Optical element, light condensation backlight system, and liquid crystal display

• US 7,746,421 B2
• Filed: 08/24/2004
• Issued: 06/29/2010
• Est. Priority Date: 10/23/2003
• Status: Active Grant

• Current Assignee: Nitto Denko Corporation
• Original Assignee: Nitto Denko Corporation
• Inventors: Hara, Kazutaka, Takahashi, Naoki, Takeda, Kentarou, Shiraogawa, Miki
• Primary Examiner(s): RUDE, TIMOTHY L
• Application Number:
  US10/576,614
• Publication Number:
  US 20070132914A1
• Time in Patent Office: 2,135 Days
• Field of Search: None
• US Class Current: 349/62
• CPC Class Codes: G02B 27/28 for polarising used in stereoscopes G02B30/25
• CPC Class Codes: G02B 5/3016 involving passive liquid crystal elements optical properties of liquid crystals G02F1/0063; polarising elements associated with active liquid crystal devices G02F1/133528
• CPC Class Codes: G02B 6/0056 for producing polarisation effects, e.g. by a surface with polarizing properties or by an additional polarizing elements
• CPC Class Codes: G02F 1/133536 Reflective polarizers G02F1/13362 takes precedence
• CPC Class Codes: G02F 1/133543 Cholesteric polarisers
• CPC Class Codes: G02F 1/13362 providing polarized light, e.g. by converting a polarisation component into another one

Abstract

An optical element comprising: a polarizing element (A), separating incident light into polarization to then emit light, and made of a cholesteric liquid crystal, and a linearly polarized light reflection polarizer (B) transmitting linearly polarized light with one polarization axis and selectively reflecting linearly polarized light with the other polarization axis perpendicular to the one polarization axis, wherein the polarizing element (A) has a distortion rate with respect to emitting light to incident light in the normal direction of 0.5 or more and a distortion rate with respect to emitting light to incident light at an angle inclined from the normal direction by 60 degrees or more of 0.2 or less, the polarizing element (A) has a function increasing a linearly polarized light component of emitting light as incidence angle is larger; is capable of condensation and collimation of incident light from a light source and capable of suppressing transmission of light in an arbitrary direction.

Claims

1. An optical element comprising:

a polarizing element (A), separating incident light into polarization to then emit light, and made of a cholesteric liquid crystal, and

a linearly polarized light reflection polarizer (B) transmitting linearly polarized light with one polarization axis and selectively reflecting linearly polarized light with the other polarization axis perpendicular to the one polarization axis, wherein

the polarizing element (A) and the linearly polarized light reflection polarizer (B) are laminated in the optical element; and

the polarizing element (A) has a distortion rate with respect to emitting light to incident light in the normal direction of 0.5 or more and

a distortion rate with respect to emitting light to incident light at an angle inclined from the normal direction by 60 degrees or more of 0.2 or less, and

wherein a reflection band width of the polarizing element (A) is 200 nm or more, and

a twist pitch length of a cholestric liquid crystal in the polarizing element (A) varies along the thickness direction;

wherein the polarizing element (A) is capable of increasing a linearly polarized light component of emitting light as incidence angle increases, and

wherein a polarization axis of the linearly polarized light of emitting light is substantially perpendicular or substantially parallel to the normal direction of a surface of the polarizing element.

2. The optical element according to claim 1, wherein, in the polarizing element (A), the linearly polarized light component of emitting light increasing as incidence angle increases has a polarization axis of linearly polarized light substantially perpendicular to the normal direction of a surface of the polarizing element.

3. The optical element according to claim 1, wherein, in the polarizing element (A), the linearly polarized light component of emitting light increasing as incidence angle increases has a polarization axis of linearly polarized light substantially parallel to the normal direction of a surface of the polarizing element.

4. The optical element according to claim 1, wherein the polarizing element (A) substantially reflects a non-transmission component of incident light.

5. The optical element according to claim 1, wherein a thickness of the polarizing element (A) is 2 μ

m or more.

6. The optical element according to claim 1, wherein the linearly polarized light reflection polarizer (B) is a grid type polarizer.

