JPH07122710B2 - Optical modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical modulator for a display panel, and more particularly to a display panel using a liquid crystal material having bistability, particularly a ferroelectric liquid crystal. The present invention relates to a liquid crystal optical modulation element suitable for gradation display.

[Prior Art] In a liquid crystal television panel using a conventional active matrix driving method, thin film transistors (TFTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TFT to establish a conduction state between a source and a drain. At this time, a video image signal is applied from the source and accumulated in the capacitor, and a liquid crystal (for example,
Twisted nematic (TN-liquid crystal) is driven, and at the same time, gradation display is performed by modulating the voltage of the video signal.

However, in such an active matrix drive type television panel using TN liquid crystal, since the TFT used has a complicated structure, there are many structural steps, and high manufacturing cost is a bottleneck. , It is difficult to form a thin film semiconductor (for example, polysilicon or amorphous silicon) forming a TFT over a wide area.

On the other hand, a passive matrix drive type display panel using TN liquid crystal is known as one that can be manufactured at a low manufacturing cost. In this display panel, one screen (one frame) is displayed as the scanning line (N) increases. The time (duty ratio) in which an effective electric field is applied to one selected point during scanning decreases at a rate of 1 / N, which causes crosstalk and does not result in a high-contrast image. In addition, since it becomes difficult to control the gradation of each pixel by voltage modulation when the duty ratio is low, it is not suitable for a display panel having a high density of wirings, particularly a liquid crystal television panel.

[Problems to be Solved by the Invention] In the related art, a stripe filter may give a feeling of vertical stripes. An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and it is an object of the present invention to provide a gradation display using a ferroelectric liquid crystal.

[Means for Solving Problems] That is, according to the present invention, a plurality of vertically long pixels in which an optical modulation substance is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are shared in each row. A scanning electrode group connected to each other and an information electrode group commonly connecting a plurality of pixels for each column, each scanning electrode having the conductive film and a transmission line having a resistance lower than that of the conductive film. An optical modulation element comprising a drive means for driving the optical modulation element so that a potential difference gradient is generated between the pair of conductive films forming the pixel by applying different voltages to two adjacent transmission lines. The optical modulation element is characterized in that the pixel is configured such that a direction in which a boundary between bright and dark changes based on a potential difference gradient changes is in a longitudinal direction of the vertically long pixel.

In the present invention, the rectangular pixel has a stripe filter of three primary colors of red, green, and blue along the direction of its long side,
It is preferable that color gradation display is performed.

[Operation] In the optical modulation element of the present invention, a rectangular pixel is provided in the first conductive film or the second conductive film, and the long side direction of the rectangular pixel is
By applying a signal whose voltage value, pulse width or number of pulses is modulated according to the gradation information, the direction in which the boundary between bright and dark spreads can be made substantially the same direction. Then, it is possible to eliminate the feeling of vertical stripes that a human feels and to perform uniform display accurately.

EXAMPLES The present invention will be described below with reference to the drawings.

The optical modulator used in the present invention includes a first optical stable state (for example, a bright state is formed) and a second optical stable state (for example, a dark state is formed) according to an applied electric field. Is preferred, that is, a substance having at least two stable states with respect to an electric field, particularly a liquid crystal having such a property.

As a liquid crystal having bistability that can be used in the optical modulator of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, and among them, a chiral smectic C phase (SmC ^* ), H phase (SmH ^* ), I Phase (Sm
I ^* ), F-phase (SmF ^* ), and G-phase (SmG ^* ) liquid crystals are suitable. About this ferroelectric liquid crystal, "LE JOURNAL DE PH
YSIQUE LETTRE ") 1975, 36 (L-69)," Ferroelectric Liquid Crystals "(" Ferroelect
ric Liquid Crystals ”);“ Applied physics Letters ”1980
No. 36 (11), "Submicro Second Bistable Electro-Optic Switching In Liquid Crystals"("Submicro Second Bistable")
Electrooptic Switching in Liquid Crystals ");
"Solid State Physics", 1981, 16 (141), "Liquid Crystal", etc., and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, examples of the ferroelectric liquid crystal compound used in the method of the present invention include desiloxybenzylidene-p'-amino-2-methylbutylcinnamate (DOBAMBC) and hexyloxybenzylidene-p'-amino-. 2-chloropropyl cinnamate (HOBACPC) and 4-o- (2
-Methyl) -butyl resorcylidene-4'-octylaniline (MBRA 8) and the like.

