U.S. patent application number 11/069958 was filed with the patent office on 2005-06-30 for liquid crystal display device and inspection method for a transparent substrate.
This patent application is currently assigned to OPTREX Corporation. Invention is credited to Fertig, Werner, Lauenstein, Christian, Ozeki, Masao.
Application Number | 20050140853 11/069958 |
Document ID | / |
Family ID | 29208166 |
Filed Date | 2005-06-30 |
United States Patent
Application |
20050140853 |
Kind Code |
A1 |
Ozeki, Masao ; et
al. |
June 30, 2005 |
Liquid crystal display device and inspection method for a
transparent substrate
Abstract
A liquid display device of D-STN mode capable of suppressing a
line-shaped ununiformity of brightness in displaying a dark
appearance is presented. A displaying cell 1 and a compensation
cell 21 are formed so that there is a difference of thickness of at
least 0.05 mm between the thickness of a glass substrate 2 at a
front side of the displaying cell 1 and the thickness of a glass
substrate at a rear side of the compensation cell 21. The
displaying cell 1 and the compensation cell 21 are arranged between
a first polarizing plate 32 and a second polarizing plate 33. A
backlight 31 is of a type emitting light whose half value width
with respect to the peak luminance is at least 5 nm.
Inventors: |
Ozeki, Masao; (Kanagawa,
JP) ; Fertig, Werner; (Babenhausen, DE) ;
Lauenstein, Christian; (Babenhausen, DE) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
OPTREX Corporation
Tokyo
JP
OPTREX EUROPE GmbH
Babenhausen
DE
|
Family ID: |
29208166 |
Appl. No.: |
11/069958 |
Filed: |
March 3, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11069958 |
Mar 3, 2005 |
|
|
|
10423940 |
Apr 28, 2003 |
|
|
|
Current U.S.
Class: |
349/74 |
Current CPC
Class: |
G01N 21/896 20130101;
G02F 1/133636 20130101; G01N 2021/9513 20130101; G02F 2203/64
20130101; G01B 11/0675 20130101 |
Class at
Publication: |
349/074 |
International
Class: |
G02F 001/1347 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2002 |
JP |
2002-126916 |
Claims
1-4. (canceled)
5. An inspection method for inspecting a distribution of thickness
of a transparent substrate, characterized in that: a first
transparent substrate having a known distribution of thickness and
a second transparent substrate to be inspected are prepared,
wherein the difference of thickness in average between first and
second transparent substrates is not more than 50 .mu.m, the first
transparent substrate and the second transparent substrate are
arranged so that light transmits through a first substrate and a
second substrate, a substrate gap of at least 0.1 mm is provided
between the first substrate and the second substrate, a light
source for irradiating light whose half value width with respect to
the peak luminance is at least 5 nm is prepared, and the degree of
distribution of thickness of the second transparent substrate is
judged from an interference intensity distribution of light passing
through the first transparent substrate and the second transparent
substrate.
6. The inspection method for a transparent substrate according to
claim 5, wherein the judgment to the interference intensity
distribution generated in the second transparent substrate is made
according to the number of interference fringes generated by
interference of light passing through the first transparent
substrate and the second transparent substrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
device and an inspection method for a transparent substrate. The
present invention relates, in particular, to a liquid crystal
display device of D-STN (Double layered Super-Twisted Nematic)
mode.
[0003] 2. Discussion of Background
[0004] Various kinds of liquid crystal display device of TN
(Twisted Nematic) mode, STN (Super-Twisted Nematic) mode or the
like have widely been used. The liquid crystal display device of TN
mode is of such a system that a nematic liquid crystal is
sandwiched by two glass substrates wherein the direction of long
axis of liquid crystal molecules is twisted 90.degree. between the
glass substrates. The device of STN mode is of a system that the
twist angle is 180.degree. or more in order to obtain a sharp
change in the intensity of transmitting light to an applicable
voltage.
[0005] When a monochrome display of a light source color and a dark
appearance is to be performed in the STN mode, a retardation plate
or a compensation cell is used in addition to a displaying liquid
crystal cell. The system performing the monochrome display of a
light source color and a dark appearance by using the compensation
cell is referred to as D-STN mode. FIG. 19 is a diagrammatically
cross-sectional view of a conventional liquid crystal display
device of D-STN mode. In the D-STN mode, a displaying cell 101 and
a compensation cell 201 are provided between a first polarizing
plate 302 and a second polarizing plate 303, and a backlight 301 is
disposed at a rear surface side of the first polarizing plate 302.
In the following, the side at which the backlight 301 is disposed
is referred to as a rear surface side and the side at which the
second polarizing plate 303 is disposed is referred to as a front
surface side.
[0006] In the displaying cell 101, a liquid crystal layer 105 is
sandwiched by a pair of glass substrates 102, 108 having
transparent electrodes 103, 107. Alignment films 104, 106 are
disposed on opposing surfaces of the transparent electrodes 103,
107.
[0007] In the compensation cell 201, a liquid crystal layer 205 is
sandwiched by a pair of glass substrates 202, 208 having alignment
films 204, 206. The liquid crystal layer 105 of the displaying cell
101 and the liquid crystal layer 205 of the compensation cell 201
have a relation that twist angles are the same and the direction of
twisting of long axis of liquid crystal molecules in either cell is
opposite to that of the other. Further, birefringence values of the
two liquid crystal layers 105, 205 are equal. In addition,
directions of alignment of liquid crystal molecules in the
displaying cell 101 and the compensation cell 201 at sides of
adjacent glass substrates are perpendicular to each other. Namely,
the direction of alignment of liquid crystal molecules in the
liquid crystal layer 105 of the displaying cell 101 at a side of
glass substrate 102 is perpendicular to the direction of alignment
of liquid crystal molecules in the liquid crystal layer 205 of the
compensation cell 201 at a side of glass substrate 208.
[0008] The glass substrate 102 at a front surface side of the
displaying cell 101 and the glass substrate 208 at a rear surface
side of the compensation cell 201 are so designed as to have the
same thickness.
[0009] If the distance (the air space) between the glass substrate
102 at a front surface side of the displaying cell 101 and the
glass substrate 208 at a rear surface side of the compensation cell
201 is too small, interference fringes (Newton rings) are generated
due to the air space. Accordingly, in the conventional technique,
the thickness of the air space formed between the glass substrate
102 and the glass substrate 208 was determined to have a specified
value. The first polarizing plate 302 is bonded to the glass
substrate 108 at a rear surface side of the displaying cell 101,
and the second polarizing plate 303 is also bonded to the glass
substrate 202 at a front surface side of the compensation cell 201.
The direction of polarization axis of the first polarizing plate
302 and the direction of polarization axis of the second polarizing
plate 302 are perpendicular to each other. Further, it is
preferable that a direction of polarization axis of the first
polarizing plate 302 and the direction of alignment of liquid
crystal molecules in the liquid crystal layer 105 of the displaying
cell 101 at a side of glass substrate 108 (a rear surface side)
form an angle of 45.degree., and the direction of polarization axis
of the second polarizing plate 303 and the direction of alignment
of liquid crystal molecules in the liquid crystal layer 205 of the
compensation cell at a side of glass substrate 202 (a front surface
side) form an angle of 45.degree., in order to obtain a display of
good light source color.
[0010] When a dark appearance is to be displayed, no voltage is
applied to the liquid crystal layer 105 of the displaying cell 101,
or a voltage lower than the predetermined threshold voltage is
applied. A first path 351 shown in FIG. 19 indicates the path of
light when the dark appearance is to be displayed. Light from the
backlight 301 presents a linearly polarized light after it has
passed through the first polarizing plate 302. Further, after
passing through the displaying cell 101, it presents an
elliptically polarized light. However, the light is again rendered
to be a linearly polarized light by means of the compensation cell
201. As a result, the light is interrupted by the second polarizing
plate 303 as shown by the first path 351 whereby the liquid crystal
display device presents the dark appearance. When the light
source-color is to be displayed, the state of alignment of liquid
crystal molecules is changed by applying a voltage to the liquid
crystal layer 105 of the displaying cell 101.
[0011] Even in a case that a retardation plate is disposed in place
of the compensation cell 201, the same display as in the D-STN mode
can be realized. However, the quality of display in using the
retardation plate is apt to suffer influence due to a temperature
change. Accordingly, the D-STN mode is often used in, for example,
an in-vehicle liquid crystal display device having a wide range of
temperature change in environment of use.
SUMMARY OF THE INVENTION
[0012] In the conventional liquid crystal display device of D-STN
mode, when a dark appearance was to be displayed by blocking light
from the backlight 301, there was a case causing leakage of light
from the second polarizing plate 303 whereby a line-shaped
ununiformity of brightness as exemplified in FIG. 20 might generate
in the display of the dark appearance. Such leakage of light was
particularly remarkable in a liquid crystal display device using a
backlight having a specified wavelength (e.g., a wavelength of red
color). Accordingly, there has been expected to overcome such
problem of the line-shaped ununiformity of brightness to improve
yield in the production of the liquid crystal display device of
D-STN mode. However, the improvement of yield was difficult because
the cause of the generation of the line-shaped ununiformity of
brightness in the display screen due to the leakage of light was
unclear.
