JPH0481774B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an image display device, and in particular,
The present invention relates to a gradation display method of a liquid crystal display element using ferroelectric liquid crystal.

[Summary of the Disclosure] This specification and drawings describe a gradation display method for a liquid crystal display element using ferroelectric liquid crystal, in which an alternating voltage is applied to a liquid crystal cell for a certain period of time to change the alignment state of liquid crystal molecules, and the voltage This invention discloses a technique that enables gradation display by matrix drive by increasing the width of the gradation displayable area.

[Prior Art] Typical gray scale display methods for ferroelectric liquid crystal devices are: (1) Gradation display method such as dither method (2) At a frequency where flicker is not perceptible, Three methods have been proposed, including a time modulation method in which each pixel is switched according to the gradation level (3) and a method in which on and off parts are mixed in one pixel using an electric field.

[Problems to be solved by the invention] However, method (1) combines several pixels into one pixel and displays this in gradation, which has the disadvantage that the display becomes rough. This method has the disadvantage that it takes a long time to construct one screen. In addition, in method (3), two threshold values V ₁ and V ₂ (V ₁ <V ₂ ) of the applied voltage V that define the gradation displayable area are used.
The width γ (=V ₂ − V ₁ ) is small, which makes gradation control difficult, and when the area is increased, matrix driving becomes almost impossible due to pixel-by-pixel variations in V ₁ and V ₂ . There was a drawback.

In particular, the present invention provides a gradation display method in method (3), which eliminates the above-mentioned conventional drawbacks by increasing the value of γ, and enables gradation control by matrix driving of ferroelectric liquid crystal. The purpose is to

[Means for Solving the Problems] The present invention provides a method for solving problems in which: between a pair of substrates on which electrodes are formed;
In the gradation display method of a ferroelectric liquid crystal element with a cell structure in which ferroelectric liquid crystals having bistable properties against electric fields are arranged, the amount of transmitted light before application of a pulsed electric field with a peak value V _io (>0) and a width ΔT is I ₀ and the amount of transmitted light after the application of the pulsed electric field is I. When the ferroelectric liquid crystal is V _io <V ₁ , I=I ₀ , and when V ₁ ≦V _io ≦V ₂ , I is V _io 2 such that it increases as the value increases and I remains constant when V ₂ ≦V _io .
By applying an alternating voltage with _an amplitude V _T between the pair of electrodes for a certain period of time, the value of V ₂ −V ₁ is adjusted to a value of V ₂ /V ₁ ≦ ₃ . A gradation display method of a ferroelectric liquid crystal element is characterized in that the gradation is made larger than the value before application of the alternating voltage within the range and the display operation is performed in this state.

[Operation] When an alternating voltage V _T is applied for a certain period of time to a cell sandwiching a ferroelectric liquid crystal that is bistable to an electric field, the alignment state of the liquid crystal molecules changes, and the color tone changes from white to black within one pixel. There will be a mixture of minute parts that take on multiple values. In other words, by applying an alternating electric field, the number of color tones during gradation display increases, which widens the two thresholds V ₁ and V ₂ of the applied voltage V _io that define the gradation display area. It turns out. Therefore, the applied voltage within one pixel
The range of the stable value I of the amount of transmitted light with respect to V _io is also expanded, and by changing the proportion of the minute portion of each color tone by changing the applied voltage V _io ,
It is possible to perform stepwise gradation display. Also,
Even if the values of V ₁ and V ₂ vary somewhat from pixel to pixel, since the width of V ₁ and V ₂ is large, matrix driving can be made possible by setting the selection scanning voltage near the middle thereof.

