U.S. patent application number 11/983869 was filed with the patent office on 2008-03-27 for reflection-type liquid crystal display device.
This patent application is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Tetsuya Hamada, Mari Sugawara, Toshihiro Suzuki, Kunihiro Tashiro, Hidefumi Yoshida.
Application Number | 20080074591 11/983869 |
Document ID | / |
Family ID | 33407437 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080074591 |
Kind Code |
A1 |
Hamada; Tetsuya ; et
al. |
March 27, 2008 |
Reflection-type liquid crystal display device
Abstract
To suppress a decrease in the contrast caused by the reflection
on the interface to the air layer without decreasing the quality of
display. A reflection-type liquid crystal display device includes a
light guide plate having a polarizing element stuck or adhered
thereto on the side facing a reflection-type liquid crystal display
panel, a source of light arranged on an end surface side of the
light guide plate, and the reflection-type liquid crystal display
panel arranged maintaining a predetermined gap relative to the
light guide plate, wherein a light-diffusing function is imparted
to the surface of the reflection-type liquid crystal display panel
on the side facing the light guide plate.
Inventors: |
Hamada; Tetsuya; (Kawasaki,
JP) ; Tashiro; Kunihiro; (Kawasaki, JP) ;
Sugawara; Mari; (Kawasaki, JP) ; Suzuki;
Toshihiro; (Kawasaki, JP) ; Yoshida; Hidefumi;
(Kawasaki, JP) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Assignee: |
Sharp Kabushiki Kaisha
Osaka
JP
|
Family ID: |
33407437 |
Appl. No.: |
11/983869 |
Filed: |
November 13, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10810483 |
Mar 26, 2004 |
|
|
|
11983869 |
Nov 13, 2007 |
|
|
|
Current U.S.
Class: |
349/112 ;
349/119 |
Current CPC
Class: |
G02B 6/0056 20130101;
G02F 1/133615 20130101; G02F 1/13362 20130101; G02F 1/133541
20210101; G02B 6/0051 20130101; G02F 1/13363 20130101; G02F
1/133528 20130101; G02F 1/133504 20130101; G02F 1/133616
20210101 |
Class at
Publication: |
349/112 ;
349/119 |
International
Class: |
G02F 1/13363 20060101
G02F001/13363; G02F 1/13357 20060101 G02F001/13357 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2003 |
JP |
2003-095011 |
Claims
1. A reflection-type liquid crystal display device comprising: at
least a reflection-type liquid crystal display panel, a first
retardation plate, a second retardation plate, a polarizing plate
and a light guide plate laminated in this order; wherein, the first
retardation plate is stuck or adhered to the reflection-type liquid
crystal display panel, the second retardation plate and the
polarizing plate are stuck or adhered to the light guide plate, and
a circular polarizer is constituted by the first retardation plate,
second retardation plate and polarizing plate.
2. A reflection-type liquid crystal display device according to
claim 1, wherein an angle subtended by delay phase axes of the
first retardation plate and of the second retardation plate is not
smaller than 0 degree but is not larger than 30 degrees, and the
sum of in-plane retardations of the first retardation plate and of
the second retardation plates is not smaller than 95 nm but is not
larger than 195 nm, which is one-fourth the region of visible light
wavelengths.
3. A reflection-type liquid crystal display device according to
claim 1, wherein an angle subtended by delay phase axes of the
first retardation plate and of the second retardation plate is not
smaller than 60 degree but is not larger than 90 degrees, and the
difference in the in-plane retardation between the first
retardation plate and the second retardation plate is not smaller
than 95 nm but is not larger than 195 nm, which is one-fourth the
region of visible light wavelengths.
4. A reflection-type liquid crystal display device according to
claim 1, wherein an angle subtended by an absorption axis of the
polarizing plate and by a delay phase axis of the second
retardation plate is .theta., an angle subtended by an absorption
axis of the polarizing plate and by a delay phase axis of the first
retardation plate is about 2.theta.+45 degrees, and the difference
in the in-plane retardation between the first retardation plate and
the second retardation plate is not smaller than 95 nm but is not
larger than 195 nm, which is one-fourth the region of visible light
wave lengths.
5. A reflection-type liquid crystal display device according to
claim 1, wherein a third retardation plate having an in-plane
retardation of not smaller than 190 nm but not larger than 390 nm
which is one-half the region of visible light wavelengths is
disposed between the polarizing plate and the second retardation
plate.
6. A reflection-type liquid crystal display device according to
claim 5, wherein the angle subtended by an absorption axis of the
polarizing plate and by a delay phase axis of the third retardation
plate is .theta., an angle subtended by the absorption axis of the
polarizing plate and by a delay phase axis of the second
retardation plate is roughly 2.theta.+45 degrees, and the
difference in the in-plane retardation between the third
retardation plate and the first and the second retardation plates
is not smaller than 95 nm but is not larger than 195 nm, which is
one-fourth the region of visible light wavelengths.
7. A reflection-type liquid crystal display device according to
claim 5, wherein the angle subtended by an absorption axis of the
polarizing plate and by a delay phase axis of the third retardation
plate is .theta., an angle subtended by a absorption axis of the
polarizing plate and by the delay phase axis of the second
retardation plate is roughly 2.theta.+45 degrees, a delay phase
axis of the second retardation plate and a delay phase axis of the
first retardation plate are nearly at right angles with each other,
and the difference in the in-plane retardation between the second
retardation plate and the first retardation plate is not smaller
than 95 nm but is not larger than 195 nm, which is one-fourth the
region of visible light wavelengths.
8. A reflection-type liquid crystal display device according to
claim 1, wherein a third retardation plate and a fourth retardation
plate having an in-plane retardation of not smaller than 190 nm but
not larger than 390 nm, which is one-half the region of visible
light wavelengths, are disposed between the polarizing plate and
the second retardation plate.
9. A reflection-type liquid crystal display device according to
claim 8, wherein an angle subtended by an absorption axis of the
polarizing plate and by a delay phase axis of the fourth
retardation plate is .theta., an angle subtended by the absorption
axis of the polarizing plate and by a delay phase axis of the third
retardation plate is roughly 2.theta.+45 degrees, the delay phase
axis of the third retardation plate and the delay phase axis of the
second retardation plate are nearly at right angles with each
other, and the difference in the in-plane retardation between the
third retardation plate and the first and the second retardation
plates is not smaller than 95 nm but is not larger than 195 nm,
which is one-fourth the region of visible light wavelengths.
10. A reflection-type liquid crystal display device according to
claim 1, wherein an undrawn film is used as the first retardation
plate.
11. A reflection-type liquid crystal display device according to
claim 1, wherein a reflection preventing film is provided on the
surface of at least the first retardation plate.
12. A reflection-type liquid crystal display device according to
claim 1, wherein a sticking layer provided between the polarizing
plate and the light guide plate has a light-diffusing function.
13. A reflection-type liquid crystal display device according to
claim 1, wherein a sticking layer provided between the first
retardation plate and the reflection-type liquid crystal display
panel has a light-diffusing function.
14. A reflection-type liquid crystal display device according to
claim 1, wherein the surfaces of the first retardation plate and of
the second retardation plate facing each other are smooth
surfaces.
15. A reflection-type liquid crystal display device according to
claim 1, wherein a viewing angle control plate for diffusing the
incident light from a particular direction is disposed between the
light guide plate and the reflection-type liquid crystal display
panel.
Description
[0001] This is a divisional of application Ser. No. 10/810,483,
filed Mar. 26, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a reflection-type liquid crystal
display device and, particularly, to a reflection-type liquid
crystal display device used for low-power-consumption units such as
portable terminals and having a feature in the constitution for
suppressing a decrease in the contrast caused by the reflection on
the interface without decreasing the quality of display.
[0004] 2. Description of the Related Art
[0005] Owing to their features such as small in size, light in
weight and low power consumption, the liquid crystal display
devices have been widely used as data equipment terminals, TVs,
portable data equipment terminals, display monitors such as video
cameras, etc.
[0006] The liquid crystal material does not emit the light by
itself and a source of light is necessary when it is used as a
display device. In the case of a reflection-type liquid crystal
display device used as a low-power-consumption unit such as a
portable terminal, in particular, the indoor illumination serves as
a source of light or a front light unit is used as a source of
light (see, for example, JP-A-2001-108986).
[0007] Here, a conventional reflection-type liquid crystal display
device will be described with reference to FIG. 17.
[0008] FIG. 17 is a sectional view schematically illustrating a
conventional reflection-type liquid crystal display device
comprising a front light unit 90 and a liquid crystal display panel
80 constituted by a liquid crystal layer 82 held between a TFT
substrate 81 and a CF (color filter) substrate 83, which are firmly
held face to face by a frame 85 maintaining a small gap or an air
layer 96.
