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

Abstract

A method for manufacturing a security device comprising the steps of: providing a lenticular grating of pitch Trl along a first direction X; defining a first image grating of first period T1, a difference dT, between the pitch Trl and the first period T1, being equal to n*Lpix, and being less than 5% of the pitch Trl; defining a second image grating of second period T2, a difference dT', between the pitch Trl and the second period T2, being equal to n'*Lpix', and being less than 5% of the pitch Trl; creating the first image grating in a first area of ​​a substrate; creating a second image grating in a second area of ​​the substrate; and applying the lenticular grating to the first and second image gratings with the first period T1 and the second period T2 along the first direction X. The resulting security device and a secure document containing it. Figure for the abstract: -

Description

Title of the invention: Method for manufacturing a security device, security device obtained, and secure document incorporating such a device. Technical field of the invention

[0001] The present invention relates to secure documents, for example an identity document.

[0002] It relates more particularly to a method of manufacturing a security device for such a document. State of the art

[0003] In the field of identity document security and fraud suppression, an interesting physical phenomenon to exploit is that of visual effects, and in particular those involving a movement or depth effect of an image.

[0004] A depth effect is based on an interference between a lenticular network and a frame, i.e. a network of images, arranged under the lenticular network, having slightly different periods.

[0005] Furthermore, it is possible to produce a movement effect through the interference of these two networks, but this effect is very sensitive to variations in the period of each of the networks.

[0006] Combining several effects makes it possible to strengthen the security of a document and makes counterfeiting more difficult for a forger.

[0007] However, it is not always possible to combine two effects, for example within the same safety element, while maintaining a good visual rendering. For example, a noticeable depth effect combined with a desired movement effect (speed, direction) can only be achieved with a limited number of parameters. A good visual rendering is therefore difficult to obtain in practice with current commercial methods.

[0008] 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 step of the lenticular network chosen, in combination with the image.

[0009] 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.

[0010] However, to increase the complexity of security devices, one objective is to have relatively small lenses, or even the smallest possible. Printer resolutions must therefore be taken into account to produce the corresponding images, and thus enable the production of certain visual effects.

[0011] The present invention thus aims to overcome at least in part the aforementioned disadvantages, and may also lead to other advantages. Description of the invention

[0012] To this end, a manufacturing process for a safety device is proposed, according to a first aspect, comprising at least the following steps:

[0013] - provide a lenticular array comprising cylindrical lenses of a pitch Tri along a first direction X;

[0014] - define a first image network having a first period Tl, a difference dT, between the step Tri and the first period Tl, being equal to n*Lpix, where n is a relative integer with absolute value Inl at least equal to 1, and Lpix is ​​a dimension of a pixel, for example in pm, the difference dT = n*Lpix being less than 5% of the step Tri of the lenticular network;

[0015] - define a second image network having a second period T2, a difference dT', between the step Tri and the second period T2, being equal to n'*Lpix', where n' is a relative integer with absolute value In'l at least equal to 1, and Lpix' a dimension of a pixel, for example in pm, the difference dT'=n'*Lpix' being less than 5% of the step Tri of the lenticular network;

[0016] - to create the first image network on a support, in a first area of ​​the support;

[0017] - create a second image network on the support, in a second area of ​​the support which is distinct from the first zone; and

[0018] - apply the lenticular grating to the first image grating and the second grating of images with the first period T1 and the second period T2 according to the first direction X.

[0019] Such a process thus makes it possible to obtain a chosen depth effect, combined with the effect of speed (movement), or even animation.

[0020] The resulting security device is therefore seen at a certain plane, which makes the use of the first plane free to integrate another type of security element.

[0021] Creating a second image network on the support, in a second area of ​​the support which is distinct from the first area, makes it possible to produce two distinct images which are visible almost simultaneously to an observer through the lenticular network.

[0022] In an example embodiment, Inl is at least equal to 2, for example between 2 and 6.

[0023] In an example embodiment, In'l is at least equal to 2, for example between 2 and 6.

