U.S. patent application number 10/522170 was filed with the patent office on 2005-11-24 for negative retardation film.
This patent application is currently assigned to Merck Patent GmbH. Invention is credited to Graham, Donald Gordon, Parri, Owain Llyr, Perrett, Tara, Skjonnemand, Karl.
Application Number | 20050259200 10/522170 |
Document ID | / |
Family ID | 31197775 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050259200 |
Kind Code |
A1 |
Skjonnemand, Karl ; et
al. |
November 24, 2005 |
Negative retardation film
Abstract
The invention relates to a negative birefringent retardation
film comprising polymerised liquid crystal (LC) material with
helically twisted structure and planar orientation, its use in
compensators and electrooptical displays like liquid crystal
displays, and to compensators and liquid crystal displays
comprising such a negative retardation film.
Inventors: |
Skjonnemand, Karl; (Holburn,
GB) ; Parri, Owain Llyr; (Poole, GB) ; Graham,
Donald Gordon; (Corfe Mullen, GB) ; Perrett,
Tara; (Bournemouth, GB) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Assignee: |
Merck Patent GmbH
Darmstadt
DE
64271
|
Family ID: |
31197775 |
Appl. No.: |
10/522170 |
Filed: |
January 24, 2005 |
PCT Filed: |
July 11, 2003 |
PCT NO: |
PCT/EP03/07532 |
Current U.S.
Class: |
349/120 |
Current CPC
Class: |
G02B 5/3016 20130101;
G02F 2413/15 20130101; G02F 1/133634 20130101 |
Class at
Publication: |
349/120 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 25, 2002 |
EP |
02016332.5 |
Claims
1. Negative birefringent retardation film comprising polymerised
liquid crystal (LC) material with helically twisted structure and
planar orientation, wherein the helical pitch of the LC material is
200 nm or less.
2. Film according to claim 1, wherein the helical pitch is from
>50 nm to 200 nm.
3. Film according to claim 2, wherein the helical pitch is from 55
to 175 nm.
4. Film according to claim 1, wherein the polymerised LC material
is polymerised or crosslinked chiral nematic or cholesteric LC
material.
5. Film according to claim 1, wherein the LC director at one
surface is parallel or antiparallel to the LC director at the
opposite surface.
6. Combination comprising a negative birefringent retardation film
according to claim 1 and a linear polariser, wherein the director
at the surface of said retardation film facing said polariser and
the polarisation direction of said polariser are oriented at an
angle of from 30 to 60.degree. relative to each other.
7. Combination according to claim 6, wherein said angle is
45.degree..
8. Use of a negative retardation film or a combination according to
claim 1 in compensators and electrooptical displays or liquid
crystal displays.
9. Compensator comprising a negative retardation film or a
combination according to claim 1.
10. Liquid crystal display comprising a negative retardation film
or a combination according to claim 1.
11. Liquid crystal display according to claim 10, characterized in
that it is a display of the VA (vertically aligned), MVA
(multi-domain vertically aligned), PVA (patterned vertically
aligned), ECB (electrically controlled birefringence), TN (twisted
nematic), HTN (highly twisted nematic) or STN (super twisted
nematic) mode.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a negative retardation film with
improved performance, its use in displays and optical elements, and
to optical elements and displays comprising such a film.
[0002] Definition of Terms
[0003] In connection with polarisation, compensation and
retardation layers, films or plates as described in the present
application, the following definitions of terms as used throughout
this application are given.
[0004] The term `film` as used in this application includes
self-supporting, i.e. free-standing, films that show more or less
pronounced mechanical stability and flexibility, as well as
coatings or layers on a supporting substrate or between two
substrates.
[0005] The term `liquid crystal or mesogenic material` or `liquid
crystal or mesogenic compound` should denote materials or compounds
comprising one or more rod-shaped, board-shaped or disk-shaped
mesogenic groups, i.e. groups with the ability to induce liquid
crystal phase behaviour. Liquid crystal compounds with rod-shaped
or board-shaped groups are also known in the art as `calamitic`
liquid crystals. Liquid crystal compounds with a disk-shaped group
are also known in the art as `discotic` liquid crystals. The
compounds or materials comprising mesogenic groups do not
necessarily have to exhibit a liquid crystal phase themselves. It
is also possible that they show liquid crystal phase behaviour only
in mixtures with other compounds, or when the mesogenic compounds
or materials, or the mixtures thereof, are polymerised.
