WO2012169881A1

WO2012169881A1 - Electrowetting optical cell and electrowetting optical element

Info

Publication number: WO2012169881A1
Application number: PCT/NL2012/050334
Authority: WO
Inventors: Hermanus Feil
Original assignee: Miortech Holding B.V.
Priority date: 2011-05-12
Filing date: 2012-05-14
Publication date: 2012-12-13
Also published as: WO2012154033A1

Abstract

The present invention relates to an electrowetting optical cell comprising a first (19) and second electrode layer (12), a containment space, and pixel walls (6, 7). The first electrode layer comprises a hydrophobic interface and the containment space comprises a polar and non-polar liquid having different conductivities, and being immiscible. The electrode layers can be powered using power receiving lines, which rearranges the polar and non-polar liquid, switching the cell between different states of optical transmittance. The first electrode layer has a boundary (19) and a central electrode part (20) electrically separate from each other. The boundary electrode part is contiguous to said pixel walls, surrounding the central electrode part. The power receiving lines connect both electrode parts such that these can be controllably powered independently from each other. The boundary electrode is a specular reflective layer or is transmissive and arranged on top of a specular reflective layer (23). The central electrode part (20) is transmissive.

Description

Title:

Electrowetting optical cell and electrowetting optical element.

Technical field

The present invention relates to an electrowetting optical cell comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first or second electrode layer for defining said cell, wherein said first electrode layer comprises a hydrophobic interface with said containment space, wherein said containment space comprises at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, said at least one polar liquid and said at least one non-polar liquid being immiscible with each other, and wherein said first and second electrode layer comprise power receiving lines for receiving power from powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said cell between different states of optical transmittance. In addition, the present invention relates to an electrowetting optical element comprising a plurality of electrowetting optical cells according to the invention. Background art

The use of electrowetting optical cells is relatively new in the field of display technologies. Electrowetting optical cells comprise a containment space between pixels walls filled with a polar and a non-polar liquid. The polar liquid is often a liquid substance based on water, while the non-polar liquid is often formed by an oily liquid. The top and bottom of the electrowetting optical cell are formed by an electrode and one of these electrodes comprises a hydrophobic surface. In an unpowered state, the hydrophobic nature of the first electrode surface causes the polar liquid to be repelled from the first surface such that the oily liquid fully covers the hydrophobic first surface. By powering the electrodes of the electrowetting optical cell, the force equilibrium changes such that the oily substance is pushed aside, covering only a (small) part of the area of the optical cell. By colouring at least one of the liquids, this enables to control the state of optical transmittance of the electrowetting cell, making it suitable to be used as a pixel in a display. In a relatively new technology, the electrowetting optical cells may also be used for the manufacturing of electronically dimmable mirrors. Electronically dimmable mirrors may for example be applied in vehicles, such as rear view mirrors of cars that automatically respond to glare. In such electrowetting optical cells, one of the electrode layers comprises a specular reflective surface which acts as a mirror surface. By switching the state of optical transmittance such as to block the mirror locally, the mirror surface can be darkened in case a bright light behind the vehicle causes glare which disturbs the driver .

Specific versions of automatically dimmable car mirrors based on electrowetting optical cells also comprise backlighting means which enables the display of indicator lights in the car mirror. This may be achieved by making part of the first electrode surface of the electrowetting optical cells transparent, and part of this surface comprising a specular reflective surface. In that case, by powering the backlight to the back of the mirror while the electrowetting optical cell is in the transmissive state, the light from the backlighting means is transmitted by the electrowetting cell.

A disadvantage of this technique is that even if the electrowetting optical cell is in the unpowered state and the oily substance fully covers the first electrode surface, part of the light from the backlighting means in conveyed by the electrowetting optical cell. Therefore, if the indicator light is not lit, the backlighting must be switched completely off in order to prevent light from the backlighting means being conveyed by the optical cell to the front of the display.

Summary of the invention The present invention has for its object to provide a electrowetting optical cell wherein the disadvantages of the prior art have been obviated, and which enables to combine different functions of the optical cell in a single cell design.

