KR20010023121A - Method of driving a bistable cholesteric liquid crystal device - Google Patents

Abstract

The present invention relates to a liquid crystal device having at least three different levels of light attenuation, comprising: a first metastable state with a first twist angle in the absence of a substantial applied field and a metastable state, and a first twist angle in the absence of a substantially applied field. Includes a bistable twisted nematic liquid crystal cell having another second twist angle and comprising a second metastable state that is metastable. The apparatus further comprises selecting a first portion for resetting the liquid crystal to a reset state with a first twist angle, a relaxed second portion with a second twist angle, and at least three different light attenuation levels, respectively. And an address generator for applying an electric field to the cell having a waveform comprising a third portion comprising any one of at least three different waveforms. At least one of the attenuation levels, the first portion of the liquid crystal cell is in a first metastable state and the second portion of the liquid crystal cell is in a second metastable state.

Description

A method of driving a bistable cholesteric liquid crystal device {METHOD OF DRIVING A BISTABLE CHOLESTERIC LIQUID CRYSTAL DEVICE}

As used herein, the term "twist" refers to the angle at which the liquid crystal director rotates from one surface of the cell to another in the plane of the liquid crystal cell.

The bistable twist nematic (BTN) effect is described in EP 0 018 180, US 4 239 345 and in "New Bistable Liquid Crystal Twist Cell", D.W. Berreman et al, J. Appl. Phys., 1981, volume 52 (4), page 3032. 1 of the accompanying drawings shows a simple example of the BTN operation mode. A cholesteric (twist nematic) material with a pitch almost twice the cell gap in a cell with parallel or antiparallel alignment directors has an initial state with a twist of 180 degrees. In the antiparallel alignment, the 180 degree twist state has the disadvantage that splay occurs. Thus, in addition to the initial stable state, there may be two additional metastable states with no splay but with less desirable twist angles, ie 0 degrees and 360 degrees. These metastable states can occur much faster than the 180-degree state and are two states used in BTN mode. For thin film cells between polarizers crossing or orthogonal, the 360 degree twisted state represents black, while the 0 degree twisted state with an optical axis in the 45 degree direction to the polarizer direction represents white.

The stable "twist-display" state is shown as 1 in a liquid crystal cell with antiparallel alignment layers 2 and 3. An initial reset pulse 4 is applied between the liquid crystal 5 and the electrodes (not shown) sandwiching the alignment layers 2, 3. The reset pulse 4 is represented by 6 in which the liquid crystal molecules in the bulk of the layer (near the vicinity of the alignment layers 2, 3) are oriented perpendicular to the cell surface, for example as shown by the molecule 7. It has sufficient amplitude and duration to cause a transition to a homeotropic state.

Then, one of the two types of select addressing pulses 8 is applied to the electrode to select the state of the desired device. One type of selective addressing pulse switches the homeotropic state 6 to a metastable black state where the liquid crystal has a ninety degree twist. This is the maximum attenuation or black state in which the cell is placed between quadrature polarizers. Another type of addressing pulse 8 switches the liquid crystal to a metastable white state 10 in which the twist is zero degrees in the homeotropic state 6 and the cell exhibits minimal attenuation or white through the orthogonal polarizer. The prior art described above provides a selection addressing pulse 8 by abruptly or gradually decreasing the voltage from the initial reset pulse 4. However, other waveforms may be used. For example, EP 0 569 029 discloses a technique for addressing a BTN liquid crystal device (LCD) by a selection pulse of a variable voltage following an initial reset pulse. JP H7-249495 discloses a technique for providing faster addressing by inserting a preset pulse between an initial reset pulse and an addressing pulse.

EP 0 579 247 discloses a technique for optimizing the position of polarizers and spectroscopy to provide optimum contrast in BTN LCDs.

Passive address black and white BTN panels are disclosed in "A Bistable Twisted Nematic (BTN) LCD Driven by a Passive Matrix Addressing", T. Nanaka et al, proceeding of Asia Display 1995, page 259.

