Process for providing marking on security papers

1. A process for providing a security document with a coloured marking, comprising providing a photosensitive preparation on a portion of said document and submitting at least selected areas of said portion to a light beam, wherein said preparation is capable of forming a film on said portion and comprises a substance capable of producing colloidal metal particles or colloidal semiconducting particles under the effect of UV irradiation, and wherein said areas are irradiated by means of an UV-light beam so as to produce said colloidal particles.

2. A process as claimed in claim 1 , wherein said preparation is an ink or varnish, is substantially transparent before said irradiation and comprises a film forming polymer and a precursor of colloidal metal particles or colloidal semiconducting particles.

3. A process as claimed in claim 2 , wherein said precursor is a precursor of Au, Ag or Cu colloidal particles.

4. A process as claimed in claim 1 , wherein said preparation is an ink or varnish, is substantially transparent before said irradiation and comprises a film forming polymer and a precursor of colloidal metal particles or colloidal semiconducting particles, and wherein said film forming polymer is a polysaccharide or polypeptide and said precursor is an inorganic gold salt or acid.

5. A process as claimed in claim 4 , wherein said film forming polymer is chitosan and said precursor is a chloroauric acid.

6. A process as claimed in claim 1 , wherein the step of providing a photosensitive preparation on a portion of said security document comprises the steps of

a) applying a chitosan solution onto said portion of said security document and

b) drying said portion, so as to form a film having a thickness of between 0.5 and 20 μm

c) applying a solution of chloroauric acid to said portion, and

d) drying said portion in the dark.

7. A process as claimed in claim 1 , wherein the step of providing a photosensitive preparation on a portion of said security document further comprises the steps of

a′) combining a chitosan solution and a chloroauric acid solution in a molar ratio HAuCL 4 /chitosan monomeric unit of between 0.1 and 1

b′) applying said combined solution onto said portion of said security document and

c′) drying said portion in the dark

d′) eventually repeating steps b′ and c′ so as to form a film having a thickness of between 0.5 and 20 μm.

8. A process as claimed in claim 1 , wherein said irradiation is performed by means of a pulsed excimer laser.

9. A process as claimed in claim 1 , wherein said irradiation is performed by means of a frequency-multiplied solid state Laser.

10. A process as claimed in claim 1 , wherein the irradiation is performed by a beam deflection method via a plurality of mirrors.

11. A process as claimed in claim 10 , wherein said irradiation is performed via a system of a beam scanning system.

12. A process as claimed in claim 1 , wherein a diffractive network is reported into said film.

13. A process as claimed in claim 1 , wherein a covering layer is applied onto said film after said irradiation, said covering layer having a high absorption in the UV range and being substantially transparent in the visible light region.

14. A process as claimed in claim 1 , and further comprising a reticulating step after development of said marking.

15. A process as claimed in claim 10 , providing an identity marking to said security document.

16. A process as claimed in claim 11 , providing an identity marking to said security document.

17. A process as claimed in claim 12 , providing an identity marking to said security document.

18. A process as claimed in claim 13 , providing an identity marking to said security document.

19. A process as claimed in claim 14 , providing an identity marking to said security document.

20. A process as claimed in claim 1 , wherein the security document is a banknote.

21. A process as claimed in claim 7 , wherein, in step d′, a film having a thickness of between 2 and 10 μm is formed.

22. A process as claimed in claim 10 , wherein said irradiation is performed via a system of piloted galvanometric mirrors.

23. A security document comprising a photosensitive preparation provided on a portion of the document, wherein said preparation is capable of forming a film on said portion and comprises a substance capable of producing colloidal metal or semiconducting particles under the effect of UV irradiation.

24. A security document according to claim 23 , wherein said preparation is an ink or varnish, is substantially transparent before irradiation, and comprises a film forming polymer and a precursor of colloidal metal particles or colloidal semiconducting particles.

25. A security document according to claim 24 , wherein said precursor is a precursor of Au, Ag or Cu colloidal particles.

26. A security document according to claim 24 , wherein said preparation is an ink or varnish, is substantially transparent before said irradiation and comprises a film forming polymer and a precursor of colloidal metal particles or colloidal semiconducting particles, and wherein said film forming polymer is a polysaccharide or polypeptide and said precursor in an inorganic gold salt or acid.

27. A security document according to claim 26 , wherein said film forming polymer is chitosan and said precursor is a chloroauric acid.

28. A security document according to claim 23 , wherein the amount of photosensitive preparation per surface unit is smaller than the amount that is necessary to produce a metallic mirror aspect.

29. A security document according to claim 23 , comprising a marking or a diffractive network formed by UV irradiation of said preparation and a covering layer applied after UV irradiation onto said film, said covering layer having a high absorption in the UV range and being substantially transparent in the visible light region.