When a device is formed using these materials, the liquid crystal compound is kept in a temperature state such that the liquid crystal compound is in the SmC ^* phase, SmH ^* phase, SmI ^* phase, SmF ^* phase, or SmG ^* phase. The element can be supported by a copper block or the like with embedded heaters.

FIG. 4 is a schematic drawing of an example of a ferroelectric liquid crystal cell. 21a and 21b are In ₂ O ₃ , SnO ₂ and ITO (Indium-Tin-O
(xide) and the like, which is a substrate (glass plate) coated with a transparent electrode, in which SmC ^* phase liquid crystal oriented so that the liquid crystal molecular layer 22 is perpendicular to the glass surface is enclosed. The thick line 23 represents a liquid crystal molecule.
_{Has a} dipole moment (P _⊥ ) 24 in a direction orthogonal to its molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and all the dipole moments (P _⊥ ) 24 are oriented in the electric field direction. Can be changed. The liquid crystal molecule 23 has an elongated shape and exhibits refractive index anisotropy in the major axis direction and the minor axis direction thereof, and therefore, for example, polarizers arranged in a crossed Nicols position above and below a glass surface may be placed. For example, it is easy to understand that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. Further, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1 μm), the helical structure of the liquid crystal molecules is unraveled (non-helical structure) even when no electric field is applied as shown in FIG. The dipole moment Pa or Pb takes either the upward (34a) or downward (34b) state. When electric fields Ea or Eb having different polarities equal to or higher than a certain threshold value are applied to such a cell for a predetermined time as shown in FIG. 5, the dipole moments are directed upward 34a or downward 34b in accordance with the electric field vector of the electric fields Ea or Eb. The liquid crystal molecules are oriented in either the first stable state 33a (bright state) or the second stable state 33b (dark state) accordingly.

There are two advantages of using such a ferroelectric liquid crystal as an optical modulation element. First, the response speed is extremely fast, and second, the alignment of liquid crystal molecules has a bistable state. Explaining the second point with reference to FIG. 5, for example, when an electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a, but this state is stable even when the electric field is cut off. Also,
When a reverse electric field Eb is applied, the liquid crystal molecules are oriented in the second stable state 33b and change their orientation. However, even when the electric field is turned off, the liquid crystal molecules are kept in this state and the memory function is maintained in each stable state. Have In order to effectively realize such a high response speed and bistability, it is preferable that the cell is as thin as possible, and generally 0.5 μ to 20 μm.
μ, particularly 1 μ to 5 μ is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using a ferroelectric liquid crystal of this type is disclosed, for example, by Clark and Lagabal in US Pat.
No. 4,367,924.

Next, details of a liquid crystal optical element as an example of the optical modulation element used in the present invention will be described with reference to FIG. FIG. 1 is a perspective view showing one substrate of the optical modulator of the present invention.

1 in FIG. 1 is one substrate. Reference numeral 2 is a display conductive film, which is laminated on the substrate 1. Reference numeral 3 denotes a transmission electrode made of a low-resistance metal film, which is laminated on the display conductive film 2 in parallel at equal intervals. The other substrate (not shown) faces the substrate 1, and a counter conductive film (counter electrode) 4 is arranged in a region corresponding to the region of the pixel A in the figure on the other substrate. . Conductive film for display 2
The above-mentioned optical modulator is sandwiched between the counter electrode 4 and the counter electrode 4.

