EP2178079B1 - Energy-saving method for marking an area of a liquid-crystal display - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a method of addressing a liquid crystal display screen and a display device implementing this method.

More specifically, the present invention relates to liquid crystal bistable displays. It is particularly applicable to nematic liquid crystal bistable displays whose two stable textures differ by a twist of about 180 °.

STATE OF THE ART

The most widely used liquid crystal displays use a nematic type liquid crystal. They consist of a layer of liquid crystal placed between two blades. Each blade comprises a substrate, often made of glass, on which a conductive electrode has been deposited, then a so-called anchoring layer, also called an alignment layer. The anchoring layer exerts on the neighboring liquid crystal molecules a return torque which tends to orient them parallel to a direction called easy axis. Anchor layers are often made by a brushed polymer deposit to create the direction of the easy axis. This is most often very close to the direction of brushing.

The thickness of the cell thus formed, called d, is made constant for example by distributing, between the blades, beads whose diameter is equal to the desired thickness (typically 1 to 6 microns).

Monostable liquid crystal devices are known. In the absence of an electric field, the liquid crystal is oriented in a single texture. This texture corresponds to an absolute minimum of the elastic energy of the liquid crystal in the cell, given the anchors on the two blades. Under an electric field, this texture is continuously deformed and its optical properties vary according to the applied voltage. Anchoring layers called "strong anchoring layers" maintain the direction of the molecules near the blades, which varies little, both in the plane of the substrate (azimuth plane) and in the direction perpendicular to it (direction zenithale): a strong anchoring of the molecules near the blades on the alignment layer corresponds to a strong azimuthal anchoring (maintaining a fixed direction in the plane of the substrate) and a strong zenith anchorage (maintaining a direction close to the plane substrate, that is to say, little or no lifting molecules to the direction perpendicular to the substrate, parallel to the electric field, and whatever the voltage applied).

At the cutting of the field, the nematic is recalled by the anchors on the two blades. It returns according to the unique stable texture without applied field. The device is monostable. Those skilled in the art will recognize the mode of operation of the most widespread nematic displays: twisted nematic (TN), supertordus (STN), electrically controlled birefringence (ECB), vertically aligned (VAN), etc. these displays can be addressed directly (very low resolution), in passive multiplexed mode (medium resolution) or in active mode (high resolution). When the addressing is multiplexed, that is to say carried out line by line, for the image to appear visually stable, the addressing signals must be sent at a frequency of several tens of hertz: as soon as the pixel is no longer energized, it relaxes to the stable state without an applied field.

• les signaux correspondant à l'image à afficher pour faire apparaître l'image
• les signaux correspondant à une teinte uniforme sur l'afficheur
• à nouveau les signaux correspondant à l'image à afficher pour faire réapparaître l'image.

It is possible to obtain on these monostable displays a blinking effect, for example in a given area of the display, called blinking area. This area may be the entire addressed surface of the display or a portion thereof. The flashing here corresponds to obtaining in said zone an image which appears then disappears, then reappears and so on, the disappearance of the image being characterized by a uniform hue on the whole of said zone. A uniform hue is a substantially identical hue for all the pixels of the area, the color of the hue can be any. To obtain this flashing effect, it is necessary to address said area of the display by sending it:

• the signals corresponding to the image to be displayed to make the image appear
• the signals corresponding to a uniform hue on the display
• again the signals corresponding to the image to be displayed to reappear the image.

The monostable displays do not have an image memory, once the signals corresponding to the uniform hue applied, the display has "forgotten" the image previously applied and it is necessary to send again the signals corresponding to said image to redisplay it.

The document WO 2006/136799 discloses a cholesteric type liquid crystal device in which a perturbation signal is applied to each pixel by driving it into a disturbed state which is distinguished in terms of luminance of each of the two stable states-i.e. the planar state and the conical focal state. A disturbed pixel returns to a planar state after a disturbance signal ceases. The method described in this document aims to create additional gray levels of the displayed image.

PURPOSE OF THE INVENTION

The object of the present invention is to improve the performance of liquid crystal display devices. In particular, the object of the invention is to make it possible, by the use of new means, to mark part or all of the information displayed on a liquid crystal display, while maintaining a reduced energy consumption compared to that of a standard liquid crystal display.

SUMMARY OF THE INVENTION

This object is achieved with a method of addressing a matrix screen according to claim 1.

BRIEF DESCRIPTION OF THE FIGURES

• la figure 1 présente schématiquement une partie d'un premier mode de réalisation de dispositif selon l'invention comprenant un afficheur bistable de type ZBD et mettant en oeuvre un procédé selon l'invention,
• la figure 2 présente schématiquement une partie d'un deuxième mode de réalisation de dispositif selon l'invention comprenant un afficheur bistable de type BiNem et mettant en oeuvre un procédé selon l'invention,
• la figure 3 décrit la structure d'un écran matriciel à adressage passif multiplexé du premier ou du deuxième mode de réalisation selon l'invention,
• la figure 4 décrit un exemple d'adressage en deux étapes de l'écran passif du deuxième mode de réalisation de dispositif selon l'invention de type BiNem,
• la figure 5 montre des variations de la luminance de pixels en fonction d'une tension RMS d'un signal de perturbation Sp de fréquence 600 Hz appliqué lors de la mise en oeuvre d'un procédé selon l'invention,
• la figure 6 illustre un exemple de réalisation de procédé selon l'invention sur un afficheur bistable où l'on marque un ensemble de colonnes (zone de marquage) ; la figure 6a présente l'afficheur au repos ; la figure 6b présente l'afficheur quand un signal de perturbation « intermédiaire » est appliqué sur une partie des colonnes, les textures initiales se distinguant encore ; la figure 6c présente l'afficheur quand un signal de perturbation « d'effacement » est appliqué, le brouillage de l'image dans la zone marquée étant total.
• la figure 7 illustre des exemples de signaux appliqués lors de la mise en oeuvre d'un procédé selon l'invention, le signal ligne étant relié à la masse, le signal colonne VC étant monopolaire (positif ou négatif) et ayant la forme d'un créneau d'amplitude Vblink,
• la figure 8 illustre autre exemple de signaux appliqués lors de la mise en oeuvre d'un procédé selon l'invention, le signal ligne étant égal à un potentiel moyen Vm et le signal colonne étant constitué d'alternance à Vm +Vblink et Vm-Vblink.
• la figure 9 illustre un exemple pour appliquer selon l'invention des tensions de perturbation sur les lignes et les colonnes pour déformer fortement et uniquement les textures d'un domaine rectangulaire.
• la figure 10 un autre exemple pour appliquer selon l'invention des tensions de perturbation sur les lignes et les colonnes pour déformer plus légèrement que dans le cas de la figure 9 les textures du domaine rectangulaire mais en économisant beaucoup d'énergie.

The various objects and characteristics of the invention will appear more clearly in the description which follows and in the appended figures in which:

• the figure 1 schematically shows a part of a first device embodiment according to the invention comprising a bistable display of ZBD type and implementing a method according to the invention,
• the figure 2 schematically shows a part of a second device embodiment according to the invention comprising a bistable display BiNem type and implementing a method according to the invention,
• the figure 3 describes the structure of a multiplexed passive addressing matrix screen of the first or second embodiment according to the invention,
• the figure 4 describes an example of two-step addressing of the passive screen of the second device embodiment according to the BiNem invention,
• the figure 5 shows variations in the luminance of pixels as a function of an RMS voltage of a perturbation signal Sp of frequency 600 Hz applied during the implementation of a method according to the invention,
• the figure 6 illustrates an exemplary embodiment of the method according to the invention on a bistable display where a set of columns (marking area) is marked; the figure 6a present the display at rest; the figure 6b presents the display when an "intermediate" perturbation signal is applied to a part of the columns, the initial textures being further distinguished; the Figure 6c presents the display when an "erase" disturbance signal is applied, the scrambling of the image in the marked area being total.
• the figure 7 illustrates examples of signals applied during the implementation of a method according to the invention, the line signal being connected to ground, the column signal VC being monopolar (positive or negative) and having the form of a slot of amplitude Vblink,
• the figure 8 illustrates another example of signals applied during the implementation of a method according to the invention, the line signal being equal to a mean potential Vm and the column signal consisting of alternating Vm + Vblink and Vm-Vblink.
• the figure 9 illustrates an example for applying according to the invention disturbance voltages on the lines and columns to strongly deform and only the textures of a rectangular domain.
• the figure 10 another example to apply according to the invention disturbance voltages on the lines and columns to deform more slightly than in the case of the figure 9 the textures of the rectangular domain but saving a lot of energy.

