EP0016787B1

EP0016787B1 - Light control device

Info

Publication number: EP0016787B1
Application number: EP19790900719
Authority: EP
Inventors: George R. Simpson; Herbert W. Sullivan
Original assignee: Bos-Knox Ltd
Current assignee: BOS-KNOX LTD.
Priority date: 1978-06-16
Filing date: 1980-01-29
Publication date: 1984-08-22
Also published as: DE2967187D1; CA1186897A; EP0016787A1; EP0016787A4; WO1980000103A1

Abstract

An electromechanical display element is provided for use in light reflective and light transmissive display arrays. The display element has a moveable electrode electrostatically controllable between a curled position removed from a stationary electrode and an uncurled position overlying the stationary electrode to modify the light reflective or transmissive character of the display element. Embodiments of the moveable electrodes are provided which readily can be manufactured for use in either type of array. Stationary electrodes having a plurality of discrete conductive regions are provided to facilitate the control of display elements in an array. Embodiments of dielectric insulators and external circuitry are provided which avoid operating problems and manufacturing complexities associated with residual electric polarization.

Description

Technical field

This invention relates to an electrostatically controllable electromechanical display device for use in light transmissive and light reflective displays.

Background art

The background art contains various examples of electrostatic display elements. One type of device, such as is shown in U.S. 1,984,683 and 3,553,364, includes light valves having flaps extending parallel with the approaching light, with each flap electrostatically divertable to an oblique angle across the light path for either a transmissive or reflective display. U.S. 3,897,997 discloses an electrode which is electrostatically wrapped about a curved fixed electrode to affect the light reflective character of the fixed electrode. Further prior art such as is described in ELECTRONICS, 7 December 1970, pp. 78-83 and I.B.M. Technical Disclosure Bulletin, vol. 13, No. 3, August, 1970, uses an electron gun to electrostatically charge selected portions of a deformable material and thereby alter its light transmissive or reflective properties.
Fig. 5 of U.S. Patent No. 3,989,357 discloses a two electrode electrically operated light control device including an electrostatically operated element comprising in superposition a stationary electrode 51, an electrode 53 moveable between a position overlying electrode 51 and a position removed from it, and non-conducting means 52 between and electrically separating the electrodes. The moveable electrode 53 is in the form of a flexible sheet with one end fixed and the other end free, and the sheet has a permanent stress biasing it into a curl away from the stationary electrode 51. The stationary electrode may also consist of raised portions which are electrically interconnected. Patent No. 4094590 describes a similar device. Patent No. 3989357 also proposes a matrix arrangement of the disclosed display devices without actually explaining how the disclosed devices are connected and actuated in a matrix.
Pages 108-109 of the Digest of Technical Papers from the 1972 SID Symposium discloses a matrix of individually actuable electrostatically operated light control devices. The individual devices employ hinged aluminum vanes which apparently are at a floating potential. The vane is either in the "Up" or horizontal position in the erase mode, or in a "Down" or nearly vertical position when "written" application of voltage to either one or both of two erase electrodes is used to hold the vane "Up". Connection of the two erase electrodes to columns and rows in a matrix of devices is disclosed. A memory electrode is located adjacent the lower position of the vane. It is pulsed for erasure.
The article "Optical display device using bistable electrets" on pages 382-3 of Applied Physics Letters, Vol. 30, No. 8 of 15 April 1977 discloses use in an optical display of a polarised electret in the form of a polypropylene film, which is moved between two electrodes by electrostatic fields between the electrodes. The electrodes are formed by 20°, V-shaped spaces between Plexiglas (a registered trade mark) blocks; one electrode being formed by ln0₂ deposited in glass so as to be transparent and the other electrode being a metallic coating. The moving element is not electrically conductive.
The present invention provides an improved electrostatically controllable electromechanical display device for light reflective or light transmissive display arrays. Each display element in the array can be individually controlled to enable the production of a variety of visual displays, including black and white and multicolor digital and pictorial displays.
According to the present invention there is provided an electrically operated light control device including an electrostatically-actuated element, said element comprising three or more mutually electrically isolated and independently addressable stationary electrodes on a substrate, a movable electrode positioned over the substrate, and non-conductive means between the movable and stationary electrodes for keeping the electrodes electrically separated, the movable electrode being in the form of a sheet of flexible material having one end fixed with respect to the substrate and the opposite end free with respect to the substrate, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the substrate and the stationary electrodes being arranged as a series on the substrate progressing from the vicinity of the fixed end of the sheet whereby the application of appropriate electrical potential differences between the movable electrode and the stationary electrodes causes the movable electrode to uncurl and progressively overlie the stationary electrodes, the movable electrode functioning to control light transmission or reflection.
In a preferred embodiment, the stationary electrodes are located on a flat surface normal to the light path, with the uncurled movable electrode lying adjacent to and covering the stationary electrodes' flat surface. The electrode can control light transmission or can affect light reflection qualities for a light reflective device. Each of the stationary electrodes of an element can be addressed separately and latched in an activated or unactivated state to cause, for example, selected elements within an array to become actuated, or to cause selected elements to remain actuated while other elements are not. Holddown electrodes are also provided. An electret non-conductive layer or a liquid layer may be employed between the electrodes to reduce the electrostatic force required in moving the movable electrode.
The invention will be further explained and preferred embodiments described, by way of example, with reference to the accompanying drawings of which:-

