous surface”;

- the specified first and / or second fingerprint (s) is (are) discontinuous (s), that is, formed (s) by separate and separated zones that together form a raster;

- two prints are intermittent and formed by several separate and separated zones, and when stroboscopic illumination, the density of the coating of the substrate with the indicated prints in the specified at least one overlap zone is substantially lower and ideally is half the density of the coating of the substrate with the indicated prints outside the overlap zone; and

- both prints are intermittent and are formed by several separate and separated zones, and when stroboscopic illumination, the density of the coating of the substrate with the indicated prints in the specified at least one overlap zone is substantially higher and, ideally, twice the density of the coating of the substrate with the indicated prints outside the overlap zone.

Brief Description of the Drawings

Other features and advantages of the invention will be more apparent from the following description of preferred embodiments of the invention. This description is presented with reference to the accompanying drawings, in which:

in FIG. 1 and 2 show a banknote including the claimed security element, wherein in FIG. 1, only the first print is visible, while in FIG. 2 only the second print is visible, top views;

in FIG. 3 shows the banknote depicted in FIG. 1 and 2, showing partial overlapping of the prints, a schematic view in longitudinal section;

in FIG. 4 illustrates the principle applied, for the manifestation for the observer of information hidden inside these prints, a schematic view;

in FIG. 5 and 6 show a banknote including the claimed security element, with prints present, each, on the large side of the banknote, views respectively from above and below;

in FIG. 7 shows a second embodiment, a view similar to FIG. 3;

in FIG. 8 is a view similar to FIG. four;

in FIG. 9 shows three versions of the first prints, top views;

in FIG. 10 shows three versions of second prints that combine with the prints shown in FIG. 9, when strobe lighting is activated, top view;

in FIG. 11 shows the overlapping prints shown in FIG. 9 and 10, and the hidden information they show, top view;

in FIG. 12-14, three prints are shown, and in FIG. 15 shows these superimposed fingerprints depicted in these three figures, and the hidden information they show, top views;

in FIG. 16-19 are views similar to FIGS. 1-4, respectively, showing another version of this embodiment;

in FIG. 20-23 are views similar to FIGS. 5-8, respectively, showing yet another version of this embodiment;

in FIG. 24-26, respectively, the first print is presented, a top view, a cross section of this print and its substrate, also a second top view with the display of information hidden in it, presented for clarity in the form of a dashed outline, but not distinguishable in reality;

in FIG. 27-29 are views similar to FIGS. 24-26 and relating to a second fingerprint that interacts with the first fingerprint depicted in the previous figures;

in FIG. 30 is a top view of how an observer perceives it when both prints of FIG. 24 and 27, operate with stroboscopic lighting;

in FIG. 31 and 32 show a substrate on which these two prints are made, depending on whether they are present or not on the same side of the substrate, cross-sectional views;

in FIG. 33 and 34 show two prints that together create a stereoscopic effect, front views;

in FIG. 35 and 36 show simplified schematic views, respectively in perspective and in section, of a device that can be used to view hidden information included in a security element.

The implementation of the invention

In the further text of the description, we will focus on a security element that contains at least two prints. In all these embodiments, the prints are made using inks, for example, in the form of lines, with guilloche, in the form of a numismatic background, etc.

These prints are made using various printing technologies, such as offset printing, screen printing, gravure printing, flexography, inkjet or laser printing, etc., and these examples are not limiting.

In particular, in the framework of the invention, we will be interested in a security element made by offset printing. However, in this case we are only talking about a simple illustration, since the use of offset printing requires a means of printing with a register, which will be explained in the description below.

So, in the present invention, it is understood that at least two prints or two patterns, when they are aligned (or register), that is, they have certain relative positions when they are joined by the edges (or are adjacent) when they are completely are on top of each other, etc., are theoretical concepts that are true for figures, plans and "layouts" (the location of individual plots on a printed sheet), but in reality they have minimal location deviations associated with their implementation on matched valuable document, as well as on several valuable documents compared with each other. For example, tolerances are usually noted between two groups of prints made simultaneously or sequentially, in the best case, of the order of +/- 0.01 mm for offset printing, while tolerances of at least +/- 0.2 mm in the worst case, when prints are performed in two separate passages, that is, which are not located in a line.

As will be explained in more detail below, prints are performed by means such as inks, which are not visible with visible light and with the naked eye, but appear in wavelength ranges beyond the visible light spectrum.

Preferably, these prints are carried out using ink that responds, for example, to a wavelength in the ultraviolet region, or using ink that responds to a wavelength in the infrared region.

