In a first aspect, the invention provides an ink formulation comprising: (a) a near
infrared absorbing
system comprising a compound of the formula M1W1.6O6, wherein M1 is one or more elements selected from the group consisting of H, He, alkali metals such as Li, Na, K, Rb, Cs, alkaline earth metals such as Be, Mg, Ca, Sr, Ba, rare-earth elements, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, AL Ga, in, Tl, Si, Ge, Sn, Pb, Sb, B, F, P, S, Se, Br, Te, Ti, Nb, V, Mo, Ta, Re, Hf, Os, Bi and I, or
ammonium; (b) a colour
change agent; and (c) a binder. The invention also provides alternative ink formulations for
laser marking, a method of forming an image using the ink formulations and a substrate coated with the ink formulation of the invention. Ink formulations according to the invention have been found to have good
laser imaging
efficacy, and an essentially negligible
impact on the background colour of the
coating.