Optical sky-sun diffuser
An embodiment 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 10 nm≦d≦240 nm; wherein: the ratio between the blue and red scattering optical densities γ≡Log [T(450 nm)]/Log [T(630 nm)] of said diffuser falls in the range 5≧γ≧2.5, where T(λ) is the Monochromatic Normalized Collinear Transmittance; in at least one propagation direction, said Monochromatic Normalized Collinear Transmittance is T(450 nm)≧0.4; in at least one propagation direction said Monochromatic Normalized Collinear Transmittance is T(450 nm)≦0.9, said propagation direction being the same or different from that at which said Monochromatic Normalized Collinear Transmittance is T(450 mm)≧0.4.
1. A solid optical diffuser which comprises a transparent solid matrix embedding a dispersion of transparent nanoparticles, wherein:
said nanoparticles have an average size d in the range 10 nm≦d≦240 nm;
the ratio between the blue and red scattering optical densities γ≡Log └T (450 nm)┘/Log └T(630 nm)┘ of said solid optical diffuser falls in the range 5≧γ≧2.5, where T(λ) is the monochromatic normalized collinear transmittance of the solid optical diffuser, which is the ratio between the transmittance of the solid optical diffuser, without the contribution of scattered light, and the transmittance of a reference sample identical to the solid optical diffuser except for the fact that it does not contain nanoparticles;
along at least a first propagation direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 nm)≧0.4; and
along at least a second propagation direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 nm)≦0.9.
2. The solid optical diffuser according to claim 1 , wherein the relative refraction index
m
≡
n
p
n
h
,
where n p is the refractive index of said nanoparticles and n h is the refractive index of said transparent solid matrix, falls in the range 0.7≦m≦2.1, and the effective particle diameter, D≡dn h , fulfills D[nm]≦132 m+115 if 0.7≦m<1; D[nm]≦240 if 1<m<1.35 and D[nm]≦−135 m+507 if 1.35≦m≦2.1.
3. The solid optical diffuser according to claim 2 , wherein, along at least the first propagation direction, the number of nanoparticles per unit area is
N
≤
N
m
ax
=
3.7
×
10
-
28
D
6
m
2
+
2
m
2
-
1
2
[
meters
-
2
]
,
D being given in meters.
4. The solid optical diffuser according to claim 3 , wherein, along at least the second propagation direction, the number of nanoparticles per unit area is
N
≥
N
m
i
n
=
4.24
×
10
-
29
D
6
m
2
+
2
m
2
-
1
2
[
meters
-
2
]
,
D being given in meters.
5. The solid optical diffuser according to claim 1 , wherein the maximum filling fraction is f≦10 −2 .
6. The solid optical diffuser according to claim 1 , moreover being shaped as a parallelepiped panel where the ratio between the largest dimension, L, and the smallest dimension, W, is L/W≧20.
7. The solid optical diffuser according to claim 1 , wherein the ratio between the blue and red scattering optical densities γ is comprised in the range 3.5≦γ≦5, and wherein:
along said first propagation direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 nm)≧0.6; and
along said second propagation direction, the monochromatic normalized collinear transmittance of the solid optical diffuser is T(450 nm)≦0.7.
8. The solid optical diffuser according to claim 1 , wherein said second propagation direction is the same as said first propagation direction.
9. The solid optical diffuser according to claim 1 , wherein said second propagation direction is orthogonal to said first propagation direction, and wherein along said second propagation direction the monochromatic normalized collinear transmittance of the solid optical diffuser is T(λ)≦0.5 for λ≦570 nm.
10. The solid optical diffuser according to claim 9 , wherein along said second propagation direction the monochromatic normalized collinear transmittance of the solid optical diffuser is T(λ)≦0.1 for λ≦570 nm.
11. The solid optical diffuser according to claim 9 , wherein the relative refraction index
m
≡
n
p
n
h
,
where n p is the refractive index of said nanoparticles and n h is the refractive index of said transparent solid matrix, falls in the range 0.7≦m≦2.1, and the effective particle diameter, D≡dn h , fulfills D[nm]≦132m+115 if 0.7≦m<1; D[nm]≦240 if 1<m<1.35 and D[nm]≦−135 m+507 if 1.35≦m≦2.1.
12. The solid optical diffuser according to claim 11 , wherein, along at least the first propagation direction, the number of nanoparticles per unit area is
N
≤
N
ma
x
=
3.7
×
10
-
28
D
6
m
2
+
2
m
2
-
1
2
[
meters
-
2
]
,
D being given in meters.
13. The solid optical diffuser according to claim 12 , wherein, along at least the second propagation direction, the number of nanoparticles per unit area is
N
≥
N
m
i
n
=
4.24
×
10
-
29
D
6
m
2
+
2
m
2
-
1
2
[
meters
-
2
]
,
D being given in meters.
14. The solid optical diffuser according to claim 9 , wherein the maximum filling fraction is f≦10 −3 .
15. The solid optical diffuser according to claim 9 , moreover being shaped as a parallelepiped panel where the ratio between the largest dimension, L, and the smallest dimension, W, is L/W≧10.
16. The solid optical diffuser according to claim 15 , configured to be side-lit by a light source so that the light generated by the light source is partially guided inside the parallelepiped panel by total internal reflection and partially scattered out of the parallelepiped panel because of the action of the nanoparticles dispersed in the panel.
17. An illumination system comprising a solid optical diffuser according to claim 9 and a light source, the solid optical diffuser and the light source being configured so that the light emitted by the light source is at least partially guided inside the solid optical diffuser.