Aspects of the present disclosure generally relate to a security feature that is based on a single axis alignment of mirrors in a structured surface that forms a micro mirror array, and more particularly, to the use of foil or other embossed or stamped devices to generate the security feature.
Optical effects such as depth, motion, and morphing have been used to generate security features for various applications, including bank notes and other types of valuable documents. Security features generally involve three different lines of defense against counterfeiting. A first line of defense includes overt security features such as watermarks, intaglio printing security threads, holograms, optically variable inks, and other features detectable by the naked eye. A second line of defense includes covert features such as magnetic inks, bar codes, luminescent printing, and retroflection. A third line of defense contains features detectable by forensic experts using laboratory equipment. The features in the first line of defense can be generally divided into optically invariable features and optically variable features or OVDs, the latter of which involves features displaying themselves in reply to an external stimulus applied to the bank note or the document by the observer.
One approach to generate or produce devices or articles with optically variable features is to form an array of magnetically-orientable platelets or flakes having reflective surfaces and aligned as concave or convex reflectors in a vehicle layer (e.g., dispersed in a fluid carrier) by the application of a magnetic field in a predefined spatial orientation. Once aligned and set (e.g., through ultra-violet (UV) curing), the array of flakes can produce different types of optical security features (e.g., optical effects).
A different approach to generate or produce devices or articles with optically variable features is to form on a foil or similar material a structured surface with an array of embossed segments or facets having reflective surfaces. The reflective facets may be referred to as micro mirrors and the overall array as a micro mirror array. The predefined alignment and orientation of the facets in the array can produce different types of optical security features as well.
In many scenarios, a Fresnel equivalent-shaped reflective structure made using the embossed foil approach can produce the same or similar optical effect to a counterpart reflective structure made of magnetically aligned flakes. There are scenarios, however, in which the optical effects or features produced by magnetically aligned flakes may not be easily reproduced by the embossed or stamped foil approach, limiting the ability of those that have implemented the embossed foil approach into their first line of defense from introducing more advanced and secure optical features that may be implemented using magnetically aligned flakes.
Accordingly, mechanisms or techniques are desirable that allow for some optical effects to be implemented as optically variable security features when using the embossed or stamped foil approach.