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

1 . 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 (D e ) in a plane of incidence (P) and deviated with respect to an incidence direction (D) by a non-zero angle of incidence (δ in ) and smaller than an acceptance angle of the optical filter (ϑ acc ) 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 (δ out ) 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 (D e ) in the plane of incidence (P) and deviated with respect to the incidence direction (D) by a not-zero angle of incidence (δ in ) and smaller than an acceptance angle of the filter (ϑ acc ) 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 (δ out ) 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 (n e ) in proximity to walls of the transparent sector adjacent to the respective absorbing sectors and a maximum value of refractive index (n i ) in a central portion of the transparent sector.

2 . The optical filter according to claim 1 , wherein a thickness (d) of the transparent sector of the plurality of transparent sectors is between 10 μm and 300 μm, between 10 μm and 200 μm, or between 10 μm and 150 μm.

3 . The optical filter according to claim 1 ,

wherein the surface of each absorbing sector of the plurality of absorbing sectors has a radius of curvature at least 5 times, at least 10 times, or at least 30 times greater than the distance between adjacent absorbing sectors; and/or

wherein the absorbing sectors of the plurality of absorbing sectors intersect the inlet surface and the outlet surface along concentric circles or along a spiral line or along substantially straight lines parallel to each other.

4 . The optical filter according to claim 1 , configured to transform a third collimated light and a fourth collimated light which respectively illuminates the inlet surface and the outlet surface along a direction in the plane of incidence (P) in a transmitted or filtered light in which

(i) for incidence directions of the input third and fourth collimated light inside an acceptance angle of the filter (ϑ acc ) measured with respect to the incidence direction (D), a transmitted light has a profile of angular luminance in the plane of incidence (P) having values close to the maximum value for angles smaller than a cut angle (ϑ cut ) measured with respect to the incidence direction (D), and zero elsewhere, said luminance profile being substantially independent or slightly dependent on the direction of the input collimated light, and

(ii) for incidence directions of the input third and fourth collimated light outside the acceptance angle of the filter, the transmitted light is substantially zero.

5 . The optical filter according to claim 1 , wherein

for a luminous transmittance T(ϑ in ) of the optical filter for a third collimated light and a fourth collimated light which respectively illuminates the inlet surface and the outlet surface along a direction in the plane of incidence (P) which forms with the incidence direction (D) an input angle (ϑ in ) such that ϑ in >Aϑ acc , the relation T(ϑ in )<BT 0 applies, where A=1.3, A=1.2, or A=1.1 and where B=0.20, B=0.15, B=0.10, or B=0.05, and where T 0 ≡<T(ϑ in )>for |ϑ in |<ϑ 1-HWHM where T(ϑ 1-HWHM )=T(0)/2, and/or

for a luminous intensity I(ϑ out ) of a light transmitted by the optical filter propagating in the plane of incidence (P) at an output angle (ϑ out ) such that ϑ out >Cϑ cut , and for a diffused input light having substantially constant luminous intensity in all directions, the relation I(ϑ out )<D I 0 applies, where C=1.3, C=1.2, or C=1.1 and where D=0.20, D=0.10, or D=0.05, and where I 0 ≡<I(ϑ in )>for |ϑ in |<ϑ 2-HWHM where I(ϑ 2-HWHM )=T(0)/2, and/or

for a luminous intensity I(ϑ out ) of a light transmitted by the optical filter propagating in the plane of incidence (P) at an output angle (ϑ out ) such that ϑ out <Eϑ cut for a diffused input light having substantially constant luminous intensity in all directions, the I(ϑ out )>F I 0 applies, where E=0.7, E=0.8, or E=0.9 and where F=0.3, F=0.4, F=0.6, or F=0.8 and where I 0 ≡<I(ϑ in )>for |ϑ in |<ϑ 2-HWHM where I(ϑ 2-HWHM )=T(0)/2.

6 . The optical filter according to claim 1 , wherein the layer of inhomogeneous transparent material of the transparent sectors comprises a plurality of layers of different refractive index.

7 . The optical filter according to claim 6 , wherein the value of the refractive index of each layer of said plurality of layers varies along the normal (N) to the absorbing sector with dependence which substantially approximates a parabolic function and/or is monotonically decreasing from the central portion of the transparent sectors to the sector walls adjacent to the respective absorbing sectors, and/or

wherein the value of the refractive index of each layer of said plurality of layers varies along the normal (N) to the absorbing sector with dependence such as to confer to the transparent sector the refractive power of a lens with focal length (f), in which 0.5 L<f<2 L, 0.7 L<f<1.6 L, 0.7 L<f<1.4 L, 0.9 L<f<1.2 L, or the focal length (f) is substantially equal to the sector length (L).

8 . The optical filter according to claim 6 , wherein

wherein a first layer positioned in a central portion is made with a transparent material having a refractive index of internal layer corresponding to the maximum value of refractive index (n i ) and a pair of layers positioned at respective adjacent absorbing sectors are made with a transparent material having a refractive index of external layer corresponding to the minimum value of refractive index (n e ), and

wherein between the sector length (L), a thickness (d) of the transparent sector, the acceptance angle of the filter (ϑ acc ) and the maximum value of refractive index (n i ) the relation applies:

L

=

G

⁢

d

tan

⁡

(

a

⁢

sin

⁢

sin

⁢

θ

acc

n

i

)

where G<3, G<2, G<1.5 and/or G>0.3, G>0.5, G>0.7, or

G

≅

π

4

.

