Patent ID: 8308198

Claim:
A judging method for judging authenticity of a water-based fluorescent ink comprising a step of irradiating an ultraviolet ray to an image formed with the ink, wherein the ink comprises water, a water-soluble fluorescent coloring material that dissolves or disperses in water, and an organic solvent, wherein the water-soluble fluorescent coloring material has, in one molecule, (a) a linking group, (b) a plurality of fluorescent groups linked via the linking group, and (c) a sulfonic acid group as a water-soluble group in the state of a free acid, and is capable of emitting a fluorescence in a visible light region by irradiation of an ultraviolet light, wherein the linking group has a structure selected from the group consisting of compounds represented by the following general formulae (1) through (6): wherein Z in formulae (1) through (3) each independently represents NR 1 R 2 , SR 3 or OR 3 ; Y in formula (2) represents H, Cl, the above-mentioned Z, SR 4 or OR 4 ; E in formula (3) represents Cl or CN; R 1 , R 2 , R 3 , and R 4 each independently represent H, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an aralkyl group, a substituted aralkyl group, or a hydroxyl group; and R 1 and R 2 may form a 5- or 6-membered ring together with a nitrogen atom, wherein in formula (4), R 5 is selected independently from a hydrogen atom, an alkyl group, a substituted alkyl group, an alkoxy group, a halogen atom, CN, a ureid group, and NHCOR 6 ; R 6 is selected from a hydrogen atom, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an aralkyl group, and a substituted aralkyl group; in formula (5), T represents an alkyl group; and W is selected from a hydrogen atom, CN, CONR 7 R 8 , a pyridium group, and a carboxyl group; R 7 and R 8 are each independently selected from a hydrogen atom, an alkyl, and a substituted alkyl group; m represents an alkylene chain having 2 to 8 carbon atoms; and in formula (6), B is selected from a hydrogen atom, an alkyl group, and a carboxyl group, and wherein the plurality of fluorescent groups of the water-soluble fluorescent coloring material have a stilbene structure.