Method for manufacturing a security device, security device obtained thereby, and secure document comprising such a device

Abstract

The invention relates to method for manufacturing a security device, the method comprising the steps of: providing a lenticular array with a pitch Trl in a first direction X; defining a first array of images with a first period T1, wherein a difference dT between the pitch Trl and the first period T1 is equal to n*Lpix and is less than 5% of the pitch Trl; defining a second array of images with a second period T2, wherein a difference dT' between the pitch Trl and the second period T2 is equal to n'*Lpix' and is less than 5% of the pitch Trl; making the first array of images in a first zone of a support; producing a second array of images in a second zone of the support; and applying the lenticular array to the first and second arrays of images with the first period T1 and the second period T2 in the first direction X. The invention also relates to a security device obtained thereby and to a secure document comprising same.

Description

[0001] A method for manufacturing a security device, the resulting security device, and a secure document containing such a device

[0002] Technical field of the invention

[0003] The present invention relates to secure documents, for example an identity document.

[0004] It relates more specifically to a manufacturing process for a security device for such a document.

[0005] State of the art

[0006] In the field of identity document security and fraud prevention, an interesting physical phenomenon to exploit is that of visual effects, and in particular those involving an effect of movement or depth of an image.

[0007] A depth effect is based on an interference between a lenticular network and a frame, i.e. a network of images, arranged below the lenticular network, having slightly different periods.

[0008] Furthermore, it is possible to produce a motion effect through the interference of these two networks, but this effect is very sensitive to variations in the period of each of the networks.

[0009] Combining several effects strengthens the security of a document and makes counterfeiting more difficult for a forger.

[0010] However, it is not always possible to combine two effects, for example within the same security element, while maintaining a good visual result. For instance, a noticeable depth effect combined with a desired movement effect (speed, direction) can only be achieved with a limited number of parameters. Therefore, a good visual result is practically difficult to obtain with current commercial methods.

[0011] Furthermore, obtaining certain visual effects depends, in particular, on the one hand on the resolution of a printer used to provide an image, and on the other hand on a pitch of the lenticular network chosen, in combination with the image.

[0012] If the lenses of the lenticular array could have a relatively large pitch, the printer resolutions to provide the corresponding image would not be a constraint.

[0013] However, to increase the complexity of security systems, one objective is to use relatively small lenses, or even the smallest possible. Printer resolutions must therefore be taken into account to produce the corresponding images, thus enabling the creation of certain visual effects.

[0014] The present invention thus aims to overcome at least in part the aforementioned disadvantages, and may also lead to other advantages.

[0015] Description of the invention

[0016] To this end, a manufacturing process for a safety device is proposed, according to a first aspect, comprising at least the following steps:

[0017] - provide a lenticular array comprising cylindrical lenses of a pitch Tri along a first direction X;

[0018] - define a first image network having a first period T1, a difference dT, between the step size Tri and the first period T1, being equal to n*L PjX , where n is an integer with absolute value |n| at least equal to 1 , and L PjX is a dimension of a pixel, for example in pm, the difference dT = n*L PjX being less than 5% of the step size of the lenticular network;

[0019] - define a second image network having a second period T2, a difference dT', between the step size Tri and the second period T2, being equal to n'*L PjX ', where n' is an integer with absolute value |n'| at least equal to 1, and L PjX 'a dimension of a pixel, for example in pm, the difference dT'=n'*L PjX 'being less than 5% of the step size of the lenticular network;

[0020] - create the first image network on a medium, in a first area of ​​the medium;

[0021] - to create a second image network on the medium, in a second area of ​​the medium that is distinct from the first area; and

[0022] - apply the lenticular network on the first image network and the second image network with the first period T1 and the second period T2 along the first direction X.

[0023] Such a process makes it possible to obtain a chosen depth effect, combined with the effect of speed (movement), or even animation.

[0024] The resulting security system is therefore seen from a certain perspective, which makes the foreground available for integrating another type of security element.

