GB2074356A

GB2074356A - Passive Electro-Optic Display Cell

Info

Publication number: GB2074356A
Application number: GB8110053A
Authority: GB
Original assignee: Asulab AG
Current assignee: Asulab AG
Priority date: 1980-04-16
Filing date: 1981-03-31
Publication date: 1981-10-28
Also published as: CH632367B; FR2480952A1; JPS56158321A; DE3114856A1; DE3114856C2; GB2074356B; CH632367GA3

Abstract

A passive electro-optic display cell comprises a layer of passive electro- optic material, e.g. liquid crystal (4), between two parallel plates (1, 2), on the inner face of each of which is a set of control electrodes (7, 11) and a screen-electrode (9, 13) separated by an insulating layer (8, 12). The screen-electrodes (9, 13) and the insulating layers (8, 12) are apertured (10, 14) opposite the corresponding control electrodes (7, 11), allowing alternative or simultaneous display of two sets of information contained in the control electrode-screen combination on the respective plates, the display being either positive or negative. <IMAGE>

Description

SPECIFICATION Passive Electro-optic Display Cell The present invention relates to a passive electro-optic display cell comprising a layer of a passive electro-optic material.

U.S. Patent No 4,137,524 discloses such a cell which comprises moreover control electrodes and a counter-electrode arranged on both sides of the layer of electro-optic material. A screen-electrode is interposed between the control electrodes and the electro-optic material. This screen-electrode is provided with an opening opposite each of the control electrodes.

If the electro-optic material used is a nematic liquid crystal, polarisers are arranged on both sides of the cell with their respective directions of polarisation disposed at an angle of 900 one with the other. Alignment layers are moreover deposited on the inner faces of the two plates for inducing, in the absence of an electric field, the well-known helical arrangement of the molecules of the liquid crystal.

In such a cell, the shape and dimensions of the openings provided in the screen determine the shape and dimensions of the display elements. The latter are constituted by zones of the cell which can be rendered individually visible or invisible with respect to the background of the display and which constitute, by their several combinations, the several signs such as digits, letters or other symbols which are to be displayed.

In the area of the background of the display, that is to say outside the areas corresponding to the screen openings, the appearance of the cell depends only on the voltage applied between the screen and the counter-electrode. If this voltage is zero or sufficiently weak, the background of the cell appears clear. If this voltage is sufficient for the field that it creates in the liquid crystal produce alignment of the molecules of the latter in a direction perpendicular to the plates, the background of the cell appears dark.

At the positions of the display elements, it is the voltage applied between the control electrodes and the counter-electrode which determines the appearance of the cell. The cell is clear in these areas if the voltage is zero or sufficiently weak, and dark if it has a sufficient value.

Consequently, the presence of the screen enables a dark display of information to be produced on a clear background, that is to say in a positive sense, or a clear display on a dark background, that is to say in a negative sense.

In other embodiments, a dichroic colorant is added to the liquid crystal, as well as, possibly, a chiralic compound. At least, one of the polarisers can then be eliminated. Contrary to the earlier case, the cell appears to be dark in the absence of electric field and to be clear in the presence of such a field.

Consequently, the operation of the cell is exactly the reverse of that which has been described above, but the cell also possesses the property of permitting either a positive or a negative display.

In any case, the presence of the screen moreover prevents the lay out of the conductive layers which connect the electrodes to the terminals of the cell being visible, in spite of the fact that the counter-electrode covers the whole rear plate. The problems arising from the configuration of the counter electrode of a conventional cell and from the mutual alignment of its electrodes and its counter-electrode are thus eliminated.

It is frequently desired to be able to display more information than a normal cell can display.

When the cell is intended to be used in an electronic watch provided with a large number of horary or non-horary functions, it is for instance desirable that it can display information presented as data indicating the time or as letters indicating the day of the week and/or the name of the month. This problem can be solved by using characters constituted by a given number of segments, but to the detriment of the aesthetic quality of these characters.

It is sometimes also desired that the cell can display information of a very different nature such as a map of the world to enable identification of the position of the horary belts the time of which is displayed. This problem can be solved by the superposition of two distinct display cells, but at the expense of increased cost and thickness of the device.