[0024] In one embodiment example, "n" is different from "n'".

[0025] In one embodiment, at least one of the first image array or the second image array is printed by offset, and for example at the same time as other elements of the security device.

[0026] In one 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.

[0027] In one embodiment, one of the first period T1 or of the second period T2 is greater than the step Tri.

[0028] Where appropriate, the corresponding value of "n" or "n'" is a positive integer.

[0029] In an example embodiment, one of the first period T1 or the second period T2 is less than the step size Tri.

[0030] Where applicable, the corresponding value of "n" or "n'" is a negative integer.

[0031] Depending on the function, the rendered image appears to scroll to the left or to the right.

[0032] In an example embodiment, the first period T1 and the second period T2 are greater than the step Tri.

[0033] Where applicable, the values ​​of "n" and "n'" are positive integers. The images rendered by each network thus appear at different depths.

[0034] Producing values ​​of "n" and "n'" both negative, and therefore corresponding negative difference dT values, generally implies, all other things 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.

[0035] In one embodiment, the first period T1 is different from the second period T2.

[0036] Also proposed, according to a second aspect, is a security device obtained by a process comprising at least some of the characteristics described above.

[0037] Such a device includes, for example:

[0038] - a support,

[0039] - a first image network having a first period Tl, a difference dT, between the step size Tri and the first period Tl, being equal to n* Lpix, and the difference dT=n* Lpix being less than 5% of the step size Tri of the lenticular grating, the first image grating being formed on the support in a first area of ​​the support,

[0040] - 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'*Lpix', and the difference dT'=n'*Lpix' being less than 5% of the step size Tri of the lenticular grating, the second image network being formed on the medium in a second area of ​​the medium which is distinct from the first area,

[0041] - a lenticular array comprising cylindrical lenses of a pitch Tri according to 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.

[0042] Such a device thus presents advantages similar to those described in connection with the above process.

[0043] In particular, such a device makes it possible to produce, for each image network observed through the lenticular network, an image having an apparent size which is arranged at an apparent distance from an observer, and producing an apparent movement effect, to the right or to the left.

[0044] Such a device has characteristics similar to those described in connection with the process.

[0045] For example, one of the first period T1 or of the second period T2 is greater than the step Tri.

[0046] For example, one of the first period T1 or of the second period T2 is less than the step Tri.

[0047] For example, the first period T1 is different from the second period T2.

[0048] A third aspect also proposes a secure document comprising a safety device having at least some of the characteristics described above.

[0049] Such a secure document is, for example, a passport, an identity card, a driving licence or other. Brief description of the figures

[0050] The invention, according to an exemplary embodiment, will be better understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and in no way limiting, with reference to the accompanying drawings in which:

[0051] [Fig.1] represents an example of an embodiment of a lenticular array with cylindrical lenses;

[0052] [Fig.2] shows a first image network and a second image network according to an example of an embodiment of the present invention, each image of the first image network representing the same pattern "A" repeated, and each image of the second image network representing the same pattern "B" repeated;

[0053] [Fig.3] illustrates a safety device obtained according to an example of implementation of the present invention;

[0054] [Fig.4] illustrates an apparent speed, on the ordinate, as a function of the pitch of the lenticular grating (constant pitch), with on the abscissa a difference dT, measured here in pixels, between the pitch of the lenticular grating Tri and the period Tl of the first image grating as illustrated [Fig.2]; a positive difference meaning that the Tri is greater than the period Tl, while a negative difference meaning that the Tri is less than the period Tl, for different print resolutions (in dpi);

[0055] [Fig.5] schematically illustrates a size and depth effect rendered for an observer having an inter-eye distance “IED”;

[0056] [Fig.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 example of an embodiment of the invention, for a period Tl of 48 pixels ([Fig.6] a)) and for a period Tl of 49 pixels ([Fig.6] b));

[0057] [Fig.7] represents an apparent “positive” ([Fig.7] a)) or “negative” ([Fig.7] b)) displacement observed under the 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 Tri lenticular array and the period of the image considered;