[0025] Creating a second image array on the substrate, in a second area distinct from the first, allows for the production of two separate images that are visible almost simultaneously to an observer through the lenticular lens. The dimension of a pixel, for example LPjX of the first image network, or L PjX ' of the second image network, designates a characteristic dimension of the pixel, for example a diameter, a width or a height.

[0026] The size of the pixel is linked to the resolution of the corresponding image network, in dpi: the pixel thus corresponds to the "dot".

[0027] An image network, or raster, here refers to an image consisting of a series of strips. Each strip contains an image from the image network, for example a pattern, or even the same pattern repeated in each strip.

[0028] The period of the image network corresponds, for example, to the width of a band.

[0029] Each band thus has a width which can be measured in number of pixels, which depends on the resolution capability (dpi).

[0030] The number of strips required to reconstruct a complete image from the pattern of each strip depends, for example, on the lenticular grating, particularly its pitch Tri, and, for example, on the smallest common multiple between the pitch Tri and the period T of the image grating. for example, according to the formula: dT .

[0031] In an example implementation, |n| is at least equal to 2, for example between 2 and 6.

[0032] In an example realization, |n'| is at least equal to 2, for example between 2 and 6.

[0033] In an example of the realization, "n" is different from "n'".

[0034] In one embodiment example, at least one of the first or second image array is offset printed, and for example at the same time as other elements of the security device.

[0035] In an example of an embodiment, at least one of the first or second image array is printed with a resolution of at least 4000 dpi (for "dots per inch"), or even at least 10000 dpi, for example between 10000 dpi and 12000 dpi.

[0036] In an example implementation, one of the first period T1 or the second period T2 is greater than the step Tri.

[0037] If applicable, the corresponding value of "n" or "n'" is a positive integer.

[0038] In one example of implementation, one of the first period T1 or of the second period T2 is less than the step Tri.

[0039] If applicable, the corresponding value of "n" or "n'" is a negative integer.

[0040] Depending on the setting, the rendered image appears to scroll to the left or to the right. In one example, the first period T1 and the second period T2 are greater than the step size Tri.

[0041] If applicable, the values ​​of "n" and "n'" are positive integers. The images rendered by each network thus appear at different depths.

[0042] Producing values ​​of "n" and "n'" both negative, and therefore corresponding negative difference dT values, generally implies, all else being equal, seeing both images rendered in the plane; so in practice it is preferable to have at least one positive difference dT, i.e. at least one of "n" or "n'" positive, or both.

[0043] In one example implementation, the first period T1 is different from the second period T2.

[0044] A second aspect also proposes a security device obtained by a process comprising at least some of the characteristics described above.

[0045] Such a system includes, for example:

[0046] - a support,

[0047] - a first image array having a first period T1, a difference dT, between the step size Tri and the first period T1, being equal to n* L PjX , and the difference dT=n* L PjX being less than 5% of the step size of the lenticular grating, the first image grating being formed on the support in a first area of ​​the support,

[0048] - a second image array having a second period T2, a difference dT', between the step size Tri and the second period T2, being equal to n'*L PjX ', and the difference dT'=n'*L PjX'being less than 5% of the step size of the lenticular grating, the second image grating being formed on the substrate in a second area of ​​the substrate which is distinct from the first area,

[0049] - a lenticular network comprising cylindrical lenses of a pitch Tri along a first direction X, the lenticular network being applied to the support and covering the first image network and the second image network with the first period T1 and the second period T2 along the first direction X.

[0050] Such a device therefore offers advantages similar to those described in connection with the process above.

[0051] In particular, such a device makes it possible to produce, for each image array observed through the lenticular lens, an image with an apparent size that is positioned at an apparent distance from an observer, and producing an apparent motion effect, either to the right or to the left. The formation of an image visible to an observer from an image array, in conjunction with a lenticular lens, occurs when the number of sampled images, that is, the number of strips, is sufficient for the combined lenses to form an image observable in a given viewing direction.

