Liquid Crystal Display Device Having Liquid Crystal Layer Divided at Different Ratios in Each Pixel

The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which optical phase difference plates are combined with a liquid crystal display element so as to improve viewing-angle-dependency of a display screen.

Liquid crystal display devices adopting nematic liquid crystal display elements, which have conventionally been widely used as numeric-value-segment-type display apparatuses such as watches and calculators, have recently been also used in word processors, note-type personal computers, car-use liquid crystal televisions, and other apparatuses.

Generally, a liquid crystal display element is provided with transparent substrates on which electrode lines for turning on or turning off pixels are formed. For example, in an active-matrix-type liquid crystal display device, active elements such as thin film transistors are provided, on the substrates together with the electrode lines, as switching means for selectively driving pixel electrodes which apply a voltage to liquid crystal. Also, in a liquid crystal display device which carries out color displaying, color filter layers of red, green, and blue are provided on the substrates.

The liquid crystal display element such as above adopts a liquid crystal displaying system which is suitably selected in accordance with the twist angle of the liquid crystal. For example, the active-driving-type twisted nematic liquid crystal displaying system (hereinafter, referred to as the TN system) and the multiplex-driving-type super-twisted nematic liquid crystal display system (hereinafter, referred to as the STN system) are widely known.

In the TN system, displaying is carried out by aligning nematic liquid crystal molecules in a 90.degree. twist, and a ray of light is directed along the twisted direction. In the STN system, displaying is carried out by taking an advantage of the fact that the transmittance suddenly changes in a vicinity of the threshold value of a voltage applied to the liquid crystal when the twist angle of the nematic liquid crystal molecules is made larger than 90.degree..

Since the STN system utilizes the birefringence effect of the liquid crystal, on the background of the display screen, a distinct color is generated due to interference of colors. In order to overcome such a problem so as to realize black-and-white displaying in the STN system, it is effective to use an optical compensation plate. A displaying system adopting the optical compensation plate can be roughly classified into two displaying systems: (1) The double layered super-twisted nematic phase compensation system (hereinafter, referred to as the DSTN system) and (2) the film-type phase compensation system (hereinafter, referred to as the film-addition-type system) in which a film having optical anisotropy is provided.

In the DSTN system, a double-layered structure is adopted wherein (1) a displaying-use liquid crystal cell and (2) a liquid crystal cell which is twist-aligned by a twist angle in a direction opposite to that of the displaying-use liquid crystal cell are provided. In contrast, the film-addition-type system adopts an arrangement wherein a film having optical anisotropy is provided. Of the two systems, the film-addition-type system is considered to be more prospective from the standpoint of light weight and low costs. Since the application of such phase compensation systems makes it possible to improve the black-and-white display characteristics, color STN liquid crystal display devices capable of color displaying have been realized by means of providing a display device of the STN system with color-filter layers.

The TN system can be roughly classified into (1) a normally-black system and (2) a normally-white system. In the normally-black system, a pair of polarization plates are positioned so that the respective polarization axes thereof are parallel to each other so as to display black during a state in which no voltage is applied to the liquid crystal layer (off state). In the normally-white system, a pair of polarization plates are positioned so that the respective polarization axes thereof are orthogonal to each other so as to display white during the off state. The normally-white system is considered to be more prospective from the standpoint of a display contrast, color reproducibility, and viewing-angle-dependency of the display, etc.

Incidentally, the described TN liquid crystal display device has a problem of increased viewing-angle-dependency in that the contrast of a displayed image is changed depending on the direction in which and the angle by which the displayed image is observed by the observer, due to the fact that (1) the liquid crystal molecules exhibit refractive index anisotropy .DELTA.n and (2) the alignment direction of the liquid crystal molecules is tilted with respect to the substrates.

FIG. 12 schematically shows a cross sectional arrangement of a TN liquid crystal display element 31. The arrangement of FIG. 12 is induced by the application of a voltage for half-tone displaying so that a liquid crystal molecule 32 slants upward slightly. In the liquid crystal display element 31, (1) linearly polarized light 35 passing through the TN liquid crystal display element 31 in a direction normal to the surfaces of substrates 33 and 34 and (2) linearly polarized light 36 and 37 respectively passing through the TN liquid crystal display element 31 in directions inclined with respect to the normal direction cross the liquid crystal molecule 32 at different angles. Thus, since the liquid crystal molecule 32 exhibits refractive index anisotropy .DELTA.n, when the linearly polarized light 35, 36, and 37 transmit through the liquid crystal molecule 32 in the respective directions, ordinary light and extraordinary light are generated. As a result, the linearly polarized light 35, 36 and 37 are respectively converted into elliptically polarized light according to a phase difference between the ordinary light and the extraordinary light. This is the cause of the viewing-angle-dependency.

Further, in an actual liquid crystal layer, the liquid crystal molecule 32 has different tilt-angles (a) in a vicinity of a midway portion of the substrate 33 and the substrate 34 and (b) in respective vicinities of the substrate 33 and the substrate 34. Also, the liquid crystal molecule 32 is twisted by 90.degree. about the axis (normal direction).

As described, the linearly polarized light 35, 36, and 37 passing through the liquid crystal layer are subjected to various birefringence effects depending on the direction or the angle of the travel. This results in complex viewing-angle-dependency.

Specifically, as such viewing-angle-dependency, the following phenomena are observed. When the viewing direction is inclined towards the standard viewing direction, i.e., the downward direction of the display surface, from the direction normal to the display screen, above a certain angle, (1) coloring of the displayed image is observed (hereinafter referred to as coloration phenomenon) and (2) the black and white of the displayed image is reversed (hereinafter referred to as reversion phenomenon). Also, when the viewing angle is inclined towards the opposite viewing direction, i.e., the upward direction of the display screen, sudden lowering of the contrast is observed.

Further, the described liquid crystal display device has a drawback in that the viewing angle becomes smaller with an increase in size of the display screen. When a large liquid crystal display screen is viewed from the front with a close distance, there is a case where different colors are observed in a displayed image on the upper portion and the lower portion of the display screen due to the effect of the viewing-angle-dependency. This is caused by a wider range of viewing angle required to encompass the entire screen surface, which is equivalent of a viewing direction increasingly far off-center.

In order to improve such viewing-angle-dependency, for example, Japanese Laid-Open Patent Applications No. 600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest an arrangement wherein an optical phase difference plate (phase difference film) having optical anisotropy is inserted as an optical element between the liquid crystal display element and one of the polarization plates.

In this arrangement, the light, having converted to elliptically polarized light from linearly polarized light in the course of its passage through the liquid crystal molecules having refractive index anisotropy, is allowed to pass through the optical phase difference plate which is provided on a side or on the both sides of the liquid crystal layer having refractive index anisotropy. This ensures that the change in the phase difference of the ordinary light and the extraordinary light with viewing angles is compensated so that the elliptically polarized light is converted again back to the linearly polarized light, thereby permitting the viewing-angle-dependency to be improved.

As such an optical phase difference plate, for example, Japanese Laid-Open Patent Application No. 313159/1993 (Tokukaihei 5-313159) discloses an optical phase difference plate wherein one of the principal refractive index directions of a refractive index ellipsoid is made parallel to the direction normal to the surface of the optical phase difference plate. However, even with this optical phase difference plate, there is a limit in suppressing the reversion phenomenon in the standard viewing direction.

In order to overcome this limit, Japanese Laid-Open Patent Application No. 75116/1994 (Tokukaihei 6-75116) suggest an optical phase difference plate having an arrangement wherein the principal refractive index direction of the refractive index ellipsoid is inclined with respect to the direction normal to the surface of the optical phase difference plate. As an optical phase difference plate having this arrangement, two types of optical phase difference plates 1 and 2 are suggested.