In accordance with the principals of the invention the above mentioned object has been achieved in that there is provided an electrowetting optical cell comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first or second electrode layer for defining said cell, wherein said first electrode layer comprises an insulating layer with a hydrophobic interface with said containment space, wherein said containment space comprises at least one polar liquid having a first conductivity and at least one non-polar liquid having a second conductivity, said at least one polar liquid and said at least one non-polar liquid being immiscible with each other, and wherein said first and second electrode layer comprise power receiving lines for receiving power from powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said cell between different states of optical transmittance, wherein said first electrode layer comprises a boundary electrode part and a central electrode part electrically separate from each other, wherein said boundary electrode part of said first electrode layer is arranged contiguous to said pixel walls, and said boundary electrode part surrounding said central electrode part such that said central electrode part is separated from said pixel walls by said boundary electrode part, and wherein said central electrode part is electrically insulated from said boundary electrode part. The electrowetting optical cell further comprises a specular reflective surface underneath the boundary electrode part and the boundary electrode part being optically transmissive for enabling reflection of incident light. The central electrode part is optically transmissive for enabling transmission of light through said first electrode layer. The boundary electrode part may also be formed by the specular reflective layer itself, for example by using a metal layer having a specular reflective surface. The power receiving lines connect both said boundary and said central electrode part such that said boundary and said central electrode part can be controllably powered independently from each other by said powering means.

The present invention is based on the insight that the interface between the polar and non-polar liquid in the unpowered state forms a curved surface. The thickness of the oil layer in the interior part of the cell differs from the thickness thereof near the pixel walls. Therefore, by making a central electrode part of the first electrode layer which can be controllably powered independently from the boundary electrode part, use can be made of the difference in optical behaviour between the central part of the cell and the boundary area near the pixel walls. For achieving this, the central electrode part is electrically insulated from said boundary electrode part, and the power receiving lines connect both said boundary and said central electrode part independently, for enabling independent powering of each of said boundary and central electrode part.

The difference in optical behaviour between the boundary of the cell and the interior part can be used, for example, as follows. A thinner layer of non-polar liquid near the pixel walls in the unpowered state may be used for providing the dimmable mirror function, while the thicker oil layer in the interior part of the cell provides sufficient blocking of backlight in the unpowered state. For enabling the mirror function, light will have to cross the thin layer of (dark) non-polar liquid twice, thereby optically making it twice as thick. Backlight for enabling the electronic display function crosses the non-polar liquid layer just once. The central electrode part may thus advantageously be used to enable the function of electronic display, while the inability to completely block backlight near the pixel walls is no problem for the dimmable mirror function. The electrowetting optical cell of the present invention is therefore most suitable for integrating the mirror function with the electronic display function.

In accordance with a specific embodiment of the present invention, the boundary electrode part comprises a specular reflective surface. This allows the electrowetting optical cell to be used as a dimmable mirror.

In accordance with a further embodiment of the present invention, the central electrode part is optically transmissive for enabling transmission of light through the first electrode layer. By making the central electrode part optically transmissive or transparent, use can be made of backlighting means and an electrowetting optical cell is obtained that can be used both as a electronically dimmable mirror while at the same time independently switching the cell on and off as a conventional pixel in for example a liquid crystal display. The skilled person will appreciate that this provides a major advantage, since this enables the electrowetting optical cell to be used in a electrowetting optical element that can function at the same time as a electronically dimmable mirror and as an electronic display.

In a further embodiment of the present invention, the electrowetting optical cell further comprises a light source arranged underneath the first electrode layer, for example a backlighting means, wherein said light source is arranged for sequentially emitting light of a plurality of primary colours, such as red, green and blue light, and wherein said powering means are arranged for powering said central electrode part such as to selectively switch said central electrode part for transmitting one or more of said plurality of primary colours of choice, for enabling perception of a pixel in a blend colour of said one or more of said plurality of primary colours of choice by a user.

According to the above embodiment a backlighting method may be applied wherein the backlight emits a sequence of red, green and blue light and the central part of the first electrode is switched to enable transmission of the colour of choice. This sequential backlighting method enables the electrowetting optical element of the present invention to be used as both an electronically dimmable mirror and a full colour display. The full colour display function is then implemented by suitably switching the central electrode part using the sequential backlighting method of backlighting with different colours sequentially and synchronising this with the switching of the central electrode for selectively transmitting the colours. The electronically dimmable mirror may be realised by suitable switching of the boundary electrode.

