U.S. patent application number 11/025076 was filed with the patent office on 2005-10-27 for reflective liquid crystal light valve.
Invention is credited to Wu, Shin-Tson, Zhu, Xinyu.
Application Number | 20050237446 11/025076 |
Document ID | / |
Family ID | 35136010 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050237446 |
Kind Code |
A1 |
Zhu, Xinyu ; et al. |
October 27, 2005 |
Reflective liquid crystal light valve
Abstract
Reflective liquid crystal light valves are disclosed. A liquid
crystal cell is also disclosed comprising a transparent electrode,
a reflective electrode, and a twisted nematic liquid crystal layer
interposed therebetween. A first alignment layer with a first
alignment direction disposed on the transparent electrode. A second
alignment layer with a second alignment direction disposed on the
reflective electrode, wherein a first included angle .phi. is
between the first and second alignment directions. A polarizing
device is disposed on the exterior of the transparent electrode to
provide an incident beam having a polarization direction, wherein a
second included angle .beta. is between the first alignment
direction and the polarization direction. A relationship between
the first included angle .phi. and the second included angle .beta.
satisfies .phi./2<.beta.<.phi./2+30.degree. or
90.degree.+.phi./2<.beta.<.phi./2+120.degree..
Inventors: |
Zhu, Xinyu; (Orlando,
FL) ; Wu, Shin-Tson; (Oviedo, FL) |
Correspondence
Address: |
Min, Hsieh & Hack LLP
c/o PortfolioIP
P.O. Box 52050
Minneapolis
MN
55402
US
|
Family ID: |
35136010 |
Appl. No.: |
11/025076 |
Filed: |
December 30, 2004 |
Current U.S.
Class: |
349/99 |
Current CPC
Class: |
G02F 1/1398 20210101;
G02F 2203/02 20130101; G02F 1/133531 20210101; G02F 1/1396
20130101 |
Class at
Publication: |
349/099 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2004 |
TW |
93111701 |
Claims
What is claimed is:
1. A reflective light valve, comprising: a transparent substrate
disposed opposite a reflective substrate with a twisted nematic
type liquid crystal material interposed therebetween; a first
alignment layer with a first alignment direction disposed on the
transparent substrate; a second alignment layer with a second
alignment direction disposed on the reflective substrate, wherein a
first included angle .phi. is between the first and second
alignment directions; and a polarizing device disposed on an
exterior of the transparent substrate to provide an incident beam
having a polarization direction, wherein a second included angle
.beta. is between the first alignment direction and the
polarization direction; wherein a relationship between the first
included angle .phi. and the second included angle .beta. satisfies
.phi./2<.beta.<.phi./2+30.de- gree. or
90.degree.+.phi./2<.beta.<.phi./2+120.degree..
2. The reflective light valve according to claim 1, wherein the
second included angle .beta. is .phi./2+1.degree. to about
3.degree..
3. The reflective light valve according to claim 2, wherein the
second included angle .beta. is .phi./2+1.5.degree..
4. The reflective light valve according to claim 1, wherein the
first included angle .phi. is between 40.degree. and
70.degree..
5. The reflective light valve according to claim 1, wherein the
transparent substrate is a glass substrate comprising a transparent
electrode formed thereon.
6. The reflective light valve according to claim 5, wherein the
transparent electrode is an indium tin oxide (ITO) or indium zinc
oxide (IZO) layer.
7. The reflective light valve according to claim 1, wherein the
reflective substrate is a silicon substrate comprising a metal
electrode formed thereon.
8. The reflective light valve according to claim 7, wherein the
metal electrode is an aluminum layer.
9. The reflective light valve according to claim 1, wherein the
twisted nematic type liquid crystal material comprises positive
dielectric anisotropic liquid crystal molecules.
10. A reflective light valve, comprising: a liquid crystal cell
comprising a transparent electrode, a reflective electrode and a
twisted nematic liquid crystal layer interposed therebetween,
wherein a retardation value (d.DELTA.n) of the twisted nematic
liquid crystal layer is about 350 nm; a first alignment layer with
a first alignment direction disposed on the transparent electrode;
a second alignment layer with a second alignment direction disposed
on the reflective electrode, wherein a first included angle .phi.
is between the first and second alignment directions; and a
polarizing device disposed on an exterior of the transparent
electrode to provide an incident beam having a polarization
direction, wherein a second included angle .beta. is between the
first alignment direction and the polarization direction; wherein a
relationship between the first included angle .phi. and the second
included angle .beta. satisfies
.phi./2<.beta.<.phi./2+30.degree. or
90.degree.+.phi./2<.beta.&l- t;.phi./2+120.degree..
