Optical structures providing dichroic effects
A document, product, or package, such as a banknote, passport or the like comprises structures having dichroic effects that change color with viewing angle in both transmission and reflection. Such structures can be useful as security features that counter the ability to effectively use counterfeit documents, products, packages, etc.
1. A document having a security feature, the document comprising:
a main body of the document; and
a pigment disposed on the main body, the pigment comprising:
an optical structure comprising at least three metal layers and at least two dielectric layers,
wherein the at least two dielectric layers are disposed on opposite sides of a first metal layer of the at least three metal layers such that each of said dielectric layers has a first side facing said first metal layer and a second side opposite said first side,
wherein remaining metal layers of the at least three metal layers are disposed on said second side of the at least two dielectric layers opposite the first side facing the first metal layer, and
wherein the pigment produces a first color in a reflection view and a second color different from the first color in a transmission view.
2. The document of claim 1 , wherein the optical structure is suspended in a medium.
3. The document of claim 2 , wherein the medium comprises a material having a refractive index in a range of 1.4 to 1.6.
4. The document of claim 2 , wherein the medium comprises an organic resin.
5. The document of claim 1 , wherein the optical structure has a thickness, and wherein a length or a width of the optical structure is in a range between 5-10 times the thickness.
6. The document of claim 1 , wherein the optical structure comprises only three metal layers.
7. The document of claim 1 , wherein one or more of the at least three metal layers comprises silver.
8. The document of claim 1 , wherein one or more of the at least three metal layers comprises gold.
9. The document of claim 1 , wherein one or more of the at least two dielectric layers comprises a material that is substantially transparent to visible light.
10. The document of claim 1 , wherein a thickness of an individual layer of the at least three metal layers is in a range between 5 nm and 100 nm.
11. The document of claim 1 , wherein a thickness of an individual layer of the at least two dielectric layers is in a range between 50 nm and 800 nm.
12. The document of claim 1 , wherein one or more of the at least two dielectric layers comprises magnesium fluoride.
13. The document of claim 1 , wherein one or more of the at least two dielectric layers comprises zinc sulfide.
14. The document of claim 1 , wherein one or more of the at least two dielectric layers comprises at least one of magnesium fluoride, silicon dioxide, ceric oxide, zinc sulfide, zirconium dioxide, titanium dioxide, tantalum pentoxide, yttrium oxide, indium oxide, tin oxide, indium tin oxide, aluminum oxide or tungsten trioxide.
15. The document of claim 1 , wherein the second color is a complementary color of the first color.
16. The document of claim 1 , wherein the second color is a non-complementary color of the first color.
17. The document of claim 1 , wherein the first color or the second color changes with a change in a viewing angle.
18. The document of claim 1 , wherein the first color or the second color does not change with a change in a viewing angle.
19. The document of claim 1 , wherein the pigment comprising the optical structure is applied on a substrate which is configured as a security thread inserted in the main body.
20. The document of claim 1 , wherein the pigment comprising the optical structure is configured as an ink applied to the main body.
21. The document of claim 1 , wherein the pigment comprising the optical structure is applied on a substrate which is configured as a foil that is attached to the main body via hot stamp.
22. The document of claim 1 , wherein the pigment comprising the optical structure is applied on a substrate which is configured as a laminate attached to the main body.
23. The document of claim 1 , wherein the pigment comprising the optical structure is incorporated in a window of the main body.
24. The document of claim 1 , further comprising two additional dielectric layers disposed on sides of the remaining metal layers opposite sides facing the at least two dielectric layers.
25. The document of claim 24 , wherein at least one of the two additional dielectric layers has a thickness in a range from 2 nm to 20 nm.
26. The document of claim 1 , wherein one or more of the at least two dielectric layers comprises a material having a refractive index in a range of 1.38 to 2.4.
27. The document of claim 1 , wherein the remaining metal layers comprise two remaining metal layers disposed on the opposite sides of the first metal layer.