In a further embodiment of the present invention, the first electrode layer comprises a colour filter. Such a colour filter may be in any particular colour, however the skilled person will appreciate that the colour of the colour filter may also be one of the primary colours which enable the electrowetting optical cell to be eventually part of a full colour arrangement of optical cells. The colour filter of the first electrode layer may preferably be arranged such as to partially overlap the central electrode part.

According to a further embodiment of the present invention, the central electrode part has a circular shape. This appears to be most beneficial to the shape of the interface between a non-polar and polar liquid in the optical cell.

According to a further embodiment of the invention the central electrode part comprises multiple central electrode part segments and each of the central electrode part segments is electrically insulated from the other central electrode part segments. In this case, the power receiving lines connect each of the central electrode part segments such that the segments can be controllably powered independently from each other by the powering means. The multiple central electrode parts may be arranged in a common opening of the surrounding electrode part. The multiple central electrode parts may also be arranged in a separate opening in the surrounding electrode part.

This embodiments enables the independent switching of different electrode segments in the interior part of the cell. The skilled person may appreciate that even different colour filters may be applied to the different central electrode part segments, such as to provide a single optical cell that can be switched or operated in full colour.

As mentioned herein above the principles of the present invention may advantageously be used in a full colour display technique. Therefore, according to a further aspect of the present invention there is provided an electrowetting optical element comprising a plurality of electrowetting optical cells as described above. The element comprises a first electrode layer and a second electrode layer, and the containment space is formed between the first and second electrode layer of the element. The pixel walls of the element are mounted on the first or second electrode layer and the pixel walls separate the plurality of cells from each other such that the cells are adjacently arranged for forming a plurality of pixels on a screen.

The different cells in the electrowetting element according to the present invention will preferably be switchable independently from each other. Each of the optical cells of the electrowetting optical element may comprise a colour filter as described hereinabove. Defining the cell comprising a colour filter as colour cells according to a further embodiment the colour filters of the colour cells may be of at least a first colour, a second colour or a third colour, and adjacent cells of the element are grouped such as to form colour pixel groups comprising at least one colour cell of the first colour, at least one colour cell of the second colour and at least one colour cell of the third colour. The skilled person will appreciate that grouping the cells in the electrowetting optical element according to this principle enables the creation of a switchable full colour electrowetting display. As will be appreciated, in accordance with a further embodiment of the invention, the boundary electro parts of each cell may comprise a specular reflective surface.

As an alternative to each electrowetting optical cell comprising a colour filter, the abovementioned principle of sequential backlighting may also be applied to the electrowetting optical cells of the electrowetting element according to the second aspect. If common backlighting means to all electrowetting optical cells sequentially radiate light of a primary colour, each central electrode part of each electrowetting optical element can be switched independently in a suitable manner for transmitting the colour of choice for a desired duty cycle. The blend colour perceived by a user for a given pixel is then determined by the mutual ratio of the length of the duty cycles for each colour for that optical cell.

According to another embodiment of this second aspect, the power receiving lines of one or more boundary electrodes of said electrowetting optical cells are interconnected such as to enable powering of said boundary electrodes simultaneously. This is exclusively advantageous where the boundary electrodes function to provide the electronically dimmable mirror function. The mirror is to be dimmed and controlled as a whole, whereas the display function formed by the central part advantageously requires independent switching of each pixel.

Brief description of the drawings. Herein below, the invention will be further illustrated by means of some specific embodiments, with reference to the enclosed drawings, wherein:

Figure 1 schematically illustrates a cross section of an electrowetting optical cell according to the present invention;

Figures 2a - 2f illustrate 3 states in side view and top view of an electrowetting cell according to the present invention;

Figure 3 schematically illustrates an arrangement of electrowetting optical cells as seen from the top;

Figure 4 schematically illustrates an electrowetting optical element, comprising a plurality of electrowetting optical cells according to the present invention, providing a potential design for a full colour electrowetting display combined with a electronic dimmable mirror;

Figures 5a - 5b illustrate further embodiments of the present invention. Detailed description of the preferred embodiment.

