Rendering Transparent Materials with a Complex Refractive Index: Semi-conductor and Conductor Thin Layers

During physical simulation of light transport, we separate materials between conductors and dielectrics. The for-mer have a complex refractive index and are treated as opaque, the latter a real one and are treated as transparent.However, thin layers with a complex refractive index can become transparent if their thickness is small compared to the extinction coefficient. This happens with thin metallic layers, but also with many pigments that are semi-conductors: their extinction coefficient (the imaginary part of their refractive index) is close to zero for part of the visible spectrum. Spectral effects inside these thin layers (attenuation and interference) result in dramatic color changes.