Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-05-08
2003-08-05
Ghyka, Alexander (Department: 2812)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S118000, C349S119000, C349S121000
Reexamination Certificate
active
06603522
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal shutter or, in particular, to a liquid shutter in which the peripheral circuit or especially the drive circuit can be simplified and the cost and power consumption can be reduced by realizing a low-voltage drive, a high-speed shutter on-off response, a high contrast, etc. The liquid crystal shutter according to this invention has the various advantages as described above, and therefore can find application in various fields such as battery-driven portable equipment, liquid crystal photoprinters and liquid crystal optical elements.
BACKGROUND ART
Conventionally, various liquid crystal shutters have been proposed. Their performance, however, is not yet satisfactory, and market demand is high for a liquid crystal shutter for a liquid crystal printer or a liquid crystal optical element high in response speed, bright, high in contrast, drivable by a simple method and capable of a gradated display.
The liquid crystal shutters under development are roughly classified, according to the type of the liquid crystal material used, into (1) a liquid crystal shutter using an ordinary nematic liquid crystal, (2) a liquid crystal shutter using a two-frequency drive nematic liquid crystal having positive or negative dielectric constant depending on the frequency, and (3) a liquid crystal shutter using a ferroelectric liquid crystal having a spontaneous polarization.
The liquid crystal shutter using the ordinary nematic liquid crystal in (1) above is further divided, according to the operating principle, into two well-known types including (a) a TN (twisted nematic) liquid crystal system in which white or black is displayed by use of the property of the optical rotatory power rotating the incident light, and the optical rotatory power is canceled by applying a voltage to the pixels and erecting the liquid crystal molecules at an angle substantially perpendicular to the substrate, and (b) a STN (super twisted nematic) liquid crystal system using a liquid crystal element having a twist angle of 180° to 260° in which black or white is displayed by applying a voltage to the pixels and thus changing the birefringence which causes a retardation of the incident light.
The prior art having the configuration described in (a) above is disclosed in, for example, Japanese Unexamined Patent Publication No. 62-150330. This conventional system is configured with a TN liquid crystal element having a twist angle of 90° and a pair of polarizing plates arranged on the two sides of the liquid crystal element in such a manner the absorption axes thereof are orthogonal to each other.
Another conventional system is disclosed, for example, in Japanese Unexamined Patent Publication No. 9-113864. In this prior art, which uses a liquid crystal element having a twist angle of 270°, polarizing plates are arranged parallel or perpendicular to the alignment direction of the upper and lower substrates thereby to utilize the optical rotatory power.
The conventional system of (b) described above includes a STN liquid crystal display apparatus called the yellow mode used as an ordinary liquid crystal display apparatus. In the conventional system such as the STN liquid crystal display apparatus having a twist angle of 240°, for example, the angles of intersection between the absorption axes of the upper and lower polarizing plates is about 60°, and the angle between the alignment direction of the substrates and the absorption axes of the polarizing plates is set to about 45°.
In the case where the a twist angle of 90° is used in the system of (a) above, the response time required to transfer from the open state to the closed state by applying a voltage is as short as several ms, while the problem is the slow response time of 10 to several tens of ms for returning from the closed state to the open state by removing the voltage. Further, in the case where the twist of 270° is used, the orientational stability of the liquid crystal element is difficult to achieve, which makes it impractically necessary to use an obliquely vapor deposited SiO film or the like special alignment layer capable of attaining a high pretilt.
In the system of (b), on the other hand, the use of a practical STN liquid crystal element having the twist angle of 225° to 250° can attain a short response time of several ms for transferring from closed to open state. In the closed state, however, a voltage is applied to the liquid crystal element to make a bluish black liquid crystal with a low contrast of about 10. In addition, a further increase in applied voltage undesirably increases the brightness again by changing the elliptical polarized state. Therefore, the applied voltage cannot be set to a very high level, with the result that the response time from open to closed state becomes as long as 10 to several tens of ms, thereby posing the problem of poor practicability as a liquid crystal shutter.
Further, based on the aforementioned study of the prior art, the present inventor has proposed a system using a liquid crystal element having a twist angle of 180° to 260°, as disclosed in Japanese Unexamined Patent Publication No. 9-119219, in which the birefringence is utilized for white display as in the STN liquid crystal system while the optical rotatory power is canceled for black display by erecting the liquid crystal molecules substantially perpendicular to the substrate.
Now, the system proposed by the present inventor will be explained with reference to
FIGS. 11 and 12
.
FIG. 12
is a sectional view showing the structure of this liquid crystal shutter, and
FIG. 11
is a plan view showing the alignment direction of liquid crystal molecules as viewed from the top in FIG.
12
. This liquid crystal shutter, as shown in
FIG. 12
, has a liquid crystal element
10
having a twist angle of 240°, a lower polarizing plate
8
and an upper polarizing plate
9
.
The liquid crystal element
10
is formed of a first substrate
1
of glass 0.7 mm thick with a first electrode
2
of ITO and an alignment layer
3
formed thereon, a second substrate
4
of glass 0.7 mm thick with a second electrode
5
of ITO providing a transparent electrode and an alignment layer
6
formed thereon, and a nematic liquid crystal
7
. The birefringence index &Dgr;n of the nematic liquid crystal
7
used in this case is 0.2, and the gap d between the first substrate
1
and the second substrate
4
is 4 &mgr;m. Therefore, the value &Dgr;nd indicating the birefringence characteristic of the liquid crystal element
10
is set to 0.8 &mgr;m,
The alignment layer
3
associated with the first substrate
1
is rubbed in the alignment direction of the lower liquid crystal molecules located on the boundary surface of the first substrate
1
arranged on the lower side, i.e. in the alignment direction
7
b
(See
FIG. 11
) of the lower liquid crystal molecule. The alignment layer
6
of the second substrate
4
, on the other hand, is rubbed in the alignment direction of the upper liquid crystal molecules located on the boundary surface of the second substrate
4
located on the upper side, i.e. in the direction
7
a
(See
FIG. 11
) of alignment of the upper liquid crystal molecule. Further, a substance having an optically rotatory power called a chiral material is added to the nematic liquid crystal
7
having a viscosity of 18 cp, so that the twist pitch P is adjusted to 8 &mgr;m to obtain the relation d/P=0.5 thereby to form a liquid crystal element having a counterclockwise twist angle of 240°.
Also, the lower polarizing plate
8
and the upper polarizing plate
9
are arranged on the two sides, respectively, of the liquid crystal element
10
in such a manner that the absorption axis
8
a
of the lower polarizing plate and the absorption axis
9
a
of the upper polarizing plate are orthogonal to each other. The absorption axis
8
a of the lower polarizing plate is arranged at an angle of 45° counterclockwise to the direction
12
of the central liquid crystal molecule indicating the alignment direction of liquid crystal
Citizen Watch Co. Ltd.
Ghyka Alexander
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