Optical sky-sun diffuser

A solid optical diffuser which comprises a transparent solid matrix embedding a dispersion of transparent nanoparticles, wherein:

• said nanoparticles have an average size d in the range 10 nm≦d≦240 nm;
• the ratio between the blue and red scattering optical densities γ≡Log [T(450 nm)]/Log [T(630 nm)] of said solid optical diffuser falls in the range 5≧γ≧2.5, where T(λ) is the monochromatic normalized collinear transmittance of the solid optical diffuser, which is the ratio between the transmittance of the solid optical diffuser, without the contribution of scattered light, and the transmittance of a reference sample identical to the solid optical diffuser except for the fact that it does not contain nanoparticles;
• along at least a first direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 mm)≧0.4;
• and
• along at least a second direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 mm)≧0.9.