1 In this optical phase difference plate, of three principal refractive indices of the refractive index ellipsoid, the direction of the smallest refractive index is made parallel to the surface of the optical phase difference plate, and the direction of one of the remaining refractive indices is inclined by an angle .theta. with respect to the surface of the optical phase difference plate, and the direction of the other remaining refractive index is also inclined by the angle .theta. with respect to a direction normal to the optical phase difference plate, wherein the value of .theta. satisfies the condition 20.degree..ltoreq..theta..ltoreq.70.degree..

2 In this optical phase difference plate, three principal refractive indices n.sub.a, n.sub.b, and n.sub.c of the refractive index ellipsoid are related to each other by the relation n.sub.a =n.sub.c >n.sub.b, and (1) the direction of the principal refractive index n.sub.b parallel to the direction normal to the surface of the optical phase difference plate and (2) the direction of the principal refractive index n.sub.a or n.sub.c on the surface of the optical phase difference plate are inclined in a clockwise direction or in a counterclockwise direction about the direction of the principal refractive index n.sub.a or n.sub.c on the surface of the optical phase difference plate. Namely, the refractive index ellipsoid is inclined with respect to the optical phase difference plate.

In the two types of the optical phase difference plates 1 and 2, the former can adopt an optical phase difference plate of either a uniaxial type or a biaxial type. On the other hand, the latter can adopt an arrangement wherein two optical phase difference plates are provided in a pair instead singly, and the inclined direction of the principal refractive index n.sub.b of each of the pair of optical phase difference plates is set to 90.degree..

In a liquid crystal display device having an arrangement wherein at least one such an optical phase difference plate is provided between the liquid crystal display element and the polarization plates, it is possible to suppress to some degree the change in contrast, the coloration phenomenon, and the reversion phenomenon which are generated depending on the viewing angle of the display screen.

Also, techniques for eliminating the reversion phenomenon have been suggested. For example, Japanese Laid-Open Patent Applications No. 186735/1982 (Tokukaisho 57-186735) discloses a so-called pixel dividing method in which display patterns (pixels) are sorted into a plurality of regions, and each region is independently subjected to alignment control so as to give a distinguishable viewing angle characteristic to each region. In this method, the liquid crystal molecules slant upward in different directions in each region, thereby eliminating the viewing-angle-dependency.

Also, Japanese Laid-Open Patent Applications No. 118406/1994 (Tokukaihei 6-118406) and Japanese Laid-Open Patent Applications No. 194645/1994 (Tokukaihei 6-194645) respectively disclose a technique in which the pixel dividing method is used in conjunction with optical phase difference plates.

In a liquid crystal display device disclosed in Japanese Laid-Open Patent Applications No. 118406/1994 (Tokukaihei 6-118406), in order to improve the contrast, optically anisotropic films (optical phase difference plates) are provided between the liquid crystal panel and the polarization plates. The compensation plates (optical phase difference plates) disclosed in Japanese Laid-Open Patent Applications No. 194645/1994 (Tokukaihei 6-194645) have substantially no surface refractive index being parallel to the surfaces of the compensation plates, and the refractive index in a direction perpendicular to the surfaces of the compensation plates is set so as to be smaller than the refractive index in the surfaces, and therefore has a negative refractive index. Thus, when a voltage is applied, positive refractive index generated in the liquid crystal display element is compensated so that the viewing-angle-dependency is lowered.

However, due to a demand for a liquid crystal display device having a wider viewing angle and a higher displaying quality in today's market, a further advancement in improving the viewing-angle-dependency is demanded. The optical phase difference plates disclosed in the above-mentioned U.S. Patents are not sufficient for meeting such a demand, and therefore there is a room for further improvements.

Also, in the pixel dividing method for eliminating the reversion phenomenon, the reversion phenomenon and the viewing-angle-dependency are eliminated because the viewing angle characteristics become substantially symmetrical when viewed from upward and downward directions. However, the pixel dividing method still has a problem in that the contrast is lowered when viewed from upward and downward directions. Thus, the displayed black is paled, and is perceived as grey. Also, the described prior art adopting the pixel dividing method has a drawback in that the viewing-angle-dependency is generated when viewed from left and right directions.

Also, in the pixel dividing method used in conjunction with the optical phase difference plates, when the viewing direction is inclined, the coloration phenomenon is generated at a 45.degree. incline. Further, because a liquid crystal element in which the pixels are sorted into regions at the same ratio is adopted, there is a limit in suppressing the lowering of contrast generated when viewed from the upward and the downward directions, for the following reason.

In the pixel dividing method, since the dividing ratio of the pixels is the same, the viewing angle characteristics of the liquid crystal display element in the standard viewing direction (direction in which the contrast improves from a direction perpendicular to the screen) and the opposite viewing direction (direction in which the contrast lowers from a direction perpendicular to the screen) are averaged. However, in practice, because the viewing angle characteristics in the standard viewing direction and the opposite viewing direction are inversely related to each other, even when the pixel dividing method is used in conjunction with the optical phase difference plates, it is difficult to uniformly suppress the lowering of contrast in the upward and downward directions. Especially, when the viewing direction is inclined towards the standard viewing direction, the reversion phenomenon and darkening of display image are generated.

It is an object of the present invention to provide a liquid crystal display device having an optical phase difference plate in which, in addition to the compensation effect of optical phase difference plates, the viewing-angle-dependency is improved more desirably by aligning in different directions divided liquid crystal layers which have been prepared by dividing a liquid crystal layer of a liquid crystal display element at different ratios with respect to each pixel, and by setting an optimum range for a change, with a wavelength of light, in refractive index anisotropy .DELTA.n of a liquid crystal material constituting a liquid crystal layer, and in which a reversion phenomenon and a coloration phenomenon are effectively improved in particular.

In order to achieve the above-mentioned object, a liquid crystal display device of the present invention includes a liquid crystal display element composed of a pair of transparent substrates, each provided with a transparent electrode layer and an alignment layer on a surface facing the other substrate, and a liquid crystal layer enclosed therebetween, a pair of polarizers provided on both sides of the liquid crystal display element, and optical phase difference plates, provided at least one, respectively positioned between the liquid crystal display element and the pair of polarizers, the optical phase difference plates, each having a refractive index ellipsoid with three principal refractive indices n.sub.a, n.sub.b, and n.sub.c related to each other by a relation n.sub.a =n.sub.c >n.sub.b, a direction of the principal refractive index n.sub.b being parallel to a direction normal to surfaces of the optical phase difference plates and a direction of the principal refractive index n.sub.a or n.sub.c in the surfaces of the optical phase difference plates being respectively inclined clockwise or counterclockwise with respect to the direction of the principal refractive index n.sub.a or n.sub.c in the surfaces of the optical phase difference plates so that the refractive index ellipsoid is inclined, wherein the alignment layer aligns in different directions divided liquid crystal layers which have been prepared by dividing the liquid crystal layer at different ratios with respect to each pixel, and the liquid crystal layer made of a liquid crystal material whose change in refractive index anisotropy .DELTA.n in accordance with a wavelength of light is set in a range which does not allow viewing-angle-dependant coloration to generate on a liquid crystal screen.

With this arrangement, in the case where linearly polarized light, upon passing through the liquid crystal layer showing birefringence, is converted into elliptically polarized light in accordance with a phase difference of ordinary light and extraordinary light generated in this process, since the optical phase difference plates whose principal refractive indices n.sub.a, n.sub.b, and n.sub.c are related to each other by a relation n.sub.a =n.sub.c >n.sub.b, and in which short axes of the refractive index ellipsoids having the principal refractive index n.sub.b are inclined with respect to a direction normal to the surfaces of the optical phase difference plates are provided between the liquid crystal display element and the pair of polarizers, a change in the phase difference of the ordinary light and the extraordinary light generated in accordance with viewing angles is compensated by the optical phase difference plates.

