Abstract:
A display device comprising a light guide ( 12 ), a front plate ( 14 ), and an intermediate electromechanically operable foil ( 16 ). Two electrode layers ( 22, 23 ) are arranged on either side of the foil ( 16 ) to induce electrostatic forces on the foil ( 16 ) and to bring selected portions of the foil into contact with the light guide ( 12 ), thereby extracting light from the light guide ( 12 ). The second electrode layer ( 22 ) is arranged on the opposite side of the light guide ( 12 ) with reference to the foil ( 16 ), and separated from the light guide ( 12 ) by means of a refractive layer ( 28 ). As no electrode layer is required on the light guide itself, the light path of rays extracted from the light guide is cleaner, and the absorption of light is reduced. The light guide can have a thickness such that the light extracted from the light guide per unit length is sufficient to allow for line-at-a-time addressing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a Continuation of co-pending, commonly assigned, U.S. patent application Ser. No. 10/557,343 entitled “LINE-AT-A-TIME FOIL DISPLAY,” filed on Nov. 21, 2005, which claims priority to PCT Application number PCT/IB2004/050695, filed May 14, 2004, the disclosures of which are hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a display device having a light guide optically coupled to a light source, a front plate facing the light guide, a first electrode layer arranged on said front plate, a second electrode layer arranged on the opposite side of the light guide with respect to the front plate and separated from the light guide by means of a refractive layer, and a movable element provided with a third electrode layer arranged between the light guide and the front plate. The electrode layers are arranged to induce electrostatic forces on the element and to bring selected portions of it into contact with the light guide, thereby extracting light from the light guide. 
       BACKGROUND OF THE INVENTION 
       [0003]    Line-at-a-time addressing is a technique well known in the art (e.g. passive matrix OLED displays), and is based upon selecting one line of the display at a time, consecutively during the frame period, and while each line is selected addressing the pixels in this line. As a consequence, each pixel can only be addressed for a fraction of the frame period (i.e. the frame period divided by number of lines). Therefore, line-at-a-time addressing requires quite a large maximum intensity per pixel, in order to obtain the desired light output. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    A conventional foil display is shown in  FIG. 1 , and comprises a light guide  1  in the form of an edge lit glass plate and a non-lit front plate  2 , with a scattering foil  3  clamped in between. On both plates there are respective sets of parallel electrodes  4 ,  5  which are arranged perpendicularly with respect to each other. By application of voltages to appropriate electrodes on the light guide, the front plate and the foil, it is possible to generate two electrostatic fields with the field vectors directed towards the light guide and the front plate respectively. The balancing of these two electrostatic forces in combination with the elastic force of the foil is used to attract the foil to either the light guide or the front plate. Typically, the foil can be attracted towards the light guide using a column electrode and towards the front plate using a row electrode. When the foil is brought into contact with the light guide, light is extracted and emitted through the front plate. If preferred, the front plate can include a color filter and/or a black matrix. 
         [0005]    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, line-at-a-time addressing is not possible. Simply put, with light rays traveling in the column direction, each light ray does not hit all the pixels in a column. 
         [0006]    Instead, a sub-frame addressing scheme is used, making use of the bi-stability of the foil. This is described in WO 00/38163, with several positive effects, the disclosure of which is hereby incorporated herein by reference. 
         [0007]    In practice, however, the control of the bi-stable switching is difficult, as non-homogeneous switching curves can cause certain pixels to remain ON or OFF. It also requires a large number of pixel switching events during addressing. Additionally, sub-frame addressing requires complex and expensive electronics. 
         [0008]    An object of the present invention is therefore to provide an improved foil display device, allowing a line-at-a-time addressing scheme. 
         [0009]    This and other objects are achieved by a device of the kind mentioned by way of introduction, wherein the light guide has a thickness such that the light extracted from the light guide per unit length is sufficient to allow for line-at-a-time addressing. 
         [0010]    The invention is based on the realization that when the second electrode layer is arranged on the far side of the light guide, the light path of rays extracted from the light guide is cleaner, and the absorption of light in the light guide is reduced. This reduced absorption allows for a thinner light guide, in turn resulting in more reflections and hence larger available light intensity per unit length. Even if each light ray is reflected more often than in conventional foil displays, the absorption will be kept at a reasonable level. 