Figure 1 is a perspective view of a known display element illustrative of the principles employed by the invention;
Figure 2 is a perspective view of a second known display element illustrative of the principles employed by the invention;
Figure 3 is a perspective view of a known light reflective display element illustrative of the principles employed by the invention;
Figure 4 is a perspective view of a known light transmissive display element illustrative of the principles employed by the invention;
Figure 5 is a schematic view of a further known display element illustrative of the principles employed by the invention;
Figure 6 is a perspective, exploded view illustrating an embodiment of a stationary electrode in a display element;
Figure 7 is a perspective, exploded view illustrating another embodiment of a stationary electrode structure in a display element;
Figure 8 is a schematic diagram of a four element matrix array for use with elements of the type shown in Figures 6 and 7;
Figure 9 is a perspective exploded view illustrating another embodiment of a stationary electrode structure in a display element;
Figure 10 is a schematic view of an embodiment of a display comprising an array of display element;
Figure 11 A, B and C are plan view of various embodiments of stationary electrode structures; and
Figure 12 is a perspective view of an embodiment used to create grey scales and primary color scales.

As shown in the drawings, the display elements of the invention can be of several configurations which can be incorporated into varied display arrays.
Figures 1 to 5 disclose basic principles employed in the applicants invention. Figure 1 depicts an illustrative display element 10 having a stationary electrode 12, to which is attached a layer of insulative material 14. A movable electrode 16 has a portion 18 adjacent to one end fixed with respect to the stationary electrode 12 and a free end 20 controllable between a curled position removed from the stationary electrode 12 and an uncurled position adjacent to the stationary electrode 12. The movable electrode 16 is electrostatically controlled by means of a source of electrical potential V and a control switch 24. When the potential V is connected across the electrodes 12 and 16, the resulting electrostatic forces cause the moveable electrode 16 to uncurl into a position overlying the stationary electrode 12, as shown by dotted lines 26. When the potential V is disconnected and the electrodes connected together, the elecrostatic forces decrease and the restitution force of the moveable electrode 16 causes the body portion 20 to curl to its relaxed, curled position removed from the stationary electrode 12.
Figure 2 shows an illustrative display element in which the insulative layer 14 is attached to the inner surface of the movable electrode.
The display element 10 of Figure 1 can be used for either a light reflective or light transmissive display device. Use in a reflective device is illustrated in Figure 3. As seen in Figure 3, when the moveable electrode 16 is curled away from the stationary electrode 12, the viewer sees the light reflected from the area 32, consisting of reflections off the exposed stationary electrode 12 and insulative layer 14, as well as off the exposed portion of inner surface 16 of the movable electrode 34. When the moveable electrode 16 is flattened to a position overlying the stationary electrode, as shown by dotted lines 26, the viewer sees only the light reflected from outer surface 36 of the moveable electrode.
As a light reflective device, the element can be used in a variety of displays such as in a black and white or a multicolor array. For example, in a black and white display the insulative material layer 14 can be black, the inner surface 34 of the moveable electrode can be black, and the outer surface 36 of the moveable electrode white. In the curled state, no light is reflected and area 32 appears to be black. When the moveable electrode is uncurled or flattened, light is reflected from the white surface. Similarly, in a colored display the exposed surfaces in one state of the device can be of one color with the exposed surfaces in the other state of another color.
The element can also be part of a light transmissive device. Use as such a device is shown in Figure 4 with the light source 40 on the opposite side of the device from the viewer who sees the transmitted light emanating from area 44. As a light gate device, light is transmitted through a translucent stationary electrode 46 and translucent insulative layer 48. In the flattened condition, an opaque moveable electrode 16 blocks the light. In a multicolor display, the curled condition reveals a color of light transmitted through either a clear or colored stationary electrode 46 and insulative layer 48. The moveable electrode 16 can be opaque, to constitute a color light gate device, or translucent and colored to effect a change of color of the transmitted light.
