U.S. patent application number 09/893776 was filed with the patent office on 2001-11-01 for liquid crystal device and electronic apparatus using the same.
This patent application is currently assigned to SEIKO EPSON CORPORATION. Invention is credited to Kuroiwa, Masahiro, Naonori, Miwa, Tsuda, Atsunari.
Application Number | 20010035928 09/893776 |
Document ID | / |
Family ID | 15198451 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035928 |
Kind Code |
A1 |
Kuroiwa, Masahiro ; et
al. |
November 1, 2001 |
Liquid crystal device and electronic apparatus using the same
Abstract
The liquid crystal device of the present invention effectively
prevents reversion of bright/dark states between a reflective
display mode and a transmissive display mode. The liquid crystal
device includes a first absorptive polarizer, which receives light
from outside;a liquid crystal cell, which receives light emitted
from the first absorptive polarizer; a second absorptive polarizer,
which receives light emitted from the liquid crystal cell; and a
reflective polarizer, which receives light emitted from the second
absorptive polarizer. The reflective polarizer has an axis of
reflection in a predetermined direction to reflect at least part of
light that has been transmitted through the first absorptive
polarizer, the liquid crystal cell, and the second absorptive
polarizer to be incident on the reflective polarizer. The
reflective polarizer partially transmits light including a linearly
polarized light component which is included in light entering the
reflective polarizer from an opposite side to the second absorptive
polarizer and which is to be transmitted through the second
absorptive polarizer. The first absorptive polarizer has an axis of
transmission in a specific direction to cause light, which has been
reflected by the reflective polarizer and transmitted through the
second absorptive polarizer, to be transmitted through the first
absorptive polarizer.
Inventors: |
Kuroiwa, Masahiro;
(Nagano-shi, JP) ; Naonori, Miwa; (Matsumoto-shi,
JP) ; Tsuda, Atsunari; (Suwa-shi, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
SEIKO EPSON CORPORATION
4-1, Nishi-shinjuku 2-chome
Shinjuku-ku
JP
|
Family ID: |
15198451 |
Appl. No.: |
09/893776 |
Filed: |
June 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09893776 |
Jun 29, 2001 |
|
|
|
09300457 |
Apr 28, 1999 |
|
|
|
Current U.S.
Class: |
349/115 ;
349/106; 349/96 |
Current CPC
Class: |
G02F 1/133536 20130101;
G02F 1/13362 20130101; G02F 2203/02 20130101 |
Class at
Publication: |
349/115 ; 349/96;
349/106 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 30, 1998 |
JP |
10-137430(P) |
Claims
What is claimed is:
1. A liquid crystal device that modulates light responsive to given
image signals, the liquid crystal device comprising: a first
absorptive polarizer, which receives light from outside; a liquid
crystal cell, which receives light emitted from the first
absorptive polarizer; a second absorptive polarizer, which receives
light emitted from the liquid crystal cell; and a reflective
polarizer, which receives light emitted from the second absorptive
polarizer, wherein the reflective polarizer has an axis of
reflection in a predetermined direction to reflect at least part of
light that has been transmitted through the first absorptive
polarizer, the liquid crystal cell, and the second absorptive
polarizer to be incident on the reflective polarizer, and the
reflective polarizer partially transmits light including a linearly
polarized light component which is included in light entering the
reflective polarizer from an opposite side to the second absorptive
polarizer and which is to be transmitted through the second
absorptive polarizer, and wherein the first absorptive polarizer
has an axis of transmission in a specific direction to cause light,
which has been reflected by the reflective polarizer and
transmitted through the second absorptive polarizer, to be
transmitted through the first absorptive polarizer.
2. A liquid crystal device in accordance with claim 1, further
comprising: a diffusing plate interposed between the second
absorptive polarizer and the reflective polarizer.
3. A liquid crystal device in accordance with claim 1, wherein the
predetermined direction of the reflection axis of the reflective
polarizer is adjusted to cause a ratio of an amount of first light
to an amount of second light to be not less than about 15% in a
state where a linearly polarized light component having a
predetermined first polarizing direction is emitted in a greatest
amount from the liquid crystal cell towards the second absorptive
polarizer, the first light being one that is reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer, the second light being one that is incident on the first
absorptive polarizer.
4. A liquid crystal device in accordance with claim 2, wherein the
predetermined direction of the reflection axis of the reflective
polarizer is adjusted to cause a ratio of an amount of first light
to an amount of second light to be not less than about 15% in a
state where a linearly polarized light component having a
predetermined first polarizing direction is emitted in a greatest
amount from the liquid crystal cell towards the second absorptive
polarizer, the first light being one that is reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer, the second light being one that is incident on the first
absorptive polarizer.
5. A liquid crystal device in accordance with claim 1, further
comprising: a backlight that is disposed opposite to the second
absorptive polarizer across the reflective polarizer, wherein light
emitted from the backlight is adjusted to have a color other than
white, in order to cause color of a first light to be close to
color of a second light, the first light being one that is emitted
from the backlight and transmitted through the reflective
polarizer, the second absorptive polarizer, the liquid crystal
cell, and the first absorptive polarizer, the second light being
one that comes from the outside and is transmitted through the
first absorptive polarizer, the liquid crystal cell, and the second
absorptive polarizer, subsequently reflected by the reflective
polarizer, then transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer.
6. A liquid crystal device in accordance with claim 5, wherein the
backlight comprises: a light source; and a color filter that
adjusts color of light emitted from the light source.
7. An electronic apparatus comprising: a display device, the
display device including: a first absorptive polarizer, which
receives light from outside; a liquid crystal cell, which receives
light emitted from the first absorptive polarizer; a second
absorptive polarizer, which receives light emitted from the liquid
crystal cell; and a reflective polarizer, which receives light
emitted from the second absorptive polarizer, wherein the
reflective polarizer has an axis of reflection in a predetermined
direction to reflect at least part of light that has been
transmitted through the first absorptive polarizer, the liquid
crystal cell, and the second absorptive polarizer to be incident on
the reflective polarizer, and the reflective polarizer partially
transmits light including a linearly polarized light component
which is included in light entering the reflective polarizer from
an opposite side to the second absorptive polarizer and which is to
be transmitted through the second absorptive polarizer, and wherein
the first absorptive polarizer has an axis of transmission in a
specific direction to cause light, which has been reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, to be transmitted through the first absorptive
polarizer.