[0052] For example, this arrangement allows an observer's left eye to see one image and their right eye to see a second image. This creates a stereoscopic effect, giving the observer a sense of depth.

[0053] Such a device has characteristics similar to those described in connection with the process.

[0054] For example, one of the first period T1 or of the second period T2 is greater than the step Tri.

[0055] For example, one of the first period T1 or of the second period T2 is less than the step Tri.

[0056] For example, the first period T1 is different from the second period T2.

[0057] A third aspect also proposes a secure document with a security device that includes at least some of the characteristics described above.

[0058] Such a secure document is, for example, a passport, an identity card, a driving licence or the like.

[0059] Brief description of the figs

[0060] The invention, according to an exemplary embodiment, will be well understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and not limitation, with reference to the accompanying drawings in which: Figure 1 represents an exemplary embodiment of a lenticular array with cylindrical lenses; Figure 2 shows a first image array and a second image array according to an exemplary embodiment of the present invention, each image of the first image array representing the same repeated pattern "A", and each image of the second image array representing the same repeated pattern "B"; Figure 3 illustrates a safety device obtained according to an exemplary implementation of the present invention;Figure 4 illustrates an apparent speed, on the ordinate, as a function of the lenticular grating pitch (constant pitch), with on the abscissa a difference dT, measured here in pixels, between the lenticular grating pitch Tri and the period T1 of the first image grating as illustrated in Figure 2; a positive difference meaning that Tri is greater than the period T1, while a negative difference meaning that Tri is smaller than the period T1, for different print resolutions (in dpi); Figure 5 schematically illustrates a size and depth effect rendered for an observer with an inter-eye distance "IED";Figure 6 shows results of measurements made under a microscope to characterize an apparent displacement, and the slope of the interpolations corresponds to the speed (or sensitivity), as a function of a rotation angle of a safety device according to an embodiment of the invention, for a period T1 of 48 pixels (Figure 6 a)) and for a period T1 of 49 pixels (Figure 6 b)); Figure 7 represents an apparent "positive" (Figure 7 a)) or "negative" (Figure 7 b)) displacement observed under a microscope according to a rotation angle of a safety device according to an embodiment of the invention, for a difference of 2 pixels between the pitch of the lenticular grating Tri and the period of the image considered; Figure 8 schematically illustrates a rotation of a safety device according to an embodiment of the invention, and the induced displacement;Figure 9 illustrates the apparent speed of a pattern as a function of the difference between the pitch of the Tri lenticular grating and the period of at least one image grating, and the possibilities for observing a non-negligible apparent speed; Figure 10 illustrates the apparent speed of a pattern as a function of the difference between the pitch of the Tri lenticular grating and the period of at least one image grating, as in Figure 9, and the point pairs allowing observation of a depth effect; and Figure 11 shows the point pairs of Figure 10 allowing observation of an interesting speed and depth effect in an example of an embodiment of the present invention.

[0061] Detailed description

[0062] The visual effect discussed in this document arises from the superimposition of at least two gratings with slightly different periods: at least one image grating, for example, produced on a substrate, such as by printing or any other method chosen to produce an image on a substrate; and a cylindrical lens array, also referred to here as a lenticular lens, covering the at least one image grating. To produce a depth effect, the lenticular lens and the at least one image grating are configured here to be arranged vertically when viewed by an observer.

[0063] This depth effect is not observable if the lenses of the lenticular array (and incidentally at least one image array positioned below the lenticular array) are arranged horizontally.

[0064] This effect is due to the laws of reflection and refraction of optical rays: this arrangement allows an observer's left eye to focus on one image and their right eye to focus on another. The difference between the observed left and right images is interpreted by the observer's brain as a distance from the object thus represented; this is what is known as "stereovision."

[0065] The greater the difference between the observed images, the more the object is interpreted as being close to the observer, or vice versa.

