Method for building a security image by multiplexing color images

1. A method for building at least one security image for a security structure of a security document from a multiplexing of determined color images, the method comprising:

selecting a marking method capable of producing sets of colors on the security document comprising different colors that can be displayed according to illumination and observation modes of the security structure;

establishing a plurality of color sets based on optical measurement that can be displayed by the security structure in a plurality of the different illumination and observation modes, each color set comprising colors from among the different colors capable of being produced by the marking method;

distributing the color sets in a plurality of groups of color sets;

establishing combinations of groups of color sets having at least one color set in common across a number of the plurality of different illumination and observation modes;

selecting a combination of a group of color sets based on the number of desired illumination and observation modes and a number of colors per color image; and

determining the color images to multiplex by means of the combination of the group of color sets selected.

2. The method according to claim 1 , wherein determining the color images to multiplex by means of the combination of the group of color sets selected comprises:

building a table of color sets from the combination of the group of color sets selected;

determining the color images to multiplex from the table of color sets; and

producing multiplexed color images by means of the table of color sets.

3. The method according to claim 1 , wherein a display of different multiplexed color images is independent of any spatial filtering system, and an observed resolution of the color images is the same as that for at least one of marking, deposition, or printing.

4. The method according to claim 1 , wherein distributing the color sets comprises at least one of:

determining the color sets comprising colors that can be observed in the same illumination and observation mode for which the difference between said colors is below a given threshold;

creating new color sets; and

determining color sets comprising colors that are identical in the same illumination and observation modes.

5. The method according to claim 4 , wherein determining the color sets comprising colors that can be observed in the same illumination and observation modes for which the difference between said colors is below a given threshold comprises an iteration of the following:

determining a reference color present in a first color set and in a given illumination and observation mode;

comparing the reference color and a second color from a second color set, the second color being observable in the same illumination and observation mode as the reference color; and

adding the second color to the color set associated with the reference color for the selected illumination and observation mode if the difference between the two colors is below the given threshold.

6. The method according to claim 1 comprising, following distributing the color sets in a plurality of groups of color sets, selecting the groups of color sets that do not comprise, in each illumination and observation mode, any color set in common.

7. The method according to claim 6 , wherein selecting the groups of color sets is carried out by iteration of the following:

selecting the group of color sets having a largest number of color sets for each illumination and observation mode; and

eliminating all the groups of color sets having color sets that are already present in the group having the largest number of color sets.

8. The method according to claim 6 , wherein establishing combinations of groups of color sets having at least one color set in common is carried out with respect to each selection of a group of color sets selected through all the illumination and observation modes.

9. The method according to claim 8 , wherein establishing a combination of groups of color sets having at least one color set in common comprises:

determining all the combinations of groups of color sets that exist among the groups of color sets from all the possible selections of groups of color sets; and

eliminating, for each illumination and observation mode, combinations of groups of color sets that involve a number of color sets that is strictly less than Π q=1:K-1 C q , wherein

C q is a number of groups of color sets for an illumination and observation mode q, and

K−1 is a complete set of illumination and observation modes except for the illumination and observation mode in question.

10. The method according to claim 9 comprising, for the combinations of groups of color sets involving a number of color sets that is strictly greater than Π q=1:K-1 C q , the following:

searching for sub-groups of color sets that belong to the same group of color sets in each illumination and observation mode;

selecting a sub-group with the lowest average color difference from the reference color for each illumination and observation mode; and

saving the other sub-groups of color sets.

11. The method according to claim 1 , wherein the marking method is laser structuring of a layer of the security document comprising metal nanoparticles.

12. The method according to claim 1 , further comprising marking the security structure using the selected marking method to produce the multiplexing of the determined color images.

13. A method for building at least one security image for a security structure of a security document from a multiplexing of determined color images, the method comprising:

establishing a plurality of color sets based on optical measurement that can be displayed by the security structure in a plurality of different illumination and observation modes, each color set comprising colors from among different colors capable of being produced on the security document by a marking method in the different illumination and observation modes;

distributing the color sets in a plurality of groups of color sets;

establishing combinations of groups of color sets having at least one color set in common across a number of the plurality of different illumination and observation modes;

selecting a combination of a group of color sets based on the number of desired illumination and observation modes and a number of colors per color image; and

determining the color images to multiplex by means of the combination of the group of color sets selected.

14. The method according to claim 13 , wherein determining the color images to multiplex by means of the combination of the group of color sets selected comprises:

building a table of color sets from the combination of the group of color sets selected;

determining the color images to multiplex from the table of color sets; and

producing multiplexed color images by means of the table of color sets.

15. The method according to claim 13 , wherein distributing the color sets comprises at least one of:

determining the color sets comprising colors that can be observed in the same illumination and observation mode for which the difference between said colors is below a given threshold;

creating new color sets; and

determining color sets comprising colors that are identical in the same illumination and observation modes.

16. The method according to claim 15 , wherein determining the color sets comprising colors that can be observed in the same illumination and observation modes for which the difference between said colors is below a given threshold comprises an iteration of the following:

determining a reference color present in a first color set and in a given illumination and observation mode;

comparing the reference color and a second color from a second color set, the second color being observable in the same illumination and observation mode as the reference color; and

adding the second color to the color set associated with the reference color for the selected illumination and observation mode if the difference between the two colors is below the given threshold.

17. The method according to claim 13 comprising:

following distributing the color sets in a plurality of groups of color sets, selecting the groups of color sets that do not comprise, in each illumination and observation mode, any color set in common;

wherein selecting the groups of color sets is carried out by iteration of the following:

selecting the group of color sets having a largest number of color sets for each illumination and observation mode; and

eliminating all the groups of color sets having color sets that are already present in the group having the largest number of color sets.

18. The method according to claim 13 , wherein establishing a combination of groups of color sets having at least one color set in common comprises:

determining all the combinations of groups of color sets that exist among the groups of color sets; and

eliminating, for each illumination and observation mode, combinations of groups of color sets that involve a number of color sets that is strictly less than Π q=1:K-1 C q , wherein

C q is a number of groups of color sets for an illumination and observation mode q, and

K−1 is a complete set of illumination and observation modes except for the illumination and observation mode in question.

19. The method according to claim 18 comprising, for the combinations of groups of color sets involving a number of color sets that is strictly greater than Π q=1:K-1 C q , the following:

searching for sub-groups of color sets that belong to the same group of color sets in each illumination and observation mode;

selecting a sub-group with the lowest average color difference from the reference color for each illumination and observation mode; and

saving the other sub-groups of color sets.

20. The method according to claim 13 , wherein the marking method is laser structuring of a layer of the security document comprising metal nanoparticles and further comprising marking the security structure using the marking method to produce the multiplexing of the determined color images.