Method of addressing a cholesteric liquid crystal display

Abstract

 Method of addressing a cholesteric liquid crystal display having a plurality of pixels arranged in rows and columns, wherein: addressing a specific one of said plurality of pixels comprises a plurality of addressing phases; during each of said plurality of addressing phases, a sequence RS, RNS of row voltage pulses and a sequence CON, COFF, CGRAY of column voltage pulses is applied; said sequence RS, RNS of row voltage pulses and sequence CON, COFF, CGRAY of column voltage pulses comprise three voltage levels of 0 volt, UL volt and UH volt; during a preparation phase as a first addressing phase of said plurality of addressing phases, a first sequence RS, RNS of row voltage pulses and a first sequence CON, COFF, CGRAY of column voltage pulses are applied; and UH is only comprised in said sequence RS, RNS of row voltage pulses and said sequence CON, COFF, CGRAY of column voltage pulses during said preparation phase, with 0 < UL = UH.

Description

[0001] The present invention refers to a method of addressing a cholesteric liquid crystal display. Specifically, the present invention refers to a method of addressing a cholesteric liquid crystal display capable of grayscale operation.

[0002] Liquid crystal displays (LCDs) are flat displays that employ the light modulating properties of liquid crystals, a state of matter having properties between those of a conventional liquid and a solid crystal. LCDs may be mainly distinguished in terms of the order or phase of the liquid crystal molecules they contain, which may be either "nematic" (with various modifications such as "twisted nematic", "super twisted nematic" and "double super twisted nematic") or "smectic". In the nematic phase, the molecules do not have any positional order but tend to point in the same direction, i. e. their longitudinal axes are parallel on the average (the term is derived from the Greek prefix nemato, meaning "threadlike"). In the smectic phase, the molecules in addition exhibit a positional order at least along one dimension, i. e. in a smectic order, the molecules form a layered structure with the molecules oriented parallel or tilted relative to the layer normal (the term is derived from the Greek word for soap, because slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal). There can be associated different geometrical properties or concepts with each of the nematic phase and the smectic phase; they are not to be confused with the respective phase itself). For example, the nematic phase may feature the property of chirality or handedness. A chiral object has a shape that can not be superimposed on its mirror image. Chirality is not a state of matter, and there can thus be no phase transition between chiral and antichiral; an object can be either chiral or antichiral, never both. The nematic phase of a chiral substance is called the cholesteric phase, originating from cholesteryl benzoate, a chiral substance where liquid crystal phases were firstly discovered. In the cholesteric phase, the molecules are helically arranged, with the helical axis extending normal to the average of the longitudinal axes of the molecules. This leads to the formation of a three-dimensional structure which can be visualized as a stack of very thin two-dimensional nematic-like layers with the director (longitudinal axes) in each layer twisted with respect to those adjacent to it. ChLCDs exhibit the above cited twisted nematic structure, where the director rotates about an axis as one moves through the material.

[0003] Cholesteric LCDs (ChLCDs) have some excelling properties, the most important one of which is the bistability of their phases, i. e. the property to be able to maintain either one of two distinct phases without application of any electric or magnetic field. This allows for information display without any power consumption. The two states are named the "planar state" and the "focal conic state" (also found in a certain type of smectic materials called smectic A). Further advantageous properties are a wide viewing angle, because no polarizer is needed, and high contrast.

[0004] When a ChLCD is in its planar state, it reflects light of a certain wavelength region, and appears, therefore, in the corresponding color. In contrast, when a ChLCD is in its focal conic state, incident light is only weakly scattered, making the pixels assume the color of the substrate behind them, usually black. That is, when there is a black absorber attached to the display backside, a front observer sees either the color of the reflected light (planar state) or the color of the absorber material (focal conic state). In both states the liquid crystal molecules are arranged in a helical structure. It should be noted that the liquid crystalline structure inside the display is divided into many tiny subdomains, enabling the coexistance of the planar state and the focal conic state inside an individual pixel of the ChLCD, making the ChLCD capable of gray scale operation.