[0066] In the context of the present invention, the lenticular network 10 comprises a plurality of cylindrical lenses 11, arranged parallel to each other along a direction X, as illustrated in Figure 1.

[0067] Each lens 11 has a width L, and a curvature R, and extends along a direction Y which is here orthogonal to the direction X.

[0068] Generally, all the lenses in a lenticular array are identical, so their width defines the "step" of the lenticular array, which is then a constant step.

[0069] However, the pitch could be "variable" if the lenses have widths that differ from each other.

[0070] For the purposes of this description, a constant "Tri" step size is assumed.

[0071] Figure 2 shows a first image network 20 according to an example of an embodiment of the present invention.

[0072] The first image network 20 comprises a plurality of images 21, one of which is identified by a frame for convenience.

[0073] The width of an image, corresponding to the width of the frame, corresponds to a period of the image network.

[0074] The first image network therefore has a first period "T1".

[0075] In the illustrated example, all 21 images are identical.

[0076] Each image 21 of the image network 20 represents here the same pattern "A".

[0077] However, it could be any other pattern. Here, the pattern has a width roughly equal to the width of the band.

[0078] However, the pattern could have a width less than the width of a band.

[0079] In the case of this example, a pattern of the same width as the band generates a string of contiguous "A"s, while a pattern of lesser width than the band would generate a string of non-contiguous "A"s.

[0080] In one example of an embodiment, a number of images 21 of the first image array 20, in association with the lenticular array, to form an image visible to an observer is given by the smallest common multiple between the period T1 of the first image array 20 and the period Tri of the lenticular array.

[0081] The formation of a complete image visible to an observer from an image grating in association with a lenticular grating then takes place when the number of images of the image grating, i.e. the number of bands, is sufficient to allow alignment without offset between a band of the image grating and a lens of the lenticular grating.

[0082] Under this condition, the size of the image formed by the first image network 20 is

[0083] For example, TrlxTl: dT .

[0084] Similarly, Figure 2 shows a second image array 30 according to an example embodiment of the present invention.

[0085] The second image network 30 comprises a plurality of images 31, one of which is identified by a frame for convenience.

[0086] The second image network therefore has a second period "T2".

[0087] In the illustrated example, all 31 images are identical.

[0088] Each image 31 of the second image network 30 represents here the same pattern "B". However, it could also be any other pattern.

[0089] Here, the pattern has a width roughly equal to the width of the band.

[0090] However, the pattern could have a width less than the width of a band.

[0091] In the case of this example, a pattern of the same width as the band generates a string of contiguous "B"s, while a pattern of lesser width than the band would generate a string of non-contiguous "B"s.

[0092] In one example of an embodiment, a number of images 31 of the second image array 30, in association with the lenticular array, to form an image visible to an observer is given by the smallest common multiple between the period T2 of the second image array 30 and the period Tri of the lenticular array.

[0093] The formation of a complete image visible to an observer from an image grating in association with a lenticular grating then takes place when the number of images in the image grating, i.e. the number of bands, is sufficient to allow alignment without offset between a band of the image grating and a lens of the lenticular grating.

[0094] Under this condition, the size of the image formed by the second image network 30 is, for example

[0095] The second image network 30 is formed in a separate area from the first image network 20 so as to form, in association with a lenticular network as illustrated in Figure 1, two images distinct from each other but visible relatively simultaneously to an observer.

[0096] Thus, as described below, the two images produced can each exhibit, when observed with the naked eye by an observer through the lenticular lens, their own scrolling speed and depth effect.

[0097] Figure 3 illustrates a safety device 100 obtained according to an example of implementation of the present invention.

[0098] As illustrated in Figure 3, the security device 100 comprises such an image network 20, 30 which is applied to a substrate 40 by any means, for example by printing. The substrate 40 can, for example, be any type of substrate commonly used in the relevant technical field.

