U.S. patent application number 10/438511 was filed with the patent office on 2004-08-26 for vertically aligned nematic mode liquid crystal display having large tilt angles and high contrast.
This patent application is currently assigned to THREE-FIVE SYSTEMS, INC.. Invention is credited to Vithana, Hemasiri.
Application Number | 20040165128 10/438511 |
Document ID | / |
Family ID | 32871861 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040165128 |
Kind Code |
A1 |
Vithana, Hemasiri |
August 26, 2004 |
Vertically aligned nematic mode liquid crystal display having large
tilt angles and high contrast
Abstract
A reflective liquid crystal on silicon (LCOS) display comprises
a transparent substrate, a reflective substrate, and liquid crystal
fluid between the substrates. The LCOS display further comprises a
matrix of pixels, arranged in a plurality, of rows and columns,
wherein an intersection of a row and a column defines a position of
a pixel in the matrix. The LCOS display has tilt angles sufficient
to overcome disclinations due to fringe fields, and, at the same
time, achieves high contrast. The surface azimuthal direction of
the molecules of the liquid crystal fluid is either substantially
parallel or perpendicular to the direction of polarization of
incoming incident linearly polarized light. Light leakage is
minimal because the effective birefringence as seen by the incoming
incident linearly polarized light is substantially zero and does
not depend on the pretilt of the molecules of the liquid crystal
fluid. Between the transparent substrate and the reflective
substrate, the twist of the molecules of the liquid crystal fluid
may vary from about 0 degrees to about 90 degrees when in the "OFF"
state.
Inventors: |
Vithana, Hemasiri;
(Chandler, AZ) |
Correspondence
Address: |
BAKER BOTTS, LLP
910 LOUISIANA
HOUSTON
TX
77002-4995
US
|
Assignee: |
THREE-FIVE SYSTEMS, INC.
|
Family ID: |
32871861 |
Appl. No.: |
10/438511 |
Filed: |
May 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60450370 |
Feb 26, 2003 |
|
|
|
Current U.S.
Class: |
349/113 |
Current CPC
Class: |
G02F 1/1393 20130101;
G02F 1/1337 20130101; G02F 1/133541 20210101; G02F 1/133528
20130101; G02F 1/133742 20210101; G02F 1/133749 20210101; G02F
1/1396 20130101; G02F 2203/02 20130101 |
Class at
Publication: |
349/113 |
International
Class: |
G02F 001/1335 |
Claims
What is claimed is:
1. A reflective liquid crystal display, comprising: a first
substrate that is substantially transparent; a second substrate
that is substantially reflective and substantially parallel with
said first substrate; and a liquid crystal fluid having a
birefringence (.DELTA.n) and a negative dielectric anisotropy,
wherein said liquid crystal fluid is between said first and second
substrates; said first substrate having a first liquid crystal
alignment layer proximate to said liquid crystal fluid, wherein
molecules of said liquid crystal fluid that are proximate to the
first liquid crystal alignment layer have a first pretilt angle
from about 2 degrees to about 15 degrees and a first azimuthal
direction; said second substrate having a second liquid crystal
alignment layer proximate to said liquid crystal fluid, wherein
molecules of said liquid crystal fluid that are proximate to the
second liquid crystal alignment layer have a second pretilt angle
from about 2 degrees to about 15 degrees and a second azimuthal
direction, the second azimuthal direction being substantially
perpendicular to the first azimuthal direction.
2. The reflective liquid crystal display of claim 1, wherein
linearly polarized incident light has a polarization direction
approximately parallel to the first azimuthal direction.
3. The reflective liquid crystal display of claim 1, wherein
linearly polarized incident light has a polarization direction
approximately perpendicular to the first azimuthal direction.
4. The reflective liquid crystal display of claim 1, wherein shades
of gray are produced by varying an electric field between said
first and second substrates from substantially no electric field to
an electric field having an optimum value.
5. The reflective liquid crystal display of claim 1, wherein a
distance (d) between inside faces of said first and second
substrates is about 3.5 micrometers.
6. The reflective liquid crystal display of claim 1, wherein a
distance (d) between inside faces of said first and second
substrates is from about 3.3 micrometers to about 3.7
micrometers.
7. The reflective liquid crystal display of claim 1, wherein the
birefringence (.DELTA.n) is about 0.0830 at about 45 degrees
Celsius.
8. The reflective liquid crystal display of claim 1, wherein the
birefringence (.DELTA.n) is from about 0.0777 to about 0.0996.
9. The reflective liquid crystal display of claim 1, wherein the
birefringence (.DELTA.n) times d is greater than .lambda./4 when an
electric field having an optimum value is applied between said
first and second substrates, where d is the distance between inside
faces of said first and second substrates and .lambda. is the
wavelength of light.
10. The reflective liquid crystal display of claim 1, wherein the
molecules of said liquid crystal fluid have a tilt angle (.theta.)
of from about 5 degrees to about 15 degrees when substantially no
electric field is applied between said first and second
substrates.
11. The reflective liquid crystal display of claim 1, wherein the
molecules of said liquid crystal fluid have a tilt angle (.theta.)
of from about 2 degrees to about 15 degrees when substantially no
electric field is applied between said first and second
substrates.
12. The reflective liquid crystal display of claim 1, wherein an
azimuthal angle (.PHI.) of the molecules of said liquid crystal
fluid varies from about 0 degrees at said first substrate to about
90 degrees at said second substrate when substantially no electric
field is applied between said first and second substrates.
13. The reflective liquid crystal display of claim 1, wherein an
azimuthal angle (.PHI.) of the molecules of said liquid crystal
fluid varies from about 0 degrees at said second substrate to about
90 degrees at said first substrate when substantially no electric
field is applied between said first and second substrates.