DETAILED DESCRIPTION OF THE INVENTION Device implementing the method according to the invention

We will now describe a first and a second device embodiment according to the invention (also called a display) implementing a method according to the invention.

• une couche de molécules de cristal liquide bistable se décomposant en pixels de cristal liquide bistable et,
• pour chaque pixel, des moyens pour appliquer un signal à ce pixel, le signal appliqué comprenant un champ électrique, ces moyens d'application étant notamment agencés pour appliquer de façon itérative un signal de perturbation Sp selon l'invention selon le procédé selon l'invention décrit par la suite.

These embodiments include a matrix screen for displaying an image, said screen comprising:

• a layer of bistable liquid crystal molecules decomposing into bistable liquid crystal pixels and,
• for each pixel, means for applying a signal to this pixel, the applied signal comprising an electric field, these application means being in particular arranged to iteratively apply a disturbance signal Sp according to the invention according to the method according to the invention described later.

Each bistable liquid crystal pixel has two possible stable states. These two stable states are stable without an electric field being applied to this pixel, the two stable states corresponding to different visual perceptions for an observer observing the matrix screen. Each stable state of a pixel corresponds to a given stable texture of liquid crystal molecules at that pixel.

The pixels are arranged in parallel pixel lines and parallel pixel columns, the lines being substantially perpendicular to the columns.

The liquid crystal of the layer is of the nematic type. The liquid crystal layer is placed between two blades, the assembly constituting a liquid crystal cell. Each blade comprises a substrate, preferably made of glass, on which a conductive electrode has been deposited, followed by a so-called anchoring layer, also called a layer. alignment. The anchoring layer exerts on the neighboring liquid crystal molecules a return torque which tends to orient them parallel to a direction called easy axis. The anchoring layers are preferably made by a brushed polymer deposit to create the direction of the easy axis. This direction of the easy axis is preferably very close to the brushing direction.

The thickness of the cell thus formed (that is to say the distance between the blades between which is included the liquid crystal layer), called d, is made constant for example by distributing, between the blades, balls of which the diameter is equal to the desired thickness (typically 1 to 6 μm).

The liquid crystal of the layer is "bistable": this type of liquid crystal operates by switching between two stable states in the absence of an electric field. An external electric field is applied only for the time necessary to switch the texture of the liquid crystal from one state to another. In the absence of an electrical control signal, the display remains in the state obtained. By its operating principle, this type of display consumes energy proportional to the number of image changes. Thus, when the frequency of these changes decreases, the power required for the operation of the display tends to zero.

ZBD technology and BiNem technology

The figure 1 illustrates, for the first embodiment of the device according to the invention, two different states of a liquid crystal pixel between two portions of the blades. This first mode uses a flexo-electric effect to switch, that is to say the sign of the applied electric field. This is the pretilt, ie the angle that the liquid crystal molecule close to the surface with it, which varies between two stable values without applied field. This bistability is obtained using a network serving as alignment layer (cf documents [1], [2], [3] and figure 1 ). This technology is called ZBD (Zenithal Bistable Display). One of the alignment layers is constituted by a periodic network allowing the vicinity of the surface of this network two orientations of the liquid crystal molecules, one planar, the other homeotropic.

The figure 2 is a sectional and sectional view of a portion of the liquid crystal cell of the second device embodiment according to the invention. In this figure, we visualize 3 adjacent liquid crystal pixels. This second bistable embodiment uses a surface effect: a break of the zenith anchorage on at least one of the alignment layers. This break allows switching between two textures whose torsions differ by an angle between 150 ° and 180 ° in absolute value. The operation of this display called BiNem is described in the following paragraph.

The BiNem display (documents [4] to [8]) is schematically presented on the figure 2 , and has a general configuration identical to that of the liquid crystal cell type ZBD which also uses substrates, electrodes, polarizers, liquid crystal. The BiNem display preferably uses two twisted textures that differ by a twist of approximately +/- 180 ° (located in absolute value between 150 ° and 180 °). A preferred but nonlimiting variant consists of a uniform or slightly twisted texture called U (illustrated on the left of the figure 2 ) in which the molecules are substantially parallel to each other, and of a strongly twisted texture called T (illustrated on the right of the figure 2 ). The least twisted U texture has a twist between 0 ° and 20 ° in absolute value. The liquid crystal layer 30 is placed between the two blades 20 and 10, which are respectively called master blade and slave blade. The master blade 20 comprises a substrate 21, an electrode 22 and an anchoring layer 24 forming a strong azimuth and zenith anchorage of the liquid crystal, that is to say an anchorage of the same type as that used in crystal displays. monostable liquid. The slave blade 10 comprises a substrate 11, an electrode 12 and an anchoring layer 14 providing a specific anchorage, corresponding to a weak zenith anchor and a medium or strong azimuthal anchoring of the liquid crystal. The usually transparent electrodes 12 and 22 are typically made of a material called ITO deposited on the substrates 11 and 21. They make it possible to apply an electric field perpendicular to the plates 10 and 20.

The addition of polarizers to each of the substrates 11 and 21, typically but not exclusively outside the cell, makes it possible to associate each texture with an optical state, for example dark for the texture U and clear for the texture T or vice versa, according to the angles of the two polarizers with respect to the directions of the anchors.

Depending on the type of rear polarizer, ie located on the other side of the liquid crystal layer relative to the observer of the display, it is possible to obtain various optical modes, transmissive, transflective or reflective (documents [9], [10]). A configuration using a single polarizer on the observer side and a mirror on the opposite side is also possible ([18], [19]).

The nematic is chiralised with a spontaneous pitch po, chosen close to four times the thickness d of the cell, to equalize the energies of the two aforementioned textures. The ratio between cell thickness d and po spontaneous pitch, d / po, is therefore about 0.25 +/- 0.1. Without a field, the states T and U are the states of minimum energy: the cell is bistable.

Under a strong electric field, an almost homeotropic texture, called H and illustrated in the middle of the figure 2 , is obtained. In the vicinity of the surface of the slave blade 10, the molecules are perpendicular to it, the zenith anchor is said to be "broken". This break is obtained thanks to the weak zenith anchoring property on this slide, which allows the molecules close to the surface to be extracted from the attraction of the alignment layer in the direction perpendicular to the substrate. The voltage voltage corresponding to the breaking of the anchoring on the slave blade 10 is called the break voltage Vcass. In fact it is to the electric break field Ecass = Vcass / d that the molecules are sensitive. Typically Ecass is less than 15V / μm at room temperature (25 ° C) for the low zenith anchor alignment layers as described in documents [11] and [12]. The breaking voltages are then: for d = 1μm Vcass <15V; for d = 2μm Vcass <30V; for d = 5μm Vcass <75V. In addition, the break voltage Vcass is always at least a few volts, even for very thin liquid crystal cells (1 .mu.m).

When the electric field is cut off, the cell evolves towards one or the other of the bistable textures U and T (see figure 2 ). When the control signals used induce a strong flow of the liquid crystal in the vicinity of the master blade 20, the hydrodynamic coupling 26 between the master blade 20 and the slave blade 10 creates a sufficient hydrodynamic flow (or flow) near the slave blade to to induce the texture T. Otherwise, the texture U is obtained by elastic coupling 28 between the two blades 10 and 20, aided by the possible inclination of the weak anchor.