[0099] In an example of realization by printing, a print resolution, for example by a printer, is preferably at least 4000 dpi (for "dots per inch", i.e. points per inch), or even at least 10000 dpi.

[0100] Current print resolutions are generally between 10000 dpi and 12000 dpi.

[0101] Thus, to create the desired visual effect, at least a first image array 20 and a second image array 30 are applied to the support 40, and the lenticular array 10 is applied to the support 40 and covers at least a first image array 20 and a second image array 30, so that at least a first image array 20 and the second image array 30 produce images visible through the lenticular array 10.

[0102] For example, with a Tri step of the lenticular grating of about 130 pm, it is possible to form about 51 pixels with a resolution of 10000 dpi.

[0103] To this end, in an example of an implementation of the invention, a method for creating a safety device includes, for example, a step of defining a speed (sensitivity), which is not a temporal speed but a dimensionless speed that qualifies a ratio of apparent displacement to a physical displacement of the support, for example here as a function of an inclination of the support around the longitudinal axis Y of the lenses (shown in Figure 1). An observation angle then varies to generate an apparent displacement of the pattern.

[0104] A relative displacement between at least one first image array and a lens array can also be obtained by translating the lens array onto a periodic image, i.e. the image array.

[0105] Within the framework of the present invention, the principle remains the same whether it is a translation with relative movement between the two networks or a rotation without physical movement between the lens network and at least one image network (periodic pattern).

[0106] The principles described below refer mainly to the first image array 20 and its period T1, but the same principles apply to the second image array 30 and its period T2.

[0107] In such an arrangement, depending on whether the step Tri of the lenticular grating 10 is smaller or larger than the period T 1 (or T2) of the image grating 20, 30, the image rendered by the image grating 20, 30 under the lenticular grating 10 appears to scroll in one direction or the other, i.e. for example to the right or to the left), while the safety device is tilted around the Y axis.

[0108] In the example shown, the pitch Tri is constant; thus, a lens covers a pattern of the image grating, for example an "A" as framed in figure 2, to within a gap dT, corresponding to the difference between the pitch Tri and the period T1: dT = Tri - T1. Thus, each lens focuses a different slice of the pattern, for example of the "A".

[0109] As a result, an observer reconstructs the pattern, here the large character "A", which moves according to the orientation of the device relative to the observer's observation position, this is also illustrated in figure 8.

[0110] As illustrated in Figure 4, the smaller the difference dT between T1 and Tri, the faster the apparent scrolling speed of the image produced by the lenticular grating appears, with an asymptote at 0 when the two gratings have the same period.

[0111] This graph shows that in practice, the smaller the difference dT between the period T1 and the step size Tri, the faster a movement appears. However, in practice, it is preferable for the difference dT to be at least 2 pixels for a resolution of 10,000 dpi. Thus, for example, image 21 has a width T1 of 53 pixels, while the lenticular grating has a step size Tri corresponding to 51 pixels.

[0112] The width of an image 21, i.e. the period T1, has an impact on the motion effect.

[0113] This parameter also affects the depth effect produced. For a given inter-eye distance (generally noted as IED, for "inter eye distance"), representing the distance between an individual's two eyes, schematically represented by the distance BC in Figure 5, an image, placed at point A, having a relatively small apparent size h2, will induce a sensation of a significant distance d2 for an observer, while an image with a larger apparent size h1 will give an impression of a distance d1, and will be perceived as closer to the observer, as illustrated in Figure 5.

[0114] Such a relationship is established, for example, by Thales' Theorem, which allows us to determine a resulting impression of depth.

[0115] For example, as shown schematically in figure 5, for an observation distance "Dobs" between an observer and the safety device, which is then, for example, located at point A, with Dobs of approximately 400 mm and an "IED" distance of approximately 63 mm (standard reference case, but this distance can be adapted depending on the case), we obtain the following results:

[0116] The perceived distance dj can be estimated more precisely with curvilinear perspective calculations.

