Patent FR-2789186-A1 (FR)
Title: Liquid crystal screen device with wider horizontal viewing angle, comprising additional optical layers made of biaxial birefringent and uniaxial positive materials
Inventor: BIGNOLLES LAURENT
IPC: G02F1/13363
Filed: 1999-02-02
Abstract: The display device comprises a layer of nematic helical liquid crystal (40) placed between cross polarisers (46,48), and a set of layers in pairs for the compensation of contrast variations as a function of the angle of observation. The compensation structure comprises two pairs of superposed uniaxial negative films such that the optical axis of one pair (42,44) is perpendicular to the plane of the cell (XY), and that of the other pair (41,43) is inclined with respect to the normal, and to the plane of cell. The compensation structure comprises additional layers in pairs, the first additional pair (54,56) made of biaxial birefringent material, and the second additional pair (55,57) made of uniaxial positive material. The optical axis of the uniaxial positive material of the second additional pair (55,57) is substantially perpendicular to the plane (XY) of cell. The biaxial material of the first additional pair (54,56) in the plane of cell presents slow axes (L54, L56), which are perpendicular to the orientation of molecules in the liquid crystal layer (40) on the near sides. The delay introduced by the second pair depends on the delay of the first pair and the coefficient of biaxialilty.  The biaxial layer (54) introduces a first optical delay in the direction of the normal (Z) to the plane (XY) of cell, which is in the range 15-60 nanometer, preferentially 20-40 nanometer, and the coefficient of biaxiality is greater than 4. The uniaxial positive material of the second additional layer (55) has an extraordinary index of refraction (ne) and an ordinary index (no), such that (ne) is greater than (no), and the introduced second delay is greater than 100 nanometer. The uniaxial positive material of the second additional layer (55) has a range of optical index such that the second delay depends on the first delay and the coefficient of biaxialilty of the first additional layer (54). The pair of first additional layers (54,56) present an index of refraction (nz) in the direction of normal to the plane (XY) of cell with principal indices (nx, ny), and the coefficient of biaxiality is given by: Nz=(nx-nz)/(nx-ny). The first and second additional layers (54,55) are situated between the liquid crystal layer (40) and superposed uniaxial negative layers (42,41). The second uniaxial layer (55) is situated between the liquid crystal layer (40) and the first biaxial layer (54). The device comprises two sets of compensation layers (42,41,54,55) and (44,43,56,57) situated between polarisers (46,48) on each side of the liquid crystal layer (40). The liquid crystal layer (40) has helical structure corresponding to a torsion of 90 deg. The passing direction of each polariser (46,48) is crossed with the near direction of alignment (A51,A53) of the liquid crystal layer (40). The inclination of optical axis of the uniaxial material of the second additional layer (55,57) with respect to the normal to the plane of cell is less than 10 deg. and preferentially perpendicular to the plane (XY).