Reflective diffractive authenticating device

In an authenticating device for use in authenticating an item of sheet material when said device is bonded to said item;

• wherein said device comprises an integrated structure of a substrate layer and a transparent overcoat layer separated from one another by a coating layer that is situated in intimate contact with said substrate and overcoat layers at the interface therebetween, said interface including a reflective diffraction grating in the form of a relief pattern, said grating having specified grating-profile, physical-amplitude and line-period parameters that operate to separate polychromatic illuminating light incident thereon into at least one pair of light beams of contrasting colors, and wherein the bond of said coating layer to said respective substrate and overcoat layers at said interface is significantly less secure than is a direct bond of said substrate to said overcoat layer;
• the improvement wherein:
• said coating layer is divided in at least one dimension into a set of separated regions, adjacent ones of said regions being separated from one another in at least one dimension by a spacing distance a, the center-to-center distance of adjacent ones of said regions in at least said one dimension being b;
• the respective values of a and b being characterized in that b is on the order of ten times larger than a, a is significantly larger than the line-period of said diffraction grating, and b is sufficiently small as to be at most barely noticeable to the naked eye of an observer;
• and
• said overcoat layer being directly bonded to said substrate layer within the separation between adjacent ones of said regions of said coating layer.