U.S. patent application number 10/929702 was filed with the patent office on 2005-02-03 for liquid crystal reflective display.
Invention is credited to Leenhouts, Frans, Van De Witte, Peter, Vogels, Joost Peter Andre.
Application Number | 20050024564 10/929702 |
Document ID | / |
Family ID | 8180749 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050024564 |
Kind Code |
A1 |
Vogels, Joost Peter Andre ;
et al. |
February 3, 2005 |
Liquid crystal reflective display
Abstract
This invention relates to a liquid crystal display device,
comprising at least one retardation film (4), a nematic liquid
crystal layer (5), and a light reflecting RGB patterned cholesteric
colour filter (6), for reflecting either essentially left-circular
or right-circular polarised light, being characterised in that said
liquid crystal layer (5) is a super twisted nematic liquid crystal
layer having a twist angle in the interval 180-270 degrees, and in
that the summed retardation R of the retardation film or
retardation films and the liquid crystal layer is equal to
approximately R=(3+2n).lambda./4, where n=0,1,2,3.
Inventors: |
Vogels, Joost Peter Andre;
(Eindhoven, NL) ; Leenhouts, Frans; (Heerlen,
NL) ; Van De Witte, Peter; (Heerlen, NL) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Family ID: |
8180749 |
Appl. No.: |
10/929702 |
Filed: |
August 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10929702 |
Aug 30, 2004 |
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10208348 |
Jul 30, 2002 |
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6795149 |
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Current U.S.
Class: |
349/117 |
Current CPC
Class: |
G02F 1/1397 20130101;
G02F 1/133533 20130101; G02F 1/133536 20130101; G02F 2203/02
20130101; G02F 1/133516 20130101 |
Class at
Publication: |
349/117 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2001 |
EP |
01202951.8 |
Claims
1. A liquid crystal display device, comprising: a polariser (1), at
least one retardation film (4), a nematic liquid crystal layer (5),
and a, preferably cross-linked, light reflecting RGB patterned
cholesteric colour filter (6), characterized in that said liquid
crystal layer (5) is a super twisted nematic liquid crystal layer
having a twist angle in the interval 180-270 degrees, and in that
the summed retardation R of the retardation film or the retardation
films and the liquid crystal layer is equal to approximately
R=(3+2n).lambda./4, where n=0,1,2,3 . . . .
2. A liquid crystal display device as in claim 1, wherein said at
least one retardation film (4) is arranged between said polariser
(1) and said super twisted nematic liquid crystal layer (5).
3. A liquid crystal display device as in claim 2, wherein said at
least one retardation film (4) comprises a quarterwave plate.
4. A liquid crystal display device as in claim 2, wherein said at
least one retardation film comprises a wideband quarterwave
plate.
5. A liquid crystal display device as in claim 4, wherein said
wideband quarterwave plate consists of two retardation films with
different optical axes and retardations.
6. A liquid crystal display device as in claim 1, wherein the
polariser (1), the at least one retardation film (4), the liquid
crystal layer (5), and the, preferably cross-linked, light
reflecting RGB patterned cholesteric colour filter (6) are arranged
on each other in sequence.
7. A liquid crystal display device as in claim 1, wherein the
absorption axis of said polariser (1) makes an angle of about
105.degree.-115.degree. with the vertical y-axis of the device,
wherein said device comprises a first and a second retardation film
(2,3), said first retardation film (2) having a retardation of
essentially 280 nm and making an angle of essentially 170.degree.
with said vertical y-axis, and said second retardation film (3)
having a retardation of esentially 140 nm and having a slow axis
making an angle of about 25.degree.-35.degree. with said vertical
y-axis,said first and second retardation films together
constituting a wide-band quarter-wave plate resulting in an
improved overall dark state, whereby the liquid crystal layer (5)
is switchable over essentially 315 nm (.apprxeq.{fraction
(l/2)}.multidot..lambda.)and comprises a right-handed
235.degree.-245.degree. twisted nematic layer with an optical
thickness of essentially 0,76 .mu.m, and whereby a left-handed
colour filter is arranged on the top of a back substrate (8).
