Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-10-28
2004-11-16
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S119000, C349S121000
Reexamination Certificate
active
06819381
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical compensation films applied to a twisted nematic liquid crystal display. This compensation film improves viewing angle, especially in the vertical direction. It also has a low average tilt angle, and large retardation value and thickness, therefore allows an easy manufacturing.
BACKGROUND OF THE INVENTION
The following terms have the definitions as stated below.
Optic axis herein refers to the direction in which propagating light does not see birefringence.
A-plate, C-plate and O-plate herein are the plates in which the optic axis is in the plane of the plate, perpendicular to the plate surface and tilted with respect to the plane of the plate, respectively.
Polarizer and Analyzer herein refer to elements that polarize electromagnetic wave. One closer to the source of the light is generally called polarizer while the one closer to the viewer is called analyzer.
Viewing direction herein is defined as a set of polar viewing angle &agr; and azimuthal viewing angle &bgr; as shown in
FIG. 1
with respect to a liquid crystal display
101
. The polar viewing angle &agr; is measured from display normal direction
103
and the azimuthal viewing angle &bgr; spans between an appropriate reference direction
105
in the plane of the display surface
107
and the projection
108
of the arrow
109
onto the display surface
107
. Various display image properties, such as contrast ratio, color and brightness are functions of angles &agr; and &bgr;.
Azimuthal angle &thgr; and tilt angle &phgr; are herein used to specify the direction of optic axis. For the transmission axes of the polarizer and the analyzer, only the azimuthal angle is used as their tilt angle is zero.
FIG. 2
shows the definition of the azimuthal angle &phgr; and tilt angle &thgr; to specify the direction of optic axis
201
with respect to the x-y-z coordinate system
203
. The x-y plane is parallel to the display surface
107
, and the z-axis is parallel to the display normal direction
103
. The azimuthal angle &phgr; is the angle between the x-axis and the projection of the optic axis
201
onto the x-y plane. The tilt angle &thgr; is the angle between the optic axis
201
and the x-y plane.
ON (OFF) state herein refers to the state with (without) an applied electric field to the liquid crystal display
101
.
Isocontrast plot herein shows a change in a contrast ratio from different viewing directions. Isocontrast line, on which the contrast ratio is constant (such as 10, 50 and 100), is plotted in polar format. The concentric circle corresponds to polar viewing angle &agr;=20°, 40°, 60° and 80° (outer most circle) and the radial lines indicates azimuthal viewing angle &bgr;=0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°.
Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizer and analyzer. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. 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 required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal 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 (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, 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 (&agr;=0°) 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.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or house hold appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCD's application as a replacement of the traditional CRT will be limited.
Among various modes of LCD, Twisted Nematic (TN) LCD is the most prevalent ones.
FIG. 3A
is a schematic of a TN-LCD
313
. A liquid crystal cell
301
is positioned between polarizer
303
and analyzer
305
. Their transmission axes
307
,
309
are crossed, meaning that the transmission (or equivalently, absorption) axes of polarizer and analyzer form angle 90±10°. Inside the liquid crystal cell, optic axis of liquid crystal shows azimuthal rotation of 90° in the OFF state across the cell thickness direction. In
FIG. 3A
, the direction of liquid crystal optic axis
311
at the cell surfaces is indicated by a single head arrow. At the surface, the liquid crystal optic axes
311
form small tilt angle &thgr;
s
with respect to the cell surfaces in order to prevent the reverse twist. Namely, the tilt is consistent with the sense (clock or counter-clock wise) of the azimuthal rotation in the liquid crystal optic axis in the cell thickness direction. Un-polarized incoming light is linearly polarized by the polarizer
303
and its plane of polarization rotates 90° while traveling through the liquid crystal cell
301
. The plane of polarization of out-coming light from the cell
301
is parallel to the transmission axis
309
of analyzer
305
and will transmit through the analyzer
305
. With sufficiently high-applied voltage, the liquid crystal becomes perpendicular to the cell plane except in the close vicinity of the bounding plates. In this ON state, the incoming polarized light essentially does not see birefringence and thus it is blocked by the analyzer. This mode (bright OFF state and dark ON state) is called Normally White Twisted Nematic Liquid Crystal Display (NW-TN-LCD) mode.
In the display normal viewing direction (&agr;=0°), one can obtain high contrast between the ON and OFF states. However, when the display is viewed from an oblique direction, the propagating light sees birefringence in ON state, thus it is not completely blocked by the analyzer. This light leakage gives low contrast ratio in the oblique direction. As is well known in the art, the degradation in the contrast ratio due to the leakage is more significant in the vertical viewing direction (azimuthal angle &bgr;=90°, 270°) than it is in the horizontal direction (&bgr;=0°, 180°) in TN-LCD. The azimuthal angle &bgr; is measured from a reference direction
205
(shown in FIG.
1
), which is chosen to be at 45° relative to the transmission axes
307
,
309
of the polarizer
303
and analyzer
305
. The isocontrast plot of the display
313
is shown in FIG.
3
B. The lines
315
,
317
,
319
are isocontrast ratio lines for contrast ratios 10, 50 and 100, respectively. The display fails to maintain contrast ratio 10 or higher in the range required for many applications. The contrast ratio quickly deteriorates as the viewing direction deviates from the normal direction. This deterioration is particularly significant in the upper vertic
Chen Janglin
Ishikawa Tomohiro
Mi Xiang-Dong
Chowdhury Tarifur R.
Kluegel Arthur E.
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