[0009] The front light unit 90 includes a source 91 of light, a
light guide plate 93 and a reflector 92 for reflecting and
collecting light from the source 91 of light toward the light guide
plate 93. A prism 94 is engraved in the surface of the light guide
plate 93, i.e., on the side of the viewer, and light being guided
is partly reflected toward the liquid crystal display panel 80.
[0010] Usually, further, a reflection prevention film 95 is
provided on the back surface of the front light unit 90 and a
polarizing plate 84 is provided on the front surface side of the
liquid crystal display panel 80.
[0011] The pitch of the prism 94 is so set relative to the pixel
pitch of the liquid crystal display panel 80 that moire fringes
appear little.
[0012] For example, the prism pitch is set to be equal to the pixel
pitch so that the moire pitch becomes an infinity or is so set that
the moire pitch becomes very fine.
[0013] In general, further, the liquid crystal display panel 80 is
so constructed as to elevate the reflection factor in order to
realize bright display. In the use of small devices such as cell
phones and PDAs, in particular, the designing aims at accomplishing
a very high reflection factor since the viewing angle need not be
broad.
[0014] In this reflection-type liquid crystal display device, a
circularly polarizing plate is usually used, the circularly
polarizing plate being made up of a retardation plate and a
polarizing plate. When no retardation is occurred in the liquid
crystal display panel or when light is reflected in front of the
liquid crystal display panel, however, the reflected light is
absorbed by the circularly polarizing plate and does not go
out.
[0015] This is because the circularly polarizing plate converts,
first, the incident light into linearly polarized light through the
polarizing plate and converts, next, the light into circularly
polarized light through the retardation plate. The circularly
polarized light reflected by the interface falls again on the
retardation plate and is converted into linearly polarized light
with its direction of polarization being turned by 90 degrees. The
reflected light which is the linearly polarized light turned by 90
degrees is absorbed by the polarizing plate and does not go
out.
[0016] The structure for arranging the circularly polarizing plate
can be contrived in the following three types as illustrated in
FIG. 18.
[0017] FIG. 18A is a view schematically illustrating the
constitution of when the circularly polarizing plate is stuck to
the side of the liquid crystal display panel, in which the
circularly polarizing plate 100 comprising a polarizing plate 101
and a retardation plate 102 is stuck onto the display surface of
the liquid crystal display panel 80 by using a sticking agent 103,
and an air layer 96 exists relative to the light guide plate
93.
[0018] FIG. 18B is a view schematically illustrating the
constitution of when the circularly polarizing plate is stuck to
the side of the light guide plate, in which the circularly
polarizing plate 100 comprising the polarizing plate 101 and the
retardation plate 102 is stuck onto the back surface of the light
guide plate 93 by using the sticking agent 104, and the air layer
96 exists relative to the liquid crystal display panel 80.
[0019] FIG. 18C is a view schematically illustrating the
constitution of when both surfaces of the circularly polarizing
plate are stuck, in which the circularly polarizing plate 100
comprising the polarizing plate 101 and the retardation plate 102
is stuck onto the back surface of the light guide plate 93 and to
the liquid crystal display panel 80 by using the sticking agents
103 and 104. In this case, there exists no air layer.
[0020] FIG. 19 is a diagram illustrating the reflected light
components on the interfaces to the air layer.
[0021] The air layer 96 in the above constitution has a refractive
index n.sub.2=1, and constituent members such as the liquid crystal
display panel 80, circular polarizing plate 100, light guide plate
93 and sticking agents 103, 104 have refractive indexes n.sub.1 and
n.sub.3 of about 1.4 to about 1.6. Therefore, the greatest
difference in the refractive index exists on the interfaces among
the constituent members and the air layer 96, and the incident
light 105 is greatly refracted by the interfaces.
[0022] The reflection increases on the interface as the refraction
increases. As the refraction exceeds a critical angle, further, a
total reflection takes place and the contrast decreases.
[0023] In the constitution illustrated in, for example, FIG. 18A,
the contrast is only about 5 to about 10. The low contrast narrows
the range of reproducing colors, and the quality of display is very
poor.
[0024] In principle, therefore, the constitution of FIG. 18C is
desired. However, when the light guide plate 93 and the liquid
crystal display panel 80 having different coefficients of thermal
expansion are stuck together, there occurs peeling due to thermal
shock. Or, when the rigid bodies are stuck together, air bubbles
tend to infiltrate. Therefore, the use of this constitution
involves difficulty except the small devices.
[0025] It has therefore been proposed to use the constitution of
FIG. 18B in which the circularly polarizing plate 100 is stuck to
the light guide plate 93, enabling the light 106 reflected by the
interface to the air layer 96 to be absorbed by the polarizing
plate 101 that constitutes the circularly polarizing plate 100 to
thereby enhance the contrast (see, for example,
JP-A-11-259007).
[0026] However, the above constitution of FIG. 18B is accompanied
by a problem of conspicuous interference rainbow due to moire
fringes between the prism 94 of the light guide plate 93 and the
pixels of the liquid crystal display panel 80, and due to
interference between the reflection structure of the liquid crystal
display panel 80 and the pixels.
[0027] The cause is attributed to a decrease in the diffusing light
component stemming from a decrease in the angle of light guided
through the light guide plate 93 as a result of sticking the
circular polarizing plate 100 onto the light guide plate 93.
[0028] When the diffusing light component is strong, on the other
hand, the moire fringes and interference fringes are averaged and
are weakened.
[0029] There still remains a problem of a decrease in the contrast
since light 107 refracted in excess of a critical angle by the
interface to the air layer 96 is totally reflected.
[0030] There further exists a problem in that the circularly
polarizing plate 100 and the liquid crystal display panel 80 are
abraded by the external pressure of input using a pen, and the
circularly polarizing plate 100 is scarred.
SUMMARY OF THE INVENTION
[0031] It is therefore an object of this invention to suppress a
decrease in the contrast caused by the reflection by the interfaces
to the air layer without decreasing the quality of display.
[0032] FIG. 1 is a view of constitution illustrating a principle of
the invention. Means for solving the problem of the invention will
now be described with reference to FIG. 1.
[0033] To solve the above problem, the invention provides a
reflection-type liquid crystal display device comprising:
[0034] a light guide plate 2 having a polarizing element 4 stuck or
adhered thereto on the side facing a reflection-type liquid crystal
display panel 1;
[0035] a source 3 of light arranged on an end surface side of the
light guide plate 2; and
[0036] the reflection-type liquid crystal display panel 1 arranged
maintaining a predetermined gap relative to the light guide plate
2; wherein
[0037] a light-diffusing function is imparted to the surface of the
reflection-type liquid crystal display panel 1 on the side facing
the light guide plate 2.
[0038] Thus, the light-diffusing function is imparted to the
surface of the reflection-type liquid crystal display panel 1 on
the side facing the light guide plate 2. Therefore, light incident
on the reflection-type liquid crystal display panel 1 may generate
interference rainbow due to the interference between the reflection
surface in the liquid crystal display panel 1 and the pixel unit.
However, a diffusion member or a ruggedness in the surface of the
reflection-type liquid crystal display panel 1 diffuses the
interference rainbow. Therefore, the interference rainbow is
weakened before it is seen by the viewer.
[0039] Here, the word "stick" means the sticking by using a
sheet-like sticking layer and the word "adhesion" means the
adhesion by using a gel-like adhering member such as an
adhesive.
[0040] To impart the light-diffusing function, in this case, the
surface of the reflection-type liquid crystal display panel 1 may
be roughened as designated at 5, or the surface of the
reflection-type liquid crystal display panel 1 may be stuck with a
film having a light-diffusing function comprising, for example, a
sticking layer containing a light-diffusing material and a
triacetyl cellulose (TAC) film.
[0041] It is desired that the film having the light-diffusing
function is subjected to the reflection-preventing treatment on the
side of the interface to the air layer.
[0042] Or, a member having a light-diffusing function may be
interposed between the polarizing element 4 and the light guide
plate 2. In this case, the light-diffusing material may be
contained in at least one of a plurality of sticking layers
constituting the polarizing element 4 and, particularly, in a
sticking layer on the side close to the light guide plate 2 and,
most preferably, in the sticking layer that comes in contact with
the light guide plate 2.
[0043] Or, the outer surface of the polarizing element 4 stuck or
adhered to the light guide plate 2 may be roughened.
[0044] In this case, light from the source 3 of light is reflected
and collected by the reflector 6 and is guided to the light guide
plate 2. Here, light reflected by the prism surfaces on the surface
of the light guide plate 2 goes out of the light guiding conditions
and goes out of the light guide plate 2 to form a ray A of light
heading toward the panel surface on the back surface side of the
light guide plate 2. The ray A of light is reflected by the back
surface of the light guide plate 2, and light A' reflected by the
surface passes through the surface of the light guide plate 2 and
enters as a ray C of light to the viewer's eyes. Here, however, the
light reflected by the surface becomes diffused light B since the
diffusing material or ruggedness is formed on the back surface of
the light guide plate 2.
