An optical memory diffraction grating having a plurality of spaced ridges each having a plurality of optically reflective facets. When illuminated with an incident light beam the grating produces a number of diffracted beams corresponding to the number of facets. The diffracted beams can be machine read...http://www.google.com.tw/patents/US5461239?utm_source=gb-gplus-share專利 US5461239 - Method and apparatus for coding and reading information in diffraction gratings using the divergence of diffracted light beams