         [0011]    The refractive layer is intended to ensure the total internal reflection of the light guide, and has a refractive index smaller than the refractive index of the light guide. It can be deposited on the light guide, and the second electrode layer can then be deposited on the refractive layer. Alternatively, the second electrode layer is arranged on a back plate, arranged on the opposite side of the light guide with reference to the front plate, and the refractive layer is then formed by an air gap separating the back plate from the light guide. 
         [0012]    Due to the increased distance between the foil and the electrodes on the far side of the light guide, a large voltage is needed to attract the foil to the active plate. However, by minimizing the distance between the light guide and the second electrode layer, the required voltage can be limited. 
         [0013]    The bi-stable character of the switching characteristics is not required for the addressing of the display, and the design can be modified to achieve a smaller bi-stable region so that addressing pulses of small magnitude may be employed. 
         [0014]    For example, the light guide can be glass plate having a thickness of 0.05-1 mm, and preferably 0.1-0.3 mm. 
         [0015]    According to a first embodiment, the second electrode layer is unstructured, while the first and third electrode layers are structured into sets of electrodes. These sets of electrodes can then be used to address the foil. In order to facilitate addressing, the first electrode layer can comprise a first set of parallel electrodes while the third electrode layer can comprises a second set of parallel electrodes, orthogonal to said first set. Thereby, a foil electrode layer divided into parallel electrodes is arranged in between one unstructured electrode and one set of parallel electrodes, perpendicular to the foil electrodes. This allows for addressing of individual pixels, defined by intersections of the electrodes. 
         [0016]    An advantage with this arrangement is that the voltage applied to the second electrode layer, which must be quite high (typically several hundred V to a few kV) due to the distance to the foil electrode, now can be a constant DC during each frame, possibly with reversing polarity for different frames. Thereby high frequency switching with high voltages is avoided. 
         [0017]    In this case, any spacers arranged between the front plate and the foil and/or between the light guide and the foil preferably extend perpendicularly with the electrodes on the foil. This accounts for less stringent alignment. 
         [0018]    According to a second embodiment, the third electrode layer is unstructured, while the first and second electrode layers are structured into sets of electrodes. These sets of electrodes can then be used to address the foil. Again, as in the first embodiment, the electrode sets can comprise parallel electrodes orthogonal to each other. Thereby, two sets of parallel electrodes, orthogonal against each other, are arranged on each side of the foil electrode, in this case unstructured, to allow for addressing of individual pixels, defined by intersections of the electrodes. 
         [0019]    With line-at-a-time addressing, the time available for pulse width modulation of the column pulses is limited. In order to achieve a desired resolution the shortest pulses must have a duration in the order of 1 μs. Therefore, it may be advantages to provide the display device with means for modulating the intensity of the light source, as described in PHNL 021414, incorporated herewith by reference. 
         [0020]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0022]      FIG. 1  is a schematic cross section of a display device according to prior art; 
           [0023]      FIG. 2  is an exploded view of a display device according to a first embodiment of the present invention; 
           [0024]      FIG. 3  is a schematic cross section of a display device according to a second embodiment of the present invention; 
           [0025]      FIG. 4  is a diagram illustrating examples of row and column pulses in a display device according to the invention; 
           [0026]      FIGS. 5   a  and  5   b  are diagrams illustrating the switching curves of a pixel of foil display according to the embodiments in  FIGS. 2 and 3 ; 
           [0027]      FIG. 6  is a schematic cross section of a display device according to a further embodiment of the present invention; and 
           [0028]      FIGS. 7   a  and  7   b  illustrate gray scale generation with a device according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIGS. 2 and 3  shows a foil display device  11  according to two different embodiments of the invention. Identical reference numerals have been used for corresponding elements of the device. With reference primarily to  FIG. 2 , which best illustrates the design of the display device, it comprises a light guide (active plate)  12 , and a front plate  14 . The front plate  14  is here a glass plates of suitable thickness, e.g. 2 mm, while the light guide  12  is a thinner glass plate preferably having a thickness range of 0.05-1 mm, in a preferred example 0.1-0.3 mm. 
         [0030]    An electromechanically operable foil  16  is clamped in between the front plate  14  and the light guide  12 . The foil can be of a flexible, light scattering material, such as parylene, with an electrode layer  17  disposed thereon, on the side facing the front plate  14 . Spacers  18 ,  19  are arranged on each side of the foil  16 , to distance it from the front plate  14  and the light guide  12 . 