Also, in the described liquid crystal display device, the divided liquid crystal layers divided at different ratios are provided, and the optical phase difference plates are provided between (1) the liquid crystal display element in which an alignment state of the divided liquid crystal layers is controlled in the described manner and (2) the pair of polarizers. With this arrangement, it is possible to eliminate the difference in viewing angle characteristics in a standard viewing direction and an opposite viewing direction which are inversely related to each other so that the viewing angle characteristics in the standard viewing direction and the opposite viewing direction become close to each other. As a result, it is possible to substantially uniformly suppress lowering of contrast and paling of the display screen, generated when the viewing angle is tilted in the upward and downward directions. Especially, it is possible to clearly display black.

However, the described compensation function is not sufficient when more desirable improvement of the viewing-angle-dependency is demanded. In order to meet such a demand, after extensive research, inventors of the present invention found that a change, with a wavelength of light, in the refractive index anisotropy .DELTA.n of the liquid crystal material constituting the liquid crystal layer plays a major role in affecting the coloration of a liquid crystal screen (display screen) in particular. This finding led to the present invention.

In the liquid crystal display device of the present invention, a change, with a wavelength of light, in the refractive index anisotropy .DELTA.n of the liquid crystal material constituting the liquid crystal layer sealed in the liquid crystal display element is set in a range which does not allow viewing-angle-dependant coloration to generate on a liquid crystal screen, thereby preventing the generation of coloration on the screen. Note that, with this arrangement, in addition to the compensation functions by the optical phase difference plates and the divided liquid crystal layer, it is also possible to further suppress a change in contrast and a reversion phenomenon.

For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.

FIG. 1 is a cross sectional view showing a separated arrangement of a liquid crystal display device in accordance with one embodiment of the present invention.

FIG. 2 is an explanatory drawing showing a pre-tilt direction of liquid crystal molecules for one pixel in the liquid crystal display device.

FIG. 3 is a perspective view showing principal refractive indices of an optical phase difference plate of the liquid crystal display device.

FIG. 4 is a perspective view of separated components of the liquid crystal display device showing an optical arrangement of polarization plates and optical phase difference plates of the liquid crystal display device.

FIG. 5 is a graph of refractive index anisotropy .DELTA.n versus wavelength of a liquid crystal material adopted as a liquid crystal layer of the liquid crystal display device.

FIG. 6 is a graph of .DELTA.n(.lambda.)/.DELTA.n(550) versus wavelength of another liquid crystal material adopted as a liquid crystal layer of the liquid crystal display device.

FIG. 7 is a perspective view showing a measuring system which measures viewing-angle-dependency of the liquid crystal display device.

FIG. 8(a) through FIG. 8(c) are graphs respectively showing transmittance vs. applied voltage of the liquid crystal display device of Fourth Example.

FIG. 9(a) through FIG. 9(c) are graphs respectively showing transmittance vs. applied voltage of the liquid crystal display device of a comparative example of Fourth Example.

FIG. 10(a) through FIG. 10(c) are graphs in accordance with Fifth Example respectively showing transmittance-applied voltage characteristics of the liquid crystal display device when an alignment dividing ratio of the liquid crystal layer is 6 to 4, 17 to 3, and 19 to 1, respectively.

FIG. 11 a graph in accordance with Fifth Example showing transmittance-applied voltage characteristics of the liquid crystal display device when the alignment dividing ratio of the liquid crystal layer is 1 to 1.

FIG. 12 is a schematic diagram showing twisted alignment of liquid crystal molecules of a TN liquid crystal display element.

The following will describe one embodiment of the present invention.

As shown in FIG. 1, a liquid crystal display device of the present embodiment is provided with a liquid crystal display element 1, a pair of optical phase difference plates 2 and 3, and a pair of polarization plates (polarizer) 4 and 5.

The liquid crystal display element 1 has a structure wherein a liquid crystal layer 8 is sandwitched between electrode substrates 6 and 7 which are provided so as to face each other. An electrode substrate 6 has an arrangement wherein transparent electrodes 10 made of ITO (Indium--Tin--Oxide) are formed on the surface, on the side of the liquid crystal layer 8, of a glass substrate (transparent substrate) 9 which is provided as a base, and an alignment layer 11 is formed on the transparent electrodes 10 thus provided. In the same manner, an electrode substrate 7 has an arrangement wherein transparent electrodes 13 made of ITO (Indium--Tin--Oxide) are formed on the surface, on the side of the liquid crystal layer 8, of a glass substrate (transparent substrate) 12 which is provided as a base, and an alignment layer 14 is formed on the transparent electrodes 13 thus provided.

For simplicity, FIG. 1 shows an arrangement of the liquid crystal display element 1 of only one pixels, yet in the entire arrangement of the liquid crystal display element 1, the strip-shaped transparent electrodes 10 and 13 respectively having predetermined widths are respectively provided on the glass substrates 9 and 12 with predetermined spaces so as to be orthogonal to each other between the glass substrates 9 and 12 when viewed from a direction perpendicular to the surface of the substrate. Each portion where the transparent electrodes 10 and 13 intersect makes up a pixel for displaying, and the pixels are provided in matrix over the entire liquid crystal display device of the present embodiment.

The electrode substrates 6 and 7 are combined with each other by seal resin 15, and the liquid crystal layer 8 is enclosed in a spacing formed by the electrode substrates 6 and 7 and the seal resin 15. Note that, although detailed explanations will be given later, as a liquid crystal material of the liquid crystal layer 8, such a material is selected so that its refractive index anisotropy .DELTA.n satisfies a predetermined condition for producing the most desirable property out of the combination of (1) the liquid crystal layer 8 of the present liquid crystal display device and (2) the compensation function of the phase difference by the phase compensation plates 2 and 3. Note that, to the transparent electrodes 10 and 13, a voltage in accordance with display data is applied by a driving circuit 17.

In the present liquid crystal display device, a unit composed of the liquid crystal display element 1, the optical phase difference plates 2 and 3, and the polarization plates (polarizer) 4 and 5 constitutes a liquid crystal cell 16.

The alignment layers 11 and 14 respectively have two regions with different states. With this arrangement, a first divided portion 8a (first divided liquid crystal layer) and a second divided portion 8b (second divided liquid crystal layer) of the liquid crystal layer 8, respectively facing the two regions are controlled in such a manner that the alignment states of the liquid crystal molecules in the first divided portion 8a and the second divided portion 8b are different. Note that, in order to improve the viewing angle characteristics when the viewing direction is inclined in the upward and downward directions and in the left and right directions, the dividing ratio of the liquid crystal layer 8 is set to be different (described later in detail). The respective two regions of the alignment layers 11 and 14 are given different alignment states by the alignment layers 11 and 14 as different pre-tilt angles are given to the liquid crystal molecules and as the tilt direction of the liquid crystal molecules are reversed in the opposite directions with respect to the direction perpendicular to the substrates.

The optical phase difference plate 2 is provided between the liquid crystal display element 1 and the polarization plate 4, and the optical phase difference plate 3 is provided between the liquid crystal display element 1 and the polarization plate 5. The optical phase difference plates 2 and 3 are prepared by (1) tilt-aligning or hybrid-aligning discotic liquid crystal on a support base made of a transparent organic polymer and (2) crosslinking of the discotic liquid crystal to each other. As a result, respective refractive index ellipsoids of the optical phase difference plates 2 and 3 are tilted with respect to the optical phase difference plates 2 and 3.

As the support base of the optical phase difference plates 2 and 3, triacetylcellulose (TAC), which is generally adopted as a polarization plate, is suitably adopted due to its reliability. Alternatively, it is also possible to adopt a colorless transparent organic polymer film having high weather resistance and chemical resistance such as polycarbonate (PC) or polyethyleneterephthalate (PET).

