U.S. patent application number 12/839015 was filed with the patent office on 2010-11-11 for vertically aligned liquid crystal display device having an optimized viewing angle.
Invention is credited to Sang-II Kim, Tae-Hwan KIM, Young-Chol Yang.
Application Number | 20100285714 12/839015 |
Document ID | / |
Family ID | 36696385 |
Filed Date | 2010-11-11 |
United States Patent
Application |
20100285714 |
Kind Code |
A1 |
KIM; Tae-Hwan ; et
al. |
November 11, 2010 |
VERTICALLY ALIGNED LIQUID CRYSTAL DISPLAY DEVICE HAVING AN
OPTIMIZED VIEWING ANGLE
Abstract
A method of improving the viewing angle of a vertically-aligned
liquid crystal display device is presented. The method involves
designing a uniaxial compensation film to provide a retardation
value of 200 nm or less for light having a wavelength of about 550
nm. Using this uniaxial compensation film, a display device can be
built by obtaining a liquid crystal panel with liquid crystal
molecules contained between glass substrates, coupling the uniaxial
compensation film to at least one of the glass substrates, and
coupling a polarization film and electrodes to the compensation
film. Preferably, the uniaxial compensation film has a thickness
less than or equal to 50 microns. Where there are multiple
compensation films, the total thickness and the total retardation
values should be considered.
Inventors: |
KIM; Tae-Hwan; (Seoul,
KR) ; Kim; Sang-II; (Kyungki-do, KR) ; Yang;
Young-Chol; (Kyungki-do, KR) |
Correspondence
Address: |
Innovation Counsel LLP
21771 Stevens Creek Blvd, Ste. 200A
Cupertino
CA
95014
US
|
Family ID: |
36696385 |
Appl. No.: |
12/839015 |
Filed: |
July 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11351434 |
Feb 9, 2006 |
7760299 |
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12839015 |
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10618142 |
Jul 10, 2003 |
7079208 |
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11351434 |
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Current U.S.
Class: |
445/24 |
Current CPC
Class: |
G02F 1/133555 20130101;
G02F 1/133637 20210101; G02F 1/13363 20130101; G02F 2202/40
20130101; G02F 2413/04 20130101; G02F 1/1393 20130101; G02F
1/133634 20130101 |
Class at
Publication: |
445/24 |
International
Class: |
H01J 9/00 20060101
H01J009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 12, 2002 |
KR |
2002-0040857 |
Claims
1. A method of building a display device, the method comprising:
providing a liquid crystal panel including liquid crystal molecules
contained between glass substrates, wherein long axes of the liquid
crystal molecules are oriented orthogonal to the glass substrates
in absence of electrical field; coupling a set of compensation
films to at least one of the glass substrates, wherein the set of
compensation films includes one or more uniaxial compensation films
and provides a total retardation value less than or equal to 200 nm
for light having a wavelength of about 550 nm; coupling a
polarization film to the set of compensation films; and coupling
electrodes to the liquid crystal panel.
2-23. (canceled)
Description
RELATED APPLICATION
[0001] This applications claims priority, under 35 U.S.C. 119, from
Korean Patent Application No. 2002-0040857 filed on Jul. 12, 2002,
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a liquid crystal display
devices.
[0004] 2. Description of Related Art
[0005] A liquid crystal display ("LCD") device includes upper and
lower panels provided with field-generating electrodes thereon, a
liquid crystal layer interposed therebetween, a pair of a polarizer
and an analyzer, compensation films, etc. The LCD generates
electric field in the liquid crystal layer by applying electric
voltages to the field-generating electrodes and adjusts the
intensity of the electric field to control the transmittance of
light passing through the liquid crystal layer, thereby displaying
desired images.
[0006] One of the most widely used types of LCD has a common
electrode and a plurality of pixel electrodes provided on
respective panels and a plurality of thin film transistors ("TFT")
for switching voltages applied to the pixel electrodes, which is
provided on the panel having the pixel electrodes.