8. A liquid crystal display device as in claim 1, wherein the
absorption axis of said polariser (1) makes an angle of about
115.degree.-125.degree. with the vertical y-axis of the device,
wherein said device comprises a first and a second retardation film
(2,3), said first retardation film (2) having a retardation of
essentially 280 nm and making an angle of about .+-.5.degree. with
said vertical y-axis, and said second retardation film (3) having a
retardation of esentially 140 nm and having a slow axis making an
angle of about 25.degree.-35.degree. with said vertical y-axis,said
first and second retardation films together constituting a
wide-band quarter-wave plate resulting in an improved overall dark
state, whereby the liquid crystal layer (5) is switchable over
essentially 315 nm (.apprxeq.1/2.multidot..lambda.)and comprises a
right-handed essentially 235.degree.-245.degree. twisted nematic
layer with an optical thickness of essentially 0,76 .mu.m, and
wherby a left-handed colour filter is arranged on the top of a back
substrate (8).
9. A liquid crystal display device as in claim 1, wherein said
device comprises a further retardation film being a twisted nematic
polymeric layer.
10. A liquid crystal display device, wherein said twisted nematic
polymeric layer and said super twisted nematic layer (5) have
mutually opposite twist directions.
11. A liquid crystal display device as in claim 9 or 10, wherein
the absorption axis of the polariser (1) makes an angle of
essentially 115.degree.-125.degree. with the vertical y-axis of the
device, wherein said device comprises a first and a second
retardation film (2,3), said first retardation film (2) having a
retardation of essentially 280 nm and making an angle of
essentially 0.degree. with said vertical y-axis, and said second
retardation film (3) having a retardation of esentially 140 nm and
having a slow axis making an angle of esentially
25.degree.-35.degree. with said vertical y-axis,said first and
second retardation films together constituting a wide-band
quarter-wave plate resulting in an improved overall dark state,
whereby said twisted nematic polymeric layer, being placed closer
to the polariser than said liquid crystal layer (5), comprises a
right-handed 230-250.degree. twisted nematic layer with an optical
thickness of 0,75-0,85 .mu.m, said liquid crystal layer being
switchable over essentially 315 nm
(.apprxeq.1/2.multidot..lambda.)and comprises a left-handed
essentially -235.degree.--245.degree. twisted nematic layer with an
optical thickness of essentially 0,82 .mu.m, and whereby a
left-handed colour filter is arranged on the top of a back
substrate (8).
Description
[0001] The present invention relates to a liquid crystal display
device, comprising at least one retardation film, a nematic liquid
crystal layer, and a reflective cholesteric colour filter
layer.
[0002] The market share of liquid crystal displays (LCD) is
continuously increasing at the cost of other display technologies.
This progress can be attributed to continuous improvements in size
and performance and the decrease in the price of the panels. The
design of LCDs for portable applications like cellular telephones
and for demanding applications like PDAs differs strongly from the
design of LCDs for high-end applications like monitors and TVs. For
displays in telecom applications, low cost and low power are
equally important as display image quality. Many challenges still
remain to further improve the properties of LCDs. A performance
issue that must be improved is, for example, the quality of the
colours for reflective passive matrix displays, as initially
described. Such display panels based on cholesteric colour filters
are an attractive alternative for absorbing colour filters.
[0003] A passive matrix display with reflective colour filters in
accordance with the prior art is described in the patent document
U.S. Pat. No. 5,555,114. This document discloses a liquid crystal
display including a cholesteric liquid crystal layer for
selectively reflecting circularly polarised light having a specific
wavelength. One embodiment of the inventive display basically
contains a polariser, a quarter wave-plate, a liquid crystal layer,
and a cholesteric colour filter layer, in sequence. The driven
liquid crystal layer may be a layer of ferro-electric, twisted
nematic or super twisted nematic liquid crystal.
