EP4015228B1 - Method for manufacturing a safety device and associated safety device - Google Patents

Description

Technical Domain

The present invention relates to the field of optical security devices, and more particularly concerns the manufacture of such a security device.

The invention applies non-exclusively to security and/or physical identity documents, such as a passport, an identity card, a driving license, a residence permit, etc.

Prior art

As is known, a security and/or identity document may include security data enabling the bearer of said document to be authenticated, such as a photograph of the face of the authorized bearer of said document.

Such security data is printed on the document so that it is visible to an operator responsible for verifying the identity of the bearer of the document. The operator can thus verify this security data. For example, the agent can verify that the photograph printed on the document corresponds to the bearer of the document.

There is now a need for a security device that allows for simple and fun authentication while being difficult to counterfeit.

The document US 2018/264866 A1 describes a method of generating a mask and a printed image that are embedded in a banknote. This document describes the preamble of claim 1.

Disclosure of the invention

The present invention relates to a method of manufacturing a safety device, according to claim 1.

The gray level of each pixel of the first grayscale image represents the luminance of the first color component of the corresponding pixel of the digital color image. In addition, the gray level of each pixel of the second grayscale image represents the luminance of the second color component of the corresponding pixel of the digital color image.

The first and second grayscale images allow for simple and fun authentication, typically using a phone equipped with a camera and the appropriate color image recomposition software, while being difficult to reproduce. The security device is thus difficult to counterfeit.

In a particular embodiment, the forming step comprises, for each of said at least one first, second, third and/or fourth grayscale images, reproducing the pixels of said image such that the reproduced pixels are positioned contiguously, each of said at least one first, second, third and/or fourth grayscale images being positioned at a different location on the physical medium of the security device.

In a particular embodiment, the forming step comprises reproducing the pixels of said at least one first, second, third and/or fourth grayscale image such that the reproduced pixels of each grayscale image are positioned in an interlaced manner with the pixels of the other grayscale images, said at least one first, second, third and/or fourth grayscale image thus being interlaced in a same location of the physical medium of the security device.

In a particular embodiment, the method further comprises a step of obtaining a lens array, the lens array being configured to allow the viewing of each of said at least one first, second, third and/or fourth different grayscale images according to a different viewing angle.

Grayscale images, etched at different angles under the lens array, make the security device difficult to reproduce.

.

Brief description of the drawings

• [Fig. 1] La figure 1 représente, de manière schématique, un dispositif de sécurité optique non couvert par le texte des revendications ;
• [Fig. 2A] La figure 2A représente, de manière schématique, un dispositif de sécurité optique non couvert par le texte des revendications ;
• [Fig. 2B] La figure 2B représente, de manière schématique, une coupe selon l'axe c-c du dispositif de la figure 2A ;
• [Fig. 3] La figure 3 représente, sous forme d'un organigramme, les principales étapes d'un procédé de fabrication conforme à un exemple de mode de réalisation de l'invention ;
• [Fig. 4] La figure 4 représente, de manière schématique, un exemple d'image source en couleurs utilisée lors de la mise en oeuvre du procédé de la figure 3 ;
• [Fig. 5] La figure 5 représente, de manière schématique, un exemple d'image source en couleurs utilisée lors de la mise en oeuvre du procédé de la figure 3, l'image source filtrée selon les composantes de couleur, et les images en niveaux de gris représentant les composantes de couleur ;
• [Fig. 6] La figure 6 représente, sous forme d'un organigramme, les principales étapes d'un procédé de génération d'une image numérique en couleurs, non couvert par le texte des revendications.

Other features and advantages of the present invention will emerge from the description given below, with reference to the attached drawings which illustrate an exemplary embodiment thereof without any limiting character. In the figures:

• [ Fig. 1 ] There figure 1 schematically represents an optical security device not covered by the text of the claims;
• [ Fig. 2A ] There Figure 2A schematically represents an optical security device not covered by the text of the claims;
• [ Fig. 2B ] There Figure 2B schematically represents a section along the cc axis of the device of the Figure 2A ;
• [ Fig. 3 ] There figure 3 represents, in the form of a flowchart, the main steps of a manufacturing process in accordance with an exemplary embodiment of the invention;
• [ Fig. 4 ] There figure 4 schematically represents an example of a color source image used when implementing the method of the figure 3 ;
• [ Fig. 5 ] There figure 5 schematically represents an example of a color source image used when implementing the method of the figure 3 , the source image filtered according to the color components, and the grayscale images representing the color components;
• [ Fig. 6 ] There figure 6 represents, in the form of a flowchart, the main steps of a method for generating a digital color image, not covered by the text of the claims.