[0058] Fig. 8 schematically illustrates a rotation of a safety device according to an example of an embodiment of the invention, and the induced displacement;

[0059] Fig. 9 illustrates the apparent speed of a pattern as a function of a difference between the pitch of the lenticular grating Tri and the period of at least one image grating, and the possibilities for observing a non-negligible apparent speed;

[0060] 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 such as Figure 9, and the point pairs allowing a depth effect to be observed; and

[0061] [Fig.11] shows the couples of [Fig.10] allowing observation of an interesting speed effect and depth effect in an example of an embodiment of the present invention. Detailed description

[0062] The visual effect referred to in this document results from the superimposition of at least two networks with slightly different periods: - at least one network of images, for example produced on a medium, for example by printing, or any other method chosen to produce an image on a medium; - a network of cylindrical lenses, also referred to here as a lenticular network, covering at least one image network.

[0063] To produce a depth effect, the lenticular array and at least one image array are configured here to be arranged vertically when viewed by an observer.

[0064] This depth effect is not observable if the lenses of the lenticular array (and incidentally at least one image array positioned under the lenticular array) are arranged horizontally.

[0065] 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 referred to as "stereovision".

[0066] The greater the difference between the observed images, the more the object is interpreted as being close to the observer, or vice versa.

[0067] 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 [Fig.1].

[0068] Each lens has a width L, and a curvature R, and extends along a direction Y which is here orthogonal to the direction X.

[0069] Generally, all the lenses of a lenticular array are identical, so that their width defines the "step" Tri of the lenticular array, which is then a constant step.

[0070] However, the pitch could be “variable” if the lenses have widths that differ from each other.

[0071] For the purposes of this description, for convenience, a constant "Tri" step is considered.

[0072] Fig. 2 shows a first image network 20 according to an example of an embodiment of the present invention.

[0073] The first image network 20 comprises a plurality of images 21, one of which is identified by a frame for convenience.

[0074] A width of an image, corresponding to a width of the frame, corresponds to a period of the image network.

[0075] The first image network therefore has here a first period “Tl”.

[0076] In the illustrated example, all 21 images are identical.

[0077] Each image 21 of the image network 20 here represents the same motif “A”.

[0078] However, it could be for any other reason.

[0079] Similarly, [Fig.2] shows a second image network 30 according to an example embodiment of the present invention.

[0080] The second image network 30 comprises a plurality of images 31, one of which is identified by a frame for convenience.

[0081] The second image network therefore has a second period "T2".

[0082] In the illustrated example, all 31 images are identical.

[0083] Each image 31 of the second image network 30 here represents the same pattern “B”.

[0084] However, it could also be for any other reason.

[0085] 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 [Fig.1] two images distinct from each other but visible relatively simultaneously by an observer.

[0086] Thus, as described below, the two images produced can each exhibit, when observed with the naked eye by an observer through the lenticular network, their own scrolling speed and depth effect.

[0087] Figure 3 illustrates a safety device 100 obtained according to an example of implementation of the present invention.

[0088] As illustrated [Fig.3], the security device 100 comprises such an image network 20, 30 which is applied to a support 40 by any means, for example by printing.

[0089] The support 40 can, for example, be any type of support that is common in the technical field under consideration.

[0090] In an example of an embodiment by printing, a printing 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.

[0091] Current print resolutions are generally between 10000 dpi and 12000 dpi.

[0092] Thus, to create the intended 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.

[0093] 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.

[0094] To this end, in an example of an implementation of the invention, a method for making 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 [Fig.1]). An observation angle then varies to generate an apparent displacement of the pattern.

[0095] A relative displacement between at least one first image network and a lens network can also be obtained by translating the lens network on a periodic image, i.e. the image network.

[0096] 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).

[0097] 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.

[0098] In such an arrangement, depending on whether the step Tri of the lenticular grating 10 is smaller or larger than the period Tl (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 inclined around the Y axis.