[0006] For the sake of simplicity, the term `liquid crystal
material` is used hereinafter for both liquid crystal materials and
mesogenic materials, and the term `mesogen` is used for the
mesogenic groups of the material.
[0007] The term `director` is known in prior art and means the
preferred orientation direction of the long molecular axes (in case
of calamitic compounds) or short molecular axis (in case of
discotic compounds) of the mesogens in a liquid crystal
material.
[0008] The term `planar structure` or `planar orientation` refers
to a film wherein the optical axis is substantially parallel to the
film plane.
[0009] The term `homeotropic structure` or `homeotropic
orientation` refers to a film wherein the optical axis is
substantially perpendicular to the film plane, i.e. substantially
parallel to the film normal.
[0010] The terms `tilted structure` or `tilted orientation` refers
to a film wherein the optical axis is tilted at an angle .theta.
between 0 and 90 degrees relative to the film plane.
[0011] The term `splayed structure` or `splayed orientation` means
a tilted orientation as defined above, wherein the tilt angle
additionally varies monotonuously in the range from 0 to
90.degree., preferably from a minimum to a maximum value, in a
direction perpendicular to the film plane.
[0012] In planar, homeotropic and tilted optical films comprising
uniaxially positive birefringent liquid crystal material with
uniform orientation, the optical axis of the film is given by the
director of the liquid crystal material.
[0013] The term `helically twisted structure` relates to a film
comprising one or more layers of liquid crystal material wherein
the mesogens are oriented with their main molecular axis in a
preferred direction within molecular sublayers, said preferred
orientation direction in different sublayers being twisted at an
angle .phi. around a helix axis. The term `helically twisted
structure with planar orientation` means a film with helically
twisted structure as described above, wherein the helix axis is
substantially perpendicular to the film plane, i.e. substantially
parallel to the film normal.
[0014] The term `A plate` refers to an optical retarder utilizing a
layer of uniaxially birefringent material with its extraordinary
axis oriented parallel to the plane of the layer, and its ordinary
axis (also called `a-axis`) oriented perpendicular to the plane of
the layer, i.e. parallel to the direction of normally incident
light.
[0015] The term `C plate` refers to an optical retarder utilizing a
layer of a uniaxially birefringent material with its extraordinary
axis (also called `c-axis`) perpendicular to the plane of the
layer, i.e. parallel to the direction of normally incident
light.
[0016] The term `O plate` refers to an optical retarder utilizing a
layer of a uniaxially birefringent material with its extraordinary
axis oriented at an oblique angle with respect to the plane of the
layer.
[0017] An A plate or C plate comprising optically uniaxial
birefringent material with positive birefringence is also referred
to as `+A/C plate` or `positive A/C plate`. An A plate or C plate
comprising a film of optically uniaxial birefringent material with
negative birefringence is also referred to as `-A/C plate` or
`negative A/C plate`.
[0018] A retardation film with positive or negative birefringence
is also shortly referred to as `positive` or `negative` retardation
film, respectively.
[0019] The Japanese term "mura" literally means unevenness. This
term is widely used and known to the expert in the field of liquid
crystal displays, and describes optical non-uniformities in liquid
crystal displays or liquid crystal films (see for example
"Automatic blemish detection in liquid crystal flat panel displays"
by William K. Pratt et al., SPIE Proceedings 3306-01, pp. 2-13).
Mura can be measured for example by an instrument of Eldim SA
(Herouville Saint Clair, France) commercially available as
MURATest.RTM., which measures unevenness of luminance and
colour.
BACKGROUND AND PRIOR ART
[0020] Retardation films with negative birefringence for use in
liquid crystal displays (LCD) are known in prior art.
[0021] For example, in LCDs of the vertically aligned (VA) mode it
is necessary to compensate light leakage that occurs in the dark
state where the LC layer in the display cell is the non-selected,
vertically aligned or homeotropic state. This can be achieved
according to prior art by the use of a negative retardation
film.
