The present invention relates to an optically compensated bend (OCB) liquid crystal display device used for the display of liquid crystal display televisions, liquid crystal display monitors, etc.
In regard to liquid crystal display devices, though the twisted nematic (TN) type liquid crystal display device has been generally used so far, the OCB type display device has been studied as a liquid crystal display device characterized in higher response (Refer to Bulletin ‘SHINGAKUGIHOU’ EDI98-144, page 199; Institute of Telecommunications Engineers, NIKKEI FLAT PANEL DISPLAY, Strategy Edition 2005, page 110).
The liquid crystal is interposed between two substrates in the OCB type crystal display device, and transparent electrodes are formed on these substrates as the voltage supplying means. On the condition before a power source is turned on, the alignment state of the liquid crystal is the state called ‘spray alignment’. When the power source is turned on, a relatively high voltage is applied to the voltage supplying means for a short time so as to transfer the alignment of the liquid crystal to the bend alignment state. It is a feature of the OCB type liquid crystal display mode that the display is carried out by means of the bend alignment state.
An aspect of the alignment of the liquid crystal will be explained by the aid of FIG. 6. As shown in FIG. 6A, liquid crystal molecules 53 of a liquid crystal layer are in the spray alignment state when the voltage V between a pixel electrode 51 and a counter electrode 52 supplied from a power source 50 is zero. Directions of orientation for upper and lower substrates 55 and 54 are the same, i.e. the parallel direction as shown by the arrows 56 and 57. Next, upon supplying a predetermined voltage, e.g. a high voltage of several ten volts between both the electrodes, the state of molecules shifts to the bend alignment state where liquid crystal molecules at the center of the liquid crystal layer stand up perpendicularly like FIG. 6B. When the voltage that can keep the liquid crystal molecules of FIG. 6B being in the bend alignment state is called ‘the counterbalanced voltage VC’, the spray alignment state is stabilized at any voltage not higher than this voltage VC. In the OCB operation, a voltage higher than the counterbalanced voltage VC (off-voltage) at which the bend alignment state is kept steady is applied to the liquid crystal molecules as shown in FIG. 6B. Upon changing the voltage between the off-voltage and an on-voltage VON higher than VC, the state of inclination of the liquid crystal molecules is changed between FIG. 6B and FIG. 6C so that the retardation value of the liquid crystal layer may be changed in order to control the transmittance in combination with polarization films 58 and 59. In addition, there is an example in which a voltage higher than VC is periodically supplied, i.e. the black insertion drive is carried out.
On the condition that the direction perpendicular to the paper surface is the horizontal surface viewed from the direction of the arrow 60 on the paper surface, observation indicates that alignment of the liquid crystal molecules is in a symmetrical relationship with respect to the horizontal direction, so that display thereof has relatively balanced visibility in the longitudinal direction. To this end, it is considered that the orientation directions 56 and 57 of both the substrates are conformed together in the vertical direction of the display surface in the case of the OCB type display.
The orientation process of the liquid crystal display device is carried out upon executing a rubbing process to the orientation film. In the OCB type liquid crystal display device, color filters disposed on the counter substrate are stripe-like extending in the direction perpendicular to the display surface. Red, green and blue stripe filters constituting one unit are sequentially aligned in parallel with each other in the horizontal direction. The counter electrode and the orientation film are deposited on the filters.
However, there is a problem in operation of the display device that transition from the spray alignment to the bend alignment is not surely executed due to the orientation film of the counter substrate side, so that the spray alignment having remained as it is looks like a luminescent spot defect.