Compensation films for LCD

Details Compensation films for LCD Compensation films for LCD

2003-04-14

2004-10-19

Pyon, Harold (Department: 1772)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C349S117000, C349S118000, C349S119000, C349S120000, C349S121000

Reexamination Certificate

active

06805924

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical compensation film useful for application to a liquid crystal display, particularly to an Ordinary-mode Normally White Twisted Nematic Liquid Crystal Display. It comprises two layers having retardation values and different tilt angles both within prescribed limitations.
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 and tilted with respect to the plane of the plate, respectively.
Polarizer and Analyzer herein refer to elements that polarize electromagnetic wave. However, one closer to the source of the light will be called polarizer while the one closer to the viewer will be called analyzer. Polarizing elements herein refers to both of polarizer and analyzer.
Viewing direction herein is defined us 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 &phgr; and tilt angle &thgr; are herein used to specify the direction of an optic axis. For the transmission axes of the polarizer and the analyzer, only the azimuthal angle &phgr; is used, as their tilt angle &thgr; is zero.
FIG. 2
shows the definition of the azimuthal angle &phgr; and tilt angle &thgr; to specify the direction of the 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°. The area enclosed within the isocontrast line with contrast ratio, for example, 10 is the viewing angle range with contrast ratio 10 or higher.
Lamination herein means a process of making a single sheet of film by uniting two or more films.
Ordinary-Mode Twisted Nematic Liquid Crystal Display herein means a Twisted Nematic Liquid Crystal Display having the direction of the liquid crystal optic axis at cell surface
311
(or
312
) substantially perpendicular to the transmission axis direction of the adjacent polarizing element
307
(or
309
).
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 analyzers, An 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 direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence (&agr;=0°) to the display and falls off rapidly as the polar viewing angle &agr; 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 nonitors 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 LCD modes, Twisted Nematic (TN) LCD is one of the most prevalent ones.
FIG. 3A
is a schematic of a TN-LCD
313
. A liquid cell
301
is positioned between a polarizer
303
and an analyzer
305
. Their transmission axes
307
,
309
are crossed, meaning that the transmission (or equivalently, absorption) axes of a polarizer and an analyzer form angle 90±10°. Inside the liquid crystal cell, the 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 the liquid crystal optic axes
311
,
312
at the cell surfaces is indicated by a single head arrow. At the surface, the liquid crystal optic axes
311
,
312
have a 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. The azimuthal angle between the of transmission axes
307
of the polarizer
303
and the optic axis
311
of the liquid crystal on the nearest cell surface is 90°. The same relation holds for the transmission axis
309
of the analyzer
305
and the liquid crystal optic axis
312
at the cell surface. 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 of bright OFF state and dark ON state combined with the angular relation between liquid crystal optic axes at the cell surfaces and transmission axes of the polarizing elements is called Ordinary-mode Normally White Twisted Nematic Liquid Crystal Display (O-mode NW-TN-LCD). On the other hand, if the transmission axes of polarizing elements are parallel to the liquid crystal optic axis

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