In addition, other devices can be constructed for other light conditions or display effects. For example, a combination reflective and transmissive display can be constructed for use in varying light conditions by use of a translucent reflective coating on the surfaces of the electrodes 46 and 48 whereby the device can be used in a reflective mode when the light source 40 is off, or in a transmissive mode when the light source is on.
In constructing operating display elements, several operating variables are to be considered in selecting the materials for use in the electrodes, the insulative layer, and the further components of a display device, such as the substrate. With respect to the moveable electrode, the material used must be capable of being curled to the correct curl size for the particular use. Other considerations include the mass since a lower mass moveable electrode will have a lower inertia and respond more quickly to a given electrostatic force. A further consideration is the stiffness of the material which affects the force needed to bend the material to effect flattening.
In general, a moveable electrode can be formed either of a metal or a plastic laminate containing a conductive material. In one embodiment, beryllium copper 25 (BeCu 25) foil, 0.0026mm thick, is curled by wrapping it about a 0.64cm mandrel and heat treating it to set the curl. The resulting curled sheet is chemically etched in to an array of 1.28cm by 1.28cm moveable electrodes. Other materials for use in opaque moveable electrodes include tin- alloys and aluminum. Materials for use in translucent electrodes include a translucent base material with a translucent deposited thin conductive layer such as deposited gold, indium oxide, or tin oxide. The materials for moveable electrodes can be provided with the curl by heat forming or can be a laminate of two or more plies bonded together while stressed to form a curl.
Stationary electrodes can be formed of a conductive material such as metal foil for a reflective display, or of a translucent layer of indium oxide or tin oxide on a translucent substrate in a transmissive display.
The insulative layer 14 can also be chosen from many materials. Materials having high dielectric constants are preferred. A polymeric film may be used. One problem encountered in the use of certain materials arises in the temporary retention of a residual electrical charge or polarization after an electric potential has been removed. For example, it has been found that in display elements of the type shown in Figure 1, the application of sufficient potential to cause the moveable electrode to flatten to a position adjacent to the stationary electrode. may induce a temporary residual polarization in the dielectric insulative layer sufficient to maintain the moveable electrode flattened for a time after the electric potential has been removed or deceased. Certain materials do not exhibit this effect or the effect is small, Cellulose, polypropylene and polyethylene are examples of such materials. Another solution is the use of dielectrics which allow the residual charge to leak off. As another solution to this residual polarization problem, an electret formed of material such as polyethylene and terephthalate (MYLAR) a registered Trademark may be used as the insulative layer. An electret material maintains a relatively constant degree of residual polarization unaffected by the further application of an electric potential across it. Since the residual charge is a constant, it can be accurately accounted for in the design of the element. As an illustration of the use of an electret in an element as shown in Figure 1, the insulative layer 14 is the electret. Since the elecret provides a portion of the attractive force to flatten the moveable electrode, the electric potential V can be of a lower potential to add a further electrostatic force sufficient to cause the moveable electrode 16 to uncurl to a position adjacent to the stationary electrode 12. The removal of the electric potential V results in the recurling return of the moveable electrode to its original curled position since the force provided by the electret is less than the restorative force of the curl bias.
A further illustrative display device is illustrated in Figure 5 where a biasing power source 54 and an incremental drive power source 56 are used to control the moveable electrode 16. The biasing power source 54, set at V volts, is at a voltage potential just below that needed to effect the uncurling of the moveable electrode 16. The incremental drive source 56, set at ΔV volts, adds sufficient further voltage potential when added to the bias potential to cause the moveable electrode to uncurl and overlie the stationary electrode 12. The use of a bias voltage continually applied across the electrode, requiring only the switching of the ΔV incremental voltage to effect a change of position of the moveable electrode, can be highly advantageous in a display system. For example, a high voltage power supply can provide the bias voltage for all elements in the array. Only a small incremental potential is necessary to control the elements which the attendant cost savings resulting from the ability to use low voltage switching hardware.