         [0031]    Two further electrode layers  22 ,  23  are provided in the device  11 , one layer  22  on the light guide  12 , on the side  12   a  facing away from the foil  16 , and one layer  23  on the face  14   a  of the front plate  14  facing the foil  16 . An insulating layer  25  is arranged on the electrode layer  23  on the front plate  14 . 
         [0032]    All electrode layers can be formed by ITO layers disposed on the mentioned surfaces. 
         [0033]    Light from a light source, such as an LED  20 , is coupled into the light guide  12  using a lens system  21 . Preferably, an optical feeding system, such as for example a feeding prism, is used, in order to couple as much light as possible into the light guide. The light is confined inside the glass plate by total internal reflection. Light may be extracted from the guide by bringing the scattering foil  16  into contact with the light guide by means of applying appropriate voltages to the electrode layers  17 ,  22 ,  23 , as will be further described below. 
         [0034]    According to the embodiment shown in  FIG. 2 , the electrode layer  22  on the front plate  14  contains a first set of parallel electrodes  24  (column electrodes), and the electrode layer  17  on the foil  16  contains a second set of parallel electrodes  26  (row electrodes), perpendicular with respect to the first set. The crossings of the electrodes of each set define the pixels of the display. The third electrode layer  23 , on the far side  12   a  of the light guide  12 , is unstructured, i.e. not divided into smaller electrodes. 
         [0035]    Preferably, the spacers  18 ,  19  are arranged perpendicularly with respect to the electrodes  26  on the foil  16 , i.e. in parallel with the electrodes on the front plate  14 . This relaxes the requirements on alignment of the spacers. 
         [0036]    The electrode layer  22  on the light guide  12  is separated from the light guide  12  by a layer  28  having a refractive index such that light coupled in the light guide is reflected by total internal reflection, and does not enter the layer  28 , nor the electrode layer  22 . This reduces absorption. 
         [0037]    According to the embodiment shown in  FIG. 3 , the first set of parallel electrodes  24  (column electrodes) is again arranged on the front plate  14 , while the second set of parallel electrodes  27  here is arranged on the light guide  12 . In this case, the electrode layer  17 ′ on the foil  16  is unstructured, facilitating manufacturing. 
         [0038]    According to the invention, the distance between the foil  16  and the electrode layer  22  is increased compared to a conventional foil display. For example, a thickness of 100 μm instead of 1 μm with an ε r =5 yields an approximately 20 times higher voltage. This means that instead of a voltage of 20 V, 400 V must be applied to the electrode layer  22  in order to generate an attractive force on the foil  16 . 
         [0039]    Addressing of a display device according to the invention is preferably performed sequentially row by row. A timing diagram of addressing pulses is shown in  FIG. 4 , and switching curves for each of the described embodiments is described in  FIGS. 5   a  and  5   b.    
         [0040]      FIG. 4  shows how row pulses  41  are applied to one row electrode  26 ;  27  at a time, in order to select a row. During the duration of this pulse, image data is applied to the columns in the form of column pulses  42  to the column electrodes  24 . Only pixels on a selected row can be activated by a column pulse  42 . 
         [0041]    In the embodiment shown in  FIG. 2 , i.e. where the foil electrode  17  is structured, a constant DC high voltage (order of 1 kV) is applied to the electrode layer  22  on the light guide  12 . At the same time, all row electrodes  26  are held at a raised potential (order of 20 V) while all column electrodes  24  are held at a lowered potential (order of −20 V). The voltage difference (V 1 ) between the foil electrode  17  and the electrode layer  23  attracts the foil  16  towards the front plate  14  (position  51  in  FIG. 5   a ). 
         [0042]    Then, the row electrode  26  of a row to be addressed is set to zero potential during a row pulse, thereby reducing the voltage difference V 1  along this selected row (position  52  in  FIG. 5   a ), and thus increasing the force towards the light guide exerted on this row. The columns  24  of pixels that should emit light are then also set to zero potential, thereby further reducing (to zero) the voltage difference V 1  in such a pixel (position  53  in  FIG. 5   a ), and thus further increasing the force on the pixel. Note that the voltage difference (V 2 ) between the foil electrode  17  and the electrode layer  22  is almost constant, due to the much larger voltage applied to the electrode layer  22 . Thus, the positions  51 ,  52 ,  53  are essentially located on a horizontal line in  FIG. 5   a.    