[0117] The principle remains the same, but it is closer to human vision and is calculated using a geometric sequence with common ratio di, according to the following relationship: h;. Dobs dt = Dobs -

[0118] 1 IED

[0119] According to one interesting option, the process may include a step to take into account a tolerance of the safety device.

[0120] Considering the tolerance on the Tri pitch of the lens to be negligible (because the pitch is more easily measured and controlled), there may still be a significant tolerance on the realization of at least one image grating, especially when it is done by printing.

[0121] These tolerances introduce an error in the period T1 of at least one image array 20, which is quantifiable. If the desired apparent size hj and the step size Tri are fixed, it is then possible to determine the period T1.

[0122] The process may, for example, then include a step of calculating the speed of the effect as well as its distance as perceived by the eye of an observer.

[0123] The results are similar to the theoretical calculations, except that printing tolerances do not allow us to observe a difference in speed and distance for a variation of 1 pixel (example T1 = 48 pixels and T1 = 49 pixels, for the same Tri step size, produce a similar effect in practice).

[0124] In the context of this example, it is therefore more interesting to make a difference of at least 2 pixels between the period T1 of at least one image network 20 and a period T2 of at least a second image network to observe a difference in speed and distance if we wish to produce a security device comprising at least two image networks with a different rendering from each other.

[0125] Nevertheless, this difference is real when visualizing the apparent displacement generated under the microscope, as illustrated for example in figure 6.

[0126] Figure 6 shows results of measurements made under a microscope to characterize an apparent displacement (on the ordinate), as a function of a rotation angle of a safety device, on the abscissa, according to an example of an embodiment of the invention, for a period T1 of 48 pixels (Figure 6 a)) and for a period T1 of 49 pixels (Figure 6 b)), for a step Tri of 51 pixels.

[0127] We observe that the resulting evolution can be extrapolated by a straight line, with a coefficient R 2 of 0.984 for figure 6 a) and 0.99 for figure 6 b).

[0128] The slope of the line corresponds to the speed (or sensitivity).

[0129] For figure 6 a), the speed (which is therefore dimensionless) is approximately 28.226, while for figure 6 b), the speed is then approximately 38.541.

[0130] It should also be noted that, contrary to theory, an apparent displacement called "negative" (period T1 greater than the pitch Tri) allows a more pronounced effect compared to an apparent displacement called "positive" of the same difference with the lens pitch, as illustrated in Figure 7.

[0131] Figure 7 represents the apparent "positive" (Figure 7a) or "negative" (Figure 7b) displacement observed under the microscope according to the angle of rotation of the safety device around the Y axis, for a difference of 2 pixels (respectively positive or negative) between the pitch of the lenticular grating T rl and the period of the image considered, for example T 1.

[0132] For an apparent “positive” displacement (Figure 7a), the velocity is then approximately 38.514 (for an interpolation line with an R 2 of 0.99), while for an apparent “negative” displacement, Figure 7b), the velocity is then approximately 49.721 (for an interpolation line with an R2 of 0.9898).

[0133] This can be explained, for example, by the influence of the rotation of the document around the Y axis made by the observer, which adds an apparent displacement, as illustrated in figure 8.

[0134] In Figure 8a), the safety device is observed at a normal angle (considered to be 0°) to the device by the observer, i.e., orthogonally. In Figure 8b), the device is tilted, rotated about the Y-axis, relative to the position in Figure 8a), i.e., the angle of observation is not 0°, assuming the observer has not moved.

[0135] The change in the angle of observation induced on a lens of the lenticular array results in a different portion of an image 21 being visible to the observer.

[0136] Thus, it is possible to play on apparent speed of movement and perceived depth of image and combine the two effects in a security device thus obtained to make it more difficult to counterfeit.

[0137] Of course, the aforementioned difference (or gap) of 2 pixels is an example related to the implementation detailed here.

