OPTICALLY VARIABLE DEVICES, THEIR PRODUCTION AND USE
Security documents often incorporate optically variable devices to prevent or hinder counterfeiters. Disclosed herein are layered optically variable devices such as colour-shift foils that employ a piezoelectric layer, and methods for their production and use. Such devices afford new techniques for a user of a security document to check quickly and easily whether the security document is a legitimate document or a counterfeit copy by placing an electrical potential difference across the security document.