[0013] Further, in the production of a transparent substrate such
as a glass substrate or the like, it is preferred that it is
possible to confirm easily unevenness in the thickness of the
transparent substrate.
[0014] It is an object of the present invention to provide a liquid
crystal display device of D-STN mode presenting a display of good
dark appearance without generating a line-shaped ununiformity of
brightness.
[0015] Further, it is an object of the present invention to provide
an inspection method for a transparent substrate, capable of
confirming easily unevenness in the thickness of a transparent
substrate.
[0016] The inventors of this application have studied the cause of
leakage of light when a dark appearance is to be displayed. As a
result, they have found the cause of leakage of light and the cause
of the line-shaped ununiformity of brightness. FIG. 21 is a diagram
showing the cause of leakage of light. A second path 352 shown in
FIG. 21 indicates an example of a path of light passing through the
second polarizing plate 303. Light from the backlight 301 does not
always trace the first path 351 as shown in FIG. 19, but a part of
light from the backlight 301 is reflected in the displaying cell
101. For example, a part of light, as shown by the second path 352,
which has passed through the liquid crystal layer 105 of the
displaying cell 101 reflects at the glass substrate 102 located at
a front surface side of the displaying cell 101, passes again
through the liquid crystal layer 105, reflects at a transparent
electrode 107 located at a rear surface side of the displaying cell
101 and passes again through the liquid crystal layer 105. In this
case, the light passes through the liquid crystal layer 105 three
times, and also passes through the glass substrate 102 at a front
surface side of the displaying cell 101 three times.
[0017] When light which has passed more than three times through
the liquid crystal layer 105 due to internal reflection in the
displaying cell 101 reaches a compensation cell 201 after having
passed through the glass substrate 102 at a front surface side of
the displaying cell 101, the compensation cell 201 cannot perform
the complete compensation to the light. Namely, it is impossible to
transform the light passing through the displaying cell 101, by
which an elliptically polarized light is presented, into a linearly
polarized light completely. As a result, the light internally
reflected passes through a second polarizing plate 303.
[0018] Further, when the light having passed more than three times
through the liquid crystal layer 105 reaches the compensation cell
201, the light may cause internal reflection at a glass substrate
208 located at a rear surface side of the compensation cell 201. In
this case, the light passes three times through the glass substrate
102 at a front surface side of the displaying cell 101 and the
glass substrate 208 at a rear surface side of the compensation cell
201 as indicated by the second path 352. Then, interference of
light is caused as a result that the light passes plural times
through glass substrates 102, 208 adjacent to each other. Although
the glass substrate 102 and the glass substrate 208 are so designed
as to have the same thickness, there results, in fact, unevenness
in the thickness of the glass substrates during production. For
example, when a glass substrate having a size of 5 cm.times.5 cm is
produced, it is presumed that a difference of thickness of from 1
to 2 .mu.m is resulted between the thickest portion and the
thinnest portion. Accordingly, when the glass substrate 208 and the
glass substrate 102 do not have uniform thicknesses, there appear a
strong portion and a weak portion of interference of light caused
by the adjacent glass substrates 102, 208, whereby a line-shaped
ununiformity of brightness as shown in FIG. 20 is recognized. Such
line-shaped ununiformity of brightness is different from Newton
rings.
[0019] As the size of a glass substrate is larger, the degree of
unevenness in the thickness of the glass substrate produced becomes
larger. For example, when a glass substrate having a size of 30
cm.times.40 cm is produced, there causes a difference of thickness
of about 10 .mu.m between the thickest portion and the thinnest
portion.
[0020] There is a case that the internally reflected light traces a
path other than the second path 352. For example, there is a case
that light reaches the compensation cell 201 after having passed 5
times or 7 times through the liquid crystal layer 105 by internal
reflection in the displaying cell 101. However, when the light
passes through the liquid crystal layer 105 5 times or 7 times, the
quantity of the light attenuates. Accordingly, light which has
passed three times through the liquid crystal layer 105 in the
internally reflected light gives a dominant influence to the
quality of a display of dark appearance. Here, description has been
made as to the light internally reflected in the displaying cell.
However, the internal reflection is generated in the compensation
cell as well, and light internally reflected in the compensation
cell 201 may pass through the second polarizing plate 303.
[0021] The inventors of this application have achieved the present
invention described hereinbelow, based on the above-mentioned
findings.
[0022] According to a first aspect of the present invention, there
is provided a liquid crystal display device comprising:
[0023] a displaying liquid crystal cell having a liquid crystal
layer sandwiched by a pair of transparent substrates with
transparent electrodes so as to maintain a state of twisted
alignment when a voltage applied to the liquid crystal layer is not
more than the threshold voltage,
[0024] a compensation cell having a liquid crystal layer sandwiched
by a pair of transparent substrates so as to maintain a state of
twisted alignment in which the direction of twisting is opposite to
that in the displaying liquid crystal cell and the twist angle is
substantially the same as that of the displaying liquid crystal,
the compensation cell having substantially the same retardation
value as the displaying liquid crystal cell, wherein the displaying
liquid crystal cell and the compensation cell are arranged so that
directions of alignment of liquid crystal molecules at sides of
adjacent transparent substrates are substantially perpendicular to
each other, and
[0025] a pair of polarizing plates disposed to sandwich the
displaying liquid crystal cell and the compensation cell so that
their polarization axes are substantially perpendicular to each
other, the liquid crystal display device being characterized in
that:
[0026] a backlight is disposed to irradiate light whose half value
width with respect to the peak luminance is at least 5 nm, and
[0027] there is a difference of thickness of at least 0.05 mm
between the thickness of the transparent substrate of the
displaying liquid crystal cell at a side of compensation cell and
the thickness of the transparent substrate of the compensation cell
at a side of displaying liquid crystal cell.
[0028] According to a second aspect of the present invention, there
is provided the liquid crystal display device as described above,
wherein the liquid crystal display device is an in-vehicle liquid
crystal display device mounted on a vehicle or a displaying liquid
crystal device for presenting information to public.
[0029] According to a third aspect of the present invention, there
is provided the liquid crystal display device as described above,
wherein there is an air space of at least 75 .mu.m between the
displaying liquid crystal cell and the compensation cell.
[0030] According to a fourth aspect of the present invention, there
is provided a liquid display device comprising:
[0031] a displaying liquid crystal cell having a liquid crystal
layer sandwiched by a pair of transparent substrates with
transparent electrodes so as to maintain a state of twisted
alignment when a voltage applied to the liquid crystal layer is not
more than the threshold voltage,
[0032] a compensation cell having a liquid crystal layer sandwiched
by a pair of transparent substrates so as to maintain a state of
twisted alignment in which the direction of twisting is opposite to
that in the displaying liquid crystal cell and the twist angle is
substantially the same as that of the displaying liquid crystal
cell, the compensation cell having substantially the same
retardation value as the displaying liquid crystal, wherein the
displaying liquid crystal cell and the compensation cell are
arranged so that directions of alignment of liquid crystal
molecules at sides of adjacent transparent substrates are
substantially perpendicular to each other, and
[0033] a pair of polarizing plates arranged to sandwich the
displaying liquid crystal cell and the compensation cell so that
their polarization axes are substantially perpendicular to each
other, the liquid crystal display device being characterized in
that:
[0034] a backlight is disposed to irradiate light whose half value
width with respect to the peak luminance is at least 5 nm, and
[0035] the transparent substrate of the displaying liquid crystal
cell at a side of compensation cell and the transparent substrate
of the compensation cell at a side of displaying liquid crystal
cell are provided with first transparent sheets of substantially
same thickness, and
[0036] the first transparent sheet of either the transparent
substrate of the displaying liquid crystal cell at a side of
compensation cell or the transparent substrate of the compensation
cell at a side of displaying liquid crystal cell is in optically
close contact with a second transparent sheet having a thickness of
at least 0.05 mm.
[0037] According to a fifth aspect of the present invention, there
is provided an inspection method for inspecting a distribution of
thickness of a transparent substrate, characterized in that:
[0038] a first transparent substrate having a known is distribution
of thickness and a second transparent substrate to be inspected are
prepared, wherein the difference of thickness in average between
first and second transparent substrates is not more than 50
.mu.m,
[0039] the first transparent substrate and the second transparent
substrate are arranged so that light transmits through a first
substrate and a second substrate,
[0040] a substrate gap of at least 0.1 mm is provided between the
first substrate and the second substrate,
[0041] a light source for irradiating light whose half value width
with respect to the peak luminance is at least 5 nm is prepared,
and
[0042] the degree of distribution of thickness of the second
transparent substrate is judged from an interference intensity
distribution of light passing through the first transparent
substrate and the second transparent substrate.
