New perspectives for the understanding of the optical properties of middle-age nano-cermets: The lustres

Lustres are famous for their shinning metallic colours displayed in the specular direction. The optical phenomena that occur in this direction are plasmon absorption and interferences, linked with the multi-layer structure of the lustre. In order to understand the optical properties of lustre, we need to study also the other directions. Doing this, we discovered that coherent phenomena still play a key role around the specular direction leading to a smooth transition from the colour observed in the specular direction to the one observed for the diffused reflection. These coherent phenomena are identified as interferences that do appear away from the specular direction and assumed to be linked with the roughness of the interfaces of the multi-layer structure.