Diffraction element and optical machine-reading device

A combination for authenticating an article of manufacture comprising

• a diffraction element having a plurality of surface portions each including a microscopic relief structure with a spatial frequency of over 50 lines per mm to form a visual image, said diffraction element including at least one pair of surface portions comprising a first surface portion having a first asymmetric relief structure and a second surface portion having a second asymmetric relief structure, said first and second relief structures being mirror symmetric and characterized by azimuth angles 180 degrees apart to form an authenticity characteristic within said visual image without disturbing said visual image, and
• an optical readout apparatus comprising a housing, a light source located in the housing for generating a light beam which is incident on said diffraction element, first and second photosensors located in said housing symmetrically with respect to said light beam which detect light diffracted by said diffraction element, each of the surface portions of the pair diffracting light to said first and second photosensors with unequal intensity such that the difference in intensity of the light diffracted to the first and second photosensors is equal but opposite for each of the surface portions of the pair, and
• an electronic circuit coupled to said first and second photosensors for generating a signal proportional to the difference between the outputs of said photosensors,
• wherein said signal outputted by said electronic circuit identifies the presence of said pair of surface portions in said diffraction element so as to authenticate said article of manufacture.