8. An electronic apparatus in accordance with claim 7, the display
device further comprising: a diffusing plate interposed between the
second absorptive polarizer and the reflective polarizer.
9. An electronic apparatus in accordance with claim 7, wherein the
predetermined direction of the reflection axis of the reflective
polarizer is adjusted to cause a ratio of an amount of first light
to an amount of second light to be not less than about 15% in a
state where a linearly polarized light component having a
predetermined first polarizing direction is emitted in a greatest
amount from the liquid crystal cell towards the second absorptive
polarizer, the first light being one that is reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer, the second light being one that is incident on the first
absorptive polarizer.
10. An electronic apparatus in accordance with claim 8, wherein the
predetermined direction of the reflection axis of the reflective
polarizer is adjusted to cause a ratio of an amount of first light
to an amount of second light to be not less than about 15% in a
state where a linearly polarized light component having a
predetermined first polarizing direction is emitted in a greatest
amount from the liquid crystal cell towards the second absorptive
polarizer, the first light being one that is reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer, the second light being one that is incident on the first
absorptive polarizer.
11. A liquid crystal device in accordance with claim 7, the display
device further comprising: a backlight that is disposed opposite to
the second absorptive polarizer across the reflective polarizer,
wherein light emitted from the backlight is adjusted to have a
color other than white, in order to cause color of a first light to
be close to color of a second light, the first light being one that
is emitted from the backlight and transmitted through the
reflective polarizer, the second absorptive polarizer, the liquid
crystal cell, and the first absorptive polarizer, the second light
being one that comes from the outside and is transmitted through
the first absorptive polarizer, the liquid crystal cell, and the
second absorptive polarizer, subsequently reflected by the
reflective polarizer, then transmitted through the second
absorptive polarizer, the liquid crystal cell, and the first
absorptive polarizer.
12. An electronic apparatus in accordance with claim 11, wherein
the backlight comprises: a light source; and a color filter that
adjusts color of light emitted from the light source.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a transflective liquid
crystal device (hereinafter also referred to as a transflective
liquid crystal device) capable of both reflective display, which
reflects incident light to display an image, and transmissive
display, which transmits incident light to display an image.
[0003] 2. Description of the Related Art
[0004] The transflective liquid crystal device is widely used as a
display device of portable information equipment. FIG. 11
schematically illustrates the structure of a conventional
transflective liquid crystal device 1000. The transflective liquid
crystal device 1000 includes an absorptive polarizer 1020, a liquid
crystal cell 1030, a light diffusing plate 1040, a reflective
polarizer 1050, and a light absorbing plate 1060. A backlight 1070
is further disposed outside the light absorbing plate 1060. The
liquid crystal cell 1030 includes a lower glass substrate 1033, an
upper glass substrate 1031, and a liquid crystal layer 1035 sealed
between these glass substrates 1031 and 1033. A plurality of
transparent signal electrodes 1034 are mounted on the upper surface
of the lower glass substrate 1033. A plurality of transparent
scanning electrodes 1032 are mounted to be perpendicular to the
plurality of signal electrodes 1034 on the lower surface of the
upper glass substrate 1031. The liquid crystal cell 1030 has a
passive matrix configuration, in which one pixel is defined by one
signal electrode 1034, one scanning electrode 1032, and the liquid
crystal layer 1035 between these electrodes 1034 and 1032. Namely
the light transmitted through the liquid crystal layer 1035 is
modulated according to the voltage applied between one signal
electrode 1034 and one scanning electrode 1032. The liquid crystal
layer 1035 may be made of a TN (twisted nematic) liquid crystal
composition or STN (super twisted nematic) liquid crystal
composition. A translucent film having the transmittance of about
50% is used for the light absorbing plate 1060.
[0005] FIG. 12 shows problems arising in the conventional
transflective liquid crystal device 1000. The absorptive polarizer
1020 has an axis of transmission 1020T that is set parallel to the
plane of the drawing, and an axis of absorption 1020A that is
perpendicular to the plane of the drawing. The reflective polarizer
1050 has, on the other hand, an axis of transmission 1050T that is
parallel to the plane of the drawing, and an axis of reflection
1050R that is perpendicular to the plane of the drawing. The
following describes the operations of the liquid crystal display
1000 on the assumption that the polarizing direction of the light
transmitted through the liquid crystal cell 1030 is rotated by 90
degrees while no voltage is applied between the signal electrodes
1034 and the scanning electrodes 1032 (that is, when the liquid
crystal cell 1030 is in an OFF state).
[0006] This liquid crystal device 1000 has two display modes, that
is, a reflective display mode using incident light 1100 from the
outside and a transmissive display mode using light 1120 emitted
from the backlight 1070. In the reflective display mode, when the
non-polarized light 1100 enters the absorptive polarizer 1020, a
linearly polarized light component having the polarization
direction parallel to the axis of absorption 1020A is mostly
absorbed by the absorptive polarizer 1020, while only a linearly
polarized light component having the polarization direction
parallel to the axis of transmission 1020T is transmitted through
the absorptive polarizer 1020 and enters the liquid crystal cell
1030. The optical rotatory power of the liquid crystal cell 1030
causes the light component entering the liquid crystal cell 1030 to
be converted into linearly polarized light having a polarizing
direction that is perpendicular to that of the incident light. The
polarizing direction of the light emitted from the liquid crystal
cell 1030 is substantially identical with the direction of the axis
of reflection 1050R of the reflective polarizer 1050, so that most
of the light emitted from the liquid crystal cell 1030 is reflected
by the reflective polarizer 1050 and re-enters the liquid crystal
cell 1030 as return light. The liquid crystal cell 1030 converts
the return light into linearly polarized light having a polarizing
direction that is perpendicular to that of the return light. At
this moment, the polarizing direction of the return light emitted
from the liquid crystal cell 1030 is substantially identical with
the direction of the axis of transmission 1020T of the absorptive
polarizer 1020, so that most of the return light emitted from the
liquid crystal cell 1030 is transmitted through the absorptive
polarizer 1020. In the reflective display mode, the pixels where
the liquid crystal cell 1030 is in the OFF state receive the light
reflected and returned as discussed above and are thereby observed
as bright pixels. The pixels where the liquid crystal cell 1030 is
in an ON state are, on the contrary, observed as dark pixels.
