DE2601272A1 - Liq. crystal display with memory characteristic - has matrix memory of liq. crystals that retain molecular orientation for period following switch off - Google Patents

Abstract

A liquid crystal matrix display is based upon energisation of liquid crystal segments to produce a given molecular orientation that is retained for a specific period. In this way the display exhibits a memory characteristic. The memory matrix is formed by strip electrodes (3,4) of SnO2 or In2O3. The boundary of the matrix is sealed with a glass solder (5) of a low melting point. Cells are formed by dielectric distance pieces (6) of SiO2 or glass and are filled by a liquid crystal layer (7). The assembly is contained between two glass covers (1,2). A silicon coating around each cell ensures that a given molecular orientation is achieved during the stand-by condition. When the control voltage is applied to the two electrodes the molecules are re-oriented to provide a display that is retained for a period following switch-off.

Description

Method and device for displaying images by means of

a liquid crystal (LC) display.

The invention relates to a method according to the preamble of the claim 1 as well as Plüssigkristall- (LC) displays for carrying out such a method.

Such a display technique is well known and has been used proven in many areas of application; See, for example, the overview article von Kobale and Krüger in "Physics in Our Time" No. 3, 1975, pages 66 to 77. However, it still does not lead to satisfactory results if you know how For example, in data display devices of a data processing system, the entered Not only wants to present information but also to store it optically. In these cases one was previously dependent on either the displayed image to constantly refresh by periodically repeated rewriting of the information or on LC substances with inherent storage capacity, usually cholesterol Compounds or mixtures with one inducing the cholesteric structure Addition to fall back. A dynamic storage "is with a high control and component expenditure burdened (external circulating memory, a separate one per image segment Drivers) and among the usable storage LC substances there are currently only Little choice, with a correspondingly limited range of values for the relevant ones Display parameters such as the temperature range for the mesomorphic state, the dielectric anisotropy, electrical conductivity, response speeds or service life.

The object of the present invention is to provide a method develop with the using a nematic LC layer without special Aufwana not only a display of information but also a storage of the represented information can be obtained; a liquid crystal display is also intended to carry out such a procedure.

For this purpose, the invention provides a method according to the preamble of claim 1 with the features specified in the characterizing part of this claim. With such a method, particularly long storage times are obtained if the The edge of the image segment is self-contained and completely encloses the image segment. The control voltage should expediently have a value between 0.7 U5 and U5 (u5 is the threshold voltage for the onset of dynamic scattering).

The proposed method is an embodiment of the invention provided that the image segment edge by a physical, essentially Wall extending perpendicular to the carrier plates with an orientation force is formed, which are the near-wall liquid crystal molecules of the image segment seeks to bring in a direction perpendicular to the rest position. Orient the panels For example, if the LC molecules close to the plate are parallel to the plate, the edge wall should Bring the LC molecules close to the wall in a position parallel to the plane of the wall.

Deviating from this, however, according to the invention, the edge of the image segment can be used can also be generated by shaping the course of the field lines in a suitable manner, preferably bulges the field lines in the border area between the image segment and the edge.

Further advantageous refinements and developments of the invention are described in the appended claims.

The invention is based on the following observation: In an in Multiplex technology addressed, large matrix display of the dynamically spreading type was after some operating time with some of the not activated pixels the Matrix shows a weak light scattering. At the controlled pixels (Image cells) there was a voltage of 3Uh, the voltage at the other image points Uh; Uh had a value just below Us. Remarkably, this one occurred Scattering effect only occurs in those image cells that have electrodes on both sides the adjacent glass solder base. It turned out that the weak Light scattering originated from SK areas which, starting from the electrode-contacting Glass solder support points, expanded and after a sufficiently long operating time the whole Covered image cell. In the microscope these areas were shown as stripe domains recognize. The domain structure caused diffraction of the incident light, see above that the areas appeared colored. The domain width was voltage independent.

When the voltage was switched off, the one with the so-called DAP effect (deformation of erect phases) comparable appearance in the observe polarized light. The size of this zone steadily decreased The boundary front was now moving in the opposite direction.

If you first create the domain information and then increase the If the operating voltage exceeds U5, the area of the entire image cell becomes turbulent. If the voltage is only increased moderately, the now perceptible scatter is clear to be distinguished from the control of other areas (spots "). Will the tension increased even further (U> 5Uh) such a strong turbulence occurs in all Areas that no difference can be recognized.

