Field of the Invention
The present invention relates generally to low-twist-angle nematic liquid crystal cells with optical compensation for wide-viewing-angle applications, and more particularly pertains to the design of liquid crystal (LC) displays (LCDs) and, particularly, with techniques for maximizing the field of view (or viewing angle) of LCDs while maintaining high contrast ratio viewing from near normal incidence and minimizing variance in relative gray levels over a wide range of viewing angles. These goals are achieved by using low-twist-angle (twist angle from about 30.degree. to about 85.degree.) nematic liquid crystal cells with optical compensation films which have a negative birefringence and tilted optical axes away from the planes of the optical compensation films.
In the following description of the present invention, TN (twisted nematic) and LTN (low angle twisted nematic) are used to represent twisted nematic liquid crystal displays with a twist angle of 90.degree. or lower than 90.degree., respectively. For LTN, this usually means a twist angle from about 85.degree. to about 30.degree.. A TN display was invented by Schadt and Helfrich in 1971 (M. Schadt and W. Helfrich, Appln. Phys. Lett. V. 18, 127 (1971)). A liquid crystal (LC) cell, in general, consists of two substrates forming a cavity between them to contain the nematic LC mixture. Between each substrate and the LC medium, there exist a conductive electrode coated on the substrate and an LC alignment layer in direct contact with the LC medium to align the adjacent LC directors into one direction. For a TN LC cell, the direction of LC directors adjacent to one of the substrates is orthogonal (or 90.degree.) to the direction of LC directors adjacent to the other substrate so that the LC directors in the cell twist 90.degree. from one substrate to the other.
Because of their maturity in manufacture and their sufficiency in performance, TN liquid crystal (LC) displays have been widely used in commercial thin-film-transistor (TFT) driven flat panel liquid crystal displays (LCDs). The strong viewing-angle dependencies of the contrast ratio, the brightness, and the grayscale of the TFT-driven TN displays have been recognized as major weaknesses for these displays. To illustrate the viewing-angle problem of the TN LC cells, a TN orientation is defined with different viewing directions.
FIG. 1 shows a TN cell 10 with two substrates 12 and 14. A rubbed polyimide film (not shown in FIG. 1) is usually used to align the LC directors. The rubbing directions of the polyimide films on substrates 12 and 14 are shown as dashed arrow 16 and solid arrow 18, respectively.
For display applications, the TN cell is placed between two polarizers 20, 22 with the transmitting axes of the polarizers being either parallel or perpendicular to the adjacent LC directors. If the transmitting axes 24, 26 of the two polarizers sandwiching the TN cell are crossed to each other, the TN display is operated in a normally-white case where the quiescent state of the TN cell is the bright state of the display. On the other hand, if the axes 24, 26 are parallel to each other, the TN display is operated in a normally-black case where the quiescent state of the TN cell represents the dark state. For the normally-white case, there are two optical Eigen modes, the ordinary-ray (o-) and the extra-ordinary-ray (e-) modes, in which the optical field propagates either parallel or perpendicular to the nematic LC directors in the TN cell, respectively. Such e- and o- modes are illustrated in FIG. 1 where the transmitting axes of the polarizers 20 and 22 are shown.
With the configuration of the TN display shown in FIG. 1, by facing the display, four viewing zones can be defined, an upper viewing zone for viewing from the 12 o'clock direction, a lower viewing zone for viewing from the 6 o'clock direction, a left viewing zone for viewing from the 9 o'clock direction, and a right viewing zone for viewing from the 3 o'clock direction. The sign of angle for the upper and right viewing zones are positive while the sign of angle for the lower and left viewing zones are negative. Traditionally, the o-mode has been used for bi-level displays. Recently, Takano, et al have carried out a detailed comparison between the o-and e-modes of NW, first-minimum TN cells for analog-grayscale full color displays (H. Takano, M. Ikezaki, and S. Suzuki, "Threshold Voltage Biased E-mode TN LCD-Optimum Optical Design for Grayscale Applications," the IV International Topical Meeting on Optics of Liquid Crystals, Oct. 7-11, 1991, Cocoa Beach, Fla.). They paid particular attention to optimizing the angular region that preserves a proper grayscale order (no grayscale reversal), i.e., minimizing the angular region of grayscale reversals for ratios of eight gray levels. They concluded that the e-mode with a near threshold-voltage bias is superior to the o-mode for analog-grayscale applications. For the rest of the description of the present invention, the e-mode is used as an example. The results are applicable to the o-mode as well.
To illustrate the viewing-angle problem of TN for analog-grayscale displays, FIG. 2 shows transmittance as a function of applied voltage for a typical TN cell when the TN cell is being viewed from five different directions. Curves 1, 2, 3, 4 and 5 in FIG. 2 correspond respectively to viewing from normal incidence, 40.degree. from the left viewing zone, 50.degree. from the right viewing zone, 30.degree. from the lower viewing zone, and 30.degree. from the upper viewing zone, where the angles in degrees are defined as the angles of viewing direction with respect to a normal to the display panel. FIG. 2 illustrates that, at a given voltage applied to the TN cell, the brightness (or the contrast ratio) of the display appears different from the above-mentioned five different viewing directions.