In the following, the term "switching" of a screen element or pixel of a BiNem-type display will be used to make the molecules of the liquid crystal pass from an initial stable texture (U or T or a coexistence thereof). two textures) towards a final stable texture (U or T or a coexistence of these textures). This name is also valid for the two stable textures of the ZBD type display.

The signal applied to the pixel is conventionally made up of several levels. The signal applied to the pixel VP is typically two-stage, but can also be multi-stage [13] or single-stage. If the voltage drop between two stages exceeds a certain absolute value, and that it operates in a sufficiently short time, the "jump" of tension is sufficient for the texture T is obtained. If the jump is not sufficient, or if the transition time is too long, the hydrodynamic flow is insufficient, the texture T becomes impossible, and the texture U is obtained.

Addressing modes

The 3 addressing modes developed for the standard liquid crystal (direct, passive multiplexed, active) can be used for the BiNem or ZBD display. The most common addressing mode is multiplexed passive addressing, but active addressing using thin-film transistors is also possible [14]. In the active and passive multiplexed modes, the display (of type Binem or ZBD) is a matrix screen formed of N x M screen elements called pixels, N being the number of rows of pixels and M the number of columns of pixels. , and addressing is done line by line.

• d'une bande ou électrode conductrice ligne 52 déposée sur une des lames et correspondant à une ligne de pixels, et
• d'une bande ou électrode conductrice colonne 50 déposée sur l'autre lame et correspondant à une colonne de pixels.

In the multiplexed passive mode, as illustrated on the figure 3 , each pixel is constituted by the intersection:

• a band or conductive electrode line 52 deposited on one of the blades and corresponding to a line of pixels, and
• a band or column conductive electrode 50 deposited on the other blade and corresponding to a column of pixels.

These strips 50, 52 perpendicular are deposited on each blade. The area between two adjacent conductive strips carried by the same substrate is called interpixel space. The area consisting of all the pixels is called matrix area. A marking area Zm is a part of this matrix area. Usually in the state of the art, the matrix area corresponds to the display area, on which area the image content that is to be displayed is displayed. Outside the matrix area, the aforementioned conductive strips 50, 52 are transformed into tracks which make the connection to the control circuits generating the addressing signal. These control circuits may be located on the substrate or remote. Conventionally, but not exclusively, the displays are addressed using components or control circuits that we will call "drivers" located for example on flexible connection elements welded to the screen. The drivers, consisting mainly of analog gates controlled by shift registers, make it possible to make the link between the control electronics and the tracks.

To display the pixel P of coordinates (n, m), where n is the number (integer) of the line on which this pixel is located, where m is the number (integer) of the column on which this pixel is located. applies a line addressing signal VLn on the line n and a column addressing signal VCm on the column m. Generally, the conductive electrodes are made of a transparent conductive material called ITO (mixed oxide of Indium and tin). But when the display is reflective, the electrodes located on the opposite side to the observer can be made with an opaque conductive material, for example aluminum.

One of the important differences to note between the passive mode and the active mode is that in the multiplexed passive mode, the voltage is applied by orthogonal electrode strips constituting the rows and the columns, the intersections of which constitute the pixels, whereas during active addressing, the electrical voltage is applied to the transistors associated with each pixel by fine wires. All the transistors of the same line are passing during the activation of this line.

Controlling a Binem display in multiplex mode

When the structure of the display is matrix as described above, the addressing is carried out line by line. When it is desired to register a given line n, an electrical signal is applied on this line, which is then called "activated". We will call this activation signal line addressing signal VLn. In the case of a standard passive multiplexing, the signal VLn is identical for all the lines, and we will call it VL.

For the BiNem, with reference to the figure 4 two phases are distinguished during activation: the first phase essentially consists of obtaining an anchoring break, ie the homeotropic texture on the line considered, by applying for example a voltage V1L> Vcass on the signal of line addressing for a duration T1, which constitutes a first level of VL. Typically in the current state of BiNem technology, V1L is between 6V and 30V over the 0 ° - 50 ° temperature range. During the second phase, a signal V2L is applied on the line for a duration T2, which constitutes a second and last level of VL. Typically in the current state of BiNem technology, V2L is between 2V and 12V over the 0 ° -50 ° temperature range. The line addressing signal is in this two-stage example, but it can also be single-stage or multi-stage. A variant uses a line signal lower than the breaking voltage, the column signal enabling switching in one or the other of the textures [20]; or, according to a two-step variant, all the pixels are first switched in the same texture, then the column voltage causes the break but only in the pixels to switch in the other texture.

Electrical signals called "data" called VC are applied simultaneously on all the columns. According to a conventional variant, the falling edge of the data signal VC is synchronized with the falling edge of the second stage of the line activation signal V2L [4]. Depending on the voltage value, and / or its shape, and / or the duration tc of the signal VCm applied simultaneously to each of the columns, the texture U or T is obtained in the pixel corresponding to the intersection of this column and the line activated. Then the next line is turned on, the other lines are off and so on from the first to the last line of the display. The time between the end of activation of a line and the beginning of the activation of the next line is called TL interline time. This time is typically but not limited to between 10 μs and 10 ms. We will call this addressing "one-step addressing". The order of activation of the lines (first n-1, then n, then n + 1) defines the scanning direction 46 (see figure 3 ). The addressing time of the display is the time required to address all its lines, so as to display new image content.

The document [15] describing the achievement of gray levels provides three variants for obtaining gray levels (FIG. 23 of document [15]) by modifying the parameters of VC.

According to a mode of use called partial addressing, it is desired to display new content in only one area of the image, the rest of the image remaining unchanged. In this case, only the lines corresponding to the area where you want to display new content are activated.

• VC(U) = +Vcol et VC(T) = -Vcol (exemple de la figure 4)
• VC(U) est le signal d'adressage colonne pour obtenir la texture U.
• VC(T) est le signal d'adressage colonne pour obtenir la texture T.
• ou bien: VC(U)= +Vcol et VC(T) = 0, ou inversement.

According to a known, but nonlimiting, preferred mode of piloting, prior to line-to-line addressing, the entire addressing of the screen (display of a complete image) or of an area of the screen is collectively performed. (partial addressing) in a given texture, usually T, by simultaneously activating all the lines or a group of lines corresponding to the zone to be addressed, with a signal Vpre. The lines are then addressed one by one, according to the conventional multiplexing method, to display the desired image or area. Only two transitions must then be made, the transition T to T on the one hand, and the transition T to U or to a mixture of U and T on the other hand. This "two-step addressing" makes it possible to better control the switching of the pixels because the pixels all start from a well-defined state at the beginning of the second step. According to the variant where the line voltage is lower than the breaking voltage, during the second step the column voltage causes the break only for the pixels whose state (texture) is to be modified. For example, the principle of multiplexed passive addressing of the BiNem display in two steps is illustrated figure 4 . The column addressing signal applied to the column m is chosen here such that tc = T2. The values VC1 to VC5 are the values of VCm applied on the column m in synchronization with the lines 1 to 5, successively activated, so as to obtain the desired final texture on the pixel at the intersection of the activated line and the column. m. In a mode where one seeks to obtain only either U or T, one can choose for example a voltage VC in the form of slot and different variants are possible:

• VC (U) = + Vcol and VC (T) = -Vcol (example of the figure 4 )
• VC (U) is the column addressing signal to obtain the U texture.
• VC (T) is the column addressing signal to obtain the texture T.
• or else: VC (U) = + Vcol and VC (T) = 0, or vice versa.

According to a known but non-limiting embodiment of the BiNem display embodiment, the brushing direction of the alignment layers is orthogonal to the direction of the lines of the display, this type of display is called "orthogonal brushing" (document [15]).

