SECURITY DEVICE

摘要

PCT No. PCT/GB91/01525 Sec. 371 Date Jul. 15, 1993 Sec. 102(e) Date Jul. 15, 1993 PCT Filed Sep. 6, 1991 PCT Pub. No. WO92/04692 PCT Pub. Date Mar. 19, 1992.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.