[0005] Formerly, the individual pixels of ChLCDs were addressed in a so called static driving scheme such as a conventional passive matrix driving scheme resulting in a slow refresh rate as the number of times per second that a ChLCD draws data (which is not to be confused with the frame rate which is usually lower than the refresh rate). In order to overcome this drawback, a so called "dynamic drive scheme" (DDS) has been proposed which enables refresh times of approximately 1ms per display row (cf. e. g. "Dynamic Drive of Cholesteric Liquid Crystal Displays"; X. Y. Huang, J. W. Doane, D. K. Yang; J. SID 5/3, 129 (1997)).

[0006] However, the DDS of Huang et al. has the disadvantage that the display driving circuitry has to be very complicated, as many different voltage levels are necessary to drive the display.

[0007] This disadvantage has been overcome with the invention of the "two-level DDS" (cf. "Bistable Cholesteric Reflective Displays: Two-Level Dynamic Drive Schemes"; A. Rybalochka, V. Sorokin, S. Valyukh; J. SID 12/2, 165 (2004)). Here, each row and column of the display is addressed either with a voltage U or zero. This makes the driving circuitry simple, because only a single voltage has to be generated. In addition to that, it is possible to use off-the-shelf display drivers that are commercially available.

[0008] However, the type two-level-DDS developed by Rybalochka et al. has the disadvantage that the proposed schemes (called therein



, and

schemes) do not provide the desired possibility of gray-level-imaging.

[0009] It is an object of the present invention to provide a method of addressing a cholesteric liquid crystal display that overcomes the above discussed disadvantages, especially is capable of grayscale operation.

[0010] This object is solved by the features of claims 1. Further advantageous are defined in the dependent claims.

[0011] The present invention (claim 1) defines a method of addressing a cholesteric liquid crystal display having a plurality of pixels arranged in rows and columns, wherein addressing a specific one of the plurality of pixels comprises a plurality of addressing phases; during each of the plurality of addressing phases, a sequence R_S, R_NS of row voltage pulses and a sequence C_ON, C_OFF, C_GRAY of column voltage pulses is applied; the sequence R_S, R_NS of row voltage pulses and sequence C_ON, C_OFF, C_GRAY of column voltage pulses comprise three voltage levels of 0 volt, U_L volt and U_H volt; during a preparation phase as a first addressing phase of the plurality of addressing phases, a first sequence R_S, R_NS of row voltage pulses and a first sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied; and U_H is only comprised in the sequence R_S, R_NS of row voltage pulses and the sequence C_ON, C_OFF, C_GRAY of column voltage pulses during the preparation phase, with 0 < U_L ≤ U_H. Preferably, the plurality of pixels are arranged as a (n x m)-matrix of a plurality of n rows and m columns, each of the plurality of pixels having a unique address given by two values (a, b), with 1 < a < n, and 1 < b < m. Alternatively, one of n and m may be "one", that is the cholesteric liquid crystal display may include only a single row or a single column. According to the present invention, there is always a preparation phase as a first addressing phase, followed by at least one further phase. Advantageously, the plurality of addressing phases are arranged successively on a time axis, starting with the preparation phase and continued with a second phase, a third phase etc. During the preparation phase, the liquid crystal is re-set, so to speak, i. e. it is completely driven to an unstable, "neutral" state called the homeotrophic state. The homeotrophic state only exists in the presence of an electric field, that is large enough to unwind the helical structure of the liquid crystal molecules, and develops into one of the above cited planar state and focal-conic state or gray state. Preferably, the voltage pulses of both the sequence of row voltage pulses and the sequence of column voltage pulses have the same polarity. It should be noted that there is a maximum or highest voltage level applied to the rows and columns, where either both of U_L and U_H are at that level (0 < U_L = U_H) or U_L < U_H (0 < U_L < U_H). To put it differently, U_H defines the highest level, and U_L with U_L > 0 is either of the same level or below it. It should be noted that the term "only" in the last feature of claim 1 refers to the phase, i. e. the preparation phase, and that in that phase U_H may be comprised in the sequences of row voltage pulses or in the sequences of column voltage pulses or in both of them.