This biasing effect and results are also obtained by the use of an electret as the insulative layer since the charge of the electret serves the same biasing function as bias power source 54. Therefore, only the incremental drive voltage ΔV is needed to actuate the moveable electrode.
The advantages of this biasing effect are also realizable when a liquid layer is present between the moveable and stationary electrodes. Surface tension forces of the liquid provide a portion of the attractive force acting on the moveable electrode. The liquid thus acts in a manner similar to a bias voltage. Suitable liquids include silicone oil and petroleum oils and derivatives.
The display device of Figure 5 can also be operated with an excess of bias voltage sufficient by itself to maintain the moveable electrode in a flattened position adjacent to the stationary electrode. In this device, the incremental drive voltage 56 is of opposite polarity, sufficient to decrease the electrostatic charge to a level allowing the moveable electrode to recurl to a position removed from the stationary electrode. This device can also take the form of a sufficiently charged electret insulative layer with the incremental drive source 56 of reverse polarity. This device is advantageous in that in the quiescent state with no ΔV potential applied, the moveable electrode is adjacent to the stationary electrode, rendering the moveable electrode less subject to accidental physical damage.
Figure 6 illustrates a display element 60 according to the invention having a stationary electrode structure 62 with a plurality of discrete conductive electrodes 66-68, insulative layer 64, and movable electrode 65. This embodiment provides independently addressable conductive electrodes of the stationary electrode structure 62 to facilitate particular control of the display element 60 for use in a display array. In the illustrated embodiment of a three electrode stationary electrode structure for example, an electrical potential can be applied independently to the X electrode 66, to the Y electrode 67, or to the hold-down electrode 68. Only when the X, Y, and hold-down electrodes are energized, will the moveable electrode 65 fully flatten. Once fully flattened, the hold-down electrode 68, when energized, provides sufficient electrostatic force to latch the moveable electrode 65 in its flattened state regardless of whether the X or Y electrodes are energized. To release the electrode 65 from its flattened state, the hold-down electrode 68 and the X and Y electrodes must all be deenergized.
When only the X electrode is energized, that is the electrode 66 proximate the fixed edge portion 61 of the movable electrode 65, the moveable electrode will partially uncurl. If, in addition to energization of the X electrode 66, the Y electrode 67 is also energized, the moveable electrode 65 will further uncurl. Energization of hold-down electrode 68, the electrode most remote from the fixed edge portion 61, will complete the uncurling of moveable electrode 65 to a fully flattened condition.
It should be noted that uncurling can not be effected by any electrode which is not immediately adjacent to the curled end portion of the moveable electrode. Therefore, the Y electrode 67 cannot cause uncurling until the X electrode 66 has been energized to cause partial uncurling.
In order that the moveable electrode be attracted by the electrostatic field of a particular stationary electrode, the moveable electrode must be sufficiently proximate to that electrode. This proximity can be achieved by causing the moveable electrode to partially overlie the particular electrode. One manner of achieving the condition of partial overlying is to shape the stationary electrodes such that the demarcations between them are not parallel to the curl axis of the moveable electrode. A chevron shape of the electrodes provides demarcations which are not parallel to the curl axis such that the moveable electrode partially overlies the adjacent electrode and thereby is located within the domain of the electrostatic field of that adjacent electrode when it is subsequently energized.
The operation of the X, Y, hold-down configuration of Figure 6 is illustrated in Figure 7 where drive voltage V can be applied between the moveable electrode 65 and any or all of the electrodes of the stationary electrode structure, X electrode 66, Y electrode 67, or hold- down electrode 68, by means of switches 70, 71 or 72 respectively. When switch 70 activates the X electrodes 66, the moveable electrode 65 uncurls partially; activation of the Y electrode 67 provides further uncurling of the moveable electrode 65. Switch 72 activates the hold-down electrode 68 to fully flatten and latch the moveable electrode 65 even if the switches 70 and 71 subsequently deactivate the X and Y electrodes 66 and 67.
