Methods for driving electro-optic displays, and apparatus for use therein

Abstract

A method for addressing a bistable electro-optic display having at least one pixel comprises applying an addressing pulse to drive the pixel to a first optical state; leaving the pixel undriven for a period of time, thereby permitting the pixel to assume a second optical state different from the first optical state; and applying to the pixel a refresh pulse which substantially restores the pixel to the first optical state, the refresh pulse being short relative to the addressing pulse.

1 . A method for addressing a bistable electro-optic medium which comprises applying to the medium an alternating current pulse having a direct current offset.  
   
   
       2 . A method according to  claim 1  wherein the alternating current pulse has substantially the form of an alternating square wave.  
   
   
       3 . A method according to  claim 1  wherein the application of the alternating current pulse having a direct current offset to the medium is continued for a period sufficient to cause the medium to assume a substantially constant optical state.  
   
   
       4 . A method according to  claim 1  wherein there is first applied to the medium an alternating current pulse having a first direct current offset, thereby causing the medium to assume a first optical state, and thereafter there is applied to the medium an alternating current pulse having a second direct current offset different from the first offset, thereby causing the medium to assume a second optical state different from the first optical state.  
   
   
       5 . A method according to  claim 1  wherein the electro-optic medium is an electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field.  
   
   
       6 . A method according to  claim 5  wherein the plurality of charged particles and the fluid are confined within a plurality of capsules.  
   
   
       7 . A method according to  claim 5  wherein the plurality of charged particles and the fluid are retained within a plurality of cavities formed within a carrier medium.  
   
   
       8 . A method according to  claim 5  wherein the plurality of charged particles and the fluid are present as a plurality of droplets surrounded by a continuous phase of a polymeric material.  
   
   
       9 . A method according to  claim 1  wherein the electro-optic medium has a first display state which is substantially opaque and a second display state which is light-transmissive.  
   
   
       10 . A method for addressing a bistable electro-optic medium which comprises applying to the medium an alternating current pulse, and varying at least one of the duty cycle and the frequency of the pulse to change the optical state of the electro-optic medium following the alternating current pulse.  
   
   
       11 . A method according to  claim 10  wherein the alternating current pulse has substantially the form of an alternating square wave.  
   
   
       12 . A method according to  claim 1  wherein the application of the alternating current pulse to the medium is continued for a period sufficient to cause the medium to assume a substantially constant optical state.  
   
   
       13 . A method according to  claim 10  wherein the duty cycle but not the frequency of the alternating current pulse is varied.  
   
   
       14 . A method according to  claim 13  wherein there is first applied to the medium an alternating current pulse having a first duty cycle offset, thereby causing the medium to assume a first optical state, and thereafter there is applied to the medium an alternating current pulse having a second duty cycle offset different from the first duty cycle, thereby causing the medium to assume a second optical state different from the first optical state.  
   
   
       15 . A method according to  claim 10  wherein the frequency but not the duty cycle of the alternating current pulse is varied.  
   
   
       16 . A method according to  claim 10  wherein the electro-optic medium is an electrophoretic medium in which a plurality of charged particles move through a fluid under the influence of an electric field.  
   
   
       17 . A method according to  claim 16  wherein the plurality of charged particles and the fluid are confined within a plurality of capsules.  
   
   
       18 . A method according to  claim 16  wherein the plurality of charged particles and the fluid are retained within a plurality of cavities formed within a carrier medium.  
   
   
       19 . A method according to  claim 16  wherein the plurality of charged particles and the fluid are present as a plurality of droplets surrounded by a continuous phase of a polymeric material.  
   
   
       20 . A method according to  claim 10  wherein the electro-optic medium has a first display state which is substantially opaque and a second display state which is light-transmissive.  
   
   
       21 . A method for addressing a bistable electro-optic medium capable of displaying at least three optical states including two extreme optical states and at least one gray state intermediate the two extreme optical states, which method comprises driving the medium from one extreme optical state to a final gray state by first applying to the medium a direct current pulse which drives the medium from the one extreme optical state to an intermediate gray state different from the final gray state, and thereafter applying to the medium an alternating current pulse which drives the medium from the intermediate gray state to the final gray state.  
   
   
       22 . A method according to  claim 21  which is carried out using drive circuitry capable of applying only potential differences of +V, 0 and −V across the electro-optic medium, where V is an arbitrary drive voltage.  
   
   
       23 . A method for addressing a bistable electro-optic display having a plurality of pixels arranged in a plurality of rows and a plurality of columns, a plurality of row electrodes each associated with one of the plurality of rows, a plurality of column electrodes each associated with one of the plurality of columns, and drive means arranged to select each of the row electrodes in turn and to apply to the column electrodes during the selection of any given row electrode voltages chosen so as to address the pixels in the row associated with the selected row electrode and write one row of a desired image on to the display, the method comprising: 
 writing a first image to the display;    receiving data representing a second image to be written to the display;    comparing the first and second images and dividing the rows of the display into a first set, in which at least one pixel of the row differs between the first and second images, and a second set, in which no pixel of the row differs between the first and second images; and    writing the second image by sequentially selecting only the row electrodes associated with the first set of rows, and applying voltages to the column electrodes to write only the first set of rows, thereby forming the second image to the display.    
   
   
       24 . An electro-optic display having a plurality of pixels, at least one of the pixels comprising a plurality of sub-pixels differing from each other in area, the display comprising drive means arranged to change the optical state of the sub-pixels independently of one another.  
   
   
       25 . An electro-optic display according to  claim 24  wherein at least two of the sub-pixels differ in area by substantially a factor of two.