[Example] FIG. 2 is a schematic diagram of a liquid crystal cell used in the present invention. In the liquid crystal cell 10 shown in FIG. 2, a striped first electrode 1 and a striped second electrode 2 are formed on each of a first substrate 11 and a second substrate 12, and a SiO2 ₂ , an insulating film 3 consisting of
Alignment film 4 made of PVA (polyvinyl alcohol)
is formed. The alignment film 4 is subjected to a rubbing treatment, and a ferroelectric liquid crystal 5 is sealed between the substrates which are arranged facing each other so that the electrodes form a matrix structure. The first electrode 1 and the second electrode 2 are
It is connected to an external power supply 6, and by applying an alternating voltage V _T from this power supply 6,
The alignment state of liquid crystal is controlled. This cell is qualitatively different from conventional gradation display cells in which binary states consisting of an on part and an off part are mixed within one pixel, and this cell is a microscopic part whose color tone ranges from white to black. It is characterized in that the colors are mixed in one pixel, and the ratio of minute portions of each color tone is changed depending on the electric field strength.

FIG. 3a is a schematic diagram of a pixel of a gradation display cell in which a conventional binary state consisting of an on part and an off part is mixed in one pixel, and FIG. 3b is a schematic diagram of a pixel according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a pixel of a cell used in FIG.
The minute portions with multivalued color tones as shown in Figure 3b are first realized by applying an alternating voltage to the cell for a certain period of time, and the stability of each minute portion with respect to one color tone is determined by the surroundings. It is thought that it is maintained by being mixed with minute parts.

The above cell is supplied with 15V (V _pp alternating voltage V _{T )} from power supply 6.
was applied to change the alignment state of liquid crystal molecules. Although the alternating voltage was applied through each electrode here, the entire cell may be sandwiched between wider electrodes and the alternating voltage may be applied from these electrodes.

FIG. 1 is a diagram showing the relationship between the applied voltage V _io (pulse width 1 msec) and the stable value I of the amount of transmitted light before and after applying the alternating voltage. In FIG. 1, a represents the characteristics before application of alternating voltage, and b
represents the characteristics after application. The relationship between the applied voltage V _io and the stable value I of the amount of transmitted light changed from a to b by applying an alternating voltage. At this time, the gradation displayable area γ before and after applying the AC electric field = V ₂ − V ₁
Before application, a γ = 2.6V, and after application, b γ = 8.0V. The threshold voltages V ₁ and V ₂ of the liquid crystal vary somewhat depending on the pixel, but in general, it is desirable that the value of γ be large in order to display gradation based on the voltage. That is, by applying the alternating voltage V _T as described above, voltage gradation display becomes easy. Note that the stable value I of the amount of transmitted light can be controlled step by step when V ₁ ≦V _io ≦ V ₂ . However, as will be described later, during matrix drive, the selection scanning voltage is set near the midpoint between V ₁ and V ₂ , and the absolute value of the display voltage is given in a range smaller than V ₁ according to the gradation signal, so the above To make maximum use of the control range, it is necessary to satisfy V ₂ /V ₁ ≦3.

FIG. 4 is a waveform diagram showing a specific optical response when V _io =10V. This optical response is achieved by sandwiching the cell between polarizers arranged in a crossed Nicol relationship and adjusting it to the darkest state, and then using a photomultiplier to adjust the amount of transmitted light I ₀ in response to a pulsed applied voltage V _io . Detected and recorded on an oscilloscope, the horizontal axis shows time t, and the vertical axis shows transmitted light amount I ₀ and applied voltage V _io . As shown in Figure 4, when a pulse-like applied voltage V _io is applied, generally when a pulse is being applied, the amount of transmitted light I ₀ increases monotonically with time, and reaches its maximum value just before the pulse ends. reach. Further, after the pulse ends, the amount of transmitted light I ₀ gradually decreases with time with a certain relaxation part, but after a sufficient time has passed, it reaches a stable value I that does not change with time. As shown in FIG. 1, the stable value I of the amount of transmitted light is V _io
At ≦V ₁ (5V), it is equal to the value before voltage application. Therefore, even if flickering occurs depending on the voltage, there is no reversal. Also, when V ₁ (5V) ≦V _io ≦V ₂ (13V),
The stable value I of the amount of transmitted light increases monotonically with respect to the applied voltage _Vio . That is, by using this area, gradation display of ferroelectric liquid crystal can be performed. Further, when V _io ≧V ₂ (13V), the stable value I of the amount of transmitted light is constant, and thus an optical response with little overshoot is exhibited.

