Create a physical model that connects the optical properties of the pigments used in cave paintings, and measured in-situ, to their physical and chemical properties: chemical composition of the minerals and binders, granulometry, texture. That model will be based on a physical model of radiative transfer inside the pigment layer. The pigments used in cave paintings are usually dry powders. We do not currently have a physical model for the reflectance of dry powders, as opposed to suspensions or compact materials. Experiments show that pigments can have different colors with the same chemical and mineral composition, implying that scattering effects are playing a role in the color. We have very few reference materials with well known physical properties and measured spectral response as well as bidirectional reflectance function, for dry powders. We do not have accurate optical constants for the materials (minerals, binders) used in the pigments, and measuring them is challenging. Painting materials are applied on a rock wall. This substrate contributes to the overall reflectance and appearance in some spectral ranges. We will have to remove its contribution.
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