Abstract: A holographic effect is generated on a holographic security device by diffraction of light by a surface relief pattern on a film. When illuminated, a moire pattern is generated from a pair of overlapping, regular arrays of lines or dots. Each array has a line of symmetry. The lines of symmetry are aligned. The device exhibits an ordered variation in the form of observed moire patterns in a direction parallel with the alignment direction of the lines of symmetry and a color variation but substantially no form variation transverse to alignment direction.

Abstract: A method of enhancing an image (11) provided by thermoplastic marking material on a substrate (4) which material can be thermally activated to take on adhesive properties. The method comprises bringing a transfer medium (7) into contact with the substrate (4) carrying the marking material, the transfer medium including a carrier (3) and a transferable layer (15, 16), the transferable layer being positioned in use between the carrier and the substrate: thermally activating the adhesive property of the marking material; and separating the carrier (13) and substrate (4) whereby areas of the transferable layer containing the activated marking material remain on the marking material, characterized in that the transferable layer exhibits or generates a visible pattern.

Abstract: A security device includes first and second diffractive structures contained within a surface relief structure. The structures are such that the device responds to illumination at a first, visible wavelength to generate a first, visible pattern while any pattern generated by the second structure is not substantially visible at that wavelength, and the device responds to illumination at a second wavelength substantially different from the first wavelength to generate a second pattern suitable for machine reading while any pattern generated by the first structure is substantially suppressed relative to the machine readable pattern at that wavelength.

Abstract: A tamper resistant security seal is a laminated tape having a transparent carrier layer (2); an optical, diffraction pattern defining layer (3,4); and an adhesive layer (6) for adhering the tape to a substrate. The optical pattern, such as a hologram, defined by the optical pattern defining layer is visible from outside the laminate. The optical diffraction pattern defining layer (3) is formed by a polymeric layer permanently bonded to the transparent carrier layer which, when heated, causes the diffraction pattern to undergo an irreversible change. The adhesive is a pressure sensitive adhesive. An additional removable support layer may be provided on the carrier. The laminate is constructed so that a reduction in temperature below 0.degree. C. will cause an irreversible change in the diffraction pattern, or is such that subsequent to such a temperature reduction, attempted removal of the tape from a substrate will cause an irreversible change in the diffraction pattern.

Abstract: An optical security device for use on a security article comprises a number of layers including an optically diffracting layer and an at least partially reflective layer which together generate a first image. A non-optically diffracting second image is provided within the device in association with the first image.