GB2420903A - Display apparatus - Google Patents

Abstract

A display apparatus having an array of pixels (10). Each pixel has a chamber having a chamber wall (12,16) and containing an optical filter liquid (22) and driving means (20) operable to move at least a portion (16) of the chamber wall. Each chamber has a viewing portion any content of which is visible in normal use and a reservoir portion (44) any content of which is hidden from view in normal use. The chamber and driving means are arranged so that, upon operation of the driving means, at least some of the filter liquid is caused to move from the viewing portion to the reservoir portion or vice versa so that the appearance of the viewing portion changes. Because of the use of movement of at least a portion of the chamber wall, rather than, for example, electric fields, to move the filter liquid, the use of conductor tracks that obscure the viewing portion can be avoided.

Description

TITLE

Display apparatus

DESCRIPTION

This invention relates to display apparatus. The invention was conceived for use as an "electronic billboard" for outdoor advertising or as a public information display for displaying, for example, arrival and departure information at an airport or railway station. However, the invention does have other uses.

The display area of an electronic billboard needs to be large, for example of the order of ten square metres or more. The display should preferably be able to show colour images of VGA resolution (640 pixels x 480 pixels) or better, and the images should be able to be changed from one to another in reasonably quick succession. The display should conveniently be able to be driven by a conventional computer, acting in a way similar to a computer monitor. The display should preferably be able to operate in bright sunlight and should not have to be oriented in a special direction to avoid incident light or to be shaded. To avoid excessive power consumption, ambient light should preferably be used as the light source for the display of information rather than the display generating light in competition to the sun or other ambient lighting. For use at night, the display should preferably be able to be front- illuminated using conventional low power lamps, or even street lighting; alternatively, it should be possible to rear illuminate the display using conventional, easily-maintained low-power lamps as used in current printed display panels situated in bus stops, airports and so on. For outdoor use, the display should preferably not be adversely affected by other environmental factors such as wide temperature ranges and altitude- dependent variation in atmospheric pressure. For ease of installation and safe support, the display should preferably be thin and lightweight, and it should preferably be easily adaptable to fit different billboard installation dimensions. Due to the large area of the display, the method used to drive each pixel, for example row-column multiplexing or direct drive intensity control voltages, should preferably take into account that this needs to be achieved over long distances where noise and line capacitance may be restrictive. In order to reduce the per pixel cost so as to achieve a commercially viable system, the display technology should preferably aim at a process where the pixels can be manufactured with hundreds fabricated onto a single sheet. Display technologies often suffer from two pixel-drive disadvantages; firstly that the conductors arriving to drive each pixel can obscure part of the pixels active area and secondly each pixel may be separated from the next by a visually passive gap, resulting in a grid effect. Pixel-drive issues such as these should preferably be minimised, for example by minimising the number of drive signals per pixel, by making the drive signals as insensitive to attenuation or noise as possible and by providing for minimal conductive tracks obscuring the pixel active area.

Current display technology includes cathode ray tube displays, vacuum fluorescent displays, light emitting diode displays, gas plasma displays and electroluminescent displays. Such displays emit varying intensity light per pixel, and for colour displays this is usually performed by emitting a combination of the primary colours red, green and blue (RGB displays). Such display technologies do not meet the requirements of outdoor large format displays. They are difficult to use in bright sunlight since they emit light rather than reflect light. They cannot be manufactured in the size required. They are heavy. In the case of discrete LED based displays they cannot be manufactured in large single sheets. The drive electronics are unlikely to operate easily for certain technologies over the long distances involved. Organic LED displays are more efficient than standard LED displays. Therefore they are more effective at competing with ambient light in daylight. However they will be difficult to manufacture as a large area sheet. Liquid crystal displays cannot be manufactured at the large size required. Generally LCD colour displays require a backlight and emit light as an RGB display. Such LCD displays would not be useful in bright sunlight. Other non-RGB- based LCD displays are reflective or transfiective, and could work using ambient light. However, these are limited in their colour rendition and cannot be made of large size. LCD technology requires the use of electrical conductors arriving at each pixel. This results in the need for visually inactive areas on the display.