[0012] According to a preferred aspect of the present invention (claim 2), (i) in case of U_H > U_L, U_L is only comprised in said sequence R_S, R_NS of row voltage pulses and said sequence C_ON, C_OFF, C_GRAY of column voltage pulses during addressing phases other than said preparation phase, and all pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row and column voltage pulses are each equal in all addressing phases; and (ii) in case of U_H = U_L, the pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row and column voltage pulses in said addressing phase are different from said pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row voltage pulses and column voltage pulses during addressing phases other than said preparation phase. That is, whereas according to claim 1, U_H is 'restricted' to the preparation phase (nothing is said about U_L), according to feature (i), U_L is 'excluded' from the preparation phase. This results in a situation where in each of the addressing phases, either U_H or U_L occurs, not both of them. In contrast, the distinction that can be made via the pulse height is compensated in feature (ii) by the variation of the pulse width.

[0013] According to a further preferred aspect of the present invention (claim 3), said plurality of addressing phases for said specific pixel comprise a 'selection and holding phase' as a second addressing phase during which a second sequence R_S, R_NS of row voltage pulses and a second sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied; and an 'evolution phase' as a third addressing phase during which a third sequence R_S, R_NS of row voltage pulses and a third sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied. Therefore, claim 3 defines two further addressing phases of the plurality of addressing phases as a "selection and holding phase", and an "evolution phase", respectively. During the selection and holding phase, the pixels are individually selected and set by applying appropriate row and column voltage pulses (shortly "row signals" and "column signals", respectively). This is done by first selecting a specific row and then a specific column by providing the corresponding signals. Having selected a row, the column signal determines whether the pixel is finally transferred to the planar state (ON-state) or to the focal-conic state (OFF-state) or the gray state. During the evolution phase (3^rd phase), as during the preparation phase (1^st phase), all rows and columns are addressed simultaneously with voltages U_H and 0V in the case of U_H > U_L. Preferably, the selection and holding phase may be time divided into a plurality of successive selection phases and holding phases. Most preferably, the selection and holding phase may be time divided into a selection phase sandwiched between two holding phases.

[0014] According to a further preferred aspect of the present invention (claim 4), during said selection and holding phase, (i) at any time, any one of said sequences of row voltage pulses is such that the corresponding one of said rows is either a selected row (in case of R_S) or a non-selected row (in case of R_NS); (ii) at any time, there is only a single selected row, wherein said specific pixel is defined by said single selected row and one of said columns; and (iii) any one of said sequences C_ON, C_OFF, C_GRAY of column voltage pulses is such that the corresponding column is either a planar-state setting column by applying the sequence C_ON of column voltage pulses, resulting in a planar state of said specific pixel at the end of said evolution phase, a focal-conic-state-setting column, by applying the sequence C_OFF of column voltage pulses resulting in focal-conic state of said specific pixel at the end of said evolution phase, or a gray-state setting column by applying the sequence C_GRAY of column voltage pulses, resulting in a gray-state of the specific pixel at the end of the evolution phase. As for feature (i), depending on the voltage scheme (or rather the strength of the present electric fields) applied (to the individual columns) subsequent to the preparation phase (during the selection and holding phase, to be precise), the corresponding pixel inside the selected row is transferred to either the planar state or the focal-conic state or the gray state. According to feature (ii), no more than one row can be selected to be the selected row. Otherwise, it would not be possible to address a single pixel by an n-tuple (a, b). According to feature (iii), the type of column, being either a planar-state setting column or a focal-conic-state setting column or a gray-state setting column, determines the type of pixel, i. e. its perceived brightness.