As shown in FIG. 3, the optical phase difference plates 2 and 3 respectively have principal refractive indices of n.sub.a, n.sub.b, and n.sub.c in three different directions. The principal refractive index n.sub.a coincides with the extended direction of the y-axis of the rectangular coordinates of x, y, and z. The principal refractive index n.sub.b is inclined by an angle .theta. in a direction of the arrow A with respect to the z-axis perpendicular to the surface of the optical phase difference plate 2 or 3 corresponding to the screen (direction normal to the surface).

In the optical phase difference plates 2 and 3, the principal refractive indices n.sub.a, n.sub.b, and n.sub.c satisfy the relation n.sub.a =n.sub.c >n.sub.b, and therefore only one optical axis is present. Thus, the optical phase difference plates 2 and 3 are uniaxial, and the refractive index anisotropy is negative. Here, since n.sub.a =n.sub.c, a first retardation value of the optical phase difference plates 2 and 3 is substantially 0 nm ((n.sub.c -n.sub.a).times.d=0), and a second retardation value, derived from the formula (n.sub.c -n.sub.b).times.d, is set to have an arbitrary value in a range of 80 nm and 250 nm. By setting the second retardation value in this range, it is ensured that the compensation function of the phase difference by the optical phase difference plates 2 and 3 is obtained. Note that, (n.sub.c -n.sub.a) and (n.sub.c -n.sub.b) respectively denote refractive index anisotropy .DELTA.n, and "d" denotes the thickness of the optical phase difference plate 2 or 3.

Also, the angle .theta. by which the principal refractive index n.sub.b of the optical phase difference plates 2 and 3 is inclined, namely, the tilt-angle .theta. of the refractive index ellipsoid is set to have an arbitrary value in a range of 15.degree..ltoreq.-.theta..ltoreq.75.degree.. By setting the tilt-angle .theta. of the refractive index ellipsoid in this range, it is ensured that the compensation function of the phase difference by the optical phase difference plates 2 and 3 is obtained, regardless of the direction (clockwise or counterclockwise) the refractive index ellipsoid is inclined.

Note that, with regard to the positioning of the optical phase difference plates 2 and 3, it is possible to arrange the optical phase difference plates 2 and 3 in such a manner that (a) only one of the optical phase difference plates 2 and 3 is provided on one side of the liquid crystal display element 1, (b) the optical phase difference plates 2 and 3 are both provided on one side of the liquid crystal display element 1 in an overlapped manner, or (c) three or more optical phase difference plates are provided.

In the present liquid crystal display device, as shown in FIG. 4, the polarization plates 4 and 5 of the liquid crystal display element 1 are provided so that respective absorption axes AX.sub.1 and AX.sub.2 of the polarization plates 4 and 5 are orthogonal to the respective long axes L.sub.1 and L.sub.2 of the liquid crystal molecules respectively contacting the alignment layers 11 and 14 (see FIG. 1). Also, in the present liquid crystal display device, since the long axes L.sub.1 and L.sub.2 are orthogonal to each other, the absorption axes AX.sub.1 and AX.sub.2 are also orthogonal to each other accordingly.

Here, as shown in FIG. 3, the direction formed by projecting the principal refractive index n.sub.b, which is inclined in a direction to give anisotropy to the optical phase difference plates 2 and 3, on the surface of the optical phase difference plate 2 or 3 is defined as the direction D. As shown in FIG. 4, the optical phase difference plate 2 is positioned so that the direction D (direction D.sub.1) is parallel to the long axis L.sub., and the optical phase difference plate 3 is positioned so that the direction D (direction D.sub.2) is parallel to the long axis L.sub.2.

With the above-mentioned arrangement of the optical phase difference plates 2 and 3 and the polarization plates 4 and 5, the present liquid crystal display device carries out a so-called normally white displaying in which white displaying is carried out by allowing light to transmit during the off-state.

Generally, in an optically anisotropic material such as liquid crystal and an optical phase difference plate (phase difference film), the anisotropy of three dimensional principal refractive indices of n.sub.a, n.sub.b, and n.sub.c is represented by a refractive index ellipsoid. The value of the refractive index anisotropy .DELTA.n differs depending on the direction in which the refractive index ellipsoid is observed.

The following will describe the liquid crystal layer 8 in detail.

As mentioned above, in order to improve the viewing angle characteristics when the viewing angle is inclined in the upward and downward directions and in the left and right directions, the liquid crystal layer 8 is divided at different dividing ratios.

Specifically, the liquid crystal layer 8 is divided into the first divided portion 8a and the second divided portion 8b at different ratios which are set so as to fall in a range of 6 to 4 and 19 to 1. Also, as shown in FIG. 2, the liquid crystal molecules are aligned by the alignment layers 11 and 14 in pre-tilt directions respectively orthogonal to the first divided portion 8a and the second divided portion 8b. The pre-tilt directions P.sub.1 and P.sub.2 of the alignment layer 11 are set in the opposite directions in the first divided portion 8a and the second divided portion 8b. Similarly, the pre-tilt directions P.sub.3 and P.sub.4 of the alignment layer 14 are set in the opposite directions. Note that, the liquid crystal layer 8 may be divided in a direction along the longitudinal direction of either transparent electrode 10 or 13.

When the liquid crystal layer 8 having the described arrangement is used in conjunction with the optical phase difference plates 2 and 3, it is possible to obtain alignment states suitable for the viewing angle characteristics of both in the standard viewing direction and in the opposite viewing direction. Thus, it is possible to suppress lowering of contrast and paling of the display screen, generated when the viewing angle is tilted in the upward and downward directions. As a result, it is possible to clearly display black which is strongly affected by lowering of contrast.

Also, in the liquid crystal display element 1, it is further preferable, with respect to the largest first divided portion 8a per pixel of the liquid crystal layer 8, that the respective tilt directions of the refractive index ellipsoids of the optical phase difference plates 2 and 3 and the respective pre-tilt directions of the liquid crystal molecules respectively positioned in vicinities of the alignment layers 11 and 14 are set in the opposite directions. The present liquid crystal display device has this arrangement.

With this arrangement, it is possible to compensate, by the optical phase difference plates 2 and 3, the deviation of optical characteristics due to the tilted liquid crystal molecules which are tilted by the effect of the alignment when a voltage is applied to the liquid crystal display element 1.

Namely, when (1) the tilt direction of the refractive index ellipsoid with respect to the surfaces of the optical phase difference plates 2 and 3 and (2) the tilt direction of the liquid crystal molecules while a voltage is being applied are opposite with respect to the largest first divided portion 8a, the optical characteristics due to the liquid crystal molecules and the optical characteristics of the refractive index ellipsoids, i.e., the optical phase difference plates 2 and 3 are opposite. As a result, the liquid crystal molecules in vicinities of inner surfaces of the alignment layers 11 and 14 do not slant upward due to the effect of the alignment even when a voltage is applied, and the deviation of the optical characteristics due to the liquid crystal molecules can be compensated by the optical phase difference plates 2 and 3.

As a result, the reversion phenomenon generated when the viewing angle is inclined towards the standard viewing direction can be suppressed, and a desirable display image with no darkening is obtained. Also, since lowering of contrast generated when the viewing angle is inclined towards the opposite viewing direction can be suppressed, a desirable display image with no paling is obtained. Further, it is also possible to suppress the generation of reversion phenomenon in the left and right directions.

Furthermore, in addition to dividing the liquid crystal layer 8 at different dividing ratios with respect to each pixel, in order to obtain the most desirable property out of the combination of the liquid crystal layer 8 and the compensation function of the phase difference by the optical phase difference plates 2 and 3, as a liquid crystal material of the liquid crystal layer 8, a liquid crystal material in which the refractive index anisotropy .DELTA.n satisfies a predetermined condition is adopted, namely, the refractive index anisotropy .DELTA.n of the liquid crystal layer 8 is set in such a range that the change in the refractive index anisotropy .DELTA.n with a wavelength of light does not cause viewing-angle-dependent coloration on the liquid crystal screen.