[0007] LCDs may operate in one of several modes. An LCD operating
in a vertically-aligned ("VA") mode contains liquid crystal
molecules aligned perpendicular to two panels. VA-mode LCDs are
sometimes preferred for their high contrast ratio and wide viewing
angle.
[0008] LCDs often suffer from light leakage, the severity of which
increases with viewing angle. The light leakage, which causes poor
visibility from the sides and a narrow viewing angle, is caused by
variations in light path and in the effective angle made by the
polarizer and the analyzer depending on the viewing directions.
[0009] Compensation films are sometimes used to neutralize the
effect of these variations. However, use of compensation films
usually significantly increases the cost of the LCD because they
are expensive and there is no efficient way to select the
compensation film that yields optimal results. A method of
determining the optimal parameters of a compensation film without
the costly trial-and-error process is needed in order to allow more
LCD applications to take advantage of compensation films.
SUMMARY
[0010] The invention is a method of improving the viewing angle of
a vertically-aligned liquid crystal display device and a display
device made using this method. In more detail, the method involves
providing liquid crystal molecules positioned between a first glass
substrate and a second glass substrate, and coupling a uniaxial
compensation film to at least one of the glass substrates such that
the uniaxial compensation film provides a retardation value of 200
nm or less for light having a wavelength of about 550 nm. When
building a display device, a liquid crystal panel with liquid
crystal molecules contained between glass substrates is first
provided. The liquid crystal molecules are positioned such that the
long axes of the liquid crystal molecules are oriented orthogonal
to the glass substrates in the absence of electrical field. Then, a
set of compensation films are coupled to at least one of the glass
substrates. The set of compensation films includes one or more
uniaxial compensation films and provides a total retardation value
of less than or equal to 200 nm for light having a wavelength of
about 550 nm. A polarization film is coupled to the set of
compensation films, and coupling electrodes are coupled to the
liquid crystal panel.
[0011] As stated above, the invention also includes a display
device built using the above method. More specifically, the
invention includes a display device including a liquid crystal
layer disposed between glass substrates so that long axes of liquid
crystal molecules are oriented orthogonal to the glass substrates,
a set of compensation films coupled to at least one of the glass
substrates, wherein the set of compensation films are selected
based on having a total retardation value less than or equal to 200
nm for light having a wavelength of about 550 nm, a polarization
film coupled to the set of compensation films, and a first
electrode and a second electrode coupled to the glass
substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and other advantages of the present invention will
become more apparent by describing preferred embodiments thereof in
detail with reference to the accompanying drawings in which:
[0013] FIG. 1 is a sectional view of an LCD according to an
embodiment of the present invention;
[0014] FIG. 2 is a layout view of a TFT array panel of an LCD
according to an embodiment of the present invention;
[0015] FIG. 3 is a sectional view of the TFT array panel shown in
FIG. 2 taken along the line III-III';
[0016] FIG. 4 is a layout view of a color filter array panel of an
LCD according to an embodiment of the present invention;
[0017] FIG. 5 is a sectional view of an LCD according to another
embodiment of the present invention;
[0018] FIG. 6 is a sectional view of a TFT array panel of an LCD
according to another embodiment of the present invention;
[0019] FIG. 7 is a sectional view of an LCD according to another
embodiment of the present invention;
[0020] FIG. 8 is a layout view of a TFT array panel of an LCD
according to another embodiment of the present invention;
[0021] FIG. 9 is a sectional view of the TFT array panel shown FIG.