Both prints emit in the visible region with a certain hue (identical or different in two prints) when they are illuminated with a specific absorption wavelength determined by the luminescent material itself.

If a paint is used that responds to wavelengths in the ultraviolet region, the range of wavelengths used will be less than 400 nm and, in particular, with absorption peaks concentrated in the range of 365 nm, 312 nm and 254 nm.

If a paint is used that responds to wavelengths in the infrared, the range of wavelengths used will exceed 780 nm and, in particular, with an absorption peak around 980 nm.

As shown in the attached FIG. 1-3, we are talking about a banknote 1, the opposite large sides 10 and 11 of which contain various prints and protective means known to the specialist, therefore, a description of these means is omitted.

On the front side 10 of this banknote there is a fingerprint 2 having two areas designated A and B.

In this embodiment, imprint 2 is in the form of a square, which is continued by sharpening on one of its sides. Zone A has a diamond-shaped contour, while Zone B corresponds to the rest.

In FIG. 2 on the same face 10 of the banknote shows only the second fingerprint 3, containing two zones A 'and C. Zone A' is aligned with zone A of the first fingerprint 2 so that they are completely overlapping, and has the same shape and such the same dimensions as zone A. As for zone C, it is located on the right side of zone A 'opposite to zone B. In the variant not described, zone C could have a shape that is different and not symmetrical with respect to zone B.

Referring to FIG. 3, it can be seen on it that fingerprints 2 and 3 partially overlap each other, forming overlapping zone 4, which consists of zones A and A ’superimposed on each other.

As mentioned above, the first print 2 is made using a material that is not visible in visible light, but appears in the first wavelength range λ ₁ outside the spectrum of visible light. As for the second print 3, it is made using a second ink, not visible in visible light, but appearing in the second wavelength range outside the visible light spectrum, and this second wavelength range λ ₂ differs from the specified first wavelength range and is beyond.

Given these characteristics, it is clear that if a banknote is illuminated from above on its front side 10, using light with a wavelength of λ ₁ , then only fingerprint 2 will appear.

Similarly, if this side is illuminated only with light with a wavelength of λ ₂ , then only imprint 3 will appear. However, as shown in FIG. 4, if you alternately illuminate side 10 of the banknote with light sources with wavelengths λ ₁ and λ ₂ , alternating them with each other with a frequency sufficient to obtain a stroboscopic effect, then the person will repeatedly see on the retina, observing area 4 of the overlap of two printed layers, will allow you to see (display, that is, make visible) hidden IC information that will be perceived and unambiguously interpreted by a person trying to authenticate a document, in this case banknote 1.

In this application, the expression "hidden information" should be understood as information that cannot be distinguished immediately, but only in specific conditions of observation, and which the observer recognizes, for example, insofar as it has a certain meaning for him.

The embodiment of FIG. 5-8, very similar to the previous option. In this case, on the front side of the banknote 10, a first fingerprint 2 is present, which is absolutely identical to the fingerprint shown in FIG. one.

However, the second print is not made on the same front side 10, but on the reverse side 11 so that zones A and A 'are aligned.

That is what is shown in FIG. 7, where the overlapping zone of the two prints 2 and 3 also has the designation 4.

Of course, this implies that the substrate itself is not completely opaque, in particular, for the color of the luminescence emitted on the reverse side (it is believed that the observer is looking at the front side) when illuminated with a strobe effect, but this applies to papers used for printing valuable documents.

You can also provide local partial or complete elimination of opacity (transparency) by chemical or physical processing or by using a known operation designed to give this property in order to improve the effect of the manifestation of hidden information.

As in the previous case, but with the arrangement of light sources with wavelengths λ ₁ and λ ₂ on both sides of the banknote and with the use of lighting with a stroboscopic effect, hidden information IC will appear due to the overlap of two prints 2 and 3 at the zone level 4. This information becomes visible in the light for a user located on one or the other side.

In FIG. Figures 9 and 10 show three different shapes of prints 2 and 3, each of which is made in the form of a raster of parallel lines, but the lines of print 2 are oriented at an angle of 90 ° with respect to the lines of print 3.

If we consider one or the other of these prints, then it is impossible to associate any specific picture with these prints. However, if we consider their overlap when repeating the image on the retina due to the stroboscopic effect, latent IC images appear that are immediately identified by the observer (Fig. 11).

In this case, from left to right, these hidden figures are formed respectively by the pound sterling sign, the dollar sign and the number 5.