Specifically, a liquid crystal material which is designed to satisfy at least one of the specified ranges in the following 1 to 3 conditions is adopted.

1 The difference (.DELTA.n(450)-.DELTA.n(650)) of the liquid crystal material, between (a) the refractive index anisotropy .DELTA.n(450) for the light having a wavelength of 450 nm and (b) the refractive index anisotropy .DELTA.n(650) for the light having a wavelength of 650 nm, is set in a range of not less than 0 and less than 0.010. More preferably, the difference (.DELTA.n(450)-.DELTA.n(650)) is set in a range of not less than 0 and not more than 0.0055.

2 The ratio (.DELTA.n(450)/.DELTA.n(550)) of the liquid crystal material, which is the ratio of (a) the refractive index anisotropy .DELTA.n(450) for the light having a wavelength of 450 nm to (b) the refractive index anisotropy .DELTA.n(550) for the light having a wavelength of 550 nm, is set in a range of not less than 1 and less than 1.07. More preferably, the ratio (.DELTA.n(450)/.DELTA.n(550)) is set in a range of not less than 1 and not more than 1.05.

3 The ratio (.DELTA.n(650)/.DELTA.n(550)) of the liquid crystal material, which is the ratio of (a) the refractive index anisotropy .DELTA.n(650) for the light having a wavelength of 650 nm to (b) the refractive index anisotropy .DELTA.n(550) for the light having a wavelength of 550 nm, is set in a range of more than 0.960 and not more than 1. More preferably, the ratio (.DELTA.n(650)/.DELTA.n(550)) is set in a range of not less than 0.975 and not more than 1.

By adopting a liquid crystal material satisfying at least one of the above 1 to 3 conditions, it is possible, due to the compensation function of the phase difference by the optical phase difference plates 2 and 3, to suppress the change in contrast, the reversion phenomenon, and the coloration phenomenon caused by the viewing-angle dependency of the display screen. Particularly, the coloration phenomenon of the display screen can be effectively improved.

To describe in more detail, by adopting a liquid crystal material satisfying the wider range of at least one of the above 1 to 3 conditions, it is possible to provide a liquid crystal display device in which a satisfactory image can be observed (although slight coloration is observed) in any viewing direction in a viewing angle of 50.degree. which is a required viewing angle in most liquid crystal display devices.

Further, by adopting a liquid crystal material satisfying at least one of the ranges which are stated to be more preferable in the above 1 to 3 conditions, it is possible to provide a liquid crystal display device in which an image without any coloration can be observed in any viewing direction in a viewing angle of 70.degree..

Also, by adopting a liquid crystal material satisfying at least one of the above 1 to 3 conditions, compared with the case of relying solely on the compensation function of the phase difference plates 2 and 3, it is possible to further suppress the change in contrast and the reversion phenomenon.

It is further preferable that in addition to satisfying at least one of the specified ranges of the above 1 to 3 conditions, the specified range of the following condition 4 is also satisfied. In the liquid crystal layer 8 of the present liquid crystal display device, the specified range of the condition 4 is satisfied.

4 The refractive index anisotropy .DELTA.n(550) of the liquid crystal material for the light having a wavelength of 550 nm is set in a range of more than 0.060 and less than 0.120. More preferably, the refractive index anisotropy .DELTA.n(550) is set in a range of not less than 0.070 and not more than 0.095.

This is because when the refractive index anisotropy .DELTA.n(550) for the light having a wavelength of 550 nm, which is in the middle of the visible light range, is not more than 0.060 or not less than 0.120, it was found that the reversion phenomenon and lowering of a contrast ratio may be generated depending on the viewing direction. Thus, by setting the refractive index anisotropy .DELTA.n(550) for the light having a wavelength of 550 nm in a range of more than 0.060 and less than 0.120, it is possible to eliminate the phase difference generated in the liquid crystal display element in accordance with viewing angles.

By satisfying the condition 4, in addition to (a) the compensation function of the phase difference by the phase difference plates 2 and 3 and (b) the improvement of the viewing-angle-dependency made by the compensation function obtained by satisfying at least one of the above 1 to 3 conditions, it is possible to further suppress lowering of the contrast ratio in the opposite viewing direction and the reversion phenomenon in the left and right directions.

Especially, by setting the refractive index anisotropy .DELTA.n(550) for the light having a wavelength of 550 nm in a range of not less than 0.070 and not more than 0.095, it is possible to eliminate even more effectively the phase difference generated in the liquid crystal display element in accordance with viewing angles, thereby ensuring that a change in contrast of the liquid crystal display image, the reversion phenomenon in the left and right directions, and the coloration phenomenon are further suppressed.

FIG. 5 shows a graph of wavelength (.lambda.) against .DELTA.n (.lambda.), wherein a wavelength-refractive index anisotropy .DELTA.n characteristic of a liquid crystal material which can be adopted as the liquid crystal layer 8 of the present liquid crystal display device is represented by the solid curve "a". Note that, in FIG. 5, for comparison, a wavelength-refractive index anisotropy .DELTA.n characteristic of a liquid crystal material adopted as the liquid crystal layer of a conventional liquid crystal display device is represented by the alternate long and short dash curve "b".

As is clear from the comparison of the curve "a" and the curve "b", the wavelength-refractive index anisotropy .DELTA.n characteristic of the liquid crystal material which can be adopted as the liquid crystal layer 8 of the present liquid crystal display device, compared with that of the liquid crystal material adopted in a conventional liquid crystal display device, shows a more gradual and substantially horizontal slope slightly declining down to the right.

Similarly, FIG. 6 shows a graph of .DELTA.n(.lambda.)/.DELTA.n(550) against wavelength (.lambda.), wherein a wavelength-refractive index anisotropy .DELTA.n characteristic of another liquid crystal material which can be adopted in the present liquid crystal display device is represented by the solid curve "c". Note that, in FIG. 6, for comparison, a wavelength-refractive index anisotropy .DELTA.n characteristic of another liquid crystal material adopted as the liquid crystal layer of a conventional liquid crystal display device is represented by the alternate long and short dash curve "d".

As is clear from the comparison of the curve "c" and the curve "d", the wavelength-refractive index anisotropy .DELTA.n characteristic of the liquid crystal material which can be adopted as the liquid crystal layer 8 of the present liquid crystal display device, compared with that of the liquid crystal material adopted in a conventional liquid crystal display device, shows a slope which is more gradual than that of the liquid crystal material adopted in a conventional liquid crystal display device.

With the described arrangements, in the liquid crystal display device of the present embodiment, in addition to the compensation function by the optical phase difference plates 2 and 3 for compensating the phase difference generated in the liquid crystal display element 1 in accordance with viewing angles, by setting the change in refractive index anisotropy .DELTA.n with a wavelength in the liquid crystal material of the liquid crystal layer 8 in such a range that no coloration of the liquid crystal screen is generated, a function for compensating the phase difference is provided. Thus, it is possible to effectively improve, in particular, the viewing-angle-dependant coloration of the liquid crystal screen, and at the same time, the change in contrast and the reversion phenomenon are also improved, thereby realizing high quality image displaying.

Note that, in the present embodiment, the explanations are based on a liquid crystal display device of a simple matrix type. However, the present invention is also applicable to a liquid crystal display device of an active matrix type adopting an active switching element such as a TFT.

The following will describe, together with comparative examples, examples in accordance with the liquid crystal display device of the present embodiment having the described arrangement.

