GB2024443A - Coloured display systems - Google Patents

Abstract

In a colour display system a first liquid crystal cell 10 selects a display column, a second liquid crystal cell 13 selects a display row (such that only the intersecting point is transmissive), and the second or a third cell 19 acts with a striped filter 20 to determine which of at least two colours is transmitted. In Fig. 1A, R and G filter strips are aligned with respective row electrodes 15 of cell 13. In Fig. 1B third cell 19 comprises selectable intedigitated electrodes 24 and 25, each finger pair 22, 23 corresponding to a row electrode 15 and to a pair of registering R and G filter strips 26. Fig. 1C is a condensed version of Fig. 1B in which cell arrangements 13 and 18 are combined in a single cell arrangement. <IMAGE>

Description

SPECIFICATION Coloured optical display systems This invention relates to coloured optical display systems. More particularly, it concerns such systems wherein the colour and form of the display are electronicaliy switchable from one state to another.

In the Complete Specification of our copending Application for Letters Patent No. 1 1,343/78 (1/61 54/M) there is described and claimed an optical filter device which is capable of being switched between at least two colours. That filter device may be defined as comprising a filter element operatively associated with a liquid crystal cell, the filter element bearing areas of filter material of one colour interspersed with areas of filter material of at least one other colour, and the liquid crystal cell being drivable in corresponding areas (conveniently by having on one side a first light-transparent electrode and on the other side at least two second light-transparent electrodes aligned respectively with the differently-coloured filter materials), so that the optical transmission characteristics of the liquid crystal cell adjacent the appropriate section of filter material may be changed (for example, by applying an electric potential between the first electrode and one (or more of the second electrodes) to change in turn the overall filtering effect of the device.

In a preferred embodiment all of the areas of filter material, and their associated electrodes, are in strip form, the strips being m-utually parallel and of regularly alternating colours -- thus, for example, red, green, red, green, and so on.

Moreover, most conveniently in each case all of the second electrodes aligned with any one colour are connected with each other, so that the device as a whole becomes a filter of that colour when those electrodes are used.

In particularly a preferred form the filter element is a three-colour filter -- such as red, green and blue - the three colours being disposed as an array of regularly interdelineated coloured strips.

In the Complete Specification of another of our copending Applications for-Letters Patent, Application No. 25,550/78 (1/61 61/M), there is described and claimed a visual display device which is capable of giving a monochrome display.

That display device may be defined as comprising two liquid crystal cells mounted face-to-face, one of which cells can be driven/activated in strips extending in one sense while the other can be driven/activated in strips extending in a second sense at an angle to the first, the optical transmission characteristics of the double cell system as a whole thus being changed only wherever two angled activated strips cross each other, so that, by selecting a suitable pair of crossing strips, and by rapidly switching from one such pair to another, the chosen visual display can be effectively built up by the device.

In a preferred form the two sets of strips cross at right-angles.

The present invention seeks to provide a coloured optical display system which may be regarded as a combination of the two aforementioned devices.

In one aspect, therefore, this invention provides a coloured optical display system comprising, in operative combination, a visual display portion and colour filter portion, wherein: a) the visual display portion is two liquid crystal cells mounted face-to-face, one of the cells being drivable in strips extending in one sense while the other cell is drivable in strips extending in a second sense at an angle to the first; and b) the colour filter portion is a filter element bearing strips of filter material of one colour interspersed with strips of filter material of at least one other colour, these filter material strips being aligned with the drivable strips in one of the visual display portion's two cells; so that in operation a desired colour strip is selected by activating the appropriate aligned cell strip, and the desired position along that strip is selected by activating the appropriate crossing cell strip, the required coloured display thus being effectively built up by switching between pairs of strips.

The visual display portion is, basically, the device as described in our aforementioned Application No. 25,550/78 (1/6161/M). Thus, it preferably comprises: first and second outer plates with a single inner plate in between, the two interplate spaces being filled with a liquid crystal substance to make two cells; and cell electrodes preferably provided on the inner surfaces of the two outer plates and on both surfaces of the inner plate) so shaped as to allow one cell to be driven/activated in mutually-parallel strips at right angles to the mutually-parallel strips in which the other cell can be driven/activated.In one embodiment electrodes provided on the inner surfaces of the two outer plates are strip electrodes (the set on one plate being at right angles to the set on the other), the inner plate having on each of its two surfaces a single sheet electrode (this way round the two sheet electrodes shield the two cells from each other); for each cell, by choosing its sheet electrode and the appropriate strip electrode there may be selected for activation the desired strip of liquid crystal substance. In a more complex embodiment (described hereinafter with reference to the accompanying drawings) one inner plate electrode is replaced by a strip electrode set, the opposing electrode then being a "multi-strip" set corresponding to the filter strips in the filter portion. Such an embodiment may be regarded as a series of the simpler embodiments mounted side-by-side.