14. A reflective liquid crystal display, comprising: a first
substrate that is substantially transparent; a second substrate
that is substantially reflective and substantially parallel with
said first substrate; and a liquid crystal fluid having a
birefringence (.DELTA.n) and a negative dielectric anisotropy,
wherein said liquid crystal fluid is between said first and second
substrates; said first substrate having a first liquid crystal
alignment layer proximate to said liquid crystal fluid, wherein
molecules of said liquid crystal fluid that are proximate to the
first liquid crystal alignment layer have a first pretilt angle
from about 2 degrees to about 15 degrees and a first azimuthal
direction; said second substrate having a second liquid crystal
alignment layer proximate to said liquid crystal fluid, wherein
molecules of said liquid crystal fluid that are proximate to the
second liquid crystal alignment layer have a second pretilt angle
from about 2 degrees to about 15 degrees and a second azimuthal
direction, the second azimuthal direction being substantially
perpendicular to the first azimuthal direction; wherein: when an
electric field is applied between said first and second substrates,
a substantial number of the molecules of said liquid crystal fluid
increase their tilt angles, and when substantially no electric
field is applied between said first and second substrates, a
substantial number of the molecules of said liquid crystal fluid
have their azimuthal direction substantially perpendicular to said
first and second substrates; whereby when the electric field is
applied between said first and second substrates, said liquid
crystal fluid changes linear polarized incident light at said first
substrate to approximately circularly polarized incident light when
at said second substrate, wherein said second substrate reflects
back the approximately circularly polarized incident light as
approximately circularly polarized light of opposite handedness,
wherein said liquid crystal fluid changes the approximately
circularly polarized reflected light to approximately linear
polarized reflected light when at said first substrate such that
the polarization directions of the linearly polarized incident
light and the linearly polarized reflected light at said first
substrate are approximately perpendicular, and whereby when
substantially no electric field is applied between said first and
second substrates, said liquid crystal fluid does not substantially
change the polarization of light passing through the liquid crystal
fluid and the polarization directions of the linearly polarized
incident light and the linearly polarized reflected light at said
first substrate are approximately parallel.
15. The reflective liquid crystal display of claim 14, wherein the
electric field is applied between said first and second substrates
for a plurality of first times and substantially no electric field
is applied for a plurality of second times.
16. The reflective liquid crystal display of claim 15, wherein the
first and second times are varied to produce shades of gray.
17. The reflective liquid crystal display of claim 15, wherein the
first and second times are varied to produce field sequential
colors.
18. A method for a reflective liquid crystal display, said method
comprising the steps of: providing a first substrate that is
substantially transparent; providing a second substrate that is
substantially reflective and substantially parallel with said first
substrate; and providing a liquid crystal fluid having a
birefringence (.DELTA.n) and a negative dielectric anisotropy
between said first and second substrates; providing a first liquid
crystal alignment layer on said first substrate, said first liquid
crystal alignment layer being proximate to said liquid crystal
fluid, wherein molecules of said liquid crystal fluid that are
proximate to the first liquid crystal alignment layer have a first
pretilt angle from about 2 degrees to about 15 degrees and a first
azimuthal direction; providing a second liquid crystal alignment
layer on said second substrate, said second liquid crystal
alignment layer being proximate to said liquid crystal fluid,
wherein molecules of said liquid crystal fluid that are proximate
to the second liquid crystal alignment layer have a second pretilt
angle from about 2 degrees to about 15 degrees and a second
azimuthal direction, the second azimuthal direction being
substantially perpendicular to the first azimuthal direction; such
that when substantially no electric field is applied between said
first and second substrates, said liquid crystal fluid does not
change the polarization state of light passing therethrough and the
polarization directions of the linearly polarized incident light
and the linearly polarized reflected light at said first substrate
are substantially parallel, when applying an electric field of an
optimum value between said first and second substrates, said liquid
crystal fluid changes linear polarized incident light at said first
substrate to approximately circularly polarized incident light when
at said second substrate, wherein said second substrate reflects
back the approximately circularly polarized incident light as
approximately circularly polarized light of opposite handedness,
wherein said liquid crystal fluid changes the approximately
circularly polarized reflected light to approximately linear
polarized reflected light when at said first substrate such that
the polarization directions of the linearly polarized incident
light and the linearly polarized reflected light at said first
substrate are approximately perpendicular, and when applying an
electric field less than the optimum value between said first and
second substrates, said liquid crystal fluid changes the
polarization state of the incident linearly polarized light to
elliptically polarized light upon passing through said liquid
crystal fluid and being reflected back from said second
substrate.
19. The method of claim 18, wherein the polarization direction of
the linearly polarized incident light is approximately parallel
with the first azimuthal direction.
20. The method of claim 18, wherein the polarization direction of
the linearly polarized incident light is approximately
perpendicular with the first azimuthal direction.
21. The method of claim 18, further comprising the step of varying
the electric field between said first and second substrates so as
to produce shades of gray by causing the polarization state of the
incident light at said second substrate to vary from approximately
linear polarization to elliptical polarization, and when there is
substantially no electric field the polarization of the incident
light at said second substrate is approximately linear.
22. The method of claim 18, wherein a distance (d) between inside
faces of said first and second substrates is about 3.5
micrometers.
23. The method of claim 18, wherein a distance (d) between inside
faces of said first and second substrates is from about 3.3
micrometers to about 3.7 micrometers.
24. The method of claim 18, wherein said liquid crystal fluid has a
birefringence (.DELTA.n) of about 0.0830 at about 45 degrees
Celsius.
25. The method of claim 18, wherein said liquid crystal fluid has a
birefringence (.DELTA.n) from about 0.0777 to about 0.0996.
26. The method of claim 18, wherein the birefringence .DELTA.n
times d is greater than .lambda./4 when an electric field having an
optimum value is applied between said first and second substrates,
where d is the distance between inside faces of said first and
second substrates and .lambda. is the wavelength of light.
27. The method of claim 18, wherein the molecules of said liquid
crystal fluid have a tilt angle (.theta.) of from about 5 degrees
to about 15 degrees when substantially no electric field is applied
between said first and second substrates.
28. The method of claim 18, wherein the molecules of said liquid
crystal fluid have a tilt angle (.theta.) of from about 2 degrees
to about 15 degrees when substantially no electric field is applied
between said first and second substrates.