[0022] Compensated LCDs comprising a negative retardation film have
been reported in prior art. For example, U.S. Pat. No. 6,141,075
discloses a VA-LCD comprising a positive and a negative
retarder.
[0023] It is also known in prior art to use negative retarders
comprising polymerised cholesteric LC (CLC) material that has a
short helical pitch and as a result shows a Bragg reflection band
in the UV range of light (UV-CLC). Such a UV-CLC layer retards
visible light with negative C symmetry and thus can be used as an
optical negative C retarder in the visible region of the spectrum.
For example, WO 01/20393 and WO 01/20394 disclose a compensator
comprising a positive and a negative retarder for use in an LCD of
the VA- or the twisted nematic (TN) mode, wherein the negative
retarder is for example made of polymerised UV-CLC material.
[0024] However in LC displays and LC films often the so-called
"mura" effect is observed. For example, in case of a UV-CLC film,
between crossed polarisers some areas of the UV-CLC films appear
lighter than others, producing a speckled, non-uniform dark state.
This can have a significant negative influence on the performance
of UV-CLC films when used as compensator in LCDs.
[0025] The aim of the present invention is to provide a negative
retarder in particular for use in LC displays that does not have
the drawbacks of retardation films of prior art, shows reduced
"mura" effect, is easy to manufacture and allows economic
fabrication even at large scales.
[0026] Another aim is to provide a compensator comprising a
negative retarder which shows good optical performance when used in
LC displays, in particular improved grey level stability at wide
viewing angles.
[0027] A further aim of this invention is to provide an
advantageous use of a negative retarder according to this
invention.
[0028] Further aims of this invention relate to liquid crystal
displays, in particular to VA mode LCDs, comprising a retardation
film according to the invention which show advantageous properties
such as good contrast, reduced colour shift and wide viewing
angles.
[0029] Other aims of the present invention are immediately evident
to the person skilled in the art from the following detailed
description.
[0030] The above aims can be achieved by providing retarders and
liquid crystal displays according to the present invention.
[0031] The inventors of the present invention have found that by
using a UV-CLC film with a reduced helical pitch, in particular
with a pitch of 200 nm or less, as negative retarder the mura
effect can be significantly reduced. Furthermore it was found that,
in an LCD comprising at least one negative UV-CLC retarder, a
liquid crystal cell and two linear polarisers, the mura effect can
be reduced by specific arrangment of the UV-CLC film. If the UV-CLC
film is positioned such that the director orientation at its
surface facing the nearest polariser is close to 45.degree.
relative to the polarisation direction, the mura effect is
minimised.
SUMMARY OF THE INVENTION
[0032] The present invention relates to a negative retardation film
comprising polymerised liquid crystal (LC) material with helically
twisted structure and planar orientation, wherein the helical pitch
of the LC material is 200 nm or less.
[0033] The invention further relates to a combination of a negative
retardation film comprising polymerised LC material with helically
twisted structure and planar orientation and a linear polariser,
wherein the director at the surface of said retardation film facing
said polariser and the polarisation direction of said polariser are
oriented at an angle of from 30 to 60.degree. relative to each
other.
[0034] The invention further relates to the use of a negative
retardation film or a combination according to the invention in
compensators and electrooptical displays like liquid crystal
displays.
[0035] The invention further relates to a compensator or a liquid
crystal display comprising a negative retardation film or a
combination of a negative retardation film and a linar polariser
according to the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 schematically depicts a UV-CLC film provided between
crossed polarisers.
[0037] FIG. 2 shows the transmission of UV-CLC films of prior art
and of the present invention between crossed polarisers with varied
film thickness for different pitch lengths.
[0038] FIG. 3 shows the transmission of UV-CLC films of prior art
and of the present invention between crossed polarisers versus
director orientation at the upper surface.
[0039] FIG. 4 shows the transmission of UV-CLC films of prior art
and of the present invention between crossed polarisers with varied
film thickness for different director orientations at the upper
surface.
