Thin Film-Based Optically Variable Security Devices : From Passive to Active

Counterfeiting costs the world economy billions of dollars every year. Aside from financial losses, counterfeiting also poses a great threat to the public's safety, for example through the existence of counterfeit passports (terrorism), pharmaceutical products (health hazards) and even airplane parts (safety issues). Optical security devices (OSDs) have therefore played a critical role in the fight against counterfeiting. OSDs, such as those based on interference introduced in the 1980s, have clearly proven their efficiency; this high efficiency being the result of the facility with which the color shift can be observed as a function of the observation angle as well as the relatively high cost of the production equipment. On the other hand, nowadays, due to the appearance of iridescent consumer products such as coextruded polymer “wrapping paper”, interference-based devices will need to be “upgraded”. It is the aim of the present thesis to show that through the use of metamerism and electrochromic materials, new types of active security devices with interesting features can be created; indeed, most present-day devices are passive in nature. It is also important to add, that although these devices are more complex to fabricate and therefore difficult to counterfeit, they do remain simple to authenticate for the man on the street. I first demonstrate that the addition of metamerism in the design of interference filters can result in innovative features. Different structures which can be used in transmission and/or in reflection are designed, fabricated, and evaluated. The first structures which are presented here are based on a combination of two different metameric interference filters. Possessing widely different transmission spectra, these filters also offer different angular color shifts and, as a result, offer an opportunity of creating hidden image effects. Despite their interesting properties, such metameric devices are shown to be highly illuminant and observer sensitive; that is the color match is lost under most observation conditions. These issues are solved by a simpler structure based on the juxtaposition of an interference filter and a non-iridescent colored material. In this case, by closely matching the transmission or reflection spectra of both elements, the sensitivity of the device to changes in light sources and observers is minimized. Since only the interference filter will change color as a function of the observation angle, a hidden image effect is yet again possible. The presence of the hidden image, as well as of the non-iridescent material, which serves as a color reference, facilitates the user's authentication process as well as automated detection by using a laser at a specific angle. Throughout this study, I present the design approach, analyze the filters' sensitivity to deposition errors, and evaluate the performance of prototype devices prepared by dual ion beam sputtering.