[0043] According to a sixth aspect of the present invention, there
is provided the inspection method for a transparent substrate as
described just above, wherein the judgment to the interference
intensity distribution generated in the second transparent
substrate is made according to the number of interference fringes
generated by interference of light passing through the first
transparent substrate and the second transparent substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 is a diagrammatic cross-sectional view showing an
embodiment of the liquid crystal display device according to the
present invention;
[0045] FIG. 2 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0046] FIG. 3 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0047] FIG. 4 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0048] FIG. 5 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0049] FIG. 6 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0050] FIG. 7 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0051] FIG. 8 is a graph showing a relative interference intensity
of each glass substrate when thicknesses of adjacent glass
substrates are both 0.5 mm;
[0052] FIG. 9 is a graph showing a relative interference intensity
of each glass substrate when thicknesses of adjacent glass
substrates are 0.5 mm and 0.55 mm;
[0053] FIG. 10 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0054] FIG. 11 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0055] FIG. 12 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0056] FIG. 13 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0057] FIG. 14 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0058] FIG. 15 is a graph showing a change of relative interference
intensity caused by two glass substrates;
[0059] FIG. 16 is a graph showing the spectral luminance of the
backlight used in Examples;
[0060] FIG. 17 is a diagram showing an example of the path of light
in the inspection method according to the present invention;
[0061] FIG. 18 is a diagram showing an example of a first glass
substrate used for the inspection method of the present
invention;
[0062] FIG. 19 is a diagrammatic cross-sectional view of a
conventional liquid crystal display device;
[0063] FIG. 20 is a diagram showing how a line-shaped ununiformity
of brightness generates;
[0064] FIG. 21 is a diagram showing a cause of leakage of light,
and
[0065] FIG. 22 is a diagram showing another cause of leakage of
light.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0066] In the following, preferred embodiments of the present
invention will be described with reference to the drawing.
[0067] FIG. 1 is a diagrammatic cross-sectional view of the liquid
crystal display device according to an embodiment of the present
invention. The liquid crystal display device is of a D-STN mode and
is provided with a first polarizing plate 32, a second polarizing
plate 33, a displaying cell (a displaying liquid crystal cell) 1
sandwiched between the first and second polarizing plates 32, 33,
and a compensation cell 21. A backlight 31 is located at a rear
surface side of the first polarizing plate 32 so as to emit light
having a half value width of at least 5 nm with respect to the peak
luminance (a width of wavelength at a half luminance of the peak
luminance). In the following description, a side at which the
backlight 31 is disposed is referred to as a rear surface side, and
a side at which the second polarizing plate 33 is disposed is
referred to as a front surface side.
[0068] In the displaying cell 1, a liquid crystal layer 5 is
sandwiched by a glass substrate (a transparent substrate) 2,
located at a front surface side, on which a transparent electrode 3
is formed and a glass substrate (a transparent substrate), at a
front surface side, on which a transparent electrode 7 is formed.
The transparent electrodes 3, 7 opposing by interposing the liquid
crystal layer 5 therebetween are provided with alignment films 4, 5
on their opposing surfaces. The alignment films 4, 6 are subjected
to a rubbing treatment. When a voltage applied to the liquid
crystal layer 5 is not more than a predetermined threshold voltage,
the displaying cell 1 maintains a state of twisted alignment in the
liquid crystal layer 5 by the presence of the alignment films 4, 6.
In the compensation cell 21, a liquid crystal layer 25 is
sandwiched by a glass substrate (a transparent substrate) 22, at a
front surface side, on which an alignment film 24 is formed and a
glass substrate (a transparent substrate) 28, at a rear surface
side, on which an alignment film 26 is formed. The alignment films
24, 26 of the compensation cell 21 are also subjected to a rubbing
treatment. The compensation cell 21 maintains a state of twisted
alignment of the liquid crystal layer 25 by the presence of the
alignment films 24, 26.
[0069] There is a difference of at least 0.05 mm between the
thickness of the glass substrate 2 and the thickness of the glass
substrate 28 which are in the displaying 1 and the compensation
cell 21 and are adjacent to each other. Namely, there is a
difference of at least 0.05 mm (50 .mu.m) between the thickness of
the glass substrate 2 located at a front surface side (a side of
compensation cell) of the displaying cell 1 and the thickness of
the glass substrate 28 located at a rear surface side (a side of
displaying cell) of the compensation cell 21. FIG. 1 shows a case
that the glass substrate 2 at a front surface side of the
displaying cell 1 is at least 0.05 mm thicker than the glass
substrate 28 at a rear surface side of the compensation cell 21.
The thickness of the glass substrate 8 at a rear surface side of
the displaying cell 1 is made equal to the thickness of the glass
substrate 2 at a front surface side of the displaying cell 1, in
this embodiment. The thickness of the glass substrate 22 at a front
surface side of the compensation cell 21 is equal to the thickness
of the glass substrate 28 at a rear surface side of the
compensation cell 21, in this embodiment. However, thicknesses of
glass substrates 2, 8 of the displaying cell 1 may not be equal.
Similarly, thicknesses of the glass substrates 22, 28 of the
compensation cell 21 may not be equal.
[0070] When each glass substrate 2, 8, 22 or 28 is produced,
unevenness of thickness may takes place in each glass substrate. It
is permissible that unevenness of thickness takes place in each of
the glass substrates 2, 8, 22 and 28 as far as it is not beyond the
normal range of unevenness in producing glass substrates. For
example, it is assumed that a glass substrate having a size of 5
cm.times.5 cm and a thickness of 0.55 mm is produced as the glass
substrate 2. In this case, a difference of thickness of, for
example, 2 .mu.m between the thickest portion and the thinnest
portion is permissible. The same condition is applicable to other
glass substrates 8, 22 and 28.
[0071] The liquid crystal layer 5 of the displaying cell 1 and the
liquid crystal layer 25 of the compensation cell 21 have the same
birefringence value. Further, the liquid crystal layer 5 of the
displaying cell 1 and the liquid crystal layer 25 of the
compensation cell 21 have such relation that when a voltage applied
to the liquid crystal layer 5 is not more than the threshold
voltage, they have the same twist angle and the direction of
twisting of long axis of liquid crystal molecules of either cell is
opposite to that of the other. In this case, directions of aligning
of liquid crystal molecules of the displaying cell 1 and the
compensation cell 21 at sides of adjacent glass substrates 2, 28
are perpendicular to each other. Namely, the direction in alignment
in the liquid crystal layer 5 of the displaying cell at a side of
glass substrate 2 is perpendicular to the direction of alignment in
the liquid crystal layer 25 of the compensation cell 21 at a side
of glass substrate 28. Further, the retardation value
.DELTA.n.multidot.d of the displaying cell 1 is equal to the
retardation value .DELTA.n.multidot.d of the compensation cell
21.
[0072] An air gap (an air space) of at least 75 .mu.m is provided
between the glass substrate 2 at a front surface side of the
displaying cell 1 and the glass substrate 28 at a rear surface side
of the compensation cell 21. The provision of the air space
prevents interference caused by reflection of light at the surface
of the glass substrate 2 at a front surface side of the displaying
cell 1 and the surface of the glass substrate 28 at a rear side of
the compensation cell 21. The first polarizing plate 32 is bonded
to the glass substrate 8 at a rear surface side of the displaying
cell 1. Similarly, the second polarizing plate 33 is bonded to the
glass substrate 22 at a front surface side of the compensation cell
21. The direction of polarization axis of the first polarizing
plate 32 and the direction of polarization axis of the second
polarizing plate 33 are perpendicular to each other. Further, it is
preferable that the direction of polarization axis of the first
polarizing plate 32 and the direction of alignment of the liquid
crystal layer 5 of the displaying cell 1 at a side of glass
substrate 8 (a rear surface side) form an angle of 45.degree., and
the direction of polarization axis of the second polarizing plate
33 and the direction of alignment of the liquid crystal layer 25 of
the compensation cell 21 at a side of glass substrate 22 (a front
surface side) form an angle of 45.degree.. Thus, by specifying
directions of polarization axes and directions of alignment, an
excellent light source color can be obtained when a display of
light source color is to be presented.
[0073] A display of dark appearance is expected, no voltage is
applied to the liquid crystal layer 5 of the displaying cell 1 or a
voltage of not more than the threshold voltage is applied to. In
this case, light emitted from the backlight 31 is rendered to be a
linearly polarized light when it is passed through the first
polarizing plate 32, and it is rendered to be an elliptically
polarized light when passed through the displaying cell 1. Then,
the light is returned again to the linearly polarized light by the
compensation cell 21 and is blocked by the second polarizing plate
33. Since there is internal reflection of a part of light emitted
from the backlight 31 in the displaying cell or the compensation
cell 21, light may leak from the second polarizing plate 33.
However, there occurs only slight unevenness in the intensity of
light passing through the glass substrates 2, 28 even though there
is unevenness in the thickness of the glass substrate 2 at a front
surface side of the displaying cell 1 or in the thickness of the
glass substrate 28 at a rear surface side of the compensation cell
21, as described after. Accordingly, an excellent dark appearance
can be presented without a line-shaped ununiformity of
brightness.
[0074] When a light source color is to be displayed, a higher
voltage than the threshold voltage should be applied to the liquid
crystal layer 5 of the displaying cell 1 so that the long axis of
each liquid crystal molecule in the liquid crystal layer 5 is
aligned to the direction of the electric field.
