Security inks containing infrared absorbing metal compounds

1. A composition comprising an oleophilic base ink composition having dispersed therein particles of an infrared-absorbing material wherein said infrared-absorbing material is selected from reduced indium tin oxide, titanium nitride, zirconium nitride, molybdenum metal, copper chromate, lanthanum hexaboride, neodymium oxide, neodymium chloride, dysprosium oxide and praseodymium oxide.

2. A composition according to claim 1 wherein the particles of the infrared-absorbing material have an average particle size of between 0.2 and 30 microns.

3. A composition according to claim 2 wherein the particles of the infrared-absorbing material comprise nanoparticles.

4. A composition according to claim 3 wherein the infrared-absorbing material absorbs radiation over a narrow range of wavelengths within the infrared region of the electromagnetic spectrum and is selected from dysprosium oxide, neodymium oxide and neodymium chloride.

5. A composition according to claim 4 wherein the infrared-absorbing material is dysprosium oxide.

6. A composition according to claim 1 wherein the infrared-absorbing material absorbs radiation over a broad range of wavelengths within the infrared region of the electromagnetic spectrum and is selected from reduced indium tin oxide, titanium nitride, zirconium nitride, molybdenum, copper chromate and lanthanum hexaboride.

7. A composition according to claim 1 wherein the infrared absorbance of the base ink is less than 50% of the absorbance of the infrared-absorbing wavelength, of any wavelength within this range.

8. A composition according to claim 1 wherein the infrared absorbing material is colourless, pale coloured or lightly coloured.