In figure 1 , a cross section of an electrowetting optical cell 1 is schematically illustrated. The electrowetting optical cell 1 comprises a bottom carrier layer 2 and a top carrier layer 3, in between which the individual electrowetting optical cells are separated from neighbouring cells by means of pixel walls such as pixel wall 6 and 7. In between the pixel walls 6 and 7 and the top carrier layer 3 and bottom carrier layer 2, the electrowetting optical cell according to the present invention comprises a containment space 8. The containment space 8 is filled with a polar liquid 15 and a non-polar liquid 16. The term polar liquid is to be interpreted as a liquid containing molecules having a non-even charge distribution across the molecule, such as to form a dipole, for example water. The top carrier layer 3 comprises a second electrode layer 12 contiguous to the containment space. The bottom carrier layer 2 comprises a first electrode layer 10 also contiguous to the containment space. The first electrode layer 10 has an insulating layer 1 1 with a hydrophobic surface, such that it will repel the polar liquid 15 when the electrodes in the first and second electrode layer are in the unpowered state, as shown in figure 1.

When powering the first and second electrode layer 10 and 12 such as to provide a potential difference between these layers, the polar liquid will be attracted to the first electrode layer 10. This will change the force equilibrium in the containment space, pushing the non-polar liquid away to cover a small area of the optical cell. This will change the optical properties of the electrowetting optical cell in case the non-polar liquid is made dark or opaque, while the polar liquid (e.g. water or a glycol mixture) is optically transmissive.

The first electrode layer 10 and the second electrode layer 12 may primarily be made of a transparent conductive material, such as indium tin oxide (ITO). The first electrode layer 10 of each cell, such as electrowetting optical cell 1 , comprises a boundary electrode part 19 and a central electrode part 20 according to the present invention. By separating the central electrode part 20 from the boundary electrode part 19, use is made of the fact that the unpowered state, the non-polar liquid comprises a first interface surface with the polar liquid 15, causing the non-polar liquid 15 to be thicker in the middle than near the sides of the optical cell. Therefore, the electrowetting optical cell in the unpowered state is near the sides of the optical cell (above boundary electrode part 19) less opaque and more transparent than in the middle of the optical cell above the central electrode part 20.

Underneath the first electrode layer, a specular reflective layer 23 is created on the bottom carrier layer 2. Underneath the central electrode part, a transparent layer 24 is created. In addition to that, on the back side of the electrowetting optical cell, backlighting means schematically illustrated with reference numeral 25 illuminate the electrowetting optical cell. The working principal of the electrowetting optical cell schematically illustrated in figure 1 is as follows. In the unpowered state, the thicker part of the non-polar liquid layer 16 will be perfectly able to completely block light from the backlighting means to cross the electrowetting optical cell to the front carrier layer 3. To the sides of the optical cell 1 , light from the backlighting means 25 will not enter the optical cell since it is blocked by the specular reflective layer 23. However, light that enters the electrowetting optical cell through the top carrier layer 3 falling onto the specular reflective surface 23, will be effectively blocked even in the thinner part of the non-polar liquid layer in the unpowered state, since it has to cross the thinner part of the unpowered non-polar liquid layer of the optical cell 1 twice before leaving again through top layer 3.

An electrically insulating structure 21 separates the central electrode part 20 from the boundary part 19 of the optical cell. The central electrode part 20 can be separately powered by means of powering lines (not shown) such as to enable advanced switching possibilities. By powering the central electrode part 20, without powering the boundary electrode part 19, the non-polar liquid will retreat on the boundary electrode part electrode part 20. Therefore, an optical path is created through the electrowetting optical cell from the backlight 25 through the bottom carrier layer 2 to the top carrier layer 3. The optical cell can be switched as a pixel in a regular electronic display.