[0075] The inventors of this application have studied in detail
about a change of relative interference intensity in a glass
substrate due to unevenness of the thickness of the glass
substrate. As a result, it has been found that influence caused by
an uneven thickness can be eliminated by providing a difference of
thickness of at least 0.05 mm between adjacent glass substrates in
the displaying cell and the compensation cell (more specifically,
the glass substrate 2 at a front surface side of the displaying
cell 1 and the glass substrate 28 at a rear surface side of the
compensation cell 21 as shown in FIG. 1), and by irradiating light
whose half value width with respect to the peak luminance of light
from the backlight is at least 5 nm.
[0076] In the following, description will be made as to how
unevenness in the thickness of a glass substrate affects the
intensity of light passing through the glass plate.
[0077] A degree of interfering light affecting the intensity of
light passing through a substrate is mentioned as an interference
intensity. In general, an interference intensity in relative sense
(a relative interference intensity) of a single glass substrate is
expressed by the following formula 1:
P=1-COS((A.times.B.times.2/.lambda.).times.2.times..pi.) Formula
1
[0078] In formula 1, a character A denotes a thickness of a glass
substrate in a unit of nm, a character B denotes a refractive index
of glass of the glass substrate and .lambda. denotes a wavelength
of light in a unit of nm. A relative interference intensity given
by two adjacent glass substrates is expressed as the product of
relative interference intensities of the glass substrates.
Accordingly, when a thickness of a second glass substrate is
expressed by A' (nm) and a refractive index of glass is expressed
by B', the relative interference intensity P with respect to a
wavelength .lambda. (nm) is expressed by the following formula
2:
P=(1-COS((A.times.B.times.2/.lambda.).times.2.times..pi.))*(1-COS((A'.time-
s.B'.times.2/.lambda.).times.2.times..pi.)) Formula 2
[0079] The relative interference intensity obtained by two glass
substrates was calculated by using formula 2. The thickness of a
first glass substrate was changed from 500,000 nm to 500,420 nm
while the thickness of a second glass substrate was kept constant
to 500,000 nm. Further, light having a half value width of 15 nm
(with respect to the peak luminance (a width of +7.5 nm with
respect to the wavelength at the peak luminance) was supposed to be
irradiated. By using LED, the light having a half value width of 15
nm is irradiated. A red-color light having a central wavelength of
640 nm is assumed.
[0080] FIG. 2 shows a change of the relative interference intensity
obtained by the combination of two glass substrates wherein the
abscissa represents the thickness T of the first glass substrate
and the ordinate represents the relative interference intensity P.
As shown in FIG. 2, the relative interference intensity by the
combination of the two glass substrates varies within a range of
from about 1.5 to about 0.5 with a change of the thickness of the
first glass substrate. For example, when the thickness of the first
glass substrate is 500,000 nm, the relative interference intensity
is about 1.5. However, when the thickness is 500,100 nm, the
relative interference intensity is about 0.5. This shows that when
unevenness of thickness results in either glass substrate,
ununiformity of strength results also in the relative interference
intensity.
[0081] FIG. 3 is a graph showing a change of the relative
interference intensity in a case that the thickness of the second
glass substrate is determined to be 520,000 nm, in the same manner
as in FIG. 2. Even in the case of FIG. 3, the relative interference
intensity by the combination of two glass substrates varies with a
change of the thickness of the first glass substrate. However, the
difference between maximum and minimum values of the relative
interference intensity is smaller than that in FIG. 2.
[0082] FIG. 4 is a graph showing a change of the relative
interference intensity in a case that the thickness of the second
glass substrate is determined to be 540,000 nm, in the same manner
as in FIGS. 2 and 3. Even in the case of FIG. 4, the relative
interference intensity by the combination of two glass substrates
varies with a change of the thickness of the first glass substrate.
However, the difference between the maximum and minimum values of
the relative interference intensity is further smaller than that
shown in FIG. 3.
[0083] FIGS. 5, 6 and 7 show graphs showing changes of relative
interference intensities in cases that the thickness of the second
glass substrate are to be 550,000 nm, 560,000 nm and 600,000 nm, in
the same manner as FIGS. 2 to 4. In the cases of FIGS. 5, 6 and 7,
the relative interference intensities do not show substantial
changes even when thickness of the first glass substrate are
changed from 500,000 nm to 500,420 nm.
[0084] It is derived from FIGS. 2 to 7 that when the difference of
thickness between adjacent glass substrates is smaller than 50,000
nm (0.05 mm) and if there is unevenness in the thickness of a glass
substrate, the relative interference intensities are also changed.
Further, it is derived that when the difference of thickness
between adjacent glass substrates is at least 50000 nm (0.05 mm),
the relative interference intensities do not show substantial
changes even though there is unevenness in the thickness of a glass
substrate. Thus, when the relative interference intensity does not
show a substantial change even though there is unevenness in the
thickness, the intensity of light leaking from the second
polarizing plate 33 shown in FIG. 1 does not show a substantial
change over the entire surface of it to thereby prevent the
occurrence of a line-shaped uniformity of brightness. Accordingly,
it is understood that the difference of thickness between adjacent
glass substrates should be at least 0.05 mm in order to prevent the
occurrence of a line-shaped ununiformity of brightness. Namely, in
the liquid crystal display device shown in FIG. 1, it is necessary
that the difference between the thickness of the glass substrate 2
at a front surface side of the displaying cell 1 and the thickness
of the glass substrate 28 at a rear surface side of the
compensation cell 21 is at least 0.05 mm.
[0085] Next, explanation will be made as to the reason why the
change of the relative interference intensity caused by the
unevenness of thickness can be suppressed by providing a difference
of thickness between adjacent glass substrates.
[0086] FIG. 8 is graphs showing the relative interference intensity
of each glass substrate in a case that thicknesses of adjacent
glass substrates are to be 500,000 nm (0.5 mm). The abscissa in
FIGS. 8(a) to 8(c) denotes a wavelength .lambda. of light. The
ordinate in FIG. 8(a) denotes the relative interference intensity
of the second glass substrate having a thickness of 500,000 .mu.m.
The ordinate in FIG. 8(b) denotes the relative interference
intensity of the first glass substrate having a thickness of
500,000 nm. The ordinate in FIG. 8(c) denotes the relative
interference intensity of the first glass substrate having a
thickness of 500,100 nm because of unevenness in the thickness.
[0087] As shown in FIGS. 8(a) to 8(c), relative interference
intensities change periodically as the wavelength .lambda. of light
is changed. The change of the relative interference intensity shown
in FIG. 8(b) is the same as the change of the relative interference
intensity in FIG. 8(a) because this change is the same as the case
that the second glass substrate has the same thickness. On the
other hand, FIG. 8(c) shows a change of the relative interference
intensity when there is a change of 100 nm in the thickness due to
unevenness. The period of the change of the relative interference
intensity is the same as that shown in FIG. 8(a) (although they are
not the same in the strict sense, the difference of periods is
negligible). Further, the phase of the waveform shown in FIG. 8(c)
is delayed by about 1/2 with respect to the waveform shown in FIG.
8(a).
[0088] The relative interference intensity of two glass substrates
can be expressed as the product of the relative interference
intensities of the glass substrates. Accordingly, the relative
interference intensity of light having a width in its wavelength
.lambda. passing through two glass substrates each having a
thickness of 500,000 nm can be expressed as a value obtained by
multiplying each relative interference intensity of the waveform
shown in FIG. 8(a) by each relative interference intensity of the
waveform shown in FIG. 8(b) for each wavelength .lambda. and
averaging the obtained values. Waveforms showing changes of the
relative interference intensities in FIGS. 8(a) and 8(b) are the
same. Accordingly, when the relative interference intensity of
either one becomes larger, the relative interference intensity of
the other becomes also larger. When the relative interference
intensity of either waveform becomes smaller, the relative
interference intensity of the other becomes also smaller. For
example, when .lambda. is about 640.07 nm, relative interference
intensities shown in FIGS. 8(a) and 8(b) become about 2. Further,
when .lambda. is about 640.2 nm for example, relative interference
intensities shown in FIGS. 8(a) and 8(b) become about 0.
[0089] Assuming that there is unevenness in the thickness of the
first glass substrate so that the glass substrate has a portion
having a thickness of 500,100 nm. The relative interference
intensity of light passing through this portion and the second
glass substrate can be expressed as a value which is obtained by
multiplying each relative interference intensity of the waveform
shown in FIG. 8(a) by each relative interference intensity shown in
FIG. 8(c), and by averaging the obtained products. Waveforms shown
in FIGS. 8(a) and 8(c) have the same period and the phase of
waveform of either one is delayed by about 1/2 from the other.
Accordingly, when the relative interference intensity of either one
becomes larger, the relative interference intensity of the other
becomes smaller. For example, when .lambda. is about 640.07 nm, the
relative interference intensity in FIG. 8(a) is 2 whereas the
relative interference intensity in FIG. 8(c) is about 0.
[0090] As described above, the waveforms shown in FIGS. 8(a) and
8(b) have such relation that when the relative interference
intensity of either one becomes larger, the relative interference
intensity of the other becomes also large, and when the relative
interference intensity of either one becomes smaller, the relative
interference intensity of the other becomes also smaller. Further,
the waveforms shown in FIGS. 8(a) and 8(c) have such relation that
when either one becomes larger, the other becomes smaller. In this
case, there is a difference between a value obtained by multiplying
each relative interference intensity of waveforms shown in FIGS.
