Illuminating device and liquid crystal display device

1. An illuminating device comprising:

a light source;

a light guide plate which converts light emitted from the light source into a surface light ray and emits the surface light ray through a front surface of the light guide plate, the light guide plate having one end surface which the light enters, the light guide plate having the light transmitted therein in a direction from the one end surface; and

an optical sheet which changes a propagation direction of the surface light ray emitted from the light guide plate,

wherein the light source is disposed in opposition to the one end surface of the light guide plate,

wherein the light guide plate has birefringence in at least an in-plane direction of the front surface and comprises a transparent medium having a slow axis in parallel to a longitudinal direction of the one end surface of the light guide plate,

wherein a polarization state converting structure to convert a polarization state of the light propagating through the light guide plate is provided in a rear surface of the light guide plate, and

wherein the polarization state converting structure includes at least one first incline surface and at least one first decline surface, the first incline surface and the first decline surface meeting at a first ridge line extending in a direction perpendicular to the longitudinal direction of the one end surface of the light guide plate.

2. The illuminating device according to claim 1 ,

wherein the optical sheet includes a base material and a prism array containing a plurality of prisms which are provided in a surface of the base material facing the light guide plate or a different surface opposing the surface of the base material facing the light guide plate,

wherein each of the prisms has at least one second incline surface and at least one second decline surface, the second incline surface and the second decline surface meeting at a second ridge line extending in a direction parallel to the longitudinal direction of the one end surface in the light guide plate, and

wherein the base material includes a transparent medium which produces no phase difference for p-polarized light emitted from the light guide plate and incident into the base material at a predetermined incident angle.

3. The illuminating device according to claim 2 ,

wherein the transparent medium included in the base material has birefringence in an in-plane direction in which the prism array is provided.

4. The illuminating device according to claim 2 ,

wherein the transparent medium included in the base material has biaxial anisotropy.

5. The illuminating device according to claim 1 ,

wherein the optical sheet includes a base material, a prism array containing a plurality of prisms provided in a surface of the base material opposing the light guide plate, and an s-polarized light high reflecting layer provided in a surface of the base material facing the light guide plate,

wherein each of the prisms has at least one second incline surface and at least one second decline surface, the second incline surface and the second decline surface meeting at a second ridge line extending in a direction parallel to the longitudinal direction of the one end surface in the light guide plate,

wherein the base material includes a transparent medium which produces no phase difference for p-polarized light emitted from the light guide plate and incident into the base material at a predetermined incident angle, and

wherein the s-polarized light high reflecting layer includes a transparent medium which reflects an s-polarized light component of light emitted from the light guide plate and incident into the s-polarized light high reflecting layer at a predetermined incident angle in order to increase a percentage of p-polarized light component in light transmitted through the optical sheet.

6. The illuminating device according to claim 5 ,

wherein the s-polarized light high reflecting layer has a thickness varying dependent on a predetermined angle and contains a layer made of a transparent material having a refractive index higher than the refractive index of the base material in the optical sheet.

7. The illuminating device according to claim 1 ,

wherein the polarization state converting structure is configured so that the inclination plane reflects a s-polarized light which has an electric vector vibration vibrating along the longitudinal direction of the one end surface of the light guide plate.

8. The illuminating device according to claim 1 , further comprising:

a light extraction structure provided in the rear surface of the light guide plate,

wherein the light extraction structure comprises at least one second incline surface and at least one second decline surface, the second incline surface and the second decline surface meeting at a second ridge line extending in a direction parallel to the longitudinal direction of the one end surface of the light guide plate.

9. The illuminating device according to claim 8 ,

wherein among the rear surface of the light guide plate, an area where the polarization state converting structure is provided is different from an area where the light extraction structure is provided, and the light extraction structure comprises

any of fine steps, unevenness, lenticules or dot-printing with white pigments.

10. The illuminating device according to claim 9 ,

wherein the light extraction structure is a first light extraction structure, and the illuminating device further comprises a second light extraction structure, and

wherein the area where the polarization state converting structure is provided is between a first area having the first light extraction structure and a second area having a second light extraction structure.

11. The illuminating device according to claim 8 ,

wherein the polarization state converting structure is a first polarization state converting structure, and the rear surface of the light guide plate is further provided with a second polarization state converting structure to convert the polarization state of the light propagating through the light guide plate,

wherein the second polarization state converting structure comprises at least one third incline surface and at least one third decline surface, the third incline surface and the third decline surface meeting at a third ridge line extending in a direction perpendicular to the longitudinal direction of the one end surface of the light guide plate, and

wherein the light extraction structure separates the first ridge line and the third ridge line.

12. The illuminating device according to claim 11 , wherein the second ridge line is between the first ridge line and the third ridge line.

13. The illuminating device according to claim 11 , wherein the second ridge line extends across an entirety of the rear surface of the light guide plate.

14. The illuminating device according to claim 11 , wherein the first ridge line and the third ridge line are substantially aligned along the direction perpendicular to the longitudinal direction of the one end surface of the light guide plate.

15. A liquid crystal display device comprising:

a liquid crystal display panel;

the illuminating an illuminating device according to claim 1 ; and

a high refractive index layer formed on the front surface of the light guide plate, the high refractive index layer having a refractive index higher than a refractive index of the light guide plate,

wherein the liquid crystal display panel is arranged in the propagation direction of the surface light ray emitted from the illuminating device,

wherein the liquid crystal display panel includes a pair of polarizers arranged with a liquid crystal layer interposed between the pair of polarizers, and

wherein one of the pair of polarizers arranged closer to the illuminating device has an absorption axis which is perpendicular to the direction in which the light is transmitted in the light guide plate.