[0040] FIG. 5 shows the transmission of a UV-CLC film of the
present invention between crossed polarisers with varied film
thickness for different director orientations at the upper
surface
DETAILED DESCRIPTION OF THE INVENTION
[0041] The negative retardation film according to the present
invention comprises helically twisted polymerised or crosslinked LC
material with a short pitch. Preferably the negative retarder is a
negative C plate retardation film comprising polymerised
cholesteric LC (CLC) material with reflection in the UV range,
which is also known as UV-CLC film or highly twisted A plate.
UV-CLC films or highly twisted A plate films and their preparation
are generally described for example in GB 2,315,072, WO 01/20393
and WO 01/20394.
[0042] In a UV-CLC film, by ensuring the CLC Bragg reflection of
the film is entirely below visible wavelengths (.lambda.<380 nm)
the film exhibits zero on-axis retardation and therefore appears
dark when placed between crossed polarisers. However, in UV-CLC
films showing the mura effect, some areas of the films appear
lighter than others, producing a speckled, non-uniform dark state.
In the lighter areas of the mura the polarisation state of the
light passing on-axis through the film is altered. As the film is
not retarding, an alternative, spatially varying, effect is causing
these variations.
[0043] According to a first preferred embodiment of the present
invention the mura effect in a UV-CLC film is reduced by reducing
the helical pitch of the film.
[0044] This first preferred embodiment relates to a negative
birefringent retarder comprising polymerised CLC material wherein
the helical pitch of the CLC material is 200 nm or less.
[0045] It was observed that within manufacturing tolerances the
film thickness of a typical UV-CLC film prepared as described in
prior art can vary in the order of about 2%.
[0046] Simulations using Berreman matrix methods have been
performed to investigate the effect of variation of the film
thickness of a UV-CLC film on its transmitted intensity when put
into an assembly as exemplarily shown in FIG. 1. The assembly of
FIG. 1 comprises a UV-CLC film (10) having an upper surface (11)
and a lower surface (12) which is sandwiched between an upper linar
polariser (21) and a lower linear polariser (22) having polarising
axes that are perpendicular to each other. The simulations were
carried out for UV-CLC films according to prior art and for UV-CLC
films according to the present invention with reduced pitch
values.
[0047] As a result it was found that there is a periodic variation
in the transmitted light intensity with film thickness, which
corresponds to the light and dark regions in the films. It was also
found that the variation of the pitch of the CLC films has a
surprisingly significant effect on the intensity of the light and
dark regions of the mura.
[0048] To achieve good contrast in an LCD the transmission of the
UV-CLC film between two crossed polarisers should preferably be
minimised. From the above simulations it was found that the
transmitted intensity of the UV-CLC film in both the light and dark
regions is reduced by reducing the pitch. Furthermore, with reduced
pitch the periodicity of the peaks is increased, so the number of
light regions is increased for a given thickness variation.
However, the reduced intensity in these light regions makes them
less visible, lowers the level of mura and improves dark state and
contrast.
[0049] The helical pitch of the CLC material in a negative
retardation film according to the present invention is preferably
from >50 nm to 200 nm, very preferably from 55 to 175 nm, most
preferably from 55 to <100 nm.
[0050] Furthermore, in an assembly as exemplarily shown in FIG. 1,
the orientation of the LC director at the upper surface (11) of a
UV-CLC film (10) was calculated for various film thickness and
pitch values. As a result, it was found that the transmission of
the UV-CLC film is highest, when the LC director at the upper
surface (11) is perpendicular to the LC director at the lower
surface (12).
[0051] In a UV-CLC film according to the present invention the LC
director at the upper surface (11) is preferably parallel or
antiparallel, i.e. oriented at 180 or 360.degree., to the LC
director at the lower surface (12).
[0052] The orientation of the upper and lower surface LC directors
can be controlled by varying the pitch and/or the thickness of the
UV-CLC film. The angle formed by the upper and lower surface LC
directors is identical to the twist angle .phi. of the CLC material
in the UV-CLC film, which is given by the helical pitch p of the
CLC material and the film thickness d according to the equation
.phi.=360.degree. d/p
[0053] Thus, the pitch and film thickness of the UV-CLC film are
preferably chosen such that the twist angle .phi. of the CLC
material is a multiple of 180.degree. or 360.degree..