8(a) and 8(b) for each wavelength .lambda., and summing the
obtained products for each .lambda., and a value obtained by
multiplying each relative interference intensity of waveforms shown
in FIGS. 8(a) and 8(c) for each wavelength .lambda. and summing the
obtained products for each .lambda.. This difference appears as a
difference of relative interference intensities in the case there
is unevenness in the thickness.
[0091] FIG. 9 shows the relative interference intensity of each
glass substrate in a case that the thickness of either of adjacent
glass substrates is 550,000 nm and the thickness of the other is
500,000 nm. The abscissa in FIGS. 9(a) to 9(c) denotes the
wavelength .lambda. of light. The ordinate in FIG. 9(a) denotes the
relative interference intensity of the second glass substrate
having a thickness of 500,000 nm, the ordinate in FIG. 9(b) denotes
the relative interference intensity of the first glass substrate
having a thickness of 550,000 nm, and the ordinate in FIG. 9(c)
denotes the relative interference intensity of the first glass
substrate having a thickness of 550,100 nm due to unevenness in the
thickness.
[0092] As shown in FIGS. 9(a) to 9(c); relative interference
intensities change periodically as the wavelength .lambda. of light
is changed. The period of the waveform shown in FIG. 9(a) is
different from those of the waveforms shown in FIGS. 9(b) and 9(c).
The waveforms shown in FIGS. 9(b) and 9(c) have the same period
(although they are not the same in the strict sense, the difference
of periods is negligible). However, there is a delay of phase of
about 1/2 between them.
[0093] The relative interference intensity of light having a width
in its wavelength passing through a glass substrate having a
thickness of 550,000 nm and a glass substrate having a thickness of
500,000 nm can be expressed as a value obtained by multiplying each
relative interference intensity of the waveform shown in FIG. 9(a)
by each relative interference intensity of the waveform shown in
FIG. 9(b) for each wavelength .lambda., and by averaging the
obtained products. Waveforms shown in FIGS. 9(a) and 9(b) are
different from each other in terms of period. Accordingly, such
relation that when the relative interference intensity of either
one becomes larger, the relative interference intensity of the
other becomes also larger or such relation that when the relative
interference intensity of either one becomes larger, the relative
interference intensity of the other becomes smaller, cannot be
maintained. For example, when .lambda. is about 640.07 in FIGS.
9(a) and 9(b), the relative interference intensity in FIG. 9(a) is
about 2, whereas the relative interference intensity in FIG. 9(b)
is about 0.2. On the other hand, when .lambda. is about 640.9,
relative interference intensities in FIGS. 9(a) and 9(b) are about
2.
[0094] Assuming that there is unevenness in the thickness of the
first glass substrate so that it has a portion having a thickness
of 550,100 nm. The relative interference intensity of light passing
through this portion and the second glass substrate can be
expressed as a value obtained by multiplying each relative
interference intensity of the waveform shown in FIG. 9(a) by each
relative interference intensity of the waveform shown in FIG. 9(c)
for each wavelength .lambda., and averaging the obtained products.
Waveforms shown in FIGS. 9(a) and 9(c) are also different from each
other in terms of period. Accordingly, such relation that when the
relative interference intensity of either one becomes larger, the
relative interference intensity of the other becomes also larger or
such relation that when the relative interference intensity of
either one becomes larger, the relative interference intensity of
the other becomes also smaller cannot be maintained, in the same
manner as the case of the combination of the waveforms shown in
FIGS. 9(a) and 9(b).
[0095] In this case, there is only a slight difference between a
value obtained by multiplying each relative interference intensity
of the waveforms shown in FIGS. 9(a) and 9(b) and summing the
obtained products for each .lambda., and a value obtained by
multiplying each relative interference intensity of the waveforms
shown in FIGS. 9(a) and 9(c) for each wavelength .lambda. and
summing obtained products for each .lambda.. Accordingly, there is
only a slight difference between relative interference intensities
even though there is unevenness in the thickness.
[0096] As described above, there is only a slight difference in the
total sum of the products of relative interference intensities for
each .lambda. between the combination of the waveforms shown in
FIGS. 9(a) and 9(b) and the combination of the waveforms shown in
FIGS. 9(a) and 9(c). However, there is a difference in the products
of relative interference intensities for each .lambda..
Accordingly, the discussion made with respect to FIG. 9 is not
applicable when a single color light without having a width in the
wavelength is used. Namely, it is impossible to control the
fluctuation of intensity of transmitting light when the light does
not having a width in wavelength .lambda..
[0097] In Examples of FIGS. 2 to 7, explanation is made as to the
relative interference intensity with respect to two glass
substrates wherein the half value width of light is 15 nm.
[0098] In the following, explanation will be made as to a case of
irradiating light whose half value width is 5 nm with respect to
the peak luminance (a width of .+-.2.5 nm with respect to the
wavelength at the peak luminance). As such light, light having a
center wavelength of 612 nm, emitted from a fluorescent lamp, is
assumed.
[0099] The relative interference intensity in a case that the
thickness of the first glass substrate is changed from 500,000 nm
to 500,420 nm in the same manner as in FIGS. 2 to 7, will be
described.
[0100] FIG. 10 shows a change of the relative interference
intensity with a change of the thickness of the first glass
substrate while the thickness of the second glass substrate is
constant as 500,000 nm. As shown in FIG. 10, the relative
interference intensity in the combination of two glass substrates
varies within a range of from about 1.5 to about 0.5 according to a
change of the thickness of the first glass substrate. For example,
when the thickness of the first glass substrate is 500,000 nm, the
relative interference intensity is about 1.5. However, when the
thickness is 500,100 nm, the relative interference intensity is
about 0.5. This shows that when unevenness of thickness takes place
in either glass substrate, there occurs unevenness of intensity in
the relative interference intensity.
[0101] FIG. 11 is a graph showing a change of the relative
interference intensity in a case that the thickness of the second
glass substrate is determined to be 520,000 nm in the same manner
as in FIG. 10. Even in the case of FIG. 11, the relative
interference intensity by the combination of the two glass
substrates varies with a change of the thickness of the first glass
substrate. However, the difference between maximum and minimum
values of the relative interference intensity is smaller than that
in FIG. 10.
[0102] FIG. 12 is a graph showing a change of the relative
interference intensity in a case that the thickness of the second
glass substrate is determined to be 540,000 nm in the same manner
as the cases shown in FIGS. 10 and 11. Even in the case of FIG. 10,
the relative interference intensity by the combination of the
second glass substrate varies with a change of the thickness of the
first glass substrate. However, the difference between maximum and
minimum values of the relative interference intensity is further
smaller than that of the case shown in FIG. 11.
[0103] FIGS. 13, 14 and 15 are graphs showing relative interference
intensities in cases that the thickness of the second glass
substrate is determined to be 550,000 nm, 560,000 nm and 600,000 nm
in the same manner as the cases shown in FIGS. 10 to 12. In the
cases shown in FIGS. 13, 14 and 15, relative interference
intensities do not show substantial changes even when the thickness
of the first glass substrate is changed from 500,000 nm to 500,420
nm.
[0104] In view of the relative interference intensities shown in
FIGS. 10 to 15, the same conclusion as the cases shown in FIGS. 2
to 7 can be derived. Namely, when the difference of thickness
between adjacent glass substrates is smaller than 50,000 nm (0.05
mm) and if there is unevenness of thickness in a glass substrate,
the conclusion that the relative interference intensities also
change, is derived. Further, when the difference of thickness
between adjacent glass substrates is at least 50,000 nm (0.05 mm),
the conclusion that the relative interference intensities do not
show substantial change is derived even though there is unevenness
of thickness in a glass substrate. Further, as described before,
when light without having no width in its wavelength .lambda. is
used, it is impossible to suppress the fluctuation of the relative
interference intensities. However, it is understood that from FIGS.
10 to 15 that the fluctuation of the relative interference
intensities can be suppressed when the half value width of light is
to be at least 5 nm and the difference of thickness between
adjacent glass substrates is to be at least 0.05 mm. In the liquid
crystal display device shown in FIG. 1, the backlight 31
irradiating light whose half value width of at least 5 nm is used,
and the difference of thickness between the glass substrate 2 at a
front surface side of the displaying cell 1 and the glass substrate
28 at a rear surface side of the compensation cell 21 is at least
0.05 mm, is provided. Accordingly, the fluctuation in the relative
interference intensities is slight and no line-shaped ununiformity
of brightness occurs even though the thickness of the glass
substrate 2 or the thickness of the glass substrate 28 is
uneven.
[0105] Relative interference intensities shown in FIGS. 2 to 15 are
derived from simulation by optical calculation. Although a
technique of simulation is used, it is possible to obtain relative
interference intensities as the case using actual liquid crystal
devices.
