Abstract (US 5591527):
Optical security articles are provided that can be used, for example, as protective transparent overlays for authenticated substrates such as documents, identification cards, and the like. The articles can include an image, such as a design or logo, to enhance the authenticity of the substrate and also provide high tamper-resistance. Typically and preferably, the image is viewable only across a narrow range of viewing angles and is viewable in ambient (diffuse) light, thus affording a readily apparent verification of the authenticity of the substrate. The technology also provides methods for making such optical security articles.

Technology Benefits:
Security article made using prismatic reflective films such that some portions of the article are transparent and reveal an image or graphics below, while some portions are reflective such that the revealed image or graphics can be viewed from only a specific narrow angle.

Technology Uniqueness:
This technology describes an optical security article comprising:
a) a first layer substantially transparent to visible light and having a first refractive index, the first layer having a structured first surface and a substantially smooth second surface, the structured first surface comprising a plurality of geometric concavities and corresponding peaks; and
b) an adhesive layer coterminating with the first layer, the adhesive layer substantially completely filling and making adhesive contact with a first portion of the plurality of geometric concavities, the adhesive layer having a second refractive index which is substantially similar to the first refractive index of the first layer, thus forming at least one transparent area,
wherein a second portion of the plurality of geometric concavities are precluded from contact with the adhesive layer by a separation layer disposed between the second portion of the plurality of geometric concavities and the adhesive layer. Preferably, the separation layer contacts only the peaks of the structured first surface and has a separation layer refractive index which is different from the first refractive index, although these conditions are not necessary to produce the desired optical reflection back through the first layer. Preferably, there are a plurality of areas having non-overlapping separation layers (i.e. all substantially in the same plane approximately parallel to the substrate to be protected), each of the separation layers having the same or very similar refractive index, and thus a plurality of transparent areas. The second portion of the plurality of geometric concavities and the separation layer thus define at least one reflective area.
This technology also describes a tamper-proof article comprising the optical security article secured by the adhesive layer to a substrate to protect the substrate from tampering and/or to enhance the appearance of authenticity of the substrate. The substrate may be essentially any substrate such as metal, wood, paper, plastic, ceramic, but is typically a photograph of an ID card or drivers license, a bank note, a stock certificate, or the like.
If the separation layer is an imaging material, such as an ink imprint of a logo, the logo is prominent when the tamper-proof article is viewed at the correct orientation, coming to the foreground over the underlying substrate. When the tamper-proof article is reoriented, the logo disappears and the underlying substrate is clearly visible. If an attempt is made to remove the optical security article, the continuous adhesive layer containing the validating logo is at least partially destroyed.

Intellectual Property Summary:
Patent No. US 5591527