[0045] When the ray A' of light having a pattern of the prism pitch
is converted into a ray B of light, the pattern of the prism pitch
is maintained but the conversion is in a direction in which the
distribution of light is uniformed. Therefore, the ray C of light
contains moire fringes due to the ray B of light and the prism of
the light guide plate 2, and the intensity of moire fringes is
small.
[0046] In this case, too, it is desired that the polarizing element
4 is subjected to the reflection-prevention treatment on the side
of the interface to the air layer.
[0047] The light-diffusing function may be provided on both the
side of the light conductor plate 2 and the side of the
reflection-type liquid crystal display panel 1.
[0048] The invention is further concerned with a reflection-type
liquid crystal display panel 1 wherein at least the reflection-type
liquid crystal display panel 1, a first retardation plate, a second
retardation plate, a polarizing plate and a light guide plate 2 are
laminated in this order, the first retardation plate is stuck or
adhered to the reflection-type liquid crystal display panel 1, the
second retardation plate and the polarizing plate are stuck or
adhered to the light guide plate 2, and a circular polarizer is
constituted by the first retardation plate, second retardation
plate and polarizing plate.
[0049] In this case, the first retardation plate is stuck to the
reflection-type liquid crystal display panel 1 to prevent the
reflection, to prevent the scars and to impart the diffusing
function. Further, the second retardation plate and the polarizing
plate are stuck to the light guide plate 2 to bring the light going
out from the second retardation plate close to the circularly
polarized light, so that the reflection by the interface to the air
layer is absorbed as much as possible by the polarizing plate, or
wavelength dispersion in the in-plane retardation is decreased so
that light reflected by the reflection-type liquid crystal display
panel 1 is efficiently absorbed by the polarizing plate.
[0050] Further, the circular polarizer is constituted by the first
retardation plate, second retardation plate and polarizing plate to
bring the total in-plane retardation of the first retardation plate
and of the second retardation plate to be a desired retardation,
i.e., to be not smaller than 95 nm but not larger than 195 nm,
which is one-fourth the range of visible light wavelengths, to
render the light going out from the first retardation plate to be a
circularly polarized light, so that the light reflected by the
reflection-type liquid crystal display panel 1 is efficiently
absorbed by the polarizing plate.
[0051] The circular polarizer stands for an element that converts
the polarized state of the incident light into nearly a circular
polarization. Even when the polarized state of light going out is
deviated from the circular polarization, the device can be regarded
to be the circular polarizer if the light emitted to the viewer is
turned by about 90 degrees and is linearly polarized in relation to
the reflection-type liquid crystal display panel 1.
[0052] The above constitution makes it possible to decrease the
intensity of reflection of black display and to enhance the
contrast.
[0053] That is, the light going out from the first retardation
plate constituting the interface to the air layer is deviated from
the circularly polarized light. When the reflection by the
interface is compared with the light reflected by the
reflection-type liquid crystal display panel 1, however, the latter
one has a large intensity of reflection, and the contrast can be
enhanced when the light going out from the second retardation plate
is circularly polarized.
[0054] In this case, in constituting a .lamda./4 plate by selecting
the sum of in-plane retardations of the first retardation plate and
of the second retardation plate to be not smaller than 95 nm but
not larger than 195 nm, which is one-fourth the region of visible
light wavelengths, it is desired that the angle subtended by the
delay phase axes of the first retardation plate and of the second
retardation plate is selected to be not smaller than 0 degree but
not larger than 30 degrees.
[0055] Desirably, in this case, the in-plane retardation of the
first retardation plate is decreased as small as possible to bring
the in-plane retardation of the second retardation plate close to
one-fourth the region of the visible light wavelengths.
[0056] If the delay phase axes of the first retardation plate and
of the second retardation plate are nearly in parallel with each
other, the in-plane retardations thereof may be added up to
constitute the circular polarizer. If the retardation remains in
the direction of thickness, the light deviates from the circular
polarization. The deviation, however, can be corrected by selecting
the angle subtended by the delay phase axes to be not smaller than
0 degree but not larger than 30 degrees.
[0057] Relying upon this constitution, the light going out from the
second retardation plate is circularly polarized or the light
emitted to the viewer is turned by about 90 degrees so as to be
linearly polarized in relation to the reflection-type crystal panel
1, to thereby enhance the contrast.
[0058] When the retardation remains in the direction of thickness,
the light is deviated from the linearly deviated light that is
turned by 90 degrees. When the device is so constituted as to
decrease the intensity of reflection, however, the action is the
same and the device can be regarded to be the circular
polarizer.
[0059] Or, in constituting a .lamda./4 plate by selecting the
difference of in-plane retardations between the first retardation
plate and the second retardation plate to be not smaller than 95 nm
but not larger than 195 nm, which is one-fourth the region of
visible light wavelengths, it is desired that the angle subtended
by delay phase axes of the first retardation plate and of the
second retardation plate is selected to be not smaller than 60
degree but not larger than 90 degrees.
[0060] When the delay phase axes of the first retardation plate and
of the second retardation plate are nearly at right angles, the
respective in-plane retardations may be subtracted to constitute
the circular polarizer. When the retardation remains in the
direction of thickness, however, the light deviates from the
circular polarization. The deviation, however, can be corrected by
deviating the delay phase axes by about 30 degrees from the state
of right angles, i.e, by selecting the angle subtended by the delay
phase axes to be not smaller than 60 degrees but not larger than 90
degrees.
[0061] With this constitution, too, the light going out from the
second retardation plate can be circularly polarized or the light
emitted to the viewer can be turned by about 90 degrees so as to be
linearly polarized in relation to the reflection-type liquid
crystal display panel 1 to thereby enhance the contrast.
[0062] In this case, too, when the retardation remains in the
direction of thickness, the light is deviated from the linearly
deviated light that is turned by 90 degrees. When the device is so
constituted as to decrease the intensity of reflection, however,
the action is the same and the device can be regarded to be the
circular polarizer.
[0063] In the above constitutions, it is desired the angle
subtended by the absorption axis of the polarizer plate and by the
delay phase axis of the second retardation plate is .theta. and
that the angle subtended by the absorption axis of the polarizer
plate and by the delay phase axis of the first retardation plate is
about 2.theta.+45 degrees.
[0064] Relying upon the above constitution, a wide-band .lamda./4
plate can be constituted by using the second retardation plate as
the .lamda./2 plate and the first retardation plate as the
.lamda./4 plate.
[0065] That is, by selecting the angle subtended by the absorption
axis of the polarizing plate and by the delay phase axis of the
first retardation plate to be about 2.theta.+45 degrees, the
.lamda./2 plate works to turn the direction of linear polarization
symmetrically to the delay phase axis irrespective of the direction
of the delay phase axis, and the .lamda./4 plate works to
circularly polarize the linearly polarized light that is incident
from a direction of roughly 45 degrees or 135 degrees relative to
the delay axis.
[0066] Owing to this constitution, the wavelength dispersion in the
in-plane retardation is decreased, and the light reflected by the
reflection-type liquid crystal display panel 1 is efficiently
absorbed by the polarizing plate.
[0067] In this constitution, the reflection by the interface to the
air layer cannot be suppressed. When the reflection by the
interface is compared with the light reflected by the
reflection-type liquid crystal display panel 1, however, the latter
one has a large strength of reflection and the contrast can be
enhanced.
[0068] In the above constitution, further, it is desired to arrange
a third retardation plate having an in-plane retardation of not
smaller than 190 nm but not larger than 390 nm which is one-half
the region of visible light wavelengths between the polarizing
plate and the second retardation plate.
[0069] Owing to the above constitution, it is allowed to constitute
a wide-band .lamda./4 plate or a wide-band .lamda./4 plate and an
optical compensation plate by using the first to third retardation
plates decreasing the wavelength dispersion in the in-plane
retardation enabling the light reflected by the reflection-type
liquid crystal display panel 1 to be efficiently absorbed by the
polarizing plate.
[0070] If the in-plate retardation of the first retardation plate
is decreased as small as possible, the light going out from the
second retardation plate can be brought close to the circularly
polarized light, and a large proportion of light reflected by the
interface to the air layer can be absorbed by the polarizing plate
enabling the contrast to be further enhanced.
[0071] Here, the optical compensation is to cancel a positive
retardation that generates in the direction of thickness of the
reflection-type liquid crystal display panel 1 relying upon a
negative retardation that generates in the direction of thickness
of the retardation plate.