In the present example, as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, five samples #1 to #5, each having a cell thickness (thickness of the liquid crystal layer 8) of 5 .mu.m, were prepared by adopting such liquid crystal materials in which the difference .DELTA.n (450)-.DELTA.n (650) between the refractive index anisotropy .DELTA.n(450) for the wavelength of 450 nm and the refractive index anisotropy .DELTA.n(650) for the wavelength of 650 nm is set to 0, 0.0030, 0.0055, 0.0070, and 0.0090 for the samples #1 to #5, respectively.

The optical phase difference plates 2 and 3 adopted in the samples #1 to #5 satisfy the following conditions: (1) the discotic liquid crystal is applied to a transparent support base (for example, triacetylcellulose (TAC), etc.), (2) the discotic liquid crystal is tilt-aligned on the support base, and is crosslinked to each other, (3) the first retardation value and the second retardation value are respectively set to 0 nm and 100 nm, and (4) the principal refractive index n.sub.b is inclined by an angle of substantially 20.degree. in a direction of the arrow A with respect to the z-axis of the x-y-z coordinates in the x-z plane, and in the same manner, the principal refractive index n.sub.c is inclined by an angle of substantially 20.degree. in a direction of the arrow B with respect to the x-axis (in other words, the tilt-angle .theta. of the refractive index ellipsoid is 20.degree.).

Also, as a comparative example of the present example, a comparative sample #100 was prepared. The comparative sample #100 has an arrangement the same as that of the samples #1 to #5 except that in the sample #100, a liquid crystal material in which the difference .DELTA.n(450)-.DELTA.n(650) is set to 0.010 is adopted as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1.

Table 1 shows results of visual inspections under the white light using the samples #1 to #5 and the comparative sample #100.

In the samples #1 to #3, a desirable image was obtained without any coloration in any viewing direction in the viewing angle of 70.degree.. In the sample #4, a desirable image was obtained without any coloration in any viewing direction up until the viewing angle of 60.degree.. In the sample #5, slight coloration was observed in the left and right viewing directions in the viewing angle of 50.degree., but the coloration was not serious enough to cause a problem in an actual use.

In contrast, in the comparative sample #100, in the viewing angle of 50.degree., in the left and right viewing directions, yellow to orange coloration was observed which is not tolerable in an actual use. In the present example, the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is tilt-aligned on the transparent support base are adopted. However, by adopting the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is hybrid-aligned rather than tilt-aligned, the results same as that obtained in the samples #1 to #5 and the comparative sample #100 were obtained as well.

In the present example, as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, five samples #6 to #10, each having a cell thickness (thickness of the liquid crystal layer 8) of 5 .mu.m, were prepared by adopting such liquid crystal materials in which the ratio .DELTA.n(450)/.DELTA.n(550), which is the ratio of the refractive index anisotropy .DELTA.n(450) for the wavelength of 450 nm to the refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm, is set to 1, 1.03, 1.05 1.06, and 1.065 for the samples #6 to #10, respectively.

As the optical phase difference plates 2 and 3 adopted in the samples #6 to #10, the optical phase difference plates 2 and 3 of the first example in which the discotic liquid crystal is tilt-aligned are adopted.

Also, as a comparative example of the present example, a comparative sample #101 was prepared. The comparative sample #101 has an arrangement the same as that of the samples #6 to #10 except that in the comparative sample #101, a liquid crystal material in which the ratio .DELTA.n(450)/.DELTA.n (550) is set to 1.07 is adopted as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1.

Table 2 shows results of visual inspections under the white light using the samples #6 to #10 and the comparative sample #101.

In the samples #6 to #8, a desirable image was obtained without any coloration in any viewing direction in the viewing angle of 70.degree.. In the sample #9, a desirable image was obtained without any coloration in any viewing direction up until the viewing angle of 50.degree.. However, in the sample #9, slight coloration was observed in the left and right viewing directions in the viewing angle of 60.degree., but the coloration was not serious enough to cause a problem in an actual use. In the sample #10, slight coloration was also observed in the left and right viewing directions in the viewing angle of 50.degree., but the coloration was not serious enough to cause a problem in an actual use.

In contrast, in the comparative sample #101, in the viewing angle of 50.degree., in the left and right viewing directions, yellow to orange coloration was observed which is not tolerable in an actual use.

In the present example, the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is tilt-aligned on the transparent support base are adopted. However, by adopting the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is hybrid-aligned rather than tilt-aligned, the results same as that obtained in the samples #6 to #10 and the comparative sample #101 were obtained as well.

In the present example, as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, five samples #11 to #15, each having a cell thickness (thickness of the liquid crystal layer 8) of 5 .mu.m, were prepared by adopting such liquid crystal materials in which the ratio .DELTA.n(650)/.DELTA.n(550), which is the ratio of the refractive index anisotropy .DELTA.n(650) for the wavelength of 650 nm to the refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm, is set to 1, 0.980, 0.975, 0.970, and 0.965 for the samples #11 to #15, respectively.

As the optical phase difference plates 2 and 3 respectively adopted in the samples #11 to #15, the optical phase difference plates 2 and 3 of the first example in which the discotic liquid crystal is tilt-aligned are adopted.

Also, as a comparative example of the present example, a comparative sample #102 was prepared. The comparative sample #102 has an arrangement the same as that of the samples #11 to #15 except that in the comparative sample #102, a liquid crystal material in which the ratio .DELTA.n(650)/.DELTA.n (550) is set to 0.960 is adopted as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1.

Table 3 shows results of visual inspections under the white light using the samples #11 to #15 and the comparative sample #102.

In the samples #11 to #13, a desirable image was obtained without any coloration in any viewing direction in the viewing angle of 70.degree.. In the sample #14, a desirable image was obtained without any coloration in any viewing direction up until the viewing angle of 50.degree.. However, in the sample #14, slight coloration was observed in the left and right viewing directions in the viewing angle of 60.degree., but the coloration was not serious enough to cause a problem in an actual use. In the sample #15, slight coloration was also observed in the left and right viewing directions in the viewing angle of 50.degree., but the coloration was not serious enough to cause a problem in an actual use.

In contrast, in the comparative sample #102, in the viewing angle of 50.degree., in the left and right viewing directions, yellow to orange coloration was observed which is not tolerable in an actual use.

In the present example, the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is tilt-aligned on the transparent support base are adopted. However, by adopting the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is hybrid-aligned rather than tilt-aligned, the results same as that obtained in the samples #11 to #15 and the comparative sample #102 were obtained as well.

In the present example, as shown in FIG. 7, the viewing-angle-dependency of the liquid crystal display device was measured by using a measuring system provided with a light-receiving element 21, an amplifier 22, and a recording device 23. The liquid crystal cell 16 is positioned so that a surface 16a on the side of the glass substrate 9 represents a reference plane x-y of the rectangular coordinates x-y-z. The light-receiving element 21 receives light with a certain stereoscopic light-receiving angle, and is positioned with a predetermined distance away from the coordinate origin in a direction making an angle .phi. (viewing angle) with respect to the z-direction perpendicular to the surface 16a.

During a measuring process, monochromatic light having a wavelength of 550 nm is projected on the liquid crystal cell 16, which has been set in the present measuring system, through the surface opposite to the surface 16a. The monochromatic light having passed through the liquid crystal cell 16 is partially made incident on the light-receiving element 21. The output of the light-receiving element 21, after amplified to a predetermined level by the amplifier 22, is recorded by the recording device 23 such as a waveform memory or a recorder.

In the present example, as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, three samples #16 to #18, each having a cell thickness (thickness of the liquid crystal layer 8) of 5 .mu.m, were prepared by adopting such liquid crystal materials whose refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm are 0.070, 0.080, and 0.095 for the samples #16 to #18, respectively.

As the optical phase difference plates 2 and 3 respectively adopted in the samples #16 to #18, the optical phase difference plates 2 and 3 of the first example in which the discotic liquid crystal is tilt-aligned are adopted.