11. The reflective light valve according to claim 10, wherein the
second included angle .beta. is .phi./2+1.degree. to about
3.degree..
12. The reflective light valve according to claim 11, wherein the
second included angle .beta. is .phi./2+1.5.degree..
13. The reflective light valve according to claim 10, wherein the
first included angle .phi. is between 40.degree. and
70.degree..
14. The reflective light valve according to claim 10, wherein the
transparent electrode is an ITO or IZO layer and the reflective
electrode is an aluminum layer.
15. The reflective light valve according to claim 10, wherein the
twisted nematic type liquid crystal layer comprises positive
dielectric anisotropic liquid crystal molecules.
16. A reflective light valve, comprising: a liquid crystal cell
comprising a transparent electrode on a transparent substrate, a
reflective electrode on a semiconductor substrate and a twisted
nematic liquid crystal layer interposed therebetween; a first
alignment layer with a first alignment direction disposed on the
transparent electrode; a second alignment layer with a second
alignment direction disposed on the reflective electrode, wherein a
first included angle .phi.is between the first and second alignment
directions; and a polarizing beam splitter disposed on an exterior
of the transparent substrate to provide an incident beam having a
polarization direction, wherein a second included angle .beta. is
between the first alignment direction and the polarization
direction; wherein a relationship between the first included angle
.phi. and the second included angle .beta. satisfies
.phi./2<.beta.<.phi./2+1.degree..about.3.degree. or
90.degree.+.phi./2<.beta.<.phi./2+91.degree..about.93.degree..
17. The reflective light valve according to claim 16, wherein the
second included angle .beta. is .phi./2+1.5.degree..
18. The reflective light valve according to claim 16, wherein the
first included angle .phi. is between 40.degree. and
70.degree..
19. The reflective light valve according to claim 16, wherein the
twisted nematic type liquid crystal layer comprises positive
dielectric anisotropic liquid crystal molecules.
20. An electronic device, comprising: a reflective light valve of
claim 16; a controller coupled to the reflective light valve; and
an input device coupled to the controller to input data to the
controller to render an image.
Description
FIELD OF THE INVENTION
[0001] The invention relates to projection displays, and more
particularly, to a reflective liquid crystal light valve for
same.
BACKGROUND OF THE INVENTION
[0002] A reflective liquid crystal light valve is an important
element in a projection display. Reflective liquid crystal light
valves typically comprise a polarizing beam splitter (PBS) and a
reflective liquid crystal cell. The size of each pixel of a high
resolution projection display is approximately equal to a cell gap
of the reflective liquid crystal cell. As such, the fringe field
between adjacent pixels can interfere with and reorient the liquid
crystal orientation and then degrade image contrast and reduce
display brightness. Therefore, to decrease the fringe field effect,
a low driving voltage is used to achieve high resolution, high
contrast ratio, and high brightness in the projection display.
[0003] U.S. Pat. No. 5,490,003 to Sprang, the entirety of which is
hereby incorporated by reference, discloses a reflective liquid
crystal display. The reflective liquid crystal display comprises a
layer of positive dielectric anisotropic liquid crystal molecules
with a twist angle and a polarizer having a polarization direction
at the bisector of the twist angle.
[0004] U.S. Pat. No. 5,936,697 to Yang, the entirety of which is
hereby incorporated by reference, discloses a self-compensated
twisted nematic (SCTN) mode reflective light valve. The reflective
light valve comprises a SCTN mode reflective liquid crystal cell
with negative dielectric anisotropic liquid crystal (LC) molecules,
and a polarizer having a polarization direction at the bisector of
the twist angle.
[0005] The conventional reflective light valve utilizing the
bisector of the twist angle, however, does not take boundary layer
residual phase retardation into consideration. Thus, in practice,
the bisector is not in the proper polarization direction for
achieving low operating voltage and high contrast ratio.
SUMMARY
[0006] According to various embodiments reflective liquid crystal
light valves with a predetermined polarization direction are
provided. An exemplary embodiment of a reflective liquid crystal
light valve comprises a liquid crystal cell comprising a
transparent electrode disposed opposite a reflective electrode with
a twisted nematic (TN) mode liquid crystal layer interposed
therebetween. The light valve can also include a first alignment
layer with a first alignment direction disposed on the transparent
electrode. A second alignment layer with a second alignment
direction is disposed on the reflective electrode, wherein a first
included angle .phi. is between the first and second alignment
directions. A polarizer is disposed on the exterior of the
transparent electrode to provide an incident beam having a
polarization direction, wherein a second included angle .beta. is
between the first alignment direction and the polarization
direction. A relationship between the first included angle .phi.
and the second included angle .beta. can satisfy
.phi./2<.beta.<.phi./2+30.degree. or
90.degree.+.phi./2<.beta.<.phi./2+120.degree..