29. The method of claim 18, wherein an azimuthal angle (.PHI.) of
the molecules of said liquid crystal fluid varies from about 0
degrees at said first substrate to about 90 degrees at said second
substrate when substantially no electric field is applied between
said first and second substrates.
30. The method of claim 18, wherein an azimuthal angle (.PHI.) of
the molecules of said liquid crystal fluid varies from about 0
degrees at said second substrate to about 90 degrees at said first
substrate when substantially no electric field is applied between
said first and second substrates.
31. A method for a reflective liquid crystal display, said method
comprising the steps of: providing a first substrate that is
substantially transparent; providing a second substrate that is
substantially reflective and substantially parallel with said first
substrate; and providing a liquid crystal fluid having a
birefringence (.DELTA.n) and a negative dielectric anisotropy,
wherein said liquid crystal fluid is between said first and second
substrates; providing a first liquid crystal alignment layer on
said first substrate, said first liquid crystal alignment layer
being proximate to said liquid crystal fluid, wherein molecules of
said liquid crystal fluid that are proximate to the first liquid
crystal alignment layer have a first pretilt angle from about 2
degrees to about 15 degrees and a first azimuthal direction;
providing a second liquid crystal alignment layer on said second
substrate, said second liquid crystal alignment layer being
proximate to said liquid crystal fluid, wherein molecules of said
liquid crystal fluid that are proximate to the second liquid
crystal alignment layer have a second pretilt angle from about 2
degrees to about 15 degrees and a second azimuthal direction, the
second azimuthal direction being substantially perpendicular to the
first azimuthal direction; wherein: when applying substantially no
electric field between said first and second substrates, a
substantial number of the molecules of said liquid crystal fluid
have their azimuthal direction substantially perpendicular to said
first and second substrates; and when applying an electric field
between said first and second substrates, a substantial number of
the molecules of said liquid crystal fluid increase their tilt
angles; whereby when applying the electric field between said first
and second substrates, said liquid crystal fluid changes linear
polarized incident light at said first substrate to approximately
circularly polarized incident light when at said second substrate,
wherein said second substrate reflects back the approximately
circularly polarized incident light as approximately circularly
polarized light of opposite handedness, wherein said liquid crystal
fluid changes the approximately circularly polarized reflected
light to approximately linear polarized reflected light when at
said first substrate such that the polarization directions of the
linearly polarized incident light and the linearly polarized
reflected light at said first substrate are approximately
perpendicular, and whereby when applying substantially no electric
field between said first and second substrates, said liquid crystal
fluid does not substantially change the polarization state of light
passing through said liquid crystal fluid such that the
polarization directions of the linearly polarized incident light
and the linearly polarized reflected light at said first substrate
are approximately parallel.
32. A reflective liquid crystal display assembly comprising: a
first substantially transparent substrate; a second substantially
reflective substrate located substantially parallel to said first
substrate; a liquid crystal fluid having a birefringence (.DELTA.n)
and a negative dielectric anisotropy, wherein said liquid crystal
fluid is between said first and second substrates; first and second
liquid crystal alignment layers on said first and second
substrates, respectively, wherein molecules of said fluid proximate
the first and second liquid crystal alignment layers have finite
pretilt angles and are oriented in first and second azimuthal
directions, respectively; wherein the assembly is configured such
that (i) when substantially no electric field is applied between
the substrates, a substantial number of fluid molecules are
oriented substantially perpendicular to said substrates, (ii) when
an electrical field of optimum value is applied between the
substrates, tilt angles of a substantial number of fluid molecules
increase, and (iii) when a less than optimum electric field is
applied between said substrates, a substantial number of fluid
molecules are oriented at intermediate tilt angles.
Description
RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
Application Serial No. 60/450,370, entitled "Method to Eliminate
the Disclination Defects Due to Fringe Fields in Vertically Aligned
Nematic Reflective LC Displays Without Hurting the Display
Contrast" by Hemasiri Vithana, filed Feb. 26, 2003, and is
incorporated herein by reference for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates generally to liquid crystal
displays (LCDs), and more particularly to reflective liquid crystal
on silicon (LCOS) displays.
BACKGROUND OF THE FIELD OF TECHNOLOGY
[0003] Liquid crystal display technology has reduced the size of
displays from full screen sizes to minidisplays of less than 1.3
inches diagonal measurement, to microdisplays that require a
magnification system. Microdisplays may be manufactured using
semiconductor integrated circuit (IC) dynamic random access memory
(DRAM) process technologies, e.g., liquid crystal on silicon
(LCOS). The LCOS microdisplays consist of a silicon substrate
backplane with a reflective surface, a cover glass and an
intervening liquid crystal layer. The LCOS microdisplays are
arranged as a matrix of pixels arranged in a plurality of rows and
columns, wherein an intersection of a row and a column defines a
position of a pixel in the matrix. To incident light, each pixel is
a liquid crystal cell above a reflecting mirror. By changing the
molecular orientation of the liquid crystal in the layer,
characterized by a tilt angle and a twist angle of the liquid
crystal director at any point in the layer, the incident light can
be made to change its polarization. The silicon backplane is an
array of pixels, typically 7 to 20 micrometers (.mu.m) in pitch.
Each pixel has a mirrored surface that occupies most of the pixel
area. The mirrored surface is also an electrical conductor that
forms a pixel capacitor with the liquid crystal display cover glass
electrode which is a transparent conductive coating on the inside
face (liquid crystal side) of the cover glass. This transparent
conductive coating is typically Indium Tin Oxide (ITO). As each
pixel capacitor is charged to a certain voltage value, the liquid
crystal fluid between the plates of the pixel capacitors changes
its molecular orientation which affects the polarization state of
the light incident to the pixels (reflections from the pixel
mirrors).
[0004] The reflective LCOS microdisplays have a high aperture
ratio, and therefore can provide greater brightness than
transmissive liquid crystal displays. Major applications of these
LCOS microdisplays are in home theater applications, e.g.,
projectors, and front and rear projection televisions (large
screen). For these applications, high contrast is very important.