[0072] In this case, it is desired that the angle subtended by the
absorption axis of the polarizing plate and by the delay phase axis
of the third retardation plate is .theta., the angle subtended by
the absorption axis of the polarizing plate and by the delay phase
axis of the second retardation plate is roughly 2.theta.+45
degrees, and the difference in the in-plane retardation between the
third retardation plate and the first and the second retardation
plates is not smaller than 95 nm but is not larger than 195 nm,
which is one-fourth the region of visible light wavelengths.
[0073] Owing to this constitution, the .lamda./2 plate is
constituted by the third retardation plate, the .lamda./4 plate is
constituted by the first and the second retardation plates, and the
wide-band .lamda./4 plate is constituted by the first to third
retardation plates.
[0074] Here, desirably, the in-plane retardation of the first
retardation plate is decreased as small as possible to bring the
in-plane retardation of the second retardation plate close to
one-fourth the region of visible light wavelengths.
[0075] Or, it is desired that the angle subtended by the absorption
axis of the polarizing plate and by the delay phase axis of the
third retardation plate is .theta., the angle subtended by the
absorption axis of the polarizing plate and by the delay phase axis
of the second retardation plate is roughly 2.theta.+45 degrees, the
delay phase axis of the second retardation plate and delay phase
axis of the first retardation plate are nearly at right angles with
each other, and the difference in the in-plane retardation between
the second retardation plate and the first retardation plate is not
smaller than 95 nm but is not larger than 195 nm, which is
one-fourth the region of visible light wavelengths.
[0076] Owing to this constitution, the .lamda./2 plate is
constituted by the second and the third retardation plates, the
.lamda./4 plate is constituted by the first retardation plate, and
the wide-band .lamda./4 plate is combined with the optical
compensation plate.
[0077] Namely, the second retardation plate is equivalent to two
pieces of .lamda./4 plates having the same delay phase axis, the
wide-band .lamda./4 plate is constituted by the third retardation
plate and by the one .lamda./4 plate, and the optical compensation
plate is constituted by the first retardation plate and by the
other .lamda./4 plate.
[0078] As a result, the wavelength dispersion in the in-plane
retardation is decreased, the light reflected by the
reflection-type liquid crystal display panel 1 is efficiently
absorbed by the polarizing plate, and the retardation is cancelled
in the direction of thickness of the liquid crystal layer that is
vertically aligned. This will now be described.
[0079] In the vertically aligned mode using the circularly
polarizing plate, the display is black when no voltage is applied
or the applied voltage is smaller than a threshold value.
Therefore, the in-plane retardation becomes roughly zero, and the
contrast can, in principle, be enhanced. In the horizontally
aligned mode using the circularly polarizing plate, on the other
hand, the display is black when a voltage is applied. Therefore,
the in-plane retardation becomes a minimum which, however, is not
zero, and the contrast becomes relatively low.
[0080] This is because in the horizontally aligned mode, a strong
anchoring effect is produced by the alignment film, and the liquid
crystal layer is not raised on the interface of the substrate even
after the voltage is applied.
[0081] Even in the vertically aligned mode, the retardation
generates in the direction of thickness for the incident light that
is tilted. However, the in-plane retardations are cancelled by each
other if there is arranged an optical compensation plate which is
equivalent to the constitution in which two pieces of .lamda./4
plates are so arranged that the delay phase axes thereof are at
right angles. Therefore, the retardation in the direction of
thickness can be used for optically compensating the
reflection-type liquid crystal display panel 1.
[0082] By combining the wide-band .lamda./4 plate and the optical
compensation plate together as described above, the light emitted
to the viewer is turned by about 90 degrees and is linearly
polarized in relation to the reflection-type liquid crystal display
panel 1, which, therefore, can be regarded as a circular polarizer
as a whole.
[0083] This constitution is not capable of suppressing the
reflection by the interface to the air layer. When the reflection
by the interface is compared with the light reflected by the
reflection-type liquid crystal display panel 1, however, the latter
one has a large intensity of reflection, and a high contrast is
maintained.
[0084] In the above constitution, further, it is desired to arrange
the third retardation plate and a fourth retardation plate having
an in-plane retardation of not smaller than 190 nm but not larger
than 390 nm, which is one-half the region of visible light
wavelengths, between the polarizing plate and the second
retardation plate. Then, the wide-band .lamda./4 plate and the
optical compensation plate are constituted by the first to the
fourth retardation plates.
[0085] Owing to this constitution, the wavelength dispersion in the
in-plane retardation is decreased, the light reflected by the
reflection-type liquid crystal display panel 1 is efficiently
absorbed by the polarizing plate, and the retardation is cancelled
in the direction of thickness of the liquid crystal layer that is
vertically aligned.
[0086] If the in-plate retardation of the first retardation plate
is decreased as small as possible, the light going out from the
second retardation plate can be brought close to the circularly
polarized light, and a large proportion of light reflected by the
interface to the air layer can be absorbed by the polarizing plate
to maintain the highest contrast.
[0087] In this case, it is desired that the angle subtended by the
absorption axis of the polarizing plate and by the delay phase axis
of the fourth retardation plate is .theta., the angle subtended by
the absorption axis of the polarizing plate and by the delay phase
axis of the third retardation plate is roughly 2.theta.+45 degrees,
the delay phase axis of the third retardation plate and the delay
phase axis of the second retardation plate are nearly at right
angles, and the difference in the in-plane retardation between the
third retardation plate and the first and the second retardation
plates is not smaller than 95 nm but is not larger than 195 nm,
which is one-fourth the region of visible light wavelengths.
[0088] Owing to this constitution, the .lamda./2 plate is
constituted by the third and fourth retardation plates, the
.lamda./4 plate is constituted by the first and the second
retardation plates, and the wide-band .lamda./4 plate is combined
with the optical compensation plate.
[0089] Here, desirably, the in-plane retardation of the first
retardation plate is decreased as small as possible to bring the
in-plane retardation of the second retardation plate close to
one-fourth the region of visible light wavelengths.
[0090] In the above constitutions, further, it is desired to use an
undrawn film as the first retardation plate.
[0091] That is, to suppress the reflection by the interface to the
air layer, the light going out from the second retardation plate
must be circularly polarized. With the constitution in which the
first retardation plate is disposed between the second retardation
plate and the reflection-type liquid crystal display panel 1,
however, the light is deviated from the circular polarization.
[0092] To increase the contrast by decreasing the deviation, it is
desired to decrease the in-plane retardation of the first
retardation plate as small as possible to bring the in-plane
retardation of the second retardation plate close to one-fourth the
region of the visible light wavelengths. For this purpose, the
undrawn film is used as the first retardation plate so that the
in-plane retardation becomes about several nanometers, and the
in-plane retardation of the second retardation plate is brought
close to 1/4 the region of the visible light wavelengths. Then, the
light going out from the second retardation plate becomes close to
the circularly polarized light, and a large proportion of light
reflected by the interface to the air layer is absorbed by the
polarizing plate to maintain a high contrast.
[0093] In the above constitutions, further, it is desired that a
reflection-preventing film is provided on the surface of at least
the first retardation plate.
[0094] Usually, it is ideal if the reflection-preventing film is
formed on the interfaces of both the first retardation plate and
the circularly polarizing plate. However, the reflection-preventing
film may be formed on the surface of at least the first retardation
plate. This decreases the reflection by the interface down to be
nearly 0 to 1/4, suppressing a decrease in the contrast caused by
the total reflection.
[0095] Here, priority is given to the surface of the first
retardation plate since the surface of the first retardation plate
is the interface which totally reflects the light, first. Namely,
this efficiently suppresses the total reflection.
[0096] In the above constitutions, further, it is desired that the
sticking layer provided between the polarizing plate and the light
guide plate 2 has a light-diffusing function.
[0097] When the display shading such as Newton rings or moire
fringes is produced due chiefly to the light guide late 2, the
light-diffusing function is imparted by the above constitution to
the interface on the side of the light guide plate 2 to relax the
display shading.
[0098] Imparting the light-diffusion function is effective in
relaxing the display shading, which at the same time, however,
causes a decrease in the contrast or causes blurred image.
Therefore, the light-diffusing function should be imparted to only
a minimum degree that is needed.
[0099] In the above constitutions, further, it is desired that the
sticking layer provided between the first retardation plate and the
reflection-type liquid crystal display panel 1 has a
light-diffusing function.
[0100] When the display shading is produced due chiefly to the
refection-type liquid crystal display panel 1, such as interference
fringes with the reflection electrodes of the reflection-type
liquid crystal display panel 1, the light-diffusing function is
imparted by the above constitution to the interface on the side of
the reflection-type liquid crystal display panel 1 to relax the
display shading.
[0101] In this case, too, imparting the light-diffusion function is
effective in relaxing the display shading, which at the same time,
however, causes a decrease in the contrast or causes blurred image.
Therefore, the light-diffusing function should be imparted to only
a minimum degree that is needed.