[0007] According to various embodiments the optimal polarization
direction of the incident beam that provides improved results is
not the bisector direction between the first and second alignment
directions. The relationship between the first included angle .phi.
and the second included angle .beta. can satisfy
.phi./2<.beta.<.phi./2+30.degree. or
90.degree.+.phi./2<.beta.<.phi./2+120.degree.. The reflective
liquid crystal light valve can thus potentially achieve lower
driving voltage and higher contrast ratio, improving display
quality.
DESCRIPTION OF THE DRAWINGS
[0008] Reflective liquid crystal light valves can be more fully
understood by reading the subsequent detailed description in
conjunction with the examples and references made to the
accompanying drawings, wherein:
[0009] FIG. 1A depicts an operating principle using a reflective
liquid crystal cell for an embodiment of a reflective light valve,
which includes an incident beam being linear polarized;
[0010] FIG. 1B depicts a schematically sectional view of the
reflective liquid crystal cell shown in FIG. 1A according to
various embodiments of the invention;
[0011] FIG. 2 schematically depicts a relationship between the
polarization direction of the incident beam and the alignment
directions according to an embodiment of a light valve;
[0012] FIG. 3A depicts a graphical plot of the relationship between
the azimuthal angle of eigenmode 1 and the residual retardation for
an embodiment of a left-handedness 60.degree.-TN liquid crystal
cell at uniform-twist and two-layer models according to various
embodiments of the invention;
[0013] FIG. 3B depicts a graphical plot of the relationship between
the azimuthal angle of eigenmode 2 and the residual retardation for
an embodiment of a left-handedness 60.degree.-TN liquid crystal
cell at uniform-twist and two-layer models according to various
embodiments of the invention;
[0014] FIG. 4A depicts a local enlarged view of FIG. 3A;
[0015] FIG. 4B depicts a local enlarged view of FIG. 3B;
[0016] FIG. 5 depicts a graphical plot of the relationship between
the residual retardation and the applied voltage for an embodiment
of a 60.degree.-TN liquid crystal cell with a retardation value
(d.DELTA.n) of 350 nm according to various embodiments of the
invention;
[0017] FIG. 6A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a 60.degree.-TN liquid crystal cell according to a
first test;
[0018] FIG. 6B depicts a local enlarged view of FIG. 6A in the dark
state;
[0019] FIG. 7A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a 57.degree.-TN liquid crystal cell according to a
second test;
[0020] FIG. 7B depicts a local enlarged view of FIG. 7A in the dark
state;
[0021] FIG. 8A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a 55.degree.-TN liquid crystal cell according to a
third test;
[0022] FIG. 8B depicts a local enlarged view of FIG. 8A in the dark
state;
[0023] FIG. 9A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a 50.degree.-TN liquid crystal cell according to a
fourth test;
[0024] FIG. 9B depicts a local enlarged view of FIG. 9A in the dark
state;
[0025] FIG. 10A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a 45.degree.-TN liquid crystal cell according to a
fifth test;
[0026] FIG. 10B depicts a local enlarged view of FIG. 10A in the
dark state;
[0027] FIG. 11A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a 40.degree.-TN liquid crystal cell according to a
sixth test;
[0028] FIG. 11B depicts a local enlarged view of FIG. 11A in the
dark state;
[0029] FIG. 12A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a 65.degree.-TN liquid crystal cell according to a
seventh test;
[0030] FIG. 12B depicts a local enlarged view of FIG. 12A in the
dark state;
[0031] FIG. 13A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a 70.degree.-TN liquid crystal cell according to an
eighth test;
[0032] FIG. 13B depicts a local enlarged view of FIG. 13A in the
dark state;
[0033] FIG. 14 depicts a schematic diagram illustrating an
embodiment of a projection display apparatus, incorporating a
controller according to various embodiments of the invention;
and
[0034] FIG. 15 depicts a schematic diagram illustrating an
electronic device incorporating an embodiment of a projection
display apparatus according to various embodiments of the
invention.
DETAILED DESCRIPTION
[0035] Reflective liquid crystal light valves according to various
embodiments are provided. An exemplary embodiment of a reflective
light valve 90, shown in FIG. 1A, comprises a reflective liquid
crystal display (e.g. a reflective TN type liquid crystal cell 100)
and a polarizing device (e.g. a polarizing beam splitter 7). The
reflective light valve 90 is well suited for the projection
display. A representative projection display is illustrated, but is
not intended to limit the disclosure.