8 taken along the line IX-IX';
[0022] FIG. 10 is a graph showing reflectance as function of
applied voltage for a transflective LCD with and without uniaxial
(C-plate) compensation films;
[0023] FIG. 11 is a graph showing transmittance as function of
applied voltage for a transflective LCD with and without uniaxial
(C-plate) compensation films;
[0024] FIGS. 12A to 12F are graphs showing isocontrast curves of a
reflective type LCD without and with one C-plate attached to the
upper panel;
[0025] FIGS. 13A to 13F are graphs showing isocontrast curves of a
transmissive type LCD without and with one C-plate attached to the
upper panel;
[0026] FIGS. 14A to 14E are graphs showing isocontrast curves of a
reflective type LCD with C-plates attached to both upper and lower
panels; and
[0027] FIGS. 15A to 15E are graphs showing isocontrast curves of a
transmissive type LCD with C-plates attached to both upper and
lower panels.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein.
[0029] In the drawings, the thickness of layers and regions are
exaggerated for clarity. Like numerals refer to like elements
throughout. It will be understood that when an element such as a
layer, film, region, substrate or panel is referred to as being
"on" another element, it can be directly on the other element or on
one or more intervening elements. In contrast, when an element is
referred to as being "directly on" another element, there are no
intervening elements present.
[0030] Then, LCDs according to embodiments of the present invention
will be described in detail with reference to the drawings.
[0031] FIG. 1 is a sectional view of a transmissive type LCD
according to an embodiment of the present invention.
[0032] An LCD according to this embodiment includes a TFT array
panel 1 and a color filter array panel 2 facing each other, and a
liquid crystal layer 3 interposed between the two panels 1 and 2.
The LCD also includes first and second polarization films 12 and 22
having nonparallel polarization axes, and first and second
protective films 13 and 23 preferably made of TAC (triacetate
cellulose) films and attached on the first and the second
polarization films 12 and 22 for protecting the polarization films
12 and 22, respectively. The LCD further includes a first uniaxial
(C-plate) compensation film 14 inserted between the TFT array panel
1 and the first protective film 13, and a second uniaxial
compensation film 24 inserted between the color filter array panel
2 and the second protective film 23.
[0033] The LCD is in a vertically-aligned (VA) mode. That is, the
liquid crystal layer 3 of the LCD includes liquid crystal molecules
aligned to make their long axes substantially perpendicular to the
two panels 1 and 2.
[0034] The first and the second protective films 13 and 23 generate
slight retardation. In addition, the uniaxial compensation films 14
and 24 have negativity and generate retardation in a range between
about 0 nm and about 200 nm for the light having a wavelength of
550 nm. Here, the uniaxiality means that nx=ny.noteq.nz and the
negativity means that nx=ny>nz, where nx, ny and nz denote the
refractive indices of x, y and z directions, respectively.
[0035] The first uniaxial compensation film 14 may be omitted.
[0036] Now, a TFT array panel and a color filter array panel of an
LCD according to embodiments are described in more detail.
[0037] FIG. 2 is a layout view of a TFT array panel for an LCD
according to an embodiment of the present invention, and FIG. 3 is
a sectional view of the TFT array panel shown in FIG. 2 taken along
the line III-III'.
[0038] As shown in FIGS. 2 and 3, a gate wire 121, 123 and 125
preferably made of a metal having low resistivity such as aluminum,
silver, etc. is formed on a transparent insulating substrate 110.
The gate wire 121, 123 and 125 includes a plurality of gate lines
121 extending in a transverse direction and a plurality of gate
electrodes 123 connected to the gate lines 121. An end portion 125
of each gate line 121 is widened for connection with an external
circuit.
[0039] A gate insulating layer 140 is formed on the entire surface
of the substrate including the gate wire 121, 123 and 125.
[0040] A plurality of semiconductor stripes 151, 153 and 159
preferably made of amorphous silicon are formed on the gate
insulating layer 140, and a plurality of ohmic contacts 161, 162,
163 and 165 preferably made of amorphous silicon heavily doped with
n-type impurity are formed on the semiconductor stripes 151, 153
and 159.