Liquid-based displays are known. In particular, patent document US5 956005 describes a monochrome display in which dyed liquid or ink is moved between a reservoir and a viewing portion of a pixel under the action of an electric field applied via three electrodes. It also describes a colour display in which three such arrangements are stacked for each pixel and contain liquids dyed cyan, magenta and yellow. The displays described in U55956005 have two main disadvantages. First, it appears that each pixel or sub-pixel has only two optical states. Therefore, the monochrome display can show only two colours' (for example white and black), and the colour display can show only eight colours (for example white, cyan, blue, magenta, red, yellow, green and black). Secondly, the display uses transparent conductive coatings for the transfer of electrical signals. The lower the resistance per unit area of these coatings, generally the less transparent they are, and so conductor tracks need to be wide or light is attenuated too much by the conductors. In the colour display, by having to employ three conductive layers per colour, and by having three colours, the attenuation of light by the nine conductive layers will be significant.

Since these conductors are thin films of conductive material, narrow long tracks are high in electrical resistance. When used to drive a capacitive loads this will lead to poor response times when switching the state of a given pixel.

The aim of the present invention, or at least specific embodiments of it, is to provide a display apparatus that overcomes, at least to some extent, at least some of the disadvantages of the displays described above.

In accordance with the present invention, there is provided a display apparatus having an array of pixels. Each pixel has a chamber having a chamber wall and containing an optical filter liquid and driving means operable to move at least a portion of the chamber wall. Each chamber has a viewing portion any content of which is visible in normal use and a reservoir portion any content of which is hidden from view in normal use. The chamber and driving means are arranged so that, upon operation of the driving means, at least some of the filter liquid is caused to move from the viewing portion to the reservoir portion or vice versa so that the appearance of the viewing portion changes. Because of the use of movement of at least a portion of the chamber wall, rather than electric fields, to move the filter liquid, the use of conductor tracks that obscure the viewing portion can be avoided.

The driving means of each pixel preferably includes an actuator for moving the portion of the chamber wall of that pixel and means for producing a drive signal for the actuator. In this case, each signal producing means may include means for modulating the amplitude of the signal so as to vary the amount of the filter fluid that is caused to move. Accordingly, a grey-scale display can be provided by amplitude- modulation. In a preferred embodiment, the drive signal is an AC signal, and the actuator is arranged to produce vibrations in accordance with the AC drive signal for causing the portion of the chamber wall to move. For each chamber there will be a fundamental resonant frequency; and each signal producing means is preferably arranged to modulate the frequency of the signal near the resonant frequency so as to vary the amount of the filter fluid that is caused to move. Accordingly, a grey-scale display can be provided by frequency-modulation, or a combination of frequency- and amplitude-modulation.

The fundamental resonant frequency for each chamber preferably differs from the fundamental resonant frequencies for the chambers of the pixels immediately adjacent that pixel at least in the axial directions of the array, so as to reduce crosstalk between adjacent pixels.

Each pixel preferably has a second such chamber the viewing portion of which can be viewed through the first-mentioned chamber; and the optical filtering characteristics of the filter liquids in the first and second chambers of each pixel are different. More preferably, each pixel has a third such chamber the viewing portion of which can be viewed through the first and second chambers; and the optical filtering characteristics of the filter liquid in the third chamber of each pixel differ from those of the filter liquids in the first and second chambers. In this way a colour display can be provided, preferably by using filter liquids of each pixel that are magenta, cyan and yellow in colour. Each pixel may furthermore have a fourth such chamber the viewing portion of which can be viewed through the first to third chambers; and the optical filtering characteristics of the filter liquid in the fourth chamber of each pixel differ from those of the filter liquids in the first to third chambers, for example it may be black in colour. In this way, black may more reliably be rendered.

in the case of plural-chamber pixels, the fundamental resonant frequency for each chamber of each pixel preferably differs from the fundamental resonant frequency/ies for the other chamber(s) of that pixel, so as to reduce crosstalk between colours in the same pixel.

In the case of plural-chamber pixels, the actuator and signal producing means for each pixel are preferably common to the plural chambers of that pixel. In the case of electrically-driven actuators, there is therefore a need for only two electrical connections per pixel. Each drive signal is preferably a superposition of signal components each based on (but optionally being frequency- modulated with respect to) the resonant frequency for a respective one of the chambers of the respective pixel.