[0007] In the transmissive display mode, on the other hand, when
the nonpolarized light 1120 enters the reflective polarizer 1050, a
linearly polarized light component having the polarization
direction parallel to the axis of reflection 105OR is mostly
reflected by the reflective polarizer 1050, while only a linearly
polarized light component having the polarization direction
parallel to the axis of transmission 1050T is transmitted through
the reflective polarizer 1050 and enters the liquid crystal cell
1030. The optical rotatory power of the liquid crystal cell 1030
causes polarizing direction of the light transmitted through the
liquid crystal cell 1030 to be converted into a direction
substantially parallel to the axis of absorption 1020A of the
absorptive polarizer 1020. Most of the light emitted from the
liquid crystal cell 1030 is accordingly absorbed by the absorptive
polarizer 1020 and is not transmitted through the absorptive
polarizer 1020. In the transmissive display mode, since the light
is absorbed in the course of the optical path, the pixels where the
liquid crystal cell 1030 is in the OFF state are observed as dark
pixels. The pixels where the liquid crystal cell 1030 is in the ON
state are, on the contrary observed as bright pixels. The
relationship between the ON/OFF state of the liquid crystal cell
1030 and the bright/dark state of the pixel in the transmissive
display mode is reverse to that in the reflective display mode. In
the transflective liquid crystal device 1000, the brightness and
darkness of display are reversed between the reflective display
mode and the transmissive display mode.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is thus to provide a
liquid crystal device that effectively prevents the reversion of
the same bright/dark states between the reflective display mode and
the transmissive display mode, and also to provide an electronic
apparatus using such a liquid crystal device.
[0009] At least part of the above and the other related objects is
attained by a liquid crystal device that modulates light responsive
to given image signals. The liquid crystal device includes a first
absorptive polarizer, which receives light from outside;a liquid
crystal cell, which receives light emitted from the first
absorptive polarizer;a second absorptive polarizer, which receives
light emitted from the liquid crystal cell; and a reflective
polarizer, which receives light emitted from the second absorptive
polarizer. The reflective polarizer has an axis of reflection in a
predetermined direction to reflect at least part of light that has
been transmitted through the first absorptive polarizer, the liquid
crystal cell, and the second absorptive polarizer to be incident on
the reflective polarizer. The reflective polarizer partially
transmits light including a linearly polarized light component
which is included in light entering the reflective polarizer from
an opposite side to the second absorptive polarizer and which is to
be transmitted through the second absorptive polarizer. The first
absorptive polarizer has an axis of transmission in a specific
direction to cause light, which has been reflected by the
reflective polarizer and transmitted through the second absorptive
polarizer, to be transmitted through the first absorptive
polarizer.
[0010] The liquid crystal device of the present invention works as
discussed below in a first state of the liquid crystal cell, in
which the light entering the first absorptive polarizer from the
outside is transmitted through the first absorptive polarizer, the
liquid crystal cell, and the second absorptive polarizer. Part of
the light that is emitted from the second absorptive polarizer and
includes a linearly polarized light component having a polarizing
direction parallel to the axis of reflection of the reflective
polarizer is reflected by the reflective polarizer, transmitted
through the second absorptive polarizer and the liquid crystal
cell, and emitted from the first absorptive polarizer. When the
light enters the reflective polarizer on the opposite side to the
second absorptive polarizer in the first state of the liquid
crystal cell, on the other hand, part of the light is transmitted
through the reflective polarizer, the second absorptive polarizer,
and the liquid crystal cell in this sequence and emitted from the
first absorptive polarizer. In this first state of the liquid
crystal cell, the light entering the first absorptive polarizer
from the outside is reflected and emitted to the outside. The light
entering the reflective polarizer is also eventually emitted to the
outside. The liquid crystal cell in the first state is accordingly
observed as a bright pixel both in the reflective display mode and
in the transmissive display mode.
[0011] The liquid crystal device works as discussed below in a
second state of the liquid crystal cell, in which the light
entering the first absorptive polarizer from the outside is
absorbed by the second absorptive polarizer. The light supplied
from the outside into the first absorptive polarizer is absorbed by
the second absorptive polarizer, so that there is no light
reflected by the reflective polarizer. Namely the light entering
the first absorptive polarizer from the outside is not emitted from
the first absorptive polarizer. When the light enters the
reflective polarizer on the opposite side to the second absorptive
polarizer in the second state of the liquid crystal cell, on the
other hand, part of the light is transmitted through the reflective
polarizer, the second absorptive polarizer, and the liquid crystal
cell in this order and enters the first absorptive polarizer. This
light is, however, absorbed by the first absorptive polarizer and
is thereby not emitted. In this second state of the liquid crystal
cell, the light entering the first absorptive polarizer from the
outside is not emitted to the outside. The light entering the
reflective polarizer is nor emitted to the outside. The liquid
crystal cell in the second state is accordingly observed as a dark
pixel both in the reflective display mode and in the transmissive
display mode.
[0012] As discussed above, the liquid crystal device of the present
invention effectively maintains the same bright/dark states in both
of the reflective and transmissive display modes.
[0013] In accordance with one preferable application, the liquid
crystal device further includes a diffusing plate interposed
between the second absorptive polarizer and the reflective
polarizer.
[0014] This arrangement effectively suppresses specular reflection
occurring on the reflective polarizer.
[0015] In accordance with another preferable application of the
liquid crystal device, the predetermined direction of the
reflection axis of the reflective polarizer is adjusted to cause a
ratio of an amount of first light to an amount of second light to
be not less than about 15% in a state where a linearly polarized
light component having a predetermined first polarizing direction
is emitted in a greatest amount from the liquid crystal cell
towards the second absorptive polarizer. The first light is one
that is reflected by the reflective polarizer and transmitted
through the second absorptive polarizer, the liquid crystal cell,
and the first absorptive polarizer. The second light is one that is
incident on the first absorptive polarizer.
