| 摘要 |
PCT No. PCT/AU89/00542 Sec. 371 Date Aug. 19, 1991 Sec. 102(e) Date Aug. 19, 1991 PCT Filed Dec. 19, 1989 PCT Pub. No. WO90/07133 PCT Pub. Date Jun. 28, 1990.A diffraction grating of reflective or transmissive lines is formed by a regular matrix of pixels each containing a respective curvilinear portion of one or more of said lines. When the pixels are illuminated, each generates a two-dimensional optical catastrophe image diffraction pattern whereby the total image diffraction pattern of the grating is optically variable but structurally stable. The invention further provides a diffraction grating of reflective or transmissive lines, comprising a multiplicity of diffraction grating regions which are at least partly separated by multiplicity of grating free regions. Each grating free region has a dimension which is at least large enough to be resolved by the human eye, the total grating free areas not exceeding about 20 to 50% of the total area of the grating. The disclosed diffraction grating is useful as a security device for currency or credit cards.
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