[0102] In the above constitutions, further, it is desired that the
opposing surfaces of the first retardation plate and of the second
retardation plate are smooth.
[0103] There is a method of imparting the light-diffusing function
by rendering rugged the interface that comes in contact with the
air layer. Upon employing the rugged structure, however, the
circularly polarizing plate and the reflection-type liquid crystal
display panel 1 are abraded by the external pressure of input by
using a pen, and their interfaces are scarred.
[0104] By forming smooth surfaces, however, the interfaces are not
scarred even when an external pressure is exerted due to input by
using a pen.
[0105] In the above constitutions, further, it is desired that a
viewing angle control plate is disposed between the light guide
plate 2 and the reflection-type liquid crystal display panel 1 to
diffuse the light incident from a particular direction.
[0106] The display shading is chiefly caused by a diffraction
phenomenon due to the neighboring light. For example, the
diffraction is seen from a given direction despite the prism shape
of the light guide plate 2 is optimized so that the diffraction
will not been seen from the direction of front surface.
[0107] The diffraction phenomenon occurs in a direction in which
the phases of neighboring light are in match, i.e., in a direction
in which the intervals become integer numbers of times of the
wavelength. Therefore, even if the prism pitch is so designed that
the phase of light is deviated relative to the front surface, the
diffraction phenomenon occurs in a direction in which the light
phases are in match.
[0108] In this case, the diffraction phenomenon can be relaxed by
using the sticking agent having a light-diffusing function
resulting, however, in a trade-off with a decrease in the contrast
and blurred image due to multiple diffusion. Therefore, the
light-diffusing function must be controlled.
[0109] By using the viewing angle control plate that diffuses the
light incident from a particular direction as in this constitution,
however, it is allowed to impart the light-diffusing function in
only a particular direction in which the diffraction phenomenon
appears conspicuously. There is no light-diffusing function in the
direction of the viewer which is the front surface, suppressing a
decrease in the contrast and suppressing the blurring of image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0110] FIG. 1 is a view of constitution illustrating a principle of
the invention;
[0111] FIG. 2 is a sectional view schematically illustrating a
reflection-type liquid crystal display device according to a first
embodiment of the invention;
[0112] FIG. 3 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to a second
embodiment of the invention;
[0113] FIG. 4 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to a third
embodiment of the invention;
[0114] FIG. 5 is a sectional view schematically illustrating a
major portion of the reflection-type liquid crystal display device
according to a fourth embodiment of the invention;
[0115] FIG. 6 is a sectional view schematically illustrating a
major portion of the reflection-type liquid crystal display device
according to a fifth embodiment of the invention;
[0116] FIG. 7 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to a sixth
embodiment of the invention;
[0117] FIGS. 8A and 8B are views illustrating the reflection-type
liquid crystal display device according to a seventh embodiment of
the invention;
[0118] FIGS. 9A and 9B are views illustrating the reflection-type
liquid crystal display device according to an eighth embodiment of
the invention;
[0119] FIGS. 10A and 10B are views illustrating the reflection-type
liquid crystal display device according to a ninth embodiment of
the invention;
[0120] FIGS. 11A and 11B are views illustrating the reflection-type
liquid crystal display device according to a tenth embodiment of
the invention;
[0121] FIGS. 12A and 12B are diagrams illustrating the intensity of
reflection and the contrast at a pole angle of 45 degrees of when
the in-plane retardation of the second retardation plate is set to
be 132 nm in the reflection-type liquid crystal display device
according to the tenth embodiment of the invention;
[0122] FIGS. 13A and 13B are diagrams illustrating the intensity of
reflection and the contrast at a pole angle of 45 degrees of when
the in-plane retardation of the second retardation plate is set to
be 138 nm in the reflection-type liquid crystal display device
according to the tenth embodiment of the invention;
[0123] FIGS. 14A and 14B are diagrams illustrating the intensity of
reflection and the contrast at a pole angle of 45 degrees of when
the in-plane retardation of the second retardation plate is set to
be 143 nm in the reflection-type liquid crystal display device
according to the tenth embodiment of the invention;
[0124] FIG. 15 is a diagram illustrating the Newton rings or moire
interference fringes at various haze values, and the results of
improvement in the interference rainbow;
[0125] FIG. 16 is a diagram illustrating the diffusion
characteristics of a viewing angle control plate;
[0126] FIG. 17 is a sectional view schematically illustrating a
conventional reflection-type liquid crystal display device;
[0127] FIGS. 18A, 18B and 18C are views illustrating arrangement
structures of the circularly polarizing plates; and
[0128] FIG. 19 is a diagram illustrating the light components
reflected by the interfaces to the air layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0129] A reflection-type liquid crystal display device according to
a first embodiment of the invention will now be described with
reference to FIG. 2.
[0130] FIG. 2 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to the
first embodiment of the present invention, which comprises a front
light unit 20, and a liquid crystal display panel 10 constituted by
a liquid crystal layer 12 held between a TFT substrate 11 having an
alignment film provided on a glass substrate via a reflection
electrode and a CF substrate 13 having an alignment film provided
on a glass substrate via a transparent electrode. The front light
unit 20 and the liquid crystal display panel 10 are firmly held by
a frame 15 being opposed to each other maintaining a small gap of
not larger than, for example, 1 mm.
[0131] The front light unit 20 includes a source 21 of light
constituted by a cold cathode tube containing Ar or Ne gas and a
trace amount of Hg, a reflector 22 for reflecting and collecting
the light from the source 21 of light toward a light guide plate
(manufactured by Fujitsu Kasei Co.) 23, and a light guide plate 23.
A polarizing element 30 is stuck to the back surface of the light
guide plate 23 which is on the side of the liquid crystal display
panel by using a sticking agent.
[0132] A prism 24 is engraved in the surface of the light guide
plate 23 which is on the side of the viewer to reflect part of the
light which guided toward the liquid crystal display panel 10.
[0133] In this case, the pitch of the prism 24 is so set relative
to the pixel pitch of the liquid crystal display panel 10 that the
moire fringes are seen little.
[0134] Further, the surface of the liquid crystal display panel 10
is blasted with sand to constitute a rough surface 14 with fine
scars having a depth between the vertex and the valley of
ruggedness of not larger than 100 .mu.m.
[0135] The degree of scars may be of such a level that generate
interference rainbow or moire fringes of a tolerable level. For
example, the haze H (cloudiness value) may be about 50% or smaller.
If the haze is too large, the brightness and contrast decrease, and
the display is blurred, too.
[0136] The haze (cloudiness value) H is an index represented by a
ratio of a diffusion transmission factor T.sub.d [%] and a total
light ray transmission factor T.sub.t [%] that are measured by
using an integrating ball-type light ray transmission
factor-measuring apparatus, and is given by,
H[%]=(T.sub.d/T.sub.t).times.100 and is indicated down to the first
decimal place.
[0137] When the polarizing element 30 is provided on the side of
the light guide plate 23 to enhance the contrast according to the
first embodiment of the invention, the light incident upon the
liquid crystal display panel 10 generates interference rainbow due
to the interference of the reflection surface in the liquid crystal
display panel 10 with the pixel portion. However, the interference
rainbow is diffused by the rough surface 14 formed on the surface
of the liquid crystal display panel 10 and enters in decreased
amounts into the viewer's eyes.
[0138] Next, the reflection-type liquid crystal display device
according to a second embodiment of the present invention will be
described with reference to FIG. 3.
[0139] FIG. 3 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to the
second embodiment of the invention. The basic constitution is the
same as that of the reflection-type liquid crystal display device
of the above first embodiment. In the second embodiment, however,
the surface of the liquid crystal display panel 10 is not roughened
as designated at rough surface 14 but is, instead, provided with a
light-diffusing film 31 which comprises a TAC film 32 and a
sticking layer 33 containing a light-diffusing material.
[0140] The light-diffusing material, in this case, may be, for
example, TiO.sub.x and its amount can be determined to maintain a
balance in the effects for decreasing the blurring amount of image,
moire fringes and interference rainbow, and may be such that
H.ltoreq.50 [%].
[0141] In this case, too, the interference rainbow is diffused by
the light-diffusing film 31 provided on the surface of the liquid
crystal display panel 10, and enters in decreased amounts into the
viewer's eyes.
[0142] Next, the reflection-type liquid crystal display device
according to a third embodiment of the present invention will be
described with reference to FIG. 4.
[0143] FIG. 4 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to the
third embodiment of the invention. The basic constitution is the
same as that of the reflection-type liquid crystal display device
of the above first embodiment. In the third embodiment, however,
the surface of the liquid crystal display panel 10 is not roughened
as designated at rough surface 14 and, besides, the polarizing
element 30 is stuck to the light guide plate 23 by using a sticking
layer 34 containing a light-diffusing material.