Depending on the manner in which it is realized, this superposition has, moreover, the drawback that the areas where display elements belonging to the first cell are superposed on display elements belonging to the second cell tend to merge with the background of the display device. For instance, when the background of the display is clear and a cross is displayed by rendering dark two rectilinear segments belonging each to a corresponding one of the cells, the center of this cross is clear.

The object of the present invention is to provide a display cell which enables the number of numeric, alphabetic or other information which can be displayed to be materially increased, without the drawbacks which result from the superposition of two cells and, with, moreover, all the advantages of the cell disclosed in U.S. Patent No. 4,137,524.

The features of the invention will be apparent from the following description, drawings and claims, the scope of the invention not being limited to the drawings themselves as the drawings are only for the purpose of illustrating a way in which the principles of the invention can be applied. Other embodiments of the invention utilising the same or equivalent principles may be used and structural changes may be made as desired by those skilled in the art without departing from the present invention and the purview of the appended claim.

The solitary drawing shows, in cross-section and by way of example, a passive electro-chromic display cell representing a preferred embodiment of the invention.

The display cell illustrated in the drawing figure comprises two transparent plates of glass, a front plate 1 and a rear plate 2, separated by an assembling frame 3, made of sintered glass for example, providing between them a space 4 filled with a nematic liquid crystal. The assemblage thus constituted is interposed between two polarisers 5 and 6, which are crossed, that is to say respective directions of polarisation of light by the polarisers are substantially perpendicular to each other.

The front glass plate 1 carries, on its inner face, a set of control electrodes 7. These electrodes 7 are covered with an insulating layer 8, which in turn carries a screen-electrode 9. The control electrodes 7, the insulating layer 8 and the screen-electrode 9 are transparent.

The insulating layer 8 and the screen-electrode 9 are provided with apertures 10, situated each opposite to a corresponding one of the electrodes 7.

Similarly to the plate 1, the rear plate 2 is transparent and is provided with a set of control electrodes 1 covered with an insulating layer 12 which in turn carries a screen-electrode 13. The control electrodes 1 the insulating layer 12 and the screen-electrode 13 are also transparent.

As with the plate 1, the insulating layer 12 and the screen-electrode 1 3 are provided with apertures 14 each situated opposite to a corresponding one of the electrodes 11.

The shape, the dimensions and the configuration of the apertures 14 are different from those of the apertures 10 provided in the insulating layer 8 and in the screen-electrode 9.

Each of the plates 1 and 2 is moreover internally covered, over its whole surface, with a layer serving for homogeneous planar alignment of the molecules of the liquid crystal, this layer not being shown in the drawing. These alignment layers are such that, in the absence of an electric field, the molecules of the liquid crystal which they contact are all substantially parallel to the plates 1 and 2 and, moveover, substantially paralled to the direction of polarisation of the corresponding polariser 5 or 6.

A protecting layer, made of silicium oxide for example (also not shown) can, if necessary, be deposited over the whole surface of the plates 1 and 2, before the deposition of the alignment layers.

Finally, a diffusor-reflector 1 5 is arranged at the rear of the cell. - The cell described and illustrated operates as follows: In a first mode of operating, the electrodes 11 and the screen 13 of the rear plate 2 are brought to a common potential, which will be designated later by P1. The cell then operates exactly as the cell disclosed in U.S. Patent No 4,137,524. Under these conditions, as a matter of fact, these electrodes 11 and the screen 13 together constitute a continuous counter-electrode covering the whole of the rear plate 2.

If the screen 9 is also placed at the potential P1, the background of the display appears clear, and each electrode 7 produces a dark visualisation of the display element which corresponds thereto when it is brought to a potential P2 sufficiently different from the potential P 1 or; on the contrary, the erasure of this display element when it is placed at this potential P1 The shape, the dimensions and the position of each display element are determined by the shape, the dimensions and the positions of the openings provided in the screen 9. By their several combinations, these display elements permit the visualisation of an amount of information selected in a determined information set.

