Color and visual appearance. Transparent, translucent, opaque

Although the word color immediately conjures up highly polysemous notions, in these pages it is approached from only one aspect of light-matter interaction, that of physics. Classic colorimetry has abundantly defined the quantities and standards that enable any practitioner, whether designer, colorist, formulator, engineer or researcher in the academic or industrial sectors, to exchange data concerning the visual appearance (such as chromaticity and gloss, for example) of materials or lighting systems. The most important international structure, based on a French initiative, is the Commission Internationale de l'Éclairage (CIE) [R 86] . Other standardization bodies, such as AFNOR, define rules of use and specify the vocabulary of applied colorimetry. Our focus here is on the fundamental phenomena that give rise to what an instrument can measure: radiation. We therefore assume a fixed reference colorimetric observer defined by the CIE, and focus our attention on the physical, physico-chemical, "exact" or phenomenological models used to describe this light-matter interaction. It is involved in stimulating the visual system as a primary external cause, and calls on knowledge from the basic sciences, mainly from many fields of optics. We highlight the crucial importance of the notion of complex dielectric function or complex refractive index [R 6 470] because of its omnipresence in all phenomena when it comes to color and, more generally, visual appearance . This notion of a complex delectrical function provides the link between the fundamental phenomena of light-matter interaction - the root causes of which lie in optics and the dimensional scale of wavelength - and the macro-scopic measurements made by our instruments, or simply what our eyes receive. Armed with this fundamental notion, we outline the path that leads from the simple to the complex,...