A solid optical sky-sun diffuser capable of reproducing - when illuminated by visible white light - the simultaneous presence of two different chromatic components: a diffused skylight, in which blue is dominant, and a transmitted sunlight, with a low blue component; said optical diffuser comprising 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 γ= Log [ T]/Log[ T] of said optical diffuser falls in the range 5 ≥ γ ≥ 2.5, where T ( λ) is a monochromatic normalized collinear transmittance of the optical diffuser, defined as the ratio between the transmittance of the optical diffuser in the propagation direction, without the contribution of scattered light, and the transmittance of a reference specimen identical to the optical diffuser except in that it does not contain the transparent nanoparticles; wherein the monochromatic normalized collinear transmittance of the diffuser at a measurement wavelength and along a measurement direction is measured by: a) traversing the diffuser by a non-polarized quasi-monochromatic light beam having a central wavelength equal to the measurement wavelength, a spectral bandwidth lower than 10nm and an angular divergence lower than 5mrad; b) measuring a first radiant power of the light beam impinging on the diffuser; c) measuring a second radiant power of the light beam propagating beyond the diffuser, along the measurement direction and within said angular divergence; d) determining a first ratio between said second and first radiant powers; e) iterating steps a) to d) on the reference specimen, so as to determine a second ratio, relating to the reference specimen; and f) determining a third ratio between the first and second ratios, the monochromatic normalized collinear transmittance being equal to the third ratio; - along at least a first propagation direction, the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm)≥ 0.4; - along at least a second propagation direction the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm)≤0.9, wherein said second propagation direction is the same as, or different than, the first propagation direction. - the ratio between the blue and red scattering optical densities γ= Log [ T]/Log[ T] of said optical diffuser falls in the range 5 ≥ γ ≥ 2.5, where T ( λ) is a monochromatic normalized collinear transmittance of the optical diffuser, defined as the ratio between the transmittance of the optical diffuser in the propagation direction, without the contribution of scattered light, and the transmittance of a reference specimen identical to the optical diffuser except in that it does not contain the transparent nanoparticles; wherein the monochromatic normalized collinear transmittance of the diffuser at a measurement wavelength and along a measurement direction is measured by: a) traversing the diffuser by a non-polarized quasi-monochromatic light beam having a central wavelength equal to the measurement wavelength, a spectral bandwidth lower than 10nm and an angular divergence lower than 5mrad; b) measuring a first radiant power of the light beam impinging on the diffuser; c) measuring a second radiant power of the light beam propagating beyond the diffuser, along the measurement direction and within said angular divergence; d) determining a first ratio between said second and first radiant powers; e) iterating steps a) to d) on the reference specimen, so as to determine a second ratio, relating to the reference specimen; and f) determining a third ratio between the first and second ratios, the monochromatic normalized collinear transmittance being equal to the third ratio; a) traversing the diffuser by a non-polarized quasi-monochromatic light beam having a central wavelength equal to the measurement wavelength, a spectral bandwidth lower than 10nm and an angular divergence lower than 5mrad; b) measuring a first radiant power of the light beam impinging on the diffuser; c) measuring a second radiant power of the light beam propagating beyond the diffuser, along the measurement direction and within said angular divergence; d) determining a first ratio between said second and first radiant powers; e) iterating steps a) to d) on the reference specimen, so as to determine a second ratio, relating to the reference specimen; and f) determining a third ratio between the first and second ratios, the monochromatic normalized collinear transmittance being equal to the third ratio; - along at least a first propagation direction, the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm)≥ 0.4; - along at least a second propagation direction the monochromatic normalized collinear transmittance of the optical diffuser is T (450 nm)≤0.9, wherein said second propagation direction is the same as, or different than, the first propagation direction.