Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-01-08
2004-12-21
Nguyen, Dung T. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S101000
Reexamination Certificate
active
06833887
ABSTRACT:
INDUSTRIAL FIELD
The present invention relates to a liquid crystal shutter having a fast response such as a liquid crystal optical printer or a liquid crystal optical device (which is used, for instance, in a color video printer or an LED combined field sequential color display), and to a method of driving the liquid crystal shutter.
BACKGROUND ART
Requirements for a liquid crystal shutter for use in a liquid crystal printer or a liquid crystal optical device are a rapid response, a bright display, a high contrast and a simple driving method, as well as a possible gradation display. However, a liquid crystal shutter satisfying all these requirements has not been developed so far.
The liquid crystal shutters which have hitherto been developed are roughly grouped into the following three categories by liquid crystal materials used:
(1) one using a general nematic liquid crystal;
(2) one using a nematic liquid crystal for two-frequency driving method having a positive or negative dielectric constant depending on the frequencies; and
(3) one using a ferroelectric liquid crystal having a spontaneous polarization.
The liquid crystal shutter using the two-frequency driving method mentioned above (2) has a rapid response but has a complicated driving circuit due to its high driving voltage and high driving frequency.
The liquid crystal shutter using the ferroelectric liquid crystal of (
3
) above operates faster than that using the two-frequency driving liquid crystal, that is, with a response time of several tens of &mgr;s, but is deficient in the stability of orientation due to use of a smectic liquid crystal phase. It also brings about a sticking phenomenon in which a display pattern remains fixed due to the DC drive and entails in principle a difficulty with the gradation control, which prevent it from being put to practical use except in certain specific applications.
The liquid crystal shutter using the general nematic liquid crystal mentioned above (1) employs the following systems depending on the principle of operation:
(a) a so-called TN (twisted nematic) liquid crystal system in which a white or black display is performed by utilizing a phenomenon called rotary polarization, rotating the incident light, in which a black or white display is performed by applying a voltage to pixels so as to orientate the liquid crystal molecules substantially orthogonal to the substrates to thereby eliminate the rotary polarization; and
(b) a so-called STN (super twisted nematic) liquid crystal system in which a white or black display is performed by utilizing birefringence causing a phase difference in the incident light, in which a black or white display is performed by applying a voltage to the display pixels to thereby vary the birefringence.
An example of the liquid crystal system of (a) above is found in Japanese Patent Laid-open Pub. No. Sho62-150330.
Reference is made to
FIGS. 10 and 11
to explain this.
FIG. 11
is a schematic sectional view of the conventional TN liquid crystal shutter, and
FIG. 10
is a top plan view showing a relationship between absorption axes of polarizing plates and the direction in which liquid crystal molecules are orientated, obtained when a liquid crystal shutter shown in
FIG. 11
is viewed from the upper polarizing plate side.
As illustrated in
FIG. 11
, the liquid crystal device comprises a first transparent substrate
1
on which are formed a transparent first electrode
2
made of indium tin oxide (ITO) and an orientation film
3
, a second transparent substrate
4
on which are formed a transparent second electrode
5
made of ITO and an orientation film
6
, and a nematic liquid crystal
7
sealed in between the first and second substrates. On the top and bottom of the liquid crystal device there are arranged an upper polarizing plate
9
and a lower polarizing plate
8
, respectively, in such a manner that their respective absorption axes are orthogonal to each other, to thereby constitute the TN liquid crystal shutter.
As shown in
FIG. 10
, in this case, the liquid crystal device has a twisted angle of 90°, with the absorption axis
13
of the lower polarizing plate
8
being parallel to the direction
10
in which lower liquid crystal molecules are orientated, that is, the direction of orientation of molecules closer to the first transparent substrate
1
, and with the absorption axis
14
of the upper polarizing plate
9
being parallel to the direction
11
in which upper liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the second transparent substrate
4
.
With no voltage applied, in this TN liquid crystal shutter, linearly polarized light transmitted through the lower polarizing plate
8
is rotated by 90° due to the rotary polarization of the liquid crystal and exits the upper-polarizing plate
9
, resulting in an opened state allowing a so-called positive display. When a 15V voltage is applied at a 5 kHz driving frequency between the first electrode
2
and the second electrode
5
, the molecules of the nematic liquid crystal are orientated in the direction orthogonal to the transparent substrates
1
and
4
to nullify the rotary polarization, thus allowing the linearly polarized light transmitted through the lower polarizing plate
8
to advance intactly through the interior of the liquid crystal device without any rotation and to be blocked by the upper polarizing plate
9
, resulting in a closed state.
An example employing method (b) above includes an STN liquid crystal display called a yellow mode for use in general liquid crystal displays. A conventional example thereof will be described with reference to
FIGS. 12 and 13
.
FIG. 13
is a schematic sectional view of a conventional STN liquid crystal display, and
FIG. 12
is a top plan view showing a relationship between the absorption axes of the polarizing films and the direction in which the liquid crystal molecules are orientated, obtained when
FIG. 13
is viewed from the upper polarizing plate side.
The configuration of the liquid crystal device shown in
FIG. 13
is similar to the configuration of the liquid crystal device shown in
FIG. 11
, and hence identical parts to those of
FIG. 11
are designated by the same reference numerals and are not again described.
On the top and bottom of the liquid crystal device having the nematic liquid crystal
7
sealed in between the first and second transparent substrates
1
and
4
there are arranged the upper polarizing plate
9
and the lower polarizing plate
8
in such a manner that their respective absorption axes intersect at 60° relative to each other, thereby constituting an STN liquid crystal display.
As shown in
FIG. 12
, in this case, the liquid crystal device has a twisted angle of 240°, with the absorption axis
13
of the lower polarizing plate
8
being angled at 45° relative to the direction
10
in which the lower liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the first transparent substrate
1
, and with the absorption axis
14
of the upper polarizing plate
9
being angled at 45° relative to the direction
11
in which the upper liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal closer to the second transparent substrate
4
.
Thus, relative to the direction
12
in which the intermediate liquid crystal molecules are orientated, that is, the direction of orientation of the liquid crystal molecules intermediate between the first transparent substrate
1
and the second transparent substrate
4
, the absorption axis
13
of the lower polarizing plate
8
forms an angle of 75° with the absorption axis
14
of the upper polarizing plate
9
forming an angle of 15°.
With no voltage applied, in this STN liquid crystal display, linearly polarized light incident at 45° relative to the liquid crystal molecules through the lower polarizing plate
8
is turned into elliptically polarized light due to the birefringence of the nematic liquid crystal
Kaneko Yasushi
Matsunaga Masaaki
Citizen Watch Co. Ltd.
Nguyen Dung T.
Westerman Hattori Daniels & Adrian LLP
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