Control of display elements such as are illustrated in Figures 6 and 7 having multiple stationary electrodes provides for use of the elements in a display array in which each element of the array can be selectively actuated without effecting the state of the remainder of the elements in the array. Such a display array is illustrated in Figure 8 in which a plurality of display elements 81, 82, 83 and 84 are assembled in columns and rows to form a display array 80. The moveable electrodes (not shown) are connected via a common lead 90 to one side of a source of electrical potential 110. Each stationary electrode structure has an X electrode, a Y electrode, and a hold-down electrode H. All X electrodes in the first column are connected via a common lead to switch X1, and all X electrodes in the second column are connected to switch X2. Similarly all Y electrodes in the first row are connected to switch Y1 and all Y electrodes in the second row are connected to switch Y2. All hold-down regions are connected in common to switch H. Thereby, each element 81-84 can be selectively actuated by selection of the appropriate switches, and latched down by the closure of hold-down switch H.
As an example of the operation of the array in Figure 8, in order to actuate element 83, hold-down switch H and switch X1 are closed to connect the hold-down and the X electrodes in the first column to the potential 110, and switch Y2 is closed to connect the Y electrodes in the second row to the potential 110. Since the element 83 is the only element in the array with both its X and Y electrodes energized, it alone is caused to full uncurl. Hold-down switch H will latch element 83 in the flattened state when the X and Y electrodes are subsequently deactivated. The fact that a moveable electrode can be effected only by a stationary electrode immediately adjacent the curled portion is of great value in simplifying the circuitry required to control an array of elements.
The display elements illustrated in Figures 6 and 7 have two independently controllable stationary electrodes in addition to the hold-down electrode. Increasing the number of independently controllable electrodes in each element permits a significant increase in the number of elements in an array without a concomitant increase in the number of switch devices required. Specifically, in order to independently address an element in an array having a number of elements N, each element having a number of independently controllable electrodes d, the number of switch elements S required is
For example, for an array of N=390,625 individually controlled picture elements, a single electrode per element would require 390,625 switches, or one switch per element. If each element has two electrodes such as in Figure 8, 1250 switches are needed to individually control and address each element. If the elements have four electrodes, only 100 switches are required. The switch devices and all other switch devices referred to in this specification can be mechanical or electronic switches including semiconductor elements which apply one of two potentials to the element to be controlled.
Figure 9 illustrates an embodiment of an element wherein moveable electrode 120 can be selectively controlled to change its state from either a flattened to a curled position, or from a curled to a flattened position when in a display array. The Figure 9 element has a stationary electrode structure formed of an X electrode 124, Y electrode 126 and two hold- down electrodes 122 and 128. Hold-down electrodes 122 (proximate the fixed edge of the moveable electrolysis partially beneath the moveable electrode 120 when it is fully curled. The X and Y electrodes, 124 and 126 respectively, are positioned between the hold- down electrodes. In other words, the electrodes are in a series progressing linearly from the fixed edge.
In operation, in order to selectively cause the moveable electrode 120 to change its state from a curled to a fully flattened condition, hold- down electrodes 122 and 128 are energized, as well as X electrodes 124 and Y electrodes 126, in the manner explained in reference to Figure 8. In this configuration, the hold-down electrode 122 lying underneath the moveable electrode 10 in its fully curled state, must be activated to partially uncurl the moveable electrode 120 to position partially overlying X electrode 124 to enable to the X electrode to cause further uncurling upon activation. When all regions 122, 124, 126 and 128 are activated, the electrode 120 will fully flatten.
In order to selectively cause the moveable electrode 120 to go from a fully flattened condition to a fully curled condition without affecting other display elements in an array, the following operation is performed. At the start, only those moveable electrodes which have their hold-down portions energized are in a fully flattened condition. To selectively release a moveable electrode first all Y electrodes in the array are deactivated. All X electrodes are then activated. The moveable electrodes thereby partially curl to a position above the Y electrode. Deactivation of the X and Y electrodes in the column and two of the desired elements will thereby release that specific moveable electrode and cause that electrode to fully curl. The hold-down electrodes can then be reactivated to secure the remaining flattened electrodes.