If, on the contrary, the screen 9 is placed at the potential P2, the background of the display appears dark and the electrodes 7 produce a clear appearance of the display element which corresponds thereto when they are placed at the potential P1, and the erasure of this display element when they are placed at the potential P2.

In the first form of display, the information belonging to the information set determined by the shape,- dimensions and configuration of the openings 10 can consequently be displayed selectively and, moreover, in positive or in negative sense.

In a second mode of operation, all the electrodes 7 and the screen 9 of the front plate 1 are placed at a common potential P 1. Under such conditions, these electrodes 7 and the screen 9 together constitute a continuous counter-electrode covering the whole of the front plate 1.

If, under these conditions, the screen 13 of the rear plate 2 is also placed at the potential P1,the background dfthe display appears also as clear, The electrodes 11 produce a dark appearance of the display elements which correspond thereto when they are placed at the potential P2 or the erasure thereof when they are placed at the potential P1.

If, on the contrary, the screen 13 is placed at the potential P2, the background of the display appears dark. The electrodes 11 produce then a clear appearance of the display elements which correspond thereto when they are placed at the potential P 1 and the erasure thereof when they are placed at the potential P2.

It can thus be seen that, in this second mode of operation, the information belonging to a second information set can be displayed in positive or in negative sense. The information of this second set is determined by the shape, dimensions and configuration of the openings 14 provided in the screen 13.

It is important to note that the information of the second set is completely different and independent from that of the first set. The information of one of these sets can be, for instance, conventional time information such as the hours, minutes and seconds of actual time, displayed- in numeric form. That of the other set can be, also by way of example, information in alphabetic form, such as the name of a day of the week or of a month, or a combination of numeric and alphabetic (alphanumeric) or other information. It can be the same time information, but presented in a-pseudo analogue form, that is to say by display elements having the form of segments arranged radially around a central point so as to simulate the appearance of the hand of a conventional watch. Many other combinations can also be imagined.

In a third mode of operation, the cell according to the invention enables the separate or simultaneous display of information belonging to the two sets of information as defined above. For producing a display in positive sense in this third mode, the two screens 9 and 1 3 are brought to said potential P 1. Consequently, the background of the display is clear. The electrodes 7 and 11 corresponding to the display elements which must remain invisible, that is to say remain clear, are also placed at this potential P1. The electrodes 7 corresponding to display elements which must be made visible, that is to say dark, are placed at the potential P2, as described above.The electrodes 11 corresponding to display elements which must also be made visible are placed at another potential P3 such that the potential difference P3-P1 is substantially equal, in absolute value, to the potential difference P2-P 1, but of reversed sign.

The display elements corresponding to the electrodes 7 which are placed at this potential P2 and are situated entirely opposite the screen 13 or the electrodes 11 placed at the potential P1 have exactly the same appearance as in the first mode of operation when the information of the first set is displayed in positive sense.

The same applies for the display elements corresponding to the electrodes 11 placed at the potential P3 and situated entirely opposite the screen 9 or the electrodes 7 placed at the potential Pi.

These elements have also exactly the same appearance as in the second mode of operation, when the information of the second set are displayed in positive sense.

The display elements, or parts of elements, corresponding to the electrodes 7 placed at the potential P2 and situated opposite or partially opposite the electrodes 11 placed at the potential P3 also have the same appearance in spite of the fact that the potential difference P2-P3 applied to these electrodes is substantially equal to twice the potential difference P2-P1.

The characteristics of the liquid crystais are, as a matter of fact, such that the appearance of a display element does not substantially change further when the voltage applied to its control electrodes goes beyond a determined value, called the saturation voltage.

For the simultaneous display, in negative sense, of information belonging to the first and second sets of information, it is necessary to place the screen 9 at the potential P 1 and the screen 1 3 at the potential P2, as in the second mode of operation mentioned above. In this third mode, however, the potentials P1 and P2 must be selected in such a way as to fulfil the two following conditions: P2-P1 P2P1kUsat; and Pm= < U threshold 2 where Usat is the saturation tension above which the dark appearance of the cell undergoes no further modification, and where Threshold is the voltage under which the clear appearance of the cell undergoes no further modification. It is obviously necessary that the liquid crystal used be such that the two voltages Usat and Uthreshold be defined.But this requirement is easily fulfilled, such liquid crystals.being well known.

