Multiple image encoding using surface relief structures as authenticating device for sheet-material authenticated item

In an article comprised of an authenticated item of sheet material which is subject to counterfeiting and an authenticating device bonded to said item, wherein said device includes a transparent material bonded to said sheet material, said material having a reflective periodic structure formed as a relief pattern on a viewable surface of said material which is proximate to said sheet material, said transparent material exhibiting a given index-of-refraction and being attached to said sheet material in a manner sufficiently secure to prevent said transparent material from being removed from said sheet material without effectively destroying said structure;

• the improvement in said device:
• wherein the period of said periodic structure is smaller in size than 100 micrometers and each period is comprised of substantially the same given set of a plural number of juxtaposed elements, respective elements of said given set having significantly different reflective characteristics from one another, and
• wherein said transparent material has a given thickness no greater than 100 micrometers and has a periodic lenticular array embossed on a surface thereof which is distal to and is oriented substantially parallel to said periodic structure, each period of said array being comprised of a lens having an aperture width that at least approximates that of a period of said structure and having a focal length substantially equal to said given thickness.