In order to avoid electrochemical effects in the liquid crystal, bipolar pulses for the Vpre signal and for the VL signal can be used.

V0 threshold voltage and Freedericksz VF voltage

In the liquid crystal cells, it is found that the field to be applied, to orient the molecules, most often has a threshold. For example, consider a positive dielectric anisotropy nematic placed in a planar and parallel anchored cell on both blades; without a field, the molecules are parallel to each other and parallel to the slides throughout the cell. An electric field, applied perpendicularly to the blades, begins to orient the molecules only when the voltage is greater than a certain threshold called the Freedericksz VF threshold or Freedericksz VF voltage (document [16]). Below VF, the liquid crystal molecules remain motionless, maintained by the nematic elasticity. From VF, as the voltage increases, the liquid crystal molecules rotate progressively in the direction of the field: first those located in the center of the cell then the others, except those close to the blades, whose alignment is maintained by anchoring.

où K11 est une des constantes élastiques du cristal liquide et Δε son anisotropie diélectrique. Selon le signe de Δε, les molécules tendent à s'orienter parallèlement (Δε >0) ou perpendiculairement au champ appliqué(Δε <0).VF est indépendante de l'épaisseur de la cellule cristal liquide et varie typiquement, pour les mélanges cristal liquide utilisés dans les afficheurs, entre 0,3V et 1V.The voltage VF can be expressed according to the following formula: $$\mathrm{VF}=\mathrm{\pi }{\left[\mathrm{K}\mathrm{11}/\left|\mathrm{\Delta \epsilon }\right|)\right]}^{\mathrm{1}/\mathrm{2}}$$

where K11 is one of the elastic constants of the liquid crystal and Δε its dielectric anisotropy. According to the sign of Δε, the molecules tend to orient themselves parallel (Δε> 0) or perpendicular to the applied field (Δε <0) .VF is independent of the thickness of the liquid crystal cell and typically varies for crystal mixtures liquid used in displays, between 0.3V and 1V.

• VF(statique) ou VFs la tension de Freedericksz correspondant à un signal appliqué en continu (c'est-à-dire avec une fréquence nulle), et
• VF(dynamique) ou VFd la tension de Freedericksz correspondant à un signal appliqué de fréquence supérieure à la fréquence de réponse du cristal liquide.

Δε being a function of the frequency of the signal applied to the liquid crystal, we call:

• VF (static) or VFs the Freedericksz voltage corresponding to a continuously applied signal (i.e. with zero frequency), and
• VF (dynamic) or VFd the Freedericksz voltage corresponding to an applied signal of frequency higher than the response frequency of the liquid crystal.

Typically VFd is slightly higher than VFs.

When the inclination of the molecules on a blade (pretilt) is high, the threshold disappears. For intermediate pretilts, typically a few degrees, the threshold remains but it is less marked. When the cells are bent or doped, but still planar, the threshold remains but the threshold voltage can vary up to about 30% compared to the theoretical VF voltage obtained with a planar and parallel anchored cell. Thus, for liquid crystal textures different from that used for measuring the Freedericksz threshold, we speak of a threshold effect, characterized by a threshold voltage, called V0, the value of which remains always relatively close to VF.

The presence of this threshold effect imposes a minimum value on the control signals of the nematic displays, it is one of their assets. It prevents the displayed images from being disturbed by spurious signals; it suffices that these parasites have an amplitude lower than the threshold voltage. This property is fundamental for multiplexing.

Disturbance signal Sp applied according to the invention.

The invention makes it possible to mark a pixel or an area of a matrix bistable display comprising two stable liquid crystal textures without an applied field, by an original method, which is not applicable on monostable displays. The concept of marking is defined by a visually detectable optical modification of this area relative to the rest of the image.

The inventors have shown that it is possible to perform this additional function on these bistable displays by not switching any of the pixels of the screen, so both quickly and with minimal energy expenditure.

An image is previously displayed on the screen by switching each pixel in one of said initial stable states.

1. a) on applique, à chaque pixel d'une zone Zm de pixels se trouvant dans un état initial correspondant à un des états stables, et pendant un premier laps de temps donné t1, un signal dit signal de perturbation Sp, ledit signal de perturbation étant supérieur à au signal seuil V0 de sorte que chaque pixel de la zone quitte son état initial, ledit signal de perturbation étant inférieur au signal de commutation Vcass de sorte que chaque pixel de la zone ayant comme état initial un des états stables ne bascule pas dans l'autre état stable, chaque pixel de la zone se trouvant alors dans un état perturbé intermédiaire entre les deux états stables ; ainsi, on génère pendant ledit premier temps t1 une perturbation visuelle de l'image préalablement inscrite dans la zone Zm; puis
2. b) on n'applique aucun signal à chaque pixel de la zone pendant un deuxième laps de temps donné t2 pour laisser revenir chaque pixel de la zone vers son état stable initial, puis
3. c) on réitère les deux étapes a) et b) précédentes, un nombre de fois supérieur à un, et avec une fréquence de réitération égale à t1 + t2.

The method according to the invention implemented by the bistable liquid crystal display comprises the following steps:

1. a) applying to each pixel of a zone Zm of pixels in an initial state corresponding to one of the stable states, and during a first given period of time t1, a signal called said perturbation signal Sp, said disturbance signal; being greater than the threshold signal V0 so that each pixel of the zone leaves its initial state, said perturbation signal being smaller than the switching signal Vcass so that each pixel of the zone having as its initial state one of the stable states does not switch over in the other stable state, each pixel of the zone being then in a disturbed state intermediate between the two stable states; thus, during said first time t1, a visual disturbance of the image previously entered in the zone Zm is generated; then
2. b) no signal is applied to each pixel of the zone for a second given period of time t2 to let each pixel of the zone return to its initial stable state, then
3. c) repeating the two preceding steps a) and b), a number of times greater than one, and with a repeating frequency equal to t1 + t2.

Thus, the proposed method is to apply, over a whole marking zone Zm comprising a set of pixels to be marked, during the time t1, an electrical signal called disturbance signal Sp having a defined amplitude not including a zero continuous range, then no longer apply a signal during time t2.

• un signal électrique colonne appliqué à la colonne sur lequel est situé ce pixel, tel qu'un signal d'adressage colonne VC ou VCm adressé tel que décrit précedement, et
• un signal électrique ligne appliqué à la ligne sur lequel est situé ce pixel, tel qu'un signal d'adressage ligne VL ou VLn adressé tel que décrit précédemment,
  ce signal de perturbation Sp étant proportionnel ou même égal à une différence entre le signal colonne et le signal ligne :
  • Sp=VL-VC
  ou bien
  Sp du pixel (n, m) = VLn - VCm

The electrical perturbation signal Sp applied by the application means to a pixel comprises:

• a column electrical signal applied to the column on which this pixel is located, such as an addressed VC or VCm column addressing signal as described above, and
• a line electrical signal applied to the line on which this pixel is located, such as an addressed VL or VLn line addressing signal as described above,
  this perturbation signal Sp being proportional or even equal to a difference between the column signal and the line signal:
  • Sp = VL-VC
  or
  Sp of the pixel (n, m) = VLn - VCm

• supérieure au seuil de Freedericksz VF et à la tension de seuil V0, et
• nettement inférieure aux seuils de commutation et à la tension de cassure Vcass.

The amplitude of the disturbance signal Sp is:

• greater than the threshold of Freedericksz VF and the threshold voltage V0, and
• significantly lower than switching thresholds and breaking voltage Vcass.

This disturbance signal Sp distorts the two textures corresponding to the two states of the pixels: their optical properties are modified, the contrast decreases to the value of the disturbance signal for which the zone takes a uniform hue.

However, as the disturbance signal is well below the switching thresholds, the orientation of the molecules near the blades does not change substantially during t1: the screen keeps in memory on the slides the initial image. It suffices to stop the disturbance signal so that the pixels return each in their equilibrium texture without a field. The image preceding the deformation is thus reconstituted at the beginning of t2 in milliseconds without any expenditure of energy.