High contrast depends upon the liquid crystal optical mode being
used in the liquid crystal display. Typically, a Vertically Aligned
Nematic (VAN) mode is one of the optical modes that can achieve a
very high contrast and many liquid crystal display manufacturers
are beginning to use this particular optical mode in their
displays.
[0005] The pretilt angle is defined as the tilt angle of the liquid
crystal director at the boundary surface. In VAN mode liquid
crystal displays, the pretilt angle is small, so the orientation of
the molecules of the liquid crystal fluid are nearly perpendicular
to the substrate surfaces when there is no electric field applied
across the display. Therefore, incoming linearly polarized light,
perpendicular to the display substrates, sees a small birefringence
as it passes through the layer. Hence this normally incident
linearly polarized light experiences little phase retardation when
going through the liquid crystal fluid, including being reflected
back from the bottom reflective substrate of the display. This
provides a very dark "OFF" state when using crossed polarizers
(e.g., polarizing beam splitter--PBS), thus very high contrast is
achieved. Upon application of an electric field across the liquid
crystal fluid, the molecules in the bulk of the liquid crystal
fluid orient themselves toward a direction defined by alignment
layers on the substrate surfaces, thereby increasing the
retardation of the layer of the liquid crystal fluid. Therefore,
linearly polarized incident light starts to experience a phase
retardation when going into the liquid crystal fluid and then being
reflected back from the bottom reflective substrate of the display.
As a result of this, the polarization state of the out-going light
(reflected light) will be elliptical and some light starts to pass
through the crossed polarizers. Increasing the electric field
increases this effect until the brightest state is achieved.
[0006] In a typical VAN mode, the orientations of the molecules of
the liquid crystal fluid at the substrate surfaces are defined by
the alignment layers on each of the substrate surfaces. This
orientation is described by a pretilt angle and a surface azimuthal
direction, which is parallel to the projection of the liquid
crystal director onto the plane of the substrate. The azimuthal
direction of the molecules of the liquid crystal fluid proximate to
the top alignment layer is opposite to the azimuthal direction of
the molecules of the liquid crystal fluid proximate to the bottom
alignment layer, i.e., anti-parallel. The azimuthal directions
defined by the alignment layers are at a 45 degree angle with the
direction of polarization of the incoming linearly polarized
incident light, as shown in FIGS. 1a and 1b. Usually the pretilt
angle of the molecules in a VAN mode display needs to be kept
small, e.g., less than 4 degrees, to achieve a very dark "OFF"
state, hence the high contrast. Although this pretilt angle is
large enough to prevent reverse tilt domains in the display, it is
not possible to overcome the defects that occur due to fringe
fields between neighboring pixels. Fringe fields become very
significant when the pixel size becomes small as is typical in LCOS
microdisplays. For example, the size of an LCOS microdisplay may
measure one inch diagonally and have a pixel size of approximately
12 .mu.m.times.12 .mu.m. When high resolution is required, e.g.,
digital cinema applications, pixel size may be further reduced to
approximately 9 .mu.m.times.9 .mu.m or even smaller. In such
situations fringe fields are quite pronounced and the liquid
crystals do not align along the direction defined by the tilt
direction of the alignment layers. Ultimately this will create
defects at the pixel boundaries, usually known as disclinations.
This is quite apparent when one pixel is in an "ON" state and an
adjacent pixel is in an "OFF" state, wherein the fringe fields are
very strong.
[0007] To overcome the above problem, it is necessary to increase
the pretilt angle generated by the alignment layers on the
substrate surfaces. Experimentally it has been determined that the
pretilt angle has to be at least 8 degrees to overcome the fringe
field effects. However, the dark state of a VAN mode liquid crystal
display with a pretilt angle of this magnitude has a significant
amount of light leakage through the crossed polarizers and the
light contrast it can achieve is not that high. Therefore, the
inherent property of VAN displays, the very dark "OFF" state,
cannot be fully achieved. This is due to the non-zero birefringence
seen by the linearly polarized incident light due to the high
pretilt angle of the liquid crystal fluid. Heretofore, it has been
necessary to use other methods such as attaching external retarders
to stop this light leakage. Generally, this is the current method
used by all the VAN liquid crystal display manufactures to solve
the above problem.
SUMMARY OF THE INVENTION
[0008] The present invention overcomes the above-identified
problems as well as other shortcomings and deficiencies of existing
technologies by providing a system, method and apparatus having
pretilt angles sufficient to overcome disclinations due to fringe
fields, and, at the same time, achieving high contrast.
[0009] In the typical VAN optical mode, the surface azimuthal
direction is 45 degrees to the direction of polarization of
incoming linearly polarized incident light. Therefore, there is an
effective birefringence as seen by the incoming incident light and
it increases with increasing pretilt angle, hence the amount of
light leakage.
[0010] If the display shown in FIG. 1a is rotated by 45 degrees
with respect to the polarization direction of the incident linearly
polarized light, i.e., the surface azimuthal direction of the
molecules of the liquid crystal fluid is either parallel or
perpendicular to the direction of polarization of incoming incident
linearly polarized light, then the light leakage is minimum because
the effective birefringence as seen by the incoming incident
linearly polarized light is zero and does not depend on the pretilt
of the molecules of the liquid crystal fluid. However, this
configuration cannot be applied to practical applications because
the "ON" state is not bright due to the same reason explained
above. But this feature is advantageously being used in the present
invention, i.e., surface azimuthal directions generated by the top
and bottom substrates are set substantially perpendicular to each
other. Also at the same time, the surface azimuthal direction
generated by one substrate is perpendicular/parallel to the
direction of polarization of incoming linearly polarized light
while the surface azimuthal direction from the other substrate is
parallel/perpendicular. Essentially this is a 90 degree twisted
structure in the "OFF" state. Because the majority of the molecules
of the liquid crystal layer do not have their azimuthal directions
oriented at 45 degrees to the direction of polarization of incoming
incident linearly polarized light, the effective birefringence as
seen by the incoming incident linearly polarized light is minimal
as compared to a VAN structure where all molecules have the same
tilt angle and their surface azimuthal directions are all oriented
at 45 degrees with respect to the polarization direction of the
incoming linearly polarized light. Therefore, in the present
invention, the light leakage is very small even though the pretilt
angle is large enough to remove the disclinations due to fringe
fields.