Description of the embodiments

The invention proposes an optical security device 100 comprising n grayscale images IM1 -IMn formed on a physical medium 110, n being a natural integer greater than or equal to 2 (see figures 1 And 2A-2B , described below). The invention further relates to the manufacture of such a device. 100% optical security and the generation of a color image from such a 100% optical security device.

The optical security device 100 is typically intended to be incorporated into a physical security and/or identity document 160, such as a passport, an identity card, a driver's license, etc.

The term "identity document" refers to any document containing information (photograph, surname, first name, etc.) that allows the authorized bearer of the document to be identified more or less securely. This identity information appears physically on the body of the document so that it can be visually verified by a person or a control machine. In addition to the identity information that is visually accessible on the body of the identity document, it is possible to store identity information in a memory contained in the body of the identity document (in a chip for example), this information being electronically accessible if necessary by suitable means to verify the identity of the person concerned.

There figure 3 depicts a method of manufacturing an optical security device 100, the method being in accordance with an exemplary embodiment of the invention.

The manufacturing process is typically implemented by a manufacturing system comprising digital data processing means and printing means.

The data processing means typically take the form of a computer, executing a computer program stored in a computer-readable information medium (or memory).

In a step S300, a digital color image 400, called a source color image 400, is obtained by the data processing means, this digital color image 400 generally being transmitted by a digital camera or by a color scanner having digitized a printed color image to obtain the digital color image 400. figure 4 schematically represents an example of a 400 color source image.

The color source image 400 may visually represent security data, such as personal data of an authorized bearer of the document 160, such as the face V or the irises of the authorized bearer. The color source image 400 is thus typically a digital photograph of the face V or the irises of the authorized bearer. Alternatively, the security data may be data relating to the document 160.

The color source image 400 comprises a plurality of P1-Pp pixels, the color of each P1-Pp pixel typically being defined according to at least three color components of a color space.

Also, the color of each pixel P1-Pp is typically defined according to a first, second and third color component of a color space.

The color space is for example the RGB space (acronym for "Red Green Blue", in Anglo-Saxon terminology), so that the three color components are a red component, a green component and a blue component. Alternatively, the color space can be the CYM space (acronym for "Cyan Yellow Magenta", in Anglo-Saxon terminology), so that the three color components are a Cyan component, a yellow component and a magenta component. Any triplet of colors orthogonal to each other in the color space can be used.

The terms "first", "second" and "third" of the expressions "first color component", "second color component" and "third color component" are introduced to distinguish the different color components of the same color space. Also, the expression "first color component" designates one of the color components of the color space different from the other color components (for example the red component, the green component or the blue component in the RGB space).

Furthermore, the position of each pixel P1-Pp in the color source image 400 is defined according to at least two spatial dimensions, typically according to two perpendicular directions by X and Y, the first direction X typically being the abscissa axis, parallel to a first side of the source image. in 400 colors and the second Y direction typically being the Y axis, parallel to a second side of the source image in 400 colors.

In a step S310, the digital data processing means obtain the n grayscale images IM1 -IMn from the color source image 400.

Each of the n grayscale images IM1-IMn comprises the same number p of pixels as the source image in colors 400, the source image in colors 400 and each grayscale image IM1-IMn having the same size, that is to say the same height H and the same width L.

Each of the n grayscale images IM1-IMn thus comprises a plurality of pixels, and each of these pixels has a grayscale representing a color component of the corresponding pixel P1-Pp of the source image in colors 400. p is a natural integer greater than or equal to two.

The term "corresponding pixel" means the pixel of the 400 color source image having the same position (according to the spatial dimensions) in the image as the pixel of the IM1-IMn grayscale image, typically the same abscissa and the same ordinate.

For each of the n grayscale images IM1-IMn, the grayscale of each pixel is determined based on the brightness level (also called luminance level or brightness level) of the color component represented by the grayscale. The lower the brightness level of the color component, the darker the grayscale, the correspondence between the brightness level and the grayscale being linear. For example, the brightness obtained according to the red component corresponds to the brightness of the source image in 400 colors observed through a red filter.

At least a first grayscale image IM1 and a second grayscale image IM2 are thus obtained in step S310.

