Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With viewing layer of specified composition
Reexamination Certificate
1999-03-02
2001-09-18
Thomas, Alexander S. (Department: 1772)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With viewing layer of specified composition
C428S001100, C349S117000, C349S121000
Reexamination Certificate
active
06291035
ABSTRACT:
The invention relates to an optical retardation film comprising two layers of an anisotropic polymer that are adjacent to each other or adjacent to both sides of a common substrate, characterized in that each layer exhibits a tilted structure with an optical symmetry axis having a tilt angle &thgr; relative to the plane of the layer.
The invention further relates to a method of preparing such an optical retardation film, to a means to produce substantially linear polarized light comprising such an optical retardation film and to a liquid crystal display comprising a display cell and such an optical retardation film.
FIGS. 1
a
to
1
c
illustrate the structure of optical retardation films according to preferred embodiments of the present invention.
FIGS. 2
a
and
2
b
show a display device according to a preferred embodiment of the present invention.
FIGS. 3
a
and
3
b
show the retardation versus viewing angle of an inventive optical retardation film compared to optical retardation films of the state of the art, measured in two directions of observation.
FIG. 4
shows the normalized retardation versus wavelength for an inventive optical retardation film compared to optical retardation films of the state of the art.
FIG. 5
shows the retardation versus viewing angle of an inventive optical retardation film, measured in two directions of observation.
FIG. 6
a
and 6b show the luminance versus viewing angle for an inventive combination of a broad waveband circular reflective polarizer and an inventive optical retardation film, compared to a combination of a broad waveband circular reflective polarizer and different optical retardation films of the state of the art.
The European Patent Application EP 0 606 940-A1 discloses a cholesteric reflective polarizer that produces circular polarized light of a high luminance over a broad range of wavelengths. This polarizer can also be combined with a quarter wave foil or plate (QWF), which transforms the circular polarized light transmitted by the cholesteric polarizer into linear polarized light.
However, when a liquid crystal display comprising a cholesteric polarizer like that described in EP 0 606 940 is watched under an increasing viewing angle, its optical properties, like e.g. the luminance and the contrast ratio, are often deteriorating.
It has therefore been desired to have available an optical retardation film that, when used together with a broad waveband cholesteric circular reflective polarizer or a combination comprising a circular reflective polarizer and a quarter wave foil, like e.g. described in EP 0 606 940, improves the optical properties of the above mentioned circular reflective polarizer or combination over a wide range of viewing angles.
One of the aims of the present invention is to provide an optical retardation film having the properties described above. Another aim of the invention is to provide a means to produce substantially linear polarized light comprising such an optical retardation film and a broadband circular reflective polarizer. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It has now been found that these aims can be achieved by providing an optical retardation film comprising two layers of an anisotropic polymer that are adjacent to each other or adjacent to both sides of a substrate, characterized in that each layer exhibits a tilted structure with an optical symmetry axis having a tilt angle &thgr; relative to the plane of the layer.
Furthermore it has been found that an optical retardation film as described above can also be used as a compensation film in conventional LC displays, like for example TN, STN or active matrix driven (AMD) TN displays, in order to compensate the viewing angle dependence of the electrooptical properties, like e.g. the contrast and grey scale, in these displays.
A retardation film with a tilted molecular structure is described in the WO 96/19770-A1, whereas the WO 96/19770-A1 discloses a retardation film with a tilted structure wherein the tilt angle varies continuously in a direction normal to the film. However, there is no hint to the combined use of such a retardation film with a broadband circular reflective polarizer in these documents.
One of the objects of the present invention is an optical retardation film comprising two layers of an anisotropic polymer that are adjacent to each other or adjacent to both sides of a common substrate, characterized in that each layer exhibits a tilted structure with an optical symmetry axis having a tilt angle &thgr; relative to the plane of the layer.
In a preferred embodiment of the present invention the optical retardation film exhibits a tilted structure wherein the tilt angles of the two layers vary from one another.
In another preferred embodiment the optical retardation film exhibits a tilted structure wherein the tilt angle in each layer varies continuously, i.e. assumes a splay configuration, wherein the preferred embodiment is characterized in that either the direction of the variation or the amount of the starting values of the variation of the tilt angle are different between the two layers.
In the first layer according to this preferred embodiment the tilt angle &thgr; varies, preferably continuously, in a direction normal to the layer from a minimum value &thgr;
min
on the side of the layer facing the second layer or, if present, the common intermediate substrate, to a maximum value &thgr;
max
on the opposite side of the layer, or the other way round.
In the second layer, the tilt angle varies, preferably continuously, in a direction normal to the layer, starting from a minimum value &thgr;
min
at the side of the layer facing the first layer or, if present, the common intermediate substrate, and ranging to a maximum value &thgr;
max
on the opposite side of the layer, as in the first layer.
In another preferred embodiment of the present invention, the tilt angle in the second layer varies in the opposite direction of the first layer, i.e. when the first layer varies from &thgr;
min
to &thgr;
max
, the second varies from &thgr;
max
to &thgr;
min
, and vice versa.
In a preferred embodiment of the present invention, the minimum tilt angle &thgr;
min
in each layer is substantially zero degrees.
In another preferred embodiment of the present invention the optical retardation film exhibits structure wherein the projection of the optical symmetry axis of the first layer into the plane of the layer and the projection of the optical symmetry axis of the second layer into the plane of the layer are twisted relative to each other at an angle &rgr; in the plane of the interface between the layers, said angle &rgr; being preferably from 0 to 90 degrees.
In a preferred embodiment of the present invention the angle &rgr; is substantially 0 degrees.
In another preferred embodiment of the present invention the retardation of the optical retardation film is from 50 to 250 nm.
Another object of the present invention is an optical retardation film as described above that is obtainable by a method comprising the following steps
A) coating a mixture comprising
a) a polymerizable mesogenic material comprising at least one polymerizable mesogen having at least one polymerizable functional group,
b) an initiator, and
c) optionally a solvent on a substrate or between a first and a second substrate in form of a layer,
B) aligning the polymerizable mesogenic material in the coated layer into a tilted and optionally a splayed structure.
C) polymerizing said mixture of a polymerizable mesogenic material by exposure to heat or actinic radiation,
D) optionally removing the substrate or, if two substrates are present, one or two of the substrates from the polymerized material, and
E) repeating the steps A), B), C) and optionally step D) at least one more time.
In a preferred embodiment of the present invention the steps A), B) and C) are carried out on both sides of one substrate.
In another preferred embodiment of the present invention the mixture of
Godden Ben
Goulding Mark
Hodges Quentin
Verrall Mark
Merck Patent Gesellschaft mit
Millen White Zelano & Branigan P.C.
Thomas Alexander S.
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