[0011] An important technical feature of the optical mode of the
present invention takes place when in the "ON" state. In the "ON"
state, the invention behaves differently from a conventional 90
degree twisted nematic (TN) mode, and the invention gives a very
good bright state as is desired in liquid crystal display
applications using a PBS. In a conventional 90 degree TN mode the
linear polarized light is "guided" by the twisted structure, both
in going in and coming out of the layer, which would give a dark
state with a PBS. According to the present invention, the bright
"ON" state is achieved without having to add chiral dopant in the
liquid crystal fluid of the display.
[0012] Although the surface azimuthal directions of the molecules
of the liquid crystal fluid at the bottom and top substrates
produce a 90 degree twist, this optical mode does not behave as
does a conventional 90 degree twisted nematic mode in the "ON"
state. On the other hand, the present invention does not behave as
a VAN mode display in the "OFF" state. The present invention
produces a much darker state than that of a regular VAN mode
display with the same pretilt angle. Therefore, the present
invention is neither a VAN nor a conventional 90 degree twisted
nematic mode display.
[0013] A gray scale, e.g., a light intensity through the polarizer
that is intermediate to the "ON" and "OFF" intensities may be
obtained by controlling the voltage that is applied across the
liquid crystal layer. In a crossed-polarizer (or PBS)
configuration, increasing the voltage increases the light passing
to the output up to a certain optimum bright-state voltage. The
value of this optimum bright state voltage depends upon the liquid
crystal material parameters, the cell gap, the pretilt and the
light wavelength range of interest. This voltage can be determined
experimentally. In addition, perceived gray scale can be controlled
by controlling the time that the liquid crystal display is in the
"ON" state and the time in the "OFF" state. In addition, color may
be generated with methods known in the art such as color filters,
use of three displays each for one color in a three-panel system,
or with one microdisplay in a field sequential color (FSC)
system.
[0014] The thickness (d) of the liquid crystal fluid (distance
between the inside faces of the top and bottom substrates) may be,
for example, about 3.5 .mu.m+/-0.2 .mu.m. The birefringence
(.DELTA.n) may be, for example, about 0.0830 at 45.degree. C.
Liquid crystal fluids that are being used in the present invention
are nematic and have negative dielectric anisotropy
.DELTA..epsilon.(=.epsilon..sub.//-.epsilon..sub..perp.<0),
where .epsilon..sub.// and .epsilon..sub..perp. are the parallel
and perpendicular (to the liquid crystal molecule) components of
the dielectric constant of the liquid crystal material. It is
contemplated and within the scope of the present invention that any
combination of thickness (d) and birefringence (.DELTA.n) may be
used so long as the condition: .DELTA.n.multidot.d>.lambda./4 is
satisfied, where .lambda. is wavelength of light incident on the
display.
[0015] Any liquid crystal fluids developed for VAN displays may be
used in the present invention. According to the present invention,
it is not necessary to introduce a chiral dopant into the liquid
crystal fluid. Typical liquid crystal fluids are for example but
not limited to: MLC-6608, MLC-6609 and MLC-6610 manufactured by
Merck. A physical property of liquid crystal materials used for VAN
displays is negative dielectric anisotropy, i.e., the perpendicular
component of the dielectric constant is larger than that of the
parallel component. Therefore, with an applied electric field, the
molecules of the liquid crystal fluid will arrange themselves
perpendicular to the direction of the electric field. For example,
the dielectric anisotropy may range from about
.DELTA..epsilon.=-3.1 to -4.2. Birefringence may range from about
0.0777 to about 0.0996, and the nematic to isotropic phase
transition temperature may be above 80.degree. C.
[0016] A technical advantage of the present invention is that with
this optical mode it is possible to have a much darker "OFF" state,
hence high contrast, even with a relatively high pretilt angle
compared to the VAN mode display with the same pretilt angle.
Because of the high pretilt angle substantially no disclination
defects will occur due to the fringe fields across the neighboring
pixels at the pixel boundaries. Another technical advantage is that
external retarders are not necessary to block the light leakage
because of the very good dark state of the present invention.
[0017] In projection applications with McNeil type polarizing beam
splitters, there is a system retarder, usually a quarter wave
plate, for each and every color channel (RGB) to compensate the
skew rays. When regular VAN mode displays are accommodated in such
applications, then this system retarder can also be used to stop
the light leakage. Essentially it is going to be a compromise state
between the skew ray compensation and light leakage. However, this
does not work well for the BLUE channel giving rise to a reasonable
amount of light leakage. Therefore, contrast of the BLUE channel is
generally lower than the other two channels (RED and GREEN). In
fact the proper retarders to stop the light leakage are the ones
with low retardation values of around 50 nanometers or less as
shown by experiments. Such retarders with good uniformity are
difficult to find and not easily available commercially. According
to the present invention, this will not be a problem because of the
very dark "OFF" state across the visible spectral region and the
system retarders can be used solely to compensate for the skew
rays. Also the BLUE channel will not suffer with low contrast
either.
[0018] Some optical architectures do not require skew ray
compensation. An example of this type of architecture are those
where a wire-grid polarizing beamsplitter is used to separate input
and output beam paths. Such wire-grid beamsplitters are
manufactured by Moxtek Inc., of Orem, Utah. Therefore, for such
applications the present invention is advantageous since the extra
cost of attaching external retarders can be eliminated.