This state hysteresis, which occurred unexpectedly, can be interpret as follows: The domain formation depends firstly on the type of geometric boundary of the cell and the surface orientation of the LC layer at from this point and, secondly, starts from the border.

If one assumes that the orientation of the crystal is on all walls, so both on the carrier plates and on the edge of the image segment, always vertically to the respective surface level, then the boundary (glass solder support points) must close a disturbance in particular in the central plane parallel to the two carrier plates the LC layer lead. If the voltage Uh is applied under these conditions, so the PK molecules rotate in the layer center plane of the entire controlled Image cell in a position parallel to the layer surface. Which direction they are within occupy the layer is not due to the field of the (plane-parallel) electrodes rather, field inhomogeneities play a role here, and of course also the am Glass solder support point defined orientation a role. That the domain area at applied voltage Uh increases steadily, is probably due to the fact that the steady ion migration and the associated material transport the individual LC molecules can oscillate constantly around their central position and thereby an energetic favorable, metastable orientation pattern grow relatively quickly into the cell area can.

Based on the registered scattering homogeneity and the interpretation The proposed method shows a way of developing and creating them on how to systematically disrupt the LC orientation, namely through appropriate Preparation of the image segment edges, can get to an information storage.

In the literature it has already been pointed out several times that in the case of displays with dynamic scatter in the course of operation on the activated Form areas of the screen that are optically scattered from the rest Separate area (lifetime spots "); cf. in particular the investigation von Alt and Freiser in J. Appl. Phys. 45 (1974) pp. 3237 to 3241, with further references. In the cited study it is reported that in a homeotropically oriented FK layer at frequencies between 0.25 fc and fc and at voltages between Uth and 2.5 Uth is the upper frequency for dynamic scattering and Uth is the threshold voltage for the formation of so-called Williams domains) a certain LC structure after a few minutes arises; this Structuring process is described by the authors "texturing" called.

The texture disintegrates after switching off the control voltage under a change their optical properties rapidly in a metastable state of rest in which the Director of the nematic LC layer on the electrode boundary on his way from one plate to the other is continuously rotated by a total of 1800.

Even if the "dexturing" is related to the present Invention observed domain formation may have certain similarities (the respective However, the structural models developed differ), but in the The mentioned work does not provide any suggestion for utilizing the described effect given; rather, the authors have described the texturing as disturbing.

The invention will now be based on particularly preferred exemplary embodiments will be explained in more detail in connection with the figures of the drawing. Corresponding to each other Parts are given the same reference numerals. They show: FIG. 1 a first Exemplary embodiment of a liquid crystal display according to the invention, FIG. 2 of the exemplary embodiment FIG. 1 shows a section, after switching off the control voltage, with the drawing Domain structure; and FIGS. 3 to 6 each show further exemplary embodiments of one according to the invention Liquid crystal display, as shown in FIG. 2.

For the sake of clarity, the figures are all for understanding the invention not absolutely necessary parts of an LC display, for example the leads to the individual electrodes, only indicated or omitted entirely.

All illustrated embodiments are matrix displays, they are operated in transmission and contain a nematic LC layer with homeotropic Plate wall orientation and negative dielectric anisotropy.

The embodiment of FIG. 1 contains two carrier plates in detail 1,2, which are individually controllable on their facing surfaces (inner surfaces), strip-shaped electrodes (image line conductor 3, image column conductor 4 of an image matrix) wear and at their edge zones over a glass solder frame 5 hermetically sealed to one another are connected. In the chamber delimited by the two plates and the glass solder frame there is a dielectric layer 6, which separates the two plates which cut out the points of intersection of the picture column conductors with the picture row conductors are. The individual cutouts each contain an LC layer 7.

The individual display parts consist of the following materials: The Both carrier plates are made of glass, the glass solder frame from a glass solder with low Melting point, the strip-shaped electrodes made of SnO2 or In203, the dielectric Layer made of SiO2 or glass and the LC layer is a commercially available nematic Substance, for example that under the name “ZLI 542” from Merck distributed mixture. The dielectric anisotropy / LC of this layer has a Value of YS £ = - -0.5, its electrical conductivity should be between 108 and 1 Ohm.cm lie. The inner surfaces of the panels as well as the surfaces adjoining the LC layer the dielectric layer are treated in a known manner in this way, for example covered with a silicone (silane) layer that the LC molecules with their Set the longitudinal axes essentially perpendicular to the plane of the surface (homeotropic wall or edge orientation). The display can be operated with direct or alternating voltage However, for reasons of service life, AC voltage operation is preferable.