The colour filter portion is, basically, the device as described in our aforementioned Application No. 11 ,343/78 (1/61 54/M). Thus, it preferably comprises a filter element having disposed thereon, in strips corresponding to and aligned with one of the visual display portion's sets of drivable strips, a series of regularly interdelineated filter strips in the chosen colours - as red, green, blue, red, green, blue, and so on. Each coloured filter strip may correspond to and be aligned with a single drivable strip. In such a case, selection of any particular drivable strip causes selection of both the colour and the position of the filter strip.

Alternativeiy, and perhaps more preferably, a full set of coloured filter strips (such as the triplet comprising a red, a green and a blue strip) corresponds to and is aligned with a single drivable strip. In this case selection of any particular drivable strip causes selection only of the position of the desired filter strip, and selection of the desired colour must be effected independently and additionally. One preferred way of carrying out this extra selection is disclosed in our aforementioned Application No. 11,343/78 (1/61 54/M); the colour filter strips are mounted upon their own liquid crystal cell, this cell having its electrodes so disposed that at any one time there may be driven strips therein corresponding to all the filter strips of one colour only.Accordingly, in this alternate embodiment of the colour display system of the invention the visual display portion and the filter portion are independent but associated portions; the form of the display is provided by the former, the colour by the latter. However, in the more complicated embodiment of the display portion already referred to, the provision of this "extra" cell is not required; the cell containing the two sets of strip electrodes acts as a combination -- a condensed version - of the filter portion's cell and the associated display portion's cell.

As appropriate, the various components, materials and sub-systems employed in the colour display system of the invention may be any of those used, or suggested for use, in this general field. Thus:- a) the liquid crystal cells may operate on the dynamic-scattering, twisted-nematic or even dyephase-change system -- though a dynamicscattering system consumes a relatively large amount of power (and switches fairly slowly), a twisted-nematic system requires the use of additional polarising filters to obtain the on-off switching effect (but is nevertheless preferred); -and a dye-phase-change system could cause problems as regards colour selection;; b) the liquid crystal cells will normally be constructed of glass, the light-transparent electrodes being made of a thin layer of metal (such as gold) or of conducting metal oxide (such as tin oxide or indium oxide) supported thereby; c) each of the second electrodes in the visual display portion of the coloured optical display system is individually addressable by the circuitry used, or suggested for use, for this purpose; d) the driving/activating potential to the liquid crystal cells' electrodes is that appropriate to the type of cell being used, and is supplied by means used, or suggested for use, for this purpose (for a twisted-nematic system, a voltage pulse of from 1 to 10 volts); e) the filter elements are prepared photographically by suitably exposing photographic colour emulsions carried by the supports for the liquid crystal cells.

Certain embodiments of the invention will now be described, though only by way of illustration, with reference to the accompanying drawings, in which: Figures la, 1b and ic show, in schematic perspective form, the main components (electrodes and filters; the supporting glass plates are not shown) of three different embodiments; and Figure 2 shows diagrammatically a cross section through part of a colour display system constructed in accordance with Figure 1 C.

The embodiment of Figure 1 a serves to illustrate the basic principle of the colour display system of the invention. A first liquid crystal cell (generally 10) having a series of vertically arranged (as viewed), mutually-parallel transparent strip electrodes (as 11) on one internal face and a simple transparent sheet electrode (12) on the opposing internal face is placed back-to back against a second cell (generally 13) having a simple transparent sheet electrode (14) on one internal face and a series of horizontally-arranged (as viewed), mutually-parallel transparent strip electrodes (as 15) on the opposed internal face.

The vertical strip electrodes (11) can be addressed independently, a selected potential being set up across the cell between the chosen electrode 11 and the sheet electrode 12, as can the horizontal strip electrodes 1 5 (operated in the same sort of way with sheet electrode 14).

Adjacent the electrode set 1 5 - possibly mounted on the outer surface of that cell face is a colour filter element 1 6 composed of alternating-colour, horizontally-arranged (as viewed), mutually-parallel red (R) and green (G) filter material strips 1 7, the set of filter strips 17 being aligned one-to-one with the set of electrode strips 1 5.

In a typical set up, the liquid crystal substance (not shown) in each cell 10 and 13 would be opaque when relaxed (undriven) but transparent when activated (driven), and the display system functions as a screen through which a light source -is observed. Activation of any particular cell -1-0 strip (using the appropriate electrode 1 will cause that strip of that cell to become transparent.