[0036] The operating principles according to various embodiments of
the reflective liquid crystal light valve 90 are illustrated in
FIG. 1A. A non-polarized incident light beam 6 from a light source
becomes a linearly-polarized light 8 after passing through a
polarizing device 7, such as a beam splitter (PBS), and being
reflected 90.degree. thereby, defining polarized light 8 as p-wave
8. It is to be understood that other polarizing devices known in
the art may also be used. The linearly-polarized light 8 then
impinges on a reflective TN type liquid crystal cell 100. As shown
in FIG. 1A, the TN type liquid crystal cell 100 comprises a
transparent front panel 1 disposed opposite a reflective rear panel
2 with a TN type liquid crystal material 5 interposed
therebetween.
[0037] FIG. 1B schematically depicts a sectional view of a TN type
liquid crystal cell, such as that labeled 100 in FIG. 1A. The front
panel 1 comprises a transparent substrate 11, a transparent
electrode 12, and a first alignment layer 13 with a first alignment
direction 3 (shown in FIG. 2). The transparent substrate 11 can be
glass. The transparent electrode 12, such as indium tin oxide (ITO)
or indium zinc oxide (IZO), is formed on the interior of the
transparent substrate 11. The first alignment layer 13 can be
formed on the transparent electrode 12. The rear panel 2 comprises
a substrate 21, a reflective electrode 22, and a second alignment
layer 23 with a second alignment direction 4 (also shown in FIG.
2). The substrate 21 can be a silicon wafer or any other suitable
semiconductor material. The reflective electrode 22, such as, for
example, aluminum or silver, is formed on the substrate 21. The
second alignment layer 23 is formed on the reflective electrode 22.
The liquid crystal material 5 is disposed between the first and
second alignment layers 13 and 23, respectively. The liquid crystal
material 5 can comprise positive dielectric anisotropic
(.DELTA..epsilon.>0) liquid crystal molecules. The liquid
crystal molecules, near the alignment layers 13 and 23, are
arranged along the alignment directions 3 and 4 shown in FIG.
2.
[0038] According to various embodiments, as depicted, for example
in FIG. 1A, the TN type liquid crystal cell 100 is designed such
that at or below a certain predetermined voltage defined as a
threshold voltage, applied to the two electrodes 12 and 22, the
incident polarized light 8 can become an s-wave 9 (or nearly
s-wave) upon reflection from the liquid crystal cell 100. The
s-wave 9 is a linearly polarized light with a direction of
polarization perpendicular to that of the p-wave 8. The s-wave 9 is
capable of passing directly through the PBS 7 to serve as a
projection beam 10. The projection beam 10 is then corrected by
projection lenses (not shown) for projection onto a screen (not
shown) for viewing. This situation represents the bright state of
the projection display.
[0039] When an external voltage is applied across the two
electrodes 12 and 22 of the liquid crystal cell 100 at or above a
certain voltage, defined as the saturation voltage, the liquid
crystal cell 100 behaves as an optically isotropic medium. In this
case, the impinging linearly polarized light 8 will be reflected
from the reflective liquid crystal cell 100, preserving the same
direction of polarization (a p-wave in this case). The reflected
p-wave cannot directly pass through the PBS 7 and will propagate
backward opposite the incident beam 6. That is, the reflected
p-wave does not project onto a screen (not shown) for viewing. This
situation represents the dark state of the projection display. In
order to get a high contrast ratio, a perfect dark state is
desired. As such, the polarization state of the incident polarized
beam 8 should be an eigenmode for the reflective liquid crystal
cell 100 in order to obtain the desired contrast.
[0040] For better understanding, two different models (i.e. a
uniform-twist model and a two-layer model) are provided to
illustrate the eigenmode of the reflective TN type liquid crystal
cell 100. According to various embodiments, positive dielectric
anisotropic (.DELTA..epsilon.>0) liquid crystal molecules are
utilized in the liquid crystal cell 100, and the pre-tilt angle at
the substrate boundary is small (3.about.5.degree.). The liquid
crystal molecules undergo a uniform twist throughout the liquid
crystal cell 100 when the applied voltage is below a threshold
voltage. When the applied voltage is around two times higher than
the threshold voltage, the liquid crystal molecules in the middle
of the liquid crystal cell 100 are aligned almost parallel to the
electric field between the panels 1 and 2. However, the boundary
layers of molecules near the front and rear substrate interfaces
can be poorly distributed due to strong surface anchoring.
