WO2005071461A1

WO2005071461A1 - Display device

Info

Publication number: WO2005071461A1
Application number: PCT/IB2005/050211
Authority: WO
Inventors: Ramon P. Van Gorkom
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-21
Filing date: 2005-01-18
Publication date: 2005-08-04
Also published as: TW200537202A

Abstract

The present invention relates to a display device (30) having a light guide (32) optically coupled to a light source (46), a passive plate (34) facing the light guide, an electrostatically operable element (36) arranged between the light guide and the passive plate, and two sets of electrodes (40, 42). The electrodes are arranged to induce electrostatic forces on the element (36) and to bring selected portions of the element into contact with the light guide (32), thereby extracting light from the light guide. The display device is characterized in that sets (31, 33) of electrodes (40, 42) are provided on one of the light guide (32) and the passive plate (34), and arranged to define a plurality of pixel areas (41), wherein a unique combination of electrodes (40, 42) belonging to different electrode sets (31, 33) have an extension in each pixel area. The display device according to the invention enables a more robust addressing a more efficient use of the bi-stable region and/or reduction of the bi-stable region, whereby the number of pixel failures due to spread in the switching curves can be reduced by a large number.

Description

Display device

The present invention relates to a display device having a light guide optically coupled to a light source, a passive plate facing the light guide, an electrostatically operable element arranged between the light guide and the passive plate, and two sets of electrodes. The electrodes are arranged to induce electrostatic forces on the element and to bring selected portions of the element into contact with the light guide, thereby extracting light from the light guide.

A conventional foil display is described in for example WO 00/38163. Such a display is shown in Fig. 1, and comprises a light guide plate 10 and a non-lit plate 12, with a scattering foil 14 clamped in between. On both plates there are respective sets of parallel electrodes 16, 18 which are arranged perpendicularly with respect to each other. The electrodes on the light guide plate are arranged in a column direction, and the electrodes on the non-lit plate are arranged in a row direction. Also the foil is provided with an electrode layer 20. The electrodes are formed by ITO layers formed on each of the mentioned surfaces. The crossings of the electrodes of each set define the pixels of the display. By application of voltages to appropriate electrodes on the light guide, the non-lit plate and the foil, the foil may be attracted to the light guide. When the foil is brought into contact with the light guide, light is extracted from the light guide. In order to minimize absorption, the light guide is made relatively thick, so as to reduce the number of reflections by the light guide surfaces. This means that the amount of light that can be extracted from the light guide per unit length, which is proportional to the number of times each light ray is reflected, is relatively small. Therefore, in order to reach sufficient brightness, a sub-field addressing scheme is used, making use of the bi-stability of the foil. However, the control of the bi-stable switching in a conventional foil display may be difficult, as the switching curves are not everywhere the same. This is shown in Fig. 2, wherein the ON-switching curve is denoted 22, the OFF -switching curve is denoted 24, and the bi-stable region is denoted 26. The spread in the switching curves decreases the global bi-stable region which may cause certain pixels to remain ON and certain pixels to remain OFF, thus causing the display to malfunction.