The display apparatus preferably further includes a white surface that can be viewed through the viewing portion or portions of each pixel.

In one embodiment, the viewing portion of each chamber includes a pair of opposed walls that are movable towards and away from each other. The chamber and the filter liquid are such that the filter liquid prefers to be contained in the viewing portion between the walls. The driving means is operable to move the walls towards each other, and the chamber and filter liquid are such that, upon such movement, the filter fluid migrates from the viewing portion to the reservoir portion.

In another embodiment, the viewing portion of each chamber includes a pair of opposed walls that are movable towards and away from each other. The reservoir portion is of variable volume. The chamber and the filter liquid are such that the filter liquid prefers to be contained in the reservoir portion. The driving means is operable to decrease the volume of the reservoir portion, and the chamber and filter liquid are such that, upon such decrease in volume, the filter fluid migrates from the reservoir portion to the viewing portion.

In either of these embodiments, the driving means may operate by a hammering action on the chamber.

In a further embodiment, each chamber is cantilevered from a support. One of the portions of the chamber is closer to the support than the other portion. The chamber and the filter liquid are such that the filter liquid prefers to be contained in said one portion of the chamber. The driving means is operable to wag the chamber, and the chamber and filter liquid are such that, upon such wagging, the filter fluid migrates from said one portion to said other portion of the chamber.

The reservoir portion of each pixel may surround the viewing portion of that pixel.

However, to avoid or reduce the gridline effect that would be produced by such an arrangement, the reservoir portion of each pixel is preferably either disposed behind the viewing portion of that pixel, or disposed to one side of that pixel and is overlapped by the viewing portion of an adjacent pixel.

Specific embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded cross-sectional side view of a sub-element of a pixel in a display embodying the invention; Figures 2A-C are assembled cross-sectional views of the sub-element of Figure 1 in a relaxed state, a partly-actuated state and a fully-actuated state, respectively; Figure 3 is a quarter-sectioned isometric view of the subelement; Figures 4A-C are face views, to a smaller scale, of the subelement in the relaxed state, the partly-actuated state and the fullyactuated state, respectively; Figure 5 is a cross-sectional side view of a portion of a display showing a pixel employing three sub-elements of the type shown in Figures 1 to 4C; Figure 6 is an isometric view of spacer element used in the display; Figure 7 is an isometric view of a portion of a masking layer used in the display of Figure 5; and Figure 8 is a face view of a portion of the display showing a number of pixels in different states.

Referring to Figures 1 to 3, a pixel sub-element 10 comprises a rectangular base plate 12, a rectangular spacer frame 14, a rectangular diaphragm 16, and rectangular anvil plate 18, an actuating hammer 20 and an optical filter liquid 22. The base plate 12 is of optically transparent, plastics material, and its upper surface has a central, flat, rectangular plateau portion 24, wall portions 26 extending downwardly at an inclined angle of, for example, 450 from the edges of the plateau portion 24 and a flat edge portion 28 surrounding the wall portions 26. The spacer frame 14 has a lower face 30 that is bonded to the edge portion 28 of the base plate 12, and a height F that is slightly greater than the height P of the plateau portion 24 above the edge portion 28 of the base plate 12. A tiny vent hole 32 extends between the inside and the outside of the spacer frame 14. The diaphragm 16 is of thin, flexible, uniform thickness, optically transparent, plastics material and is stretched taut across and bonded to the upper face 34 of the spacer frame 14. The anvil plate 18 is of relatively rigid, optically transparent, plastics material having generally the same rectangular size as the plateau portion 24 of the base plate 12, and its flat lower face is bonded to the centre of the diaphragm 16. The hammer 20 has a shaft 36 extending from the side of the sub-element 10 to a head 38 facing the centre of the anvil plate 18, and a tuning portion 40 extending beyond the head 38. The filter liquid 22 may be yellow, cyan, magenta or black and is disposed between the base plate 12 and the diaphragm 16. The surface energy of the liquid 22 and the contacting surfaces of the base plate 12 and the diaphragm 16 is selected such that the surface tension forces render the liquid 22 trapped between the two surfaces such that the fluid is substantially unaffected by gravity or other external accelerations, within reason, acting on the entire assembly.