In the embodiment of FIG. 12-15, this is not about two prints, but about three.

These prints are labeled 2, 2 'and 2' '

Here, too, if we consider each of these prints, it is impossible to associate any specific pictures with them. However, if we consider their overlap in FIG. 15, manifested in stroboscopic lighting, it is possible to quickly identify the hidden information formed by the number 5.

All three of these prints can be made on the same side of the security element. However, one or two of them can be applied to the second side of the protective element. Preferably, each print is made using a material whose manifestation wavelength is different from the other two. At the same time, it is possible to envisage the appearance of an imprint for two of these layers with the help of light of the same wavelength. The same applies to the case of only two layers, if these two layers are applied on opposite sides.

In FIG. 16-19 show another embodiment similar to the embodiment shown in FIG. 1 and 2, that is, both prints 2 and 3 are made on the same front side of banknote 1.

However, in this case, the common contour of each of the prints 2 and 3 is strictly identical, and each print contains a zone of perfect overlap A and A 'forming zone 4.

As in the embodiment of FIG. 1 and 2, alternately illuminating the front side of the banknote with the help of light with wavelengths λ ₁ and λ ₂ , they reveal hidden information that occupies zone A and A '.

As for the embodiment shown in FIG. 20 and 21, its common feature with the embodiment just described is that fingerprints 2 and 3 are perfectly superimposed on each other, but fingerprint 2 is made on the front side 10, and fingerprint 3 is made on the back side 11.

At a wavelength of λ ₁ , if we consider fingerprint 2, the density of the lines or dots of this fingerprint is determined so that the density A, that is, dA, is equal to the density B, that is, dB. Similarly, speaking of fingerprint 3 illuminated by a light source with a wavelength of λ ₂ , the density of lines or points of this layer 3 is determined so that the density related to zone A ′ (i.e., dA ′) is equal to the density of zone C ( i.e. dC).

When illuminating the entire complex in sequence using light sources with wavelengths λ ₁ and λ ₂ , three options for overlays are possible:

So, when dA + dA 'exceeds dB, and dA + dA' exceeds dC, and zones B and C do not overlap, the hidden information is partially reproduced. In the second case, when dA or dA 'is less than or equal to the sum dA + dA', which is less than dB + dC, and zones B and C are at least partially overlapping, information appears in the form of a negative. Finally, in the third case, that is, when dA + dA 'exceeds dB + dC, and they themselves are greater than or equal to dB or dC, while zones B and C are at least partially overlapping, the information appears in the form of positive.

In FIG. 24 shows the print 2 applied to the front side 10 and consisting of a set of individual points. Fingerprint 2 is shown in section in FIG. 25. In FIG. 26, the hidden IC information is shown as a dashed outline for clarity, but in fact it is not distinguishable.

In FIG. 27 and 28, a second print 3 is shown, also formed by dots separated from each other, printed so that some of these points overlap with the print 2. The print 3 can be made either on the front side 10 or on the back side 11. FIG. 29, hidden IC information is shown as a dashed outline for clarity, but in fact it is not distinguishable.

Referring to FIG. 30-32, regardless of whether fingerprints are made on the same side 10 (Fig. 31 or on opposite sides 10 and 11 (Fig. 32), hidden information IC, displaying the number 50, appears due to the fact that in this zone 4 of the overlapping density of the dots is essentially half lower in this case than on the rest of the area of the overlapping fingerprints 2 and 3 of the security element.

In FIG. 33 and 34 show other types of prints 2 and 3, which are observed to simulate a stereoscopic binocular vision of subject S in decor D, as an observer would see in real life.

In this case, the subject S is in the center and displays the seated person shown in outline. Decor D or the background, also shown by lines, is formed by steps in the lower part and wall panels in the upper part. The two images associated with the decor are viewed from slightly different angles.

In this case, the hidden information IC is the relief of the central subject, created by interpreting the pictures in the observer’s brain, observed due to the stroboscopic effect, when the frequency of alternation of illumination makes it possible to smooth out any relative movement of the pictures between themselves and thus create a stereoscopic effect. In this case, if the density dA is the density of the central subject of fingerprint 2 at a wavelength of λ ₁ , and dA 'is the density of the subject of fingerprint 3 at a wavelength of λ ₂ , then when applied, the sum of dA plus dA' is equal to dA or dA '(the subjects are the same) . In addition, the sum of the density dB of the decor of fingerprint 2 at a wavelength of λ ₁ and the density dC of the same decor of fingerprint 3 at a wavelength of λ ₂ exceeds the separately considered dB and dC.