Similarly, activation of any particular cell 13 strip (using the appropriate electrode 1 5) will cause that strip of that cell to become transparent. It will easily be seen that activation of any particular cell 10 strip (using the appropriate electrode 11) combined with activation of any particular cell 13 strip (using the appropriate electrode 1 5), will result in the two-cell combination becoming transparent only at the spot where the two actiavated strips cross. Moreover, because each electrode 1 5 is aligned with either a red or a green filter strip 17, so that the formed transparent strip in cell 13 is effectively a coloured strip the actual colour of which depends on exactly which electrode strip 13 is being utilised, so the two-cell combination will become not only transparent but also coloured only where the two activated strips cross. By suitably switching from one electrode 11 to another, and from one electrode 1 5 to another, the transparent spot can be moved about and changed in colour; if this is done fast enough an apparently "permanent" display of one or both colours is built up on the screen.

The definition of the display improves as the number of electrode strips 11 and 1 5 increases; the Figure shows only four strips 11 and five strips 1 5 (with a corresponding five filter strips 17) for the whole screen, but in reality a screen might bear as many as one hundred strips per cm in each direction.

The version of the colour display system shown in Figure 1b is very similar to that shown in Figure 1 a, save that the single filter element (16 in Figure 1a) has been replaced by a more complicated filter portion (generally 18). This more complex set-up consists of a liquid crystal cell (generally 19) and a filter element (20). The cell 19 has a transparent sheet electrode (21) on one internal face and a set of horizontally-arranged (as viewed), mutually parallel transparent strip electrodes (as 22/23) each of which is coupled to a bus bar (24 or 25) so that every alternate electrode 22/23 is coupled the same bus 24/25.The filter element 20 (conveniently mounted on the outer surface of the face of the cell bearing electrode strips 22/23) carries pairs of filter strips (as 26), one of each pair being red (R) the other green(G), and these filter strips 26 are aligned one-to-one with the strip electrodes 22/23.

Using the same sort of liquid crystal substance (not shown) as previously described -- thus, opaque when relaxed, but transparent when activated - it can easily be seen that activation of all of the cell strips associated with all of the filter strips 26 of any one colour (by applying an appropriate potential across the sheet electrode 20 and, using the bus 24 or 25, all of the corresponding electrode strips 22 or 23) causes the entire filter portion 1 8 to appear transparent and of the chosen colour.

When the filter portion 1 8 is used in combination with the two-cell (1-0, 13) display portion, the arrangement being such that the electrode strip pairs (as 22/23), and thus the filter strip pairs (as 26R/26G), are aligned on a one-to one basis with the cell 13 electrode strips 15, -operation of the two-cell display portion selects a transparent spot (and thus controls the form of any display built up by that spot), while operation of the filter portion 1 8 allows a choice of colour for the spot (and thus for the whole or any part of'the built-up display).

It will be apparent, incidentally, that the advantage of the Figure 1 b embodiment over a comparable (in terms of number of coloured filter strips 17) Figure 1 a embodiment is the improvement in ease of control of, say five electrode strips 15 and two sets of filter strips 22/23 over the comparable ten (five times two) filter strips (as 17). In a real situation, where there are perhaps one hundred electrode strips 1 5 per cm. and three sets of filter strips 26, there is a significant advantage in controlling one hundred and three integers as opposed to three hundred ! The embodiment of Figure 1 C - and of Figure 2 - is a somewhat simplified version of that of Figure 1b.The components are: a first liquid crystal cell (10), with vertical transparent electrode strips (as 11) on one inner face and a transparent sheet electrode (12) on the opposed inner face; a second liquid crystal cell (13/19), with horizontal vertical transparent strips (as 1 5) on one inner face and, aligned therewith on a oneto-one basis, horizontal transparent electrode strip pairs (as 22/23) on the opposed inner face (connected alternately to one or other of bus bars 24/25): and, aligned with electrode strip pairs 22/23 on a one-to-one basis, a set of pairs of filter strips (as 26) alternating in the chosen colours red (R) and green(G).

Employing throughout liquid crystal substances of the same sort as already described (opaque when relaxed, but transparent when driven), the mode of operation of the Figure Ic embodiment should be clear from the Figure. Briefly, it is as follows: a) Applying a suitable potential across a chosen vertical electrode strip 11 and sheet electrode 12 causes the selected cell 10 strip to become transparent.

b) Applying a suitable potential across a chosen horizontal electrode strip 1 5 and, via bus 24 or 25, all of one or other of every pair of horizontal electrode strips 22/23, causes the selected cell 1 3/1 9 strip to become transparent and suitably coloured.

c) Where the two strips cross, so there is formed a colour red transparent spot.

d) By selecting the correct electrode strips 11, 15 and 22/23, and by switching from selection to selection, there can be built up a coloured display.

Figure 2 shows diagrammatically the physical construction of a colour display system of the type shown in Figure 1 c. Only part of the whole device is shown.