Referring in particular to Figure 2A, in the relaxed state of the subelement 10, the head 38 of the hammer 20 applies very little or no force to the anvil plate 18, and the arrangement is such that a uniform thickness of the filter liquid 22 covers generally the whole area of the plateau portion 24 of the base plate 12, but without any substantial amount of the filter liquid 22 extending beyond the plateau portion 24. This provides the normal maximum amount of filtering of light passing through the sub- element 10 in the region of the anvil plate 18, as represented in Figure 4A.

Referring in particular to Figure 2C, if under the action of the hammer 20, the anvil plate 18 is depressed downwardly so that the lower surface of the diaphragm 16 below the anvil plate 18 substantially touches the plateau portion 24 of the base plate 12, the filter liquid 22 is displaced outwardly to form a rectangular ring 42 occupying a reservoir 44 formed between the inclined wall portions 26 of the base plate 12, the portion of the diaphragm 16 beyond the anvil plate 18 and the frame 14. The effect of surface tension is sufficiently greater than the effect of gravity, whatever, the orientation of the sub- element 10, that the filter liquid that has been displaced from the plateau portion 24 is retained in the V-shaped groove formed between the inclined wall portions 26 of the base plate 12 and the portion of the diaphragm 16 beyond the anvil plate 18.

This is the fully-actuated state of the sub-element 10, which provides substantially no filtering of light passing through the sub-element 10 in the region of the anvil plate 18, as represented in Figure 4C. If the action of the hanmier 20 is removed, the sub-element will revert to the relaxed state shown in Figures 2A and 4A.

It will be appreciated that if the action of the hammer 20 is to press the diaphragm 16 to a position part-way between those shown in Figures 2A and 4A on the one hand and Figures 2C and 4C on the other hand, then there will be partial filtering of the light passing through the sub- element 10 in the region of the anvil plate 18. Figures 2B and 4B show the case where the sub- element is half-way between the relaxed and fully-actuated states.

In order to form a monochrome display, a plurality of the sub-elements 10 are arranged in a two-dimensional array. The sub-elements 10 are covered with a masking plate having a plurality of apertures each in alignment with a respective one of the anvil plates 18, and a reflective backsurface and/or backlight is disposed behind the sub-elements 10. Each subelement 10 may have a respective two-state actuator for moving the respective hammer 20 to change the respective sub- element 10 between its relaxed state and its fully-actuated state. By supplying appropriate drive signals to the actuators, it is therefore possible to write information on the display. It will be appreciated that it is also possible to form a grey-scale display in a similar fashion, but using multi-state or progressive actuators that can cause the sub- elements to assume states between their relaxed and fully-actuated states. It will furthermore be appreciated that it is possible to form a colour display by providing three or four layers of the sub-elements and actuators between the masking plate and the back-surface/backlight, with the colours of the filter liquids 22 used in three of the layers being yellow, cyan and magenta, respectively, and with the colour of the filter liquid 22 used in the optional fourth layer being black. However, a 640-by-480 pixel three-layer display would require a large number (921,600) of actuators. An arrangement that reduces the required number of actuators will now be described with reference to Figures 5 to 8.

Referring to Figure 5, a colour display 46 has a two-dimensional array of pixels 48, one of which is shown in full in the drawing. Each pixel 48 comprises a stack of three sub-elements I OY, 1 OC, 1 OM similar to the sub-elements described above with reference to Figures 1 to 4C. The lower sub-element bY contains yellow filter liquid 22Y and has its base plate 12 mounted on a common back-plate 50. The back-plate is semi-transparent and white in colour so that it can reflect light ambient light passing through the pixel 48 and also transmit light from a back-light 51. The middle sub-element 1 OC contains cyan filter liquid 22C and is mounted above the yellow sub-element 1OY by a rectangular spacer frame 52, as shown in Figure 6, that has a notch 54 through which the shaft of the hammer 20Y for the yellow sub-element 1 OY projects. The upper sub- element 1OM contains magenta filter liquid 22M and is mounted above the cyan sub-element IOC by a further rectangular spacer frame 52 having a notch 54 through which the shaft of the hammer 20C for the cyan sub- element 1 OC projects. The array of pixels 48 is covered by a common faceplate 56, as shown in Figure 7, that has a two-dimensional array of rectangular apertures 58, each arranged to align with and be of the same size as the anvil plates 18 of a respective one of the pixels 48. A rectangular wall 60 surrounds each aperture 58 for engaging the magenta sub-element IOM of the respective pixel 48, and each wall 60 has a notch 62 through which the shaft of the hammer 20M for the magenta sub-element 1 OM projects.