The measurement was conducted using the samples #16 to #18 set in the measuring system of FIG. 7, and the light-receiving element 21 fixed to a position with a certain angle .phi.. In the measurement, the output level of the light-receiving element 21 against an applied voltage to the samples #16 to #18 was measured.

In the measurement, the light-receiving element 21 was positioned with an angle .phi. of 50.degree., and supposing that the y-direction and the x-direction respectively direct to the left side and the bottom side of the screen, the position of the light-receiving element 21 was changed so as to carry out the measurement from the upward direction, the downward direction, and the left and right directions.

The results of the measurement are shown in FIG. 8(a) through FIG. 8(c). FIG. 8(a) through FIG. 8(c) are graphs respectively showing transmittance of light against a voltage applied to the samples #16 to #18 (transmittance vs. applied voltage).

FIG. 8(a), FIG. 8(b), and FIG. 8(c) respectively show the results when the measurement was carried out from the upward direction, the downward direction, and the left and right directions respectively indicated in FIG. 2.

In FIG. 8(a) through FIG. 8(c), the curves L1, L4, and L7 indicated by the alternate long and short lines represent the sample #16 adopting, as the liquid crystal layer 8, a liquid crystal material having the refractive index anisotropy .DELTA.n(550) of 0.070. The curves L2, L5, and L8 indicated by the solid lines represent the sample #17 adopting, as the liquid crystal layer 8, a liquid crystal material having the refractive index anisotropy .DELTA.n(550) of 0.080. The curves L3, L6, and L9 indicated by the dotted lines represent the sample #18 adopting, as the liquid crystal layer 8, a liquid crystal material having the refractive index anisotropy .DELTA.n(550) of 0.095.

Also, as a comparative example of the present example, two samples #103 and #104 were prepared. The comparative samples #103 and #104 have an arrangement the same as that of the samples #16 to #18 except that in the comparative samples #103 and #104, such liquid crystal materials are adopted as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, whose refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm are set to 0.060 and 0.120 for the comparative samples #103 and #104, respectively. The comparative samples #103 and #104 thus prepared were set in the measuring system of FIG. 7, and the light-receiving element 21 was fixed to a position with a certain angle .phi. in the same manner as the present example so as to measure the output level of the light-receiving element 21 against a voltage applied to the comparative samples #103 and #104.

In the measurement, as in the present example, the light-receiving element 21 was positioned with an angle .phi. of 50.degree., and supposing that the y-direction and the x-direction respectively direct to the left side and the bottom side of the screen, the position of the light-receiving element 21 was changed so as to carry out the measurement from the upward direction, the downward direction, and the left and right directions.

The results of the measurement are shown in FIG. 9(a) through FIG. 9(c). FIG. 9(a) through FIG. 9(c) are graphs respectively showing transmittance of light against a voltage applied to the comparative samples #103 and #104 (transmittance vs. applied voltage).

FIG. 9(a), FIG. 9(b), and FIG. 9(c) respectively show the results when the measurement was carried out from the upward direction, the downward direction, and the left and right directions respectively indicated in FIG. 2.

In FIG. 9(a) through FIG. 9(c), the curves L10, L12, and L14 indicated by the solid lines represent the comparative sample #103 adopting, as the liquid crystal layer 8, a liquid crystal material having the refractive index anisotropy .DELTA.n(550) of 0.060, and the curves L11, L13, and L15 indicated by the dotted lines represent the comparative sample #104 adopting, as the liquid crystal layer 8, a liquid crystal material having the refractive index anisotropy .DELTA.n(550) of 0.120.

When the samples #16 to #18 of the present example and the comparative samples #103 and #104 of the comparative example were compared with respect to transmittance versus applied voltage in the upward direction, it was found that in FIG. 8(a), the transmittance, as indicated by the curves L1, L2, and L3, decreased by a sufficient amount as the applied voltage was increased. In contrast, in FIG. 9(a), compared with the curves L1, L2, and L3 of FIG. 8(a), the transmittance, as indicated by the curve L11, did not decrease by a sufficient amount even though an applied voltage was increased. On the other hand, the curve L10 exhibited the reversion phenomenon in which the transmittance increased after an initial drop as the applied voltage was increased.

In the same manner, when the samples #16 to #18 of the present example and the comparative samples #103 and #104 of the comparative example were compared with respect to transmittance versus applied voltage in the downward direction, it was found that in FIG. 8(b), the transmittance, as indicated by the curves L4, L5, and L6, decreased to substantially 0 as the applied voltage was increased. Also, in FIG. 9(b), as in FIG. 8(b), the transmittance, as indicated by the curve L12, decreased to substantially 0 with an increased applied voltage. However, the curve 13 exhibited the reversion phenomenon.

Also, when the samples #16 to #18 of the present example and the comparative samples #103 and #104 of the comparative example were compared with respect to transmittance versus applied voltage in the left and right directions, while the transmittance, as indicated by the curves L7, L8, and L9 of FIG. 8(c), and the transmittance, as indicated by the curve L14 of FIG. 9(c), all decreased to substantially 0 with an increased applied voltage, only the curve L15 of FIG. 9(c) exhibited the reversion phenomenon.

Further, visual inspections were carried out using the samples #16 to #18 and the comparative samples #103 and #104 under the white light. The results are as follows.

A desirable image with no coloration was obtained in the samples #16 to #18 and the comparative sample #103 when observed from any direction in a viewing angle of 50.degree.. In contrast, yellow to orange coloration was observed in the comparative sample #104 when observed from the left and right directions in a viewing angle of 50.degree..

It can be seen from the results of the measurements that, as shown in FIG. 8(a) through FIG. 8(c), when such liquid crystal materials are adopted as the liquid crystal layer 8, whose refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm are respectively set to 0.070, 0.080, and 0.095, the transmittance thereof are decreased by a sufficient amount without no reversion phenomenon, thus making the viewing angle wider. Also, since no coloration phenomenon is generated, the displaying quality of the liquid crystal display device is greatly improved.

In contrast, as shown in FIG. 9(a) through FIG. 9(c), when such liquid crystal materials are adopted as the liquid crystal layer 8, whose refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm are respectively set to 0.060 and 0.120, the viewing-angle-dependency is not improved satisfactorily.

In the present example, the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is tilt-aligned on the transparent support base are adopted. However, by adopting the optical phase difference plates 2 and 3 having an arrangement wherein the discotic liquid crystal is hybrid-aligned rather than tilt-aligned, the results same as that obtained in the samples #16 to #18 and the comparative samples #103 and #104 were obtained as well.

Further, when the dependency of a transmittance-applied voltage characteristic on the tilt-angle .theta. was examined by changing the tilt-angle .theta. of the refractive index ellipsoid of the optical phase difference plates 2 and 3, it was found that as long as the tilt-angle .theta. is in a range of 15.degree..ltoreq..theta..ltoreq.75.degree., the same transmittance-applied voltage characteristic was maintained regardless of the alignment state of the discotic liquid crystal of the optical phase difference plates 2 and 3. Note that, in the case where the tilt-angle .theta. exceeds the range of 15.degree..ltoreq..theta..ltoreq.75.degree., it was found that the viewing angle does not widen in the opposite viewing direction.

Also, when the dependency of a transmittance-applied voltage characteristic on the second retardation value was examined by changing the second retardation value of the optical phase difference plated 2 and 3, it was found that as long as the second retardation value falls in a range of 80 nm to 250 nm, the same transmittance-applied voltage characteristic was maintained regardless of the alignment state of the discotic liquid crystal. Note that, in the case where the second retardation value exceeds the range of 80 nm to 250 nm, it was found that the viewing angle does not widen in the lateral direction (left and right directions).