[0054] According to a second preferred embodiment of the present
invention the mura effect of a UV-CLC film can be reduced by proper
arrangement of the film when it is placed between crossed
polarisers or adjacent to a polariser. This is an especially
suitable method for reducing mura in an UV-CLC film where the upper
and lower surface LC directors are non-parallel.
[0055] This second preferred embodiment relates to a combination of
a negative birefringent retarder comprising polymerised CLC
material and a linear polariser, wherein the director at the
surface of said retardation film facing said polariser and the
polarisation direction of said polariser are oriented at an angle
of approximately from 30 to 600 relative to each other.
[0056] Especially preferably the angle between the surface director
of the UV-CLC film and the polarisation direction of the respective
adjacent polariser is from 35 to 550, in particular from 40 to 500,
very preferably 45.degree..
[0057] Through an optical microscope it was observed that if a
UV-CLC film sandwiched between two linear polarisers is rotated
relative to the polarisers, the light areas become dark and
surprisingly the dark areas remain dark.
[0058] To verify that simulations are recreating the observed
effect, calculations were performed for an assembly as exemplarily
shown in FIG. 1 comprising a UV-CLC film (10) between crossed
linear polarisers (21,22) which is rotated relative to the
polarisers. As a result it was found that upon rotation of the CLC
film, the mura effect generated by thickness variations is
minimised and virtually disappears when the UV-CLC film is arranged
such that the LC director at its lower surface (12) is oriented at
45.degree. to the lower polariser (22).
[0059] The following embodiments of the present invention are
preferred
[0060] the negative retardation film comprises a polymerised or
crosslinked chiral nematic or cholesteric LC material,
[0061] the helical pitch of the CLC material in the negative
retardation film is from >50 nm to 200 nm, very preferably from
55 to 175 nm, most preferably from 55 to <100 nm,
[0062] the thickness of the negative retardation film is from 1 to
4 .mu.m, preferably from 2 to 3.5 .mu.m,
[0063] the optical retardation of the negative retardation film is
from 50 to 400 nm, preferably from 100 to 250 nm,
[0064] An LCD comprising a negative retardation film as described
above an below is another aspect of the invention. Especially
preferred is an LCD comprising the following components
[0065] a liquid crystal (LC) cell formed by two transparent
substrates having surfaces which oppose each other, an electrode
layer provided on the inside of at least one of said two
transparent substrates and optionally superposed with an alignment
layer, and an LC medium which is present between the two
transparent substrates that is switchable between at least two
different states by application of an electric field,
[0066] a first-linear polariser on one side of the LC cell,
[0067] a second linear polariser on the side of the LC cell
opposite to that of said first linear polariser,
[0068] one or more negative retardation films situated between the
LC cell and the first polariser and/or between the LC cell and the
second polariser,
[0069] it being possible for the above components to be separated,
stacked, mounted on top of each other, coated on top of each other
or connected by means of adhesive layers,
[0070] wherein at least one of said negative retardation films is a
film according to the present invention and/or a film that is
arranged relative to the polarisers as described above and
below.
[0071] Very preferred is an LCD, wherein the switchable LC medium
exhibits a negative dielectric anisotropy and the LC molecules of
the switchable liquid crystal medium exhibit a substantially
homeotropic orientation when no electric field is applied.
[0072] Very preferably the LCD is a display of the vertically
aligned (VA), multidomain VA (MVA) or patterned VA (PVA) mode.
Displays of these types are generally known in the art.
[0073] The individual optical components of the LCD according to
the invention, such as the LC cell, the individual retarders and
the linear polarisers, can be separated or can be laminated to
other components. They can be stacked, mounted on top of each other
or be connected e.g. by means of adhesive layers. In case of films
comprising polymerised LC material, it is also possible that stacks
of two or more films are prepared by coating the LC material of one
film directly onto another film, the latter serving as
substrate.