The distal ends of the shafts of the three hammers 20Y,20C,20M of each pixel 48 are cantilevered from an actuator rod 64 disposed to one side of the pixel 48. The rod 64 extends through a respective hole in the backplate 50 and is coupled to a vibratory actuator 66 having a voice coil or piezoelectric element for vibrating the rod 64 longitudinally at the frequency of an input AC drive signal 68. As the rod 64 vibrates, it can cause the hammers 20Y,20C,20M to tap on their anvil plates 18 and accordingly cause the anvil plates 18 to be depressed so that the states of the sub-elements 1 OY, 1 OC, 1 OM can be controlled. It will be appreciated that each hammer 20Y,20C,20M tapping on its anvil plate 18 will act as a vibratory spring- mass-damper system which will be complex due to the hammer 20Y,20C,20M being resisted by its anvil plate 18 through part of each vibration, but not through the whole vibration as the hammer 20Y,20C,20M bounces off its anvil plate 18. Nevertheless each hammer 20Y,20C,20M will have a fundamental resonant frequency, and the hammers 20Y,20C,20M of each pixel 48 are tuned so that the resonant frequency of each is sufficiently different from the fundamental resonant frequency and any significant harmonic resonant frequencies of the others. The hammers 20Y,20C,20M of each pixel 48 can be tuned in a number of ways. For example, as shown in Figure 5, the tuning portions of the hammers 20Y,20C,20M are of different lengths. Additionally or alternatively, the hammers 20Y,20C,20M may have different stifThesses, and preferably they have similar energy transfer characteristics.

In the absence of any vibration, the pixel 48 will appear black. On the assumption that the Q factors of the three hammer systems are sufficiently high and the fundamental resonant frequencies of the three hammer systems are sufficiently different that cross-talk is negligible, it will be appreciated that if the rod 64 is vibrated with sufficient amplitude at the fundamental resonant frequency fy of the yellow hammer system (say for simplicity as a result of a unity- amplitude drive signal 68 equal to sin2Rfi), the yellow sub-element I OY will be fully actuated, but the other sub-elements 1 OC, 1 OM will remain in their relaxed states, and the pixel will appear blue. Similarly, if the rod 64 is vibrated with sufficient amplitude at the fundamental resonant frequencyf of the cyan hammer system (say for simplicity as a result of a unity-amplitude drive signal 68 equal to sin2rfct), the cyan sub-element 1 OC will be fully actuated and the pixel will appear red; and if the rod 64 is vibrated with sufficient amplitude at the fundamental resonant frequency fM of the magenta hammer system (say for simplicity as a result of a unity-amplitude drive signal 68 equal to sin2Jt), the magenta sub-element 1OC will be fully actuated and the pixel will appear green. These elementary drive signals may be added. For example, with a drive signal of sin2fi + sin2.i, the yellow and cyan sub- elements 1OY,1OC will both be fully actuated and the pixel will appear magenta; and with a drive signal of sin2!f + sin2Jt + sin2 lrfMr, all three sub-elements 1 OY, 1 OC, 1 OM will be fully actuated and the pixel will appear white. It will therefore be appreciated that the pixel 48 can render black, any of the three primary colours, any of the three secondary colours and white.

