Metameric interference security image structures.

We study innovative interference security image structures based on metamerism. We have designed, fabricated, and evaluated different structures that can be used in transmission or in reflection. These metameric structures are either a combination of two different interference filters or of an interference filter and a noniridescent colored material. In the latter case, by closely matching the spectra, the sensitivity of the device to changes in light sources and observers is minimized. Because of the intrinsic color shift of interference filters, one can create a hidden image that appears at a specific observation angle. The presence of the hidden image, as well as in some cases of the noniridescent material, which serves as a color reference, increases the complexity of such devices while facilitating the user's authentication process as well as automatized detection by using a laser at a specific angle. We present the design approach, analyze the filters' sensitivity to deposition errors, and evaluate the performance of prototype devices prepared by dual ion beam sputtering.