Security liquid electrostatic ink composition

1. A security liquid electrophotographic (LEP) ink composition comprising:

a luminescent component comprising a transition metal oxide, a transition metal hydroxide, a transition metal hydride, a transition metal halide, or a combination thereof;

a resin; and

a carrier liquid,

wherein the LEP ink composition is colourless when viewed under visible light and luminesces to emit visible light on exposure to IR light.

2. The LEP ink composition according to claim 1 , wherein the luminescent component absorbs IR light having a wavelength in the range of 940 nm to 1060 nm.

3. The LEP ink composition according to claim 1 , wherein the luminescent component emits visible light on exposure to IR light having an intensity above a threshold value.

4. The LEP ink composition according to claim 3 , wherein the threshold value for the intensity of IR light is 1 mW/mm 2 or greater.

5. The LEP ink composition according to claim 1 which forms a LEP image which is invisible to the human eye.

6. The LEP ink composition according to claim 1 , further comprising a charge director.

7. The LEP ink composition according to claim 1 , wherein the resin comprises a copolymer of an alkylene monomer and an acrylic acid monomer or a methacrylic acid monomer.

8. A method of providing a security liquid electrophotographic (LEP) ink composition, the method comprising:

providing a first luminescent component and a second luminescent component, wherein the first luminescent component and the second luminescent component are selected to provide a LEP ink composition having pre-selected emission spectrum on exposure to IR light; and

combining the first luminescent component and the second luminescent component with a resin and a carrier liquid to provide a LEP ink composition which is colourless when viewed under visible light.

9. The method according to claim 8 , wherein the first luminescent component or the second luminescent component is selected to emit visible light at a pre-selected wavelength on exposure to IR light.

10. A method of printing a security LEP image, the method comprising:

providing the LEP ink composition of claim 8 ;

liquid electrophotographically printing the LEP ink composition onto a print substrate to provide a security LEP image disposed on the print substrate, wherein the security LEP image is invisible to the human eye when viewed under visible light.

11. A security liquid electrophotographic (LEP) ink composition comprising:

a luminescent component;

a resin; and

a carrier liquid;

wherein the LEP ink composition is colourless when viewed under visible light and luminesces to emit visible light on exposure to IR light;

and wherein the luminescent component includes a Cu(I) complex, a Cu(II) complex, a Cr(III) complex, a Re(I) complex, a Re(III) complex, a Ru(II) complex, an Os(II) complex, an Ir(III) complex, a Pt(II) complex, a Pd(II) complex, an Au(I) complex, or an Au(III) complex.