International audienceThe different areas of a concave object illuminate each other by a multiple light reflection process, called interreflections, depending on the geometries of the object and the lighting. For an accurate prediction of the radiance perceived from each point of the object by an observer or a camera, an interreflection model is necessary, taking into account the optical properties and the shape of the object, the orientation(s) of the incident light which can produce shadows, and the infinite number of light bounces between the different points of the object. The present paper focusses on the irradiance of two adjacent planar panels (V-cavity) illuminated by collimated light from any direction of the hemisphere, or by diffuse light. According to the reflectance of the material and the angle of the cavity, the loss of irradiance near the fold due to the shadowing effect is partly compensated by the gain in radiance due to the interreflections

The color of a surface structured at the mesoscopic scale differs from the one of a flat surface of the same material because of the light interreflections taking place in the concavities of the surface, as well as the shadowing effect. The color variation depends not only on the surface topology but also on the spectral reflectance of the material, its matte or glossy finishing, and the angular distribution of the incident light. For an accurate prediction of the radiance perceived from each point of the object by an observer or a camera, we must take into account comprehensively the multiple paths of light which can be reflected, scattered or absorbed by the material and its surface. In this paper, we focus on the light reflection component due to the material-air interface, in the special case of a surface structured with parallel, periodical, specular Vshaped ridges, illuminated either by collimated light from any direction of the hemisphere, or by diffuse light. Thanks to an analytical model, we compute the radiance reflected in every direction of the hemisphere by accounting for the different interreflections, according to the angular reflectance of the panels and the aperture angle of the cavity. We can then deduce the apparent reflectance of the cavity when viewed from a large distance.

The color of a surface structured at the mesoscopic scale differs from the one of a flat surface of the same material because of the light interreflections taking place in the concavities of the surface, as well as shadowing effects. The color variation does not only arise in scattering materials, but also in absence of scattering, e.g. in metals and clear dielectrics, just as a consequence of multiple specular reflections between neighboring flat facets of the surface. In this paper, we investigate such color variation in the case of an infinitely long V-shaped groove, having in mind the visual appearance of a surface composed of many structures of that sort, all parallel and identical. We develop a full model of multiple specular reflections, accounting for ray position and orientation and polarization effects occurring at each reflection. We compare that situation with two approximate models, more usual and easier to compute, where light is assumed to remain unpolarized all along, or where the p-and s-polarized components are treated separately. Spectral reflectances were predicted for various materials and angles of cavities, under diffuse illumination. In most cases, the three models predict very similar bi-hemispherical reflectances, but the hemispherical-directional reflectances can vary noticeably in certain observation directions. This study might help achieving more physically-realistic rendering of dielectric or metallic ridged surfaces in computer graphics.