On the other hand, by leaving the central electrode part 20 unpowered while switching the boundary electrode part 19, the non-polar liquid will retreat on the area on top of the central electrode part 20, providing optical access through the optical cell 1 in the region directly above the boundary electrode part. Therefore, light entering through the top carrier layer 3 will cross the electrowetting optical cell , be reflected on the specular reflective surface 23, and crossing the electrowetting optical cell again creating a mirror image to the user. By switching only the boundary electrode part in the powered state, the mirror will be fully visible. As a result, with the electrowetting optical cell according to the present invention, the display function of the electrowetting optical cell can be independently controlled from the mirror function of the electrowetting optical cell. Such a cell may advantageously be used for creating an automatically dimmable car mirror while at the same time enabling a display function in the mirror.

Figures 2a - 2f illustrate three operating states in side view and top view of an electrowetting cell as described above

Figure 2a illustrates an electrowetting cell in an unpowered state. Figure 2a shows the central electrode part 20 with transparent layer 24 underneath, and surrounding electrode part 19 having specular reflective layer 23 underneath. For the purpose of simplicity, pixel walls, top and bottom carrier layers and polar liquid of figure 1 are not shown. The central electrode 20 and the surrounding electrode 19 are both transparent. On top of the central electrode 20 and the surrounding electrode 19 the non- polar liquid layer 16 and the surrounding electrode 19 the non-polar liquid layer 16 is applied for blocking incident light 27a from light source 29 and light 26 from light source 25 underneath the electrowetting cell. The electrowetting cell of figure 2a is in an unpowered state, none of the central electrodes 20 and the surrounding electrode 19 are powered, i.e. connected to a voltage source. As a consequence the non-polar liquid 16 spreads across the surface of both central and boundary electrodes 20, 19.

Figure 2b shows the same electrowetting cell of figure 2a in the unpowered state in top view. Because the non-polar liquid 16 is spread across the surface of both central and surrounding electrodes 20, 19, the electrowetting cell appears entirely dark.

Figure 2c shows the electrowetting cell of figure 2a in a reflective state, where the central electrode 20 is not powered and the surrounding electrode 19 is in a powered state, i.e. connected to a voltage source. Due to the electrostatic operation of the electrowetting cell, the non-polar liquid 16 retreats to the unpowered part of the electrowetting cell, leaving the surrounding electrode 19 exposed to incident light 27a from light source 29. Since the transparent surrounding electrode 19 is placed on top of a specular reflective layer 23, the incident light 27a is reflected. The reflection or reflected light 27b can be viewed by an observer. Incident light 26 from the light source 25 underneath the electrowetting cell however cannot pass through the transparent layer 24 and the central electrode 20.

Figure 2d shows the electrowetting cell of Figure 2c in top view. Due to the specular reflective layer 23 being exposed, the reflection of the light source 29 or any object is visually reflected 27b, however no light from the back of the electrowetting cell is allowed to pass, so the central part of the electrowetting cell remains dark.

Figure 2e shows the electrowetting cell of Figure 2a in a transparent state. In this state the central electrode 20 is in a powered state, where the surrounding electrode 19 is in an unpowered state. As a consequence the non-polar liquid 16 retreats to the surface of the surrounding electrode 19. In this state the incident light 27a from a light source or object 29 is no longer reflected by reflective layer 23. Incident light 26 from light source 25 on the back of the electrowetting cell is now allowed to pass through the transparent layer 24 and transparent central electrode 20.

Figure 2f shows the electrowetting cell in transparent state of Figure 2e. The central part of the electrowetting cell now appears as a light spot, whereas the remaining part of the electrowetting cell surface is dark due to the non-polar liquid 16 which is spread over the surrounding electrode 19.

By usefully controlling electrowetting cells in a combination of states as described above in a display, electrowetting cells in transparent state can display information to a viewer in the form of messages or images, whereas electrowetting cells alternating between the unpowered state and reflective state can perform the function of dimming excessive incident light.

Figure 3 schematically illustrates an arrangement of electrowetting optical cells in a electrowetting optical element or screen, as viewed from the top. Each electrowetting optical cell is defined by the pixel walls, such as pixel wall 32. The pixel walls are created such that the shape and surface of each electrowetting optical cell is randomized. The advantage of randomizing the shape and surface of the electrowetting optical cells, is that optical interference patterns in the mirror function of an electrowetting element according to the present invention are effectively reduced. As the skilled person will appreciate, the manner of randomizing the shape and surface of the pixels of the electrowetting optical element may differ from the illustration in figure 3 at the choice of the designer.