An object of the present invention is to provide a foil display which can be more robustly addressed. This and other objects are achieved by a device of the kind mentioned by way of introduction, wherein the both sets of electrodes are provided on one of the light guide and the passive plate, and arranged to define a plurality of pixel areas wherein a unique combination of electrodes belonging to different electrode sets have an extension in each pixel area. Thus, the two electrode sets can be arranged either on the passive (non-lit) plate or on the light guide (active plate). All electrodes are provided with a voltage which is then reduced or removed for one electrode in each set. By reducing the voltage for one electrode in each set, the pixel area where both these electrodes extend will be provided with . this lower voltage over a larger area than any other pixel area, and the force on the foil will thus be reduced or eliminated in this particular pixel area. Alternatively, a voltage is applied to one electrode in each set, while the other electrodes are set at lower or zero potential. This results in a increase of the force exerted on the foil in a particular pixel area. The invention is based on the understanding that by arranging both electrode sets on the same plate, it is possible to address the display by only changing the force acting on the movable element towards one plate, while the force towards the other plate is held constant. For example in a case where both electrode sets are arranged on the passive plate, the force towards the passive plate may be altered, while the force exerted on the element towards the light guide is held constant. Thus, pixels can be switched ON by decreasing the force towards the passive plate, whereby portions of the electrostatically operable element are brought into contact with the light guide. The constant force towards the light guide may for example be achieved by arranging a common electrode on the light guide. The arrangement of the two sets of structured electrodes on one single plate according to the invention enables a more robust addressing. Preferably, the electrodes of one set are arranged mainly in the row direction, while the electrodes of the other set are arranged mainly in the column direction. As mentioned, a pixel of the display is defined by an area where two electrodes belonging to different sets extend. Thus, a certain pixel is a unique area in which one electrode from each set of electrodes is present. Preferably, each electrode covers approximately half of a particular pixel area. Consequently, the contribution from each electrode will be equal, if equal voltages are applied on both electrodes. An advantage with thisαs that when one of the two equal voltages applied to the electrodes is removed (e.g. the row voltage), the reduction in force is a factor 2 ~ 1.4. In conventional addressing, where row and column electrodes are arranged on different sides of the field creating the force, and provided with plus or minus a given potential, the voltage difference would have been reduced by a factor 2 if one of the electrodes were grounded. The reduced voltage drop can be used to achieve a more robust addressing, less sensitive to the exact overlapping of the bi-stability regions, whereby the number of pixel failures may significantly be reduced. Thus, a very robust addressing for the sub-field addressing scheme may be achieved. If, for example, a row is selected by removing the¹ row . voltage, and a number of pixels in the selected row are switched ON by removing the column voltage, the other pixels in this selected row, and these columns (in unselected rows) are not affected as much as in a conventional foil display. In each pixel area, the electrodes of the two sets can be arranged in one common layer or in two different layers, i.e. the row electrodes in one layer and the column electrodes in another layer. According to a first embodiment of the present invention, both the sets of structured electrodes are arranged on the passive plate of the display. In this embodiment, the force exerted on the movable element towards the light guide may be held constant, whereby addressing occurs by removing the force towards the passive plate by means of the two sets of structured electrodes on the passive plate. An advantage with this arrangement is that fewer elements have to be arranged on the light guide compared to a conventional foil display. Accordingly, the light guide can be kept more "clean", which is advantageous in terms of luminance and contrast. Preferably, the light guide is provided with an electrode layer, for providing an electrostatic force. Thus, a constant voltage level may be applied to the electrode, whereby a constant electrostatic force acting on the movable element is achieved, which force attracts the element towards the light guide. The electrode layer may be unstructured or structured. An advantage with the latter is that reliability is increased because the Si0 -layer of the light guide can cause short circuit between the electrode on the movable element and the electrode layer on the light guide via the spacers. The light guide electrode layer may for example be structured into independently addressable blocks of rows or columns. This enables block or row resets, i.e. OFF-switching of complete blocks or rows, which in turn enables more efficient addressing schemes. The above mentioned light guide electrode may be arranged on the side of the light guide facing the movable element, or on the opposite side of the light guide with respect to the movable element, i.e. on the "outside" of the light guide. An advantage with the latter is that the luminance and contrast properties can be further improved. According to a second embodiment of the invention, the electrode layout of the light guide and the passive plate is reversed, i.e. the two sets of structured electrodes are arranged on the light guide of the display. This however requires an off addressing scheme and means for modulating the light source. In this second embodiment, the passive plate is advantageously provided with an electrode layer for providing a common electrode. According to a third embodiment of the invention, the display device further comprises two additional sets of structured electrodes, whereby two sets of structured electrodes are arranged on each of the passive plate and the light guide. This enables further advantageous addressing opportunities. The movable element of the display is preferably provided with structured electrode layer. The electrode layer may for example be structured into sets of electrodes. This enables more flexible addressing possibilities. The display device according to the present invention is advantageously addressed using a a sub-field addressing scheme, whereby the device according to the invention enables more robust sub-field addressing compared to a conventional dynamic foil display. However, the display device may alternatively be addressed using a line-at-a-time addressing scheme.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. Fig. 1 is a schematic cross section of a display according to prior art. Fig. 2 illustrates the switching curves according to prior art. Figs. 3a-3b are schematic side views illustrating a pixel of a display according to a first embodiment of the invention. Figs. 4a-4d are schematic top views of pixels illustrating electrode layouts according to the invention. Fig. 5 schematically illustrates the switching curves and switching points according to the embodiment in Figs. 3 and 4.