Therefore, the TN type liquid crystal can be defined as a
uniform-twist model when the applied voltage is below the threshold
voltage and as a two-layer model when the applied voltage is about
two times higher than the threshold voltage.
[0041] In the uniform-twist model, there are two eigenmodes for the
TN type liquid crystal cell. Both eigenmodes are linearly polarized
and orthogonal. In the mentioned eigenmodes, the azimuthal angles
of linear polarization are determined by ".theta." in the following
equation (1): 1 tan = - cos X 1 - ( sin X 2 X ) 2 sin X X ( 1 )
[0042] In the above equation, .GAMMA.=2.pi.d.DELTA.n/.lambda. and
X={square root}{square root over (.phi..sup.2+(.GAMMA./2).sup.2)},
wherein .GAMMA. is the phase of uniformly twisted TN type liquid
crystal molecules, d is the cell gap between two substrates 1 and
2, .DELTA.n is the birefringence of the liquid crystal material,
.lambda. is the wavelength of the incident beam, and .phi. is the
twist angle of the liquid crystal molecules (i.e. the included
angle between the first and second alignment directions 3 and 4).
Here, the left-handedness twist angle (for example,
counterclockwise direction) is defined to be positive and the
right-handedness (for example, clockwise direction) twist angle as
negative. Referring to FIG. 2, the positive included angle .phi. is
between the first and second alignment directions 3 and 4. Numeral
25 denotes the bisector of the included angle .phi..
[0043] In the two-layer model, each boundary layer is referred to
as a non-twisted uniaxial layer with residual phase
.psi.=2.pi..alpha./.lambda- ., wherein .alpha. is the retardation
of each boundary layer. Retardation a decreases as the applied
voltage increases. Similarly, there are two eigenmodes for the
reflective TN type liquid crystal cell using the two-layer model.
Both of the mentioned eigenmodes are linearly polarized and
orthogonal. In the mentioned eigenmodes, the azimuthal angles of
linear polarization are determined by ".theta." in the following
equation (2): 2 tan = - cos cos X cos 2 cos 2 + sin 2 sin ( 2 )
[0044] When an intermediate voltage (the applied voltage between
the threshold voltage and two times thereof) is applied, no
approximation is made because of more complicated cases.
Nevertheless, the azimuthal angles of the eigenmodes should be
between the uniform-twist and two-layer models.
[0045] FIG. 3A is a graphical plot of the relationship between the
azimuthal angle of eigenmode 1 and the residual retardation for an
embodiment of a left-handedness 60.degree.-TN (i.e. twist angle
.phi. is 60.degree.) liquid crystal cell at uniform-twist and
two-layer models according to various embodiments. FIG. 4A is a
local enlarged view of FIG. 3A. FIG. 3B is a graphical plot of the
relationship between the azimuthal angle of eigenmode 2 and the
residual retardation for the left-handedness 60.degree.-TN liquid
crystal cell in the uniform-twist and two-layer models according to
various embodiments. FIG. 4B is a local enlarged view of FIG. 3B.
Referring to FIGS. 3A, 3B, 4A and 4B, the azimuthal angles of
eigenmodes 1 and 2 gradually reach the bisector 25 (i.e.
.phi./2=30.degree.) of the twist angle or perpendicular to the
bisector 25 (i.e. 90.degree.+.phi./2=120.degree.). This is the
reason that the cited references (U.S. Pat. Nos. 5,490,003 and
5,936,697) employ the bisector effect to achieve a dark state in
simulation.
[0046] The bisector used in the conventional technology, however,
does not achieve low operating voltages and/or high contrast ratios
because of the poor polarization direction for achieving low
operating voltage and high contrast ratio in practice. According to
the conventional technology, even when the applied voltage reaches
three times the threshold voltage, the residual retardation is
still much greater than 0. As a result, the azimuthal angles of the
two eigenmodes are not exactly parallel to the bisector or
perpendicular to the bisector. One reason for the poor result is
that the conventional technology does not take boundary layer
residual phase retardation into consideration.
[0047] Various tests were preformed and the parameters of the
liquid crystal molecules used in the tests of the specification are
listed in Table 1.
1 TABLE 1 Parameter of LC molecules Value Refractive index n.sub.e
1.65 Refractive index n.sub.o 1.55 Ferroelectric index
.epsilon..sub.p 12.0 Ferroelectric index .epsilon..sub.v 4.0
Coefficient of elasticity k.sub.11 11.5E-12N Coefficient of
elasticity k.sub.22 6.5E-12N Coefficient of elasticity k.sub.33
16.0E-12N Pre-tilt angle 3.degree.
