Currently, liquid crystal display devices (hereinafter also referred to as “LCD”) that are most commonly used are twisted-nematic type LCDs (hereinafter also referred to as “TN-LCD”) of normally white (hereinafter also referred to as “NW”) mode. This NW mode TN-LCD has a liquid crystal cell comprises a nematic liquid crystal sandwiched between a pair of glass substrates forming transparent electrodes. The nematic liquid crystal has such an aligned structure that it has a helical axis in the normal line direction of the glass substrates and the twisted angle is about 90°. The liquid crystal cell is sandwiched between a pair of linear polarizers disposed so that their absorption axes are orthogonal to each other. When no electric voltage is applied to the NW mode TN-LCD, it operates in a white state because linearly polarized incident light is rotated by 90° due to the optical rotation characteristics of the liquid crystal cell and then exits. When an electric voltage is applied, liquid crystal molecules arise with respect to the glass substrates, and the optical rotation characteristics are lost. As the result, since linearly polarized incident light passes through the cell as it is, the NW mode TN-LCD operates in a black state. The use of the white and black states and an intermediate state therebetween enables gradation displaying.
However, the nematic liquid crystal used in the LCD has a rod-like molecular structure and exhibits a positive refractive index anisotropy having a large refractive index in the direction of the molecular axis. Therefore, the polarization state of light obliquely passing through the LCD changes in a direction different from the normal line direction thereof due to the retardation caused by the refractive index anisotropy of the liquid crystal. Accordingly, when the image produced by the LCD is viewed at an angle deviating from the normal line direction thereof, the LCD exhibits unfavorable viewing angle characteristics, that is phenomena such as reduced contrast and inverted gradation displaying. For improving viewing angle characteristics, a significant effect therefore can be achieved by improving the viewing angle characteristics during black displaying, i.e., when an electric voltage is applied. When an electric voltage is applied, the liquid crystal molecules are aligned nearly vertically with respect to the glass substrates. Therefore, the aligned molecules are regarded as exhibiting a positive refractive index anisotropy having an optical axis in the normal line direction of the glass substrates, and methods for compensating for such an anisotropy have been reported in which to use a retardation film having an optical axis in the normal line direction thereof and a negative refractive index anisotropy (see Patent Document Nos. 1 and 2 below). However, since in an actual LCD, the liquid crystal molecules near the glass substrates remain tilted due to the binding force of the alignment films formed thereon even when an electric voltage is applied, it is difficult to compensate sufficiently for the polarization state caused by the liquid crystal cell only using the retardation film having an optical axis in the normal line direction thereof and a negative refractive index anisotropy.
Methods have also been proposed in which to use a liquid crystal film of disc-shaped liquid crystal molecules with an optical axis tilted from the normal line direction thereof and a negative refractive index anisotropy in order to also compensate for the above-described tilted liquid crystal molecules (see Patent Document Nos. 3 and 4 below). However, although these methods can improve the compensation for the liquid crystal cell when applied with an electric voltage, it is costly to synthesize and form disc-shaped liquid crystal molecules into a film, and compensation becomes insufficient in an intermediate gradation displaying state that is a more complex aligned configuration wherein the number of the tilted liquid crystal portion increases, due to a lower applied electric voltage. It is thus difficult to improve the inverted gradation phenomenon while the viewing angle characteristics of display contrast can be improved. As the result, only insufficient viewing angle characteristics can be obtained.
As alternative methods, it has been reported that the viewing angle characteristics can also be improved using a retardation plate having an optical axis tilted from the normal line direction thereof and still having the same positive refractive index anisotropy as the liquid crystal does (see, Patent Document Nos. 5, 6 and 7).
Although these reports describe that the use of a retardation plate having a positive refractive index anisotropy and an optical axis tilted from the normal line direction thereof is effective in restraining the gradation inversion of a liquid crystal cell, the methods described in these reports can only provide insufficient viewing angle characteristics for those of display contrast.
Patent Document No. 8 proposes a liquid crystal display device with the combination of a retardation plate wherein liquid crystal molecules having a positive refractive index anisotropy are obliquely aligned and a uniaxial film, which device comprises a polarizer/an obliquely aligned film/a uniaxial film/a TN liquid crystal cell/a uniaxial film/an obliquely aligned film/a polarizer. However, this configuration is not enough to completely compensate for the viewing angle of display contrast and can only provide for incomplete viewing angle characteristics.
As described above, with regard to the optical compensation plate used for improving the viewing angle characteristics of an TN-LCD, no optical compensation plate has not been found yet which can drastically improve viewing angle characteristics including not only display contrast but also gradation inversion. Therefore, further improvements have been required for the optical compensation plate.
(1) Patent Document 1: Japanese Laid-Open Patent Publication No. 2-015239
(2) Patent Document 2: Japanese Laid-Open Patent Publication No. 3-103823
(3) Patent Document 3: Japanese Laid-Open Patent Publication No. 63-239421
(4) Patent Document 4: Japanese Laid-Open Patent Publication No. 6-214116
(5) Patent Document 5: Japanese Laid-Open Patent Publication No. 5-080323
(6) Patent Document 6: Japanese Laid-Open Patent Publication No. 7-306406
(7) Patent Document 7: WO96/10773
(8) Patent Document 8: Japanese Laid-Open Patent Publication No. 10-123506