Color LCD device

Details Color LCD device Color LCD device

2001-05-15

2003-02-04

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S108000, C349S109000

Reexamination Certificate

active

06515727

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color liquid crystal display (LCD) device, and more particularly to a color LCD device having different electrode structures for red, green, and blue colors to achieve an improvement in color characteristics for an intermediate color, that is, gray.
2. Description of the Related Art
As well known, liquid crystal is a material exhibiting intermediate characteristics mesophase between solid and liquid. Such liquid crystal has a property for transmitting or shielding light in accordance with an alignment thereof. Accordingly, a desired character or image may be displayed by controlling a liquid crystal, arranged between facing electrodes, to transmit or shield light in accordance with an appropriate adjustment for the arrangement of a particular portion in the liquid crystal conducted based on a variation in the voltage applied to those electrodes. An LCD device is a display device using such a property of liquid crystal. Such an LCD device occupies an important position in the display device field in that it consumes a considerably low power consumption as compared to other devices while being capable of having various sizes from a super-miniature size to a large size and having diverse display patterns.
LCD devices utilize various characteristics to conduct a driving operation in a particular mode for displaying a desired character or image. A mode utilizing birefringence characteristics is known. For the most basic one of modes utilizing birefringence characteristics, an electrically controlled birefringence (ECB) mode is known, in which a liquid crystal controls light using upper and lower electrodes as it is aligned in a horizontal or vertical direction in accordance with a voltage applied thereto.
FIG. 1
is a view illustrating an ECB mode. In
FIG. 1
, the reference numeral
101
denotes transparent electrodes made of an indium tin oxide (ITO) and adapted to form an electric field in accordance with a voltage applied thereto. Also, the reference numeral
102
denotes glass substrates each adapted to provide a base for the transparent electrodes
101
, the reference numeral
103
denotes polarizers for selectively transmitting or shielding light, and the reference numeral
104
denotes a liquid crystal exhibiting electro-optical operation characteristics in accordance with the electric field formed between the transparent electrodes.
For the ECB mode, there are two modes, that is, a vertical alignment (VA) ECB mode in which the state of liquid crystal is changed from a “first state” to a “third state” in accordance with an application of voltage, and a homogeneous ECB mode, in which the state of liquid crystal is changed from the “third state” to the “first state”.
Also, there is an LC mode in which the state of liquid crystal is changed between a “first state” and a “second state” in accordance with an application of voltage using a transverse electric field system, as shown in FIG.
2
. In
FIG. 2
, the reference numeral
201
a
denotes plus (+) (or minus (−)) electrodes,
201
b
minus (or plus) electrodes,
202
glass substrates,
203
polarizers, and
204
a liquid crystal, respectively.
The basic principle of the LC mode is to adjust light in accordance with an anisotropy in the refractive index of the liquid crystal and an angular relation between the mean direction of the liquid crystal and the direction of each polarizer. The transmittance T in a forward direction in such an LC mode can be expressed as follows:
T
=sin
2
(2&thgr;)sin
2
(&dgr;/2)
&dgr;=2nd &Dgr;
n
eff
/&lgr;
where, “&thgr;” represents the angle between the transmission axis of the polarizer at the side of incident light and the direction of the liquid crystal, “d” represents a cell gap, “&Dgr;n
eff
” represents an effective refractive index, and “&lgr;” represents the wavelength of the incident light.
As apparent from the above expressions, it can be found that there may be two modes for adjusting light, that is, a mode for adjusting “&thgr;” (in-plane switching (IPS) mode) and a mode for adjusting “&dgr;” (phase difference) (ECB mode). In the ECB mode for adjusting “&dgr;” (phase difference) (normally, “&thgr;” is 45°), the phase difference &dgr; is varied as “&Dgr;n
eff
” varies in accordance with an application of voltage to the liquid crystal. In this case, the variation in the phase difference &dgr; is differently exhibited at different wavelengths, respectively. In particular, a considerable color characteristic difference is exhibited at a wavelength corresponding to gray.
FIG. 3
illustrates respective variations in transmittance depending on the voltage applied in the VA-ECB mode at different wavelengths.
In order to compensate for the color characteristic difference exhibited at the wavelength corresponding to gray, as shown in
FIG. 3
, a scheme has been proposed in which different cell gaps are used for different colors to achieve an improvement in color characteristics. This scheme is illustrated in FIG.
4
.
In
FIG. 4
, the reference numeral
401
denotes a common electrode,
402
color filters, and
403
black matrices, respectively. The black matrices
403
are formed at boundaries of pixels to prevent a color diffusion. These black matrices
403
serve to suppress an interference effect exhibited among neighboring colors, thereby keeping the purity of each color. In
FIG. 4
, the reference numerals
404
and
407
denote glass substrates, and the reference numerals
405
and
408
denote polarizers, respectively. Also, the reference numeral
406
denotes pixel electrodes respectively corresponding to red, green and blue colors, and the reference numeral
409
denotes a liquid crystal.
However, this method, in which different cell gaps are used for different colors respectively, has a problem associated with manufacturing processes. For example, where color filters are designed to have different thicknesses for different colors, respectively, it is necessary to change the characteristics of materials used in order to compensate for absolute transmittances thereof. Furthermore, it is difficult to secure a desired uniformity of cell gaps. It is also difficult to secure a desired alignment uniformity of the liquid crystal during a rubbing process. As a result, the above mentioned method has a problem in that it is difficult to secure a desired yield of the LCD process.
A lateral field induced (LFI) VA mode is also known as a mode applying the birefringence mode. For this mode, a common electrode area patterned electrode structures are provided on an upper plate and a lower plate, respectively, as shown in
FIGS. 5A and 5B
. The upper plate has a structure rubbed in a slit direction. In
FIGS. 5A and 5B
, the reference numeral
501
denotes an ITO electrode,
502
glass substrates,
503
polarizers, and
504
a liquid crystal.
In the LFI-VA mode, the alignment of the liquid crystal
504
is substantially vertical when no voltage is applied, thereby preventing light from passing through the liquid crystal. When a voltage is applied, a multi-domain structure is formed in accordance with a combined function of a transverse electric field and a vertical electric field formed by the electrode structure of the lower plate
502
. The simplest diagram of this structure is illustrated in FIG.
5
C.
FIG. 5C
schematically illustrates the mean direction of the liquid crystal exhibited at the front side when a voltage is applied in the LFI-VA mode.
In this structure, the direction of the liquid crystal at the boundary of each lower plate electrode and an associated slit is slightly twisted in accordance with an application of voltage. Also, domains arranged at both sides of each electrode exhibit different ECB characteristics in that they have different 90° viewing angle directions, respectively. Referring to
FIG. 6
, it can be found that the variation in transmittance characteristics exhibited in accordance with a variation in voltage is differently exhibited at different wavelengths

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