[0074] The LCDs according to the present invention may further
comprise one or more further optical components such as polarisers
or compensation or retardation films, like for example a planar A
plate, an O plate or a positive or negative C plate retardation
film with twisted, homeotropic, planar, tilted or splayed
structure. Particularly preferred are optical films comprising
polymerised or crosslinked LC material. Tilted or splayed LC films
are described for example in U.S. Pat. No. 5,619,352, WO 97/44409,
WO 97/44702, WO 97/44703 and WO 98/12584. Homeotropic LC films are
described for example in WO 98/00475. Planar LC films are described
for example in WO 98/04651.
[0075] The LCD according to the present invention may be a
reflective or transmissive display, and may further comprise a
light source, like a conventional backlight, or a reflective layer
on the side of the LC cell opposite to that of the first linear
polariser. In case of a reflective display with a reflective layer
on one side of the LC cell the second linear polariser may be
omitted.
[0076] The negative retarder of the present invention is preferably
prepared from a polymerisable LC material by in-situ
polymerisation. In a preferred method of preparation the
polymerisable LC material is coated onto a substrate, oriented into
the desired orientation and subsequently polymerised for example by
exposure to heat or actinic radiation as described for example in
WO 01-20394, GB 2,315,072 or WO 98/04651. In case of a UVCLC film
the polymerisable LC material preferably comprises one or more
achiral polymerisable mesogenic compounds and at least one chiral
compound. The chiral compound can be selected from
non-polymerisable chiral compounds, like e.g. conventional chiral
dopants, polymerisable chiral non-mesogenic or polymerisable chiral
mesogenic compounds.
[0077] In general, the polymerisable LC material preferably
comprises at least one polymerisable mesogen having one
polymerisable functional group (monoreactive compound) and at least
one polymerisable mesogen having two or more polymerisable
functional groups (di- or multireactive compound).
[0078] Alternatively it is possible to prepare the retardation film
from a readily synthesized LC polymer that is applied onto a
substrate, for example at a temperature above its glass transition
temperature or its melting point, or from solution e.g. in an
organic solvent, aligned into the desired orientation, and
solidified e.g. by evaporating the solvent or by cooling below the
glass temperature or melting point of the LC polymer. If for
example a LC polymer with a glass temperature that is higher than
ambient temperature is used, evaporation of the solvent or cooling
leaves a solid LC polymer film. If for example an LC polymer with a
high melting point is used, the LC polymer can be applied as a melt
onto the substrate which solidifies upon cooling. LC side chain
polymers or LC main chain polymers can be used, preferably LC side
chain polymers. The LC polymer should preferably be selected such
that its glass transition or melting temperature is significantly
higher than the operating tempature of the retarder. For example,
LC side chain polymers comprising a polyacrylate, polymethacrylate,
polysiloxane, polystyrene or epoxide backbone with laterally
attached mesogenic side chains can be used. The LC polymer may also
comprise side chains with reactive groups that can be crosslinked
after or during evaporation of the solvent to permanently fix the
orientation. The LC polymer may also be subjected to mechanical or
heat treatment after application to the substrate to improve
alignment. The above methods and suitable materials are known to
those skilled in the art.
[0079] The negative retarder according to the present invention can
be used for compensation of conventional LCDs, in particular those
of the DAP (deformation of aligned phases) or VA (vertically
aligned) mode, like e.g. ECB (electrically controlled
birefringence), CSH (colour super homeotropic), VAN or VAC
(vertically aligned nematic or cholesteric) displays, MVA
(multi-domain vertically aligned) or PVA (patterned vertically
aligned) displays, in displays of the bend mode or hybrid type
displays, like e.g. OCB (optically compensated bend cell or
optically compensated birefringence), R-OCB (reflective OCB), HAN
(hybrid aligned nematic) or pi-cell (.pi.-cell) displays,
furthermore in displays of the TN (twisted nematic), HTN (highly
twisted nematic) or STN (super twisted nematic) mode, in AMD-TN
(active matrix driven TN) displays, or in displays of the IPS (in
plane switching) mode which are also known as `super TFT`
displays.
[0080] Especially preferred are VA, MVA, PVA, OCB and pi-cell
displays.
[0081] The examples below serve to illustrate the invention without
limiting it.
EXAMPLE 1
Negative UV-CLC Retarder with Reduced Pitch
[0082] Simulations using Berreman matrix methods have been
performed to investigate the effect of the film thickness variation
of various UV-CLC films on the transmitted intensity of the UV-CLCs
film between crossed polarisers.