From Fig. 2 it can be seen which structure is after control of a pixel in the FK area there and after the control voltage has been switched off remaining domains most likely have: The individual SK molecules - in In the figure, the directions of its longitudinal axes are indicated by dotted lines 8 - are from their original position, perpendicular to the panel walls, to the layer center plane (dash-dotted line 9) deflected more and more. On the dielectric Boundaries they are also perpendicular, at least in the vicinity of the layer center plane. Field inhomogeneities then ensure the double rib structure idealized in the figure (upper ribs 10, lower ribs 11) that for the viewer as a pattern from one another parallel vertical stripes appear.

These strips run along the rib centers or between adjacent ones Ribs, their locations are marked with arrows in Fig. 2.

The dielectric layer always needs the distance between the two To fill out the carrier plates completely, it is sufficient if, starting from a of the two carrier plates, protrudes sufficiently far from the opposite carrier plate. Such a modified embodiment is shown in FIG.

In this case, the end faces of the dielectric layer become rounded, an orientation pattern very similar to the domain structure of FIG. 2 is obtained.

In the exemplary embodiments of FIGS. 4 to 6, the inventive intended disruption at the edge of the image segment instead of a physical wall caused a suitable design of the field lines.

In the display embodiment of FIG. 4, the individual image striplines are each support plate on both sides by an additional, also strip-shaped Edge strip conductor 12) bordered. The strip lines are from the Edge strip conductors electrically isolated and also closer to the opposite one Carrier plate approached. The electric field is in the border area between the image point and the edge deformed, in particular the field lines arising on the edge of the plate are deformed drawn into the bilapoint area on their way to the counter electrode in the present case (bulged, dashed lines 17). This creates a with the same texture in principle as the domain structure of FIG. 2. To generate the required Field deformations can also increase the voltage applied to the edge electrodes be less than the voltage applied to the picture electrodes and either together be switched off with the control voltage for the image segment electrode or else are maintained during the subsequent storage period.

Additional electrodes can be avoided by using the strip conductor either except for its lateral edge zone or exclusively on this edge zone covered with a dielectric layer (dielectric center strip 14 in Fig. 5, dielectric edge strips 15 in Fig. 6), whose dielectric constant (DK) from the DK of the LC layer, in the present case in particular from the DK component in the direction of the longitudinal axis of the LC molecules. Then there is on the basis of the theorem that the normal component of the dielectric displacement vector continuously passes through interfaces between two dielectric media, in the LC layer between the dielectric layer and the opposite carrier plate a different field strength than in the rest of the area of the crossing point, so that the desired Can adjust the course of the field lines.

The invention is not limited to the illustrated embodiments limited. In addition to matrix displays, there are also other displays, for example alphanumeric displays with a segmented front electrode and a continuous one Back electrode, in question.

Furthermore, one does not always need from a homeotropically oriented one To assume an LC layer with negative dielectric anisotropy, it is also that Use of homogeneously oriented LC layers with a like> 0 is conceivable. Independent information storage could also be made by not completely closed Realize image segment edges, for example in matrix displays with between the image strip conductors a carrier plate placed dielectric longitudinal webs. It may be even possible, solely through suitable shaping of the edge zone of the segment electrodes to build up the Bildsegmentrana without the aid of other aids.

Finally, it should be pointed out that, in principle, the It is possible to have the display with a magnetic instead of an electric field to operate.

