Liquid crystal display device with retardation plates
View Patent ↗A liquid crystal display device includes an OCB mode liquid crystal display panel, and an optical compensation element which is disposed outside of the liquid crystal display layer. The optical compensation element includes a polarizer plate, a first retardation plate which is disposed between the polarizer plate and the liquid crystal layer, and a second retardation plate which is disposed between the polarizer plate and the first retardation plate and has a biaxial refractive index anisotropy. The optical compensation element compensates a difference of a polarization state that differs between azimuth directions of light passing through the liquid crystal layer and compensates a shift of the polarization state of light, which passes through the retardation plate, from an azimuth direction of an absorption axis of the polarizer plate.
1. A liquid crystal display device comprising:
an OCB mode liquid crystal display panel comprising a first substrate including a first alignment film, a second substrate including a second alignment film, and a liquid crystal layer which is held between the first alignment film and the second alignment film, a rubbing direction of the first alignment film and the second alignment film agrees with a reference azimuth direction; and
a first optical compensation element which is disposed outside the first substrate and a second optical compensation element which is disposed outside the first substrate, the first optical compensation element and the second optical compensation element optically compensating a retardation of the liquid crystal layer in a predetermined display state in which a voltage is applied to the liquid crystal layer;
wherein the first optical compensation element includes: a first polarizer plate having a first absorption axis which intersects at 45° with respect to the reference azimuth direction, a first retardation plate which is disposed between the first polarizer plate and the liquid crystal display panel, includes a first slow axis parallel with the reference azimuth direction, and imparts a phase difference of a ¼ wavelength; a second retardation plate which is disposed between the first polarizer plate and the first retardation plate, includes a second slow axis parallel with the first absorption axis, and has a biaxial refractive index anisotropy; a third retardation plate corresponding to an A-plate which is disposed between the first retardation plate and the liquid crystal display panel and includes a third slow axis perpendicular to the reference azimuth direction, and a fourth retardation plate corresponding to a C-plate which is disposed between the first retardation plate and the third retardation plate;
the second optical compensation element includes: a second polarizer plate having a second absorption axis which intersects at 135° with respect to the reference azimuth direction; a fifth retardation plate which is disposed between the second polarizer plate and the liquid crystal display panel, includes a fifth slow axis perpendicular to the reference azimuth direction, and imparts a phase difference of a ¼ wavelength; a sixth retardation plate which is disposed between the second polarizer plate and the fifth retardation plate, includes a sixth slow axis perpendicular to the second absorption axis, and has a biaxial refractive index anisotropy; a seventh retardation plate corresponding to an A-plate which is disposed between the fifth retardation plate and the liquid crystal display panel and includes a seventh slow axis perpendicular to the reference azimuth direction; and an eighth retardation plate corresponding to a C-plate which is disposed between the fifth retardation plate and the seventh retardation plate, and
the second retardation plate has the refractive index anisotropy which is set in such a manner as to compensate (a) a difference of a polarization state due to an influence of optical rotary power that differs between azimuth directions of light passing through the liquid crystal layer and to compensate (b) a shift of the polarization state of light, which passes through the first retardation plate, from an azimuth direction of the first absorption axis of the first polarizer plate.
2. The liquid crystal display device according to claim 1 , wherein an Nz coefficient in the second retardation plate is set in a range of between 0.7 and 0.9, the Nz coefficient being given by
Nz =( nx−nz )/( nx−ny ),
wherein nx and ny are refractive indices in mutually perpendicular directions in a plane of the second retardation plate, and nz is a refractive index in a normal direction to the second retardation plate.
3. The liquid crystal display device according to claim 2 , wherein an Nz coefficient in the sixth retardation plate is set in a range of between 0.15 and 0.3, the Nz coefficient being given by
Nz =( nx−nz )/( nx−ny ),
wherein nx and ny are refractive indices in mutually perpendicular directions in a plane of the sixth retardation plate, and nz is a refractive index in a normal direction to the sixth retardation plate.