To further quantify the viewing-angle problems of a typical TN cell, eight different voltage levels were applied to the TN cell to achieve eight approximately equally-spaced gray levels starting from the brightness to the darkest states of the display. The change of these eight levels as a function of viewing angles in the horizontal and vertical viewing directions are shown in FIGS. 3 and 4, respectively, for a typical TN cell. As shown in FIG. 3, at a horizontal viewing angle of either +40.degree. or -40.degree., the transmittance of the gray level 8 (darkest level near normal incidence) is higher than that of the gray level 7. In this case, we have contrast or gray-level reversal between gray level 8 (g8) and gray level 7 (g7) for these viewing directions. The display will appear annoying if a grayscale reversal occurs between any two gray levels from level 1 to level 8.
FIG. 5 shows iso-contrast curves as a function of viewing angle for a typical TN display. It can be seen that the contrast ratio decreases when the viewing angle deviates further from normal incidence. The TN cell usually has the best contrast ratio near normal incidence. FIG. 5 also shows that, outside the thick solid curves, image (or grayscale) reversal occurs so that the display appears annoying when viewed from these viewing zones having image reversals.
The narrow viewing-angle characteristics of a TN cell are caused by a change of retardation of the TN cell when the viewing direction is changed. To a first degree of approximation, the retardation of a nematic LC cell at normal incidence is proportional to d.DELTA.n, where d is the cell gap and .DELTA.n is the birefringence of the LC medium. When the LC cell is viewed from an oblique direction, the retardation becomes larger because the effective cell-gap becomes larger at this oblique viewing direction. Therefore, it is expected that the viewing-angle characteristics of a TN cell can be improved if one uses a low d.DELTA.n for the TN cell. However, as the d.DELTA.n of the TN cell is lower than approximately 0.48 .mu.m, the brightness of the display is reduced. In order to maintain high brightness, we can reduce the twist angle of the LC cell from 90.degree. as the d.DELTA.n is reduced. Therefore, LTN cells can have wider viewing angles than TN cells because the LTN cells have lower values of d.DELTA.n than TN cells for about the same brightness. The major problem associated with LTN cells is that the contrast ratio around normal incidence decreases as the twist angle of the LTN cell decreases. Therefore, a higher operating voltage is required for LTN cells to achieve similar contrast ratios at normal incidence as the regular TN cells. Higher operating voltage implies higher cost and larger power consumption. In other words, LTN cells may have about the same brightness as TN cells and have wider viewing angles than TN cells but the best viewing zone for LTN cells is no longer along the direction of normal incidence. Since the displays are most frequently viewed along the normal incidence, it is important to shift the best viewing zone of LTN cells into near normal incidence without raising the operating voltage.
One prior art approach by Hirakata, et al. compensates a 70.degree.-twist LTN cell with uni-axial optical compensation films having positive birefringence (J. I. Hirakata, H. Abe, I. Hiyama, K. Kondo, SID 95 DIGEST, (1995) p. 563.). FIG. 6 illustrates this prior art approach. Imagine the paper containing FIG. 6 being the display. We view the display just as we view FIG. 6, and the incident light impinges from the back side of the display with the viewer situated at the front side. The transmitting axes of the polarizer at the back side and the analyzer at the front side are parallel to the x-axis and y-axis, respectively, as shown in FIG. 6. The LC directors align along the n.sub.1 direction adjacent to the back substrate. The LC directors align along the n.sub.2 direction adjacent to the front substrate. The LC directors in the LTN cell twist from the n.sub.1 direction toward the n.sub.2 direction with a total twist angle. The angle, .alpha., between n.sub.1 and the x-axis is approximately equal to the angle (also .alpha.) between n.sub.2 and the y-axis. From FIG. 6, it can be seen that 2.alpha.+ is approximately equal to 90.degree.. Between the polarizer and the LC cell, there is one sheet of optical compensation film. The optical compensation film is uni-axial with its optical axis along the direction c.sub.1. The direction of c.sub.1 is perpendicular to the direction of n.sub.1. There is another sheet of similar optical compensation film between the LC cell and the analyzer. The optical axis of this second compensation film is along the c.sub.2 direction. The direction of c.sub.2 is also perpendicular to the direction of n.sub.2. Both c.sub.1 and c.sub.2 are in the plane of the compensation film. The d.DELTA.n of the 70.degree.-twist cell is about 0.38 .mu.m, and each compensation film has a positive birefringence and a retardation about 0.023 .mu.m. Retardation films with such a small retardation of about 0.023 .mu.m are expensive because it is difficult to maintain a uniform small retardation within about .+-.3% across the whole display area in order to achieve a display with good uniformity.