         [0043]    Pixels in position  53 , where both row and column electrodes are set to zero potential, will not be exposed to any attractive force towards the front plate  14 , and the foil will in these places therefore move towards the light guide  12  as a result of the constant attractive force. Other pixels, in positions  51  or  52 , will all remain attracted towards the front plate, although to a different extent. 
         [0044]    According to this embodiment, a thin light guide plate may therefore be used with relatively low switching voltages on the row and column electrodes. 
         [0045]    In the embodiment shown in  FIG. 4 , the unstructured electrode  17  of the foil  16  is kept as a constant potential. Further, a positive voltage is applied to the column electrodes  24 , to thereby attract the foil to the front plate  14 , and thus keep all pixels in the off-state (position  54  in  FIG. 5   b ). A row is selected by increasing the voltage difference (V 1 ) between the foil electrode layer  17 ′ and the electrode layer  22 ′, by applying a positive voltage pulse  41  to a row electrode  27 . This selection pulse increases the electrostatic force towards the light guide  12 , and brings the pixel to state  55  in  FIG. 5   b.    
         [0046]    The pixels in such a selected row can now be switched ON, i.e. moved to state  56  in  FIG. 5   b , by applying a negative pulse  42  to the column electrodes, thereby increasing the voltage difference (V 2 ) between the foil  16  and the electrode layer  23 . Pixels in unselected rows may switch to an intermediate state (position  57  in  FIG. 5   b ), but no contact is made with the light guide  12 . Thus light is only extracted in a pixel area where a row is selected, and where the column voltage correspond to the on-value. At the end of the row selection pulse  41 , the voltage difference V 1  is again increased, and all pixels are again attracted to the column plate, i.e. switched to the off-state  51 . The following row can now be selected. 
         [0047]    Addressing according to this embodiment will require switching of relatively high row voltages (see above) leading to complicated driver electronics. 
         [0048]    According to a further embodiment, shown in  FIG. 6 , the layer  28  can be realized by arranging the electrode layer  22 ,  22 ′ on a third plate  30 , and separating this third plate  30  from the light guide  12  with additional spacers  32 . In other words, the layer  28  is in this case an air gap. The distance between the far side of the light guide  12   a  and the electrode layer  22 ,  22 ′ should be kept small, preferably in the range 0.1-0.5 μm. 
         [0049]    When line-at-a-time addressing is implemented, and pixels only remain in the on state for the duration of the column pulse  62 , gray scales can be generated by varying the length of the column pulse. This is illustrated in  FIG. 7   a.    
         [0050]    A white pixel corresponds to a column pulse  72  with essentially the same length as the row selection pulse  71 , and by reducing the pulse width it is feasible to make lower gray scales, as illustrated by column pulses  73  and  74 . 
         [0051]    For a VGA display (480 rows) the time available for each row selection pulse is the frame time divided by the numbers of rows, 10 ms/480≈20 μs. In order to generate the required number of different gray levels, it will therefore be necessary to apply column pulses with durations smaller than 1 μs. As shown in  FIG. 7   b , this issue may be circumvented by a simultaneous modulation of the light intensity  75  during each row selection pulse. This technique is described in more detail in PHNL021414, incorporated herewith by reference. The grayscale level is then again adjusted by varying the length of the pulse  72 - 74 . However, the low intensity  75  of the light source  20  during part of the row selection pulse  71  provides the possibility to generate a low gray scale with a longer pulse width. 
         [0052]    An alternative approach for generating gray scales is to modulate the contact area between a pixel and the light guide. In contrast to a conventional foil display, in the display according to the invention the foil is switched by controlling the force towards the light guide, and there is no force towards the front plate present during the switching process. Therefore, modulation of the contact area of a pixel with the light guide can be achieved by simply varying the electrostatic force towards the active plate. 
         [0053]    Many variations of the above described embodiments can of course be realized by the skilled person, without departing from the scope of the invention as defined by the claims. 
         [0054]    For example, the position of the row and the column electrodes can be exchanged. However, in the embodiment shown in  FIG. 4 , with high voltage electrodes  26 ′, it is preferable to use these electrodes  26 ′ as row electrodes, as row selection pulses have a much lower duty cycle. 
         [0055]    Although line-at-a-time addressing has been used in the described embodiments, the inventive display may also be used with a sub-frame addressing scheme, as a bi-stability still exists. In that case, a light guide  12  of conventional thickness may be used. 
         [0056]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.