9 . The optical filter according to claim 6 , wherein the plurality of layers of different refractive index is constituted by a first layer positioned in a central portion of a respective transparent sector and a pair of second layers, wherein each second layer of the pair of second layers is arranged alongside the first layer.

10 . The optical filter according to claim 9 ,

wherein the first layer positioned in a central portion is made with a transparent material having a refractive index of internal layer corresponding to the maximum value of refractive index (n i ) and the pair of second layers is made with a transparent material having a refractive index of external layer corresponding to the minimum value of refractive index (n e ), and

wherein between the sector length (L), a thickness (d) of the transparent sector, a total internal reflection angle (θ TIR ) and the maximum value of refractive index (n i ) the relation applies:

L

=

G

⁢

d

tan

⁡

(

a

⁢

sin

⁢

sin

⁢

θ

TIR

n

i

)

with the total internal reflection angle (θ TIR ) equal to a total internal reflection angle at the interface between the internal layer and the external layer and where G<3, G<2, G<1.5 and/or G>0.3, G>0.5, G>0.7, or G≅1.

11 . The optical filter according to claim 1 , wherein the refractive index profile of each transparent sector varies continuously according to a substantially parabolic and/or monotonically decreasing trend from the central portion of the transparent sectors to the sector walls adjacent to the respective absorbing sectors, and/or

wherein the refractive index profile of each transparent sector varies continuously according to a trend such as to locally confer to the transparent sector a refractive power of a GRIN cylindrical lens with focal length (f), wherein 0.5 L<f<2 L, 0.7 L<f<1.6 L, 0.7 L<f<1.4 L, 0.9 L<f<1.2 L, or the focal length (f) is substantially equal to the sector length (L).

12 . The optical filter according to claim 11 , wherein

between the sector length (L), a thickness (d) of the transparent sector, the acceptance angle of the filter (ϑ acc ) and the maximum value of refractive index (n i ) the relation applies:

L

=

G

⁢

d

tan

⁡

(

a

⁢

sin

⁢

sin

⁢

θ

acc

n

i

)

where G<3, G<2, G<1.5 and/or G>0.3, G>0.5, G>0.7, or

G

≅

π

4

.

13 . An artificial light illumination device for reproducing the light of the sky and the sun, the artificial light illumination device comprising:

a direct light source configured to emit visible light along directions in a range across a main direction, the light emitted by the direct light source having a first calibrated color temperature (CCT);

an optical filter according to claim 1 placed downstream of the direct light source with respect to the main direction; and

a diffused light source positioned downstream of the optical filter with respect to the main direction, the diffused light source being configured to transmit, at least in part, a filtered light in output from the filter and being configured to produce a diffused light component, having an angular luminance profile with a divergence at least 2 times, 3 times, or 4 times greater than a divergence of the filtered light, and/or by a CCT at least 1.2 times, 1.3 times, 1.5 times, or 1.8 times greater than the first CCT, and/or a CCT equal to 5600 Kelvin.

14 . 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 first collimated light which impinges on the inlet surface along an incidence direction (D) at a local region of the inlet surface, the transparent sector focuses the first collimated light along a first focal line orthogonal to a local plane of incidence (P), and/or

given a second collimated light which impinges on the outlet surface along an incidence direction (D) at a local region of the outlet surface, the transparent sector focuses the second collimated light along a second focal line orthogonal to a local plane of incidence (P),

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 in proximity to walls of the transparent sector adjacent to the respective absorbing sectors and a maximum value in a central portion of the transparent sector.

15 . The optical filter according to claim 14 , wherein each transparent sector of the plurality of transparent sectors is locally arranged and configured to have such a refractive power that,

given the first collimated light which impinges on the inlet surface along the incidence direction (D) at the local region of the inlet surface, the transparent sector focuses the first collimated light along the first focal line orthogonal to the local plane of incidence (P) and positioned at a first distance (f1) from the inlet surface measured along the incidence direction (D), wherein 0.5 L<f1<2 L, 0.7 L<f1<1.6 L, 0.7 L<f1<1.4 L, 0.9 L<f1<1.2 L, or the first focal distance (f1) is substantially equal to the sector length (L); and/or

given the second collimated light which impinges on the outlet surface along the incidence direction (D) at the local region of the outlet surface, the transparent sector focuses the second collimated light along the second focal line orthogonal to the local plane of incidence (P) and positioned at a second distance (f 2 ) from the outlet surface measured along the incidence direction (D), wherein 0.5 L<f 2 <2 L, 0.7 L<f 2 <1.6 L, 0.7 L<f 2 <1.4 L, 0.9 L<f 2 <1.2 L, or further preferably wherein the second focal distance (f 2 ) is substantially equal to the sector length (L).

16 . The optical filter according to claim 14 , wherein a thickness (d) of the transparent sector of the plurality of transparent sectors is between 10 μm and 300 μm, between 10 μm and 200 μm, or between 10 μm and 150 μm.

17 . The optical filter according to claim 14 ,

wherein the surface of each absorbing sector of the plurality of absorbing sectors has a radius of curvature at least 5 times, at least 10 times, or at least 30 times greater than the distance between adjacent absorbing sectors; and/or

wherein the absorbing sectors of the plurality of absorbing sectors intersect the inlet surface and the outlet surface along concentric circles or along a spiral line or along substantially straight lines parallel to each other.