[0041] A data wire 171, 173, 175, 177 and 179 preferably made of a
metal having low resistivity such as aluminum, silver, etc. is
formed on the ohmic contacts 161, 162, 163 and 165 and the gate
insulating layer 140.
[0042] The data wire 171, 173, 175, 177 and 179 includes a
plurality of data lines 171 intersecting the gate lines 121 to
define a plurality of pixel areas, a plurality of source electrodes
173 which are branches of the data lines 171 and connected to the
ohmic contacts 163, a plurality of drain electrode 175 separated
from the source electrodes 173 and formed on the ohmic contacts 165
opposite to the source electrodes 173 with respect to the gate
electrodes 123, and a plurality of storage electrodes 177
overlapping the gate lines 121 to form storage capacitors. An end
portion 179 of each data line 171 is widened for connection with an
external circuit.
[0043] A passivation layer 180 is formed on the data wire 171, 173,
175, 177 and 179. The passivation layer 180 has a plurality of
first contact holes 181 exposing the drain electrodes 175, a
plurality of second contact holes 182 exposing the end portions 125
of the gate lines, a plurality of third contact holes 183 exposing
the end portions 179 of the data lines 171, and a plurality of
fourth contact holes 184 exposing the storage electrodes 177.
[0044] A plurality of pixel electrodes 190 and a plurality of
contact assistants 95 and 97 are formed on the passivation layer
180. The pixel electrodes 190 are connected to the drain electrodes
175 and the storage electrodes 177 via the first and the fourth
contact holes 181 and 184, respectively, and the contact assistants
95 and 97 are connected to the exposed end portions 125 of the gate
lines 121 and the exposed end portions 179 of the data lines 171
via the second and the third contact holes 182 and 183,
respectively. The pixel electrodes 190 and the contact assistants
95 and 97 are preferably made of transparent material such as ITO
(indium tin oxide) or IZO (indium zinc oxide).
[0045] FIG. 4 is a layout view of a color filter array panel of an
LCD according to an embodiment of the present invention.
[0046] A black matrix 220 is formed on an insulating substrate 210,
a plurality of color filters 230 are formed on the black matrix
220, and a common electrode 270 is formed on the color filters 230.
The common electrode 270 is preferably made of a transparent
conductive material such as ITO or IZO.
[0047] FIG. 5 is a sectional view of a reflective type LCD without
separate light source according to another embodiment of the
present invention.
[0048] An LCD according to this embodiment includes a TFT array
panel 1 and a color filter array panel 2 facing each other, and a
liquid crystal layer 3 interposed between the two panels 1 and 2.
The LCD further includes a polarization film 22 and a protective
film 23 attached on the polarization film 22 for protecting the
polarization film 22. The LCD also includes a uniaxial compensation
film 24 and a reverse dispersion phase difference film 25 inserted
between the color filter array panel 2 and the protective film
23.
[0049] The LCD is in a VA mode. The protective film 23 generates
slight retardation, and the uniaxial compensation film 24 has
negativity and generates retardation ranging 0 nm to 200 nm for the
light with 550 nm wavelength.
[0050] FIG. 6 is a sectional view of a TFT array panel of an LCD
according to another embodiment of the present invention.
[0051] Referring to FIG. 6, a gate wire 121, 123 and 125, a gate
insulating layer 140, a plurality of semiconductor stripes 151 and
153, a plurality of ohmic contacts 161, 162, 163 and 165, a data
wire 171, 173, 175, 177 and 179, a passivation layer 180, a
plurality of pixel electrodes 190, and a plurality of contact
assistants 95 and 97 are formed on a substrate 110.
[0052] The surface of the passivation layer 180 has embossment
including prominences/protrusions and depressions, and the pixel
electrodes 190 are preferably made of a metal having good
reflectance such as aluminum.
[0053] FIG. 7 is a sectional view of a transflective LCD according
to another embodiment of the present invention.