The electrodes 7 and 11, corresponding to display elements which must remain invisible, are placed at the potentials P1 and P2, respectively, as in the second mode of operation mentioned above, in a negative display.

The electrodes 7 and 11, respectively, corresponding to display elements which must be made visible are placed at the potential Pm.

The display elements corresponding to electrodes 7 placed at this potential Pm and situated entirely opposite the screen 13 or the electrodes 11 placed at the potential P2 appear as clear since the difference between these potentials P2 and Pm is equal to or lower than the threshold voltage.

The same applies for the display elements corresponding to electrodes 11 placed at this potential Pm and situated entirely opposite the screen 9 or electrodes 7 placed at the potential P1,since the difference between these potentials P1 and Pm is also lower than the threshold voltage.

The display elements, or parts of elements, corresponding to electrodes 7 and 11, respectively, placed at the potential Pm and situated opposite, or partially opposite, electrodes 11 and 7, respectively, also placed at this potential Pm, appear obviously also as clear since the potential difference between these electrodes is zero.

Consequently, it can be seen that, in this third mode of operation, the cell according to the invention enables the simultaneous display, in positive or negative sense, of information belonging to the first and to the second sets of information. The areas where display elements of the first set are superposed with elements of the second set have an appearance identical to that of the areas where these elements are separated.

The following table summarizes the operation of the cell in this third mode. It indicates the clear or dark appearance of the background of the display and of two display elements El and E2 corresponding respectively to electrodes 7 and 1 for the several combinations of potentials applied to these electrodes and to the screens 9 and 13.

9 13 7 11 Ground El E2 P1 P1 P1 P1 clear clear clear P1 P1 P2 P1 clear dark clear P1 P1 P1 P3 clear clear dark P1 P1 P2 P3 clear dark dark P1 P2 P1 P2 dark dark dark P1 P2 Pm P2 dark clear dark P1 P2 P1 Pm dark dark clear P1 P2 Pm Pm dark clear clear It will be appreciated that the potentials applied to the screens and to the electrodes in the three modes of operation described above have been given only by way of example. Other combinations of potentials could produce the same results. These potentials could in particular be alternative. In this case, it would not be the absolute value of their differences which would produce the visualisation or the erasure of the display elements, but the root-mean-square values of these differences.

The cell described, which is of the type called "twist nematic", operates in reflexion due to the presence of the diffusor-reflector 1 5. It will be appreciated that it can operate in transmission if the diffusor-reflector 1 5 is dispensed with.

Moreover, the same arrangement of screen-electrodes provided with apertures can be applied with the same advantages to cells of the so-called "Heilmeyer" type, in which dichroic molecules are mixed with the liquid crystal as well as to cells of the so-called "White-Taylor" type, which further contain a chiralic compound.

It is also to be noted that the insulating layers 8 and 12 could be continuous and not be provided with apertures. In this case the manufacture of the cell would be slightly simplified, but the contrast would be slightly reduced. This decreased contrast could however be compensated for by a slight increase of the control voltages applied to the cell.

Finally, other electro-optic materials-can be used, especially a liquid crystal having a dynamic diffusion or a dipolar suspension.

Claims

1. A passive electro-optic display cell comprising a layer of a passive electro-optic material having opposite first and second faces, wherein the cell comprises a first set of control electrodes and a second set of control electrodes arranged, respectively, opposite said first and said second faces of said passive electro-optic material, a first screen-electrode arranged between said first face and said first set of electrodes and which is provided with openings arranged each opposite a corresponding one of the electrodes of said first set of control electrodes, and a second screen-electrode arranged between said second face and said second set of electrodes and which is provided with openings arranged each opposite a corresponding one of the electrodes of said second set of control electrodes.

2. A passive electro-optic display cell constructed and arranged substantially as herein particularly described with reference to the accompanying drawing.