[0138] More generally, within the framework of the present invention, in order to have a movement that appears fluid, it is preferable to be able to accommodate as many images 21 as possible, therefore to have a print resolution that is as high as possible where appropriate, for example at least 10000 dpi.

[0139] In the case of printing, a printing tolerance of T 1 is at least 10pm.

[0140] In an interesting example of implementation where speed and therefore stereoscopic effect are important, T1 is preferably close to Tri.

[0141] With two image arrays, characterized by their periods T1 and T2, it is more advantageous for them to be viewed at different depths and at different speeds. Under these conditions, it is beneficial for the difference between two periods T1 and T2 to be at least 2 pixels.

[0142] Thus, to observe a non-negligible speed, it is advantageous to have a difference dT between the pitch of the lenticular grating Tri and the period T1 of the image grating, where dT = n*Lpj X , with n an integer at least equal to 1 and L PjX the size of a pixel, of approximately 5% maximum compared to the Tri step of the lenticular network.

[0143] In practice, it is advantageous that n be at least equal to 2, or even for example between 2 and 6; for example about 2 pixels (n = 2) with a print resolution of 10000dpi.

[0144] If the device has two image networks to form two distinct patterns, then it is also interesting that the difference dT' between the step size Tri and the second period T2 is equal to n'*L PjX ', where similarly n' is an integer at least equal to 1, for example at least equal to 2, for example between 2 and 6, and L PjX ' the size of a pixel, i.e. dT'=n'*L PjX ', i.e. less than 5% of the Tri step of the lenticular network.

[0145] In a particular embodiment, n and n' are different so as to produce different visual effects between the two patterns, i.e. that produced by the first image network 20 and that produced by the second image network 30.

[0146] In practice, L PjX and L PjX ' are generally identical because the two image networks are usually created together for convenience. However, it could be different depending on the embodiment chosen.

[0147] Such a criterion (dT <5%) implies 4 possibilities illustrated in figure 9 to observe a non-negligible speed.

[0148] Figure 9 illustrates the apparent speed of a pattern (dimensionless), on the ordinate, as a function of a difference dT between the pitch of the lenticular grating Tri and the period of an image grating, for example T1, on the abscissa.

[0149] In this example, at least one array of 20 images is printed with a resolution of 10000 dpi.

[0150] Figure 10 is based on the same graph as Figure 9, in which examples of pairs allowing observation of a non-negligible difference in depth are represented, each pair being represented by two circles connected to each other.

[0151] To observe a non-negligible difference in depth, it is useful to have a difference dT between the step Tri and the period T1 of at least 2 pixels.

[0152] With a 5% difference in time (dT) between the lenticular grating's pitch and the period of an image grating, a depth and speed effect is produced. Furthermore, it is possible to use at least a second image grating, thus accentuating these effects.

[0153] As illustrated in Figure 11, if one wishes to combine a speed effect and a depth effect, the number of interesting pairs among those represented in Figure 10 is reduced.

[0154] Thus, for example, for a lenticular grating 10 of step Tri = 130 pm, in order to be able to produce a scrolling speed visible to the naked eye by an observer, T1 is for example close to 130 pm, but different from it.

[0155] Considering a printer resolution of 10000 dpi, this corresponds to 10000 pixels for 2.54 cm, or L PjX = 2.54 pm or approximately 3.93 pixels for 10pm, a step of 130pm allows for approximately 51 pixels.

[0156] T1 and Tri are for example different by 2 pixels (i.e. about 5 pm here), i.e. a difference dT of about 4% compared to the pitch of the Tri lenticular array.

[0157] A difference dT is understood here as a shift to the left or right, i.e. the period T1 (or T2) can be greater or less than the step Tri.

[0158] With a printer with a resolution of 15000 dpi and lenses with a Tri pitch of 200 pm, we would obtain the following results:

[0159] 15000 dpi corresponds to 15000 pixels per 2.54 cm, or L PjX = 1.70 pm;

[0160] A step size of 200 pm allows for printing approximately 118 pixels

[0161] For a difference of about 5 pm, about 3 pixels, or 2.5%, the speed effect is visible.

