Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-29
2004-05-25
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S181000
Reexamination Certificate
active
06741311
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a reflective type liquid crystal display, more particularly to a reflective-type fringe field switching mode LCD(hereinafter, reflective FFS-LCD) capable of improving a reflection rate thereof.
BACKGROUND OF THE INVENTION
A twisted nematic(TN) mode LCD having nematic liquid crystal compositions of positive dielectric anisotropy has been used for the conventional reflective type LCD. This reflective type TN-LCD has a low power consumption property and is used for relatively small size LCDs of an electronic clock, a digital clock and so on. However, the reflective TN-LCD has chronic problems of a poor viewing angle and a low contrast ratio.
Consequently, a reflective FFS-LCD device is now in the process of research and development to ensure a good viewing angle property, a high reflective rate and an aperture ratio. The composition of this conventional reflective FFS-LCD device is approximately illustrated in
FIGS. 1 and 2
.
Referring to
FIGS. 1 and 2
, a lower substrate
10
is opposed to an upper substrate
15
at a selected distance. A liquid crystal layer
17
having a plurality of liquid crystal molecules is interposed between the lower substrate
10
and the upper substrate
15
. A counter electrode
11
a
and a pixel electrode
11
b
forming a fringe field to operate the liquid crystal molecules, are disposed on the inside surface of the lower substrate
10
. A color filter(not shown) is disposed on the inside surface of the upper substrate
15
. A first homogeneous alignment layer
12
is interposed between the lower substrate
10
including the counter electrode
11
a
and the pixel electrode
11
b
, and the liquid crystal layer
17
. A second homogeneous alignment layer
16
is interposed between the upper substrate
15
including the color filter and the liquid crystal layer
17
. At this time, the first and the second homogeneous alignment layers
12
,
16
have rubbing axes R
1
,R
2
respectively, and the rubbing axis R
1
of the first homogeneous alignment layer
12
and the rubbing axis R
2
of the second homogeneous alignment layer
16
form 180°, i.e. anti-parallel to each other. In addition, the rubbing axis R
1
forms a selected angle with a line f which on the substrate surface, projects the fringe field that is formed between the counter electrode
11
a
and the pixel electrode
11
b
. A polarizer
8
is attached on the outside surface of the upper substrate
5
so that polarizing axis
8
a
thereof is equal to the rubbing axis R
1
of the first homogeneous alignment layer
12
. A &lgr;/4 plate
19
polarizing an incident light or a reflected light by &lgr;/4 is disposed on the outside surface of the lower substrate
10
. On the outside of the &lgr;/4 plate
19
, a reflective plate
20
reflecting the light which passes through the &lgr;/4 plate
19
, is disposed. At this time, the &lgr;/4 plate
19
is disposed so that a fast (or slow) axis thereof is at an angle of 45° with the first rubbing axis (R
1
).
Such conventional reflective-type FFS-LCD operates as following.
First, referring to
FIG. 1
, when voltage difference does not occur between the counter electrode
11
a
and the pixel electrode
11
b
, the liquid crystal molecules (not shown) are arranged so that the rubbing axes R
1
, R
2
and the long axes thereof are parallel. Consequently, a natural light
22
a
becomes an incident light
22
b
proceeding to the same direction as a polarization axis
18
a
by passing through a polarizer
18
. Thereafter, the direction of the incident light
22
b
is not changed while passing through the liquid crystal layer
17
which the rubbing axes R
1
, R
2
and the long axes of the liquid crystal molecules are arranged side by side thereon. The incident light
22
b
which has passed through the liquid crystal layer
17
, is at an angle of 45° with the fast (or slow) axis of the &lgr;/4 plate
19
, thereby becoming a right-circularly polarized light
22
c
passing through the &lgr;/4 plate
19
. The right-circularly polarized light
22
c
is reflected b a reflective plate
20
, thereby becoming a left-circularly polarized reflected light
23
a.
The reflected light
23
a
becomes the reflected light
23
a
proceeding to a crossing direction with the axis of the polarized light
18
a
, passing through the &lgr;/4 plate
19
having the fast (or slow) axis at an angle of 45° with the proceeding direction thereof. The proceeding direction of the reflected light
23
b
which has passed through the &lgr;/4 plate
19
, is orthogonal to the long axis of the liquid crystal layer
17
, thereby passing through the liquid crystal layer
17
without being changed. The reflected light
22
b
which has passed the liquid crystal layer
17
, is orthogonal to the polarizing axis
18
a
, thereby not passing through the polarizer
18
. Accordingly, a screen becomes dark.
Next, like
FIG. 2
, when a fringe field F is formed between the counter electrode
11
a
and the pixel electrode
11
b
, the liquid crystal molecules (not shown) are twisted into the fringe field form. Consequently, the optical axes of the liquid crystal molecules (not shown) are at a selected angle with the polarizing axis
18
a
. A natural light
25
a
passes through the polarizer
18
, thereby becoming an incident light
25
b
proceeding to the same direction as the polarizing axis
18
a
. Thereafter, the incident light
25
b
is at an angle of 45° with a long axis of a liquid crystal molecule which is arranged in a fringe field F form. Therefore, an incident light
25
c
which has passed through the liquid crystal layer
17
, becomes the incident light
25
c
which is at an angle of 45° with the polarizing axis
18
a
. Here, the incident light
25
c
which has passed through the liquid crystal layer
17
coincides with the fast(or slow) axis
19
a
of the &lgr;/4 plate
19
, thereby passing through the &lgr;/4 plate
19
without change in the proceeding direction thereof. The incident light
25
c
which has passed through the &lgr;/4 plate
19
, is reflected by the reflective plate
20
, thereby becoming a reflected light
26
a.
The proceeding direction of the reflected light
26
a
coincides with the fast (or slow) axis of the &lgr;/4 plate
19
, thereby passing through the &lgr;/4 plate
19
without changing the proceeding direction thereof. The proceeding direction of the reflected light
26
a
which has passed through the &lgr;/4 plate
19
, is at an angle of 45° with the long axis of the liquid crystal molecule on the liquid crystal layer
17
and therefore the proceeding direction of a reflected light which has passed through the liquid crystal layer
17
, coincides with the polarizing axis
18
a
. Therefore, a screen is in a white state.
Conventional reflective type liquid crystal display device has not required a backlight as light source and optical component such as the &lgr;/4 plate
19
has been added to the outside of a substrate thereof to improve a contrast.
However, the manufacturing cost is increased as the optical component such as the &lgr;/4 plate is added. Moreover, the &lgr;/4 plate absorbs some of an incident light or a reflected light, thereby deteriorating transmissivity of the LCD, i.e. the reflectance.
SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide a reflective type liquid crystal display device having a good contrast ratio and a good reflectance without an additional optical component.
To accomplish the aforementioned object of this invention, the present invention according to a first embodiment provides a reflective type FFS-LCD including: a liquid crystal layer having a plurality of the liquid crystal molecules; a first substrate disposed on one side of the liquid crystal layer and in which a counter electrode and a pixel electrode, both for generating a fringe field to drive the liquid crystal molecules are formed; a second substrate disposed on the other side of the liquid crystal layer; a first homogeneous alignment layer interposed between the liquid crystal layer and the first substrat
Hong Seung Ho
Lee Seung Hee
Boe-Hydis Technology., Ltd.
Chowdhury Tarifur R.
Ladas & Parry
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