Other hues, saturations and luminances are possible by modulating the amplitudes of the drive signal components or by modulating their frequencies with respect to the fundamental resonant frequencies. For example: a drive signal 68 of Y2sin2rf will produce dark blue; a drive signal 68 of Y2sin2rkt + sin2ifMt will produce a "lime" green; and a drive signal 68 of Y2sin2fi + sin2rjt + /2sin2fMt will produce pink. Also: a drive signal 68 of sin2ij (where J,c is a frequency slightly greater or smaller than the cyan resonant frequency that produces a 50% response by the cyan sub-element 1OC) will produce dark red; a drive signal 68 of sin2ifi + S1fl2jt will produce purple; and a drive signal 68 of sin2If}1 + sin21fd + sin2if,jt will produce aquamarine.

All of these hues, saturations and luminances are obtained as the result of a single actuator 66 and a single drive signal per pixel 48.

The resonant frequencies of the hammer systems in each pixel 48 are preferably chosen to differ from the resonant frequencies of the hammer systems in the immediately adjacent pixels, at least in the primary directions of the two-dimensional array, so as to reduce cross-talk between adjacent pixels.

As mentioned above, the hammers 20Y,20C,20M and actuator rod 64 of each pixel 48 are driven by a respective actuator 66. Each actuator 66 may comprise a plunger of high magnetic permeability that is bonded to the lower end of the actuator rod 64 and is encircled by and displaceable within a coil. The coil may be formed by interconnected tracks on different layers of a multi-layer printed circuit board, and the printed circuit board may also form the back plate 50.

Each coil may be driven by a multi-level digital signal 68 and may have a capacitor connected across its ends to filter out much of the higherorder harmonics in the digital signal 68 and enable the coil/capacitor circuit to ring within the range of the fundamental resonant frequencies of the hammer systems of the pixel 48.

Each pixel 48 may be individually driven directly, for example by a graphics card in a PC that supplies the required individual digital signal for each pixel. However, that would require one - 10 - signal connection per pixel from the graphics card to the display. Each pixel 48 will have a degree of persistence due to the energy stored in the coil/capacitor circuit, the energy of the hammer systems and the viscosity of the filter liquid 22. It is therefore possible to drive each pixel 48 for only part of the time that it is required to be in its partly or fully actuated states. Accordingly, it is possible to employ time-division multiplexing such that the pixels are arranged in groups of, say, eight rows, and only one row of pixels in each group is driven at one time, so that the number of signal connections can be reduced to one- eighth at the expense of a three-bit multiplexing signal.

It is also possible to provide each pixel with an integrated circuit that stores the digital sequence of the drive signal that is to be supplied to that pixel. The integrated circuits can then be updated serially using a two dimensional addressing scheme. However, for a display having a large number of pixels, the cost of the equally large number of integrated circuits could be prohibitive.

As a development of this, the pixels may be arranged as, say, 8 x 8 subarrays with each sub-array of sixty-four pixels sharing a respective integrated circuit which stores the digital sequences of the drive signals to be applied to the pixels in the sub-array. The integrated circuits can then be updated serially using a two dimensional addressing scheme, and each integrated circuit can drive the pixels in its sub-array either simultaneously or using time-division multiplexing.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many modifications and developments may be made thereto within the scope of the present invention. - 11

Claims (23)

1. A display apparatus having an array of pixels, each pixel having a chamber having a chamber wall and containing an optical filter liquid and driving means operable to move at least a portion of the chamber wall, each chamber having a viewing portion any content of which is visible in normal use and a reservoir portion any content of which is hidden from view in normal use, the chamber and driving means being arranged so that, upon operation of the driving means, at least some of the filter liquid is caused to move from the viewing portion to the reservoir portion or vice versa so that the appearance of the viewing portion changes.

2. A display apparatus as claimed in claim 1, wherein the driving means of each pixel includes: an actuator for moving the portion of the chamber wall of that pixel; and means for producing a drive signal for the actuator.

3. A display apparatus as claimed in claim 2, wherein each signal producing means includes means for modulating the amplitude of the signal so as to vary the amount of the filter fluid that is caused to move.

4. A display apparatus as claimed in claim 2 or 3, wherein the drive signal is an AC signal, and the actuator is arranged to produce vibrations in accordance with the AC drive signal for causing the portion of the chamber wall to move.