Next, a specific gradation display method will be described. FIG. 5 schematically shows the matrix electrode structure in the liquid crystal cell described above, in which Y ₁ to _Yo represent scanning electrodes, and X ₁ to X _n represent display electrodes. Also,
FIG. 6 is a time chart of the voltages applied to the k-th scanning electrode Y _k and the l-th display electrode X _l , the selected pixels V _Y -V _X , and the non-selected pixels. here,
It is assumed that black is written to a pixel on the display screen with a negative voltage, and white is written with a positive voltage. It is also assumed that the mode of each pixel is all unified to the white state before the start of scanning.

In FIG. 6, E of the scanning electrode Y _k is an erasing pulse, and -V _E is the erasing pulse voltage.
Further, W _Y is a write pulse for _Yk , and V _WY is a write pulse voltage. The voltage V _Y applied to the scanning electrode Y _k is in the range 0≦t≦Δt |V _E |≧V ₂
It is a _constant value −V _{E that} satisfies
It is 0 in the area of .

On the other hand, _WX of the display electrode _Xl is a writing pulse for _Xl , and _VWX represents the writing pulse voltage.
Further, C is a crosstalk prevention pulse, which is used to prevent a voltage in the same direction from being continuously applied to non-selected points for a long time.

The voltage V _X applied to the display electrode X _l is 0≦t≦Δt
It is 0 in the region where Δt≦t≦2Δt and −V ₁ ≦
The displacement voltage V WX satisfies V _WX ≦V ₁ and changes depending on the brightness of the pixel, and is −V _WX in the region of 2Δt≦ _t ≦3Δt.

As described above, by applying the voltage V _Y of the scanning electrode and the voltage _{V X} of the display electrode, _as shown in FIG.
In the region of Δt, a voltage of −V _E where |V _E |≧V ₂ is applied, and the pixel is inverted to black. Next, in the region of Δt≦t≦2Δt, a voltage that satisfies V ₁ ≦V _WY −V _WX ≦3V ₁ and changes depending on the brightness of the pixel is applied, and the pixel becomes V _io −I shown in FIG. In accordance with the characteristics, the light transmission state has a uniform gradation, that is, it transitions to a gradation white state. Furthermore, in the following region of 2Δt≦t≦3Δt, −
A voltage of V ₁ ≦V _io ≦V ₁ is applied to the pixel, but as shown in FIG. 1, the stable value I of the amount of transmitted light does not change when |V _io |≦V ₁ . Note that the gradation writing voltage (V _WY −V _WX ) applied to the selected point is determined by applying an opposite voltage in the region of Δt≦t≦2Δt, and then applying this positive voltage to the selected pixel continuously. , also plays a role in preventing the gradation from becoming uncontrollable.

On the other hand, as shown in FIG. 6d, in the non-selected pixels, no voltage is applied in the region of 0≦t≦Δt;
Pixels are not inverted. Also, in the region of Δt≦t≦2Δt
Although a voltage of V _WX is applied, since -V ₁ ≦V _WX ≦V ₁ , the stable value I of the amount of transmitted light does not change. Continued 2Δt≦t≦
A voltage of −V _WX is applied even in the region of 3Δt, but −V ₁ ≦
−V _WX ≦V ₁ , so the stable value of the amount of transmitted light I
does not change.

In this way, by applying the voltages shown in FIGS. 6a and 6b to the scanning electrode _Yk and the display electrode _Xl , the selected pixel enters a light transmitting state according to the gradation signal,
The state of non-selected pixels does not change.

FIG. 7 shows a time chart of applied voltages when scanning electrodes and display electrodes are further scanned continuously in a time-division manner. On the scanning electrode side in Fig. 7a, the sixth
The applied voltage with the waveform shown in figure a is applied to each scan electrode Y ₁ ,
Y ₂ , ..., _Yo are applied in this order with a phase difference of 3Δt. In addition, on the display electrode side in FIG. 7b, the applied voltage with the waveform shown in _FIG . _6b is applied to each display _electrode X ₁ , Applied according to brightness.