[0015] According to a further preferred aspect of the present invention (claim 5), the sequence R_S of voltage pulses applied to the single selected row is different from the sequences R_NS of voltage pulses applied to each of said non-selected rows.

[0016] According to a another preferred aspect of the present invention (claim 6), there are defined: across-pixel voltages P_ON which each are a difference P_ON = R_S - C_ON between a sequence R_S of row voltage pulses of the single selected row and a sequence C_ON of column voltage pulses of one of the planar-state setting columns; across-pixel voltages P_OFF which each are a difference P_OFF = R_S - C_OFF between a sequence R_S of row voltage pulses of the single selected row and a sequence C_OFF of column voltage pulses of one of the focal-state-setting columns; across-pixel voltages P_GRAY which each are a difference P_GRAY = R_S - C_CRAY between said sequence R_S of row voltage pulses of said single selected row and a sequence C_GRAY of column voltage pulses of one of said gray-state setting columns; across-pixel voltages P_Hd1 which each are a difference P_Hd1 = R_NS - C_ON between a sequence R_NS of row voltage pulses of one of the non-selected rows and a sequence C_ON of column voltage pulses of one of the planar-state setting columns; and across-pixel voltages P_Hd2 which each are a difference P_Hd2 = R_NS - C_OFF between a sequence R_NS of row voltage pulses of one of the non-selected rows and a sequence C_OFF of column voltage pulses of one of the focal-state-setting columns; and across-pixel voltages P_HD which each are a difference P_HD = R_NS - C_CRAY between said sequence R_NS of row voltage pulses of said non-selected rows and said sequence C_GRAY of column voltage pulses of one of said gray-state setting columns. Furthermore, each of the across-pixel voltages P_ON, P_OFF, P_GRAY, P_Hd1, P_Hd2, and P_HD is a DC-free voltage that contains at least one continuous waiting time with zero voltage, and a ratio between a waiting time of the across-pixel voltage P_OFF to a waiting time of either the across-pixel voltage P_Hd1 or the across-pixel voltage P_Hd2 is either 4:1, 4:2 or 3:2. That is, an "across-pixel voltage" is a potential difference between a potential applied to a column and a potential applied to a row (either a selected or a non-selected row), the rows and columns thus can uniquely define or address (and thereby control) the specific pixel. Therefore, the totality of pixels form a two-dimensional matrix and are addressable by two values, a row number and a column number (the above n-tuple (a, b)).

[0017] According to a another preferred aspect of the present invention (claim 7), in a method according to the preferred aspect defined in claim 6, (i) each of said sequences C_ON and C_OFF of column voltage pulses comprises a first subsequence of a single non-zero voltage pulse, and a second subsequence of two consecutive non-zero voltage pulses (ii) in one of said sequences C_ON and C_OFF of column voltage pulses, there is a separation of a single zero voltage pulse between said first subsequence and said second subsequence, whereas in the other one of said sequences C_ON and C_OFF of column voltage pulses, said separation is two zero voltage pulses, (iii) said sequence C_GRAY of column voltage pulses is generated by shifting, in said one of said sequences C_ON and C_OFF of column voltage pulses in which said second separation is two consecutive zero voltage pulses, said second sequence towards said first subsequence about a distance smaller than said separation; and (iv) an intensity of a said gray-state is determined by an amount of said distance. To put it more simply, gray-levels can easily be achieved by shifting the position of the broader non-zero voltage pulse in the column voltage signal to a position between its respective position in the sequences C_ON and C_OFF of column voltage pulses. The resulting across-pixel voltage drives the corresponding pixel inside the selected row into a gray-state.

[0018] According to a another preferred aspect of the present invention (claim 8), each of said sequences R_S, R_NS of row voltage pulses and sequences C_ON, C_OFF, C_GRAY of column voltage pulses comprise six voltage pulses.