5. A display apparatus as claimed in claim 4, wherein: for each chamber there is a fundamental resonant frequency; and each signal producing means arranged to modulate the frequency of the signal near the resonant frequency so as to vary the amount of the filter fluid that is caused to move.

6. A display apparatus as claimed in claim 5, wherein the fundamental resonant frequency for each chamber differs from the fundamental resonant frequencies for the chambers of the pixels immediately adjacent that chamber at least in the axial directions of the array.

7. A display apparatus as claimed in any preceding claim, wherein: each pixel has a second such chamber the viewing portion of which can be viewed through the first-mentioned chamber; and the optical filtering characteristics of the filter liquids in the first and second chambers of each pixel are different.

8. A display apparatus as claimed in claim 7, wherein: each pixel has a third such chamber the viewing portion of which can be viewed through the first and second chambers; and the optical filtering characteristics of the filter liquid in the third chamber of each pixel differ from those of the filter liquids in the first and second chambers.

9. A display apparatus as claimed in claim 8, wherein the three filter liquids of each pixel are magenta, cyan and yellow in colour.

10. A display apparatus as claimed in claim 8, wherein: each pixel has a fourth such chamber the viewing portion of which can be viewed through the first to third chambers; and the optical filtering characteristics of the filter liquid in the fourth chamber of each pixel differ from those of the filter liquids in the first to third chambers.

11. A display apparatus as claimed in claim 10, wherein the four filter liquids of each pixel are magenta, cyan, yellow and black in colour.

12. A display apparatus as claimed in any of claims 7 to 11 when directly or indirectly dependent on claim 5, wherein the fundamental resonant frequency for each chamber of each pixel differs from the fundamental resonant frequency/ies for the other chamber(s) of that pixel.

13. A display apparatus as claimed in any of claims 7 to 12 when directly or indirectly dependent on claim 2, wherein the actuator and signal producing means for each pixel are common to the plural chambers of that pixel.

14. A display apparatus as claimed in claim 13 when dependent on claim 12, wherein each drive signal is a superposition of signal components each based on the fundamental resonant frequency for a respective one of the chambers of the respective pixel.

15. A display apparatus as claimed in any preceding claim, further including a white surface that can be viewed through the viewing portion or portions of each pixel.

16. A display apparatus as claimed in any preceding claim, wherein: the viewing portion of each chamber includes a pair of opposed walls that are movable towards and away from each other; the chamber and the filter liquid are such that the filter liquid prefers to be contained in the viewing portion between the walls; the driving means is operable to move the walls towards each other; and the chamber and filter liquid are such that, upon such movement, the filter fluid migrates from the viewing portion to the reservoir portion.

17. A display apparatus as claimed in any of claims 1 to 15, wherein: the viewing portion of each chamber includes a pair of opposed walls that are movable towards and away from each other; the reservoir portion is of variable volume; the chamber and the filter liquid are such that the filter liquid prefers to be contained in the reservoir portion; the driving means is operable to - 13 - decrease the volume of the reservoir portion; and the chamber and filter liquid are such that, upon such decrease in volume, the filter fluid migrates from the reservoir portion to the viewing portion.

18. A display apparatus as claimed in claim 16 or 17, wherein the driving means operates by a hammering action on the chamber.

19. A display apparatus as claimed in any of claims 1 to 15, wherein: each chamber is cantilevered from a support; one of the portions of the chamber is closer to the support than the other portion; the chamber and the filter liquid are such that the filter liquid prefers to be contained in the said one portion of the chamber; the driving means is operable to wag the chamber; and the chamber and filter liquid are such that, upon such wagging, the filter fluid migrates from said one portion to said other portion of the chamber.

20. A display apparatus as claimed in any preceding claim, wherein the reservoir portion of each pixel surrounds the viewing portion of that pixel.

21. A display apparatus as claimed in any of claims 1 to 19, wherein the reservoir portion of each pixel is disposed behind the viewing portion of that pixel.

22. A display apparatus as claimed in any of claims 1 to 19, wherein the reservoir portion of each pixel is disposed to one side of that pixel and is overlapped by the viewing portion of an adjacent pixel.

23. A display apparatus substantially as described with reference to the drawings.