15 claims 6 figures Blank page

Claims (15)

P a t e n t a n s p r ü c h e 1. Method for displaying images by means of a liquid crystal display with two carrier plates with a nematic liquid crystal layer with a given orientation (rest position) and a given Sign for the susceptibility anisotropy, preferably a liquid crystal layer with homeotropic orientation with respect to the carrier plates and negative dielectric Anisotropy, hermetically seal and on their facing surfaces each wear an electrode coating, to build up the image to be displayed from individual image segments the electrode coatings have at least one separately controllable Segment electrode included, that is to say Purpose of additional image storage in the liquid crystal layer around each Image segment at least during the time of the control of the associated segment electrodes an edge (edge of the image segment) is generated which contains the liquid crystal molecules adjacent to it seeks to deflect the image segment from its rest position, and that a control voltage (Ua) is created with a value at which, based on the generated image segment edge, Form light-scattering domains, their structure even after switching off the control voltage is essentially maintained over a period of time (storage time). 2. The method according to claim 1, d a d u r c h g e k e n n -z e i c This means that the edge of the image segment is self-contained and that the image segment is complete encloses. 3. The method according to claim 1 or 2, d a d u r c h g e k e n n -z e i c h e t that G, 7 U5 h Ua C Us where Ua is the control voltage and U5 is the Is the threshold voltage for the onset of dynamic scattering. 4. The method according to any one of claims 1 to 3, d a d u r c h g e k it is noted that the domains generated are strip-shaped, the strip width practically does not depend on the applied voltage, and have a double rib profile (Fig. 2). 50 The method according to any one of claims 1 to 4, d a d u r c h g e k It is noted that the edge of the image segment is affected by a physical, in the Wall with an orientation force extending substantially perpendicular to the support plates is formed, which are the near-wall liquid crystal molecules of the image segment seeks to bring in a direction perpendicular to the rest position. 6. The method according to any one of claims 1 to 4, d a d u r c h g e k It should be noted that the image segment edge is formed by a suitably shaped Course of the field lines, preferably through a bulge of the field lines in the border area between the image segment and the edge. 7. Liquid crystal display for performing the method according to claim 5, that the walls of dielectric Web (6) are formed, which at least one of the two carrier plates (1,2) go out. 8. Liquid crystal display according to claim 7, d a d u r c h g e k e n n z e i c h n e t that the dielectric webs (6) are made of a printed glass solder are formed. 9. Liquid crystal display according to claim 7 or 8, wherein the electrode coatings of both carrier plates each strip-shaped segment electrodes (picture line conductor, Image column ladder of an image matrix), d a d u r c h e k e n n -z e i c h n e t that the dielectric webs (6) on one of the segment electrodes (3) provided carrier plates (1) are applied in the form of a grid from which the points of intersection of the picture line conductors with the picture column conductors are recessed. 10. Liquid crystal display for carrying out a method according to Claim 6, d a d u r c h g e k e n n n z e i c h n e t that to produce a bulge the field lines (13) in the border area between the image segment and the edge of the electrode coating at least one of the two carrier plates in addition to its segment electrodes (3) additional Contains electrodes (edge electrodes 12) with those in the liquid crystal layer at the location of the edge electrodes at least during the control of the image segment Field can be built up, the strength of which depends on the field strength at the location of the segment electrodes is different. 11. Liquid crystal display according to claim 10, d a d u r c h g e k e n n z e i c h n e t that the edge electrodes (12) and the segment electrodes (3) lie on the same level and can be connected to different voltages. 12. Liquid crystal display according to claim 10, d a d u r c h g e k e n n z e i c h n e t that the edge electrodes (12) and the segment electrodes (3) lie on different levels and applied to the same or different voltages can be. 13. Liquid crystal display according to claim 12, d a d u r c h g e k e n n z e i h n e t that the segment electrodes (3) of the opposite carrier plate are more closely approximated than the edge electrodes (12) of the same carrier plate. 14. Liquid crystal display for carrying out a method according to Claim 6, d a d u r c h g e k e n n n z e i c h n e t that to produce a bulge the field lines (13) in the border area between the image segment and the edge of an image segment associated segment electrodes (3,4) at least one of the two carrier plates (1,2) except for an edge zone with a dielectric, the rest position of the liquid practically non-changing layer (14), whose dielectric constant (DK) from the DK of the liquid crystal layer, in particular from the DK component in the longitudinal direction of liquid crystal molecules, is different. 15. Liquid crystal display for carrying out a method according to Claim 6, d a d u r c h g e k e n n n z e i c h n e t that to produce a bulge the field lines (13) in the border area between the image segment and the edge of an image segment associated segment electrodes (3,4) at least one of the two carrier plates (1,2) are covered on their edge zone with a dielectric layer (15) whose dielectric constant (DK) from the DK of the liquid crystal layer, in particular from the DK component in Longitudinal direction of the liquid crystal molecules, is different.