Also, based on the results of the visual inspections obtained by using the comparative samples #103 and #104, three samples #19 to #21 were prepared. The samples #19 to #21 have an arrangement the same as that of the samples adopted in the present example except that in the samples #19 to #21, such liquid crystal materials are adopted as the liquid crystal layer 8 of the liquid crystal cell 16 provided in the liquid crystal display device of FIG. 1, whose refractive index anisotropy .DELTA.n(550) for the wavelength of 550 nm are set to 0.065, 0.100, and 0.115 for the samples #19 to #21, respectively. The samples #19 to #21 thus prepared were set in the measuring system of FIG. 7, and the light-receiving element 21 was fixed to a position with a certain angle .phi. in the same manner as above so as to measure the output level of the light-receiving element 21 in response to a voltage applied to the samples #19 to #21. The visual inspections were carried out under the white light.

The results showed that in the samples #20 and #21 whose refractive index anisotropy .DELTA.n(550) are respectively 0.100 and 0.115, when the angle .phi. is 50.degree., a slight increase in the transmittance was observed in response to an increase in the voltage in the left and right directions. However, no reversion phenomenon was observed in the inspection such that the slight increase in the transmittance observed here is tolerable in an actual use. Also, in the samples #20 and #21, no problem was presented in the upward direction. In contrast, in the sample #19 whose refractive index anisotropy .DELTA.n(550) is 0.065, as in the comparative sample #103, the transmittance exhibited a curve which rises after an initial drop. However, compared with that of the comparative example #103 of FIG. 9(a), the rise in the transmittance is smaller, and therefore is tolerable in an actual use. Also, in the sample #19, no problem was presented in the left and right directions.

In the visual inspections, in the samples #20 and #21, slight yellow to orange coloration was observed, but only to the extent which does not pose a problem. Similarly, in the sample #19, slight blue coloration was observed, but only to the extent which does not pose a problem.

In addition to the measurements described above, the transmittance in a direction normal to the surface of the liquid crystal cell 16 during white displaying was measured by applying a voltage of substantially 1V using the sample #19 and the comparative sample #103. The results are as follows: in the comparative sample #103, a decrease in transmittance was observed which is not tolerable in an actual use, whereas in the sample #19, a slight decrease in transmittance was observed, but only to the extent which is tolerable in an actual use.

In the present example, in the same manner as Fourth Example, the viewing-angle-dependency of the liquid crystal display device was measured by using the measuring system of FIG. 7.

In the present example, three samples #22 to #24 were prepared by adopting liquid crystal materials whose ratios of the first divided portion 8a and the second divided portion 8b in the liquid crystal cell 16 of FIG. 1 are set to 6 to 4, 17 to 3, and 19 to 1, respectively.

As the optical phase difference plates 2 and 3 respectively adopted in the samples #22 to #24, the optical phase difference plates 2 and 3 of the first example in which the discotic liquid crystal is tilt-aligned are adopted.

The measurement was conducted using the samples #22 to #24 set in the measuring system of FIG. 7, and the light-receiving element 21 fixed to a position with a certain angle .phi.. In the measurement, the output level of the light-receiving element 21 against an applied voltage to the samples #22 to #24 was measured.

In the measurement, the light-receiving element 21 was positioned with an angle .phi. of 30.degree., and supposing that the y-direction and the x-direction respectively direct to the left side and the bottom side of the screen, the position of the light-receiving element 21 was changed so as to carry out the measurement from the upward direction, the downward direction, the left direction, and the right direction. The measurement was also conducted by positioning the light-receiving element 21 in a direction of the z-axis.

The results of the measurement are shown in FIG. 10(a) through FIG. 10(c). FIG. 10(a) through FIG. 10(c) are graphs respectively showing transmittance of light against a voltage applied to the samples #22 to #24 (transmittance vs. applied voltage). FIG. 10(a), FIG. 10(b), and FIG. 10(c) are the results of measurement for the sample #22 with a dividing ratio of 6 to 4, the sample #23 with a dividing ratio of 17 to 3, and the sample #24 with a dividing ratio of 19 to 1, respectively.

In FIG. 10(a) through FIG. 10(c), the curves L21, L22, and L23 respectively indicated by the solid line, broken line, and dotted line represent the transmittance-applied voltage characteristics in the direction of z-axis, the downward direction, and the right direction, respectively. The curves L24 and L25 respectively indicated by the alternate long and short lines and the alternate long and two short lines represent the transmittance-applied voltage characteristics in the upward direction and left direction, respectively.

As is clear from FIG. 10(b), it was found that in the transmittance-applied voltage characteristics in the half-tone displaying range, the curves L22, L23, L24, and L25 are in vicinities of L21. Thus, in the half-tone displaying range, a substantially uniform viewing angle characteristic can be obtained in the viewing directions of up, down, left and right.

In the measurement in downward direction, low transmittance of a constant value, i.e., 7 percent was maintained during the on state, and no reversion phenomenon was observed. In contrast, in the measurement in upward direction, it was found that the transmittance was lower than that in the downward direction, and is sufficiently low during the on state.

Also, as shown in FIG. 10(a) through FIG. 10(c), when the dividing ratio of 6 to 4, in the half-tone displaying range during the on state, the curve L22 (downward direction) and the curve L24 (upward direction) are close to each other, and as the dividing ratio becomes larger, the curves L22 and L24 come even closer. Also, as shown in FIG. 10(c), when the dividing ratio of 19 to 1, the curve L22 (downward direction) comes close to the curve L21 (z-axis direction), and as the dividing ratio becomes smaller, the curves L22 and L21 come even closer. This suppresses the darkening of display image in the downward direction (standard viewing direction).

Also, when the dividing ratio is set in a range of 7 to 3 and 9 to 1, as in the dividing ratio of 17 to 3, an improvement can be made as a desirable viewing angle characteristic which is well-balanced in the downward direction and the upward direction is obtained.

Note that, in the present liquid crystal display device, two optical phase difference plates 2 and 3 are provided on the both sides of the liquid crystal display element 1. However, the viewing angle characteristic can also be improved in the described manner by providing only one optical phase difference plate. In the case where only one optical phase difference plate is provided, although an improvement is made as a well-balanced viewing angle characteristic is obtained in the upward and downward directions, the viewing angle characteristics become asymmetrical in the left and right directions. In contrast, in the case where two optical phase difference plates are provided, the viewing angle characteristics can be improved not only in the upward and downward directions as in the case of providing one optical phase difference plate but also in the left and right directions as symmetric viewing angle characteristics are obtained.

Also, as a comparative example of the present example, a comparative sample #105 whose ratio of the first divided portion 8a to the second divided portion 8b are set to 1 to 1 was prepared. In the same manner as above, the comparative sample #105 was set in the measuring system of FIG. 7 and the viewing angle characteristics were measured. The results of the measurement are shown by the graph of transmittance versus applied voltage in FIG. 11.

In the graph of FIG. 11, the curves L31, L32, L33, L34, and L35 respectively indicated by the solid line, the broken line, the dotted line, the alternate long and short lines, and the alternate long and two short lines represent the transmittance-applied voltage characteristics in the direction of z-axis, the downward direction, right direction, upward direction, and left direction, respectively.

It was found from the results of the measurement that in the left and right directions, a sufficiently low transmittance is obtained during the on state, and therefore no problem is presented in the viewing angle characteristic. On the other hand, in the upward and downward directions, it was found that the transmittance is not sufficiently low during the on state. Thus, in the liquid crystal display device of the present comparative example, the viewing-angle-dependency is presented in the upward and downward directions.

Also, it is preferable, in the case where a divided liquid crystal layer is composed of the first divided portion 8a and the second divided portion 8b, that the ratio of the first divided portion 8a to the second divided portion 8b is set in a range of 6 to 4 and 19 to 1. This improves the viewing angle characteristics. Further, by setting the ratio of the first divided portion 8a to the second divided portion 8b in a range of 7 to 3 and 9 to 1, it is possible to realize remarkably desirable viewing angle characteristics.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.