[0106] FIG. 1 shows the case that the glass substrate 2 at a front
surface side of the displaying cell 1 is at least 0.05 mm thicker
than the glass substrate 28 at a rear surface side of the
compensation cell 21. However, either one of two adjacent glass
substrates 2, 28 may be thickened. Accordingly, the thickness of
the glass substrate 28 at a rear surface side of the compensation
cell 21 may be at least 0.05 mm thicker than the thickness of the
glass substrate 2 at a front surface side of the displaying cell
1.
[0107] In the preparation of the liquid crystal display device of
the present invention, thicknesses of the glass substrates 2, 28
are determined so that there is a difference of thickness of at
least 0.05 mm between the thickness of the glass substrate 2 of the
displaying cell 1 at a side of compensation cell 21 and the
thickness of the glass substrate 28 of the compensation cell 21 at
a side of displaying cell 1, and the glass substrates 2, 28 are
prepared according to such determination. In this case, the glass
substrates 2, 28 may have a certain degree of unevenness of
thickness, such unevenness being resulted usually in the
preparation of glass substrates. The displaying cell 1 is prepared
by using the glass substrate 2. Similarly, the compensation cell 21
is prepared by using the glass substrate 28. In this case, the
direction of twisting in the state of twisted alignment of the
displaying cell 1 is opposite to that of the compensation cell
21.
[0108] Then, the first polarizing plate 32 is disposed at a rear
surface side of the displaying cell 1, and the second polarizing
plate 33 is disposed at a front surface side of the compensation
cell 21. In a state that the displaying cell 1 and the compensation
cell 21 are arranged adjacent to each other, the polarization axis
of the first polarizing plate 32 should be perpendicular to the
polarization axis of the second polarizing plate 33. Further, it is
preferable that an angle of 45.degree. is formed between the
direction of polarization axis of the first polarizing plate 32 and
the direction of alignment in the liquid crystal layer 5 of the
displaying cell at a side of glass substrate 8 (a rear surface
side), and an angle of 45.degree. is formed between the direction
of polarization axis of the second polarizing plate 33 and the
direction of alignment in the liquid crystal layer 25 of the
compensation cell 21 at a side of glass substrate 22 (a front
surface side). Further, the displaying cell 1 and the compensation
cell 21 are arranged so that directions of alignment of liquid
crystal molecules at sides of adjacent glass substrates are
perpendicular to each other. Further, a backlight for irradiating
light whose half value width with respect to the peak luminance is
at least 5 nm is disposed at a rear surface side of the first
polarizing plate 32 in the displaying cell 1.
[0109] Although the twist angle and the birefringence value of
liquid crystal in the displaying cell 1 are preferably made equal
to those of the compensation cell 21, errors may take place in the
preparation of them. Further, errors may take place in retardation
values of the compensation cell 21 and the displaying cell 1 as
long as the retardation value of the compensation cell 21 is not
more than the retardation value of the displaying cell 1,
specifically, the retardation value of the compensation cell 21 is
in a range of from 80% to 100% of that of the displaying cell
1.
[0110] Similarly, even in the determination of angles, directions
of alignment etc., errors may take place. For example, in a case
that the first polarizing plate 32 and the second polarizing plate
33 are arranged so that their polarization axes are perpendicular,
errors may take place. Such errors are permissible if the angle
formed by their axes is not more than 5.degree.. Further, errors
may take place when the displaying cell 1 and the compensation cell
21 are arranged so that directions of alignment of liquid crystal
molecules at sides of adjacent glass substrates are perpendicular
to each other. Such errors are permissible if the angle formed by
directions of alignment is not more than 5.degree.. Further, errors
may take place even when the displaying cell 1 and the compensation
cell 21 are arranged so that the direction of the polarization axis
of the first polarizing plate 32 and the direction of alignment in
the liquid crystal layer 5 of the displaying cell 1 at a side of
glass substrate 8 (a rear surface side) provide an angle of
45.degree., and the direction of the polarization axis of the
second polarizing plate 33 and the direction of alignment in the
liquid crystal layer 25 of the compensation cell 21 at a side of
glass substrate 22 (a front surface side) provide an angle of
45.degree.. Such errors are permissible if the angle formed by the
direction of alignment and the polarization axis is from 40.degree.
to 50.degree..
[0111] FIG. 1 shows an embodiment that the glass substrate 2 at a
front surface side (a side of compensation cell) of the displaying
cell 1 is produced as a single glass sheet whose thickness is at
least 0.05 mm thicker than the thickness of the glass substrate 28
at a rear surface side (at a side of the displaying cell) of the
compensation cell 21. A substrate formed by bonding (optically
adhering) a transparent sheet (a second transparent sheet) having a
thickness of at least 0.05 mm to a glass sheet (a first transparent
sheet) having the same thickness as the glass substrate 28 (the
glass sheet) at a rear surface side of the compensation cell 21,
may be used as the glass substrate 2 at a front surface side of the
displaying cell 1. Similarly, a substrate formed by bonding a
transparent sheet having a thickness of at least 0.05 mm to a glass
sheet having the same thickness as the glass substrate 2 (glass
sheet) at a front surface side of the displaying cell 1, may be
used as the glass substrate 28 at a rear surface side of the
compensation cell 21. In this case, the glass substrate 28 of the
compensation cell 21 is at least 0.05 mm thicker than the glass
substrate 2 of the displaying cell 1. The thickness of the glass
sheet to be attached with the transparent sheet is equal to the
thickness of the glass substrate 2 (or the glass substrate 28). In
this case, however, errors may take place in the preparation.
[0112] Although it is preferable that refractive indices of glass
sheets used for the glass substrates 2, 28 and transparent sheets
to be bonded to the glass sheets are equal, they may be different.
Even when the refractive indices are different from each other, the
occurrence of a line-shaped ununiformity of brightness can be
prevented when for instance, indices of the glass sheets and the
transparent sheets are in a range of from 1.34 to 1.59. Table 1
shows the refractive index of each transparent substrate.
1 TABLE 1 Material Refractive index Glass 1.52 Cytop 1.34
Polycarbonate (PC) 1.59 Polymethyl methacrylate 1.49 (PMMA)
Polystyrene (PS) 1.59
[0113] As shown in Table 1, the refractive index of glass is 1.52.
Refractive index of cytop, polycarbonate (PC), polymethyl
methacrylate (PMMA) and polystyrene (PS) are in a range of from
1.34 to 1.59. The cytop is a trademark registered in Japan of a
fluorine type transparent resin manufactured by Asahi Glass
Company, Limited. The occurrence of a line-shaped ununiformity of
brightness can be prevented even when a transparent sheet having a
thickness of at least 0.05 mm made of a material having a
refractive index of from 1.34 to 1.59 as shown in Table 1 is bonded
to a glass sheet having a refractive index of 1.52. Further, the
transparent sheet is not always necessary to have a refractive
index of from 1.34 to 1.59. Further, the transparent sheet may be a
transparent material exemplified in Table 1, instead of the glass
sheet.
[0114] Description will be made as to how the liquid crystal
display device of the present invention is used. The liquid crystal
display device of the present invention can be used as, for
example, a displaying portion of an instrument panel or a center
panel of a vehicle. Further, it can be used as a displaying portion
of a meter panel of an automatic two-wheeled vehicle or the like.
Further, it may be used as an in-vehicle liquid crystal display
device mounted on a vehicle or a liquid crystal display device for
presenting information in public facilities or the like. For
example, it may be used as a displaying panel for presenting
various kinds of information instead of an advertising board, or as
a guide panel for presenting various kinds of information to
passengers in a train.
[0115] The quality of display of D-STN mode is influenced little by
temperature. Accordingly, the liquid crystal display device of the
present invention is in particular suitable for applications
requiring a wide width of change of environmental temperature, such
as applications to a vehicle, an outdoor equipment and so on.
[0116] In the present invention, the fluctuation of the relative
interference intensity is suppressed by providing a difference of
at least 0.05 mm between the thickness of the glass substrate 2 of
the displaying cell 1 and the thickness of the glass substrate 28
of the compensation cell. Even in a case that the thickness of the
glass substrate 2 at a front surface side of the displaying cell 1
is made equal to the thickness of the glass substrate 28 at a rear
surface side of the compensation cell 21, a good quality of display
without a line-shaped ununiformity of brightness can be realized by
suppressing unevenness in the thickness. As understood from FIGS. 2
to 4 and FIGS. 10 to 12, periods of the fluctuation of the relative
interference intensities are 200 nm (0.2 .mu.m). Even in a case
that the thickness of the glass substrate 2 at a front surface side
of the displaying cell 1 is made equal to the thickness of the
glass substrate 28 at a rear surface side of the compensation cell
21, a good quality of display without a line-shaped ununiformity of
brightness can be realized if the glass substrates 2, 28 are
prepared so that a difference between the thickest portion and the
thinnest portion of the glass substrates is not more than the
period of fluctuation (0.2 .mu.m) of the relative interference
intensity.
[0117] In the following, description will be made as to the reason
with reference to FIG. 2, as an example.
[0118] FIG. 2 shows a change of the relative interference intensity
in a case that the thickness of the second glass substrate is
500,000 nm and the thickness of the first glass substrate is
changed from 500,000 nm. By changing the thickness of the first
glass substrate from 500,000 nm to 500,100 nm, the relative
interference intensity is changed from about 1.5 to about 0.5.