[0016] Unlike the conventional liquid crystal display, this
arrangement enables non-reversed transmissive display without
unduly affecting the advantageous characteristics (including
brightness) of reflective display.
[0017] In accordance with still another preferable application, the
liquid crystal device further includes a backlight disposed
opposite to the second absorptive polarizer across the reflective
polarizer. Light emitted from the backlight is adjusted to have a
color other than white, in order to cause color of a first light to
be close to color of a second light. The first light is one that is
emitted from the backlight and transmitted through the reflective
polarizer, the second absorptive polarizer, the liquid crystal
cell, and the first absorptive polarizer. The second light is one
that comes from the outside and is transmitted through the first
absorptive polarizer, the liquid crystal cell, and the second
absorptive polarizer, subsequently reflected by the reflective
polarizer, then transmitted through the second absorptive
polarizer, the liquid crystal cell, and the first absorptive
polarizer
[0018] In this structure, it is preferable that the backlight
includes a light source and a color filter that adjusts color of
light emitted from the light source.
[0019] This arrangement enables the color of a transmitted light
component that is included in the light emitted from the backlight,
transmitted through the reflective polarizer, and emitted from the
first absorptive polarizer to be adjusted close to the color of a
reflected light component that is supplied from the outside to the
first absorptive polarizer, reflected by the reflective polarizer,
and emitted from the first absorptive polarizer. This reduces a
difference in color tone of the display between the reflective
display mode and the transmissive display mode.
[0020] Any one of the above liquid crystal devices may be mounted
as a display device on a variety of electronic apparatuses.
[0021] These and other objects, features, aspects, and advantages
of the present invention will become more apparent from the
following detailed description of the preferred embodiments with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 schematically illustrates the structure of a
transflective liquid crystal device 100 in a first embodiment
according to the present invention;
[0023] FIG. 2 illustrates the structure of a reflective polarizer
160;
[0024] FIG. 3 shows the relationship between an axis of
transmission 120T of a first absorptive polarizer 120, an axis of
transmission 140T of a second absorptive polarizer 140, and an axis
of reflection 160R of the reflective polarizer 160;
[0025] FIGS. 4A and 4B show functions of the liquid crystal device
100 in the first embodiment;
[0026] FIG. 5 is a graph showing a variation in reflectivity in a
reflective display mode and a variation in transmittance in a
transmissive display mode in the liquid crystal device 100 of the
first embodiment;
[0027] FIG. 6 schematically illustrates the structure of another
liquid crystal device 200 in a second embodiment according to the
present invention;
[0028] FIG. 7 shows the relationship between the axis of
transmission 120T of the first absorptive polarizer 120, the axis
of transmission 140T of the second absorptive polarizer 140, an
optical axis 2100A of a .lambda./2 phase plate 210, and the axis of
reflection 160R of the reflective polarizer 160;
[0029] FIG. 8 schematically illustrates the structure of still
another liquid crystal device 300 in a third embodiment according
to the present invention;
[0030] FIG. 9 is a graph showing xy chromaticity coordinates of the
reflected light and transmitted light that are observed in the
first embodiment shown in the graph of FIG. 5, in an XYZ color
specification system;
[0031] FIGS. 10A through 10C show examples of electronic apparatus
to which the liquid crystal device of the present invention is
applied;
[0032] FIG. 11 schematically illustrates the structure of a
conventional transflective liquid crystal device; and
[0033] FIG. 12 shows problems arising in the conventional
transflective liquid crystal device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A. First Embodiment
[0034] FIG. 1 schematically illustrates the structure of a
transflective liquid crystal device 100 in a first embodiment
according to the present invention. The liquid crystal device 100
includes a first absorptive polarizer 120, a liquid crystal cell
130, a second absorptive polarizer 140, a light scattering plate
(diffusing plate) 150, and a reflective polarizer 160. A backlight
170 is further disposed outside the reflective polarizer 160.
Although there is shown a gap between each element in FIG. 1, the
gap is only for the clarity of illustration. In an actual device,
the respective elements are in close contact with one another
without any gaps. This is also adopted in other embodiments and
modifications discussed later.
[0035] The liquid crystal cell 130 includes a lower glass substrate
133, an upper glass substrate 131, and a liquid crystal layer 135
sealed between these glass substrates 131 and 133. A plurality of
transparent signal electrodes 134 are mounted on the upper surface
of the lower glass substrate 133. A plurality of transparent
scanning electrodes 132 that are arranged perpendicularly to the
signal electrodes 134 are mounted on the lower surface of the upper
glass substrate 131. The liquid crystal layer 135 is composed of a
TN (twisted nematic) liquid crystal composition or STN (super
twisted nematic) liquid crystal composition. The liquid crystal
cell 130 has a simple matrix configuration, in which one pixel is
defined by one signal electrode 134, one scanning electrode 132,
and the liquid crystal layer 135 between these electrodes 134 and
132. Although there is shown a relatively wide gap between the
upper glass substrate 131 and the lower glass substrate 133 in FIG.
1, this is only for the clarity of illustration. In an actual
device, the upper glass substrate 131 face the lower glass
substrate 133 across a narrow gap of several to ten-odd
micrometers. The liquid crystal cell 130 has color filters, an
alignment layer, a driving circuit, and other related elements, in
addition to the elements illustrated in FIG. 1. For example, color
filters are interposed between the lower glass substrate 133 and
the signal electrodes 134 to be arranged perpendicularly to the
scanning electrodes 132. The color filters of the respective
colors, red (R), green (G), and blue (B) are arranged repeatedly in
this sequence corresponding to the respective signal electrodes
134, that is, arranged in stripe. The color filters may
alternatively be mounted on the upper glass substrate 131. The
arrangement of the color filters is not limited to the stripe
configuration but may have a mosaic configuration. These elements
are not essential for the explanation of the present invention and
are thus omitted from the illustration.
[0036] The first absorptive polarizer 120 and the second absorptive
polarizer 140 respectively have the function of transmitting a
predetermined linear polarized light component while absorbing the
other linear polarized light components. The polarizers used in the
conventional transmission liquid crystal devices and reflection
liquid crystal devices can be applied for these absorptive
polarizers 120 and 140.