EP 0 613 116 discloses a technique for providing a short address time by optimizing the temporal position after a reset pulse of a short selection pulse. This is shown in FIG. 2 of the accompanying drawings. The BTN LCD is arranged as a square pixel array to form a panel having row electrodes common to each pixel row receiving strobe waveforms and column electrodes common to each pixel column receiving data signals. A representative strobe or row waveform is shown at 12, the voltage appearing at the liquid crystal layer is 13, and the amplitude or coefficient of this voltage is shown at 14. The light transmittance of the cell is shown at 15.

The liquid crystal exhibits a metastable black state at the start of the addressing interval shown in FIG. 2. The reset pulse 4 resets the liquid crystal to the homeotropic state 6, followed by a selection pulse 16. At the same time, a data waveform for selecting a desired liquid crystal state is applied to the column electrode indicated by 17. This generates an electric field between the liquid crystal across the voltage, and thus the amplitude of the liquid crystal relaxing to the white metastable state 10.

During the sequential addressing of that pixel, an additional reset pulse 4 'resets the liquid crystal to the homeotropic state 6, and an additional selection pulse 16' cooperates with the appropriate data waveform 17 'to black the liquid crystal. Generates a pulse amplitude 18 'that relaxes to a steady state 9.

A disadvantage of known BTN LCDs is that since they have a bistable mode, only black and white states can be addressed. Various techniques can be used to address intermediate gray levels. For example, a black and white region may be mixed by sub pixelation in each pixel, and the sub pixel becomes smaller than the resolution of the eye. However, such a configuration requires more driving circuit and high speed switching liquid crystal mixing unless it is limited to a relatively small low-definition display panel. In addition, higher construction precision is required.

Another known technique is to apply different voltages or voltage trains to the pixel to provide different amounts of black and white areas. For example, this can be achieved by non-uniform pixels with different properties where different areas of the pixel require different switching voltages. This technique is known as multi-threshold modulation, requires sub-pixelization with additional manufacturing steps, and requires higher configuration precision. Thus, none of the known types of BTN LCDs can achieve gray level addressing without the disadvantages associated with space or time multiplexing and increased addressing speed, increased configuration complexity, or both.

EP 0 234 624 discloses a super twisted nematic (STN) LCD that can display gray levels by multi-domain technology. The electro-optical (voltage / transmission) properties of this device indicate hysteresis. Different gray levels can be achieved by applying and maintaining liquid crystal cell fields of different amplitudes.

1 is a schematic diagram showing operation of a conventional BCD LCD type.

Fig. 2 is a diagram showing a waveform and liquid crystal mode in the form of a conventional BCD LCD.

3 is a schematic diagram of a passive address type BTN LCD constituting an embodiment of the present invention;

4 is a cross-sectional view of the device of FIG.

FIG. 5 illustrates the orientation of various optical component parts within the apparatus of FIG. 3. FIG.

6 shows various waveforms and the resulting performance of the device of FIG. 3.

7 is a diagram of a conventional addressing waveform.

8 is a diagram of various waveform shapes that may be used in the device of FIG.

9 illustrates another set of waveforms and the resulting optical performance for use with the apparatus of FIG. 3.

FIG. 10 illustrates another set of waveforms for use with the apparatus of FIG. 3. FIG.

11 is a micrograph showing the performance of the device of FIG. 3 using the waveform shown in FIG. 9.

According to a first aspect of the invention, it has at least three different light attenuation levels, has a first twist angle in the absence of a substantial applied electric field, is metastable and corresponds to the first attenuation level of at least three different light attenuation levels. A second metastable state having a first metastable state and a second twist angle that is different from the first twist angle when there is no cell applied electric field and is metastable and corresponding to a second attenuation level of at least three different optical attenuation levels A liquid crystal device comprising a bistable twisted nematic liquid crystal cell having a state is provided. The device comprises a first portion for resetting the liquid crystal to a reset state having a first twist angle, a second portion for allowing the liquid crystal to relax to a relaxed state having a second twist angle, across the cell, and the at least An address generator for applying an electric field having a waveform comprising a third portion comprising one of said at least three different waveforms to respectively select three different light attenuation levels, said at least three different light attenuation levels In at least one of, the first portion of the liquid crystal is characterized in that the first metastable state, the second portion of the liquid crystal is characterized in that the second metastable state.