Figure 3 also illustrates the boundary electrode part 28 and the central electrode part 30 of the first electrode layer. As in the embodiment of figure 1 , the boundary electrode part 28 of the electrowetting optical cell provides the mirror function, having a specular reflective surface underneath. The central electrode part 30 enables backlight from the back of the electrowetting optical element (not shown) to penetrate through the central electrode part 30 in case the central electrode part is powered up and the non-polar liquid (not shown) does not cover the central electrode part 30.

Additional advantages may be achieved by using colour filters for the central electrode parts of each of the electrowetting optical cells. The use of colour filters enables to provide a full colour electrowetting optical display, while at the same time providing the function of electronically dimmable mirror. In an electrowetting optical element containing an arrangement or grouping of multiple electrowetting optical cells adjacent to each other, use can be made of multiple colour filters having a different colour, e.g. each colour filter being of one of the primary colours red, yellow and blue (or red, green and blue). By grouping the electrowetting optical cells with different colour filters in a suitable way, this technique can be used for effectively creating a full colour display wherein each pixel is basically formed by adjacently arranged optical cells with different colour filters. Using variation of the duty cycle for powering a central electrode part of each optical cell while using a common display frequency, the colour of the pixel as perceived by the viewer can be changed with sufficient flexibility, e.g. enabling the same number of possible colours that may be provided in a conventional display. The skilled person will appreciate that every individual central electrode part must be individually controllably powered by connecting the electrode to a voltage source using for example a central processing unit or a screen controller for powering and unpowering each of the powering lines of the electrowetting optical cells of the electrowetting optical element.

An example of a full colour electrowetting optical element with integrated electronically dimmable mirror is illustrated in figure 4. Electrowetting optical element 40 in figure 4 illustrates a plurality of pixel walls, such as pixels wall 52. Note that in the embodiment illustrated in figure 4, the electrowetting optical cells are not randomized. Figure 4 illustrates how the electrowetting optical cells having different colour filters may be grouped such as to form a full colour display, in addition to an electronically dimmable mirror. In figure 4, the pixel walls, such as pixel wall 52, define the individual electrowetting optical cells. Each optical cell may be constructed in a similar way as illustrated in the embodiment of figure 1 , and comprises a boundary electrode part 56 and a central electrode part 55.

As can be seen for electrowetting optical cells 40, 44, and 48, the electrowetting optical cells are grouped such that a triplet of optical cells (40, 44, 48) contains central electrode parts having colour filters 42, 46 and 50 of a different primary colour (for indicating a different primary colour in figure 4, use is made of different shading in the colour filters 42, 46 and 50).

Basically, the triplet of electrowetting optical cells 40, 44 and 48 together form a full colour pixel of which the colour can be changed by controlling the duty cycle or powering the central electrode parts during each full cycle of display frequency.

In figure 5a, a further embodiment of an electrowetting optical cell of the present invention is schematically illustrated. Here, the electrowetting optical cell comprises a boundary electrode part 35, pixel walls 52, and three adjacent central electrode parts 37, 38 and 39 which can be individually controllably powered from each other. As will be appreciated, the three central electrode parts 37, 38 and 39 may comprise a different colour filter of a primary colour, in addition to being arranged for allowing penetration of backlight. This enables to provide a full colour electrowetting optical cell. As illustrated in figure 5b, the central electrode parts 37, 38, 39 can be of arbitrary shape and location. The central electrode parts 37, 38, 39 can be located separately within the electrowetting cell. An arbitrary number of central electrode parts 37, 38, 39 can be placed in one electrowetting cell.

For the purpose of comprehensiveness, it is noted here that numerous modifications and variations of the present invention are possible in the light if the above teachings. It is therefore understood that, within the scope of the appended claims, the invention may be practices otherwise than as specifically described therein.