[0099] 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 on [Fig.2], to within a gap dT, corresponding to the difference between the pitch Tri and the period Tl: dT = Tri - Tl.

[0100] Thus, each lens focuses a different slice of the pattern, for example of the "A".

[0101] As a result, an observer reconstructs the pattern, here the character "A" enlarged, which moves according to the orientation of the device with respect to the observer's observation position, this also being illustrated [Fig.8].

[0102] As illustrated in [Fig.4], the smaller the difference dT between Tl 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.

[0103] This graph shows that in practice, the smaller the difference dT between the period Tl and the step size Tri, the faster a movement appears. In practice, however, it is preferable for the difference dT to be at least 2 pixels for a resolution of 10000 dpi. Thus, for example, an image 21 has a width Tl of 53 pixels, while the lenticular grating has a step size Tri corresponding to 51 pixels.

[0104] The width of an image 21, i.e. the period Tl, has an impact on the motion effect.

[0105] This parameter also affects the depth effect produced.

[0106] For a given inter-eye distance (generally denoted IED, for "inter-eye distance"), representing the distance between an individual's two eyes, schematically represented by the distance BC in [Fig. 5], an image, positioned at point A, having a relatively small apparent size h2, will induce a sensation of significant distance d2 for a observer, while an image of larger apparent size hl, will give an impression of distance dl, and will be perceived as closer to the observer, as illustrated [Fig.5].

[0107] Such a relationship is established for example by Thales' Theorem, which makes it possible to determine a resulting impression of depth.

[0108] For example, as schematically shown [Fig.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 a distance "IED" of approximately 63 mm (standard reference case, but this distance can be adapted depending on the case), the following results are obtained: Tl (in pixels) 48 49 53 Apparent size h; (in pixels) 816 1249.5 1351.5 Apparent size h; (in mm) 2 3.17 3.43 Perceived distance d; 387 380 378

[0109] The perceived distance d; can be estimated more precisely with curvilinear perspective calculations.

[0110] 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: [°H11 d, = DobS-^

[0112] According to an interesting option, the process may include a step of taking into account a tolerance of the safety device.

[0113] Considering the tolerance on the Tri pitch of the lens to be negligible (because the pitch is more easily measurable and controllable), there may still be a significant tolerance on the realization of at least one image grating, particularly when it is carried out by printing.

[0114] These tolerances induce an error on the period Tl of at least one image network 20 which is quantifiable.

[0115] If the desired apparent size h and the step size Tri are fixed, it is then possible to determine the period TL Theoretical Tl (in pixels) 48 49 53 Measured Tl (in pixels) 47.98 48.88 52.89 Theoretical apparent size (in pixels) 816 1249.5 1351.5 Apparent size measured (in pixels): 812.4, 1177, 1426.5

[0116] The method may for example then include a step of calculating the speed of the effect as well as its distance perceived by the eye of an observer.

[0117] The results are similar to the theoretical calculations, except that the printing tolerances do not allow us to observe a difference in speed and distance for a variation of 1 pixel (example Tl = 48 pixels and Tl = 49 pixels, for the same Tri step size, produce a similar effect in practice).

[0118] In the context of the present 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.

[0119] Nevertheless, this difference is real when visualizing the apparent displacement generated under the microscope, as illustrated for example [Fig.6].

[0120] Fig. 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 Tl of 48 pixels ([Fig. 6] a)) and for a period Tl of 49 pixels ([Fig. 6] b)), for a step Tri of 51 pixels.

[0121] It can be seen that the evolution obtained can be extrapolated by a straight line, with an R2 coefficient of 0.984 for [Fig.6] a) and 0.99 for [Fig.6] b).

[0122] A slope of the line corresponds to the speed (or sensitivity).

[0123] For [Fig.6] a), the velocity (which is therefore dimensionless) is about 28.226, while for [Fig.6] b), the velocity is then about 38.541.

[0124] It should also be noted that, contrary to theory, an apparent displacement called "negative" (period Tl greater than the pitch Tri) allows a more pronounced effect compared to an apparent displacement called "positive" of the same deviation with the lens pitch, as illustrated [Fig.7].

