Conception et fabrication de dispositifs de sécurité optiques

Since many years, counterfeiting has been a growing problem affecting our society and its effect is constantly intensifying. A multitude of devices have been developed in order to counter this growing problem. Therefore, the current trend is to constantly make such devices more complicated by using the latest technologies (that are often expensive), in order to discourage counterfeiters.

The use of optical interference filters (interference security image structures - ISIS) as an optical security device has gained in popularity due to its color shifting characteristics as a function of the observation angle. This color change, which is termed iridescence, is not reproducible by reprographic techniques such as photocopying, printing, photography, etc. In order to increase the security offered by such filters, the present work proposes to: (1) Increase their complexity as well as performance by adding new properties to the simple color shift. (2) Preserve their simplicity of use in order to facilitate their identification by the general public.

In this master's thesis, I present a multitude of designs which are the result of a long reflection process. First, simple filters showing different color shifts, such as a red to yellow or a reverse color shift from blue to red, are presented. In order to increase there complexity, additional properties are added. For example, the combination of two filters in order to obtain three different color shifts.

Many devices based on the metameric phenomenon will also be presented. Filters possessing this characteristic have different transmission and reflection spectra, and yet they present the same color under a specific angle and illumination source. This makes it possible to create a hidden image effect, that appears only when the device is tilted. It is also possible to add a specific spectral property such as a large difference in transmission at a specific wavelength. This offers a second level of verification for example, by the use of a laser. A colorimetric analysis shows that such devices are highly affected by deposition errors and changes in illumination.

The previous model was then improved by using the following innovative concept. In this case, the filters are metameric in transmission with a coloured polymer. Their spectra being very similar, they can be made to be metameric under many standard light sources. Since the polymer doesn't change color as a function of the observation angle, the hidden image effect is still possible.

Finally other filters based on the addition of a thin metallic layer of a few nanometers are shown. This addition permits an impressive visual phenomenon which offers two different colors in reflection depending on the side from which the device is observed and a color in transmission which is independent on the observation side.

All these filters were developed by the use of an interference filter design program. They were deposited by dual ion beam sputtering (DIBS), a very stable system offering a high quality of the deposited materials - we used SiO₂ and Ta₂O₅ as dielectric materials and tantalum as the metallic layer. The optical properties of the film materials and of the filters were evaluated by variable angle spectroscopic ellipsometry and spectrophotometry; the latter measurements performed at different angles were used to determine colors of the filters in transmission and reflection.

To increase their aesthetic appearance, different patterning techniques were used. More precisely: masking during deposition, laser ablation, photolithography, deposition on a low adherence polymer and finally simple cutting. All these methods have their strengths and weaknesses, but photolithography was by far the technique offering the best results.

All of these results demonstrate that it is still possible to innovate in the domain of interference filters applied to security. The proposed devices are simple to use and offer a clearly identifiable optical effect. Furthermore, their increased fabrication complexity ensures that they will be harder to reproduce.