According to another aspect of the invention, the second portion of the waveform may comprise a space, ie a space that can relax to a sufficiently small size.

According to another aspect of the invention, the relaxed state may be a second metastable state.

According to another aspect of the invention, the at least three different light attenuation levels may comprise a maximum attenuation level, a minimum attenuation level and at least one intermediate attenuation level. At least one of the first portion and the third portion may include at least one pulse. The at least one pulse may comprise a unipolar pulse. The at least one pulse may comprise a bipolar pulse. Each bipolar pulse may include first and second subpulses that are substantially equal in amplitude and opposite in polarity.

According to another aspect of the present invention, the third portion may include a first portion and a second portion. The first portion may include a first pulse and a second pulse. The second pulse may be spaced apart from the first pulse. The amplitude of the first pulse may be less than the amplitude of the second pulse. The second portion may comprise a plurality of pulses having a progressively decreasing amplitude. The pulses of the second portion may be continuous.

According to another aspect of the invention, the address generator may comprise a data signal generator and a strobe signal generator. The cell includes a liquid crystal layer disposed between a plurality of strobe electrodes for receiving a strobe signal from the strobe signal generator and a plurality of data electrodes for receiving a data signal from the data signal generator, wherein the data electrode is The strobe electrode can be crossed to define a pixel. The data signal generator may supply data signals with amplitudes smaller than Fredricksz transition voltages of the first metastable state and the second metastable state. The liquid crystal may have an initial stable state and the amplitude of the data signal may be smaller than the Fredericks transition voltage of the initial stable state. All the data signals may include pulses of the same amplitude. All of the data signals may include symmetrical bipolar pulses. All of the data signals may include unipolar pulses. The effective voltages of all the data signals may be the same. The data signals for selecting another light attenuation level among the light attenuation levels may have different pulse widths.

According to another aspect of the invention, the amplitude of the strobe signal may be greater than the Fredrik's transition of the less twisted metastable state of the first metastable state and the second metastable state. The amplitude of the strobe signal may be four times less than the Fredrik's transition of the less twisted metastable state of the first metastable state and the second metastable state.

According to another aspect of the present invention, the liquid crystal cell may be disposed between the first polarizer and the second polarizer. The first polarizer and the second polarizer may be linear polarizers having a substantially orthogonal polarization direction. The polarization direction may be mutually oriented between 80 ° and 100 °.

According to another aspect of the invention, the liquid crystal cell may be disposed between the first alignment layer and the second alignment layer to provide a substantially anti-parallel alignment substantially 45 ° to the polarization direction. The anti-parallel alignment can be oriented between 40 ° and 50 ° with respect to the polarization direction. The first and second alignment layers are characterized by having a first alignment direction and a second alignment direction oriented at an angle comprised between 135 ° and 225 °. The included angle may be between 170 ° and 190 °. The included angle may be between 175 ° and 185 °. The included angle may be between 178 ° and 182 °.

According to another aspect of the present invention, each of the first and second alignment layers may be arranged to provide a pretilt between 1 ° and 25 °. The pretilt may be between 3 ° and 15 °. The pretilt may be between 5 ° and 10 °. The liquid crystal cell may have a thickness of 1 to 3 micrometers.

According to another aspect of the invention, Δn · d is between 0.1 and 0.3 micrometers, where d is the thickness of the liquid crystal cell, Δn = ne-no, ne and no, respectively, the liquid crystal in the second metastable state It is characterized in that the abnormal refractive index and the normal refractive index of).

According to another aspect of the present invention, the difference between the twist of the liquid crystal layer in the first metastable state and the second metastable state is substantially 360 °. The liquid crystal layer may be twisted at substantially 0 ° and 360 ° in the first metastable state and the second metastable state, respectively. The liquid crystal layer twist of each of the first metastable state and the second metastable state may be substantially different by about 180 ° from the twist in the initial stable state.