[0054] An LCD according to this embodiment includes a TFT array
panel 1 and a color filter array panel 2 facing each other, and a
liquid crystal layer 3 interposed between the two panels 1 and 2.
The LCD also includes a pair of first and second polarization films
12 and 22, and a pair of first and second protective films 13 and
23 attached on the polarization films 12 and 22, respectively. The
LCD further includes a first uniaxial (C-plate) compensation film
14 and a first reverse dispersion phase difference film 15 inserted
between the TFT array panel 1 and the first protective film 13, and
a second uniaxial (C-plate) compensation film 24 and a second
reverse dispersion phase difference film 25 inserted between the
color filter array panel 2 and the second protective film 23.
[0055] The LCD is in a VA mode. The first and the second protective
films 13 and 23 generate slight retardation, and the first and the
second uniaxial compensation films 14 and 24 have negativity and
generate retardation in a range from 0 nm to 200 nm for the light
with 550 nm wavelength. The first uniaxial compensation film 14 may
be omitted.
[0056] FIG. 8 is a layout view of a TFT array panel of an LCD
according to an embodiment of the present invention, and FIG. 9 is
a sectional view of the TFT array panel shown in FIG. 8 taken along
the line IX-IX'.
[0057] Referring to FIG. 8, a gate wire 121, 123 and 125, a gate
insulating layer 140, a plurality of semiconductor stripes 151 and
153, a plurality of ohmic contacts 161, 162, 163 and 165, a data
wire 171, 173, 175, 177 and 179, and a passivation layer 801 are
formed on a substrate 110.
[0058] A plurality of transparent electrodes 90 and a plurality of
contact assistants 95 and 97 preferably made of ITO or IZO are
formed on the passivation layer 801. An interlayer insulating layer
802 having an embossed surface is formed on the transparent
electrodes 90. A plurality of reflecting electrodes 80 are formed
on the interlayer insulating layer 802, and each reflecting
electrodes 80 has a window 82 for light transmission.
[0059] Various characteristics of various types of LCDs with
various types of compensation films were measured.
[0060] The LCDs used for the measurement have conditions shown in
TABLE 1 and TABLE 2.
TABLE-US-00001 TABLE 1 Mode VA Dopant natural pitch of 67 microns
Twist Angle 90 degrees Pretilt Angle 89 degrees K11 13.0 pN K22 5.1
pN K33 14.7 pN .epsilon..parallel. 3.6 .epsilon..perp. 7.4 Cell Gap
2.89 microns
[0061] Here, K11, K22 and K33 are elastic coefficients of
spreading, twisting and bending measured in pico-newton (pN) and
.epsilon..sub..parallel. and .epsilon..perp. are permittivity
parallel to and perpendicular to the director, respectively.
TABLE-US-00002 TABLE 2 .DELTA.nd for Thickness 550 nm n.infin.
A(nm.sup.-.sup.2) (microns) wavelength Liquid VA ne 1.5369 7651.0
2.89 240 nm Crystal no 1.4607 5569.0 Reverse ne 1.5934 -268.8 52.14
142.86 nm dispersion.lamda./ no 1.59 0 4 plate TAC nx ny nz 80 --
1.4800 1.4798 1.4791 C-Plate nx ny nz 20 80 nm 1.504 1.504
1.500
[0062] Here, ne is the refractive index parallel to the director
(i.e. for extraordinary ray) and no is the refractive index
perpendicular to the director (i.e. for ordinary ray), while
.DELTA.n=ne-no. In addition, the dispersion relation is given
by:
n ( .lamda. ) = n .infin. + A .lamda. 2 , ##EQU00001##
where n.sub..infin. is the refractive index for infinite wavelength
and A is a constant.
[0063] FIGS. 10 and 11 are graphs respectively showing reflectance
and transmittance as function of applied voltage for a
transflective LCD with and without uniaxial (C-plate) compensation
films.
[0064] The curves show that the presence of the uniaxial
compensation films hardly affects the reflectance and the
transmittance of the LCD.