[0019] Further advantages and features of the present invention will become apparent from the following detailed description of preferred embodiments in combination with the attached drawings. In the drawings, there are:

Fig. 1a first sequences of voltage pulses applied to rows and columns during the holding and selection phase, and Fig. 1b a complete set of first sequences of voltage pulses (for the preparation, selection and holding, and evolution phase), according to a first embodiment of the present invention;

Fig. 2a first sequences of voltage pulses applied to rows and columns during the holding and selection phase, and Fig. 2b a complete set of first sequences of voltage pulses (for the preparation, selection and holding, and evolution phase), according to a second embodiment of the present invention; and

Fig. 3a first sequences of voltage pulses applied to rows and columns during the holding and selection phase, and Fig. 3b a complete set of first sequences of voltage pulses (for the preparation, selection and holding, and evolution phase), according to a third embodiment of the present invention.

Figs. 1a and 1b explain first sequences of voltage pulses (DDS_4_1), Figs. 2a and 2b explain second sequences of voltage pulses (DDS_4_2) and Figs. 3a and 3b explain third sequences of voltage pulses (DDS_3_2) according to the present invention. In addition, while Figs. 1a, 2a, and 3a show sequences of voltage pulses applied during the "holding and selection" phase, Figs. 1b, 2b, and 3b show the sequences of voltage pulses applied during all addressing phases for the exemplary case of a (2x2)-display. In the following "sequences of voltage pulses" is abbreviated to "voltage signal"

[0020] Each of Figs. 1a, 2a, and 3a is a matrix or table of voltage signals showing in the first row 'column voltage signals' C_ON, C_OFF, and C_GRAY, in the first column 'row voltage signals' R_S, and R_NS, and in the other table elements differences between one of the column voltage signals and one of the row voltage signals, wherein:

R_S

is the voltage signal applied to a selected row (there is only a single selected row at a time);

R_NS

is the voltage signal applied to non-selected rows, which are all rows other than the selected row;

C_ON

is the voltage signal applied to a column (planar-state setting column) in order to transfer the corresponding pixel inside the selected row to the planar state (ON state);

C_OFF

is the voltage signal applied to a column (focal-conic state setting column) in order to transfer the corresponding pixel inside the selected row to the focal-conic state (OFF state);

C_GRAY

is the voltage signal applied to a column in order to achieve gray levels;

P_ON

= R_S-C_ON;

P_OFF

= R_S-C_OFF;

P_Gray

= R_S-C_Gray;

P_Hd1

= R_NS-C_ON;

P_Hd2

= R_NS-C_OFF; and

P_Hd

= R_NS-C_Gray

[0021] The across-pixel voltage signal P_ON = R_S-C_ON transfers the corresponding pixel in the selected row to the planar state (ON state). The across-pixel voltage signal P_OFF = R_S-C_OFF transfers the corresponding pixel in the selected row to the focal conic state (OFF state). The across-pixel voltage signals P_Hd1 = R_NS-C_ON and P_Hd2 = R_NS-C_OFF are applied to all pixels in not-selected rows. Furthermore, gray-levels can easily be achieved by shifting the position of the broader pulse in the column-signal between its position in C_ON and C_OFF (see the column signal C_Gray). The resulting across-pixel voltage P_Gray = R_S-C_Gray transfers the corresponding pixel in the selected row to a gray state.

[0022] As can be seen in all figures, each voltage signal (also called 'addressing frame') is divided into six subparts, where a voltage of U_H is applied to subparts depicted as gray rectangles, a voltage of U_L is applied to subparts depicted as gray squares, and 0 volt is applied to subparts depicted as white boxes. Furthermore, the pulses forming the voltage signals R_S, R_NS, C_ON, C_OFF, and C_Gray have the same polarity, whereas the pulses forming the across-pixel voltage signals, being each a difference, have different polarity. Specifically, all across-pixel voltages are free of DC-voltage (as many positive pulses as negative pulses per frame), otherwise the liquid crystal could be damaged.

