Bend aligned namatic liquid crystal imaging display with compensation film

#This invention relates to a display comprising a bend aligned nematic liquid crystal cell, a polarizer, and a compensation film containing a positively birefringent material oriented with its optic axis tilted in a plane perpendicular to the bend aligned nematic liquid crystal cell surface plane.

#Liquid crystals (LC) are widely used for electronic displays. In these display systems, an LC layer is typically situated between a polarizer layer and an analyzer layer. The analyzer is oriented such that its absorbing axis is perpendicular to that of the polarizer. Incident light polarized by the polarizer passes through a liquid crystal cell is affected by the molecular orientation in the liquid crystal, which can be altered by the application of a voltage across the cell. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than that required for the luminescent materials used in other display types such as cathode ray tubes. Accordingly, LC technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light weight, low power consumption and long operating life are important features.

#Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cell, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about the normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.

#FIGS. 1A and 1B are schematics showing the operation of the bend aligned nematic liquid crystal cell display. FIG. 1C is a cross sectional view of Hybrid Aligned Nematic liquid crystal cell (HAN cell).

#FIG. 2 is a schematic diagram of a prior art display device with a biaxial plate.

#FIGS. 3A, 3B, 3C, and 3D are cross sectional diagrams of the exemplary displays in accordance with the invention.

#Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention.

#FIGS. 1A and 1B show the operation of a bend aligned nematic liquid crystal cell 50 in a cross sectional view. A bend aligned nematic liquid crystal cell 50 is one in which positively birefringent nematic liquid crystals 12 are oriented with the bend structure in a plane perpendicular to the cell surface plane (the X-Y plane). In the examples shown in FIGS. 1A and 1B, the liquid crystals 12 take the bend structure oriented in the X-Z plane, which is perpendicular to the liquid crystal cell surface plane (the X-Y plane). When the field is OFF, the liquid crystals 12 take the orientation in the cell as exemplified in FIG. 1A. FIG. 1B shows that when the field is ON, the liquid crystals 12 orient to the electric field direction indicated by the arrow 28. In the middle part of the cell, the liquid crystals 12 are almost perpendicular to the cell surface plane (the X-Y plane). The bend aligned nematic liquid crystal cell 50 shows symmetry for an optical ray such as 14 propagating in an oblique direction in a liquid crystal director plane (the X-Z plane) as described before. In the ON state, such as shown in FIG. 1B, a plate with negative out of plane phase retardation can be effective to compensate the vertically oriented portion of the cell. However, this does not accomplish complete compensation as the liquid crystals 12 at the bottom part 18 and upper part 24 near the substrates 10 experience a distortion. It is an object of this invention to compensate for the ON state of the bend aligned nematic liquid crystal cell to yield a high contrast in an extended viewing angle. As a result, one obtains a liquid crystal display with a wide viewing angle and fast response time. The compensation is achieved by providing a compensation film containing a positively birefringent material oriented with the optic axis tilted in a plane perpendicular to the liquid crystal cell surface plane.

#In FIG. 3A through 3D, possible configurations of the display 100 according to the invention are shown. A display 100 shown in FIG. 3A comprises a bend aligned nematic liquid crystal cell 50, the first pair of positive A-plates 52A, 52B, the second pair of positive A-plates 58A, 58B, a pair of negative C-plates 56A, 56B, the compensation films 55A, 55B and the crossed polarizers 60, 61 on both sides of the bend aligned nematic liquid crystal cell 50. The transmission axes of polarizers 60, 61 are orthogonally crossed with respect to each other in the X-Y plane. The angle between the transmission axes of polarizers 60, 61 is considered as orthogonal when it is within 85 to 95 degrees. The optic axes of the A-plates, 58A and 58B, are in the X-Y plane and parallel to the transmission axes of adjacent polarizers, 60 and 61, respectively. Compensation films, 55B and 55A, are placed between the negative C plates, 56A and 56B, and additional positive A-plates 52A and 52B, respectively. These positive A-plates 52A, 52B are placed next to the bend aligned nematic liquid crystal cell 50 such that their optic axes are in the Y-direction and are perpendicular to the X-Z plane in which liquid crystals are oriented. The function of the positive A-plates 52A, 52B is to offset the phase retardation due to projections of the liquid crystals in the X direction. The A-plate 52A (or 52B) can also be placed between the compensation film 55A (or 55B) and the negative C-plate 56A (or 56B) as shown in FIG. 3B. FIG. 3C is another example of the display 100 according to the invention. In this case, two positive C-plates (plates with its optic axis lying in the plate normal, namely in Z-direction, with positive birefringence) 62A and 62B are placed next to the polarizers 60 and 61, respectively. The optic axes of the positive A-plates 58A, 58B are placed perpendicularly to the transmission axes of their nearer polarizers. The C-plates 62A, 62B and A-plates 58A, 58B can be both negative instead of being both positive. Compensation films, 55B and 55A, are sandwiched by negative C-plates 56A, 56B and positive A-plates 52A, 52B for phase retardation offset, respectively. The placement of the offset A-plates 52A, 52B can be altered, as is the case in FIGS. 3A and 3B. FIG. 3D shows a reflective type display 102 in accordance with the present invention comprising the hybrid aligned nematic liquid crystal cell 51, the reflective plate 64, and the compensation film 55A. The C-plate 56A and the A-plate 58A are arranged so as to compensate the polarizer 60.