Optical sheet comprising a base composed of a stack of transmissive sheets, and a back-light device and liquid crystal device incorporating the optical sheet

1. An optical sheet comprising:

a light-transmissive base; and

an optical functional layer provided on a light-emitting surface of the transmissive base,

wherein the base is composed of a stack of transmissive sheets bonded while placing an adhesive material layer in between,

wherein the optical functional layer is a beam-condensing layer having a plurality of irregularity portions consecutively arranged on a surface thereof, and

wherein the optical sheet is a beam-condensing sheet which improves directionality of an irradiated light.

2. The optical sheet as claimed in claim 1 , wherein the optical sheet is a prism sheet and each of the irregularity portions is a prism having a triangular cross-sectional geometry.

3. The optical sheet as claimed in claim 1 , wherein the adhesive material layer is composed of any one of ultraviolet-curing adhesive, pressure-sensitive adhesive and hot-melt adhesive.

4. The optical sheet as claimed in claim 1 , wherein the irregularity portions show different refractive indices in a direction of ridges and in a direction of arrangement.

5. The optical sheet as claimed in claim 1 , wherein the stack comprises:

a first sheet base supporting the optical functional layer; and

a second sheet base opposed to the first sheet base while placing the adhesive material layer in between.

6. The optical sheet as claimed in claim 5 , wherein at least either one of the first sheet base and the second sheet base comprises an ultraviolet absorber.

7. The optical sheet as claimed in claim 5 , wherein a refractive index of the adhesive material layer is equivalent to, or smaller than refractive indices of the first and second sheet bases.

8. The optical sheet as claimed in claim 5 , wherein the adhesive material layer comprises a blue dye.

9. The optical sheet as claimed in claim 1 , wherein a pitch of arrangement of the irregularity portions is 110 μm or larger.

10. The optical sheet as claimed in claim 1 , wherein a pitch of arrangement of said irregularity portions is 110 μm or more and 350 μm or less.

11. A liquid crystal display device having a liquid crystal display panel, a light source disposed on a back surface side of the liquid crystal display panel, and an optical sheet disposed between the liquid crystal display panel and the light source, wherein:

the optical sheet includes:

a light-transmissive base; and

an optical functional layer provided on a light-emitting surface of the base, wherein:

the base is composed of a stack of transmissive sheets bonded while placing an adhesive material layer in between,

the optical functional layer is a beam-condensing layer having a plurality of irregularity portions consecutively arranged on a surface thereof, and

the optical sheet is a beam-condensing sheet which improves directionality of an irradiated light.

12. The optical sheet as claimed in claim 5 , wherein a surface of the second sheet base opposite to a surface facing the first sheet base is roughened.

13. The optical sheet as claimed in claim 12 , wherein the roughened surface comprises projections having a height of 0.20 μm or greater above the average center level.

14. The optical sheet as claimed in claim 13 , wherein the density of projections is between 70/mm 2 and 400/mm 2 .

15. The optical sheet as claimed in claim 5 , wherein the first and second sheet bases are composed of the same material.

16. The optical sheet as claimed in claim 1 , wherein an angle formed between directions of molecular alignment of the transmissive sheets is 20° or less.