A solid sun-sky optical diffuser capable of reproducing, when illuminated with visible white light, the simultaneous presence of two different chromatic components: a diffuse zenithal light, in which blue is dominant, and a transmitted sunlight, with a low blue component; 5 said optical diffuser comprising a transparent solid matrix that integrates a dispersion of transparent nanoparticles having an average size d in the range of 10 nm = d = 240 nm; wherein: - the ratio between the optical densities of blue and red dispersion γ = Log [T (450 nm)] / Log [T (630 nm)] of said optical diffuser is in the range 5 = γ = 2.5, in which T (λ) is a monochromatic normalized collinear transmittance of the optical diffuser, defined as the ratio between the optical diffuser transmittance in the propagation direction, without the contribution of the scattered light, and the transmittance of a reference sample identical to the optical diffuser except that it does not contain transparent nanoparticles; in which the monochromatic normalized collinear transmittance of the diffuser at a measurement wavelength and along a measurement direction is measured: a) by crossing the diffuser an almost monochromatic non-polarized beam of light having a central wavelength equal to the measurement wavelength, a spectral bandwidth less than 10 nm and an angular divergence less than 5 mrad; b) measuring a first radiant power of the incident light beam on the diffuser; c) measuring a second radiant power of the light beam that propagates beyond the diffuser, along the measurement direction and within said angular divergence; d) determining a first relationship between said second and first radiant powers; e) iterating steps a) to d) in the reference sample, in order to determine a second relationship, in relation to the reference sample; and f) determining a third relationship between the first and second relationships, the monochromatic normalized collinear transmittance being equal to the third relationship; - along at least one first propagation direction, the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm) = 0.4; - along at least one second propagation direction the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm) = 0.9, in which said second propagation direction is the same as, or different than, the First propagation address.
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