[0023] A characteristic feature of the voltages signals P_OFF, P_Hd1, and P_Hd2 in each of the cases DDS_4_1 (Fig. 1 a), DDS_4_2 (Fig. 2a) and DDS_3_2 (Fig. 3a) are the waiting times, i. e. subparts with neither a positive nor a negative voltage applied. For instance, in DDS_4_1, there is a waiting time of four subparts in P_OFF and a waiting time of one subpart in P_Hd1 and P_Hd2, in DDS_4_2, there is a waiting time of four subparts in P_OFF and a waiting time of two subparts in P_Hd1 and P_Hd2, and in DDS_3_2, there is a waiting time of three subparts in P_OFF and a waiting time of two subparts in P_Hd1 and P_Hd2. Therefore, in the notation DDS_x_y, x and y refer to the number of subparts forming the waiting times in P_OFF and P_Hd1/ P_Hd2, respectively. The ratio x/y of these waiting times (for example 4:1 in DDS_4_1) has to be chosen according to the electro-optical characteristics of the ChLCD. The electro-optical characteristic curve of a ChLCD determines which of our three novel waveforms is best suited to achieve good contrast.

[0024] Figs. 1b, 2b and 3b show the complete voltage signals for a 2x2-display. The row voltage signals are denoted by Row1 and Row2. The column voltage signals are denoted by Col1 and Col2. The resulting across-pixel voltage signals are denoted by P11, P12, P21 and P22.

[0025] The pixel P11 (first row, first column) is driven to the planar state (ON-state). The pixels P12 and P21 are driven to the focal conic state (OFF-state). Pixel P22 is driven to a gray state. The highest voltage U_H is only used during the preparation phase. During the preparation phase all pixels are simultaneously addressed with an identical waveform.

[0026] The selection and holding phase is divided into two parts T1 and T2 (because there are two rows). During the interval T1, the first row is selected. During T2 the second row is selected. In the evolution phase all pixels are again addressed simultaneously with an identical waveform.

Claims

1. Method of addressing a cholesteric liquid crystal display having a plurality of pixels arranged in rows and columns, wherein:

- addressing a specific one of said plurality of pixels comprises a plurality of addressing phases;

- during each of said plurality of addressing phases, a sequence R_S, R_NS of row voltage pulses and a sequence C_ON, C_OFF, C_GRAY of column voltage pulses is applied;

- said sequence R_S, R_NS of row voltage pulses and sequence C_ON, C_OFF, C_GRAY of column voltage pulses comprise three voltage levels of 0 volt, U_L volt and U_H volt;

- during a preparation phase as a first addressing phase of said plurality of addressing phases, a first sequence R_S, R_NS of row voltage pulses and a first sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied; and

- U_H is only comprised in said sequence R_S, R_NS of row voltage pulses and said sequence C_ON, C_OFF, C_GRAY of column voltage pulses during said preparation phase, with 0 < U_L ≤ U_H.

2. Method according to claim 1, wherein:

- in case of U_H > U_L,

- U_L is only comprised in said sequence R_S, R_NS of row voltage pulses and said sequence C_ON, C_OFF, C_GRAY of column voltage pulses during addressing phases other than said preparation phase; and

- all pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row and column voltage pulses are each equal in all addressing phases; and

- in case of U_H = U_L,

- said pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row voltage pulses and column voltage pulses in said addressing phase are different from said pulse widths of said sequences R_S, R_NS, C_ON, C_OFF, C_GRAY of row voltage pulses and column voltage pulses during addressing phases other than said preparation phase.

3. Method of claim 1 or 2, wherein said plurality of addressing phases for said specific pixel comprise:

- a selection and holding phase as a second addressing phase during which a second sequence R_S, R_NS of row voltage pulses and a second sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied; and

- an evolution phase as a third addressing phase during which a third sequence R_S, R_NS of row voltage pulses and a third sequence C_ON, C_OFF, C_GRAY of column voltage pulses are applied.