According to another aspect of the present invention, the thickness ratio of the liquid crystal cell to the bulk pitch may be between 0.2 and 1.2. And the thickness ratio is between 0.5 and 0.95. The thickness ratio may be between 0.6 and 0.9.

According to another aspect of the invention, the apparatus may comprise a matrix matrix of pixels, wherein the address generator is arranged to supply each frame of image data as n consecutive subframes, where n is each i th sub And an integer greater than 1, where i is an integer of 0 < i <

As a result, it is possible to achieve a gray level representing a BCD LCD without requiring the high speed switching of the liquid crystal material without requiring a special configuration of the LCD. The problems associated with subdivided pixels or the disadvantages of using geometrically unbalanced pixels are avoided and the problems associated with many other forms of liquid crystal, such as active matrix twisted nematic LCDs, super-twisted nematic LCDs, ferromagnetic and antiferromagnetic LCDs. It is possible to provide a relatively inexpensive display with a good viewing angle while avoiding.

Hereinafter, the present invention will be described with reference to the accompanying drawings.

Like reference numerals in the drawings denote like components.

3 illustrates a passive matrix BTN LCD constituting an embodiment of the present invention. The apparatus includes a waveform generator, shown as data signal generator 20, and a strobe signal generator 21 for supplying an addressing waveform to a rectangular pixel matrix. Data and strobe signal generators 20 and 21 have inputs connected to timing inputs 22 for receiving timing signals. The data signal generator 20 has a data input 23 for receiving data to be displayed.

The data signal generator is connected to the n-row electrode 24, and the strobe signal generator 21 is connected to the m column electrode 25. Each row electrode 24 is common to the pixels of that row, and each column electrode 25 is common to the pixels of that column. The pixel is partitioned at the intersection between the row and column electrodes, which is the portion where the row and column electrodes overlap, as indicated, for example, by 26.

The data signal generator 20 is arranged to simultaneously supply the data signals Vd1, ..., Vdn to the n column electrodes 24 to refresh the pixels of the row one row at a time. The strobe signal generator 21 is arranged to supply the strobe signals Vs1, ..., Vsm to the m row electrodes 25 one at a time to strobe new image data into the pixels one column at a time.

4 is a cross-sectional view showing the configuration of the BTN LCD of FIG. 3 in a transmissive display format rather than a reflective display format. The device comprises a polarizer 30 fixed to the outer surface of a transparent substrate, for example made of glass. Substrate 31 is accompanied by row electrodes 25 made of transparent conductors such as indium tin oxide (ITO) and an alignment layer 3 comprising, for example, rubbed polyimide. The polarizer 32 is fixed to the outer surface of the transparent substrate made of glass, for example. Substrate 33 carries column electrodes 24 made of transparent conductors, such as indium tin oxide (ITO), and an alignment layer 2 comprising, for example, rubbed polyimide. In order to partition the liquid crystal layer, the device is assembled with alignment layers 2 and 3 facing each other, spaced apart by spacers 34. The gap resulting therefrom is filled with cholesteric liquid crystal 5 to form a sealed layer in any suitable manner.

The polarization and alignment directions of polarizers 30 and 32 and alignment layers 2 and 3 are shown in FIG. 5. Specifically, the polarization directions of the polarizers 30 and 32 are shown as 35 and 36 respectively, and the plane alignment direction of the alignment layers 2 and 3 is shown as 37. The polarization directions 36 and 35 are substantially orthogonal, and the alignment direction 37 is directed in the direction antiparallel to the polarization directions 35 and 36 and inclined by about 45 °. The liquid crystal 5 has in fact a twisted metastable state and a bit twisted metastable state. The twisted state is shown by the direction of the liquid crystal molecules shown at 38, in which the state has a twist of substantially 360 degrees. The bit-twisted state is shown at 39, with all molecules aligned in 37 directions. In the twisted metastable state, the light 40 is polarized in the 35 direction by the polarizer 30, and this polarization is virtually unaffected by the path through the liquid crystal 5. As a result of the orthogonal polarization 36, absorption of light occurs, causing the pixels to appear black or opaque.

