Abstract: This aim of the invention is to develop an ink composition comprising at least one non-photochromic pigment or dye, a photochromic pigment or dye and a solvent, the photochromic pigment or dye changing absorption spectrum under the effect of a first light emitting in a wavelength belonging to the ultra-violet range, so that when it is subsequently illuminated by a second light only emitting in a wavelength belonging to the visible range, it has a color different from that which it had, under the illumination of said second light, prior to illumination by said first light, said color also differing from that of the non-photochromic pigment or dye when the latter is illuminated by said second light.

Abstract: This method consists in introducing a programmed defect at the writing level; the defect will be detected at the reading level by means of a key corresponding to the writing control algorithm which is known only to the user.

Abstract: The invention relates to a circuit for feeding ink to an ink-jet printing head.Consideration is given to a circuit comprising in particular a first auxiliary circuit (R1), the function of which is to measure the ink pressure between the feed pipe (20) and the ink supply pipe (18) and a second auxiliary circuit R2 which serves if necessary to clear the orifice (41) for discharging ink drops (100).This circuit can also comprise a third auxiliary circuit R3 for delivering a signal which is representative of the viscosity of the ink.Measurement of the pressure and of the viscosity permits control of the principal parameters governing the operation of the printer which is equipped with the circuit.The invention applies to any ink-jet printer.