A nanoparticle sized between 10-180 nm composed of M(III).sub.2O.sub.3, M(II)O and M(II)M(III).sub.2O.sub.4, wherein M(III) is a trivalent metal and M(II) is a divalent metal, or Fe.sub.2O.sub.3, MnO and M(II)O, wherein M is a divalent metal selected from the group consisting of Fe, Ni, Co, Cu, Pt, Au, Ag, Ba and a rare earth metal.

A document (15) includes at least: one first image (5) including at least one photoluminescent pigment which is invisible under illumination in visible light and is visible under illumination by at least one first source (20) of non-visible light emitting in the short ultraviolet domain, one second image including at least one photoluminescent pigment which is invisible under illumination in visible light and is visible under illumination by at least one second source of non-visible light emitting in the long ultraviolet domain, one layer, named a filtering layer, arranged facing the second image and including at least one material, named a filtering material, suitable for allowing the wavelengths emitted by the first source of non-visible light to be filtered without preventing the first image from being seen under illumination by the first light source.

A method for the secure protection of a portion of the surface of an official document bearing markings, in which: a transparent security layer (17) is selected, the layer including at least one semi-transparent printed image including at least one transparent binder and at least one thermochromic pigment (14, 13, 12, 11); the semi-transparent printed image reveals at least one semi-transparent visible pattern in a first non-activated state at least a first temperature, such that the markings can be seen through the image, the pattern not being visible in a second activated state at least a second temperature different from the first temperature. A device for the secure protection of a portion of the surface of an official document and such an official document are also described.

A fluid for ink jet printing characters on a substrate that become magnetized in the presence of a magnetic field, the fluid HAVING a suspension of nanoparticles dispersed in a solvent, wherein the fluid comprises a viscosity from 1 to 50 cps and a surface tension of 20-45 dynes/cm, further wherein each nanoparticle is sized between 10-180 nm and comprises M(III).sub.2O.sub.3, M(II)O and M(II)M(III).sub.2O.sub.4, wherein M(III) is a trivalent metal and M(II) is a divalent metal, or Fe.sub.2O.sub.3, MnO and M(II)O, wherein M is a divalent metal selected from the group consisting of Fe, Ni, Mn, Co, Cu, Pt, Au, Ag, Ba and a rare earth metal.

A security element for a security and/or valuable document includes a matrix based on an organic polymeric material and at least one electrically conductive pigment dispersed in the matrix and at least one organic luminescent substance dispersed in the matrix. In the presence of the conductive pigment the luminescent substance is capable of non-contact excitation of light emission where the particle size is less than 200 nm. The luminescent substance is not encapsulated and is directly surrounded by the matrix and is embedded therein.

Systems and methods for document and product authentication are provided using a combination of interacting absorption and emission materials that are formed into covert optically encoded markings. The markings are formed from at least one emitter and at least one absorber, that exhibit a first emission in response to a first excitation of the emitter and a change in that emission in response to excitation of the absorber such that various combinations of emitter and absorber materials create a machine readable response that cannot be detected by the human eye.

The present invention relates to the field of the protection of value documents and value commercial goods against counterfeit and illegal reproduction. In particular, the present invention relates to security threads or stripes to be incorporated into or onto security documents. The security threads or stripes comprise i) an optically variable layer; ii) a magnetic code; and iii) a non-metallized substrate, wherein the magnetic code has a color matching the color impression of the optically variable layer at one viewing angle and wherein the optically variable layer and the magnetic code are jointly visible from one side of the security thread or stripe.

Laser-personalizable security articles include multi-layer security documents. The multi-layer security document includes an optically transparent cover layer, a composite image and an imagable layer adjacent to the cover layer. The first surface of the cover layer is at least partially a microstructured surface, where the microstructured surface forms microlenses or a lenticular surface. The composite image is made by a collection of complete or partial images viewed through the microstructured surface of the cover layer. The composite image is located on or within the second surface of the cover layer. The imagable layer is a laser imagable layer. When imaged, a personalized second composite three dimensional image is created on or in the imagable layer.

A security element for manufacturing value documents, such as banknotes, checks or the like, comprises a top on which a microrelief structure is developed that has at least two sub-regions that each comprise a plurality of groove- and/or rib-shaped structural elements that lie adjacent to one another and extend along a longitudinal direction, and are reflecting or backscattering. The longitudinal directions of the sub-regions are different, and the structural elements are each not resolvable with the naked eye with respect to the width transverse to the longitudinal direction, and fan out incident parallel light achromatically in a fan that lies transverse to the longitudinal direction and has an opening angle of at least 30.

A method of visualizing a latent image includes: preparing a latent image composed of a plurality of paramagnetic seeds immobilized on a base in the form of an image, and a dispersion liquid of paramagnetic colloidal particles; immersing the latent image in the dispersion liquid of paramagnetic colloidal particles; and applying a magnetic field to the latent image and the dispersion liquid of paramagnetic colloidal particles.