In the case of the bit-twisted metastable state shown at 39, the birefringence of the liquid crystal 5 is one in which the polarization of the light from the polarizer 30 is substantially rotated 90 ° to align with the polarization direction 36. Therefore, the pixel appears white or transparent.

FIG. 6 is a diagram showing waveforms of data and strobe signals supplied by the data and strobe signal generators 20 and 21. The strobe or thermal waveform 11 includes a first portion, a second portion, and a third portion subsequent to and including the first and second portions. The second part includes a space or reset period. The first portion of the third portion comprises a partial reset pulse 42. The second part of the third part comprises a change-favouring pulse 43.

The column or data waveforms contain bipolar pulses of constant amplitude but have a variable width to select the desired gray level of the pixel. The data pulses are bipolar and have no total DC component. The strobe pulse is a monopolar and its polarity can be changed periodically, for example between frames, in order to preserve the DC component and prevent deterioration of the liquid crystal 5.

The resulting waveform across the liquid crystal is shown at 13 and the amplitude and module of the waveform are shown at 14.

To refresh a row of pixels, during the first portion of the strobe signal waveform, and thus the waveform of the electric field across the pixels of the row, the reset pulse 41 is applied by the strobe signal generator 21 while the other rows are refreshed. It is supplied to the electrode 25. The final voltage module, shown at 44, is greater than the Fredericksz transition voltage such that the liquid crystal is reset to a homeotropic state 6 with zero twist. During the second portion of the strobe signal waveform, hence the pixel pick waveform, the pixels in the row to be refreshed receive only the data signal waveform. Since they are sufficiently small in amplitude, the liquid crystal can relax to a state with a 360 ° twist, which is believed to be a twisted, i.e. black metastable state, but it is or is a partial homootropic state with a 360 ° twist, or otherwise It may also include.

In the third part of the strobe signal waveform, and therefore the pixel field waveform, a partial reset pulse 42 is applied to the row electrode 25, along with the data waveform to supply a voltage at which the module 45 selects relatively light gray. Supply. Specifically, the height and width of the pulses 42 are selected to achieve what is soon believed to be partial blanking or resetting. Therefore, the degree of blanking or resetting varies in different areas of the pixel. This change can be caused by director perturbation in the cholesteric or by inhomogeneous flow during the Frederiks transition. In areas where there is sufficient blanking caused by the pulses, a bit-twisted homootropic or prewhite state is produced, and in a region where the partial reset pulse 42 has a lesser effect, the liquid crystal is almost a 360 ° twist, indicating a preblack state. It will be blanked with a homootropic state. The relative abundance of the prewhite and preblack regions is determined by the data voltage applied simultaneously with the partial reset pulse 42.

At this time, a change preference pulse 43 is applied and is considered to serve as a "kick white" pulse to generate white in the areas that are in the prewhite state. The regions in the preblack state are not affected by the pulse 43 and relax to the black state. The change preference pulse 43 is selected such that the simultaneously applied data voltage is not influenced by the last state selected.

Another addressing technique differs from the method described above in that the pulses 42 and 43 are selected in such a way that the final transmission is substantially determined by the voltage applied during the pulse 43. It is believed that the pulse 43 contributes to the Fredericks transition initiated during the pulse 42 and affects the ratio of the prewhite region to the preblack region.

The local difference in the degree of blanking is thought to be caused by the voltage change immersed by thermal perturbation or dynamic inequality in the cholesteric liquid crystal, or both. Specifically, during the reset and relaxation of the liquid crystal, the liquid crystal molecules cannot move as a whole in the same way, so that they temporarily have different orientations at different positions. By varying the module 45 of the waveform of the electric field applied to the liquid crystal, analog selection of the gray level can be achieved. For example, the data waveform shown 46 causes the module 47 to be a waveform that selects or addresses a gray level darker than the level addressed by the module 45.

FIG. 7 schematically illustrates the waveform of the type used in the conventional addressing arrangement described above, and FIG. 8 schematically illustrates the waveform of various types that may be used in accordance with the present invention. 7 shows a waveform disclosed in EP 0 018 180 which discloses a waveform in which the reset pulse abruptly attenuates to address black as shown at 50 and a waveform attenuates more slowly as shown at 50 in address white (a). As shown.