[0037] The reflective polarizer 160 has the function of reflecting
a predetermined linear polarized light component while transmitting
the other linear polarized light components. The reflective
polarizer 160 is made of, for example, a birefringent dielectric
multi-layered film. The details of the birefringent dielectric
multi-layered film are disclosed in International Publication No.
WO97/01788 and International Application-based Japanese Patent
Laid-Open Gazette No. 9-506985, the disclosures of which are herein
incorporated by reference for all purposes.
[0038] FIG. 2 illustrates the structure of the reflective polarizer
160. The reflective polarizer 160 is basically a birefringent
dielectric multi-layered film prepared by alternately placing two
different types of polymer layers 161 and 162 one upon another. One
of the two different polymers is selected among the materials
having a high modulus of photoelasticity, whereas the other polymer
is selected among the materials having a low modulus of
photoelasticity. It is here noted that the selected materials
should have substantially equal ordinary indexes in the orientated
state. For example, PEN (2,6-polyethylene naphthalate) is selected
for the material having the high modulus of photoelasticity, and
coPEN (70-naphthalate/30-terephthalate copolyester) is selected for
the material having the low modulus of photoelasticity. Films of
these two different polymers were alternately laid one upon another
to form a film laminate, and the film laminate was stretched to
approximately 5 times in the direction of the x axis in the
rectangular coordinate system shown in FIG. 2. The observed index
of refraction in the x-axis direction was 1.88 in the PEN layer and
1.64 in the coPEN layer. The observed index of refraction in the
y-axis direction was about 1.64 in both the PEN layer and the coPEN
layer. When light enters the film laminate from the direction of
its normal, a light component vibrating in the y-axis direction is
transmitted through the film. This is an axis of transmission. A
light component vibrating in the x-axis direction is, on the other
hand, reflected only when the PEN layer and the coPEN layer satisfy
a predetermined condition. This is an axis of reflection. The
predetermined condition is that the sum of an optical path (that
is, the product of the index of refraction and the thickness of the
film) of the PEN layer and an optical path of the coPEN layer is
equal to half the wavelength of light. Lamination of several tens
layers or preferably more than 100 layers of both the PEN layer and
the coPEN layer to the thickness of about 30 .mu.m enables
reflection of substantially all the light component vibrating in
the direction of the axis of reflection. Changing the number of
layers varies the resulting reflectivity. The reflective polarizer
thus manufactured has the polarization ability only for the light
of a single design wavelength. In order to attain the polarization
ability in a wider range of the wavelength, a plurality of
reflective polarizers having different design wavelengths are laid
one upon another while their axes of reflection are aligned.
[0039] The sufficiently thick laminate of the reflective polarizer
is brighter by at least 30% than a known reflection polarizer that
is prepared by combining a conventional polarizer (absorptive
polarizer) with an aluminum reflector. There are two reasons. One
reason is that the reflective polarizer is a dielectric mirror and
thereby reflects almost 100% of a specific linearly polarized light
component, although the metal aluminum has the reflectivity of not
greater than 90%. The other reason is that the conventional
absorptive polarizer takes advantage of a dichroic dyestuff, such
as a halogen like iodine or a dye and wastes at least 10% of the
light because of its relatively low dichromatic ratio.
[0040] Another liquid crystal polymer having a cholesteric phase
may be combined with a .lambda./4 phase plate and used for the
reflective polarizer. The details of such a reflective polarizer
are disclosed, for example, Japanese Patent Laid-Open Gazette No.
8-271892, the disclosure of which is herein incorporated by
reference for all purposes.
[0041] The reflective polarizer 160 used in the embodiment does not
attain a 100% degree of polarization, so that the reflectivity of
the linearly polarized light component having the polarizing
direction that is parallel to the axis of reflection is several
tens percents, while the transmittance of the linearly polarized
light component having the polarizing direction that is parallel to
the axis of transmission is also several tens percents. The
reflective polarizer 160 reflects part of linearly polarized light
components having polarizing directions other than that parallel to
the axis of reflection and transmits part of linearly polarized
light components having polarizing directions other than that
parallel to the axis of transmission. The degree of polarization
here is defined either by the reflectivity of light in the
direction of the axis of reflection or by the transmittance of
light in the direction of the axis of transmission.
[0042] The diffusing plate 150 (see FIG. 1) has the function of
diffusing light. The diffusing plate 150 may be omitted from the
liquid crystal device. In this case, the light specularly reflected
by the reflection plate 160 is emitted outside as the return light.
The diffusing plate 150 has the function of preventing the specular
reflection. A plastic film with beads dispersed therein, for
example, may be used for the diffusing plate 150. In one possible
modification, the diffusing plate may be replaced by the second
absorptive polarizer 140 and the reflective polarizer 160 bonded to
each other via an optical adhesive with beads dispersed therein.
The diffusing plate 150 may be interposed between the first
absorptive polarizer 120 and the liquid crystal cell 130,
interposed between the liquid crystal cell 130 and the second
absorptive polarizer 140, or mounted on the upper surface of the
first absorptive polarizer 120.
[0043] The backlight 170 includes a light source 171 and a light
guide plate 172. Light emitted from the light source 171 is guided
and diffused by the light guide plate 172, in order to enable the
light to enter all the pixels in the liquid crystal cell 130. The
light guide plate 172 may be a diffusing plate or a laminate of
light-collecting prisms. The light source 171 may be a cold-cathode
tube or a LED (light-emitting diode). An EL (electroluminescence)
surface light source may be used for the backlight 170, instead of
the combination of the light source 171 with the light guide plate
172.
[0044] FIG. 3 shows the relationship between an axis of
transmission 120T of a first absorptive polarizer 120, an axis of
transmission 140T of a second absorptive polarizer 140, and an axis
of reflection 160R of the reflective polarizer 160. The axis of
transmission 120T of the first absorptive polarizer 120 is set to
be at right angles to the axis of transmission 140T of the second
absorptive polarizer 140. In this example, the axis of transmission
120T of the first absorptive polarizer 120 is set to be inclined 45
degrees counterclockwise against the horizontal direction (the
direction of the x axis) in the drawing. The axis of reflection
160R of the reflective polarizer 160 is set to be rotated clockwise
by an angle of .theta.ax from the axis of transmission 140T of the
second absorptive polarizer 140. The reflective polarizer 160 has
an axis of transmission 160T that is at right angles to the axis of
reflection 160R.