[0019] The present invention is directed to a reflective liquid
crystal display, comprising: a first substrate that is
substantially transparent; a second substrate that is substantially
reflective and substantially parallel with said first substrate;
and a liquid crystal fluid having a birefringence (.DELTA.n) and a
negative dielectric anisotropy, wherein said liquid crystal fluid
is between said first and second substrates; said first substrate
having a first liquid crystal alignment layer proximate to said
liquid crystal fluid, wherein molecules of said liquid crystal
fluid that are proximate to the first liquid crystal alignment
layer have a first pretilt angle from about 2 degrees to about 15
degrees and a first azimuthal direction; said second substrate
having a second liquid crystal alignment layer proximate to said
liquid crystal fluid, wherein molecules of said liquid crystal
fluid that are proximate to the second liquid crystal alignment
layer have a second pretilt angle from about 2 degrees to about 15
degrees and a second azimuthal direction, the second azimuthal
direction being substantially perpendicular to the first azimuthal
direction.
[0020] The polarization direction of the linearly polarized
incident light may be approximately parallel with the first surface
azimuthal direction. The polarization direction of the linearly
polarized incident light may be approximately perpendicular with
the first surface azimuthal direction.
[0021] Shades of gray may be produced by varying the electric field
between said first and second substrates from substantially no
electric field to an electric field having an optimum value.
[0022] The present invention is also directed to a reflective
liquid crystal display, comprising: a first substrate that is
substantially transparent; a second substrate that is substantially
reflective and substantially parallel with said first substrate;
and a liquid crystal fluid having a birefringence (.DELTA.n) and a
negative dielectric anisotropy, wherein said liquid crystal fluid
is between said first and second substrates; said first substrate
having a first liquid crystal alignment layer proximate to said
liquid crystal fluid, wherein molecules of said liquid crystal
fluid that are proximate to the first liquid crystal alignment
layer have a first pretilt angle from about 2 degrees to about 15
degrees and a first azimuthal direction; said second substrate
having a second liquid crystal alignment layer proximate to said
liquid crystal fluid, wherein molecules of said liquid crystal
fluid that are proximate to the second liquid crystal alignment
have a second pretilt angle from about 2 degrees to about 15
degrees and a second azimuthal direction, the second azimuthal
direction being substantially perpendicular to the first azimuthal
direction; wherein: when an electric field is applied between said
first and second substrates, a substantial number of the molecules
of said liquid crystal fluid increase their tilt angles, and when
substantially no electric field is applied between said first and
second substrates, a substantial number of the molecules of said
liquid crystal fluid have their azimuthal direction substantially
perpendicular to said first and second substrates; whereby when the
electric field is applied between said first and second substrates,
said liquid crystal fluid changes linear polarized incident light
at said first substrate to approximately circularly polarized
incident light when at said second substrate, wherein said second
substrate reflects back the approximately circularly polarized
incident light as approximately circularly polarized light of
opposite handedness, wherein said liquid crystal fluid changes the
approximately circularly polarized reflected light to approximately
linear polarized reflected light when at said first substrate such
that the polarization directions of the linearly polarized incident
light and the linearly polarized reflected light at said first
substrate are approximately perpendicular, and whereby when
substantially no electric field is applied between said first and
second substrates, said liquid crystal fluid does not substantially
change the polarization of light passing through the liquid crystal
fluid and the polarization directions of the linearly polarized
incident light and the linearly polarized reflected light at said
first substrate are approximately parallel.
[0023] The electric field may be applied between said first and
second substrates for a plurality of first times and substantially
no electric field may be applied for a plurality of second times,
wherein the first and second times may be varied to produce shades
of gray. The first and second times may also be varied to produce
field sequential colors.
[0024] The present invention is directed to a method for a
reflective liquid crystal display, said method comprising the steps
of: providing a first substrate that is substantially transparent;
providing a second substrate that is substantially reflective and
substantially parallel with said first substrate; and providing a
liquid crystal fluid having a birefringence (.DELTA.n) and a
negative dielectric anisotropy between said first and second
substrates; providing a first liquid crystal alignment layer on
said first substrate, said first liquid crystal alignment layer
being proximate to said liquid crystal fluid, wherein molecules of
said liquid crystal fluid that are proximate to the first liquid
crystal alignment layer have a first pretilt angle from about 2
degrees to about 15 degrees and a first azimuthal direction;
providing a second liquid crystal alignment layer on said second
substrate, said second liquid crystal alignment layer being
proximate to said liquid crystal fluid, wherein molecules of said
liquid crystal fluid that are proximate to the second liquid
crystal alignment layer have a second pretilt angle from about 2
degrees to about 15 degrees and a second azimuthal direction, the
second azimuthal direction being substantially perpendicular to the
first tilt orientation direction; such that when substantially no
electric field is applied between said first and second substrates,
said liquid crystal fluid does not change the polarization state of
light passing therethrough and the polarization directions of the
linearly polarized incident light and the linearly polarized
reflected light at said first substrate are substantially parallel,
when applying an electric field of an optimum value between said
first and second substrates, said liquid crystal fluid changes
linear polarized incident light at said first substrate to
approximately circularly polarized incident light when at said
second substrate, wherein said second substrate reflects back the
approximately circularly polarized incident light as approximately
circularly polarized light of opposite handedness, wherein said
liquid crystal fluid changes the approximately circularly polarized
reflected light to approximately linear polarized reflected light
when at said first substrate such that the polarization directions
of the linearly polarized incident light and the linearly polarized
reflected light at said first substrate are approximately
perpendicular, and when applying an electric field less than the
optimum value between said first and second substrates, said liquid
crystal fluid changes the polarization state of the incident
linearly polarized light to elliptically polarized light upon
passing through said liquid crystal fluid and being reflected back
from said second substrate.
[0025] Varying the electric field between said first and second
substrates may be used to produce shades of gray by causing the
polarization state of the incident light at said second substrate
to vary from approximately linear polarization to elliptical
polarization, and when there is substantially no electric field the
polarization of the incident light at said second substrate is
approximately linear.