The first grayscale image IM1 comprises a plurality of pixels each having a grayscale representing the first color component of the corresponding pixel P1-Pp of the color source image 400, and the second grayscale image IM2 comprising a plurality of pixels each having a grayscale representing the second color component of the corresponding pixel P1-Pp of the color source image 400.

The number n of grayscale images is typically three, three being the number of dimensions of the color space, so that each color component is represented by the grayscales of pixels in a different grayscale image IM1-IMn.

Also, when the color of each pixel P1-Pp of the color source image 400 is defined according to the three color components, a third grayscale image IM3 is typically obtained in step S310, each pixel of the third grayscale image IM3 having a grayscale representing the third color component of the corresponding pixel of the color source image 400.

Furthermore, a so-called global grayscale image IM4 can be obtained in step S310, each pixel of the global grayscale image IM4 having a gray level representing the M color components of the corresponding pixel P1-Pp of the color source image 400.

The overall grayscale image IM4 and the grayscale images IM1, IM2 and IM3 are related by an equation. Thus, knowing three of these four images, it is possible to calculate the fourth image. This is why two images among the grayscale images IM1, IM2 and IM3 are sufficient provided that image IM4 is known.

When the color space is RGB, the equation used for a given pixel of the overall image seen through a video screen is typically NGg = 0.299 NGr + 0.587 NGv + 0.114 NGb, where NGg is the gray level of said pixel of the overall image IM4, NGr is the gray level of the corresponding pixel of the gray level image representing the red component, NGv is the gray level of the corresponding pixel of the gray level image representing the green component, NGb is the gray level of the corresponding pixel of the gray level image representing the blue component. The aforementioned coefficients are related to the sensitivity of the human eye to red, green and blue colors (the human eye is more sensitive to green than to red, and more sensitive to red than blue, with a bright green thus contributing more than other colours to the overall brightness). However, these coefficients are only examples and different coefficients can be considered.

In the case where the image is observed directly, the equation used is for example NGg = 0.2125 NGr + 0.7154 NGv + 0.0721 NGb.

Also, in addition to the first grayscale image IM1 and the second grayscale image IM2, it is possible to obtain at step S310 the third grayscale image IM3 and/or the overall grayscale image IM4.

The overall grayscale image allows for a natural black and white visualization of the image with the naked eye, while grayscale images representing only one color component distort the source image more.

There figure 5 shows a source image 400 in color, the overall grayscale image IM4, as well as the source image filtered according to the red component 520, the green component 530 and the blue component 540, and the grayscale image IM1 representing the red component, the grayscale image IM2 representing the green component and the grayscale image IM3 representing the blue component, determined according to the filtered source images.

In a step S320, the N grayscale images IM1-IMn obtained in step S310 are formed on the physical medium 110 of the security device 100, by the printing means of the manufacturing system.

The IM1-IMn grayscale images are typically formed by laser engraving. Alternatively, the grayscale images are formed by printing, the printing typically being digital printing such as inkjet or thermal transfer, or by retransfer or other equivalent digital process.

The forming step S320 may comprise, typically for each grayscale image IM1-IMn obtained in step S310, a reproduction S322 of the pixels of the grayscale image IM1-IMn such that the reproduced pixels of the image are positioned contiguously, with no gap being formed between two adjacent pixels of the image.

Each of the grayscale images IM1-IMn obtained in step S310 is thus positioned at a different location of the physical medium 110 of the security device 100.

Moreover, each of these IM1-IMn grayscale images is directly visible in its entirety by a user, without the aid of an additional element such as a lens array.

There figure 1 schematically represents an optical security device 100, not covered by the text of the claims and comprising grayscale images IM1-IM4 formed according to step S322 of contiguous reproduction of the pixels.

In this example, the grayscale images IM1-IM4 are positioned side by side at a lower portion of the support 110. However, another configuration could be envisaged.

The pixels of each image having been reproduced on the support 110 so as to be positioned contiguously, each image formed in its entirety at a different location E1-E4 of the support 110.

Furthermore, the pixels P1-Pp of the 400 color source image from which the grayscale images IM1-IM4 of this example were obtained are typically defined according to three color components.

Also, the pixels of the first grayscale image IM1 each have a gray level representing the first color component of the corresponding pixels of the color source image 400, the pixels of the second grayscale image IM2 each have a gray level representing the second color component of the corresponding pixels of the color source image 400, the pixels of the third grayscale image IM3 each have a gray level representing the third color component of the corresponding pixels of the color source image 400, and the pixels of the fourth grayscale image IM4 each have a gray level representing the first, second, and third color components of the corresponding pixels of the color source image 400.