[0045] The axis of transmission 120T of the first absorptive
polarizer 120 is set in the direction identical with the polarizing
direction of linearly polarized light that is to be rotated in the
liquid crystal cell 130. In the linearly polarized light
transmitted through the first absorptive polarizer 120, a linearly
polarized light component passing through the cell area in the OFF
state has the polarizing direction rotated by 90 degrees, whereas a
linearly polarized light component passing through the cell area in
the ON state has the unchanged polarizing direction. The axis of
transmission 140T of the second absorptive polarizer 140 is set in
the direction perpendicular to the axis of transmission 120T of the
first absorptive polarizer 120T, in order to transmit the linearly
polarized light component passing through the cell area in the OFF
state. The axis of reflection 160R of the reflective polarizer 160
is set to partly reflect the linearly polarized light component
transmitted through the cell area in the OFF state and the second
absorptive polarizer 140.
[0046] FIGS. 4A and 4B show functions of the liquid crystal device
100 in the first embodiment. FIG. 4A shows bright display (the cell
area in the OFF state), and FIG. 4B shows dark display (the cell
area in the ON state). The description first regards the case in
which the backlight 170 does not emit light, that is, the
reflective display mode. The first absorptive polarizer 120
transmits only linearly polarized light components having the
polarizing direction that is parallel to the axis of transmission
120T, among non-polarized rays of light 181 and 182 entering the
first absorptive polarizer 120, and causes the transmitted,
linearly polarized light components to enter the liquid crystal
cell 130.
[0047] Referring to FIG. 4A, linearly polarized light 181a, which
is emitted from the first absorptive polarizer 120 and enters the
cell area in the OFF state, is subjected to rotation of the
polarizing direction by 90 degrees in the liquid crystal cell 130
and enters the second absorptive polarizer 140 as linearly
polarized light 181b. Since the polarizing direction of the
linearly polarized light 181b is parallel to the direction of the
axis of transmission 140T of the second absorptive polarizer 140,
the linearly polarized light 181b is mostly transmitted through the
second absorptive polarizer 140 and enters the reflective polarizer
160 as linearly polarized light 181c. The linearly polarized light
181c entering the reflective polarizer 160 can be divided into two
polarized light components whose polarization directions are
parallel to the axis of reflection 160R and the axis of
transmission 160T of the reflective polarizer 160, respectively.
The linearly polarized light component having the poraization
direction parallel to the axis of reflection 160R is reflected by
the reflective polarizer 160 and re-enters the second absorptive
polarizer 140 as return light 181d. The return light 181d
re-entering the second absorptive polarizer 140 can be divided into
two polarized light components whose polarization directions are
parallel to the axis of transmission 140T and an axis of absorption
140A of the second absorptive polarizer 140, respectively. The
polarized light component having the porarization direction
parallel to the axis of absorption 140A is mostly absorbed, while
only the polarized light component having the porarization
direction parallel to the axis of transmission 140T re-enters the
liquid crystal cell 130 as linearly polarized light 181e. The
linearly polarized light 181e re-entering the liquid crystal cell
130 is subjected to rotation of the polarizing direction by 90
degrees in the liquid crystal cell 130 and enters the first
absorptive polarizer 120 as linearly polarized light 181f. Since
the polarizing direction of the linearly polarized light 181f is
parallel to the axis of transmission 120T of the first absorptive
polarizer 120, the linearly polarized light 181f is mostly
transmitted through the first absorptive polarizer 120 and emitted.
The cell area in the OFF state is accordingly displayed as a bright
pixel in the reflective display mode.
[0048] Referring to FIG. 4B, on the other hand, linearly polarized
light 182a, which is transmitted through the first absorptive
polarizer 120 and enters the liquid crystal cell 130 in the ON
state, is transmitted through the liquid crystal cell 130 without
rotation of the polarizing direction and enters the second
absorptive polarizer 140 as linearly polarized light 182b. The
linearly polarized light 182b entering the second absorptive
polarizer 140 has the polarizing direction that is parallel to the
axis of absorption 140A of the second absorptive polarizer 140
(that is, the direction perpendicular to the axis of transmission
140T). The linearly polarized light 182b is thus mostly absorbed by
the second absorptive polarizer 140 and is not transmitted through
the first absorptive polarizer 120. The cell area in the ON state
is accordingly displayed as a dark pixel in the reflective display
mode.
[0049] The liquid crystal cell 130 can be set in an intermediate
state between the ON state and the OFF state. When the liquid
crystal cell 130 is in the intermediate state, the state of FIG. 4A
and the state of FIG. 4B are mixed with each other to attain the
display of intermediate tone.
[0050] The following description regards the case in which the
backlight 170 (see FIG. 1) emits light, that is, the transmissive
display mode. The reflective polarizer 160 transmits polarized
light components having the polarizing direction that is parallel
to the axis of transmission 160T of the reflective polarizer 160,
among non-polarized rays 191 and 192 emitted from the backlight
170, and causes the transmitted, polarized light components to
enter the second absorptive polarizer 140 as polarized light
components 191a and 192a. The reflective polarizer 160, however,
has a low degree of polarization and thereby causes polarized light
components having polarizing directions other than that parallel to
the axis of transmission 160T to be partially transmitted. The
polarized light entering the second absorptive polarizer 140 can be
divided into two polarized light components having the porarization
directions parallel to the axis of transmission 140T and the axis
of absorption 140A of the second absorptive polarizer 140,
respectively. Only the polarized light component having the
porarization direction parallel to the axis of transmission 140T
enters the liquid crystal cell 130.
[0051] Referring to FIG. 4A, linearly polarized light 191b, which
is emitted from the second absorptive polarizer 140 and enters the
cell area in the OFF state, is subjected to rotation of the
polarizing direction by 90 degrees in the liquid crystal cell 130
and enters the first absorptive polarizer 120 as linearly polarized
light 191c. Since the polarizing direction of the linearly
polarized light 191c is parallel to the axis of transmission 120T
of the first absorptive polarizer 120, the linearly polarized light
191c is mostly transmitted through the first absorptive polarizer
120 and emitted. The cell area in the OFF state is accordingly
displayed as a bright pixel in the transmissive display mode, in
the same manner as in the reflective display mode.
