Birefringent marking

1. A birefringent marking comprising at least one layer of liquid crystal material having a unique, non-uniform pattern of regions of different thickness.

2. A birefringent marking according to claim 1 , wherein the liquid crystal material is a vitrified, polymerised or crosslinked liquid crystal material.

3. A birefringent marking according to claim 1 , wherein the liquid crystal material is a nematic or smectic liquid crystal material.

4. A birefringent marking according to claim 1 , wherein the liquid crystal material is a cholesteric liquid crystal material.

5. A birefrin gent marking according to claim 1 , further comprising a reflective layer.

6. A birefringent marking according to claim 5 , wherein the reflective layer is a metallic or metallized layer, hot stamping foil, holographic image, pearlescent or interference layer or pearlescent or interference pigment.

7. A birefringent marking according to claim 4 , further comprising an absorptive layer.

8. A birefringent marking according to claim 1 , further comprising at least one optical retardation layer.

9. A method of preparing a birefringent marking according to claim 1 comprising applying droplets of a solution of a liquid crystal material to a substrate and allowing the solvent to evaporate.

10. A method according to claim 9 , wherein the liquid crystal material is a polymerisable or crosslinkable material that is polymerised or crosslinked during or after evaporation of the solvent.

11. A method of applying a hidden image to a surface or substrate, said image being invisible when viewed under unpolarised light and visible when viewed under polarised light, comprising providing a birefringent marking comprising a liquid crystal material having regions of different thickness to the surface.

12. A decorative or security application comprising a birefringent marking according to claim 1 .

13. A security, authentification or identification marking or device comprising a birefringent marking according to claim 1 .

14. A document of value comprising a marking or device according to claim 13 .

15. A birefringent marking according to claim 2 , wherein the liquid crystal material is obtained by polymerising or crosslinking a liquid crystal material comprising one or more compounds of formulae Ia–II and/or lla–IIb

wherein

P is a polymerisable group,

x and y are each independently 1 to 12,

A is 1,4-phenylene that is optionally mono- di or trisubstituted by L 1 or 1,4-cyclohexylene,

v is 0 or 1,

Z 0 is —COO—, —OCO—, —CH 2 CH 2 — or a single bond,

Y is a polar group,

R 0 is an unpolar alkyl or alkoxy group, and

L 1 and L 2 are each independently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 C atoms.

16. A hidden image being invisible when viewed under unpolarised light and visible when viewed under polarised light, comprising a birefringent marking according to claim 1 .