However, in only providing a change of thickness of 100 nm, a state
that a portion of low relative interference intensity is sandwiched
between portions of high relative interference cannot be obtained.
Accordingly, even though the relative interference intensity is
changed largely from about 1.5 to about 0.5, the portion of low
relative interference intensity is not conspicuous and it is
difficult that the portion is recognized as a line-shaped
ununiformity of brightness. When the thickness of the first glass
substrate is further increased to 500,200 nm, the relative
interference intensity is again returned to about 1.5. In this
case, the portion of low relative interference intensity is
sandwiched between portions of high relative interference intensity
so that the portion of low relative interference intensity is
conspicuous and it is recognized as a line-shaped ununiformity of
brightness. Further, a difference of thickness between the thickest
portion and the thinnest portion is not more than the period of
fluctuation (0.2 .mu.m) of the relative interference intensity,
there is no possibility that a portion of low (or high) relative
interference intensity is sandwiched between portions of high (or
low) relative interference intensity. Accordingly, even when
thicknesses of the glass substrates 2, 28 adjacent to each other in
the displaying cell 1 and the compensation cell 21 are made equal,
the occurrence of a line-shaped ununiformity of brightness can be
prevented if the glass substrates 2, 28 are prepared so that a
difference of thickness between the thickest portion and the
thinnest portion is not more than 0.2 .mu.m.
[0119] With reference to FIG. 21, the cause of leakage of light
when a dark appearance is to be displayed, has been described.
However, another cause of leakage of light can be considered. Such
phenomenon will be described with reference to FIG. 22.
[0120] Coherence length is in inverse proportional to the relative
linewidth .DELTA..lambda./.lambda. of the light, which means it is
increasing with decreasing linewidth. Roughly, coherence length is
the same multiple of the wavelength, as wavelength is a multiple of
linewidth.
[0121] Typical red LED yields .DELTA..lambda./.lambda. approx. 60
nm/600 nm=1/10, resulting in a coherence length of
10.times..lambda., approx. 6000 nm=6 .mu.m.
[0122] In FIG. 22, interference is strong, if 2.times.thickness
differences between glass 102 and 208 is less than the coherence
length of approx. 6 .mu.m. Interference gradually disappears, if
thickness difference exceeds the coherence length. Practical
thickness difference will be a multiple of this, therefore total
avoidance of the interference is possible.
[0123] FIG. 22 shows a pair of paths, which we consider to be
significant for the effect in DSTN.
[0124] The first path includes 1.times.cell 101+2.times.LC layer
105+2.times.alignment film 106+2.times.alignment film
104+2.times.transparent electrode 103+2.times.glass
102+1.times.cell 201.
[0125] The second path includes 1.times.cell 101+1.times.cell
201+2.times.LC layer 205+2.times.glass 208+2.times.alignment film
204+2.times.alignment film 206.
[0126] The path difference is 2.times.glass 102-2.times.glass 208
(LC layer difference is neglectable).
[0127] The number of internal reflections in each path is 2. If we
calculate the reflection coefficient to be in the range of 5%, the
relative intensity is 0.25% of the main light path. Any bigger
number of internal reflections along the light path will strongly
reduce the relative intensity further.
[0128] In the search for other significant "parasitic" light paths
(which means, they are caused by internal reflections), we can
exclude all, which have an equal number of paths through each LC
layer. These will be blocked by the ordinary compensation effect of
DSTN. We can further exclude all multiple paths through the
polarizers. In addition, almost no reflection is caused by the
interface of glass to polarizer. We neglect the influence of ITO
and other coating thicknesses of the internal layers in both cells
and especially the different number and kind of coatings in cell
101 compared to cell 201.
[0129] This finally reduces the options for "effective" parasitic
light paths to the one, which we mentioned above.
[0130] Now, the present invention will be described in detail with
reference to Examples. However, it should be understood that the
present invention is by no means restricted by such specific
Examples.
EXAMPLE 1
[0131] A displaying cell was prepared in a manner as follows. A
transparent electrode is disposed on each of two glass substrates
of 0.7 mm thick, and an alignment film was formed on each of the
transparent electrodes. A rubbing treatment was conducted to
alignment films. In the rubbing treatment, the direction of rubbing
was determined so as to form a twist angle of 180.degree. in a
state that the two glass substrates were combined by interposing
liquid crystal therebetween. After the rubbing, the glass
substrates were piled up so that the transparent electrodes each
having the alignment film oppose to each other and liquid crystal
was injected between the glass substrates in a shield state. As the
liquid crystal, a commercially available liquid crystal ("ZLI4431":
.DELTA.n=0.1643 manufactured by Merck) was used. The cell gap was 6
.mu.m and .DELTA.n.multidot.d was 0.986 .mu.m.
[0132] A compensation cell was prepared in a manner as follows. An
alignment film was formed on each of two glass substrates of 0.55
mm thick, and a rubbing treatment was conducted to alignment films.
In the rubbing treatment, the direction of rubbing was determined
so as to form a twist angle of 180.degree. in the same manner as in
the displaying cell. After the rubbing, the glass substrates were
piled up so that the alignment films oppose to each other, and
liquid crystal was injected between the glass substrates in a
shield state. The liquid crystal used was "ZLI4431" manufactured by
Merck in the same manner as in the displaying cell. The cell gap of
the compensation cell was 6 .mu.m and .DELTA.n.multidot.d was 0.986
.mu.m. However, in the compensation cell, the direction of twisting
of liquid crystal molecules was opposite to that of the displaying
cell.
[0133] Then, polarizing plates were bonded to the glass substrate
to be located at a rear surface side of the displaying cell and the
glass substrate to be located at a front surface side of the
compensation cell, and the displaying cell and the compensation
cell were arranged so that their surfaces on which the polarizing
plates were not bonded, were opposed to each other. The polarizing
plate was bonded to the displaying cell so that the direction of
polarization axis of the polarizing plate and the direction of
alignment in the liquid crystal layer of the displaying cell at a
side of rear glass substrate form an angle of 45.degree.. The
polarizing plate was bonded to the compensation cell so that the
direction of polarization axis of the polarizing plate and the
direction of alignment in the liquid crystal layer of the
compensation cell at a side of front glass substrate form an angle
of 45.degree.. Further, the displaying cell and the compensation
cell were arranged so that directions of alignment of liquid
crystal molecules at sides of adjacent glass substrates were
perpendicular to each other, and directions of polarization axes of
the two polarizing plates are perpendicular to each other. The
thickness of each of the glass substrates of the displaying cell
was 0.7 mm, and the thickness of each of the glass substrates of
the compensation cell was 0.55 mm. Accordingly, the difference of
thickness between the adjacent glass substrates in the displaying
cell and the compensation cell is 0.15 mm which is larger than 0.05
mm.
[0134] A backlight was disposed at a rear surface of the is
polarizing plate bonded to the displaying cell. LED was used as the
backlight. FIG. 16 shows the spectral characteristics of the
backlight used in this Example wherein the abscissa .lambda.
represents the wavelength and the ordinate I represents the
luminance of light. The half value width in the spectral
characteristics shown in FIG. 16 is 5 nm or more.
[0135] When no voltage was applied to the liquid crystal layer of
the displaying cell, a display of good dark appearance was obtained
without generating a line-shaped ununiformity of brightness.
COMPARATIVE EXAMPLE
[0136] A liquid crystal display device was prepared in the same
manner as Example 1 except that glass substrates of 0.55 mm thick
were used for the glass substrates of the displaying cell. Since
the thickness of each glass substrate of the compensation cell is
0.55 mm, the difference of thickness between adjacent glass
substrates in the displaying cell and the compensation cell is
smaller than 0.05 mm. In this liquid crystal display device,
ununiformity of display was found in the picture screen in a state
that no voltage was applied to the liquid crystal layer of the
displaying cell, and a good quality of display could not be
obtained.
EXAMPLE 2
[0137] Glass substrates of 0.55 mm thick were used for the glass
substrates of the displaying cell, and glass substrates of 0.5 mm
thick were used as the glass substrates of the compensation cell.
Then, a liquid crystal display device was prepared in the same
manner as Example 1. The difference of thickness between adjacent
glass substrates in the displaying cell and the compensation cell
is 0.05 mm. A display of good dark appearance was obtained without
generating a line-shaped ununiformity of brightness in a case that
no voltage was applied to the liquid crystal layer of the
displaying cell.
EXAMPLE 3
[0138] A glass substrate of 0.55 mm thick was used as the glass
substrate at a front surface side of the displaying cell, and a
glass substrate of 0.5 mm thick was used as the glass substrate at
a rear surface side of the displaying cell. Further, glass
substrates of 0.5 mm thick were used for the glass substrates of
the compensation cell. Then, a liquid crystal display device was
prepared in the same manner as Example 1. The difference of
thickness between adjacent glass substrates in the displaying cell
and the compensation cell is 0.05 mm. A display of good dark
appearance could be obtained without generating a line-shaped
ununiformity of brightness in a case that no voltage was applied to
the liquid crystal layer of the displaying cell.