In FIG. 35 and 36, an example 5 shows a device 5 that allows the development of hidden IC information within the security element described above.

In this case, this device is made in the form of a housing 5 with a lower part 50 and with an upper part 51, which has a through observation window 52. The lower part and the upper part are separated by a hole or slit 53, made at least partially in the housing 5, into which you can enter a flat valuable document on which can be applied manifest claimed security element for authentication.

Of course, in the housing 5 there are light sources with wavelengths λ ₁ and λ ₂ or more, if necessary. These light sources are indicated by 6 and 7.

These diode-type light sources are located and connected to an electronic circuit board that contains an on / off button 80 with a battery, such as a lithium-ion battery, and a USB port 82. Of course, other power options, such as a network or a standard non-rechargeable one, can be provided battery.

Another option not presented herein is the case when the light sources are on the same side and when the prints 2 and 3 are located on the same side. In this case, the shape of the housing 5 itself may change. The presence of a slot 53 is not necessary for the introduction of an authenticated sample. Indeed, this gap is here to provide the partial shade necessary to distinguish the colors of luminescent materials. Indeed, if you authenticate in a place already in partial shade, or using a removable shutter in the right place, this will allow you to get the same result.

Claims (11)

1. A security element that contains a substrate (1), on which there is at least one imprint (2; 2 ';2''3), and which contains the first imprint (2), not visible in visible light, but manifested in the first wavelength range (λ ₁ ) outside the visible light spectrum, and the second imprint (3) not visible in visible light, but appearing in the second wavelength range (λ ₂ ) outside the visible light spectrum, wherein the first and second prints (2, 3) are located in areas that partially overlap each other, thus forming at least one zone (4) overlap, characterized in that it contains hidden information (IC), which is formed by the specified at least one overlapping zone (4), wherein the hidden information (IC) is manifested in stroboscopic lighting, in which the lights are alternated in two wavelength ranges corresponding to the first and the second wavelength ranges (λ ₁ and λ ₂ ). 2. The security element according to claim 1, characterized in that said second wavelength range (λ ₂ ) differs from said first wavelength range (λ ₁ ) and is outside it. 3. The security element according to claim 1 or 2, characterized in that it contains at least one other imprint (2 ', 2''), not visible in visible light, but manifesting itself in the wavelength range outside the spectrum of visible light, wherein this wavelength range differs from other wavelength ranges (λ ₁ and λ ₂ ) and is beyond them, and this at least one other fingerprint (2 ', 2'') has, along with other fingerprints (2, 3), at least one common overlapping zone (4), wherein said at least one common overlapping zone (4) forms m nshey least one hidden information (IC), which manifests itself when the stroboscopic illumination, in which alternate light in the wavelength ranges corresponding to said first, second and another range of wavelengths. 4. The security element according to one of paragraphs. 1-3, characterized in that the specified hidden information (IC) is a picture, number or number, text, code, embossed imprint created by the brain of the observer, or a combination thereof. 5. The security element according to one of paragraphs. 1-4, characterized in that all the prints (2, 3; 2 ', 2' ') are present on the same side (10; 11) of the specified substrate (1). 6. The protective element according to one of paragraphs. 1-4, characterized in that at least one of these prints (2; 3; 2 '; 2``) is present on one side (10) of the specified position (1), while at least one second print (2 ; 3; 2 '; 2' ') is present on the opposite side (11). 7. The protective element according to one of paragraphs. 1-6, characterized in that said first and / or second fingerprint (s) (2; 3; 2 '; 2' ') is (are) solid (s), that is, it forms (s) a “homogeneous surface”. 8. The protective element according to one of paragraphs. 1-7, characterized in that said first and / or second fingerprint (s) (2; 3; 2 '; 2' ') is (are) intermittent (s), that is, formed (s) at least two separate and divided zones that together form a raster. 9. The security element according to claim 8, characterized in that the two prints (2, 3) are discontinuous and are formed by several separate and separated zones, while under strobe lighting, the density of the coating of the substrate (1) by said prints (2, 3) in said at least one overlap zone (4) is substantially lower and ideally half the coverage density of the substrate with said prints outside the overlap zone (4). 10. The security element according to claim 8, characterized in that both prints (2, 3) are intermittent and are formed by several separate and separated zones, while under strobe lighting, the density of the coating of the substrate (1) by said prints (2, 3) in said at least one overlap zone (4) substantially exceeds and ideally doubles the density of the substrate coating with said prints outside the overlap zone (4).

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