[0144] In the third embodiment, the light directed by the prism 24
toward the side of the liquid crystal display panel is diffused by
the sticking layer 34 containing the light-diffusing material
provided on the back surface of the light guide plate 23, and is
converted in a direction in which the distribution of light is
uniformed, and the moire intensity is weakened.
[0145] Next, the reflection-type liquid crystal display device
according to a fourth embodiment of the present invention will be
described with reference to FIG. 5.
[0146] FIG. 5 is a sectional view schematically illustrating a
major portion of the reflection-type liquid crystal display device
according to the fourth embodiment of the invention. The basic
constitution is the same as that of the third embodiment.
Therefore, described below is the constitution of the polarizing
element only which makes a difference.
[0147] The polarizing element 40 of this case is a polarizing plate
constituted by a plurality of pieces of retardation films, i.e.,
constituted by a .lamda./4 plate 41, a sticking layer 42, a
.lamda./2 plate 43, a sticking layer 44, a TAC/PVA/TAC film 45 and
a sticking layer 46 containing a light-diffusing material, which
are stuck to the light guide plate 23 by the sticking layer 46
containing the light-diffusing material.
[0148] In the fourth embodiment, the light-diffusing material is
contained in the sticking layer closest to the light guide plate
23. The light emitted going out from the light guide plate 23
toward the liquid crystal display panel 10 is reflected by the
interface due to a difference in the refractive index between the
light guide plate 23 and the polarizing element 40, and becomes a
cause of moire fringes. By containing the light-diffusing material
in the sticking layer of the side close to the light guide plate
23, however, the light reflected by the interface is diffused at a
place where the reflection takes place first on the interface, and
the moire intensity is effectively weakened.
[0149] Next, the reflection-type liquid crystal display device
according to a fifth embodiment of the present invention will be
described with reference to FIG. 6.
[0150] FIG. 6 is a sectional view schematically illustrating a
major portion of the reflection-type liquid crystal display device
according to the fifth embodiment of the invention. The basic
constitution is the same as that of the third embodiment.
Therefore, described below is the constitution of the polarizing
element only which makes a difference.
[0151] The polarizing element 50 of this case, too, is a polarizing
plate constituted by a plurality of pieces of retardation films,
i.e., constituted by a .lamda./4 plate 51 of which the surface is
roughened (AG-treated: anti-glare-treated), a sticking layer 52, a
.lamda./2 plate 53, a sticking layer 54, a TAC/PVA/TAC film 55 and
a sticking layer 56, which are stuck to the light guide plate 23 by
the sticking layer 56.
[0152] The constitution of the fifth embodiment is suited in a case
where the light reflected by the interface on the side close to the
light guide plate 23 is relatively weak, and the light reflected by
the interface of the polarizing element 50 relative to the air
layer is large.
[0153] Next, the reflection-type liquid crystal display device
according to a sixth embodiment of the present invention will be
described with reference to FIG. 7.
[0154] FIG. 7 is a sectional view schematically illustrating the
reflection-type liquid crystal display device according to the
sixth embodiment of the invention. The basic constitution is the
same as that of the third embodiment. In the sixth embodiment,
however, the light-diffusing film 31 comprising the TAC film 32 and
the sticking layer 33 containing the light-diffusing material is
provided on the surface of the liquid crystal display panel 10,
too, like in the above second embodiment.
[0155] In the sixth embodiment, the moire fringes of the light
guide plate 23 itself are decreased by the diffusing action of the
sticking layer 34 containing the light-diffusing material on the
side of the polarizing element 30 while the interference rainbow is
effectively decreased by the diffusing action of the sticking layer
33 containing the light-diffusing material on the side of the
liquid crystal display panel 10.
[0156] The moire fringes of the light guide plate 23 and of the
liquid crystal display panel 10, too, can be decreased by the
diffusing action of the two.
[0157] In this case, the degree of diffusion or haze (cloudiness
value) of the sticking layer 34 containing the light-diffusing
material on the side of the light guide plate 23 may be smaller
than the degree of diffusion of the third to the fifth embodiments
that use the above single sticking layer 34 containing the
light-diffusing material.
[0158] Next, the reflection-type liquid crystal display device
according to a seventh embodiment of the present invention will be
described with reference to FIGS. 8A and 8B. Described below are,
however, the constitutions of the polarizing element and the
light-diffusing films only.
[0159] FIG. 8A is a view illustrating a modified example of the
polarizing element 40 constituted by a .lamda./4 plate 41, a
sticking layer 42, a .lamda./2 plate 43, a sticking layer 44, a
TAC/PVA/TAC film 45 and a sticking layer 46 containing a
light-diffusing material, and, further, having a
reflection-preventing film 47 provided on the surface of the
.lamda./4 plate 41 on the surface of the polarizing element 40.
[0160] The reflection-preventing film 47 works to suppress the
reflection on the interface to the air layer on the side of the
light guide plate 23, and decreases the moire fringes and improves
the contrast.
[0161] Namely, light emitted to the liquid crystal display panel 10
from the light guide plate 23 is reflected by the surface of the
liquid crystal display panel 10. In the case of the black display,
the reflected light is absorbed by the polarizing element 40 to
produce a black display.
[0162] However, the light that is reflected by the interface of the
polarizing element 40 to the air layer does not travel toward the
liquid crystal display panel 10 but travels toward the viewer. This
light is viewed being added up to the light of the black display
and, hence, black is seen floating. However, provision of the
reflection-preventing film 47 lowers the reflection by the
interface, and a high contrast is maintained.
[0163] FIG. 8B is a view illustrating a modified example of the
light-diffusing film 31 having a reflection-preventing film 35
provided on the surface of the light-diffusing film 31 which is
constituted by the TAC film 32 and the sticking layer 33 containing
the light-diffusing material.
[0164] In this case, too, the reflection-preventing film 35
suppresses the reflection by the interface to the air layer on the
side of the liquid crystal display panel 10, decreasing the moire
fringes and enhancing the contrast.
[0165] Next, described below with reference to FIGS. 9A and 9B is
the reflection-type liquid crystal display device according to an
eighth embodiment of the invention. The basic constitution is the
same as that of the above first embodiment. Therefore, described
below chiefly is the constitution of the polarizing element.
[0166] FIG. 9A is a view schematically illustrating the
constitution of the reflection-type liquid crystal display device
according to the eighth embodiment of the invention. A first
retardation plate 61 is stuck onto the liquid crystal display panel
10 by using a sticking material 62 containing the light-diffusing
material, and a reflection-preventing film 63 is provided on the
first retardation plate 61.
[0167] On the other hand, a polarizing plate 66 to which a second
retardation plate 64 is stuck with the sticking layer 65, is stuck
to the light guide plate 23 through the sticking layer 67
containing the light-diffusing material, and the two are opposed to
each other via an air layer 68.
[0168] In this case, a circular polarizer 69 is constituted by the
first retardation plate 61, second retardation plate 64 and
polarizing plate 66.
[0169] When the first retardation plate 61 has a retardation A, the
retardation of the second retardation plate 64 is set to be
.lamda./4 plate.+-.A, so that they as a whole work as a .lamda./4
plate.
[0170] Or, the first retardation plate 61 may be the .lamda./4
plate and the second retardation plate 64 may be the .lamda./2
plate, so that they as a whole work as a wide-band .lamda./4
plate.
[0171] In the eighth embodiment, the first retardation plate 61 is
stuck to the liquid crystal display panel 10 to prevent the
reflection, to prevent the scars and to impart the diffusing
function. Further, the second retardation plate 64 and the
polarizing plate 66 are stuck to the light guide plate 23 to bring
the light going out from the second retardation plate 64 close to
the circularly polarized light, enabling a large proportion of
light reflected by the interface to the air layer to be absorbed by
the polarizing plate 66 thereby to enhance the contrast.
[0172] Or, the whole plates are rendered to work as a wide-band
.lamda./4 plate to decrease the wavelength dispersion in the
in-plane retardation, enabling the light reflected by the liquid
crystal display panel 10 to be efficiently absorbed by the
polarizing plate 66.
[0173] Next, described below with reference to FIGS. 10A and 10B is
the reflection-type liquid crystal display device according to a
ninth embodiment of the invention. The basic constitution is the
same as that of the above eighth embodiment. In this case, a third
retardation plate is inserted between the second retardation plate
and the polarizing plate.
[0174] FIG. 10A is a view schematically illustrating the
constitution of the reflection-type liquid crystal display device
according to the ninth embodiment of the invention. Like in the
above eighth embodiment, the first retardation plate 61 is stuck
onto the liquid crystal display panel 10 by using the sticking
material 62 containing the light-diffusing material, and the
reflection-preventing film 63 is provided on the first retardation
plate 61.
