Substrate film for opticla sheet, optical sheet and backlight unit

Abstract

An object of the present invention is to provide a substrate film for an optical sheet, an optical sheet and a backlight unit capable of markedly enhancing utilization efficiency of rays of light and improving luminance. The present invention is characterized by being a substrate film for an optical sheet which is a made of a transparent resin and which is formed into a rectangular shape, which substrate film for the optical sheet having an optical anisotropy, and having an absolute value of the angle of the crystal orientation with respect to the short side orientation being π/16 or greater and 3π/16 or less. The crystal orientation preferably follows the phase advancing axial orientation. The substrate film for the optical sheet has a retardation value such that average intensity of transmitted beam I A becomes equal to or greater than 0.75 so as to be capable of converting polarization orientation of outgoing rays of light of the optical waveguide plate or the like to the transmission axial orientation of the polarizing plate or the like. The retardation value is preferably 140 nm or greater and 390 nm or less. The transparent resin is preferably polyethylene terephthalate or polycarbonate. The optical sheet of the present invention has the substrate film for the optical sheet, and an optical layer.

What is claimed is: 
     
         1 . An edge light type back light unit for use in a liquid crystal display device in which rays of light are dispersed and lead to a front face side, the edge light type back light comprising:
 a rectangular optical waveguide plate;   a lamp disposed along an edge of a longer side of the optical waveguide plate and that emits said rays of light; and   a light diffusion sheet formed into a rectangular shape, provided with the lamp along an edge of a longer side thereof, and overlaid on a surface of the optical waveguide plate,   the light diffusion sheet comprising:
 a substrate film for the light diffusion sheet which is made of a transparent resin and which is formed into a rectangular shape by punching from a stretched original film, the substrate film for the light diffusion sheet having an optical anisotropy, and having an absolute value of the angle of the crystal orientation with respect to a short side orientation being greater than or equal to π/16 and less than or equal to 3π/16; and 
 an optical layer overlaid on one face of the substrate film for the light diffusion sheet; and 
   wherein the transparent resin of the substrate film is polyethylene terephthalate; and   wherein a retardation value of the substrate film is greater than 680 nm.   
     
     
         2 . The edge light type back light according to claim  1 , wherein
 the optical layer comprises multiple particles of a light diffusing agent and a binder therefor, and is formed by a polymer composition coating, and   an amount of the light diffusing agent, included per 100 parts of substrate polymer in the polymer composition coating, is 10 parts or greater and 200 parts or less as determined on a basis of solid content, and   the multiple particles of the light diffusing agent are spherical beads.   
     
     
         3 . The edge light type back light according to  claim 1 , wherein
 provided that a polarizer P is disposed on one face side of the substrate film and an analyzer Q is disposed on another face side of the same, and the linear polarized light that passed through the polarizer P (amplitude=1, intensity of transmitted beam=1) transmits the substrate film and then pass through the analyzer Q,   when the phase advancing axial orientation and phase retarding axial orientation of crystallographic axis of the substrate film front face is designated as x orientation and y orientation; the thickness of the substrate film is designated as d, refractive index in the x orientation and the y orientation is designated as nx and ny (nx≠ny); angle between the crystal orientation and the transmission axial orientation of the polarizer P is designated as ψ; and angle between the transmission axial orientation of the polarizer P and the transmission axial orientation of the analyzer Q is designated as β,   displacement of the light passed through the analyzer Q is represented by the following mathematical formula (1); intensity of transmitted beam is represented by the following mathematical formula (2); and average intensity of transmitted beams I A  at three wavelengths of RGB is represented by the following mathematical formula (3):
     E ( d,t )=sin φ cos Θ( d,t )sin(β+φ)+cos φ cos {Θ( d,t )+δ( d )} cos(β+φ)  (1)
 
     I =(1/ To )∫ 0   To   E   2   dt   (2)
 
     I   A =( I   1   +I   2   +I   3 )/3  (3)
 
   wherein, I is wavelength; c is light velocity; t is time; k is 2π/λ; Θ(d,t) is k(nxd−ct); and δ(d) is k(ny−nx)d. I 1 , I 2  and I 3  are intensity of transmitted beam of the light passed through the analyzer Q at a wavelength of the incident light of the polarizer P of I 1 : 430 nm, I 2 : 550 nm, I 3 : 610 nm, respectively,   and provided that the polarizer P and the analyzer Q are arranged to give the angles of β being π/4 and ψ being −(β/2),   the sheet has a retardation value such that average intensity of transmitted beam I A  derived from the above mathematical formula (3) becomes equal to or greater than 0.75,   wherein the retardation value Re is derived from the formula of: Re=(ny−nx)d.   
     
     
         4 . The edge light type back light according to  claim 1 , wherein a sticking preventive layer including beads dispersed in a binder is provided on another face of the substrate film for the light diffusion sheet. 
     
     
         5 . A production method of a light diffusion sheet, the method comprising:
 producing a substrate film for the light diffusion sheet by punching from a stretched original film to give a rectangular shape; and   forming an optical layer by overlaying and curing a polymer composition for the optical layer on a front face of the substrate film for the light diffusion sheet produced,   wherein the substrate film for the light diffusion sheet produced has an optical anisotropy and has an absolute value of the angle of the crystal orientation with respect to a short side orientation being greater than or equal to π/16 and less than or equal to 3π/16.   
     
     
         6 . A production method of an edge light type back light unit for use in a liquid crystal display device in which rays of light are dispersed and lead to a front face side, the edge light type back light comprising:
 a rectangular optical waveguide plate;   a lamp disposed along an edge of a longer side of the optical waveguide plate and that emits said rays of light; and   alight diffusion sheet formed into a rectangular shape, provided with the lamp along an edge of a longer side thereof, and overlaid on a surface of the optical waveguide plate   the method comprising using as the light diffusion sheet, a light diffusion sheet produced by the production method according to  claim 5 .