[0052] Referring to FIG. 4B, on the other hand, linearly polarized
light 192b, which is transmitted through the second absorptive
polarizer 140 and enters the cell area in the ON state, is
transmitted through the liquid crystal polarizer 130 without
rotation of the polarizing direction and enters the first
absorptive polarizer 120 as linearly polarized light 192c. Since
the polarizing direction of the linearly polarized light 192c is
parallel to the axis of absorption 120A of the first absorptive
polarizer 120, the linearly polarized light 192c is mostly absorbed
by the first absorptive polarizer 120 and is not transmitted
through the first absorptive polarizer 120. The cell area in the ON
state is accordingly displayed as a dark pixel in the transmissive
display mode, in the same manner as in the reflective display mode.
As discussed above, the transflective liquid crystal device 100 of
the first embodiment has the same bright/dark states in both of the
reflective display mode and the transmissive display mode.
[0053] FIG. 5 is a graph showing a variation in reflectivity in the
reflective display mode and a variation in transmittance in the
transmissive display mode in the liquid crystal device 100 of the
first embodiment. The data in the graph of FIG. 5 are plotted, with
angle (hereinafter referred to as the combination angle) .theta.ax
between the axis of transmission 140T of the second absorptive
polarizer 140 and the axis of reflection 160R of the reflective
polarizer 160 as abscissa and reflectivity, and transmittance as
ordinate, under the condition that all the pixels in the liquid
crystal cell 130 are in the OFF state, that is, displayed in white.
The transmittance and reflectivity in the graph of FIG. 5 are
results of the measurement using NPF-EG1228DU (manufactured by
NITTO DENKO Co., Ltd.) as the first absorptive polarizer 120 and
the second absorptive polarizer 140 and RDF-C (manufactured by 3M
Corp.) as the reflective polarizer 160. The RDF-C has the functions
of both the diffusing plate 150 and the reflective polarizer 160
shown in FIG. 1. The standard light source C is used for the
measurement of the transmittance and the reflectivity. The
reflectivity here is defined as the ratio of the intensity of
reflected light under the condition of the display in the brightest
reflective display mode (that is, the reflective display) in the
liquid crystal device 100 placed at a predetermined position from
the standard light source C to the intensity of reflected light
from a standard white plate placed at the same position.
[0054] In this liquid crystal device 100, the combination angle
.theta.ax is set equal to about 20 degrees as shown in FIG. 3. The
graph of FIG. 5 gives the reflectivity of about 22.4% and the
transmittance of about 2.1% for this combination angle .theta.ax.
In the conventional liquid crystal display described as the prior
art, the reflectivity can be enhanced to about 29%. Although the
liquid crystal device 100 of the first embodiment has a little
lower reflectivity but attains substantially equivalent brightness.
The liquid crystal device 100 further has the significant
advantage, that is, no reversion of the bright/dark states between
the reflective display mode and the transmissive display mode.
[0055] A transflector (for example, an Al/Ag deposit film) may be
used, in place of the reflective polarizer, to prevent reversion of
the bright/dark states between the reflective display mode and the
transmissive display mode. In this case, however, the reflectivity
is about 15% at most. Compared with this liquid crystal device
including the transflector, the liquid crystal device 100 of the
embodiment attains the sufficiently bright reflective display.
[0056] As described above, the liquid crystal device 100 of the
first embodiment ensures the display without reversion of the
bright/dark states between the reflective display mode and the
transmissive display mode, while maintaining the advantageous
characteristics (that is, the reflectivity) of the reflective
display.
[0057] It is desirable that the reflectivity of the liquid crystal
device 100 is not less than about 15%. For that purpose, the
combination angle .theta.ax should be set in the range of about 0
degree to 35 degrees: the range R.theta.ax shown in FIG. 5. When
the combination angle .theta.ax is equal to 0 degree, the
reflectivity is about 27.5%, which attains the extremely bright
reflective display. The combination angle .theta.ax sufficiently
close to 0 degree may, however, cause uneven polarization of the
reflective polarizer 160 to be observed in the transmissive
display. The combination angle .theta.ax is thus preferably in the
range of about 0 degree to 30 degrees and more preferably in the
range of about 15 degrees to 25 degrees. The uneven polarization of
the reflective polarizer 160 can be relieved by placing a polarizer
having an axis of transmission that is parallel to the axis of
transmission 160T of the reflective polarizer 160 between the
reflective polarizer 160 and the backlight 170.
[0058] The above description regards the example, in which the axis
of transmission 120T of the first absorptive polarizer 120 is set
in the direction inclined 45 degrees counterclockwise against the x
axis, and the axis of transmission 140T of the second absorptive
polarizer 140 is set to be at right angles to the axis of
transmission 120T as shown in FIG. 3. The axis of transmission 140T
may be set parallel to the axis of transmission 120T. In this case,
the liquid crystal cell 130 is displayed as a bright pixel in the
ON state and as a dark pixel in the OFF state. The axis of
transmission 120T is not restricted to the direction inclined
counterclockwise 45 degrees counterclockwise against the x axis,
but is set arbitrarily depending upon the structure of the liquid
crystal cell 130.
[0059] The above description regards the specific arrangement in
which the reflective polarizer 160 has the axis of reflection 160R
and the axis of transmission 160T that are arranged perpendicularly
to each other. This arrangement is, however, not essential, and the
axis of reflection 160R and the axis of transmission 160T may be
not perpendicular to each other.
[0060] The above description refers to the specific arrangement of
the liquid crystal cell in which the polarizing direction of the
light passing through the cell area in the OFF state is rotated by
90 degrees, while the polarizing direction of the light passing
through the cell area in the ON state is not rotated. This
arrangement is, however, not essential. Another available liquid
crystal cell changes the polarizing conditions of light in the ON
state and in the OFF state, like an STN-type liquid crystal cell
that takes advantage of the birefringence. Any liquid crystal cell
can be used as long as the polarizing direction of light passing
through the cell area in the ON state is substantially
perpendicular to the polarizing direction of light passing through
the cell area in the OFF state.