[0048] In one test, the results of which are shown in FIG. 5, the
residual retardation of an embodiment of a 60.degree.-TN liquid
crystal cell with a retardation value (d.DELTA.n) of 350 nm is
plotted verses different applied voltages. Referring to FIG. 5,
when the applied voltage is even at 5V.sub.rms, the residual
retardation is still about 50 nm. Referring to FIGS. 4A and 4B, the
azimuthal angles of the eigenmodes 1 and 2 are about 0.5.degree.
larger than the bisector when the residual retardation is 50 nm.
That is, the azimuthal angles of the eigenmodes 1 and 2 are about
30.5.degree. and 120.5.degree., respectively.
[0049] In projection displays, it is desirable to decrease the
driving voltage in order to minimize the fringe field effect.
Because the azimuthal angles of the two eigenmodes deviate from the
direction of the bisector (or the direction perpendicular to the
bisector), the polarizing direction of PBS 7 can be oriented to be
parallel or perpendicular to the azimuthal angles of the eigenmodes
of the TN type liquid crystal cell at the desired driving voltage.
An example is provided to illustrate a feature of the disclosure.
Please refer to FIG. 5. A dark state with a driving voltage of
3.5V.sub.rms has a corresponding residual retardation of about 75
nm. From FIGS. 4A and 4B, it is found that the azimuthal angles of
the eigenmodes are about 1.5.degree. greater than bisector
(30.degree./120.degree.) when the residual retardation is 75 nm.
Thus, referring to FIG. 2, the included angle .beta. between the
polarization direction 71 of PBS 7 and the first alignment
direction 3 of the alignment layer 13 is set at about
.phi./2+1.5.degree. or .pi./2+.phi./2+1.5.degree.. In such a
situation, a perfect dark state occurs at the low driving voltage
of about 3.5V.sub.rms.
[0050] From FIGS. 4A, 4B, and 5, a relationship among the applied
voltage, the residual retardation, and the azimuthal angle is
obtained. As the applied voltage increases, the residual
retardation decreases so that the corresponding azimuthal angles of
the two eigenmodes change accordingly. According to various
embodiments in order to obtain a reflective liquid crystal light
valve with a lower driving voltage and a higher contrast ratio, a
relationship between the included angle .phi. and the included
angle .beta. should satisfy .phi./2<.beta.<.phi./2+30.degree.
or 90.degree.+.phi./2<.beta.<.phi./2+120.degree.. As such,
the included angle .phi. is between the first and second alignment
directions 3 and 4, the included angle .beta. is between the first
alignment direction 3 and the polarization direction 71 of PBS 7
and numeral 25 denotes the bisector of the included angle .phi..
According to various embodiments the included angle .phi. can be
between 40.degree. and 70.degree.. In further embodiments, the
included angle .beta. can be .phi./2+1.degree. to about 3.degree.,
and in still further embodiments, .phi./2+1.5.degree.. That is, the
polarization direction of the polarizing device 7, such as a PBS,
disposed on the exterior of the transparent panel that provides
improved results 1 is not the conventional bisector 25.
[0051] Note that, when the cell 100 uses right-handed TN type
liquid crystal molecules, the included angle .beta. satisfies
-.phi./2>.beta.>-.phi./2-30.degree. or
.pi./2-.phi./2>.beta.>- .pi./2-.phi./2-30.degree.. For
convenience, all angles are based on the first alignment direction
3 of the front panel 1 (i.e. the first alignment layer 13), as
shown in FIG. 2, and all counterclockwise angles are defined to be
positive.
[0052] The following experimental data are provided for better
understanding of various embodiments of a reflective light valve
having a lower driving voltage and a higher contrast ratio than
that of the conventional technology.
[0053] FIG. 6A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a left-handed 60.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 350 nm at different polarization angles
.beta., according to a first test. FIG. 6B is a local enlarged view
of FIG. 6A in the dark state (i.e. the region that reflectance is
about 0). In the first test, a green incident light (.lambda.=550
nm) is used to impinge the reflective liquid crystal cell 100 shown
in FIG. 1A. The solid line denotes the bisector
(.beta.=.phi./2=30.0.degree.) in FIGS. 6A and 6B.
[0054] Because the PBS 7 has a limited extinction ratio (ER) of
about 1000:1, the contrast ratio (CR) of the reflective light valve
is affected by the extinction ratio of PBS as 3 CR = 1 ( 1 / ER ) +
R ,
[0055] wherein R is the normalized reflectance. For example, when
the normalized reflectance is R=0.00005, the contrast ratio is
CR=1/(0.001+0.00005)=952.