The marking of the screen area is thus achieved by disappearing, during t1, then reappearing during t2, the image in this area.

The typical duration of t1 is between 0.1 and a few tens of seconds, and the typical duration of t2 is between 0.1s and a few minutes, so that the typical duration of t1 + t2 is between 0.2s and a few minutes.

Effect of disturbance signal Sp: change of luminance

In the particular case of a BiNem display, these two stable states correspond to the U and T textures.

• un état clair aussi appelé état passant, et
• un état sombre aussi appelé état bloquant.
  Soit Lib la luminance (exprimée en candelas/m²) issue d'un pixel dont la texture correspond à l'état dit clair ou passant (« bright ») de ce pixel, et Lid la luminance (exprimée en candelas/m²) issue d'un pixel dont la texture correspond à l'état sombre ou bloquant (« dark ») de ce pixel. Nous choisirons pour définir les luminances émises par l'afficheur, les luminances mesurées dans la direction perpendiculaire aux substrats. Lorsque l'afficheur est en mode réflectif, la mesure de luminance dépend du type d'éclairage illuminant la cellule. Nous choisirons pour caractériser la luminance de l'afficheur en mode réflectif un éclairage diffus, et l'on recueille la luminance réfléchie au travers de l'afficheur dans la direction perpendiculaire au substrat et obstruant la zone autour du détecteur, de manière à ne pas recueillir dans le détecteur de rayons issu de la réflexion spéculaire de la source d'éclairage sur la face avant (interface air/afficheur) de l'afficheur (en anglais « specular excluded »). Cette méthode de mesure de luminance de l'afficheur en mode réflectif utilisant un éclairage diffus dite « specular excluded » est décrite dans le document [17]. Elle permet d'obtenir une mesure de contraste (rapport entre la luminance de l'état passant et la luminance de l'état bloquant) et une mesure de réflectance (rapport entre la luminance de l'afficheur dans l'état passant et la luminance obtenue en remplaçant l'afficheur par un diffuseur de type Lambertien). La réflectance s'exprime en %.
  L'état passant a une luminance Lib perçue par l'observateur supérieure à la luminance Lid de l'état bloquant.
  Le signal de perturbation Sp appliqué à la zone de marquage pendant un laps de temps t1 a pour effet d'orienter les molécules de cristal liquide dans le volume en fonction de ce signal Sp appliqué et donc différemment de leur orientation initiale sans champ appliqué. Cette nouvelle orientation provoque une modification de la luminance issue de chaque pixel de l'afficheur. On appel ;
• Lpb la luminance (exprimée en candelas/m²) issue d'un pixel se trouvant dans un état perturbé intermédiaire entre les deux états stables du fait de l'application de signal de perturbation, ce pixel se trouvant initialement dans l'état passant avant l'application du signal de perturbation, et
• Lpd la luminance (exprimée en candelas/m²) issue d'un pixel se trouvant dans un état perturbé intermédiaire entre les deux états stables du fait de l'application de signal de perturbation, ce pixel se trouvant initialement dans l'état bloquant avant l'application du signal de perturbation.

In general, the two stable states include:

• a clear state also called a passing state, and
• a dark state also called a blocking state.
  Let Lib be the luminance (expressed in candelas / m ² ) coming from a pixel whose texture corresponds to the so-called "bright" state of this pixel, and Lid the luminance (expressed in candelas / m ² ) from a pixel whose texture corresponds to the dark or blocking ("dark") state of this pixel. We will choose to define the luminances emitted by the display, the luminances measured in the direction perpendicular to the substrates. When the display is in reflective mode, the luminance measurement depends on the type of illumination illuminating the cell. We will choose to characterize the luminance of the display in reflective mode diffuse illumination, and the luminance reflected is reflected through the display in the direction perpendicular to the substrate and obstructing the area around the detector, so as not to collect in the ray detector from specular reflection of the light source on the front panel (air / display interface) of the display (in English "specular excluded"). This method of luminance measurement of the display in reflective mode using diffuse lighting called "specular excluded" is described in document [17]. It makes it possible to obtain a contrast measurement (ratio between the luminance of the on state and the luminance of the blocking state) and a reflectance measurement (ratio between the luminance of the display in the on state and the luminance obtained by replacing the display with a Lambertian type diffuser). Reflectance is expressed in%.
  The passing state has a luminance Lib perceived by the observer greater than the luminance Lid of the blocking state.
  The perturbation signal Sp applied to the marking zone during a period of time t1 has the effect of orienting the liquid crystal molecules in the volume as a function of this applied signal Sp and therefore differently from their initial orientation without an applied field. This new orientation causes a change in luminance from each pixel of the display. We call;
• Lpb the luminance (expressed in candelas / m ² ) coming from a pixel in an intermediate disturbed state between the two stable states due to the application of disturbance signal, this pixel being initially in the forward state before the application of the disturbance signal, and
• Lpd the luminance (expressed in candelas / m ² ) from a pixel in a disturbed state intermediate between the two stable states due to the application of disturbance signal, this pixel being initially in the blocking state before the application of the disturbance signal.

Thus, Lpb and Lpd are the luminances of the disturbed pixels having stable initial states corresponding to the luminances respectively Lib and Lid.

RMS (Root Mean Square) voltage is also known as the rms value of this voltage. The figure 5 shows the evolution of the luminance ratio Lpb / Lib and the luminance ratio Lpd / Lib as a function of the voltage RMS of the perturbation signal Sp applied. On the figure 5 , the luminances Lpb and Lpd are both normalized with respect to Lib the luminance of the initial state passing without disturbance signal applied.

In this example, the effect of the disturbance signal Sp is threefold.

Sp decreases the luminance Lpb of the progressively increasing state as the value of Sp increases, up to a value corresponding to the "equilibrium" state of the liquid crystal molecules under applied field. Any increase in the applied voltage will hardly change the resulting liquid crystal texture. The effective value of the luminance corresponding to this equilibrium state, called the "equilibrium" luminance Lo, is a function of the position of the polarizers of the cell.

Sp increases the luminance Lpd of the blocking state progressively as the value of Sp increases, up to the same value called "equilibrium" luminance Lo.

c'est-à-dire que l'image préalablement inscrite est totalement effacée. Un signal Sp tel que l'image préalablement inscrite est effacée est appelé signal de perturbation Sp « d'effacement ». Tel qu'illustré sur la figure 6c, en appliquant un signal de perturbation « d'effacement » sur un pixel P6 initialement dans un état passant et sur un pixel P5 initialement dans un état bloquant, le pixel P6 initialement dans l'état stable passant se retrouve dans état perturbé identique à l'état perturbé dans lequel se retrouve le pixel P5 initialement dans l'état stable bloquant, ces états perturbés de pixels initialement dans deux états stables différents correspondant à une même perception visuelle pour l'observateur observant l'écran.When this "equilibrium" luminance Lo is reached, we have Lpb = Lpd = Lo. The contrast between the on state and the blocking state Lpb / Lpd is equal to 1: $$\mathrm{Lpb}/\mathrm{lpd}=1$$

that is, the previously registered image is completely erased. A signal Sp such that the previously inscribed image is erased is called the "erasure" disturbance signal Sp. As illustrated on the Figure 6c by applying an "erase" disturbance signal on a pixel P6 initially in an on state and on a pixel P5 initially in a blocking state, the pixel P6 initially in the passing stable state is found in the same disturbed state perturbed state in which the pixel P5 initially found in the blocking stable state, these disturbed states of pixels initially in two different stable states corresponding to the same visual perception for the observer observing the screen.

• un pixel P1 dans un état stable bloquant est représenté en noir,
• un pixel P2 dans un état stable passant est représenté en gris clair, et
• un pixel P3, P4, P5 et P6 dans un état perturbé est représenté en gris plus ou moins foncé.

