Liquid crystal cell

Details Liquid crystal cell Liquid crystal cell

2000-11-17

2003-07-29

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S117000

Reexamination Certificate

active

06600531

ABSTRACT:

This application claims the benefit of Korean Patent Application No. 1999-051594, filed on Nov. 19, 1999, the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein, and the benefit of Korean Patent Application No. 1999-0052213, filed on Nov. 23, 1999, the entirety of which is also hereby incorporated by reference for all purposes as if fully set forth herein, both under 35 U.S.C. § 119.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal cell design for a liquid crystal display (LCD) device, and more particularly, to a liquid crystal cell that uses a twisted nematic (TN) liquid crystal.
2. Discussion of the Related Art
Conventional LCD devices include display panels. Such display panels have upper and lower substrates that are attached with each other, and a liquid crystal, such as a twisted nematic (TN) liquid crystal, interposed there between. Such display panels are operationally divided into a plurality of liquid crystal cells. On exterior surfaces of the upper and the lower substrates, polarizers and retardation films, or compensation films are selectively attached.
A major consideration in the design of liquid crystal cells is the characteristics of the particular liquid crystal that is used. A good liquid crystal should have a short response time, a low gray scale, a wide viewing angle, and should be operational at low voltages. However, it is very difficult to find a liquid crystal that has all of these characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of TN liquid crystals, a low twisted nematic (LTN) liquid crystal has advantages of a short response time and a good gray scale. However, it typically has a low contrast ratio and relatively poor color dispersion properties. Other twisted nematic (TN) liquid crystals have twisted angles of 90 degrees, or those employing an in-plating switching (IPS) mode. Those liquid crystals can provide a wide viewing angle, but afterimages are produced during moving images and their brightness is relatively low. The anti-ferroelectric liquid crystal (AFLC) or an optical compensated birefringence (OCB) have advantages of a wide viewing angle and a short response time, although there are problems with contrast ratios and cell gap alignment.
Of particular interest to this invention is the difficulty of determining the optimum design parameters of a liquid crystal cell. A liquid crystal cell design should take into consideration many parameters, including liquid crystal arrangement and the transmittance axis directions of the polarizers. However, as there are simply too many important factors to consider it is humanly impossible to consider them all. Accordingly, computer simulation is usually used to process the design parameters and to arrive at an optimum liquid crystal cell.
One such computer simulation is the parameter space approach. The parameter space approach provides a graph that illustrates transmittance with respect to the product of a cell's thickness and birefringence when under a non-electric field condition. In the parameter space graph, the optimum parameter values of cell thickness and birefringence product “d&Dgr;n”, or “d.DELTA.n” where the transmittance is highest can be easily found. The d.DELTA.n is calculated using a Jones matrix formulation.
As the Jones matrix formulation (and the generalized geometric optics approximation [GGOA]) has been fully discussed elsewhere it need not be repeated in detail here. An important point to note is that in the Jones matrix formulation the liquid crystal director (the direction in which the molecules line up) is assumed to be uniform over the entire cell. However, it is well known that the tilt angle decreases in the middle of the liquid crystal cell due to elastic energy minimization, especially for high pretilt angle cases. However, since an average tilt angle can be used without producing any significant error in predicting the properties of the LCD, most computer simulations assume that the tilt angle is zero.
The basic configuration and operation of a twisted nematic liquid crystal device will be provided. Then, a more detailed description of the parameter space method will be given. As shown in
FIG. 1
, first and second polarizers
10
and
16
, respectively, having first and second transmittance axis directions
40
and
42
that are perpendicular to each other, are opposed with and spaced apart from each other. Between the two polarizers
10
and
16
are first and second transparent substrates
12
and
14
, which are also opposed with and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to
FIG. 1
, the first and the second transparent substrates
12
and
14
include first and second orientation films
20
and
22
, respectively, on their opposing surfaces. Between the first and the second orientation films
20
and
22
is a positive TN liquid crystal
18
.
The positive TN liquid crystal has a characteristic that it becomes arranged according to the direction of an applied electric field. The first and the second polarizer
10
and
16
, respectively, transmit light that is parallel with their transmittance-axis directions
40
and
42
, but reflect or absorb light that is perpendicular to their transmittance-axis directions
40
and
42
.
The first and the second orientation films
20
and
22
were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and the second transparent substrates
12
and
14
to become tilted several degrees from each substrate surface. First and second rubbing directions
50
and
52
of the first and the second orientation films
20
and
22
are, respectively, parallel with the transmittance-axis directions of the first and the second polarizer
10
and
16
. When no electric field is applied to the positive TN liquid crystal
18
, the orientation of the liquid crystal molecules becomes twisted from one substrate to the other at a definite angle, that being the twisted angle of the positive TN liquid crystal
18
.
During operation, a back light device
24
irradiates white light onto the first polarizer
10
. The first polarizer
10
transmits only the portion of the light that is parallel with the first transmittance-axis direction
40
. The result is a first linearly polarized light
26
that passes through the polarizer
10
. The first linearly polarized light
26
then passes through the positive TN liquid crystal
18
via the first transparent substrate
12
.
As the first polarized light
26
passes through the positive TN liquid crystal
18
, the first linearly polarized light
26
changes its phase according to the twisted alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light
26
becomes an elliptically (possibly circularly) polarized light
28
.
The elliptically polarized light
28
passes through the second transparent substrate
14
, and meets the second polarizer
16
. When the elliptically polarized light
28
passes through the second polarizer
16
, the second polarizer
16
transmits only the portion of the elliptically polarized light
28
that is parallel to the second transmittance-axis direction
42
. A polarized light
30
is then emitted. At the above-mentioned operation mode, a white state is displayed.
Turning now to
FIG. 2
, when a voltage supplier
35
induces an electric field through the positive TN liquid crystal
18
, the positive TN liquid crystal molecules rotate and become arranged such that the longitudinal axes of the molecules become perpendicular to the surfaces of the first and second substrates
12
and
14
. Accordingly, the first linearly polarized light
26
passes through the first transparent substrates

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