Secure document identification technique

Claim 1

In combination: an individual item of sheet material incorporating an element purporting to be a particular one of a given type of authenticating device, wherein said given type of authenticating device exhibits predetermined macroscopic reflectivity characteristics and wherein said particular authenticating device includes a unique pattern of microscopic lines having widths in the order of micrometers that are randomly positioned in at least one dimension with each separation distance between adjacent lines being in the order of tens of micrometers; a record medium forming part of said item of sheet material and having recorded thereon digital codes representing the locations of the respective line positions of at least some of said pattern lines in at least said one dimension within a certain preselected area of said particular authenticating device; wherein said element is comprised of a relatively high index-of-refraction inorganic dielectric coating layer situated at the interface between relatively low index-of-refraction plastic substrate and overcoat layers, said interface being in the form of a surface relief pattern defining a diffraction-grating-line structure having a value of line spacing period with respect to at least some of the free-space wavelengths included in polychromatic illuminating light such that both zero and first diffraction order light can propagate in said inorganic dielectric coating layer but only zero diffraction order light can propagate in said plastic substrate and overcoat layers; and wherein said dielectric coating layer inherently forms a random pattern of cracks that define said pattern lines, such pattern lines being preferentially oriented substantially parallel to said lines of said grating-line structure.