[0026] The present invention is also directed to a method for a
reflective liquid crystal display, said method comprising the steps
of: providing a first substrate that is substantially transparent;
providing a second substrate that is substantially reflective and
substantially parallel with said first substrate; and providing a
liquid crystal fluid having a birefringence (.DELTA.n) and a
negative dielectric anisotropy, wherein said liquid crystal fluid
is between said first and second substrates; providing a first
liquid crystal alignment layer on said first substrate, said first
liquid crystal alignment layer being proximate to said liquid
crystal fluid, wherein molecules of said liquid crystal fluid that
are proximate to the first liquid crystal alignment layer have a
first pretilt angle from about 2 degrees to about 15 degrees and a
first azimuthal direction; providing a second liquid crystal
alignment layer on said second substrate, said second liquid
crystal alignment layer being proximate to said liquid crystal
fluid, wherein molecules of said liquid crystal fluid that are
proximate to the second liquid crystal alignment layer have a
second pretilt angle from about 2 degrees to about 15 degrees and a
second azimuthal direction, the second azimuthal direction being
substantially perpendicular to the first azimuthal direction;
wherein: when substantially no electric field is applied between
said first and second substrates, a substantial number of the
molecules of said liquid crystal fluid have their orientation
substantially perpendicular to said first and second substrates;
and when applying an electric field between said first and second
substrates, a substantial number of the molecules of said liquid
crystal fluid change tilt towards parallel to the substrates;
whereby when applying the electric field between said first and
second substrates, said liquid crystal fluid changes linear
polarized incident light at said first substrate to approximately
circularly polarized incident light when at said second substrate,
wherein said second substrate reflects back the approximately
circularly polarized incident light as approximately circularly
polarized light of opposite handedness, wherein said liquid crystal
fluid changes the approximately circularly polarized reflected
light to approximately linear polarized reflected light when at
said first substrate such that the polarization directions of the
linearly polarized incident light and the linearly polarized
reflected light at said first substrate are approximately
perpendicular, and whereby when no electric field is applied
between said first and second substrates, said liquid crystal fluid
does not substantially change the polarization state of light
passing through said liquid crystal fluid such that the
polarization directions of the linearly polarized incident light
and the linearly polarized reflected light at said first substrate
are approximately parallel.
[0027] The present invention is also directed to a reflective
liquid crystal display assembly comprising: a first substantially
transparent substrate; a second substantially reflective substrate
located substantially parallel to said first substrate; a liquid
crystal fluid having a birefringence (.DELTA.n) and a negative
dielectric anisotropy, wherein said liquid crystal fluid is between
said first and second substrates; first and second liquid crystal
alignment layers on said first and second substrates, respectively,
wherein molecules of said fluid proximate the first and second
liquid crystal alignment layers have finite pretilt angles and are
oriented in first and second azimuthal directions, respectively;
wherein the assembly is configured such that (i) when substantially
no electric field is applied between the substrates, a substantial
number of fluid molecules are oriented substantially perpendicular
to said substrates, (ii) when an electrical field of optimum value
is applied between the substrates, tilt angles of a substantial
number of fluid molecules increase, and (iii) when a less than
optimum electric field is applied between said substrates, a
substantial number of fluid molecules are oriented at intermediate
tilt angles.
[0028] Other technical advantages of the present disclosure will be
readily apparent to one skilled in the art from the following
figures, descriptions, and claims. Various embodiments of the
invention obtain only a subset of the advantages set forth. No one
advantage is critical to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] A more complete understanding of the present disclosure and
advantages thereof may be acquired by referring to the following
description taken in conjunction with the accompanying drawings,
wherein:
[0030] FIGS. 1a and 1b are schematic representations of a prior art
VAN mode liquid crystal display in the "OFF" and "ON" states,
respectively;
[0031] FIG. 2 is a schematic elevational view of a portion of the
liquid crystal display showing the azimuthal direction and pretilt
angles of an exemplary molecule in a liquid crystal fluid;
[0032] FIGS. 3a and 3b are schematic representations of a liquid
crystal display in the "OFF" and "ON" states, respectively,
according to the present invention; and
[0033] FIG. 4 is a collection of graphical representations of the
tilt angles and azimuthal angles of liquid crystal fluid molecules
as a function of the molecules' locations between the substrates in
"OFF," and "ON" states, according to the present invention.
[0034] While the present invention is susceptible to various
modifications and alternative forms, specific exemplary embodiments
thereof have been shown by way of example in the drawings and are
herein described in detail. It should be understood, however, that
the description herein of specific embodiments is not intended to
limit the invention to the particular forms disclosed, but on the
contrary, the intention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0035] The present invention is directed to a reflective liquid
crystal microdisplay comprising a matrix of pixels of a liquid
crystal fluid having light modifying properties controlled by
voltage values stored in capacitors comprising the areas
representing the pixels in the matrix of pixels of the liquid
crystal microdisplay. The molecules of the liquid crystal fluid
have a surface azimuthal direction that is either approximately
parallel or perpendicular to the direction of polarization of
incoming incident linearly polarized light.
[0036] According to the present invention, when in the "OFF" state
(no electric field across the pixel capacitors), the molecules of
the liquid crystal fluid have pretilt angles (measured from the
perpendicular directions of the faces of the substrates) just
sufficient to minimize disclinations due to fringe fields. The
azimuthal direction of the molecules of the liquid crystal fluid at
the transparent substrate is at approximately 90 degrees from the
azimuthal direction of the molecules of the liquid crystal fluid at
the reflective substrate. This configuration results in a very dark
state when in the "OFF" state, resulting in high contrast.
[0037] Application of a voltage to the liquid crystal layer changes
the tilt of the liquid crystal molecules in the bulk of the layer
in a direction towards parallel to the substrates, due to the
negative dielectric anisotropy of the material (see FIG. 4).
[0038] Referring now to the drawings, the details of exemplary
embodiments of the invention are schematically illustrated. Like
elements in the drawings will be represented by like numbers, and
similar elements will be represented by like numbers with a
different lower case letter suffix. Referring to FIGS. 1a and 1b,
depicted are schematic representations of a prior art VAN mode
liquid crystal display in the "OFF" and "ON" states, respectively.