When we set specific voltage values based on the time chart shown in Figure 7 and displayed an image using matrix drive, we found that a ferroelectric liquid crystal display with a ferroelectric liquid crystal level, which was considered impossible with conventional analog methods, It has been confirmed that it is possible to display In particular, in this example, by setting V _WY =2V ₁ , it was possible to efficiently obtain gradations from white to black.

[Effects of the Invention] According to the present invention, by applying an alternating voltage to a liquid crystal cell, it is possible to mix minute portions with multivalued color tones from white to black within one pixel, and also to display gradation. The possible region γ can be enlarged, and gradation display can be realized by matrix driving of ferroelectric liquid crystal. In this case, even if there is some variation in the threshold values V ₁ and V ₂ of the applied voltages, it will not be affected, so matrix driving is fully possible even when the area is increased. Therefore, when constructing a ferroelectric liquid crystal element, there is no need for digital gradation such as dithering, the displayed image does not become coarse, and the number of scanning electrodes and display electrodes can be dramatically reduced. be able to. Further, unlike the time modulation method, the time required to construct one screen is not delayed.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the applied voltage V _io and the stable value I of the amount of transmitted light, Figure 2 is a schematic diagram of a liquid crystal cell, and Figure 3
The figure is a schematic diagram of a pixel, Figure 4 is a waveform diagram showing optical response, Figure 5 is a schematic diagram of a matrix electrode structure, and Figure 6 is a schematic diagram of a matrix electrode structure.
The figure and FIG. 7 are time charts of applied voltage. 1: first electrode, 2: second electrode, 3: insulating film, 4: alignment film, 5: ferroelectric liquid crystal, 6: power supply,
10: Liquid crystal cell, 11: First substrate, 12: Second
substrate, Y ₁ , Y ₂ ,..., _Yo : scanning electrode, X ₁ , X ₂ ,
..., X _n : Display electrode.

Claims (1)

[Claims] 1. A gradation display method for a ferroelectric liquid crystal element having a cell structure in which a ferroelectric liquid crystal having bistability to an electric field is arranged between a pair of substrates on which electrodes are formed, comprising: When the amount of transmitted light before the application of a pulsed electric field with a high value V _io (>0) and a width ΔT is I ₀ , and the amount of transmitted light after the application of the pulsed electric field is I, when the ferroelectric liquid crystal has V _io <V ₁ , I = I ₀ , V ₁ ≦V _io ≦V ₂ , I increases with the increase of V _io , and V ₂ ≦V _io , I has two thresholds V ₁ and V ₂ such that I is constant; By applying an alternating voltage of amplitude V _T between a pair of electrodes for a certain period of time, the value of V ₂ - V ₁ is made larger than the value before applying the alternating voltage within the range of V ₂ /V ₁ ≦3, and this 1. A gradation display method for a ferroelectric liquid crystal element, characterized in that a display operation is performed in a certain state. 2 The above electrodes are formed in a matrix, and the voltage V _Y of the k-th scanning electrode Y _k is a constant value −V _E that satisfies |V _E |≧V ₂ in the region of 0≦t≦Δt, and Δt≦
In the region of t≦2Δt, V _WY = 2V ₁ , which is a constant value,
In the region where _2Δt ≦t≦3Δt, _the voltage _V
According to claim 1, the variable V _WX satisfies V _WX ≦V ₁ and changes depending on the brightness of the pixel, and is −V _WX in the region of 2Δt≦t≦3Δt. gradation display method for ferroelectric liquid crystal elements. 3. Claim 1, characterized in that minute portions with multi-valued color tones from white to black are mixed in one pixel, and the ratio of the minute portions of each color tone is changed depending on the electric field strength. Or the gradation display method of a ferroelectric liquid crystal element according to item 2.