Color and Visual Appearance. The Metallic Look

Here, we focus on the reflection of light from a smooth or rough metal surface and the simulation of its visual appearance. The fundamental data governing the optical behavior of metals and conductive media are recalled, and numerous values of the complex refractive indices of metallic elements are given in the appendix. The optical properties of a number of alloys and minerals with metallic lustre are also presented. These data, more than rare in the scientific literature, have made it possible to calculate the visual appearance of a few metallic and semi-metallic elements in a high state of purity. The theoretical rudiments presented are applied to the simulation of the visual appearance of binary alloys; the example given concerns bronzes (copper-tin alloys) as well as electrum. Classical methods for obtaining the fundamental data of complex refractive indices are briefly described. Matter is not the only factor involved in the interaction with light, and the observer's view of it cannot be fully described without considering the effects of illumination. We show how the effect of orthotropic ambient lighting modifies the perception of metallic luster, a kind of "non-glare" in this case. This is done by determining a particular surface-defining integral, called SWR. We give a graphical version obtained by numerical integration in the case where the imaginary part of the refractive index is non-zero. There is an analytical answer to the calculation of this integral in the case where the refractive index is a pure real number. The influence of directional lighting makes visual interpretation, the understanding of what is seen, more habitual; this is indeed the role of the images obtained by spectral image synthesis based on the exclusive use of this notion of complex refractive index and presented throughout the text.

It is recommended, though not absolutely necessary for the specialist, to have read the first part of this text, entitled "Transparent, translucent and opaque" [AF3252] .