7. The optical element according to claim 1, wherein the linearly polarized light reflection polarizer (B) is a multilayer thin film laminate with two or more layers made of two or more kinds of materials having a difference between refractive indices.

8. The optical element according to claim 7, wherein the thin multilayer laminate is a vapor-deposited thin film.

9. The optical element according to claim 1, wherein the linearly polarized light reflection polarizer (B) is a multi-birefringence layer thin film laminate with two or more layers made of two or more kinds of materials each having a birefringence.

10. The optical element according to claim 9, wherein the thin multilayer laminate is a stretched resin laminate with two or more layers containing two or more kinds of resins each having a birefringence.

11. An optical element comprising a ½

wavelength plate (C) sandwiched between the polarizing element (A) and the linearly polarized light reflection polarizer (B) in the optical element according to claim 1.

12. The optical element according to claim 11, wherein the ½

wavelength plate (C) is a broad band wavelength plate working as an almost ½

wavelength plate over the entire visible light band.

13. The optical element according to claim 12, wherein the ½

wavelength plate (C) has a front retardation values, which is expressed by (nx−

ny)×

d, in the range of a ½

wavelength ±

10% at wavelengths in the light source wavelength band (ranging from 420 to 650 nm),

where a direction in which an in-plane refractive index is maximized is defined as X axis and a direction perpendicular to the X axis is defined as Y axis, where refractive indices in each axis directions are defined as nx and ny, respectively, and a thickness is defined as d (nm).

14. The optical element according to claim 11, wherein the ½

wavelength plate (C) controls a retardation in the thickness direction and reduces a change in retardation caused by a change in angle.

15. The optical element according to claim 14, wherein the ½

wavelength plate (C) has an Nz coefficient, which is expressed by Nz=(nx−

nz)/(nx−

ny), in a relation of −

2.5<

Nz≦

1,

where a direction in which an in-plane refractive index is maximized is defined as X axis, a direction perpendicular to the X axis is defined as Y axis and a thickness direction of the film is defined as Z axis, where refractive indices in each axis directions are defined as nx, ny and nz.

16. The optical element according to claim 1, wherein a polarizing plate is disposed outside of the linearly polarized light reflection polarizer (B) so that the polarized light transmission axis of the linearly polarized light reflection polarizer (B) and the polarization axis direction of the polarizing plate coincide with each other.

17. The optical element according to claim 1, wherein layers are laminated with a transparent adhesive or pressure sensitive adhesive.

18. A light condensation backlight system, in which at least a light source is provided for the optical element according to claim 1.

19. The light condensation backlight system according to claim 18, comprising a primary light condensor wherein a condensed light is in the angular range of ±

60 degrees from the normal direction.

20. The light condensation backlight system according to claim 19, wherein the primary light condensor is a microprism sheet array disposed on the light source.

21. A light condensation backlight system, in which at least a light source is provided for the optical element according to claim 11.

22. The light condensation backlight system according to claim 21, comprising a primary light condensor wherein in condensed light is in the angular range of ±

60 degrees from the normal direction.

23. The light condensation backlight system according to claim 22, wherein the primary light condensor is a microprism sheet array disposed on the light source.

24. A light condensation backlight system, in which at least a light source is provided for the optical element according to claim 16.

25. The light condensation backlight system according to claim 24, comprising a primary light condensor wherein in condensed light is in the angular range of ±

60 degrees from the normal direction.

26. The light condensation backlight system according to claim 25, wherein the primary light condensor is a microprism sheet array disposed on the light source.

27. A liquid crystal display, in which at least a liquid crystal cell is provided for the light condensation backlight system according to claim 25.

28. The liquid crystal display according to claim 27, comprising a diffusing plate neither backscattering nor depolarizing laminated on the viewing side of the liquid crystal cell.

29. The optical element according to claim 1, wherein the laminated optical element has more than one cholesteric liquid crystal layer wherein each has a central wavelength that is different from each other.

30. The optical element according to claim 1, wherein the twist pitch length continuously alters in the thickness length.