4. Method of claim 3, wherein during said selection and holding phase:

- at any time, any one of said sequences of row voltage pulses is such that the corresponding one of said rows is either a selected row or a non-selected row;

- at any time, there is only a single selected row, wherein said specific pixel is defined by said single selected row and one of said columns; and

- any one of said sequences C_ON, C_OFF, C_GRAY of column voltage pulses is such that the corresponding column is either a planar-state setting column by applying said sequence C_ON of column voltage pulses, resulting in a planar state of said specific pixel at the end of said evolution phase, a focal-conic-state setting column by applying said sequence C_OFF of column voltage pulses, resulting in focal-conic-state of said specific pixel at the end of said evolution phases.; or a gray-state setting column by applying said sequence C_GRAY of column voltage pulses, resulting in a gray-state of said specific pixel at the end of said evolution phase.

5. Method of claim 4, wherein said sequence R_S of voltage pulses applied to said single selected row is different from the sequences R_NS of voltage pulses applied to each of said non-selected rows.

6. Method of claim 4 or claim 5, wherein:

- there are defined:

- across-pixel voltages P_ON which each are a difference P_ON = R_S - C_ON between a sequence R_S of row voltage pulses of said single selected row and a sequence C_ON of column voltage pulses of one of said planar-state setting columns;

- across-pixel voltages P_OFF which each are a difference P_OFF = R_S - C_OFF between said sequence R_S of row voltage pulses of said single selected row and a sequence C_OFF of column voltage pulses of one of said focal-conic-state setting columns;

- across-pixel voltages P_GRAY which each are a difference P_GRAY = R_S-C_CRAY between said sequence R_S of row voltage pulses of said single selected row and a sequence C_GRAY of column voltage pulses of one of said gray-state setting columns;

- across-pixel voltages P_Hd1 which each are a difference P_Hd1 = R_NS - C_ON between a sequence R_NS of row voltage pulses of one of said non-selected rows and said sequence C_ON of column voltage pulses of one of said planar-state setting columns; and

- across-pixel voltages P_Hd2 which each are a difference P_Hd2 = R_NS - C_OFF between said sequence R_NS of row voltage pulses of one of said non-selected rows and said sequence C_OFF of column voltage pulses of one of said focal-state setting columns;

- across-pixel voltages P_HD which each are a difference P_HD = R_NS - C_CRAY between said sequence R_NS of row voltage pulses of one of said non-selected rows and said sequence C_GRAY of column voltage pulses of one of said gray-state setting columns;

- each of said across-pixel voltages P_ON, P_OFF, P_GRAY, P_Hd1, P_Hd2, and P_HD is a DC-free voltage that contains at least one continuous waiting time with zero voltage; and

- a ratio between a waiting time of said across-pixel voltage P_OFF to a waiting time of either said across-pixel voltage P_Hd1 or said across-pixel voltage P_Hd2 is either 4:1, 4:2 or 3:2.

7. Method of one of claims1 to 6, wherein:

- each of said sequences C_ON and C_OFF of column voltage pulses comprises a first subsequence of a single non-zero voltage pulse, and a second subsequence of two consecutive non-zero voltage pulses;

- in one of said sequences C_ON and C_OFF of column voltage pulses, there is a separation of a single zero voltage pulse between said first subsequence and said second subsequence, whereas in the other one of said sequences C_ON and C_OFF of column voltage pulses, said separation is two zero voltage pulses;

- said sequence C_GRAY of column voltage pulses is generated by shifting, in said one of said sequences C_ON and C_OFF of column voltage pulses in which said second separation is two consecutive zero voltage pulses, said second sequence towards said first subsequence about a distance smaller than said separation; and

- an intensity of a said gray-state is determined by an amount of said distance.

8. Method of one of claims 1 to 7, wherein each of said sequences R_S, R_NS of row voltage pulses and sequences C_ON, C_OFF, C_GRAY of column voltage pulses comprise six voltage pulses.

Drawing