[0175] On the other hand, a third retardation plate 70 is stuck
between the second retardation plate 64 and the polarizing plate 66
by using the sticking layer 65 and the sticking layer 71, which
are, then, stuck to the light guide plate 23 with the sticking
layer 67 containing the light-diffusing material, and the two are
opposed to each other via the air layer 68.
[0176] In this case, the circular polarizer 69 is constituted by
the first retardation plate 61, the second retardation plate 64,
the third retardation plate 70 and the polarizing plate 66.
[0177] When the first retardation plate 61 has a retardation A, the
retardation of the second retardation plate 64 is set to be
.lamda./4 plate.+-.A, and the third retardation plate 70 is a
.lamda./2 plate so that they as a whole work as a wide-band
.lamda./4 plate.
[0178] Or, the first retardation plate 61 is the .lamda./4 plate,
the second retardation plate 64 is the .lamda./2 plate and the
third retardation plate 70 is the .lamda./2 plate so that they as a
whole work as a wide-band .lamda./4 plate and as an optical
compensation plate.
[0179] In the ninth embodiment, the wavelength dispersion in the
in-plane retardation is decreased so that the light reflected by
the reflection-type liquid crystal display panel 1 is efficiently
absorbed by the polarizing plate.
[0180] Prevention of reflection, prevention of scars, diffusing
function and the action of bringing the light going out from the
second retardation plate 64 close to the circularly polarized
light, are the same as those of the eighth embodiment described
above.
[0181] Next, described below with reference to FIGS. 11A and 11B is
the reflection-type liquid crystal display device according to a
tenth embodiment of the invention. The basic constitution is the
same as that of the above eighth embodiment. In this case, the
third retardation plate and a fourth retardation plate are inserted
between the second retardation plate and the polarizing plate.
[0182] FIG. 11A is a view schematically illustrating the
constitution of the reflection-type liquid crystal display device
according to the tenth embodiment of the invention. Like in the
above eighth embodiment, the first retardation plate 61 is stuck
onto the liquid crystal display panel 10 by using the sticking
material 62 containing the light-diffusing material, and the
reflection-preventing film 63 is provided on the first retardation
plate 61.
[0183] On the other hand, the third retardation plate 70 and the
fourth retardation plate 72 are stuck between the second
retardation plate 64 and the polarizing plate 66 by using the
sticking layer 65, the sticking layer 71 and a sticking layer 73,
which are, then, stuck to the light guide plate 23 with the
sticking layer 67 containing the light-diffusing material, and the
two are opposed to each other via the air layer 68.
[0184] In this case, the circular polarizer 69 is constituted by
the first retardation plate 61, the second retardation plate 64,
the third retardation plate 70, the fourth retardation plate 72 and
the polarizing plate 66.
[0185] The tenth embodiment will now be described in detail
concerning its constitution inclusive of delay phase axes and the
effect.
[0186] In this case, first, the first retardation plate 61 is made
of an undrawn TAC film having an in-plane retardation of several
nanometers, e.g., 5.5 nm, the surface of the TAC film being
subjected to the hard-coat low-reflection (HCLR) treatment to form
a smooth reflection-preventing film 63.
[0187] The second retardation plate 64, on the other hand, has an
in-plane retardation of 132 to 143 nm so as to approach .lamda./4,
and the angle subtended by the delay phase axis of the first
retardation plate 61 and by the delay phase axis of the second
retardation plate 64 is selected to be 0 degree to 180 degrees.
[0188] The in-plane retardations of the third retardation plate 70
and of the fourth retardation plate 72 are 275 nm, respectively.
The angle .theta. subtended by the absorption axis of the
polarizing plate 66 and by the delay phase axis of the fourth
retardation plate 72 is selected to be 10 degrees, the angle
subtended by the delay phase axis of the fourth retardation plate
72 and by the delay phase axis of the third retardation plate 70 is
selected to be 55 degrees, and the angle subtended by the delay
phase axis of the third retardation plate 70 and by the delay phase
axis of the second retardation plate 64 is selected to be 90
degrees.
[0189] Therefore, the angle subtended by the absorption axis of the
polarizing plate 66 and by the delay phase axis of the third
retardation plate 70 is 65 degrees, i.e., 2.theta.+45 degrees.
[0190] FIGS. 12A to 14B illustrate the results of measuring the
reflection intensity and the contrast at a pole angle of 45 degrees
of the reflection-type liquid crystal display devices.
[0191] The reflection intensities shown in the drawings are those
of the black display based on a standard white plate, while the
contrasts represent the ratio of reflection intensities of black
and white displays.
[0192] Here, as for the retardation in the direction of thickness,
.DELTA.R.sub.th=0 nm represents a case where the absolute value of
the retardation .DELTA.R.sub.thLC of the liquid crystal layer is in
agreement with the absolute value of the retardation
.DELTA.R.sub.thF of the retardation plate, .DELTA.R.sub.th=-50 nm
represents a case where the retardation .DELTA.R.sub.thLC of the
liquid crystal layer is smaller by 50 nm than the retardation
.DELTA.R.sub.thF of the retardation plate, and .DELTA.R.sub.th=50
nm represents a case where the retardation .DELTA.R.sub.thLC of the
liquid crystal layer is larger by 50 nm than the retardation
.DELTA.R.sub.thF of the retardation plate.
[0193] Namely, an ideal optical compensation in the vertical
alignment mode is .DELTA.R.sub.th=0 nm. When there is a dispersion
in the thickness of the liquid crystal layer and in the retardation
plate, however, .DELTA.R.sub.th under goes a change. Therefore, an
optimum value is found in a range of .DELTA.R.sub.th=+50 nm
presuming that the sum of dispersion in the film thickness is about
.+-.10% at the greatest.
[0194] Here, the retardation in the direction of thickness is
presumed to be 137.5 nm to 275 nm for the liquid crystal layer and
137.5 nm to 275 nm for the retardation plate.
[0195] The liquid crystal layer is vertically aligned. Therefore,
if the refractive index of the liquid crystal molecules in the
direction of major is denoted by n.sub.e, the refractive index of
the liquid crystal molecules in the direction of minor axis by no
and the light path of the liquid crystal layer or cell gap by
d.sub.LC, then the retardation .DELTA.R.sub.thLC of the liquid
crystal layer is given by,
.DELTA.R.sub.thLC=(n.sub.e-n.sub.o).times.d.sub.LC=.DELTA.n.times.d.sub.L-
C
[0196] When the reflection electrode has a rugged structure,
however, the liquid crystal layer is aligned in a tilted manner,
and the light path d.sub.LC differs depending upon the light that
is incident and the light that is going out. Therefore, the
refractive index n and the light path d.sub.LC must be
corrected.
[0197] On the other hand, if the refractive indexes of the
retardation plate in the plane directions are denoted by n.sub.x
and n.sub.y, the refractive index in the direction of thickness by
n.sub.z and the light path in the retardation plate or film
thickness by d.sub.F, then, the retardation .DELTA.R.sub.thF of the
retardation plate is given by,
.DELTA.R.sub.thF=[(n.sub.x+n.sub.y)/2-n.sub.z].times.d.sub.F
[0198] FIGS. 12A and 12B are diagrams illustrating the results of
measurement of when the in-plane retardation of the second
retardation plate is 132 nm. Here, the visible light wavelength
.lamda. is set to be 550 nm which is close to a peak of visual
sensitivity, and corresponds to a value obtained by subtracting the
in-plane retardation 5.5 nm of the TAC film which is the first
retardation plate from 137.5 nm which is the retardation of
.lamda./4.
[0199] When .DELTA.R.sub.th=0 nm, the angle subtended by the delay
phase axis of the first retardation plate and by the delay phase
axis of the second retardation plate is 0 degree or 180 degrees,
and the reflection intensity of black display is a minimum and the
contrast CR is a maximum. When .DELTA.R.sub.th=.+-.50 nm, the
subtended angle is 30 degrees or 150 degrees, i.e., 0 degrees (180
degrees)+30 degrees, and the reflection intensity of black display
is a minimum and CR is a maximum.
[0200] This means that when there is no deviation in the
retardation in the direction of thickness, the delay phase axes are
nearly in parallel and the incident light is circularly polarized.
When there is a deviation in the retardation in the direction of
thickness, the deviation in the retardation can be corrected by
deviating the delay phase axes within a range of roughly 30 degrees
from a parallel state.
[0201] FIGS. 13A and 13B illustrate the results of measurement of
when the in-plane retardation of the second retardation plate is
138 nm. In this case, too, the visible light wavelength is selected
to be 550 nm which is close to a peak of visual sensitivity, and
corresponds to 137.5 nm of the retardation of .lamda./4 without
taking into consideration the in-plane retardation of the TAC film
which is the first retardation plate.
[0202] When .DELTA.R.sub.th=0 nm, there is no great difference over
the whole angular range. When .DELTA.R.sub.th=.+-.50 nm, on the
other hand, the reflection intensity of black display becomes a
minimum and CR a maximum at a subtended angle of 45 degrees or 135
degrees.