7 (b) and 7 (c) show two types of waveforms disclosed in EP 0 569 029. After the reset pulse 41 is followed by a variable pulse 52 adjacent to (b) and spaced apart from the reset pulse 41 in (c).

FIG. 8A shows the reset pulse 41 followed by the space followed by the partial reset pulse 42 and the change preference pulse 43. In (a) the pulses 42 and 43 are soon spaced apart.

Various waveform structures within the scope of the present invention are shown in Figures 8 (b) to (i). Specifically, the pulses 42 and 43 may soon be spaced apart from each other and may be adjacent immediately, each of which may consist of a combination of pulses.

For example, as shown in b and c, the partial reset pulse 42 may be divided into two parts 42a and 42b, which may be attached as shown in b and shown in c. As noted, they may be spaced apart at regular time intervals. The first portion 42a is lower in amplitude than the second portion 42b, and the second portion 42b is formed so that only the data transmitted in the first portion 42a can affect the gray level selection.

The single change preference pulse 43 may comprise a series of pulses each of which decreases the amplitudes shown in d, e and f, together with the pulses 42, 42a and 42b shown in a, b and c. The partial reset pulse 42 and the change preference pulse 43 may not be distinct in waveform as illustrated, for example, in d to i.

9 shows a waveform diagram and light transmission for a waveform of the type shown in FIG. 8C. The thermal waveform 12a, together with the first portion 42a of the bipolar partial reset pulse 42, as shown at 15a, causes substantially all of the pixels to switch to a transparent or white state. The column or data waveform of 12a controls the addressing of intermediate or gray levels, as illustrated by transmission curve 15b. A column or data waveform of 12c substantially switches the entire pixel to black as illustrated in 15c.

The operation illustrated in FIG. 9 includes a nematic liquid crystal mixture ZLI-4792 with a thickness of 2 micrometers and a chiral dopant R-1011 doped with a cholesteric pitch of 3 micrometers (both materials are purchased from Merck). Possible) with a device of the type illustrated in FIGS. The alignment layers 2 and 3 comprise antiparallel polyimide which has been inclined at about 5 degrees in advance. The row waveform 11 includes a reset pulse 41 having a 3 millisecond period and a 37 volt amplitude. Next is a zero millisecond interval of 11 milliseconds, followed by a bipolar pulse 42a having a 0.25 millisecond period and 4.5 volts amplitude. Next, there is a 0.25 millisecond interval, followed by a portion 42b having a 0.25 millisecond period and a 35 volt amplitude. There is then a 0.5 millisecond interval followed by a pulse 43 having a 0.25 millisecond period and a 9 volt amplitude. Next, there is an interval of 30 milliseconds and the waveform is repeated. The thermal waveform includes a bipolar pulse with an amplitude of 2.5 volts and a 25 millisecond period.

FIG. 10 illustrates a data waveform providing ten gray levels including black and white for the display of the type illustrated in FIGS. In this case, the polarization directions 35 and 36 are 4 degrees and 86 degrees, and the alignment direction 37 is 45 degrees. Alignment layers 2 and 3 comprise antiparallel polyimide, such as RN 715, available from Merck, pre-sloped at approximately 7 degrees. The liquid crystal layer thickness is 1.4 micrometers and the liquid crystal layer comprises a 1.3% weight ratio nematic liquid crystal mixture ZLI-4792 doped with a chiral dopant R-1011. The row waveform 11 includes a bipolar reset pulse 41 having a 3 millisecond period and a 30 volt amplitude, followed by a 4.2 millisecond interval. The unipolar pulse 42 has a 88 microsecond period and 17.1 volts amplitude followed by an interval of 0.204 milliseconds. Next, a bipolar pulse train 43 with a 1 millisecond period and 5.3 volts amplitude is shown. The row waveform then repeats after a 24.308 millisecond interval. The polarity of the pulse 41 changes during the cycle, but the polarity of the pulse 42 changes with each iteration. The pulse train 43 includes individual pulses having an 8 millisecond width in polarity changed by 4 milliseconds within the thermal waveform 12. The particular data pulse waveform needed to achieve 10 gray levels (numbered from 1 to 10 for the waveform) from black to white is shown simultaneously with the partial reset pulse 42.

