Abstract: An organic phosphor which is a solid insoluble in water and organic solvents and which may be used in a phosphorescent printing ink or a paper coating composition comprises molecules of at least one phosphorescent activator held within a matrix of a condensation resin that does not itself absorb ultraviolet or visible radiation at the excitation and emission frequencies of the phosphorescence activator(s). The phosphorescence activator(s) is/are a compound or mixtures of compounds of the general formula(A).sub.m --Ar--CO--X (I)wherein Ar denotes a fused ring polycyclic aromatic group;m is zero or an integer of from 1 up to the maximum number of ring positions available for substitution;X denotes an alkyl group or a group of the formula (A).sub.m --Ar-- or (B).sub.n Ph--wherein Ph is a phenyl group and n is zero or an integer of from 1 to 5; andA and B represent ring substituents, or an acid addition salt thereof when said compound is basic.

Abstract: A credential, such as a passport or an identification card, is provided, for example by impregnation or coating, with a phosphorescent composition which includes at least two phosphorescence activators which exhibit different emission characteristics both with respect to wavelength and lifetime so that, when the composition is irradiated, the initial afterglow changes color, for example from green to blue.