Alternatively, the forming step S320 may comprise reproducing S324 the pixels of the grayscale images IM1-IMn obtained in step S310 such that the reproduced pixels of each grayscale image IM1-IMn are positioned in an interlaced manner with the pixels of the other grayscale images IM1-IMn, typically according to a distribution direction DR.

More precisely, each grayscale image IM1-IMn can be divided into q lines of pixels, the pixels of the same line having the same abscissa (or the same ordinate).

The lines of the grayscale images IM1-IMn are then interleaved according to the distribution direction DR in a periodic manner. More precisely, the first line of the first image is followed by the first line of the second image and so on up to the first line of the nth image so as to form a first series of lines, the first line of the nth image being followed by the second line of the first image which is followed by the second line of the second image, and so on up to the last (qth) line of the nth image. A number q of series of lines are thus formed.

The grayscale images IM1-IMn thus formed are interleaved in the same location E of the physical medium 110 of the security device 100. Such a configuration thus makes it possible to reduce the space required for the formation of the grayscale images IM1-IMn, which is advantageous in the context of security and/or identity documents whose size is restricted.

When the grayscale images IM1-IMn are formed so as to be interleaved, a lens array 200 is typically obtained (step S326).

The lens array 200 is typically attached to the physical support 110 or created by hot laminating the surface of the physical support 110.

The lens array 200 is configured to allow each of the different grayscale images IM1-IMn to be viewed from a different viewing angle.

Also, each lens 202 of the lens array 200 typically has the shape of a half-cylinder, the length of each lens 202 being equal to the length of a pixel line (and therefore at the height of the grayscale images IM1-IMn) and the width of each lens 202 being equal to the width of a pixel multiplied by the number n of grayscale images IM1-IMn.

The lenses 202 of the lens array 200 are positioned contiguously along the distribution direction DR, with each lens 202 positioned above a series of lines.

The periodicity of the interlacing and the lens array 200, as well as the positioning of the lens array 200 relative to the interlacing, implies, when the security device 100 is oriented according to a given orientation (and therefore viewed according to a given viewing angle), the focusing, by the lens array 200, of the incident light rays on the lines of pixels of the same image among the grayscale images IM1-IMn.

Each grayscale image IM1-IMn is thus associated with a given viewing angle and, when the security device 100 is oriented according to this viewing angle, each lens 202 of the lens array 200 focuses the light rays on the line of pixels of the associated image of the series of lines on which the lens 202 is positioned.

Each grayscale image IM1-IMn can thus be viewed in its entirety according to the associated viewing angle.

The formation of the grayscale images (step S324) is typically implemented by laser etching after the creation or fixing of the lens array 200, the laser etching (step S326) being carried out through the lens array 200.

THE Figures 2A And 2B schematically represent an optical security device 100, not covered by the text of the claims and comprising grayscale images IM1-IM4 formed according to step S324 of interlaced reproduction of the pixels.

The security device 100 comprises in this example four interlaced grayscale images IM1-IM4.

Furthermore, the pixels of the 400 color source image from which the grayscale images IM1-IM4 of this example were obtained are typically defined according to three color components, such that the pixels of the first grayscale image IM1 each have a gray level representing the first color component of the corresponding pixels of the color source image 400, the pixels of the second grayscale image IM2 each have a gray level representing the second color component of the corresponding pixels of the color source image 400, the pixels of the third grayscale image IM3 each have a gray level representing the third color component of the corresponding pixels of the color source image 400, and the pixels of the fourth grayscale image IM4 each have a gray level representing the first, second, and third color components of the corresponding pixels of the color source image 400.

The interlacing of the grayscale images IM1-IM4 forms three series S1-S3 of pixel lines. Each lens 202 of the lens array 200 is positioned on a different series S1-S3 of pixel lines. The invention is described in the case where the number of series is equal to three, but the invention is nonetheless applicable when a greater number of series is used.

A simple and fun authentication of the security document 100 or of the bearer of the security document 100 can be carried out from the grayscale images IM1-IMn formed on the physical medium 110 of the security document 100 manufactured according to the method of the figure 3 .

There figure 6 represents a method of generating a digital color image, called a reconstructed image, from a security device 100 manufactured according to the manufacturing method of the figure 3 , the generation method not being covered by the text of the claims.

The generation method is typically implemented by a generation device, having the conventional architecture of a computer and comprising digital data processing means, an optical sensor, such as a camera, and a display means such as a screen.

