A method of printing a stealth white image includes applying a stealth ink containing a red coloring material that emits visible light at exposure to ultraviolet radiation and a green coloring material that emits visible light at exposure to ultraviolet radiation to a substrate which contains a fluorescent brightener to form the stealth white image thereon, wherein the stealth white image demonstrates an a* value of from −2.0 to 2.0 and a b* value of from −10.0 to 0 at exposure to ultraviolet radiation having a wavelength of 370 nm according to CIE 1976 L*a*b* colorimetric system.

A method of printing a stealth white image includes applying a stealth ink containing a red coloring material that emits visible light at exposure to ultraviolet radiation and a green coloring material that emits visible light at exposure to ultraviolet radiation to a substrate which contains a fluorescent brightener to form the stealth white image thereon, wherein the stealth white image demonstrates an a* value of from −2.0 to 2.0 and a b* value of from −10.0 to 0 at exposure to ultraviolet radiation having a wavelength of 370 nm according to CIE 1976 L*a*b* colorimetric system.

A substrate is printed with front side and back side markings in a first color ink. The front side when viewed with reflected light includes a first marking printed in a first saturation and a first symbol printed in the first saturation. The back side when viewed with reflected light includes: a second marking printed in a second saturation; a second symbol printed in the second saturation; a third marking printed in the first saturation; and a third symbol printed in the first saturation. The first marking, the second marking, and the third marking are printed to provide an interlocking pattern of low saturation color and high saturation color when viewed with transmitted light. The second symbol is visible, and the first symbol and the second symbol are not distinguishably visible, when viewed with transmitted light.

A substrate is printed with front side and back side markings in a first color ink. The front side when viewed with reflected light includes a first marking printed in a first saturation and a first symbol printed in the first saturation. The back side when viewed with reflected light includes: a second marking printed in a second saturation; a second symbol printed in the second saturation; a third marking printed in the first saturation; and a third symbol printed in the first saturation. The first marking, the second marking, and the third marking are printed to provide an interlocking pattern of low saturation color and high saturation color when viewed with transmitted light. The second symbol is visible, and the first symbol and the second symbol are not distinguishably visible, when viewed with transmitted light.

A 3D physical unclonable functions (PUF) system produced based on harnessing the out-of-plane crumpling of a layer of 2D material during shrinkage of a substrate carrying such layer. The structural details of the so-formed 3D PUF pattern are extracted from the tags in a layer-by-layer fashion using confocal laser microscopy imaging and then reconstructed to form the 3D PUF keys and stored in the database, serving as a secure anti-counterfeiting PUF that demonstrates encoding capacity in excess of 10.sup.40,000,000. Authentication is performed with a customized trained Siamese neural network framework in a matter of few minutes in a fashion that does not depend on rotation, linear translation, tilt, variations of contrast and/or resolution of the extracted optical images.

A 3D physical unclonable functions (PUF) system produced based on harnessing the out-of-plane crumpling of a layer of 2D material during shrinkage of a substrate carrying such layer. The structural details of the so-formed 3D PUF pattern are extracted from the tags in a layer-by-layer fashion using confocal laser microscopy imaging and then reconstructed to form the 3D PUF keys and stored in the database, serving as a secure anti-counterfeiting PUF that demonstrates encoding capacity in excess of 10.sup.40,000,000. Authentication is performed with a customized trained Siamese neural network framework in a matter of few minutes in a fashion that does not depend on rotation, linear translation, tilt, variations of contrast and/or resolution of the extracted optical images.

The present invention relates to the field of magnetic assemblies and processes for producing optical effect layers (OELs) comprising magnetically oriented non-spherical magnetic or magnetizable pigment particles on a substrate. In particular, the present invention relates to magnetic assemblies processes for producing said OELs as anti-counterfeit means on security documents or security articles or for decorative purposes.

A platelet-shaped magnetic effect pigment is provided for use in a printing ink, and includes a layer construction with a magnetic layer and at least one optical functional layer, such that the magnetic layer is based on a magnetic material having a column-shaped nanostructure and the magnetic columns respectively have a largely uniform preferential magnetic direction deviating from the platelet plane.

A platelet-shaped magnetic effect pigment is provided for use in a printing ink, and includes a layer construction with a magnetic layer and at least one optical functional layer, such that the magnetic layer is based on a magnetic material having a column-shaped nanostructure and the magnetic columns respectively have a largely uniform preferential magnetic direction deviating from the platelet plane.

The present invention relates to method of manufacturing a tamper-proof medium for thermal printing, wherein a liquid treatment composition comprising at least one acid is deposited on a substrate which comprises a thermochromic coating layer comprising at least one halochromic leuco dye.