[0125] Figure 7 represents the apparent “positive” (Fig. 7 a) or “negative” (Fig. 7 b) 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 Tri lenticular grating and the period of the image considered, for example TL

[0126] For an apparent “positive” displacement, [Fig.7] a), the velocity is then about 38.514 (for an interpolation line with an R2 of 0.99), while for an apparent “negative” displacement, [Fig.7] b), the velocity is then about 49.721 (for an interpolation line with an R2 of 0.9898).

[0127] 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 [Fig.8],

[0128] In [Fig. 8] a), the safety device is observed at a normal angle (considered to be 0°) to the device by the observer, i.e., orthogonally. In [Fig. 8] b), the device is inclined, rotated about the Y-axis, relative to the position in [Fig. 8] a), i.e., the angle of observation is different from 0°, assuming that the observer has not moved.

[0129] The change in the angle of observation induced on a lens of the lenticular array causes a different portion of an image 21 to be visible to the observer.

[0130] Thus, it is possible to play on an apparent speed of movement and a perceived depth of image and combine the two effects in a security device thus obtained to make it more difficult to counterfeit.

[0131] Of course, the aforementioned difference (or gap) of 2 pixels is an example related to the embodiment detailed here.

[0132] 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.

[0133] In the case of printing, a printing tolerance of Tl is at least lOpm.

[0134] In an interesting embodiment where speed and therefore stereoscopic effect are important, Tl is preferably close to Tri.

[0135] With two image networks, characterized by their period T1 and T2, it is more interesting that they are seen at different depths, and that their speeds are different.

[0136] Under these conditions, it is interesting that a difference between two periods T1 and T2 is at least 2 pixels.

[0137] Thus, to observe a non-negligible speed, it is interesting to have a difference dT between the step of the lenticular network Tri and the period Tl of the image network, where dT = n*Lpix, with n an integer at least equal to 1 and Lpix the size of a pixel, of about 5% maximum with respect to the step Tri of the lenticular network.

[0138] 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.

[0139] If the device comprises two image networks, to form two distinct patterns, it is then further interesting that a difference dT' between the step Tri and the second period T2 be equal to n'*Lpix', where similarly n' is an integer at least equal to 1, by example at least equal to 2, for example between 2 and 6, and Lpix' the size of a pixel, i.e. dT'=n'*Lpix', i.e. less than 5% of the step size of the lenticular network.

[0140] 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.

[0141] In practice, Lpix and Lpix' are generally identical because the two image networks are usually created together for convenience. However, they could differ depending on the embodiment chosen.

[0142] Such a criterion (dT<5%) implies 4 possibilities illustrated [Fig.9] for observing a non-negligible speed.

[0143] Fig. 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 Tl, on the abscissa.

[0144] In this example, at least one array of images 20 is printed with a resolution of 10000 dpi.

[0145] [Fig. 10] is based on the same graph as [Fig. 9], in which examples of pairs allowing observation of a non-negligible depth difference are represented, each pair being represented by two circles connected to each other.

[0146] To observe a non-negligible difference in depth, it is interesting to have a difference dT between the step Tri and the period Tl of at least 2 pixels.

[0147] With a difference dT of 5% between the pitch of the lenticular grating and the period of an image grating, a depth and speed effect is therefore produced.

[0148] It is also possible to use at least a second image network, which makes it possible to accentuate these effects.

[0149] As illustrated [Fig.1 1], if one wishes to combine a speed effect and a depth effect, the number of interesting pairs among those represented [Fig. 10] is reduced.

[0150] 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, Tl is for example close to 130 pm, but different from it.

[0151] Considering a printer resolution of 10000 dpi, corresponds to 10000 pixels for 2.54 cm, i.e. Lpix = 2.54 pm or about 3.93 pixels for lOpm, a step of 130pm allows to realize about 51 pixels.