[0203] This means that when there is no deviation in the
retardation in the direction of thickness, the incident light on
the average is deviated from the circularly polarized light in all
directions. When there is a deviation in the retardation in the
direction of thickness, the deviation in the retardation can be
corrected by further deviating the delay phase axes from the
parallel state.
[0204] Here, however, the retardation conditions for the liquid
crystal layer and the retardation plates have been so set that
.DELTA.R.sub.th=0 nm. Therefore, no advantage is obtained when
.DELTA.R.sub.th=0 nm but advantage is obtained only when
.DELTA.R.sub.th is deviated. A maximum CR obtained by this
constitution is 1/10 to 1/20 that of the case of when the in-plane
retardation of the second retardation plate shown in FIG. 12 is 132
nm.
[0205] FIGS. 14A and 14B illustrate the results of measurement of
when the in-plane retardation of the second retardation plate is
143 nm. In this case, too, the visible light wavelength .lamda. is
selected to be 550 nm which is close to a peak if visual
sensitivity and which corresponds to a value obtained by adding the
in-plane retardation 5.5 nm of the TAC film which is the first
retardation plate to 137.5 nm which is the retardation of
.lamda./4.
[0206] When .DELTA.R.sub.th=0 nm, the reflection intensity of black
display is a minimum and the contrast CR is a maximum at an angle
of 90 degrees subtended by the delay phase axis of the first
retardation plate and by the delay phase axis of the second
retardation plate. When .DELTA.R.sub.th=.+-.50 nm, on the other
hand, the reflection intensity of black display is a minimum and CR
is a maximum at a subtended angle of 60 degrees or 120 degrees,
i.e., at 90 degrees.+-.30 degrees.
[0207] This means that when there is no deviation in the
retardation in the direction of thickness, the incident light is
circularly polarized when the delay phase angles are at right
angles. When there is a deviation in the retardation in the
direction of thickness, the deviation in the retardation can be
corrected by further deviating the delay phase axes in a range of
about 30 degrees from the state where the delay phase axes are
intersecting at right angles.
[0208] From the results of FIGS. 12A to 14B, it can be comprehended
that the contrast CR is greatly improved by constituting the
circular polarizer 69 by taking the TAC film that serves as the
first retardation plate into consideration, i.e., by constituting
the circular polarizer 69 by taking into consideration the in-plane
retardations and delay phase axes of the first retardation plate
and of the second retardation plate.
[0209] Next, described below is the reflection-type liquid crystal
display device according to an eleventh embodiment of the
invention.
[0210] The reflection-type liquid crystal display device of the
eleventh embodiment is the one in which the reflection-preventing
film 63 is removed from the surface of the first retardation plate
61 of the tenth embodiment.
[0211] Here, the in-plane retardation of the second retardation
plate 64 is 132 nm, and the angle subtended by the delay phase axes
of the first retardation plate 61 and of the second retardation
plate 64 is 0 degree. Measurement of the reflection intensity at a
pole angle of 45 degrees indicates that the reflection intensity of
black display at .DELTA.R.sub.th=0 nm is about 4 times as great as
that of the tenth embodiment but CR is one-fourth.
[0212] This indicates that the interface to the air layer dominates
the reflection by the interface, and the contrast can be improved
to a considerable degree if the reflection by the interface is
suppressed by using the reflection-preventing film.
[0213] The reflection-type liquid crystal display device of the
eleventh embodiment exhibits inferior characteristics to the liquid
crystal display device of the tenth embodiment but features an
enhanced contrast compared to that of the prior art.
[0214] Next, the reflection-type liquid crystal display device
according to a twelfth embodiment of the invention will be
described with reference to FIG. 15.
[0215] The reflection-type liquid crystal display device according
to the twelfth embodiment is the one in which the sticking layer 62
containing the light-diffusing material on the side of the liquid
crystal display panel 10 and the sticking layer 67 containing the
light-diffusing material on the side of the light guide plate 23
have haze values (cloudiness values) of 20 [%] to 60 [%] of the
reflection-type liquid crystal display device of the tenth
embodiment.
[0216] FIG. 15 is a diagram which illustrates the results of
improvement in the Newton rings or moire interference fringes at
each of the haze values and the interference rainbow due to the
reflection electrode.
[0217] For comparison, there are fabricated a reflection-type
liquid crystal display device without light-diffusing materials in
the respective sticking layers and a reflection-type liquid crystal
display device having fine ruggedness formed on the surface of the
first retardation plate 61 and subjected to the anti-glare (AG)
treatment only.
[0218] As will be obvious from the drawing, use of the sticking
layer 67 containing the light-diffusing material on the side of the
light guide plate 23 decreases the display shading such as Newton
rings and moire fringes caused chiefly by the light guide plate. On
the other hand, use of the sticking layer 61 containing the
light-diffusing material on the side of the liquid crystal display
panel 10 decreases the display shade caused chiefly by the
reflection-type liquid crystal display panel, such as interference
rainbow due to the reflection electrode.
[0219] Here, the light-diffusing function increases with an
increase in the haze values enabling the display shading to be
further decreased. If the haze value is increased to be not smaller
than 60 [%], however, the contrast decreases and the image is
further blurred.
[0220] It is therefore desired that the sticking layer containing
the light-diffusing material having such a diffusion degree that
the haze value is about 40 [%], is used for both or for at least
either one of the sticking layer 62 containing the light-diffusing
material on the side of the liquid crystal display panel 10 and the
sticking layer 67 containing the light-diffusing material on the
side of the light guide plate 23.
[0221] The display shading is observed in the reflection-type
liquid crystal display devices for comparison without the
light-diffusing material in the sticking layers.
[0222] Decreased interference fringes are exhibited by the
reflection-type liquid crystal display devices which have fine
ruggedness formed on the surface of the first retardation plate 61
and which are subjected to the anti-glare (AG) treatment. However,
scars occur due to the abrasion of the first retardation plate 61
having ruggedness in the surface and the abrasion of the second
retardation plate having a flat surface.
[0223] Described below next with reference to FIG. 16 is the
reflection-type liquid crystal display device according to a
thirteenth embodiment of the invention.
[0224] The reflection-type liquid crystal display device of the
thirteenth embodiment is the same as that of the above tenth
embodiment except that a viewing angle control plate 74 such as
LUMISTY (trade name, manufactured by Sumitomo Kagaku Co.) for
diffusing an incident beam from a particular direction is arranged
between the light guide plate 23 and the polarizing plate 66.
[0225] FIG. 16 is a diagram illustrating the diffusion
characteristics of the viewing angle control plate 74. The viewing
angle control plate 74 is so designed as to permit the transmission
of light incident within a range of .+-.25 degrees from the
direction perpendicular to the substrate and to diffuse the light
incident in a range of .+-.25 to 55 degrees.
[0226] Though the angular range of diffusion can be arbitrarily
set, the angle is set within the above range since the occurrence
of display shading is conspicuous in the above angular range.
[0227] This is because, the display shading is caused by the
diffraction phenomenon and the diffraction is seen from a certain
direction even if the prism shape is optimized so that the
diffraction will not be seen from the direction of front
surface.
[0228] As a result of observing how the display shading can be
seen, it is learned that, though the display is slightly shaded,
the contrast is high and the image is not blurred within a range of
.+-.25 degrees from the direction perpendicular to the substrate
which is the direction from the viewer. In the range of .+-.25 to
55 degrees, further, the display shading is not almost seen
decreasing the offensive feeling caused by the display shading in
the visible range.
[0229] Though various embodiments of the invention was described
above, it should be noted that the invention is in no way limited
to the constitutions of the above embodiments only but can be
modified in a variety of ways.
[0230] In the first embodiment, for example, the surface of the
liquid crystal display panel is scarred by blasting the sand.
However, the scars can be formed not only by the sand blasting.
[0231] In the above embodiments, further, the polarizing plate and
the light guide plate are stuck together, and the retardation plate
and the liquid crystal display panel are stuck together, by using
sticking layers. However, the polarizing plate and the light guide
plate, and the retardation plate and the liquid crystal display
panel, may be stuck together not only by using the sticking layers
but also by using an adhesive such as a UV-curable adhesive.
[0232] Further, though not described in detail, the constitutions
of the eighth and the ninth embodiments comply with that of the
above tenth embodiment, and the constitutions described in the
Section of "Summary of the Invention" can be employed.
[0233] According to the invention, as described above, since moire
fringes and the interference rainbow can be decreased, it is
possible to improve the quality of display in the reflection-type
liquid crystal display device using the front light unit.
Therefore, the invention contributes largely toward particularly
realizing the reflection-type liquid crystal display device with
high quality of display for middle-sized and large-sized
display.
* * * * *