Patent ID: 7192474

Claim:
An aqueous ink capable of providing a secure marking including a dark, machine-readable image exhibiting fluorescence when exposed to fluorescent-exciting radiation, said ink being of suitable viscosity and surface tension for use in ink jet printing, comprising: (a) a first colorant comprising a fluorescent dye and/or pigment emitting light within a characteristic emission band when excited by fluorescent-exciting radiation; (b) a second colorant comprising a carbon black infrared radiation absorbing composition alone or with one or more dyes or pigments having a light absorption band at longer wavelengths than the characteristic emission band of the first colorant or also overlapping the emission wavelength of the first colorant in such a way as to result in a dark color, preferably black, and (c) an aqueous liquid vehicle comprising water and a water-soluble vehicle in sufficient amounts to achieve an ink viscosity and surface tension effective for application of the ink to a substrate in a predetermined pattern by ink jet printing.