Brightness enhancing film with embedded diffuser

1. An optical film, comprising:

a birefringent substrate;

a prismatic layer carried by the substrate, the prismatic layer having a major surface comprising a plurality of side by side linear prisms extending along a same prism direction; and

an embedded structured surface disposed between the substrate and the prismatic layer comprising closely-packed structures arranged such that ridges are formed between adjacent structures, the structures being limited in size along two orthogonal in-plane directions;

wherein the embedded structured surface has a topography characterizable by a first and second Fourier power spectrum associated with respective first and second orthogonal in-plane directions, and wherein

the first Fourier power spectrum includes one or more first frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a first baseline, each first frequency peak having a first peak ratio of less than 0.8, the first peak ratio being equal to an area between the first frequency peak and the first baseline divided by an area beneath the first frequency peak, and

the second Fourier power spectrum includes one or more second frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a second baseline, each second frequency peak having a second peak ratio of less than 0.8, the second peak ratio being equal to an area between the second frequency peak and the second baseline divided by an area beneath the second frequency peak; and

wherein the embedded structured surface is characterized by a total ridge length per unit area in plan view of less than 200 mm/mm 2 .

2. The optical film of claim 1 , wherein the embedded structured surface separates two optical media that differ in refractive index by at least 0.05.

3. The optical film of claim 1 , wherein the closely-packed structures are characterized by equivalent circular diameters (ECDs) in plan view, and wherein the structures have an average ECD of less than 15 microns.

4. The optical film of claim 1 , wherein the prism direction and one of the first and second orthogonal in-plane directions are the same.

5. The optical film of claim 1 , wherein at least some of the closely-packed structures comprise curved base surfaces.

6. An optical film, comprising:

a birefringent substrate;

a prismatic layer carried by the substrate, the prismatic layer having a major surface comprising a plurality of side by side linear prisms extending along a same prism direction; and

an embedded structured surface disposed between the substrate and the prismatic layer comprising closely-packed structures, the embedded structured surface defining a reference plane and a thickness direction perpendicular to the reference plane;

wherein the embedded structured surface has a topography characterizable by a first and second Fourier power spectrum associated with respective first and second orthogonal in-plane directions, and wherein

the first Fourier power spectrum includes one or more first frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a first baseline, each first frequency peak having a first peak ratio of less than 0.8, the first peak ratio being equal to an area between the first frequency peak and the first baseline divided by an area beneath the first frequency peak, and

the second Fourier power spectrum includes one or more second frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a second baseline, each second frequency peak having a second peak ratio of less than 0.8, the second peak ratio being equal to an area between the second frequency peak and the second baseline divided by an area beneath the second frequency peak; and

wherein the closely-packed structures are characterized by equivalent circular diameters (ECDs) in the reference plane and mean heights along the thickness direction and wherein an aspect ratio of each structure equals the mean height of the structure divided by the ECD of the structure; and

wherein an average aspect ratio of the structures is less than 0.15.

7. The optical film of claim 6 , wherein the embedded structured surface is characterized by a total ridge length per unit area in plan view of less than 200 mm/mm 2 .

8. The optical film of claim 6 , wherein at least some of the closely-packed structures comprise curved base surfaces.

9. The optical film of claim 6 , wherein the structures have an average ECD of less than 15 microns.

10. The optical film of claim 6 , wherein the structures have an average ECD of less than 10 microns.

11. An optical film, comprising:

a birefringent substrate;

a prismatic layer carried by the substrate, the prismatic layer having a major surface comprising a plurality of side by side linear prisms extending along a same prism direction; and

an embedded structured surface disposed between the substrate and the prismatic layer comprising closely-packed structures having curved base surfaces;

wherein the embedded structured surface has a topography characterizable by a first and second Fourier power spectrum associated with respective first and second orthogonal in-plane directions, and wherein

the first Fourier power spectrum includes one or more first frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a first baseline, each first frequency peak having a first peak ratio of less than 0.8, the first peak ratio being equal to an area between the first frequency peak and the first baseline divided by an area beneath the first frequency peak; and

the second Fourier power spectrum includes one or more second frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a second baseline, each second frequency peak having a second peak ratio of less than 0.8, the second peak ratio being equal to an area between the second frequency peak and the second baseline divided by an area beneath the second frequency peak; and

wherein the embedded structured surface provides an optical haze of less than 95%.

12. The optical film of claim 11 , wherein the embedded structured surface is characterized by a total ridge length per unit area in plan view of less than 200 mm/mm 2 .

13. The optical film of claim 11 , wherein the closely-packed structures are characterized by equivalent circular diameters (ECDs) in plan view, and wherein the structures have an average ECD of less than 15 microns.

14. The optical film of claim 13 , wherein the structures have an average ECD of less than 10 microns.

15. The optical film of claim 11 , wherein the embedded structured surface is characterized by a total ridge length per unit area in plan view of less than 200 mm/mm 2 .

16. An optical film, comprising:

a birefringent substrate;

a prismatic layer carried by the substrate, the prismatic layer having a major surface comprising a plurality of side by side linear prisms extending along a same prism direction; and

an embedded structured surface disposed between the substrate and the prismatic layer comprising closely-packed structures;

wherein the embedded structured surface has a topography characterizable by a first and second Fourier power spectrum associated with respective first and second orthogonal in-plane directions, and wherein

the first Fourier power spectrum includes one or more first frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a first baseline, each first frequency peak having a first peak ratio of less than 0.8, the first peak ratio being equal to an area between the first frequency peak and the first baseline divided by an area beneath the first frequency peak; and

the second Fourier power spectrum includes one or more second frequency peak not corresponding to zero frequency and being bounded by two adjacent valleys that define a second baseline, each second frequency peak having a second peak ratio of less than 0.8, the second peak ratio being equal to an area between the second frequency peak and the second baseline divided by an area beneath the second frequency peak; and

wherein the embedded structured surface provides an optical haze in a range from 10 to 60% and an optical clarity in a range from 10 to 40%.

17. The optical film of claim 16 , wherein the embedded structured surface is characterized by a total ridge length per unit area in plan view of less than 200 mm/mm 2 .

18. The optical film of claim 16 , wherein the closely-packed structures are characterized by equivalent circular diameters (ECDs) in plan view, and wherein the structures have an average ECD of less than 15 microns.

19. The optical film of claim 18 , wherein the structures have an average ECD of less than 10 microns.

20. The optical film of claim 16 , wherein the first peak ratio is less than 0.5 and the second peak ratio is less than 0.5.