The azimuthal direction of the liquid crystal molecules defined by
the alignment layers on the substrate surfaces 102, 104, are
anti-parallel to each other, and are at a 45 degree angle with the
direction of polarization of incoming incident linearly polarized
light. In a VAN mode liquid crystal display, the pretilt angle
.theta. needs to be kept small, e.g., less than 4 degrees, to
achieve a very dark "OFF" state, hence high contrast.
[0039] Referring now to FIG. 2, depicted is a schematic elevational
view of a portion of a liquid crystal display showing the azimuthal
direction angle (.PHI.) and tilt angle (.theta.) of an exemplary
molecule in a liquid crystal fluid. A glass (transparent) substrate
202 and a reflective (mirror) substrate 204 are parallel and have
liquid crystal fluid, generally represented by the numeral 206,
therebetween. The distance (thickness of liquid crystal fluid)
between these parallel substrates is generally represented by "d."
The distance (thickness), d, preferably may be about 3.5
.mu.m+/-0.2 .mu.m. The liquid crystal fluid 206 preferably may have
a birefringence (.DELTA.n) of about 0.0830 at 45.degree. C. Liquid
crystal fluids that are being used in the present invention are
nematic and have negative dielectric anisotropy
.DELTA..epsilon.(=.epsilon..sub.//-.epsilon..sub.195 <0), where
.epsilon..sub.// and .epsilon..sub..perp. are the parallel and
perpendicular (to the nematic director) components of the
dielectric constant of the liquid crystal fluid. It is contemplated
and within the scope of the present invention that any combination
of distance (thickness) (d) and birefringence (.DELTA.n) may be
used with the condition: .DELTA.n.multidot.d>.lambda./4 is
approximately satisfied for an efficient bright state at a
convenient voltage, (where .lambda. is the wavelength of light
incident on the display). For example, with a cell gap of
approximately 3.5 .mu.m, using MLC-6608, and a pretilt of
approximately 8 degrees from surface-normal, the optimum bright
state voltage is approximately 4.0V for Red (640 nm), 3.5V for
Green (540 nm) and 3.15V for Blue (470 nm).
[0040] A single molecule 206a of the liquid crystal fluid 206 is
shown for exemplary purposes. The tilt angle .theta. is measured
from the Z-axis which is perpendicular to the substrates 202 and
204. The azimuthal angle .PHI. is measured from the X-axis in the
XY plane and is the angle between the X axis and the projection of
the molecules of the liquid crystal fluid 206 on the XY plane.
According to the present invention, polarization direction of the
linearly polarized incident light at the glass substrate will be
either approximately parallel or perpendicular to the azimuthal
direction generated at the glass substrate 202.
[0041] Referring to FIG. 3a, depicted is a schematic representation
of a liquid crystal display in the "OFF" state, according to the
present invention. The molecules of the liquid crystal fluid 206
are depicted in the "OFF" state wherein the pretilt angle .theta.
(defined above) is only large enough to remove disclinations due to
fringe fields. Preferably the pretilt angle .theta. may be from
about 2 degrees to about 15 degrees. Most preferably the pretilt
angle .theta. may be from about 5 degrees to about 15 degrees. The
polarization state of the incident light 308 will not be affected
substantially by the molecules of the liquid crystal fluid 206 and
will be reflected back from the substrate 204 as substantially
linearly polarized light with the polarization direction
substantially parallel with that of the incident linearly polarized
light.
[0042] Referring to FIG. 3b, depicted is a schematic representation
of a liquid crystal display in the "ON" state, according to the
present invention. The configuration of the liquid crystal fluid
206, formed under optimum voltage drive, will cause the linearly
polarized incident light entering the substrate 202 to become
approximately circularly polarized light at the substrate 204. The
approximately circularly polarized incident light at the substrate
204 will be reflected back as approximately circularly polarized
light, but with opposite handedness, and when it reaches the
substrate 202, this reflected light will be approximately linearly
polarized with a polarization direction approximately perpendicular
to that of the linearly polarized incident light.
[0043] Referring now to FIG. 4, depicted are graphical
representations of the tilt angles and azimuthal angles of
molecules of the liquid crystal fluid 206 as a function of the
location of these molecules located between the substrates in "OFF"
and "ON" states, according to the present invention. Tilt angle
.theta. is depicted on the left vertical axis from 0 to 90 degrees,
and the azimuthal angle .PHI. is depicted on the right vertical
axis from 0 to 90 degrees. The locations of the molecules of the
liquid crystal fluid 206 between the substrates 202 and 204 are
depicted on the horizontal axis from 0 to d. Curve 402 depicts the
tilt angle .theta. of the molecules of the liquid crystal fluid 206
when they are in the "OFF" state. Curve 404 depicts the azimuthal
angle .theta. of the molecules of the liquid crystal fluid 206 when
they in the "OFF" state. Curve 410 depicts the tilt angle .theta.
of the molecules of the liquid crystal fluid 206 when they are in
the "ON" state. Curve 412 depicts the azimuthal angle .PHI. of the
molecules of the liquid crystal fluid 206 when they are in the "ON"
state. Corresponding tilt curves for an intermediate gray shade
would fall between curves 402 and 410 and azimuthal curves would
fall between curves 404 and 412.
[0044] The invention, therefore, is well adapted to carry out the
objects and attain the ends and advantages mentioned, as well as
others inherent therein. While the invention has been depicted,
described, and is defined by reference to exemplary embodiments of
the invention, such references do not imply a limitation on the
invention, and no such limitation is to be inferred. The invention
is capable of considerable modification, alternation, and
equivalents in form and function, as will occur to those having
ordinarily skills in the pertinent arts and having the benefit of
this disclosure. The depicted and described embodiments of the
invention are exemplary only, and are not exhaustive of the scope
of the invention. Consequently, the invention is intended to be
limited only by the spirit and scope of the appended claims, giving
full cognizance to equivalents in all respects.
* * * * *