FIG. 11 shows the results obtained in a, b, and c using the apparatus described above with the waveform shown in FIG. 9. Likewise, the gray level indicated by the transmission curve 15b is shown in b of FIG. 11. Thus, it is possible to maintain a good contrast ratio between black and white and provide an intermediate gray level that can be distinguished distinctly from both black and white.

According to the present invention, it is possible to achieve gray level addressing of a BTN LCD without the need for high speed switching of the liquid crystal material. The disadvantages associated with subdividing the pixels or using non-uniformly distributed pixels are eliminated, and without the problems associated with other types of liquid crystals such as active matrix twisted nematic LCDs, supertwisted nematic LCDs, ferromagnetic and antiferromagnetic LCDs It is possible to provide an inexpensive display.

Claims (46)

A liquid crystal device having at least three different levels of light attenuation, comprising: a first metastable state having a first twist angle in the absence of a substantial applied electric field and a metastable state in which there is no substantial applied field, and a first twist angle different from the first twisted angle in the absence of a substantially applied field A liquid crystal device comprising a bistable twisted nematic liquid crystal cell having a second metastable state which has a two twist angle and is metastable, A first portion for resetting the liquid crystal to a reset state having a first twist angle, a second portion capable of relaxing the liquid crystal to a relaxed state having a second twist angle, and the at least three different light attenuation levels, respectively An address generator for applying to said cell an electric field having a waveform comprising a third portion comprising one of at least three different waveforms for selection, A first portion of the liquid crystal is the first metastable state, and a second portion of the liquid crystal is the second metastable state. The liquid crystal device of claim 1, wherein the second portion of the waveform comprises a space. The liquid crystal device according to claim 1, wherein the relaxed state is a second metastable state. The liquid crystal device of claim 1, wherein the at least three different light attenuation levels comprise a maximum attenuation level, a minimum attenuation level and at least one intermediate attenuation level. The liquid crystal device of claim 1, wherein at least one of the first portion and the third portion comprises at least one pulse. 6. The liquid crystal device of claim 5, wherein the at least one pulse comprises a monopolar pulse. 6. The liquid crystal device of claim 5, wherein the at least one pulse comprises a bipolar pulse. 8. The liquid crystal device according to claim 7, wherein each bipolar pulse includes a first subpulse and a second subpulse having substantially the same amplitude and opposite polarities. The liquid crystal device of claim 1, wherein the third portion includes a first portion and a second portion. 10. The liquid crystal device of claim 9, wherein the first portion comprises a first pulse and a second pulse. The liquid crystal device of claim 10, wherein the second pulse is spaced apart from the first pulse. The liquid crystal device of claim 10, wherein an amplitude of the first pulse is smaller than an amplitude of the second pulse. 10. The liquid crystal device according to claim 9, wherein the second portion includes a plurality of pulses having a gradually decreasing amplitude. The liquid crystal device of claim 13, wherein the pulses of the second portion are continuous. The liquid crystal device of claim 1, wherein the address generator comprises a data signal generator and a strobe signal generator. 16. The liquid crystal display of claim 15, wherein the cell includes a liquid crystal layer disposed between the plurality of strobe electrodes for receiving a strobe signal from the strobe signal generator and the plurality of data electrodes for receiving a data signal from the data signal generator. And the data electrode crosses the strobe electrode to define a pixel. 17. The apparatus of claim 16, wherein the data signal generator is arranged to supply data signals whose amplitudes are less than Fredericksz transition voltages of the first metastable state and the second metastable state. The liquid crystal device characterized by the above-mentioned. 18. The liquid crystal device according to claim 17, wherein the liquid crystal has an initial stable state and the amplitude of the data signal is smaller than the Fredericks transition voltage of the initial stable state. 