The response speed of an element is related to the size of the element. Sub-dividing an element into a plurality will promote increased response speed. Therefore, the element at a particular address in an array advantageously may be subdivided into two or more elements electrically connected in common.
Figure 10 illustrates the further use of a biasing power source such as described with reference to Figure 5. In the display array 240 of Figure 10, four display elements comprise moveable electrodes 242, 243, 244 and 245 and corresponding stationary electrode structures having hold-down electrode 246, X, row electrode 248, X₂ row electrode 250, Y, column electrodes 252, and Y₂ column electrodes 254. Bias voltage V_i is continually applied between the movable and stationary electrodes of all elements. Further bias voltage V, can be selectively applied in series with V, via switch 247. Incremental drive voltage V₃ can be selectively applied in series with V, and V₂ via the other switches 249. In order to cause a curled moveable electrode to change state, all three potentials V" V₂ and V₃ must be applied. To release a flattened electrode, the V₂ and V₃ potentials must be removed. The V, potential therefore represents a relatively large bias voltage which can be applied across all elements. The V₂ potential reflects the residual polarization of the insulative layer in each element. The V₃ potential is of an incremental level to drive an element already biased by V_i and V₂. The level of V₃ potential is set to allow for the inherent deviations in the amount of potential required to cause a change in state in various individual display elements stemming from manufacturing variations in such element parameters as insulative layer thickness, dielectric characteristics and curl diameter. It has been found that the V₃ potential may be in the order of ten percent of the V₁+V₂ level. In the biasing configuration of Figure 10, the curls can be controlled to selectively cause their change of state from a curled to an uncurled position by control of V₃ alone, once the biasing voltages V₁ and V₂ have been applied. The control switches required in a display array can be operated at the lower V₃ voltage, with fewer switches needed at the higher V₁ or V₂ voltages, with attendant savings in manufacturing cost.
The present invention can be used to create a digitally controlled two color, or black and white, display with desired gray scales, or a color display with desired intensities of the three primary colors. Various procedures for creating the gray scale and color shades are discussed here. Figure 11 shows a plan view of element arrangements to create gray scales. Figure 11 a shows the use of curls 148 which have square or rectangular shapes in the plan view. Figures 11 b and 11 c respectively, show the use of triangular shapes. To create an 80% black gray scale, 20% of the elements are curled, when the curled position represents white, and 60% of the elements are curled to create 40% black gray scale. In all these examples, the dotted lines, 144 represent the curl axes of the elements and the straight solid lines represent the element perimeters. The arrows 146 show the curl direction.
Various shade scales can be accomplished by grouping plural elements. The number of shade combinations available in a group is S=2^N where N is the number of differently shaded elements. Thus, four elements will provide 16 shade combinations, ranging from no actuation to all elements fully actuated.
Another procedure for the creation of two different color scales and primary color shades is through the control of the duty (up and down) cycles of elements. Therefore, a black and white element (where white is the curled position) when cycled faster than the ability of the eye to perceive the movement, would appear to be the percentage black of the duty cycle devoted to the coiled up state vs. the flat (black) state. Where S is the number of different shade combinations achieved from N different discrete and additive duty cycles, then S=2^N. Therefore, for four different discrete and additive duty cycles 16 different shades can be created.
Figure 12 shows another way to make use of the present invention to create gray scales and primary color shade scales. Separately driven X and Y electrodes 150, 152 pull the selected moveable electrode 158 to the first hold-down electrode 154 representing a gray or shade scale. Additional separately driven electrodes X₂ and Y₂, 156 and 157 are used to pull the selected electrode to the second hold-down electrode 154 to create another gray or shade scale. Additional X, Y and hold-down electrodes to create additional selectable shades or gray scales can be provided.