Figs. 3a-3b schematically show a pixel of a display device 30 according to a first embodiment of the present invention. Identical reference numerals have been used for corresponding elements of the device. The display device 30 which is partially shown in Figs. 3a-3b comprises a light guide (active plate) 32 and a passive plate 34. The active plate 32 and the passive plate 34 are preferably made by glass. An electromechanically operated foil 36 is further clamed in between the active plate 32 and the passive plate 34. The foil 36 may for example be of a flexible light scattering material, such as parylene, provided on one side with an conducting layer 37, making it electrostatically operable. Spacers 38, 39 are arranged on each side of the foil 36 to separate it from the active plate 32 and the passive plate 34. The display device further comprises row electrodes 40 and column electrodes 42. In Figs. 3a and 3b, one row electrode 40 and one column electrode 42 is shown. The electrodes can be formed by ITO layers. According to the first embodiment of the invention, both the row and column electrodes 40, 42 are arranged on the passive plate 34, as shown in Figs. 3a and 3b. In Fig. 3 a, both the row and column electrodes 40, 42 are arranged in one layer on the passive plate 34. In this case, because the row and column electrodes cross each other, different metal layers are used which connect to the ITO using vias in the SiQ₂ of the plate. In Fig. 3b, the row and column electrodes 40, 42 are arranged in different ITO layers i.e. the row electrode in one layer and the column electrode in another layer. These techniques are known from for example AMLCD panels. The layout of the row and column electrodes is further detailed in Figs. 4a-4d.

Fig. 4a shows a set of row electrodes 31 and a set of column electrodes 33. A unique combination of a row electrode 40 belonging to the electrode set 31 and a column electrode 42 belonging the electrode set 33 has an extension in a pixel area 41. The pixel area 41 is limited by the spacers 39, which thus helps define the area of the pixel. Also, Fig. 4b-4d each show a top view of a pixel 41 with further exemplifying electrode layouts. In Figs. 4a-4d, both the row electrode 40 and the column electrode 42 have an essentially comb-shaped structure. The comb-shaped row electrode 40 and the comb-shaped column electrode 42 are arranged interleaved with each other so that each electrode covers approximately half of the pixel area. However, other electrode layouts may alternatively be used. For example, the pixel area may simply be divided in half, wherein the row electrode extends in one half of the pixel area, and the column electrode in the other half. Returning to Figs. 3a-3b, the display device 30 further comprises a common electrode 44 arranged on the active plate 32. Upon operation of the display, light from a light source, such as a LED 46, is coupled into the active plate 32. The light is confined inside the active plate by total internal reflection. Light may be extracted from the active plate 32 by bringing the foil 36 into contact with the active plate by means of applying appropriate voltages to the electrodes 37, 40, 42, and 44, as will be further described below. Fig. 5 schematically shows the switching curves and switching points corresponding to the electrode arrangement in Figs. 3a and 3b. The voltage difference between the active plate common electrode 44 and the foil electrode 37 is labeled V_active, and the voltage difference between the foil electrode 37 and the pixel area 41, i.e. the combination of row electrode 40 and column electrode 42 on the passive plate 34, is labeled passive- The hereinafter detailed operation is described with respect to a sub-field addressing scheme. All pixels are initially switched OFF by a robust OFF action, e.g. by setting Vactive = 0 V by connecting the common electrode 44 to ground, and increasing V_passj_Ve by applying both row and column electrodes with the same voltage (V_r0w3 V_coι), e.g. 50 V. This corresponds to position 50 in Fig. 5, located in the OFF region 52. It is here assumed that the foil is connected to ground (V_f0π = 0). The voltage difference V_active is then increased to a constant potential (for example 40 V). This creates a force on the foil towards the active plate 32, moving the pixel to a position 56 in the bi-stable region 58. The pixels thus remain in their current state, hence OFF. The row electrode 40 of a row to be addressed is then set to zero potential (V_row ⁼ 0) during a row pulse, which decreases the voltage difference V_passive- When the row electrode is grounded, the potential of the pixels in this row is reduced to V_passive ^:

