Infrared dyes having an infrared absorption maximum above 1130 nm with minor colouration in the visual spectrum. The infrared dyes are especially useful for producing multicolour laser marked security documents having reduced background coloration and enhanced colour gamut.

The problem of the present invention is to overcome drawbacks of conventional optical filters such as near-infrared cut filters and to provide an optical filter which generates little scatted light even during light absorption and has excellent transmittance property. The optical filter of the present invention is characterized by containing a squarylium-based compound and a compound which absorbs or quenches fluorescence of the squarylium-based compound. The optical filter of the present invention preferably contains a near-infrared absorbing dye containing a squarylium compound (A) and at least one compound (B) selected from the group consisting of a phthalocyanine-based compound (B-1) and a cyanine-based compound (B-2).

The problem of the present invention is to overcome drawbacks of conventional optical filters such as near-infrared cut filters and to provide an optical filter which generates little scatted light even during light absorption and has excellent transmittance property. The optical filter of the present invention is characterized by containing a squarylium-based compound and a compound which absorbs or quenches fluorescence of the squarylium-based compound. The optical filter of the present invention preferably contains a near-infrared absorbing dye containing a squarylium compound (A) and at least one compound (B) selected from the group consisting of a phthalocyanine-based compound (B-1) and a cyanine-based compound (B-2).

The invention provides a novel class of cyanine dyes that are functionalized with sulfonic acid groups and a linker moiety that facilitates their conjugation to other species and substituent groups which increase the water-solubility, and optimize the optical properties of the dyes. Also provided are conjugates of the dyes, methods of using the dyes and their conjugates and kits including the dyes and their conjugates.