[0065] FIGS. 12A to 12F are graphs showing isocontrast curves of a
reflective type LCD without and with one C-plate attached to the
upper panel. FIGS. 12A to 12F show the isocontrast curves for the
cases 2 to 7 in the TABLE 3, respectively.
TABLE-US-00003 TABLE 3 Reflective Type Number/ Viewing Angle Areal
Thickness(um)/ Reflectance Front (up/down/left/right) Isocontrast
Case Mode .DELTA.nd of C plate (%) view CR CR 2:1 Ratio (CR 10:1) 1
TN None 11.7 19.9 47/34/80/66 0.861 2 VA None 16.9 26.0 68/68/51/51
0.757 3 VA One/20/80 nm 16.9 25.8 80/80/79/79 1 4 VA One/40/160 nm
16.9 24.0 55/55/68/68 1.324 5 VA One/60/240 nm 16.9 22.1
42/42/50/50 0.987 6 VA One/80/320 nm 16.9 26.6 35/35/42/42 0.723 7
VA One/100/400 nm 16.9 26.6 31/31/36/36 0.603
[0066] FIGS. 13A to 13F are graphs showing isocontrast curves of a
transmissive type LCD without and with one C-plate attached to the
upper panel. FIGS. 13A to 13F show the isocontrast curves for the
cases 2 to 7 in TABLE 4, respectively.
TABLE-US-00004 TABLE 4 Transmissive type Number/ Viewing Angle
Areal Thickness(um)/ Transmittance Front (up/down/left/right)
Isocontrast Case Mode .DELTA.nd of C plate (%) View CR CR 2:1 Ratio
(CR 10:1) 1 TN None 7.4 378.4 59/59/80/80 1.065 2 VA None 13.0
881.6 80/47/80/80 1.404 3 VA One/20/80 nm 13.0 880.3 80/40/80/79
1.55 4 VA One/40/160 nm 13.0 881.9 60/34/70/63 1.410 5 VA
One/60/240 nm 13.0 880.7 50/31/57/54 1.177 6 VA One/80/320 nm 13.0
881.0 44/30/50/49 0.925 7 VA One/100/400 nm 13.0 882.0 39/27/45/44
0.797
[0067] FIGS. 14A to 14E are graphs showing isocontrast curves of a
reflective type LCD with two C-plates respectively attached to
upper and lower panels. FIGS. 14A to 14E show the isocontrast
curves for Cases 8 to 12, respectively.
TABLE-US-00005 TABLE 5 Reflective type Viewing Angle Areal
Thickness(um)/ Reflectivity Front (up/down/left/right) Isocontrast
Case Mode .DELTA.nd of C plate (%) view CR CR 2:1 Ratio (CR 10:1) 8
VA 10/40 nm .times. 2 16.9 23.2 75/78/62/62 0.861 9 VA 20/80 nm
.times. 2 16.9 25.8 80/80/79/79 1 10 VA 30/120 nm .times. 2 16.9
24.3 69/69/80/80 1.209 11 VA 40/160 nm .times. 2 16.9 24.0
55/55/68/68 1.324 12 VA 50/200 nm .times. 2 16.9 24.3 47/47/58/58
1.242
[0068] FIGS. 15A to 15E are graphs showing isocontrast curves of a
transmissive type LCD with two C-plates respective attached to
upper and lower panels. FIGS. 15A to 15E show the isocontrast
curves for Cases 8 to 12 in TABLE 6, respectively.