^( -V_foπϊ + iK_o, -^)²)^ = Λ _o,² +0²}/2 = V_coJ l ϊ , where V_col is the potential of the column electrodes. (Assuming that each of the row and column electrode covers half of the pixel area each, as described above.) In other words, the voltage difference V_paSsi_ve in Fig. 5 is reduced by a factor of 2 = 1.4, i.e. around 35 V. This decreases the force towards the passive plate exerted on the foil along this row, and moves it to a position 60, still in the bistable region. Next, but still within the row pulse, the column electrodes 42 of pixels that should emit light are also set to zero potential during a column pulse, whereby the potential of pixels in the selected row will be zero (both row and column electrodes set to zero). This means that the voltage difference V_passi_Ve is also zero, and the pixel moves to a position 62 in the ON region 64. This results in that the foil in these places due to the constant force towards the active plate will be attracted to the active plate, whereby the pixels are switched ON, i.e. light is emitted from the active plate at these selected pixels. After the column and row pulse, all pixels return to position 56 in the bi-stable region 58, and pixels that have been switched ON will remain ON. A new row is then selected by a new row pulse, and the procedure is repeated until the entire display has been addressed. It should be noted that the voltage difference between the foil and any of the electrodes preferably changes sign every frame, in order to minimize charging. As mentioned above, the voltage difference V_acti_Ve is constant throughout the addressing, whereby the positions 56, 60 and 62 are located on a horizontal line in Fig. 5. After a period of light generation, during which the light source is turned on, a robust OFF action may again be applied to place all pixels in the OFF-state. As may be seen in Fig. 5, the center switching point 60 is, due to the inventive electrode arrangement, positioned closer to the switching point 56 than the switching point 62, due to the fact that the reduction in voltage difference V_passive will be first by a factor 2. The vicinity of the two switching points 56 and 60 in the bi-stable region makes the switching more robust. The embodiment of the display device according to the invention described above in relation to Figs. 3-5 may alternatively be used for line-at-a-time addressing. In that case, the switching voltages and the bi-stable region are selected so that the switching points are outside of the bi-stable region. The invention is not limited to the embodiments described above. Those skilled in the art will recognize that variations and modifications can be made without departing from the scope of the invention as claimed in the accompanying claims. For example a resistance lowering metalization may be added on each of the electrode layers, which lowers the resistance of the ITO. This means that the display can be faster addressed.

Claims

CLAIMS:

1. A display device (30) comprising: a light guide (32) optically coupled to a light source (46), a passive plate (34) facing the light guide (32), an electrostatically operable element (36) arranged between said light guide (32) and said passive plate (34), a first electrode (40) and a second electrode (42), a pixel area (41), defined by the combination of the first electrode (40) and the second electrode (42), both electrodes (40), (42) having an extension in said pixel area (41), both said electrodes (40), (42) are provided on either the light guide (32) or on the passive plate (34), and are arranged for receiving a potential for inducing electrostatic forces on the element (36) in order to bring the pixel area (41) into contact with the light guide (32), thereby extracting light from the light guide (32).

2. A display device (30) as claimed in claim 1, wherein the electrodes (40) and (42) are arranged as row and column electrodes.

3. A display device (30) as claimed in claim 1 or 2, wherein the display device comprises a set (31) of electrodes (40) and a set (33) of electrodes (42).

4. A display according to claim 1, 2 or 3, wherein the extensions of said electrodes (40, 42) in each pixel area (41) are essentially equal in size.

5. A display device according to claim 1, 2, 3 or 4, wherein the two electrodes

(40, 42) of a pixel are arranged in an interleaved comb structure.

6. A display device according to any one of the preceding claims, wherein said both sets of electrodes (40, 42) are provided on said passive plate (34).

7. A display device according to claim 6, further comprising an electrode layer (44) arranged on said active plate (32).

8. A display device according to claim 7, wherein said electrode layer (44) is arranged on the opposite side of the light guide (32) with respect to the passive plate (34).

9. A display device according to claim 1, 2, 3, 4 or 5, wherein said both sets of electrodes (40, 42) are provided on said light guide (32).

10. A display device according to claim 9, further comprising an electrode layer arranged on said passive plate (32).

11. A display device according to claim 6, further comprising two additional sets of structured electrodes, whereby said additional two sets of structured electrodes are provided on said light guide (32).

12. A display device according to any one of the preceding claims, wherein said movable element (36) is provided with a structured electrode layer.

13. A display device according to any one of the preceding claims, wherein said display is addressed using a sub-field addressing scheme.

14. A display device according to any one of claims 1-12, wherein said display is addressed using a line-at-a-time addressing scheme.