[0004] However, this display, as described in the patent document
U.S. Pat No. 5,555,114, has the problem that for twisted nematic
liquid crystal, a display of high contrast that cover the entire
range of the visible spectrum cannot be obtained.
[0005] Consequently, the object of the present invention is to
provide a display device with twisted nematic liquid crystals,
overcoming the above-described problems with the prior art, yet
maintaining a fairly simple structure.
[0006] These and other objects are achieved by a liquid crystal
display device, comprising a polariser, at least one retardation
film, a nematic liquid crystal layer being comprised in an
electro-optical cell, and a, preferably cross-linked, light
reflective cholesteric colour filter layer, for reflecting either
essentially left-circular or right-circular polarised light, being
characterised in that said liquid crystal layer is a super twisted
nematic liquid crystal layer having a twist angle in the interval
180-270 degrees, and in that the summed retardation R of the
retardation film or the retardation films and the liquid crystal
layer is equal to approximately R=(3+2n).lambda./4, where n=0,1,2,3
. . . . By utilising super twisted nematic liquid crystals in the
display, a multiplexable display is achieved.
[0007] Moreover, said retardation film or retardation films are
preferably arranged between a front polariser and said super
twisted nematic liquid crystal layer.
[0008] Said retardation layer preferably comprises a wide-band
quarter-wave plate. Suitably, said wide-band quarter-wave plate
consists of a quarter wave plate in combination with a half wave
plate, thereby enabling the use of standard, well-tested layer
components. Suitably, the front polariser, the retardation film or
retardation films, the liquid crystal layer and the reflective
cholesteric colour filter layer are arranged on each other in
sequence.
[0009] In accordance with a preferred embodiment of the invention,
said device further comprises a further retardation layer which is
a twisted nematic polymeric film, being arranged between the front
substrate and the super twisted nematic layer, said twisted nematic
polymeric layer and super twisted nematic layer have mutually
opposite twist directions.
[0010] Further preferred embodiments of the invention are described
in the remaining sub-claims.
[0011] A currently preferred embodiment of the present invention
will now be described in closer detail, with reference to the
accompanying drawings.
[0012] FIG. 1 is a cross-section view of a display in accordance
with the invention.
[0013] FIG. 2 is a diagram showing the optical principle mechanism
of a display in accordance with FIG. 1.
[0014] FIG. 3a is a diagram showing the optical principle mechanism
in accordance with a computer simulation of a display in accordance
with the invention, being optimised for obtaining high
brightness.
[0015] FIG. 3b is a diagram showing the transmission in relation to
wavelength for incident light, for a display device in accordance
with FIG. 3a.
[0016] FIG. 4a is a diagram showing the optical principle mechanism
in accordance with a computer simulation of a display in accordance
with the invention, being optimised for obtaining high
contrast.
[0017] FIG. 4b is a diagram showing the transmission in relation to
wavelength for incident light, for a display device in accordance
with FIG. 4a.
[0018] FIG. 1 is a schematic, cross-sectional view of a part of a
liquid-crystal reflective display device, comprising a display cell
in this case comprising a super twisted nematic liquid crystal
layer 5, also referred to as a LC layer, having a twist angle
within the interval 180.degree.-270.degree., essentially being
sandwiched between two glass substrates, a front substrate 7 and a
back substrate 8. On the side of the front substrate opposing said
liquid crystal layer 5, a polariser 1 and one or more retardation
films 4 are arranged. Further, here said combination of retardation
films 4 comprises a half wave-plate 2 and a quarter wave-plate 3.