By way of nonlimiting example, the result of the marking of a zone Zm of the display corresponding to a set of adjacent columns of pixels (columns contiguous to the right of the display), called the marking zone, is shown on the Figures 6a, 6b and 6c . In these figures:

• a pixel P1 in a stable blocking state is represented in black,
• a P2 pixel in a steady state passing is represented in light gray, and
• a pixel P3, P4, P5 and P6 in a disturbed state is represented in more or less dark gray.

The figure 6a corresponds to the display in its initial state. The on state corresponds here to the state T and the blocking state to the state U. The figure 6b shows the image obtained with an "intermediate" Sp interference signal, ie with a decrease in the luminance of the on state and an increase of the luminance of the blocking state, but without having reached the "equilibrium" luminance in the marking zone. As illustrated on the figure 6b by applying an "intermediate" perturbation signal on a pixel P4 initially in an on state and on a pixel P3 initially in a blocking state, the pixel P4 initially in the passing stable state is found in a disturbed state different from the state disturbed in which the pixel P3 is initially found in the blocking stable state, these perturbed states of pixels initially in two different stable states corresponding to different visual perceptions for the observer observing the screen. Thus, the on state has darkened, the blocking state is significantly less black, but the textures obtained from the starting textures U and T for this "intermediate" value of the perturbation signal are still optically distinct. Although still visible, the image has a degraded contrast and the eye perceives perfectly the marking of this zone. The Figure 6c shows the image obtained with a signal Sp such that the "equilibrium" luminance is reached. For this value, the textures from U and T appear optically identical, the previously registered image is no longer visible, it is erased. Of course and a fortiori, the marking is perfectly noticeable. The perturbation signal Sp is then called the "erase" perturbation signal.

Static marking mode or flashing marking mode

• soit avec une perturbation de l'image (étape a) pendant le temps t1) et une absence de signal de perturbation (étape b) pendant le temps 2) telles que la périodicité soit inférieure à une durée de persistance rétinienne de l'observateur, de manière à obtenir un effet visuel de marquage statique de la zone, ce que nous appellerons mode de fonctionnement statique ou marquage statique, ce mode étant le plus économique en énergie.
• soit en alternant perturbation de la zone Zm pendant le temps t1 et image non perturbée pendant le temps t2, à une fréquence sensible pour l'observateur ; ce mode sera appelé mode clignotant ou marquage clignotant; le temps t1+t2 sera alors par exemple supérieur à 0,1 seconde ; ainsi, on réitère les étapes a) et b) un nombre de fois supérieur à 1, de manière à obtenir un effet visuel de clignotement de la zone Zm provoqué par une alternance de l'état perturbé et de l'état initial stable pour chaque pixel de la zone.

The zone Zm that one wishes to mark can be it in two ways:

• with a disturbance of the image (step a) during time t1) and an absence of disturbance signal (step b) during time 2) such that the periodicity is less than a duration of retinal persistence of the observer, in order to obtain a visual effect of static marking of the zone, which we will call static operating mode or static marking, this mode being the most economical in energy.
• either by alternating disturbance of the zone Zm during the time t1 and undisturbed image during the time t2, at a sensible frequency for the observer; this mode will be called flashing mode or flashing marking; the time t1 + t2 will then for example be greater than 0.1 seconds; thus, the steps a) and b) are repeated a number of times greater than 1, so as to obtain a flashing visual effect of the zone Zm caused by an alternation of the disturbed state and the stable initial state for each pixel of the area.

Frequency of the disturbance signal

• soit en appliquant un signal de perturbation Sp constitué d'une tension unique continue de durée t1 et d'amplitude Vblink inférieure à Vcass mais supérieure à la tension de seuil V0 du cristal liquide (fréquence nulle du signal Sp) ; dans ce cas, le signal de perturbation Sp comprend un signal électrique de tension constante.
• soit en appliquant, pour éviter une polarisation de la cellule, malgré l'augmentation de consommation d'énergie que cela implique, un signal de perturbation Sp périodique de fréquence non nulle fp. Par exemple un train d'impulsions de fréquence suffisamment élevée donnera à la zone une teinte homogène pendant la perturbation (typiquement une fréquence supérieure à 50 Hz, soit une période du signal de perturbation pp inférieure à 20ms). Si la fréquence est inférieure à 50 HZ, l'oeil de l'observateur va pouvoir percevoir des fluctuations pendant le temps de perturbation t1.

During the time or times t1, the disturbance of the marking zone can be obtained according to two variants:

• either by applying a disturbance signal Sp consisting of a single continuous voltage of duration t1 and amplitude Vblink less than Vcass but greater than the threshold voltage V0 of the liquid crystal (zero frequency of the signal Sp); in this case, the disturbance signal Sp comprises an electrical signal of constant voltage.
• or by applying, in order to avoid polarization of the cell, despite the increase in energy consumption that this implies, a periodic sp interference signal of non-zero frequency fp . For example, a pulse train of sufficiently high frequency will give the zone a homogeneous tint during the disturbance (typically a frequency greater than 50 Hz, ie a period of the perturbation signal pp less than 20 ms). If the frequency is less than 50 Hz, the observer's eye will be able to perceive fluctuations during the disturbance time t1.

The two modes are compatible with the desired effect, that is to say a disruption of the image during the time or times t1, then a return to the previously displayed image, during times t2 and after marking.

For a period pp between 20 ms and the response time of the liquid crystal (typically a few ms, ie a frequency fp between 50 Hz and 500 Hz), the liquid crystal will more or less follow the applied signal, and the eye will perceive a luminance average corresponding to the different orientations of the liquid crystal. The visual effect obtained, which will always be a difference of the luminance with respect to those of the stable states of the previously displayed image, will be a homogeneous shade depending on the shape of the signal applied during the period pp.

For a period pp less than the response time of the liquid crystal, typically a frequency fp greater than 500 Hz, the latter will be oriented according to the RMS value (Root Mean Square) of the periodic signal applied. The behavior of the liquid crystal becomes independent of the shape of the applied signal and its frequency, only the RMS value of the signal counts. In this case too, the disturbance will correspond to a hue homogeneous in time.

In all cases, the perturbation signal applied must have an RMS voltage lower than the break voltage Vcass and greater than the threshold voltage V0 of the liquid crystal.

First variant: Marked area equal to all pixels in a set of rows or columns

A first variant of the invention is to mark (statically or flashing) a zone Zm consisting of a set of q adjacent lines (referenced respectively Lx1, Lx2, .... Lxq) or a set of q adjacent columns (referenced respectively Cx1, Cx2, ... Cxq), the marking concerning all the pixels of the row or the column concerned. Thus, the zone Zm comprises a set of adjacent lines or a set of adjacent columns.

For example the static marking or the blinking of a set of columns can be obtained by applying only a column signal on the columns of the flashing zone, the lines being grounded or at a fixed potential.

According to a first option, it is possible to apply a column signal VC, for example monopolar (positive or negative) having the form of a slot and amplitude Vblink, for example equal to 2.5 V during the duration t1 (case of the continuous signal, for example). example tl = 500 ms), as described figure 7 , simultaneously on a set of excited columns. The line signal VL is for example equal to 0V on all the lines, obtained for example by putting all the lines to ground. The time t2 between two disturbances is for example equal to one second.

A variant (not shown in the figures) to avoid storage of the charges in the display is to apply a bipolar signal (+ Vblink hanging half of t1 then - Vblink during the other half of t1, or vice versa) . Another variant (not shown in the figures) is to apply + Vblink during t1 for a disturbance and the following disturbance a signal -Vblink during t1.

With reference to the figure 8 a second option is to use a sp frequency signal of non-zero frequency fp , monopolar or bipolar. A bipolar sp interference signal has the advantage of eliminating the disadvantages of a continuous electric polarization that can cause storage of charges in the screen.