B. Second Embodiment
[0061] FIG. 6 schematically illustrates the structure of another
liquid crystal device 200 in a second embodiment according to the
present invention. The liquid crystal device 200 has a similar
structure to that of the liquid crystal device 100 of the first
embodiment, except that a .lambda./2 phase plate 210 is interposed
between the second absorptive polarizer 140 and the diffusing plate
150 in the liquid crystal device 100 of the first embodiment.
[0062] FIG. 7 shows the relationship between the axis of
transmission 120T of the first absorptive polarizer 120, the axis
of transmission 140T of the second absorptive polarizer 140, an
optical axis 210OA of the .lambda./2 phase plate 210, and the axis
of reflection 160R of the reflective polarizer 160. In the liquid
crystal device 200 of the second embodiment, the axis of
transmission 120T of the first absorptive polarizer 120 is set to
be inclined 45 degrees counterclockwise against the x axis. The
axis of transmission 140T of the second absorptive polarizer 140 is
set to be at right angles to the axis of transmission 120T of the
first absorptive polarizer 120. The optical axis 210OA of the
.lambda./2 phase plate 210 is set to be inclined 45 degrees
clockwise against the axis of transmission 140T. Linearly polarized
light, having the polarizing direction parallel to the axis of
transmission 140T, entering the .lambda./2 phase plate 210 is
accordingly converted into linearly polarized light rotated 90
degrees clockwise , that is, linearly polarized light having an
axis of polarization 210T that is parallel to the axis of
transmission 120T of the first absorptive polarizer 120. The axis
of reflection 160R of the reflective polarizer 160 is set to be
inclined about 20 degrees clockwise against the axis of
polarization 210T.
[0063] Setting the axis of transmission 120T of the first
absorptive polarizer 120, the axis of transmission 140T of the
second absorptive polarizer 140, the optical axis 210OA of the
.lambda./2 phase plate 210, and the axis of reflection 160R of the
reflective polarizer 160 as shown in FIG. 7 enables the liquid
crystal device 200 to ensure the display without reversion of the
bright/dark states between the reflective display mode and the
transmissive display mode, while maintaining the advantageous
characteristics of the reflective display, like the liquid crystal
device 100 of the first embodiment.
[0064] The above description refers to the specific arrangement in
which the .lambda./2 phase plate 210 is interposed between the
second absorptive polarizer 140 and the diffusing plate 150. The
.lambda./2 phase plate 210 may, however, alternatively be
interposed between the diffusing plate 150 and the reflective
polarizer 160. This arrangement exerts almost the same effects. A
.lambda./2 phase plate may be used in place of the .lambda./2 phase
plate.
C. Third Embodiment
[0065] FIG. 8 schematically illustrates the structure of still
another liquid crystal device 300 in a third embodiment according
to the present invention. The liquid crystal device 300 has a
similar structure to that of the liquid crystal device 100 of the
first embodiment, except that a color filter plate 310 is
interposed between the reflective polarizer 160 and the backlight
170 in the liquid crystal device 100 of the first embodiment. The
modification from the first embodiment is instead to attain the
following effect.
[0066] FIG. 9 is a graph showing xy chromaticity coordinates of the
reflected light and transmitted light that are observed in the
first embodiment shown in the graph of FIG. 5, in an XYZ color
specification system. The graph of FIG. 9 shows variations in
chromaticity coordinates of the transmitted light and the reflected
light against the combination angle .theta.ax in the case of bright
display. As clearly understood from the graph, while the color of
the reflected light hardly changes, the color of the transmitted
light is different from the color of the reflected light and
significantly varies with the combination angle .theta.ax. In the
liquid crystal device 300 of the third embodiment, the color filter
plate 310 is placed between the backlight 170 and the reflective
polarizer 160, in order to make the color of the transmitted light
close to the color of the reflected light. This arrangement
effectively reduces a difference in color tone of the display
between the reflective display and the transmissive display.
[0067] Although the liquid crystal device 300 includes the color
filter plate 310, the emission spectra of the light source 171
included in the backlight 170 may be adjusted to reduce the color
tone difference while omitting the color filter plate.
D. Examples of Electronic Apparatus
[0068] The liquid crystal device of the present invention is
favorably applicable for a display device included in a variety of
portable equipment that are used in various environments and
desired to have a little power consumption. FIGS. 10A through 10C
show examples of electronic apparatus to which the liquid crystal
device of the present invention is applied.
[0069] FIG. 10A shows a cellular phone having a display unit 802 in
an upper section on a front face of a main body 801. FIG. 10B shows
a watch having a display unit 804 on the center of a main body 803.
FIG. 10C shows a portable information apparatus having a display
unit 806 in an upper section of a main body 805 and an input unit
807 in a lower section thereof.
[0070] These information equipment are used in a variety of
environments, indoors and outdoors, and are thus desirable to be
driven with batteries over a long time period. It is accordingly
preferable that the display device used for these display units
802, 804, and 806 has a little power consumption. One known example
of the display device having a little power consumption is a
reflection liquid crystal device taking advantage of natural light.
The known reflection liquid crystal device, however, can not be
used practically in dark surroundings. The liquid crystal device of
the present invention can be used in two different modes, that is,
the reflective display mode and the transmissive display mode. The
liquid crystal device of the present invention ensures the display
without reversion of the bright/dark states between the reflective
display mode and the transmissive display mode, while maintaining
the brightness in the reflective display. The liquid crystal device
of the present invention is thus effectively applied for the
electronic apparatuses.
[0071] The above embodiments regard the liquid crystal cells of the
simple matrix configuration. The present invention is, however,
also applicable to liquid crystal cells of an active matrix
configuration. Although not specifically mentioned in the above
embodiments, the present invention is applicable to liquid crystal
cells for both color display and monochromatic display.
[0072] The examples of the electronic apparatus discussed above are
not restrictive but only illustrative. The liquid crystal device of
the present invention is applicable to a variety of other
electronic apparatuses having a display unit.
[0073] It should be clearly understood that the above embodiments
are only illustrative and not restrictive in any sense. The scope
and spirit of the present invention are limited only by the terms
of the appended claims.
* * * * *