[0056] Referring to FIG. 6B, an applied voltage of about 5V.sub.rms
provides R=0.00005 when the bisector (.beta.=30.0.degree.) is used
as the polarization angle. According to the first test, because the
boundary layers are taken into consideration, an angle .beta. of
about 31.5.degree. provides an improved result. For example, the
driving voltage of the dark state drops to 3.5V.sub.rms, when
.beta. is about 31.5.degree.. As shown by the first test, about
3.5V.sub.rms results in the same CR=952. Thus, according to an
embodiment, the reflective light valve has lower driving voltage
than the conventional technology.
[0057] Further, using a driving voltage at 3.5V.sub.rms of the
conventional technology can only obtain a contrast ratio of
CR=1/(0.001+0.0012)=455. In contrast, according to various
embodiments described herein, a driving voltage of 3.5V.sub.rms
obtains a much higher contrast ratio of, for example, 952.
[0058] Accordingly, the first test verifies that a polarization
angle .beta. of .phi./2+1.degree. to about 3.degree., and in
certain embodiments, .phi./2+1.5.degree. is advantageous. An
embodiment of the reflective light valve can thus provide a high
contrast ratio with a low driving voltage, thereby reducing power
consumption.
[0059] FIG. 7A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a left-handed 57.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 350 nm at different polarization angles
.beta., according to a second test. FIG. 7B is a local enlarged
view of FIG. 7A in the dark state (i.e. the region that reflectance
is about 0). In the second test, a green incident light
(.lambda.=550 nm) is used to impinge the reflective liquid crystal
cell 100 shown in FIG. 1A. The solid line denotes the bisector
(.beta.=.phi./2=28.5.degree.) in FIGS. 7A and 7B.
[0060] Referring to FIG. 7B, an applied voltage of about
4.8V.sub.rms provides R=0.0001 when the bisector
(.beta.=28.5.degree.) is used as the polarization angle. The
contrast ratio of the reflective liquid crystal cell 100 at
4.8V.sub.rms is CR=1/(0.001+0.0001)=909. According to the second
test, an angle .beta. of about 30.degree. provides improved
results. For example, the driving voltage of the dark state drops
to about 3.4V.sub.rms when .beta. is about 30.degree.. As shown by
the second test, about 3.4V.sub.rms results in the same CR=909.
Thus, according to an embodiment, the reflective light valve as
described herein has a lower driving voltage than the conventional
technology.
[0061] FIG. 8A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a left-handed 55.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 350 nm at different polarization angles
.beta., according to a third test. FIG. 8B is a local enlarged view
of FIG. 8A in the dark state (i.e. the region that reflectance is
about 0). In the third test, a green incident light (.lambda.=550
nm) is used to impinge the reflective liquid crystal cell 100 shown
in FIG. 1A. The solid line denotes the bisector
(.beta.=.phi./2=27.5.degree.) in FIGS. 8A and 8B.
[0062] Referring to FIG. 8B, an applied voltage of about
4.8V.sub.rms provides R=0.0001 when the bisector
(.beta.=27.5.degree.) is used as the polarization angle. The
contrast ratio of the reflective liquid crystal cell 100 at
4.8V.sub.rms is CR=1/(0.001+0.0001)=909. According to the third
test, an angle .beta. of about 29.degree. provides an improved
result. For example, the driving voltage of the dark state drops to
about 3.4V.sub.rms when .beta. is about 29.degree.. As shown by the
third test, about 3.4V.sub.rms to reach the same CR=909. Thus,
according to an embodiment of the reflective light valve has a
lower driving voltage than the conventional technology.
[0063] FIG. 9A depicts a graphical plot of the relationship between
the normalized reflectance and the applied voltage for an
embodiment of a left-handed 50.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 350 nm at different polarization angles
.beta., according to a fourth test. FIG. 9B is a local enlarged
view of FIG. 9A in the dark state (i.e. the region with reflectance
of about 0). In the fourth test, a green incident light
(.lambda.=550 nm) is used to impinge the reflective liquid crystal
cell 100 shown in FIG. 1A. The solid line denotes the bisector
(.beta.=.phi./2=25.degree.) in FIGS. 9A and 9B.
[0064] Referring to FIG. 9B, an applied voltage of about 5V.sub.rms
provides R=0.0001 when the bisector (.beta.=25.degree.) is used as
the polarization angle. The contrast ratio of the reflective liquid
crystal cell 100 at 5V.sub.rms is CR=1/(0.001+0.0001)=909.
According to the fourth test, an angle .beta. of about 26.5.degree.
provides an improved result. For example, when .beta. is about
26.5.degree., the driving voltage of the dark state drops to about
3.4V.sub.rms. As shown by the fourth test about 3.4V.sub.rms
results in the same CR=909. Thus, according to an embodiment, the
reflective light valve as described herein has a lower driving
voltage than the conventional technology.
