Collimator comprising a prismatic layer stack, and lighting unit comprising such collimator

1. A collimator comprising a prismatic film stack, with the prismatic film stack comprising:

a first flexible prismatic film comprising 1D arranged first prisms with first truncated prism tops having first prism top angles (α 1 ), and first grooves having first groove angles (β 1 ); and

a second flexible prismatic film resting directly on the first truncated prism tops and comprising 1D arranged second prisms, with second prism tops having second prism top angles (α 2 ), and second grooves, having second groove angles (β 2 );

wherein the first primsatic film and the second prismatic film are in a crossed configuration, the first and second prism tops of the first and the second primatic films point in the same direction, and wherein the first and second prism top angles (α 1 , α 2 ) and the first and second groove angles (β 1 , β 2 ) are selected from the range of 120-160°; and

wherein at least one of the first and second groove are truncated to form a flat top portion.

2. The collimator according to claim 1 , wherein the prism top angles (α 1 , α 2 ) and the groove angles (β 1 , β 2 ) are selected from the range of 130-150°.

3. The collimator according to claim 1 , wherein the first truncated prism tops having shortest truncated prism top distances (dt 1 ) between two adjacent truncated first prism tops, wherein the first prismatic film has a first prismatic film pitch (p 1 ), and wherein the first prismatic film has a ratio of the first prismatic film pitch (p 1 ) and shortest truncated prism top distances (dt 1 ) of 1<p 1 /dt 1 ≦2.

4. The collimator according to claim 1 , wherein the second prism tops are truncated and have shortest truncated prism top distances (dt 2 ) between two adjacent truncated second prism tops, wherein the second prismatic film has a second prismatic layer pitch (p 2 ), and wherein the second prismatic film has a ratio of the second prismatic layer pitch (p 2 ) and shortest truncated prism top distances (dt 2 ) of 1<p 2 /dt 2 ≦2.

5. The collimator according to claim 1 , wherein the first grooves are truncated, wherein the first prisms have base widths (dg 1 ) between two adjacent truncated first grooves, wherein the first prismatic film has a first prismatic film pitch (p 1 ), and wherein the first prismatic film has a ratio of the first prismatic film pitch (p 1 ) and base widths (dg 1 ) of 1p 1 /dt 1 ≦2.

6. The collimator according to claim 1 , wherein the second grooves are truncated, wherein the second prisms have base widths (dg 2 ) between two adjacent truncated second grooves, wherein the second prismatic film has a second prismatic layer pitch (p 2 ), and wherein the second prismatic film has a ratio of the second prismatic film pitch (p 2 ) and base widths (dg 2 ) of 1<p 2 /dg 2 ≦2.

7. The collimator according to claim 1 , wherein the stack further comprises an optical layer arranged downstream of the second prismatic film.

8. The collimator according to claim 1 , wherein the stack further comprises a diffuser layer arranged upstream of the first prismatic film.

9. The collimator according to claim 1 , wherein the first prisms have first longitudinal axes and the second prisms have second longitudinal axes, and wherein the first longitudinal axes and the second longitudinal axes each have mutual angles in the range of 80-100°.

10. The collimator according to claim 1 , wherein the first prismatic film has a first prismatic film pitch (p 1 ) and wherein the second prismatic film has a second prismatic film pitch (p 2 ), and wherein the first and the second prismatic film pitches (p 1 , p 2 ) are selected from the range of 5-1000 μm.

11. The collimator ( 1 ) according to claim 10 , wherein the first and the second prismatic film pitches (p 1 , p 2 ) are selected form the range of 5-200 μm.

12. A collimator comprising a prismatic optical layer stack, with the optical layer stack comprising:

a first prismatic layer comprising arranged first prisms, with first truncated prism tops having first prism top angles and first grooves having first groove angles;

a second prismatic layer resting directly on the first truncated prism tops and comprising arranged second prisms, with second prism tops having second prism top angles and second grooves having second groove angles;

the first prismatic layer and the second prismatic layer in a crossed configuration;

the first and second prism tops of the first and the second prismatic layers point in the same direction;

wherein the first and second prism top angles and the first and second groove angles are selected from the range of 120-160°;

wherein the first and second grooves and second prism tops are truncated to form a flat top portion; and

wherein each of the first and second prismatic layers is flexible and approximately 50-200 μm thick.

13. A collimator comprising a prismatic optical layer stack, with the optical layer stack comprising:

a first prismatic layer comprising arranged first prisms with first truncated prism tops having first prism top angles and first grooves having first groove angles;

a second prismatic layer resting directly on the first truncated prism tops and comprising arranged second prisms, with second prism tops having second prism top angles and second grooves having second groove angles;

the first prismatic layer and the second prismatic layer in a crossed configuration;

the first and second prism tops of the first and the second prismatic layers point in the same direction;

wherein the first and second prism top angles and the first and second groove angles are selected from the range of 120-160°;

wherein at least one of the first and second grooves are truncated to form a flat top portion; and

wherein each of the first and second prismatic layers is flexible and is approximately 50-200 μm thick.