Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-07-20
2003-12-09
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S117000, C349S113000
Reexamination Certificate
active
06661483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device that produces a display by utilizing light reflected from a reflective layer, and more particularly to a liquid crystal display device wherein both white and black display colors are achromatic colors, and wherein reflectivity in white display state is high and reflectively in black display state is low.
2. Description of the Related Art
Liquid crystal displays, which are thin and light compared with other types of display, are widely used in various applications including displays for portable information terminals. A liquid crystal display contains a liquid crystal cell as a light receiving type display element. The liquid crystal cell does not emit light by itself, but produces a display by changing its light-transmitting properties by being driven with an operating voltage of 1 to 9 volts. Accordingly, a reflective mode liquid crystal display, which displays images by reflecting ambient light using a reflector mounted underneath the liquid crystal, is an extremely low power consumption display device. It is also known that if a super twisted nematic (STN) liquid crystal cell is used in the liquid crystal display device, the price of the liquid crystal display device can be reduced because the construction of the liquid crystal display device can be simplified.
Conventionally, STN reflective liquid crystal displays utilize optical birefringence, whereas twisted nematic (TN) reflective liquid crystal displays utilize optical rotatory power. In STN reflective liquid crystal displays, therefore, optical compensation is difficult since the polarization state of the emergent light varies depending on the amount of birefringence in the liquid crystal layer. As shown in
FIG. 9
, a prior art STN reflective liquid crystal display device
1
comprises a liquid crystal cell
3
sandwiched between two polarizers
4
and
5
to enhance optical compensation. In the liquid crystal display device
1
as shown in
FIG. 9
, a phase retardation plate
6
is interposed between the liquid crystal cell
3
and the first polarizer
4
. The second polarizer
5
is located between a reflector
7
and the liquid crystal cell
3
. The liquid crystal cell
3
is constructed by sandwiching a liquid crystal layer
8
between two transparent substrates
9
and
10
. Each substrate
9
,
10
comprises an alignment film and electrodes formed on a substrate base.
When constructed as a color display using color filters, the reflective liquid crystal display device
1
which uses two polarizers involves the problem that optical reflectivity enough to provide sufficient brightness for display is difficult to obtain because of the light loss associated with the color filters. Furthermore, in the reflective liquid crystal display device
1
, since the two polarizers
4
and
5
are mounted on the outside of the liquid crystal cell
3
, the reflector
7
also has to be mounted on the outside of the liquid crystal cell
3
. As a result, light loss is caused due to the presence of the substrate
10
of the liquid crystal cell
3
on the second polarizer
5
side.
In the reflective liquid crystal display device
1
using two polarizers, the substrate 10 about 1 mm thick and the second polarizer
5
about 0.2 mm thick are interposed between the reflector
7
and the liquid crystal layer
8
. Consequently, when light is incident obliquely on the reflective liquid crystal display device
1
of
FIG. 9
, as shown in
FIG. 10
, the incident light when reflected passes through a pixel different from the pixel it passed through when it entered the cell. In this case, if the reflective liquid crystal display device
1
is viewed perspectively, it will be seen that parallax is caused such that the shade
16
of a displayed object appears to be projected on the reflector
7
. Poor viewability caused by such parallax is also a problem with the reflective liquid crystal display device
1
of FIG.
9
.
In view of the above situation, there is proposed, as shown in
FIG. 11
, a reflective liquid crystal display device
11
in which one polarizer is omitted to achieve an improvement in brightness corresponding to the omission of one polarizer. In
FIG. 11
, the same constituent elements as those in
FIG. 9
are designated by the same reference numerals and an explanatory description thereof will not be given here. In the reflective liquid crystal display device of
FIG. 11
, only one polarizer
4
is arranged on the side of the liquid crystal cell
3
opposite from the reflector, and the second polarizer
5
is omitted. The applicant further proposes in Japanese Unexamined Patent Publication JP-A 7-84252 (1995) a reflective liquid crystal display device having only one polarizer and characterized by the provision of a reflector
7
within the liquid crystal cell
3
. The reflective liquid crystal display device
11
having the structure as shown in
FIG. 11
can thus eliminate the problem of light loss caused by the presence of the substrate
10
of the liquid crystal cell
3
on the second polarizer
5
side. Reflective liquid crystal display devices of the type in which the reflector
7
is disposed within the liquid crystal cell
3
are also disclosed in Japanese Unexamined Patent Publications JP-A 10-161110 (1998) and JP-A 10-170906 (1998). In the reflective liquid crystal display device of
FIG. 11
, the problem of poor viewability caused by parallax is also solved because of the absence of the substrate base and the second polarizer
5
between the liquid crystal layer
8
and the reflector
7
.
FIG. 12A
is a schematic diagram for explaining how the light loss occurs in the reflective liquid crystal display device
1
of
FIG. 9
that has two polarizers.
FIG. 12B
is a schematic diagram for explaining how the light loss is reduced in the reflective liquid crystal display device
11
of
FIG. 11
that has a single polarizer. The explanation of
FIGS. 12A and 12B
is given assuming that the transmittance of light per polarizer is 45% and that the transmittance of the polarization component parallel to the absorption axis of the polarizer is 0%. Further, in the explanation of
FIGS. 12A and 12B
, light absorption into color filters is not considered. In the examples of
FIGS. 12A and 12B
, of the polarization component orthogonal to the absorption axis of the polarizer, which accounts for 50% of the incident light, 10% is absorbed by the polarizer; this means that the transmittance of the polarization component orthogonal to the absorption axis, per polarizer, is 90%.
In the reflective liquid crystal display device
1
of
FIG. 9
that uses two polarizers, since the incident light on the device
1
emerges from it after passing the polarizers a total of four times, the reflectivity is 32.8% as shown by expression (1). On the other hand, in the reflective liquid crystal display device
11
of
FIG. 11
in which only one polarizer is used and the reflector is disposed within the liquid crystal cell
3
, the incident light on the device
11
emerges from it after passing through the polarizer two times; therefore, the reflectivity is 40.5% as shown by expression (2). As can be seen from the above results, the reflective liquid crystal display device
11
having only one polarizer has the potential of providing up to about 23.5% improvement in reflectivity over the reflective liquid crystal display device
1
having two polarizers.
Reflectivity of liquid crystal display device of FIG.
9
=0.9×0.9×0.9×0.9×50%=32.8% (1)
Reflectivity of liquid crystal display device of FIG.
11
=0.9×0.9×50%=40.5% (2)
In the liquid crystal display device
11
having a single polarizer and a single reflector, however, the omission of one polarizer makes optical compensation all the more difficult, and the display background color which should be white or black is caused to shift. Specifically, in the case of an STN liquid crystal cell that utilizes
Moriwaki Hiroyuki
Tanaka Mitsuhiro
Conlin David G.
Edwards & Angell LLP
Hartnell, III George W.
Parker Kenneth
Sharp Kabushiki Kaisha
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