In the liquid crystal optical element configured as described above, a potential gradient is generated in the electric field between the counter electrode 4 and the inner surface of the display conductive film 2 by the signal voltage applied to the transmission electrode 3. At this time, the transfer electrode 3b is connected to the reference potential point V _E.
When a predetermined signal voltage Va is applied to the transmission electrode adjacent to the transmission electrode connected to the reference potential point for the transmission electrodes 3a and 3c (for example, 0 volt), the transmission electrode as shown in FIG. In-plane length direction l ₁ of the conductive film 2 between the spaces 3a and 3b or 3b and 3c
A potential gradient of Va can be applied to and l ₂ .

At this time, when the inversion threshold voltage Vth of the ferroelectric liquid crystal is set to Va, if -Vb is applied to the counter electrode 4, as shown in FIG. A ferroelectric liquid crystal corresponding to m ₁ and m ₂ has a potential difference Va + Vb greater than the inversion threshold voltage Vth.
Is applied, and the regions corresponding to m ₁ and m ₂ can be inverted from the bright state to the dark state, for example.

Therefore, in the present invention, the gradation can be expressed by applying Vb with a value according to the gradation for each pixel. At this time, the voltage value of the voltage signal −Vb applied to the counter electrode 4 may be modulated according to the gradation information, or its pulse width may be modulated according to the gradation information, or its pulse Gradation can be controlled by modulating the number.

Further, in the present invention, prior to applying the above-mentioned gradation signal,
After performing an erasing step to put the pixel into one of a bright state and a dark state, an inversion voltage for inverting the state is applied to the ferroelectric liquid crystal under the control of gradation. It is necessary.

Next, FIG. 3 shows a state diagram as seen from directly above in the vicinity of the pixel A in FIG. As an example, the pitch P of the transmission electrodes is 21.
The width W of the counter conductive film is 0 μm and 70 μm. Further, the counter conductive film 4R has red, the counter conductive film 4G has green, and the counter conductive film 4B has a striped filter formed by a method such as vacuum deposition or dyeing of organic pigments of blue, respectively.

The picture element, which is a set of three pixels of red, green, and blue, is formed into a substantially square shape, and can express all colors. That is, each color pixel has a rectangular shape, and a striped filter is arranged in the direction of the long side of the pixel to widen the boundary between light and dark.

[Effects of the Invention] As described above, according to the optical modulation element of the present invention, good gradation display is performed, and the stripe filter eliminates the sense of vertical stripes felt by humans, and a uniform image can be obtained. .

[Brief description of drawings]

FIG. 1 is a perspective view showing one substrate used for the optical modulation element of the present invention, FIGS. 2 (a) and 2 (b) are explanatory views schematically showing the potential gradient between the conductive films, and FIG. FIGS. 4 and 5 are schematic views showing the gradation of a pixel in the invention, and FIG. 4 and FIG. 5 are perspective views schematically showing a ferroelectric liquid crystal element used in the invention. 1 ... Substrate 2 ... Display conductive film 3 ... Transfer electrode 4 ... Counter conductive film A ... Pixel

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Claims (3)

[Claims] 1. A scanning electrode group in which a plurality of vertically long pixels in which an optical modulation substance is arranged between a pair of conductive films facing each other are arranged in a matrix, and a plurality of pixels are commonly connected in each row, and in each column. An information electrode group in which a plurality of pixels are commonly connected is provided, and each of the scan electrodes has the conductive film and a transmission line having a resistance lower than that of the conductive film. An optical modulation element including a driving unit that drives the optical modulation element so that a potential difference gradient is generated between the pair of conductive films forming the pixel by application, and a bright / dark boundary generated based on the potential difference gradient is generated. The optical modulation element, wherein the pixel is configured such that the changing direction is the longitudinal direction of the vertically long pixel. 2. The optical modulation according to claim 1, wherein the vertically long pixel has any one of filters of three primary colors of red, green and blue, and color gradation display is performed. element. 3. The optical modulation element according to claim 1, wherein the optical modulation substance is a ferroelectric liquid crystal.