TABLE-US-00006 TABLE 6 Transmissive Type Viewing Angle Areal
Thickness(um)/ Transmittance Front (up/down/left/right) Isocontrast
Case Mode .DELTA.nd of C plate (%) View CR CR 2:1 Ratio (CR 10:1) 8
VA 10/40 nm .times. 2 13.0 881.0 80/39/80/78 1.565 9 VA 20/80 nm
.times. 2 13.0 881.8 59/34/75/63 1.430 10 VA 30/120 nm .times. 2
13.0 881.2 49/31/58/54 1.215 11 VA 40/160 nm .times. 2 13.0 881.6
43/29/51/48 0.958 12 VA 50/200 nm .times. 2 13.0 882.1 38/27/46/44
0.817
[0069] In TABLES 3 to 6, the areal isocontrast ratio is an
isocontrast area for the contrast ratio of 10:1 divided by that in
Case 3 of the reflective type LCD. White and black voltages in VA
mode are 3.5V and 1.8V for the reflective type and 4.5V and 1.8V
for transmissive type, respectively. The abbreviation "CR" stands
for contrast ratio.
[0070] The ratios such as 2:1, 5:1, 10:1, 20:1 and 22:1 in the
legends of FIGS. 12A to 15E indicate contrast ratios and the values
(for example, 68/68/51/51 in FIG. 12A) at the bottom of FIGS. 12A
to 15E indicate upper/lower/left/right side viewing angles giving
the contrast ratio of 2:1.
[0071] The measurement values of TABLES 3 to 6 shown in FIGS. 12A
to 12F, 13A to 13F, and 14A to 15E can be summarized as
follows.
[0072] Without uniaxial compensation film (C-plate), the
transmittance, the reflectance, the contrast ration at the front
view, and the viewing angle of the VA mode are superior to those of
the TN mode.
[0073] Compared with Case 2, which does not include a uniaxial
compensation film, Cases 3 and 4 of the VA mode show both improved
viewing angle and improved isocontrast curve, and Cases 3 and 4 of
the transmissive mode show improved isocontrast curves. In
contrast, Cases 5, 6 and 7, each of which includes a compensation
film causing a retardation greater than 160 nm, show deteriorated
viewing angle and deteriorated isocontrast curves. Data indicates
that a uniaxial compensation film providing a retardation value
larger than 160 nm has a detrimental effect on the LCD device.
[0074] With uniaxial compensation films attached to both upper and
lower panels, the isocontrast curves for the transmissive-type LCDs
are improved until the sum of the retardation values of the two
compensation films equals to 160 nm. When the combined retardation
value exceeds 160 nm, both the isocontrast curves and the viewing
angles become worse. For the reflective-type LCDs, only one of the
two compensation films contributes to the total retardation since
light is not transmitted through both of films. Therefore, the
actual retardation values of the compensation films for the cases 8
to 11 are equal to or smaller than 160 nm. This explains why case
12 for the reflective-type LCD shows improved isocontrast curves in
spite of having a retardation value of 200 nm. For the
transmissive-type LCD, although there is no experimental example
for the case of 200 nm retardation, the results shown in TABLES 3
to 6 suggest that the isocontrast curves will be improved where
retardation values are equal to or less than 200 nm.
[0075] The above-described experimental results show a correlation
between the total thickness of the uniaxial compensation film(s)
and the viewing angle and/or the contrast ratio. It appears that
the total thickness of the uniaxial compensation film(s) affects
the viewing angle and/or the contrast ratio more than the number or
the physical arrangement of the uniaxial compensation film(s).
[0076] Of the cases above, Case 3 has optimal characteristics both
for the transmissive type and the reflective type LCDs. Although
some cases show better contrast ratio at the front view than Case
3, the difference is small. Overall, Case 3 resulted in a better
viewing angle than the other cases. Therefore, it can be said that
Case 3 is optimized.
[0077] In conclusion, the total retardation of the uniaxial
compensation film(s) equal to or under 200 nm improves the
isocontrast curve and/or the viewing angle. This improvement is
irrelevant to the number of uniaxial compensation films used and
the type (reflective or the transmissive) of LCD.
[0078] According to the present invention, uniaxial compensation
film(s) generating a predetermined retardation is used to improve
the viewing angle of the LCD.
* * * * *