[0139] A liquid crystal display device was prepared in the same
manner as Comparative Example 1 except that glass substrates of
0.55 mm thick were used for the glass substrates of the displaying
cell and the glass substrates of the compensation cell. Namely, the
liquid crystal display device was prepared so that thicknesses of
adjacent glass substrates of the displaying cell and the
compensation cell were equal. In this case, however, polishing was
conducted to the adjacent glass substrates in the displaying cell
and the compensation cell so that the difference between the
thickest portion and the thinnest portion was 0.2 .mu.m or less. A
display of good dark appearance could be obtained without
generating a line-shaped ununiformity of brightness in a case that
no voltage was applied to the liquid crystal layer of the
displaying cell, unlike the case of Comparative Example 1.
[0140] In the following, description will be made as to an
inspection method for a transparent substrate wherein a change of
the relative interference intensity of two transparent substrates
is utilized.
[0141] Assuming that two glass substrates (transparent substrates)
are arranged adjacent to each other, such as the glass substrates
2, 28 as shown in FIG. 1, wherein either one of the glass
substrates has no distribution of thickness, namely, it has no
unevenness of thickness, and the other has a certain distribution
of thickness, namely, it has a thick portion and a thin portion.
When the thickness of the glass substrate is changed 0.1 .mu.m (a
half of the period of fluctuation of the relative interference
intensity), there appears a change in the relative interference
intensity from a high state to a low state. Namely, the color of
interference light changes from a dark state to a thin state. When
the thickness is changed 0.2 .mu.m (i.e., the period of fluctuation
of the relative interference intensity), the relative interference
intensity is changes from a high (dark) state to a low (thin) state
and successively to be a high (dark) state, whereby such change is
recognized as a single line-shaped ununiformity of brightness. The
inspection method for a transparent; substrate according to the
present invention is a method utilizing this.
[0142] When a distribution of thickness is to be inspected, a first
glass substrate having a predetermined thickness and a second glass
substrate to be inspected are arranged adjacent to each other, and
the presence or absence of a line-shaped ununiformity of brightness
(interference fringes) is confirmed. The difference of thickness in
average between the first glass substrate and the second glass
substrate should be 50 .mu.m or less. The two glass substrates are
arranged so that they are adjacent to each other and light passes
through each plane of the two glass substrates. For example, they
are arranged so that planes of the glass substrates are opposed to
each other. In this case, a substrate gap of at least 0.1 mm should
be provided between the first glass substrate and the second glass
substrate. As the first glass substrate, a very flat substrate
without having any deviation of thickness (without having a
distribution of thickness) is used. The second glass substrate to
be inspected should be produced to have the same thickness as the
first glass substrate. Namely, the second glass substrate has
substantially the same thickness as the first glass substrate while
it may have a distribution of thickness.
[0143] After the two glass substrates have been arranged in the
above-mentioned manner, a distribution of the interference
intensity of light passing through the two glass substrates is
observed. The distribution of interference intensity is observed as
an interference fringe(s) (a line-shaped ununiformity of
brightness). The interference fringe(s) is different from Newton
ring(s).
[0144] FIG. 17 is a diagram showing Examples of path of light
passing through two glass substrates, wherein FIG. 17(a) shows a
path of light propagating linearly in the two glass substrates,
FIG. 17(b) shows a path of light which enters into the glass
substrate located at an opposite side of an observer; reflects on a
rear surface of the glass substrate; comes out again the glass
substrate at a opposite side of the observer, and passes through
the glass substrate located at a side of the observer, and FIG.
17(c) shows a path of light which passes through the glass
substrate located at a side of the observer to enter into the glass
substrate located at an opposite side of the observer; reflects on
the surface of the glass substrate, and passes through again the
glass substrate at a side of the observer. In order to confirm the
distribution of the interference intensity of light in this case,
the distribution of the interference intensity of light tracing
paths as shown in FIGS. 17(a) to 17(c) should be observed, for
example.
[0145] In the inspection method of the present invention, a light
source for irradiating light whose halt value width with respect to
the peak luminance is at least 5 nm is prepared so as to let such
light pass through the two glass substrates. It is preferable to
use natural light for such light.
[0146] When the second glass substrate has no distribution of
thickness and has a thickness completely equal to the thickness of
the first glass substrate, no line-shaped ununiformity of
brightness takes place. If the second glass substrate has a certain
distribution of thickness and the difference of thickness between
the two glass substrates approaches a degree capable of causing an
optical interference, a line-shaped ununiformity of brightness (an
interference fringe) is apt to take place due to the interference
of light passing through the two glass substrates. When the
difference of thickness reaches about 0.2 .mu.m, a single
line-shaped ununiformity of brightness is observed. When the
difference of thickness between the two glass substrates is further
increased, the number of lines in the line-shaped ununiformity of
brightness will increase. For example, when the difference of
thickness is expanded to 0.4 .mu.m, two lines are observed in the
line-shaped ununiformity of brightness. Further, there appears a
single line in the line-shaped ununiformity of brightness every
time the difference of thickness expands to about 0.2 .mu.m.
Accordingly, the inspection as to how much the difference of
thickness expands can be done by observing the number of lines in
the line-shaped ununiformity of brightness. When no line is
observed, the judgment that the distribution of thickness in the
second glass substrate is less than 0.2 .mu.m, can be made. Namely,
the judgment that the difference between the thickest portion and
the thinnest portion is less than 0.2 .mu.m, can be made.
[0147] In the observation of a line or lines in the line-shaped
ununiformity of brightness, the judgment that the thickness of the
second glass substrate increases about 0.2 .mu.m every time a
single line is added, can be made.
[0148] Accordingly, the maximum value in the change of thickness of
the second glass substrate can be calculated approximately as a
value obtained by multiplying the number of lines in the
line-shaped ununiformity of brightness by 0.2 .mu.m. For example,
when 10 lines are observed, the judgment that the difference of
thickness between the second glass substrate and the first glass is
substrate is about 2 .mu.m at the maximum, can be made.
Accordingly, the judgment that the difference of thickness between
the thickest portion and the thinnest portion of the second glass
substrate is about 2 .mu.m at the maximum, can be made. The
above-mentioned approximate calculation can be realized in only the
case that there is a difference of thickness between two glass
substrates and the thickness of the second glass substrate is
gradually increased or decreased. In case that the thickness of the
second glass substrate is repeatedly thick and thin, the value of
the difference of thickness between the two glass substrates is
smaller than a value obtained by multiplying the number of lines in
the line-shaped ununiformity of brightness by 0.2 .mu.m.
[0149] Plural kinds of first glass substrates should be prepared so
as to meet thicknesses of glass substrates to be produced. For
example, when a glass substrate of 0.55 mm thick is produced and
inspected, a glass substrate of 0.55 mm thick without having any
distribution of thickness should be prepared as the first glass
substrate. When a glass substrate of 0.5 mm thick is produced and
inspected, a glass substrate of 0.55 mm thick without having any
distribution of thickness should be prepared as the first glass
substrate.
[0150] The inspection method of the present invention allows
inspection to the thickness of a substrate or the presence or
absence of unevenness of thickness of a substrate used for a liquid
crystal display device, an organic EL display device or various
kinds of flat panel display. When various kinds of first substrates
as reference are prepared, a highly precise inspection can easily
be done. In this case, it is preferable that the thickness of the
second transparent substrate is 30 .mu.m or more in average. If the
substrate is too thin, interference takes place by the substrate
itself. It is further preferable that the thickness is 100 .mu.m or
more.
[0151] The observation of the line-shaped ununiformity of
brightness may be by visual check by human eyes or automatic
inspection using a CCD camera.
[0152] The first glass substrate is not necessarily a very flat
substrate without a deviation of thickness if the state of
distribution of thickness of it is known. For example, even when
the glass substrate has a portion having a different thickness and
if the state of that portion is known, the other portion having a
constant thickness can be used for inspection.
[0153] As shown in FIG. 18, even when a glass substrate whose
thickness is changed in a wedge-like form is used as the first
glass substrate, it is possible to confirm a change in the
thickness of the second glass substrate by finding the number of
lines in a line-shaped ununiformity of brightness 301 resulted in a
direction perpendicular to the direction 300 along which the
thickness is unchanged.
[0154] A substrate to be inspected is not limited to a substrate
for a liquid crystal display device, an organic EL display device
or various kinds of flat panel display. A glass substrate usable
with another substrate in parallel may be an object to be
inspected. For example, a glass substrate usable solely such as a
cover glass for a solar cell may be an object to be inspected.
Further, a transparent substrate other than glass (e.g., a plastic
substrate) may be an object to be inspected.
[0155] According to the liquid crystal display device of the
present invention, a display of good quality can be provided
without generating a line-shaped ununiformity of brightness even
when there is leakage of light in presenting a display of dark
appearance. Further, according to the inspection method for a
transparent substrate according to the present invention,
unevenness of thickness in a substrate can easily be confirmed.
[0156] The entire disclosure of Japanese Patent Application No.
2002-126916 filed on Apr. 26, 2002 including specification, claims,
drawings and summary is incorporated herein by reference in its
entirety.
* * * * *