When the column drivers can only deliver a positive column signal, the obtaining of positive and negative alternations on the pixels of the columns can be obtained by putting the columns belonging to the static marking zone (or flashing marking) to a mean potential Vm, the column signal consisting of alternating Vm + Vblink and Vm-Vblink. The lines are set to the average potential Vm, if necessary using an optimized and specific Vm generation circuit. It is also necessary to apply Vm also to the columns located outside the marking area (or blinking), so that they do not see the optical disturbances caused by the application of Vm lines.

Of course the variants allowing the blinking of a set of columns are applicable to a set of lines.

Sensitivity of the disturbance to the RMS voltage

The disturbance signal Sp used on the figure 6 is a monopolar column signal of frequency 600 Hz, the lines being grounded. As previously described, the figure 6a corresponds to the display in its initial state. The on state corresponds to the state T and the blocking state to the state U. The figure 6b shows the image obtained with a disturbance signal Sp of RMS value 1.5V in the marking area (or flashing area). The textures obtained from the starting textures U and T for an RMS value of the applied signal of 1.5V are still optically distinct. This RMS voltage value therefore corresponds here to an "intermediate" Sp perturbation signal. Although still visible, the image has a luminance of the passing state decreased, a gradient contrast and the eye perfectly perceives the marking of this area. The Figure 6c shows the image obtained with a signal Sp of RMS value 2.5 V. For this value, the textures resulting from U and T appear identical, almost all the molecules are lifted by the applied field, the image is no longer visible. This RMS voltage value therefore corresponds here to a disturbance signal Sp "of erasure". Of course and a fortiori, the marking is perfectly noticeable.

The disturbance signal Sp therefore preferably comprises an electrical signal (VL-VC) having an RMS value of voltage greater than 1.65 times the Freedericksz VF voltage of the liquid crystal layer.

Threshold voltage

For a value of Sp less than or equal to what is called the threshold voltage V0 of the liquid crystal, the liquid crystal molecules do not react to the applied field. Optically this translates into a luminance of the states passing and dark in field equal to that without field. This threshold voltage is a function of the liquid crystal, the texture in which it is located, the frequency and the form factor of the applied signal. For the two stable textures used here, the threshold voltage is almost identical. The threshold voltage V0 is greater than or equal to the voltage VFs.

For the curve of the figures 5 a liquid crystal such as VFs = 0.6 V has been used, and it can be seen that the luminances of the passing and blocking states remain constant approximately up to an RMS voltage, which therefore corresponds to the threshold voltage V0, close to VFs. that is 0.6 V.
Voltage V5%

After numerous investigations, the inventors have shown that a marking was perceptible as soon as the luminance Lpb varied at least 5% with respect to its luminance Lib without an applied signal. When a part of the display (static or flashing marking area) is subjected to a perturbation signal Sp for a given time t1, we will call V5% the value of the maximum voltage applied to a pixel initially in the on state such as this disturbs the luminance of the pixel in question by 5%. Beyond the frequency corresponding to the response time of the liquid crystal used, V5% is an RMS voltage. This voltage V5% is a function of the liquid crystal used, the frequency of the applied signal Sp as well as its form factor, and the time t1 during which the disturbance signal is applied; V5% is greater than or equal to VFs and V0.

Thus, for each pixel of the zone Zm initially in the on state and to which a perturbation signal Sp is applied, the disturbed state of this pixel initially in the on state has a luminance Lpb perceived by the lower observer of at least 5%, see 10% or even 20% with respect to the luminance Lib of the initial state passing from this pixel.

Example of Figure 5

On the example of the figure 5 , the signal Sp is a frequency signal 600Hz. At first, for values of Sp such that the RMS value is lower than the threshold voltage (here V0 ≈VFs = 0.6V), no alteration of the displayed image is observed.

Then, from an RMS value of Sp greater than the threshold voltage V0, progressively as Sp increases, the liquid crystal molecules rise, which has the optical consequence of a decrease in luminance Lpb.

A drop of 5% of the luminance of the on state (Lpd / Lib = 0.95) corresponds to an applied voltage of approximately V5% = 0.8V, ie at a voltage slightly higher than VFs.

For "intermediate" voltage values, there is a continuous decrease of the contrast (Lpb / Lpd), ie a decrease of Lpb together with an increase of Lpd. For a value of applied RMS voltage equal to 1.65 times VFs is 1V, the ratio Lpb / Lib = 0.85. 0.85 less than 0.95, 0.95 corresponding to a fall of 5% of the initial luminance. In this example, the luminance decrease of the on state is greater than 5% when the applied signal has an RMS value equal to 1.65 times VFs of the liquid crystal. So for static or flashing marking perceptible, a Sp value of 1.65 times VFs is sufficient. For static marking
or flashing more marked, one will choose to apply a signal having a voltage RMS equal to 2.5 times VFs.

From a certain critical value Vcri of the voltage of the perturbation signal, the perturbation signal Sp is an "erasure" signal, and almost all the molecules are raised, the textures of the passing and dark states no longer evolve. optically, the luminances Lpb and Lpd of the perturbed states become equal to the equilibrium luminance Lo. This luminance value Lo is a function, inter alia, of the angles of the polarizers used. In the described experimental configuration, this value is equal to 0.52 multiplied by Lib.

In this example, the minimum RMS voltage value to obtain the erasure of the previously registered image is about 2V = 3.3 times VFs. The marking of the zone is in this case maximum.

Second variant: marking of the intersection of N 'lines and M' columns of a screen of N rows and M columns

A second variant of the invention is to mark (statically or flashing) a zone Zm comprising an intersection of a set of N 'adjacent lines and a set of M' adjacent columns.

We have 1 ≤ N'≤N and 1 ≤ M'≤M. The lines or columns of which at least one pixel belongs to the zone to be marked are called "excited". Contiguous lines and columns, that is adjacent, are chosen here. It is of course possible to simultaneously mark several zones of the display, each zone being at least equal to one pixel.

The signal applied to a pixel is the difference between the signal on its line and that on its column. The difficulty with respect to the previous case is to optically disturb only the zone situated at the intersection of the excited lines and columns, while the other pixels of the excited lines and columns not belonging to the zone to be marked are not disturbed. This result will be obtained by taking advantage of the existence of the threshold voltage of the display V0.

A first option is illustrated on the figure 9 . The zone marked Zm is in black, the strip of excited lines is horizontal and in light gray, and the band of excited columns is vertical and dark gray. During the disturbance time t1, 3 times V0 RMS is applied to the excited lines and + V0 RMS to the other lines, 0V RMS to the excited columns, and 2xV0 RMS to the non-excited columns. So we apply an RMS voltage V0 to the whole screen except the pixels of the zone to be marked Zm which receive 3 times V0 RMS, which is more than enough to obtain a disturbance of the previously registered image. The pixels subjected to V0, threshold voltage, will not react to this voltage and remain stable.

One can also apply V0 to all the rows, and V0 to the non-excited columns and -V0 to the excited columns.

No voltage is then applied to any row and column during the time t2. This mode allows a very visible disturbance, but it has the disadvantage of applying a potential difference across the display, which consumes about as much energy as a refresh image. It should be noted, however, that in the case of flash marking, the frequency of flashing is about ten times lower than that of refreshing a conventional monostable display. Thus the power consumed by the invention is in this case ten times lower than that consumed by a conventional display.

A second option is illustrated on the figure 10 . The zone marked Zm is in black, the strip of excited lines is horizontal and in light gray, and the band of excited columns is vertical and dark gray. This second option makes it possible to apply a potential difference only on the excited lines and columns, which is less energy consuming. V0 is applied twice to the excited lines and V0 to the other lines, 0V to the excited columns, and V0 to the non-excited columns. Thus, a zero voltage or V0 is applied to the pixels outside the flashing zone Zm, and V0 is applied twice to the pixels of the flashing zone Zm. The disturbance signal here is 2 times V0 compared to 3 times V0 for the previous variant. The perturbation obtained for this second variant is less than that of the first variant if one is in the case where 2 times V0 does not make it possible to obtain the total erasure of the image previously inscribed. But a repeated scrambling of the image, even without its disappearance, is enough to attract the eye which is sensitive to the temporal variation of luminance.