The data processing means typically take the form of a computer, executing a computer program stored in a computer-readable information medium (or memory).

The generating device is typically a portable device, such as a smartphone or tablet.

In a step S600, at least two grayscale images IM1-IMn formed on the physical medium 110 of the security device 100 are optically acquired, by means of the optical sensor of the generation device. For example, each grayscale image IM1-IMn formed on the physical medium 110 is optically acquired in step S600. Alternatively, each grayscale image associated with a single color component of the color source image 400 is optically acquired. Alternatively, two grayscale images associated with a single color component of the color source image 400 and the overall grayscale image IM4 are optically acquired. Alternatively, two grayscale images associated with a single color component of the color source image 400 are optically acquired as well as the color source image, an overall grayscale image IM4 being obtained from the acquisition of the color source image. A digital grayscale image is thus created by the generation device for each acquired IM1-IMn grayscale image.

Multiple digital grayscale images may thus be obtained, typically one digital image for each grayscale image associated with a single color component of the source image, or two digital images for two grayscale images associated with a single color component and one digital image for the overall IM4 grayscale image. Alternatively, a video is obtained using the sensor, and then digital images are extracted from the video.

In order for this acquisition to be implemented, an operator positions the optical sensor of the generation device and/or the safety device so that the optical sensor can successively acquire each grayscale image IM1-IMn.

The relative positioning of the sensor of the generating device relative to the safety device 100 can be guided by means of positioning marks displayed by the display means of the generating device.

The positioning marks are more precisely superimposed on the image displayed by the display means, this image representing in real time the environment captured by the optical sensor. The operator can thus modify the relative positioning so that the grayscale image to be captured is superimposed on the positioning marks. The positioning marks are thus configured to guide the relative positioning of the generation device with respect to the security device, and the digital images acquired or extracted from the acquired video have a size suitable for the generation of the reconstructed image.

When the grayscale images IM1-IMn are formed to be interlaced and the security device 100 comprises a lens array 200, the acquisition step comprises acquiring each image at a different acquisition angle, the acquisition angles corresponding to the different viewing angles, which is the viewing angle associated with the grayscale image IM1-IMn. Alternatively, a video is acquired at multiple acquisition angles, and then the digital images are extracted from the video.

In a step S610, the reconstructed image is generated, from the grayscale images IM1-IMn acquired in step S600, by the digital data processing means of the generation system.

The color component(s) represented by the gray levels of the pixels of each acquired IM1-IMn grayscale image is found using a component location information element.

The location information element is for example represented on the physical medium 110 of the security device 100 or encoded in a coding element represented on the physical medium 110, the coding element typically being a 2D or 3D barcode.

Alternatively, when the IM1-IMn grayscale images are formed so as to be interlaced, the location information element is typically the acquisition angle of the grayscale image. The lines of the IM1-IMn grayscale images are in fact typically interlaced so that each IM1-IMn grayscale image is associated with a predetermined viewing angle, a correspondence table between the grayscale images gray level and viewing angles being accessible by the generating device.

The color of each pixel of the reconstructed color digital image is typically obtained from the gray level of the pixel of each digital image whose gray levels correspond to a single color component of the color source image 400. More specifically, for each digital image whose gray levels are associated with a single component, the associated component is determined using the component location information element, and then, for each pixel of the digital image, the brightness level of the component is determined from the gray level of the pixel.

The color of each pixel in the reconstructed color digital image is then obtained from the brightness levels of each component, associated with the corresponding pixels in the grayscale digital images.

Alternatively, when a grayscale image associated with the first red color component, a grayscale image associated with the second green color component, and the overall grayscale image IM4 are optically acquired, the grayscale of the third blue color component is determined for each pixel using the above equation NGg = 0.299 NGr + 0.587 NGv + 0.114 NGb, and then the brightness level of each component is determined from the grayscale of the associated pixel, and the color of each pixel of the reconstructed digital color image is then obtained from the brightness levels of each component associated with the corresponding pixels of the grayscale digital images.

The image obtained from the grayscale images IM1-IMn acquired in step S600 is typically a two-dimensional (2D) image. The generation step S610 may then comprise a three-dimensional (3D) reconstruction step from the obtained 2D image.

When the 400 color source image is a digital photograph of the authorized wearer's face V, and therefore the 2D image obtained is the authorized wearer's face V, the 3D reconstruction includes a measurement of at least a distance on the 2D image, such as the pupillary distance and/or the distance between the two ears.