[0152] Tl and Tri are for example different by 2 pixels (i.e. about 5 pm here), i.e. a difference dT of about 4% with respect to the pitch of the Tri lenticular grating.

[0153] A difference dT is understood here as a left or right shift, i.e. the period Tl (or T2) can be greater or less than the step Tri.

[0154] With a printer with a resolution of 15000 dpi and lenses with a Tri pitch of 200 pm, the following results would be obtained: - 15000 dpi corresponds to 15000 pixels for 2.54 cm, i.e. Lpix = 1.70 pm; - A step size of 200 pm allows for printing approximately 118 pixels - For a difference of approximately 5 pm, about 3 pixels, or 2.5%, the effect of Speed ​​is visible.

[0155] To combine a non-negligible speed effect with a depth effect, it is therefore necessary to pay attention to the parameters selected for the printed period Tl (or T2) relative to the pitch of the lenticular grating Tri.

[0156] The calculation of the apparent speed is discretized according to the printing resolution used.

[0157] It is noted that the difference of the period Tl with the lenticular period (Tr) to observe the speed is preferably 2 pixels at 10000dpi but is preferably 3 pixels at 15000dpi.

[0158] For a depth effect, a pair of parameters, T1, T2, must be selected that produce different "perceived distances" when these distances are measured experimentally (the theoretical distance is modified according to tool wear, etc.). The more these perceived distances differ from each other, the greater the depth effect will be.

[0159] However, there must not be too large a difference otherwise the size of the observable pattern is too impacted (the apparent size).

[0160] In an interesting implementation example, the apparent size h; is at least 2 mm in practice.

[0161] This amounts to a difference between the pairs of solutions as illustrated in [Fig.9] of two pixels for 10000dpi. Delta pix 1 2 3 4 5 Apparent size eh; for 10000 dpi 5.5mm 2.9mm XXX Apparent size eh; for 12700 dpi 8.3mm 4.0mm 2.7mm XX Apparent size eh; for 15000 dpi 12.8mm 5.5mm 3.4mm 2.5mm Apparent size eh; for 20000 dpi 9.6mm 5.5mm 3.8mm 2.9mm 2.3m

[0162] Finally, for an apparent "negative" displacement, the pattern will be observed in the plane (due to the absence of possible stereovision). Two patterns chosen such that they have two apparent negative displacements will both be seen in the plane regardless of the pixel gap between their periods.

Claims

Demands

1. A method for manufacturing a safety device comprising at least the following steps: - providing a lenticular grating comprising cylindrical lenses of a pitch Tri along a first direction X; - defining a first image grating having a first period T1, a difference dT, between the pitch Tri and the first period T1, being equal to n*Lpix, where n is an integer with an absolute value Inl at least equal to 1, and Lpix a dimension of one pixel, the difference dT = n*Lpix being less than 5% of the pitch Tri of the lenticular grating; - defining a second image grating having a second period T2, a difference dT', between the pitch Tri and the second period T2, being equal to n'*Lpix', where n' is an integer with an absolute value 1 ni at least equal to 1, and Lpix' a dimension of one pixel, the difference dT'=n'*Lpix' being less than 5% of the pitch Tri of the lenticular grating;- create the first image array on a support, in a first area of ​​the support; - create a second image array on the support, in a second area of ​​the support which is distinct from the first area; and - apply the lenticular lens to the first image array and the second image array 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 Tl, a difference dT, between the pitch Tri and the first period Tl, being equal to n* Lpix, and dT=n* Lpix is ​​less than 5% of the pitch Tri of the lenticular array, the first image array being formed on the support in a first area of ​​the support,

6.

7.

8.

9. - 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'*Lpix', and dT'=n'* Lpix' is less than 5% of the step size Tri of the lenticular array, the second image array being formed on the support in a second area of ​​the support 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. A device according to claim 5, wherein one of the first period T1 or the second period T2 is greater than the step size Tri. 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. Device according to any one of claims 5 to 7, wherein the first period T1 is different from the second period T2. Secure document comprising a security device according to any one of claims 5 to 8.

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