Third variant: dynamic cursor

A third variant of the invention is to mark (statically or flashing) a mobile marking zone Zm, called "cursor".

• dans une première variante, le procédé selon l'invention comprend un déplacement de la zone de pixels Zm sur l'écran après chaque itération des étapes a) et b), ou
• dans une deuxième variante, le procédé selon l'invention comprend un déplacement de la zone de pixels Zm sur l'écran après un nombre Q d'itération des étapes a) et b) supérieur à 1, la zone Zm étant de préférence marquée de façon clignotante pendant les Q itérations pendant lesquelles Zm est immobile.

By applying the concepts developed above, it is possible to set up a dynamic cursor in a BiNem screen. The marking zone Zm comprising all the disturbed pixels is called "cursor". All of the pixels of the cursor Zm are disturbed so as to be different from the rest of the image displayed on the screen, this cursor Zm moving from one point to another on the screen so as to recreate a "mouse" function of a computer. The method according to the invention then further comprises a displacement of the pixel area Zm on the screen between at least two iterations of steps a) and b):

• in a first variant, the method according to the invention comprises a displacement of the zone of pixels Zm on the screen after each iteration of the steps a) and b), or
• in a second variant, the method according to the invention comprises a displacement of the pixel area Zm on the screen after a number Q of iteration of steps a) and b) greater than 1, the zone Zm preferably being marked with flashing during the Q iterations during which Zm is stationary.

The lines and columns addressed by the disturbance signal Sp will be different each time the position of the "cursor" will have to change. The speed of movement of the cursor will be adapted to obtain a satisfactory tracking of the cursor by the eye of the observer. Of course, for this application only the pixels corresponding to the cursor will be addressed, the other pixels continuing to display the image because of the bistability of the screen.

For this application, the interest of the invention is obvious. Indeed, you just have to mark the cursor area by moving it or blinking it at most 5 times per second.

If the cursor is not useful, the bistable display according to the invention consumes no electric power, whereas a monostable liquid crystal screen according to the state of the prior art consumes a power P because it must refresh its image 50 times a second.

Ainsi, grâce à l'invention proposée, il est possible d'ajouter un curseur à un écran bistable en n'augmentant que marginalement sa consommation.If the slider is useful, a monostable liquid crystal display according to the state of the prior art still consumes the same power P, while the bistable display according to the invention must receive the same energy per pixel, but 5 times less often and only on the lines and columns of the cursor. The power required to display a 5 * 5mm ² slider in a bistable A4 format screen is: $$\mathrm{P\; bistable\; cursor}=\mathrm{P\; classic\; screen}\left(\mathrm{NOT}/\mathrm{NOT}+\mathrm{M\text{'}}/\mathrm{M}\right)*\left(\mathrm{5}/\mathrm{50}\right)\approx \mathrm{P\; classic\; screen}/\mathrm{250}$$

Thus, thanks to the proposed invention, it is possible to add a cursor to a bistable screen by only marginally increasing its consumption.

CITES DOCUMENTS

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Claims (16)

1. A method for addressing a matrix screen, said screen comprising:

   - a layer of bistable liquid crystal divided into bistable liquid crystal pixels, and

   - means, for each pixel, for applying a signal to said pixel, said applied signal comprising an electric field,

   - with each bistable liquid crystal pixel having two possible stable states that are stable without an electric field being applied to this pixel, the two stable states corresponding to different visual perceptions for an observer observing said screen, a stable state, referred to as passing state, having a luminance Lib perceived by an observer, the other stable state, referred to as blocking state, having a luminance Lid perceived by an observer, with the luminance Lib being greater than the luminance Lid, with a ratio of the two luminances defining an initial Lib/Lid contrast,

   - an image being previously displayed on said screen by switching each pixel to one of said initial stable states and having said initial contrast,

   - a marking zone (Zm) being defined as comprising a set of pixels to be marked, with each of said pixels to be marked being found in an initial state corresponding to one of the stable states, said zone comprising a pixel (P6) initially in said passing state and a pixel (P5) initially in said blocking state,

   said method comprising the following steps:

   A. applying, to all of said marking zone (Zm), and during a first given time period (t1), a signal referred to as disturbance signal (Sp),

   - said disturbance signal (Sp) being higher than a threshold signal so that each pixel of said zone leaves its initial state, said disturbance signal being lower than a switching signal so that each pixel of the zone with one of said stable states as an initial state does not switch to the other stable state, each pixel of said marking zone then being found in a disturbed state,

   - said disturbance signal having a defined amplitude so that the disturbed state of a pixel initially in said passing state has a luminance Lpb perceived by the observer of less than 5% relative to the luminance Lib of said initial passing state of said pixel, the disturbed state of a pixel initially in the blocking state has a luminance Lpd perceived by the observer that is higher than the luminance Lid of the initial blocking state of said pixel, with a ratio of the two luminances in a disturbed state defining a disturbed Lpb/Lpd contrast lower than the initial contrast, said disturbance signal inducing a disturbance that can be optically perceived by an observer of said zone, then

   B. applying no signal to each pixel of said marking zone during a second given time period (t2) so as to allow each pixel of said zone to return to its initial stable state, then

   C. repeating the two preceding steps A and B.
2. The addressing method according to claim 1, wherein steps A and B are repeated more than once so as to obtain a visual effect of blinking of said zone that is caused by alternating the disturbed states and the initial states for each pixel of said zone.
3. The addressing method according to claim 1, wherein steps A and B are repeated with a frequency lower than a duration of retinal persistence of the observer so as to obtain a visual effect of static marking of said zone.
4. The method according to any one of the preceding claims, comprising a displacement of said zone of pixels on said screen between at least two repetitions of steps A and B.
5. The method according to any one of the preceding claims, wherein the pixels are arranged in lines of parallel pixels and in columns of parallel pixels, with the lines being substantially perpendicular to the columns.
6. The method according to claim 5, wherein said zone comprises a set of adjacent lines or a set of adjacent columns.
7. The method according to claim 5, wherein said zone comprises an intersection of a set of adjacent lines and a set of adjacent columns.
8. The method according to any one of claims 5 to 7, wherein said disturbance signal applied to a pixel comprises a column signal applied to the column on which said pixel is located and further comprises a line signal applied to the line on which said pixel is located and is proportional to a difference between the column signal and the line signal.
9. The method according to any one of the preceding claims, wherein said disturbance signal is an erasure signal for which the disturbed state of a pixel initially in one of the stable states is identical to the disturbed state of another pixel initially in the other stable state, with the disturbed states of two pixels initially in two different stable states corresponding to the same visual perception for the observer observing said screen.
10. The method according to any one of claims 1 to 8, wherein said disturbance signal is an intermediate disturbance signal for which the disturbed state of a pixel initially in one of the stable states is different to the disturbed state of another pixel initially in the other stable state, with the disturbed states of two pixels initially in two different stable states corresponding to different visual perceptions for the observer observing said screen.
11. The method according to any one of the preceding claims, wherein said disturbance signal comprises a constant voltage electric signal.
12. The method according to any one of claims 1 to 10, wherein said disturbance signal comprises a periodic signal.
13. The method according to claim 12, wherein the frequency of said disturbance signal is between 50 Hz and 500 Hz.
14. The method according to claim 12, wherein the frequency of said disturbance signal is higher than 500 Hz.
15. The method according to any one of the preceding claims, wherein said disturbance signal comprises an electric signal with an effective RMS voltage value of more than 1.65 times a Freedericksz voltage of the liquid crystal layer.
16. The method according to any one of the preceding claims, wherein said disturbance signal is bipolar.

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