An embodiment of a solid optical sky-sun diffuser, which comprises a transparent solid matrix embedding a dispersion of transparent nanoparticles having an average size d in the range 10 nm≤d≤240 nm; wherein: the ratio between the blue and red scattering optical densities γ≡Log [T(450 nm)]/Log [T(630 nm)] of said diffuser falls in the range 5≥γ≥2.5, where T(λ) is the Monochromatic Normalized Collinear Transmittance; in at least one propagation direction, said Monochromatic Normalized Collinear Transmittance is T(450 nm)≥0.4; in at least one propagation direction said Monochromatic Normalized Collinear Transmittance is T(450 nm)≤0.9, said propagation direction being the same or different from that at which said Monochromatic Normalized Collinear Transmittance is T(450 mm)≥0.4.