Claims

1. Electrowetting optical cell comprising a first electrode layer and a second electrode layer, a containment space formed between said first and said second electrode layer, pixel walls mounted on said first or second electrode layer for defining said cell, wherein said first electrode layer comprises an insulating layer with a hydrophobic interface with said containment space, wherein said containment space comprises at least one polar liquid and at least one non-polar liquid, said at least one polar liquid and said at least one non-polar liquid being immiscible with each other, and

wherein said first and second electrode layer comprise power receiving lines for receiving power from powering means for controllably powering said first and second electrode layers for rearranging said polar liquid relative to said non-polar liquid for switching said cell between different states of optical transmittance,

wherein said first electrode layer comprises a boundary electrode part and a central electrode part electrically separate from each other, wherein said boundary electrode part of said first electrode layer is arranged contiguous to said pixel walls, and said boundary electrode part surrounding said central electrode part such that said central electrode part is separated from said pixel walls by said boundary electrode part, and

the electrowetting optical cell further comprising a specular reflective surface acting as boundary electrode or underneath the transparent boundary electrode part and the boundary electrode part being optically transmissive for enabling reflection of incident light,

wherein said central electrode part is optically transmissive for enabling transmission of light through said first electrode layer,

wherein said power receiving lines connect both said boundary and said central electrode part such that said boundary and said central electrode part can be controllably powered independently from each other by said powering means.

2. Electrowetting optical cell according to claim 1 , wherein said first electrode layer comprises a colour filter.

3. Electrowetting optical cell according to claim 1 or 2, wherein said colour filter of said first electrode layer is arranged such as to at least partially overlap said central electrode part.

4. Electrowetting optical cell according to any of the previous claims, wherein said central electrode part has a circular shape.

5. Electrowetting optical cell according to any of the previous claims, wherein said central electrode part comprises multiple central electrode part segments, wherein each of said central electrode part segments electrically insulated from said other central electrode part segments, and said power receiving lines connect each of said central electrode part segments such that said segments can be controllably powered independently from each other by said powering means.

6. Electrowetting optical cell according to claim 5, wherein the multiple central electrode parts are arranged in a common opening in the surrounding electrode part.

7. Electrowetting optical cell according to any of the previous claims, further comprising a light source arranged on the first electrode side of the electrowetting cell, wherein said light source is adapted for sequentially emitting light of a plurality of primary colours, such as red, green and blue light, and wherein said powering means are arranged for powering said central electrode part such as to selectively switch said central electrode part for transmitting one or more of said plurality of primary colours of choice, for enabling perception of a pixel in a blend colour of said one or more of said plurality of primary colours of choice by a user.

8. Electrowetting optical element comprising a plurality of electrowetting optical cells in accordance with any of the previous claims, wherein the respective specular reflective layers of each of the plurality of electrowetting optical cells form a contiguous common specular reflective layer.

9. Electrowetting optical element according to claim 8, wherein power receiving lines of one or more of said cells are connected to said powering means such as render said state of optical transmittance of said one or more of said cells to be controllably adjustable.

10. Electrowetting optical element according to claim 8 or 9, wherein each of one or more colour cells of said plurality of cells comprise a colour filter in accordance with claim 2 or 3.

1 1. Electrowetting optical element according to claim 10, wherein said colour filters of said colour cells are of at least a first colour, a second colour, or a third colour, and wherein adjacent cells of said element are grouped such as to form colour pixel groups comprising at least one colour cell of said first colour, at least one colour cell of said second colour, and at least one colour cell of said third colour.

12. Electrowetting optical element according to any of the claims 8 or 9, further comprising backlighting means, wherein said backlighting means are arranged for sequentially emitting light of a plurality of primary colours, such as red, green and blue light, and wherein said powering means are arranged for powering said central electrode parts of each of said plurality of electrowetting optical cells such as to selectively switch said central electrode parts for transmitting one or more of said plurality of primary colours of choice, for enabling perception of a pixel for each of said plurality of electrowetting optical cells in a blend colour of said one or more of said plurality of primary colours of choice by a user.

13. Electrowetting optical element according to any of the claims 8 - 12, wherein power receiving lines of one or more boundary electrodes of said electrowetting optical cells are interconnected such as to enable powering of said boundary electrodes simultaneously.