[0065] FIG. 10A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a left-handed 45.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 355 nm at different polarization angles
.beta., according to a fifth test. FIG. 10B is a local enlarged
view of FIG. 10A in the dark state (i.e. the region with
reflectance of about 0). In the fifth test, a green incident light
(.lambda.=550 nm) is used to impinge the reflective liquid crystal
cell 100 shown in FIG. 1A. The solid line denotes the bisector
(.beta.=.phi./2=22.5.degree.) in FIGS. 10A and 10B.
[0066] Referring to FIG. 10B, an applied voltage of about
4.7V.sub.rms provides R=0.0002 when the bisector
(.beta.=22.5.degree.) is used as the polarization angle. The
contrast ratio of the reflective liquid crystal cell 100 at
4.7V.sub.rms is CR=1/(0.001+0.0002)=833. According to the fifth
test, an angle .beta. of about 24.degree. provides an improved
result. For example, when .beta. is about 24.degree., the driving
voltage of the dark state drops to about 3.4V.sub.rms. As shown by
the fifth test, about 3.4V.sub.rms results in the same CR=833.
Thus, according to an embodiment, the reflective light valve as
described herein has a lower driving voltage than the conventional
technology.
[0067] FIG. 11A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a left-handed 40.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 365 nm at different polarization angles
.beta., according to a sixth test. FIG. 11B is a local enlarged
view of FIG. 11A in the dark state (i.e. the region with
reflectance of about 0). In FIGS. 11A and 11B, the solid line
denotes the bisector (.beta.=.phi./2=20.beta.). Similar to the
above tests, when the polarization angle .beta. is set at
.phi./2+1.5.degree. (i.e. .beta.=21.5.degree.), an embodiment of
the reflective light valve of the sixth test can provide a high
contrast ratio with a lower driving voltage than the conventional
technology.
[0068] FIG. 12A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a left-handed 65.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 345 nm at different polarization angles
.beta., according to a seventh test. FIG. 12B is a local enlarged
view of FIG. 12A in the dark state (i.e. the region with
reflectance of about 0). In FIGS. 12A and 12B, the solid line
denotes the bisector (.beta.=.phi./2=32.5.degree.- ). Similar to
the above tests, when the polarization angle .beta. is set at
.phi./2+1.5.degree. (i.e. .beta.=34.degree.), an embodiment of the
reflective light valve of the seventh test can provide a high
contrast ratio with a lower driving voltage than the conventional
technology.
[0069] FIG. 13A depicts a graphical plot of the relationship
between the normalized reflectance and the applied voltage for an
embodiment of a left-handed 70.degree.-TN liquid crystal cell with
retardation (d.DELTA.n) of 345 nm at different polarization angles
.beta., according to an eighth test. FIG. 13B is a local enlarged
view of FIG. 13A in the dark state (i.e. the region with
reflectance of about 0). In FIGS. 13A and 13B, the solid line
denotes the bisector (.beta.=.phi./2=35.degree.). Similar to the
above tests, when the polarization angle .beta. is set at
.phi./2+1.5.degree. (i.e. .beta.=36.5.degree.), an embodiment of
the reflective light valve of the eighth test can provide a high
contrast ratio with a lower driving voltage than the conventional
technology.
[0070] An embodiment of a reflective light valve 90 shown in FIG.
1A can be coupled to a controller 142, forming a display device 143
as shown in FIG. 14. The controller 142 can comprise source and
gate driving circuits (not shown) to control the reflective light
valve 90 to render images in accordance with an input. The display
device 143 and associated controller 142 may be directed to a
reflective projection display apparatus.
[0071] FIG. 15 depicts a schematic diagram illustrating an
electronic device 151 incorporating an embodiment of the reflective
light valve 90. An input device 154 is coupled to the controller
142 of the display device 143 shown in FIG. 15 to form an
electronic device 151. The input device 154 can include a processor
or the like, inputting data to the controller 142 to render an
image. The electronic device 151 may be a portable device such as a
notebook computer, tablet computer, cellular phone, or a display
monitor device, or non-portable device such as a desktop computer
or a projection TV.
[0072] While the invention has been described by way of example and
in terms of various embodiments, it is to be understood that the
invention is not limited thereto. On the contrary, it is intended
to cover various modifications and similar arrangements as would be
apparent to those skilled in the art. Therefore, the scope of the
appended claims should be accorded the broadest interpretation so
as to encompass all such modifications and similar
arrangements.
* * * * *