The present invention relates to liquid crystal display apparatus.
A display method, wherein an electric field is supplied to a liquid crystal in a direction approximately parallel to the surface of a substrate using a pair of comb-shaped electrodes, has been disclosed in JP-B-63-21907 (1988), U.S. Pat. No. 4,345,249, WO91/10936, JP-A-6-222397 (1994), and JP-A-6-160878 (1994). In this type of display method, the electrode is not required to be transparent, and so an opaque metallic electrode having a high electro-conductivity is used.
The display method disclosed in the above referred prior art, i.e. the display method wherein an electric field is supplied to a liquid crystal in a direction approximately parallel to the surface of a substrate (hereinafter called an in-plane switching method), has the feature of providing a wide viewing angle. However, none of the prior art teaches how to make a high contrast ratio and a decreased nonuniformity of display compatible.
In accordance with the in-plane switching method, the electric field is supplied in a direction approximately parallel to the surface of the substrate. Therefore, in contrast to conventional twisted nematic display method, the driving voltage depends on the cell gap, and the threshold voltage, Ec, is approximately expressed by the following equation (math. 1). ##EQU1##
where,
d.sub.LC : Cell gap PA2 K.sub.2 : Elastic constant of twist of the liquid crystal PA2 .DELTA..epsilon.: Dielectric anisotropy of the liquid crystal PA2 .epsilon..sub.0 : Dielectric constant in vacuum
Investigation by the present inventors has revealed that, in accordance with the in-plane switching method, the driving voltage depends on the cell gap, and accordingly, variation of the cell gap in the display plane is readily expressed as a variation of the surface brightness, and nonuniformity of display occurs readily. The term, nonuniformity of display refers to a nonuniformity in brightness caused by variation the cell gaps.
If the dispersing amount of the spacer beads is increased as a countermeasure against the nonuniformity of display, the nonuniformity of display can be decreased by an increase in the amount of the spacer beads, which contributes to formation of the cell gap. However, it also has been revealed that leakage of light caused by variation in the liquid crystal orientation in the vicinity of the spacer beads is increased with an increase in the amount of the spacer beads at an aperture portion of the display, creating the problem that the contrast is decreased.
If a planarization film composed of organic high polymers is formed as another countermeasure on the whole surface of a substrate having a group of electrodes, the nonuniformity of display can be decreased with a decrease of the surface nonuniformity. However, it also has been revealed that the contrast ratio is decreased because the driving voltage is increased, and so as problem arises in that a sufficient voltage can not be supplied to the liquid crystal.