Optical filter and artificial light illumination device for reproducing the light of the sky and the sun using the same

An optical filter comprising

• an inlet surface and an outlet surface which are substantially flat and parallel,
• a plurality of locally planar and mutually locally parallel visible light absorbing sectors extending between the inlet surface and the outlet surface for a sector length (L), and
• a plurality of solid transparent sectors made with at least one solid material transparent to visible light, each transparent sector of the plurality of transparent sectors being interposed between two absorbing sectors to create an alternation of transparent sectors and absorbing sectors adjacent to and in contact with each other,
• wherein each visible light absorbing sector of the plurality of visible light absorbing sectors borders at most two transparent sectors of the plurality of transparent sectors, and/or wherein a distance between adjacent absorbing sectors of the plurality of absorbing sectors is substantially constant,
• wherein each transparent sector of the plurality of transparent sectors is locally arranged and configured to have such a refractive power that,
• given a third collimated light which impinges on the inlet surface at a local region of the inlet surface and along an inlet direction (De) in a plane of incidence (P) and deviated with respect to an incidence direction (D) by a non-zero angle of incidence (δin) and smaller than an acceptance angle of the optical filter (ϑacc) measured with respect to the incidence direction (D), in output from the outlet surface it is produced at least one component of transmitted light that exits from the outlet surface along a direction in the plane of incidence (P) and deviated with respect to the incidence direction (D) by a deviation angle (δout) such that
• δ
• in
• ❘
• "\[LeftBracketingBar]"
• δ
• in
• ❘
• "\[RightBracketingBar]"
• =
• -
• δ
• out
• ❘
• "\[LeftBracketingBar]"
• δ
• out
• ❘
• "\[RightBracketingBar]"
• applies and wherein a luminous transmittance for the third collimated light is significantly higher than 50%, higher than 60%, or higher than 70% net of losses due to reflections on the inlet and outlet surfaces, and/or
• given a fourth collimated light which impinges on the outlet surface at a local region of the outlet surface and along an inlet direction (De) in the plane of incidence (P) and deviated with respect to the incidence direction (D) by a not-zero angle of incidence (δin) and smaller than an acceptance angle of the filter (ϑacc) measured with respect to the incidence direction (D), in output from the inlet surface it is produced at least one component of transmitted light that exits from the inlet surface along a direction in the plane of incidence (P) and deviated with respect to the incidence direction (D) by a deviation angle (δout) such that
• δ
• in
• ❘
• "\[LeftBracketingBar]"
• δ
• in
• ❘
• "\[RightBracketingBar]"
• =
• -
• δ
• out
• ❘
• "\[LeftBracketingBar]"
• δ
• out
• ❘
• "\[RightBracketingBar]"
• applies and wherein the luminous transmittance for the fourth collimated light is significantly higher than 50%, higher than 60%, or higher than 70% net of losses due to reflections on the inlet and outlet surfaces,
• wherein a plane orthogonal to the inlet surface and containing a normal (N) to an absorbing sector of the plurality of absorbing sectors at the local region of the inlet surface defines the local plane of incidence (P), and the intersection between the local plane of incidence (P) and a surface of an absorbing sector defines the incidence direction (D) substantially common to the entire inlet surface,
• wherein each transparent sector of the plurality of transparent sectors comprises a layer of inhomogeneous transparent material having a non-uniform refractive index profile with gradient substantially parallel to the normal (N) to the absorbing sector, having a minimum value of refractive index (ne) in proximity to walls of the transparent sector adjacent to the respective absorbing sectors and a maximum value of refractive index (ni) in a central portion of the transparent sector.