Diffraction grating

A security device having improved contrast properties comprising a diffraction grating of reflective or transmissive lines, comprising a multiplicity of diffraction grating regions, which are at least partly separated by a multiplicity of grating free regions, each grating free region having a dimension which is at least large enough to be resolved by the human eye, the grating free regions not exceeding about 20 to 50% of the total area of the grating.

A security device comprising a plastic film grating configured to form part of a currency note or credit card, wherein said grating is a pixellated diffraction grating in which each pixel is an individual diffraction grating of reflecting or transmissive lines, said security device, when illuminated, generating an image that is optically variable with position of view and that retains its structural stability in that the observed intensity of diffracted light from a particular point on the grating surface changes smoothly when the grating surface is crinkled in the vicinity of the observed point.

A security device comprising a plastic film grating configured to form part of a currency note or credit card, wherein said grating is a pixellated diffraction grating in which each pixel is an individual diffraction grating of reflecting or transmissive lines, said security device, when illuminated, generating an optically variable but structurally stable image, wherein each individual diffraction grating generates a two-dimensional optical catastrophe image diffraction pattern when illuminated.

A method of providing an item susceptible to counterfeiting with a diffraction grating, comprising scanning an optically invariable image to form a matrix of pixellated portions of said image, then producing from said pixellated portions a pixellated diffraction grating in which each pixel is an individual diffraction grating of reflective or transmissive lines each including a said pixellated portion of said image, said diffraction grating, when illuminated, generating an optically variable but structurally stable image, and attaching said grating to the item, the method further comprising selecting parameters for each individual diffraction grating from one or more of brightness, stability, color, orientation, distance and angle at which the optically variable image has maximum clarity and the light source giving maximum effect to the optically variable image.

A method of forming a diffraction grating, comprising scanning an optically invariable image to form a matrix of pixellated portions of said image, producing from said pixellated portions a pixellated diffraction grating in which each pixel is an individual diffraction grating of reflective or transmissive lines each including a said pixellated portion of said image, said diffraction grating, when illuminated, generating an optically variable but structurally stable image, the method further comprising forming said reflecting or transmissive lines in a manner which generates, when the diffraction grating is illuminated, a two-dimensional optical catastrophe image diffraction pattern.

A method of providing an item susceptible to counterfeiting with a diffraction grating, comprising scanning an optically invariable image to form a matrix of pixellated portions of said image, then producing from said pixellated portions a pixellated diffraction grating in which each pixel is an individual diffraction grating of reflective or transmissive lines each including a said pixellated portion of said image, said diffraction grating, when illuminated, generating an optically variable but structurally stable image, and attaching said grating to the item, the method further comprising forming each individual diffraction grating so that they are at least partly separated from each other by a multiplicity of grating free regions, each grating free region having a dimension which is at least large enough to be resolved by the human eye, the grating free regions not exceeding about 20% to 50% of the total area of the grating.