The system comprises a series of transparent colourless glass plates (30,31,32 and 33), with suitable end pieces (34,35 and 36) interposed there between. Within the cell formed by plates and pieces 30,31 and 34 is a liquid crystal substance (not shown) activated by control electrodes (11 and 12). Electrode 11 is a set of independently-addressable, vertically-arranged (as viewed), mutually-parallel transparent electrode strips supported on the inner face of plate 30, while electrode 12 is a transparent sheet electrode supported on the face of the plate 31 opposed to plate 30. Within the cell formed by plates and pieces 31,32 and 35 is a liquid crystal substance (not shown) activated by control electrodes (15) and 22/23).Electrode 15 is a set of independently-addressable, horizontallyarranged (as viewed), mutually-parallel transparent electrode strips supported on the other face of plate 31, while electrode 22/23 is a set of pairs of horizontally-arranged (as viewed), mutually parallel transparent electrode strips supported on the inner face of plate 32 and aligned on a one-to-one basis with the strips 15, every alternate strip 22/23 being electrically connected (via one or other of bus bars 24 or 25, not shown in this Figure). Finally,-sandwiched between plates 32 and 33 (with end piece 36) is a filter element composed of a set of alternatelycoloured, horizontally-arranged (as viewed), mutually-parallel red and green filter strips (as 26R, 26G), the pairs of filter strips 26R and 26G being aligned on a one-to-one basis with the pairs of electrode strips 22/23.

Claims (2)

1. A coloured optical display system comprising, in operative combination, a visual display portion and a colour filter portion, wherein: a) the visual display portion is two liquid crystal cells mounted face-to-face, one of the cells being drivable in strips extending in one sense while the other cell is drivable in strips extending in a second sense at an angle to the first; and b) the colour filter portion is a filter element bearing strips of filter material of one colour interspersed with strips of filter material of at least one other colour, these filter material strips being aligned with the drivable strips in one of the visual display portion's two cells; so that in operation a desired colour strip is selected by activating the appropriate aligned cell strip, and the desired position along that strip is selected by activating the appropriate crossing cell strip, the required coloured display thus being effectively built up by switching between pairs of strips.

2. A display system as claimed in claim 1, wherein the cell electrodes are so shaped as to allow one cell to be drivergactivated in mutuallyparallel strips at right angles to the mutuallyparallel strips in which the other cell can be driven/activated.

2. A display system as claimed in claim 1, wherein the visual display portion comprises: first and second outer plates with a single inner plate in between, the two interplate spaces being filled with a liquid crystal substance to make two cells; and cell electrodes so shaped as to allow one cell to be driven/activated in mutually/parallel strips at right angles to the mutually-parallel strips in which the other cell can be driven/activated.

3. A display system as claimed in claim 2, wherein the electrodes provided on the inner surfaces of the two outer plates are strip electrodes (the set on one plate being at right angles to the set on the other), the inner plate having on each of its two surfaces a single sheet electrode.

4. A display system as claimed in claim 2-, wherein the electrodes provided on the inner surfaces of the two outer plates are strip electrodes (the set on one plate being at right angles to the set on the other), the inner plate having on one of its two surfaces a single sheet electrode and on the other of its two surfaces a set of strip electrodes corresponding to the strip electrodes on the opposing outer plate's inner surface.

5. A display system as claimed in any of the preceding claims, wherein the filter portion comprises a filter element having disposed thereon, in strips corresponding to and aligned with one of the visual display portion's sets of drivable strips, a series of regularly interdelineated filter strips in the chosen colours.

6. A display system as claimed in claim 5, wherein a full set of coloured filter strips corresponds to and is aligned with a single drivable strip.

7. A display system as claimed in claim 6, wherein the colour filter strips are mounted upon their own liquid crystal cell, this cell having its electrodes so disposed that at any one time there may be driven strips therein corresponding to all the filter strips of one colour only.

8. A display system as claimed in any of the preceding claims, wherein the liquid crystal cells operate on the twisted-nematic system.

9. A display system as claimed in any of the preceding claims and substantially as described hereinbefore with reference to the appropriate one or more Figures of the accompanying drawings.

New claims or amendments to claims filed on 30/8/79 Superseded claims: 1,2.

New or amended claims; 1, 2.

1. A coloured optical display system comprising, in operative combination, a visual display portion and a colour filter portion, wherein: a) the visual display portion is two liquid crystal cells mounted face-to-face so as to comprise first and second outer plates with a single, common inner plate in between, the cell electrodes being so shaped that one of the cells is drivable in strips extending in one sense while the other cell is drivable in strips extending in a second sense at an angle to the first; and b) the colour filter portion is a filter element bearing strips of filter material of one colour interspersed with strips of filter material of at least one other colour, these filter material strips being aligned with the drivable strips in one of the visual display portion's two cells; so that in operation a desired colour strip is selected by activating the appropriate aligned cell strip, and the desired position along that strip is selected by activating the appropriate crossing cell strip, the required coloured display thus being effectively built up by switching between pairs of strips.