Optical Sky-Sun Diffuser

An embodiement 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 10nm < d < 240nm; wherein: the ratio between the blue and red scattering optical densities gamma=Log[T(450nm)/Log[T(630nm)] of said diffuser falls in the range 5 > gamma>2.5, where T(lambda) is the Monochromatic Normalized Collinear Transmittance; in at least one propagation direction, said Monochromatic Normalized Collinear Transmittance is T(450nm)>0.4; in at least one propagation direction said Monochromatic Normalized Collinear Transmittance is T(450nm)<0.9, said propagation direction being the same or different from that at which said Monochromatic Normalized Collinear Transmittance is T(450nm)>0.4 .