17. The liquid crystal device according to claim 16, wherein all the data signals comprise pulses of the same amplitude. 17. The liquid crystal device of claim 16, wherein all of the data signals comprise symmetrical bipolar pulses. 17. The liquid crystal device according to claim 16, wherein all the data signals comprise unipolar pulses. The liquid crystal device of claim 16, wherein the effective voltages of all the data signals are the same. 20. The liquid crystal device according to claim 19, wherein the data signals for selecting another light attenuation level among the light attenuation levels have different pulse widths. 17. The liquid crystal device of claim 16, wherein the amplitude of the strobe signal is greater than a Fredrik's transition of the less twisted metastable state of the first metastable state and the second metastable state. 25. The liquid crystal device of claim 24, wherein an amplitude of the strobe signal is four times less than a Fredrix transition of a less twisted metastable state of the first and second metastable states. The liquid crystal device according to claim 1, wherein the liquid crystal cell is disposed between the first polarizer and the second polarizer. 27. The liquid crystal device of claim 26, wherein the first polarizer and the second polarizer are linear polarizers having a substantially orthogonal polarization direction. 28. The liquid crystal device according to claim 27, wherein the polarization directions are mutually oriented between 80 and 100 degrees. 28. The liquid crystal cell of claim 27, wherein the liquid crystal cell is disposed between a first alignment layer and a second alignment layer to provide a substantially antiparallel alignment substantially 45 [deg.] To the polarization direction. Liquid crystal device. 30. The liquid crystal device according to claim 29, wherein the anti-parallel alignment is oriented between 40 and 50 degrees with respect to the polarization direction. 30. The liquid crystal device according to claim 29, wherein the first alignment layer and the second alignment layer have a first alignment direction and a second alignment direction oriented at an angle comprised between 135 ° and 225 °. 32. The liquid crystal device of claim 31, wherein the included angle is between 170 ° and 190 °. 33. The liquid crystal device of claim 32, wherein the included angle is between 175 ° and 185 °. The liquid crystal device of claim 33, wherein the included angle is between 178 ° and 182 °. 30. The liquid crystal device according to claim 29, wherein each of the first alignment layer and the second alignment layer is arranged to provide a pretilt between 1 ° and 25 °. 36. The liquid crystal device of claim 35, wherein the pretilt is between 3 [deg.] And 15 [deg.]. 37. The liquid crystal device of claim 36, wherein the pretilt is between 5 [deg.] And 10 [deg.]. The liquid crystal device according to claim 1, wherein the liquid crystal cell has a thickness of 1 to 3 micrometers. 2. The abnormal refraction of the liquid crystal according to claim 1, wherein Δn · d is between 0.1 and 0.3 micrometers, wherein d is the thickness of the liquid crystal cell, and Δn = ne-no, ne and no are each in the second metastable state. It is a coefficient and a normal refractive index, The liquid crystal device characterized by the above-mentioned. The liquid crystal device according to claim 1, wherein the difference between the twist of the liquid crystal layer in the first metastable state and the second metastable state is substantially 360 degrees. 41. The liquid crystal device according to claim 40, wherein the liquid crystal layer is twisted at substantially 0 degrees and 360 degrees in the first metastable state and the second metastable state, respectively. 41. The liquid crystal device according to claim 40, wherein the liquid crystal layer twist in each of the first metastable state and the second metastable state is substantially different from the twist in the initial stable state. The liquid crystal device according to claim 1, wherein the thickness ratio of the liquid crystal cell to the bulk pitch is between 0.2 and 1.2. 44. The liquid crystal device according to claim 43, wherein said thickness ratio is between 0.5 and 0.95. 45. The liquid crystal device according to claim 44, wherein the thickness ratio is between 0.6 and 0.9. The apparatus of claim 1, wherein the apparatus comprises a matrix matrix of pixels, wherein the address generator is arranged to provide each frame of image data as n consecutive subframes, where n is each i th subframe (i +). and n is an integer greater than 1 to include the nth row (where i is an integer of 0 < i < n and m is a nonnegative integer).