Between said front substrate 7 and said super twisted nematic
liquid crystal layer 5, a first electrode layer 10 and a LC
orientation layer 12 are arranged in sequence. Further, between
said highly twisted nematic liquid crystal layer 5 and said back
substrate 8, an LC orientation layer 13 a second electrode layer
11, and a reflective cholesteric colour filter layer 6 are arranged
in sequence. On the side of the back substrate opposing said
reflective cholesteric colour filter layer 6 an absorption layer 9
is arranged. Said first and second electrode layers 10, 11 and the
orientation layers 12, 13 are formed in accordance with prior art
for super twisted nematic displays, and will not be described
closer herein. However, said first and second electrode layers 10,
11 are manufactured by a transparent material, such as indium tin
oxide (ITO), and are connected with a power source 14, in order to
enable the generation of an electric field over the super twisted
nematic liquid crystal layer 5 to create an ON and an OFF state of
said display on per se known manner. Further, the orientation
layers are provided on both sides of the super twisted nematic
liquid crystal layer in order to give the layer a desired stable
orientation. The orientation layers 12, 13 may for example comprise
a polymer, e.g. PI, being treated in accordance with prior art.
[0019] The above-described reflective cholesteric colour filter
layer 6 basically combines a reflector function, a polariser
function and a colour filter function. The cholesteric liquid
crystal phase is a chiral mesophase having a natural periodicity.
The liquid crystals are arranged in a screwlike fashion. When the
product of the birefringence of the cholesteric material and the
periodicity of the liquid crystals matches the wavelength of light
in the material i.e.
.lambda.=n.multidot.p
[0020] (where .lambda. is the wavelength of the incident light, n
is the refraction index of the material and p is the pitch of the
material) the light will be reflected by means of Bragg reflection.
Further, the screwlike liquid crystal is either left- or
right-handed. Depending on the handedness of the cholesteric liquid
crystals, either left-handed or right-handed circularly polarised
light will be reflected. Normally, the cholesteric layer will
reflect only light with wavelengths between p.multidot.n.sub.e and
p.multidot.n.sub.o, where n.sub.e and n.sub.o are the extraordinary
and the ordinary refractive indices of a uniaxially oriented phase
respectively. Preferably, said reflective cholesteric colour filter
layer is based on a photosensitive cholesteric material. For this
kind of materials, the periodicity of the cholesteric phase can be
influenced by UV light. The cholesteric colour filter can be
patternwise exposed to UV light with a greyscale mask. This process
is advantageous in that only one exposure step is necessary in
order to generate colours as described in patent document
WO/0034808. This is a strong reduction compared to the
manufacturing process of the conventional absorbing colour filter
which usually involves three litographic steps for the subsequent
deposition of red, green and blue filters.
[0021] As described above, the retardation film 4 (also referred to
as retardation layer) comprises two films, a half wave-plate 2, and
a quarter wave-plate 3, together constituting a wide-band
quarter-wave plate 4, resulting in a better overall dark state.
[0022] A more schematic drawing, disclosing the principal optically
active components of the display is disclosed in FIG. 2.
[0023] A first embodiment of this invention, as shown in FIGS. 3a
and 3b, will hereinafter be described. This configuration, as
schematically shown in FIG. 3a, comprises a right-handed
240.degree. twisted nematic layer with an optical thickness of 0,76
.mu.m. The display is of the normally black type. In this mode the
off-state of the driven LC layer 5 is dark and the on-state is
bright. Although this embodiment is described for the normally
black mode, it is also possible to make a configuration in the
normally white mode. Between the front polariser 1 and the front
substrate 7, the above two retardation films 2, 3 are inserted. A
film 3 with the retardation of 140 nm in combination with a
polariser makes circularly polarised light. By adding a half-wave
plate 2 of 280 mn in between, a wide-band quarter-wave plate is
created, resulting in a better overall dark state. The slow axis of
the film 3, which is adjacent to the substrate, makes an angle of
30.degree. with the vertical y-axis of the device. The half-wave
plate 2 makes an angle of 170.degree. with the vertical y-axis. The
absorption axis of the polariser makes an angle of 110.degree. with
the vertical y-axis. The LC layer 5 switches over 315 nm
(.apprxeq.1/2.multidot..lambda.). On the top of the back substrate
8, a left-handed colour filter is used. By using the above
configuration a display, optimised for the highest brightness of
the on-state is obtained, as seen in FIG. 3b.