Claims

1. An electrically operated light control device including an electrostatically-actuated element (60), said element comprising three or more mutually electrically isolated and independently addressable stationary electrodes (66, 67, 68) on a substrate, a movable electrode (65) positioned over the substrate, and non-conductive means (64) between the movable and stationary electrodes for keeping the electrodes electrically separated, the movable electrode (65) being in the form of a sheet of flexible material having one end fixed with respect to the substrate and the opposite end free with respect to the substrate, the sheet of flexible material having a permanent stress which biases the sheet into a curl away from the substrate, and the stationary electrodes (66, 67, 68) being arranged as a series on the substrate progressing from the vicinity of the fixed end of the sheet whereby the application of appropriate electrical potential differences between the movable electrode (65) and the stationary electrodes (66, 67, 68) causes the movable electrode (65) to uncurl and progressively overlie the stationary electrodes (66, 67, 68), the movable electrode (65) functioning to control light transmission or reflection.

2. A device according to claim 1 in which a source of electrical potential difference is connectable between the movable and stationary electrodes through switching means (70, 71, 72) and in which the permanent stress in the flexible sheet is a mechanical stress which is insufficient to overcome the electrostatic force created when an appropriate electrical potential difference is applied between the movable electrode (65) and the stationary electrode (66) on the substrate adjacent the fixed end of the movable electrode (65) to cause the movable electrode (65) to overlie the said stationary electrode (66).

3. A device according to claim 1 or 2 in which the electrodes are arranged so that, in the process of uncurling, the movable electrode (65) partially overlies each successive electrode (67, 68) after the electrode (66) closest to its fixed end before it completely overlies the preceding electrode.

4. A device according to claim 3 in which the separations between the electrodes (66, 67, 68) on the substrate are not parallel to the axis of curl of the movable electrode (65).

5. A device according to claim 4 in which the separations between the electrodes (66, 67, 68) on the substrate are chevron shaped.

6. A device according to any of claims 1 to 4 in which the furthest electrode (68) from the fixed end of the movable electrode is a holding electrode.

7. A device according to claim 6 in which a plurality of elements (81-84) are arranged in an array of columns and rows, in each row all of the stationary electrodes (Y) in a first corresponding position on the substrate are connected together, and in each column all of the stationary electrodes (X) in a second corresponding position on the substrate are connected together.

8. A device according to any of claims 1 to 7 in which the non-conductive means (64) includes a sheet of electret material which is capable of retaining an electrostatic charge to provide an electrostatic force to act upon the movable electrode, the element being actuated by the resultant force of the electrostatic force provided by the electret material and the electrostatic forces created when electrical potentials are applied to the electrodes.

9. A device according to any of claims 1 to 8 in which the non-conductive means comprises a layer of liquid between the flexible material (65) and the substrate providing an attractive force when the flexible material overlies the substrate which force opposes a portion of the curl bias of the flexible material, and the element being actuated by the resultant sum of the attractive force provided by the liquid and electrostatic forces.