Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Utility Patent
1999-06-03
2001-01-02
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S181000
Utility Patent
active
06169589
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a color liquid crystal display, more particularly, to a color liquid crystal display performing color display using birefringence of a liquid crystal device, without using color filters.
2. Description of the Related Art
There are two kinds of color liquid crystal displays, that is, one embodies a color filter and the other performs color display using birefringence of a liquid crystal without using a color filter.
In a liquid crystal display having a color filter therein, since a pixel is composed of three elements of R, G, B, the amount of transmitted light is reduced to around one third and the liquid crystal display usually requires a small fluorescent light as a back light, the liquid crystal display with a color filter is not suitable for a reflection-type color liquid crystal display.
On the other hand, a liquid crystal display which performs color display utilizing birefringence of a liquid crystal is suitable for the reflection-type color liquid crystal display, because color displaying can be performed with one pixel only by changing the voltage applied to a liquid crystal device.
As the color display utilizing the birefringence of the liquid crystal, the followings are known:
(1) A color liquid crystal display composed only of a liquid crystal device and a pair of polarizing films.
(2) A color liquid crystal display composed of a liquid crystal device, a retardation film and a pair of polarizing films.
(3) A color liquid crystal display composed of a liquid crystal device, a twisted retardation film and a pair of polarizing films.
As a liquid crystal device for a color liquid crystal display, a homogeneous liquid crystal device having a twisted angle of zero degrees, a TN (twisted nematic) liquid crystal device having a twisted angle of 90 degrees, and an STN (super twisted nematic) liquid crystal device having a twisted angle between 180 to 270 degrees have been developed.
A conventional example of a color liquid crystal display using a twisted retardation film and adopting an STN liquid crystal device as a liquid crystal device will be explained with reference to
FIG. 16
to FIG.
18
.
FIG. 18
is a schematic sectional view of the above-described color liquid crystal display,
FIG. 16
is a plan view showing a relation between the absorption axes of lower polarizing films and the molecular alignment direction in the liquid crystal, obtained when
FIG. 18
is viewed from the upper polarizing film
9
side, and
FIG. 17
is also a plan view showing a relation between the absorption axes of the upper polarizing film and the twisted retardation film.
Such a color liquid crystal display is disclosed, for instance, in Japanese Patent Laid-open No. Hei 7-5457.
In the color liquid crystal display, as shown in
FIG. 18
, a liquid crystal device
20
is formed by holding a nematic liquid crystal
7
in a twisted alignment between a pair of substrates composed of a first substrate
1
which is formed with an alignment layer
3
and a first electrode
2
made of ITO (Indium Tin Oxide) and a second substrate
4
which is formed with an alignment layer
6
and a second electrode
5
made of ITO.
Further, a pair of polarizing films, that is, a lower polarizing film
8
and an upper polarizing film
9
, are disposed holding the above-described liquid crystal device
20
thereinbetween, a twisted retardation film
10
is disposed between the liquid crystal device
20
and the upper polarizing film
9
, and a reflecting plate
11
is disposed outside of the lower polarizing film
8
.
Absorption axes (or transmission axes) of the pair of polarizing films
8
and
9
are disposed in parallel. Here, the twisted angle of the liquid crystal device
20
is 250 degrees. The absorption axis
8
a
of the lower polarizing film
8
shown by a broken line with arrows in
FIG. 16
intersects with the lower molecular alignment direction
7
a
in the liquid crystal, that is the alignment direction of the liquid crystal in the first substrate
1
, at an angle of 45 degrees. The absorption axis
9
a
of the upper polarizing film
9
shown by a solid line with arrows in
FIG. 17
is disposed to intersect with the upper molecular alignment direction
10
b
in the twisted retardation film
10
at an angle of 45 degrees.
Incidentally, the reference numeral
7
b
in
FIG. 16
shows the upper molecular alignment direction in the liquid crystal, that is the alignment direction in the liquid crystal of the second substrate
4
, and the reference numeral
10
a
in
FIG. 17
shows the lower molecular alignment direction in the twisted retardation film
10
.
The &Dgr;nd value of the liquid crystal device
20
expressed by the product of a difference of the birefringence &Dgr;n of the nematic liquid crystal
7
and a cell gap d, that is a space between the first substrate
1
and the second substrate
2
, is 843 nm. A twisted angle of the twisted retardation film
10
is 250 degrees in the reverse direction of the twisted angle of the liquid crystal device
20
. The &Dgr;nd value of the twisted retardation film, which is expressed by the product of a difference &Dgr;n of the birefringence of the twisted retardation film
10
and the thickness d, is also 843 nm.
As shown in
FIG. 17
, since the absorption axis
9
a
of the upper polarizing film
9
is disposed to intersect with the upper molecular alignment direction
10
b
in the twisted retardation film
10
at an angle of 45 degrees, linearly polarized light incident from the upper polarizing film
9
becomes elliptic polarized after passing through the twisted retardation film
10
.
However, since the upper molecular alignment direction
7
b
of the liquid crystal in the liquid crystal device
20
deviates from the lower molecular alignment direction
10
a
in the twisted retardation film by 90 degrees, the elliptic polarized light generated at the liquid crystal device
20
and the twisted retardation film
10
is placed back into its original state of linearly polarized light and reaches the lower polarizing film
8
. Since the absorption axis
8
a
of the lower polarizing film
8
is parallel to the absorption axis
9
a
of the upper polarizing film
9
, a white display is shown.
When voltage is applied between the first electrode
2
and the second electrode
5
, the liquid crystal molecules
7
are activated and the apparent &Dgr;nd value of the liquid crystal device
20
is decreased. Accordingly, the elliptic polarized light generated at the twisted retardation film
10
cannot be completely canceled by the liquid crystal device
20
, and reaches the lower polarizing film
8
without changing its elliptical polarization state. Accordingly, light beams having a specific wavelength penetrate therethrough and generate several colors.
The light that has passed through the lower polarizing film
8
is reflected by a reflecting plate
11
and emits upwards again after passing through the lower polarizing film
8
, the liquid crystal device
20
, the twisted retardation film
10
and the upper polarizing film
9
. Thus, a reflection-type color display can be obtained. That is, it can display in white when no voltage is applied, but according to a voltage increase, it can display in several colors such as yellow, violet, red and so on.
Next, a conventional color liquid crystal display using a retardation film and adopting an STN liquid crystal device as a liquid crystal device will be explained with reference to
FIG. 19
to FIG.
21
.
FIG. 21
is a schematic sectional view of the color liquid crystal display,
FIG. 19
is a plan view showing a relation between absorption axes of a lower polarizing film and the molecular alignment direction in the liquid crystal obtained when
FIG. 21
is viewed from the upper polarizing film
9
side, and
FIG. 20
is also a plan view showing a relation between absorption axes of an upper polarizing film and the phase delay axis of each retardation film.
Such a color liquid crystal display is disclosed, for instance, in Japanese Patent Laid-o
Armstrong, Westerman Hattori, McLeland & Naughton
Citizen Watch Co. Ltd.
Parker Kenneth
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