[0083] The simulations were carried out for an assembly as shown in
FIG. 1, comprising a UV-CLC film (10) having an upper surface (11)
and a lower surface (12) which is sandwiched between an upper
linear polariser (21) and a lower linear polariser (22) having
polarising axes that are perpendicular to each other.
[0084] The simulations were carried out for UV-CLC films (a)-(d)
according to the present invention with a helical pitch of 140 nm
(a), 160 nm (b), 180 nm (c) and 200 nm (d), respectively, in
comparison with negative UV-CLC films (e)-(f) of prior art with a
helical pitch of 220 nm (e) and 240 nm (f), respectively, and with
an isotropic film (g). The films and their pitch values are shown
in the table below.
1 Film a b c d e F g Pitch (nm) 140 160 180 200 220 240
isotropic
[0085] The results of the simulations are shown in FIG. 2, which
depicts the intensity/thickness profiles for UV-CLC films
(a)-(g).
[0086] FIG. 2 demonstrates that there is a periodic variation in
the transmitted light intensity with film thickness, which
corresponds to the light and dark regions in the UV-CLC films. It
can also be seen that the variation of the pitch of the UV-CLC
films has a surprisingly significant effect on the intensity of the
light and dark regions of the mura.
[0087] From FIG. 2 it is evident that by reducing the pitch of a
UV-CLC film the transmitted intensity in both the light and dark
regions is reduced. The periodicity of the peaks is increased with
reduced pitch so the number of light regions is increased for a
given thickness variation. However, the reduced intensity in these
light regions makes them less visible, lowers the level of mura and
improves dark state and contrast. The UV-CLC films (a)-(d) with
reduced pitch according to the present invention are therefore
especially suitable for use as negative retarders.
EXAMPLE 2
Orientation of Negative UV-CLC Retarder relative to Polarisers
[0088] By using the data from Example 1 as shown in FIG. 2, the
orientation of the LC director at the upper surface of UV-CLC films
(a)-(d) according to the present invention and UV-CLC films (e)-(f)
of prior art was calculated for each film thickness and cholesteric
pitch in an assembly as exemplarily shown in FIG. 1.
[0089] The results are shown in FIG. 3, which depicts the
transmission of the UV-CLC films (a)-(f) (10) versus the LC
director orientation at the upper surface (11) relative to the LC
director orientation at the lower surface (12). The director at the
lower surface (12) is parallel to the polariser direction of the
lower polariser (22). It is clearly seen that the intensity peaks
occur when the upper surface LC director is perpendicular to the
lower surface LC director.
[0090] When observing the UV-CLC films (a)-(f) through an optical
microscope it can be seen that if the films are rotated relative to
the polarisers, the light areas become dark and surprisingly the
dark areas remain dark.
[0091] To verify that the simulations are recreating the observed
effect, calculations were performed for the UV-CLC films (a)-(f) of
example 1 when placed between two crossed linear polarisers in an
assembly as exemplarily shown in FIG. 1. The results are depicted
in FIG. 4 for UV-CLC film (a) according to the present invention
with a pitch of 140 nm, and UV-CLC film (e) of prior art with a
pitch of 220 nm, wherein the LC director at the lower surface (12)
is aligned at 45.degree. (a45, e45) and 0.degree. (a0, e0),
respectively, relative to the polarisation direction of the lower
polariser (22).
[0092] FIG. 4 clearly shows that upon rotation of the sample, the
mura effect generated by thickness variations virtually disappears
when the UV-CLC film is placed with the lower surface director 450
to the lower polariser. Therefore, lamination of a UV-CLC film
according to the present invention with the lower surface director
at an angle of 45.degree. to the lower polariser will reduce the
mura.
[0093] The combination of reduced pitch and optimum orientation of
the sample is shown in FIG. 5 for UV-CLC film (a) according to the
present invention with a pitch of 140 nm, wherein the director at
the lower surface is oriented at an angle of 0.degree. (a0), 220
(a22), 450 (a45) and 67.degree. (a67) relative to the lower
polariser.
* * * * *