The 2D photo is then projected onto a 3D volume in the shape of a white head, the 3D volume being further deformed as a function of the distance measurement(s) made. Alternatively, the volume of the head is encoded in an information element represented or encoded on the physical medium 110.

In a step S620, the reconstituted color image is displayed by the display means of the generation device. This color display makes it possible to authenticate in a simple and fun manner the security device 100 and/or the bearer of the document 160 incorporating the security device 100, from grayscale images.

A security check may further be implemented in a step S630, based on the digital color image generated in step S610. This security check typically comprises a comparison of the generated digital color image with a reference image.

The reference image is typically an image stored in a database accessible by the generation device, this image typically being the source image in 400 colors.

Alternatively, the reference image corresponds to the acquisition at step S600 of the overall grayscale image IM4, or of a color image formed on the physical medium 110 of the security device 100 (typically the color source image 400).

Alternatively, when the color source image 400 represents personal data of an authorized bearer of the document 160, the reference image is acquired at the time of a security check during which the generation method is implemented, typically by filming or photographing the personal data of the bearer of the document 160 (for example the face or the irises).

When the displayed color reconstructed image is a 3D image reconstructed in step S610, the display angle of the 3D image can be determined based on the viewing angle of the document 160.

More specifically, the display angle may be equal to the viewing angle of the document 160. This viewing angle may be determined from an image of the document 160 acquired by the optical sensor. For example, when the document 160 comprises rectangular faces, the viewing angle may be determined based on the deformation of a rectangular face in the image of the document 160.

It is thus possible to display in augmented reality the reconstructed image in colors, the 3D image being displayed according to a display angle depending on the relative position of the document 160 with respect to the optical sensor of the generation device. Also, when the reconstructed image is the face of the authorized bearer, the operator has the impression that he is rotating around a face in 3D color when he moves his generation device with respect to the document 160.

Claims (4)

1. Method for manufacturing a security device (100), comprising the following steps:

   - obtaining (S300) a digital colour image (400), said digital colour image (400) comprising a plurality of pixels (P1-Pp), the colour of each pixel (P1-Pp) being defined in accordance with at least a first, a second and a third colour component of a colour space,

   - obtaining (S310) at least a first and a second grayscale image (IM1-IMn) from said digital colour image (400) :

   -- said first grayscale image (IM1) comprising a plurality of pixels each having a grayscale level representing the first colour component of the corresponding pixel of the digital colour image (400), and

   -- said second grayscale image (IM2) comprising a plurality of pixels each having a grayscale level representing the second colour component of the corresponding pixel of the digital colour image (400),

   - obtaining (S310) a third and/or a fourth grayscale image (IM1-IMn) from said digital colour image (400):

   - said third grayscale image (IM3) comprising a plurality of pixels each having a grayscale level representing the third colour component of the corresponding pixel of the digital colour image (400), and

   - said fourth grayscale image (IM4) comprising a plurality of pixels each having a grayscale level representing the first, second and third colour components of the corresponding pixel of the digital colour image (400),

   - forming (S320) said at least first and second grayscale images (IM1-IMn) on a physical medium (110) of said security device (100), and forming said third grayscale image and/or said fourth grayscale image on the physical medium (110) of said security device (100), the method being characterized in that said at least first, second, third and/or fourth grayscale images (IM1-IMn) are formed on the physical medium (110) of the security device (100) by laser etching.
2. Manufacturing method according to Claim 1, wherein the forming step (S320) comprises, for each of said at least first, second, third and/or fourth grayscale images (IM1-IMn), reproducing (S322) the pixels of said image (IM1-IMn) such that the reproduced pixels are positioned contiguously, each of said at least first, second, third and/or fourth grayscale images (IM1-IMn) being positioned at a different location (E1-E4) of the physical medium (110) of the security device (100).
3. Manufacturing method according to Claim 1, wherein the forming step (S320) comprises reproducing (S324) the pixels of said at least first, second, third and/or fourth grayscale images (IM1-IMn) such that the reproduced pixels of each grayscale image (IM1-IMn) are positioned so as to be interleaved with the pixels of the other grayscale images (IM1-IMn), said at least first, second, third and/or fourth grayscale images (IM1-IMn) thus being interleaved in the same location (E) of the physical medium (110) of the security device (100).
4. Manufacturing method according to Claim 3, furthermore comprising a step of obtaining (S326) a lens array (200), the lens array (200) being configured to allow different visualization of each of said at least first, second, third and/or fourth grayscale images (IM1-IMn) at a different viewing angle.

Images