[0024] A second embodiment of this invention, as shown in FIGS. 4a
and 4b, will hereinafter be described. This configuration, as
schematically shown in FIG. 4a, comprises a right-handed
240.degree. twisted nematic layer with an optical thickness of 0,76
.mu.m. The display is of the normally black type. In this mode the
off-state of the driven LC layer is dark and the on-state is
bright. Although this embodiment is described for the normally
black mode, it is also possible to make a configuration in the
normally white mode. Between the front polariser 1 and the front
substrate 7, the above two retardation films 2, 3 are inserted. A
film 3 with the retardation of 140 nm in combination with a
polariser makes circularly polarised light. By adding a half-wave
plate 2 of 280 nm in between, a wide-band quarter-wave plate is
created, resulting in a better overall dark state. The slow axis of
the film 3, which is adjacent to the substrate, makes an angle of
30.degree. with the vertical y-axis. The half-wave plate 2 makes an
angle of 0.degree. with the vertical y-axis. The absorption axis of
the polariser makes an angle of 120.degree. with the vertical
y-axis. The LC layer 5 switches over 315 nm
(.apprxeq.1/2.multidot..lambda.). On the top of the back substrate
8, a left-handed colour filter is used. By using the above
configuration a display, optimised for the highest contrast ratio
is obtained, as seen in FIG. 4b.
[0025] In accordance with a third embodiment of the invention (not
shown) the above described single super twisted nematic LC layer is
exchanged for two super twisted nematic LC layers, having opposite
twisting directions. In this specific example, the first layer,
placed closer to the polariser, comprises a right-handed
230-250.degree. (for example 245.degree.) twisted nematic layer
with an optical thickness of 0,75-0,85 .mu.m (for example 0,79
.mu.m) and the second layer, placed closer to the cholesteric
colour filter layer, comprises a left-handed -240.degree. twisted
nematic layer with an optical thickness of 0,82 .mu.m. However,
many 9other configurations are possible. The display is of the
normally black type. In this mode the off-state of the driven LC
layer is dark and the on-state is bright. Although this embodiment
is described for the normally black mode, it is also possible to
make a configuration in the normally white mode. Between the front
polariser 1 and the front substrate 7, the above two retardation
films 2, 3 are inserted. A film 3 with the retardation of 140 nm in
combination with a polariser makes circularly polarised light. By
adding a half-wave plate 2 of 280 nm in between, a wide-band
quarter-wave plate is created, resulting in a better overall dark
state. The slow axis of the film 3, which is adjacent to the
substrate, makes an angle of 40.degree. with the vertical y-axis.
The half-wave plate 2 makes an angle of 12.degree. with the
vertical y-axis. The absorption axis of the polariser makes an
angle of 100.degree. with the vertical y-axis. On the top of the
back substrate 8, a left-handed colour filter layer is used.
[0026] The present invention should not be considered as being
limited to the above-described embodiment, but rather includes all
possible variations covered by the scope defined by the appended
claims. Many combinations of twist angle and optical thickness
being suitable for reflective STNs with cholesteric colour filters,
besides the ones described above, are possible in order to optimise
the display for different purposes. Consequently, the invention is
by no means restricted to a twist angle of 240.degree., but other
configurations differing in twist angle and optical thickness may
be combined with cholesteric colour filters to give either normally
black or normally white multiplexable reflective colour
displays.
[0027] Further, it shall be understood that the above-specified
numerical values for angles, retardations etc. are not absolute,
but may vary with in an interval of at least approximately 10%.
* * * * *