[0162] To combine a non-negligible speed effect with a depth effect, attention must therefore be paid to the parameters selected for the printed period T1 (or T2) relative to the pitch of the lenticular network Tri.

[0163] The calculation of apparent speed is discretized according to the printing resolution used. It is noted that the difference between the period T1 and the lenticular period (Tr) for observing the speed is preferably 2 pixels at 10000 dpi, but is preferably 3 pixels at 15000 dpi.

[0164] To create a depth effect, a pair of parameters, T1 and T2, must be selected, producing different "perceived distances" when these distances are measured experimentally (the theoretical distance is modified depending on tool wear, etc.). The more these perceived distances differ from each other, the greater the depth effect will be.

[0165] However, the difference must not be too large, otherwise the size of the observable pattern (the apparent size) is too significantly impacted. In an interesting implementation example, the apparent size hj is at least 2 mm in practice.

[0166] This amounts to a difference between the pairs of solutions as illustrated in figure 9 of two pixels per 10000dpi.

[0167] Finally, for an apparent "negative" displacement, the pattern will be observed in the plane (due to the absence of possible stereovision). Two patterns chosen to have two apparent negative displacements will both be seen in the plane regardless of the pixel difference between their periods.

Claims

DEMANDS 1. A method for manufacturing a safety device comprising at least the following steps: - provide a lenticular array comprising cylindrical lenses of a pitch Tri along a first direction X; - define a first image network having a first period T1, a difference dT, between the step size Tri and the first period T1, being equal to n*L PiX , where n is an integer with absolute value |n| at least equal to 1 , and L PjX one dimension of a pixel, the difference dT = n*L PjX being less than 5% of the step size of the lenticular network; - define a second image network having a second period T2, a difference dT', between the step size Tri and the second period T2, being equal to n'*L PjX ', where n' is an integer with absolute value |n| at least equal to 1, and L PjX ' one dimension of a pixel, the difference dT'=n'*L PjX'being less than 5% of the step size of the lenticular network; - create the first image network on a medium, in a first area of ​​the medium; - to create a second image network on the medium, in a second area of ​​the medium that is distinct from the first area; and - apply the lenticular network on the first image network and the second image network with the first period T1 and the second period T2 along the first direction X.

2. A method according to claim 1, wherein one of the first period T1 or the second period T2 is greater than the step size Tri 3. A method according to any one of claims 1 or 2, wherein one of the first period T1 or the second period T2 is less than the step size Tri 4. A method according to any one of claims 1 to 3, wherein the first period T1 is different from the second period T2.

5. A safety device obtained by the method according to any one of claims 1 to 4, the device comprising: - a support, - a first image array having a first period T1, a difference dT, between the step size Tri and the first period T1, being equal to n* L PjX , and dT=n* L PjX is less than 5% of the step size of the lenticular grating, the first image grating being formed on the support in a first area of ​​the support, - a second image array having a second period T2, a difference dT', between the step size Tri and the second period T2, being equal to n'*L PjX ', and dT'=n'* L PjX 'is less than 5% of the step size of the lenticular grating, the second image grating being formed on the substrate in a second area of ​​the substrate which is distinct from the first area, - a lenticular network comprising cylindrical lenses of a pitch Tri along a first direction X, the lenticular network being applied to the support and covering the first image network and the second image network with the first period T1 and the second period T2 along the first direction X.

6. Device according to claim 5, wherein one of the first period T1 or the second period T2 is greater than the step Tri.

7. A device according to any one of claims 5 or 6, wherein one of the first period T1 or the second period T2 is less than the step size Tri 8. Device according to any one of claims 5 to 7, wherein the first period T1 is different from the second period T2.

9. Secure document comprising a security device according to any one of claims 5 to 8.

Images