Driving methods for electrophoretic displays

Abstract

This application is directed to driving methods for electrophoretic displays. The driving methods comprise grey level waveforms which greatly enhance the pictorial quality of images displayed. The driving method comprises: (a) applying waveform to drive each pixel from its initial color state to the full first color then to a color state of a desired level; or (b) applying waveform to drive each pixel from its initial color state to the full second color then to a color state of a desired level.

Description

TECHNICAL FIELD

There is a strong desire to use microcup-based electrophoretic display front planes for e-books because they are easy to read (e.g., acceptable white levels, wide range of viewing angles, reasonable contrast, viewability in reflected light, paper-like quality, etc) and require low power consumption. However, most of the driving methods developed to date are applicable to only binary black and white images. In order to achieve higher pictorial quality, grey level images are needed. The present invention presents driving methods for that purpose.

SUMMARY OF THE INVENTION

The first aspect of the invention is directed to a driving method for a display device having a binary color system comprising a first color and a second color, which method comprises

• • a) applying waveform to drive each pixel from its initial color state to the full first color then to a color state of a desired level; or
  • b) applying waveform to drive each pixel from its initial color state to the full second color then to a color state of a desired level.

In one embodiment of the first aspect of the invention, the first color and second colors are two contrasting colors. In one embodiment, the two contrasting colors are black and white. In one embodiment, mono-polar driving is used which comprises applying a waveform to a common electrode. In one embodiment, bi-polar driving is used which does not comprise applying a waveform to a common electrode.

The second aspect of the invention is directed to a driving method for a display device having a binary color system comprising a first color and a second color, which method comprises

• • a) applying waveform to drive each pixel from its initial color state to the full first color state, then to the full second color state and finally to a color state of a desired level; or
  • b) applying waveform to drive each pixel from its initial color state to the full second color state, then to the full first color state and finally to a color state of a desired level.

In one embodiment of the second aspect of the invention, the first color and second colors are two contrasting colors. In one embodiment, the two contrasting colors are black and white. In one embodiment, mono-polar driving is used which comprises applying a waveform to a common electrode. In one embodiment, bi-polar driving is used which does not comprise applying a waveform to a common electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a typical electrophoretic display device.

FIG. 2 illustrates an example of an electrophoretic display having a binary color system.

FIGS. 3 a and 3b show two mono-polar driving waveforms.

FIGS. 4 a and 4b show alternative mono-polar driving waveforms.

FIGS. 5 a and 5b show two bi-polar driving waveforms.

FIG. 6 is an example of waveforms of the present invention.

FIG. 7 shows repeatability of the reflectance achieved by the example waveforms.

FIG. 8 demonstrates the bistability of images achieved by the example waveforms.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an electrophoretic display (100) which may be driven by any of the driving methods presented herein. In FIG. 1, the electrophoretic display cells 10a, 10b, 10c, on the front viewing side indicated with a graphic eye, are provided with a common electrode 11 (which is usually transparent and therefore on the viewing side). On the opposing side (i.e., the rear side) of the electrophoretic display cells 10a, 10b and 10c, a substrate (12) includes discrete pixel electrodes 12a, 12b and 12c, respectively. Each of the pixel electrodes 12a, 12b and 12c defines an individual pixel of the electrophoretic display. Although the pixel electrodes are shown aligned with the display cells, in practice, a plurality of display cells (as a pixel) may be associated with one discrete pixel electrode.

It is also noted that the display device may be viewed from the rear side when the substrate 12 and the pixel electrodes are transparent.

An electrophoretic fluid 13 is filled in each of the electrophoretic display cells 10a, 10b and 10c. Each of the electrophoretic display cells 10a, 10b and 10c is surrounded by display cell walls 14.

The movement of the charged particles 15 in a display cell is determined by the voltage potential difference applied to the common electrode and the pixel electrode associated with the display cell in which the charged particles are filled.

As an example, the charged particles 15 may be positively charged so that they will be drawn to a pixel electrode or the common electrode, whichever is at an opposite voltage potential from that of charged particles. If the same polarity is applied to the pixel electrode and the common electrode in a display cell, the positively charged pigment particles will then be drawn to the electrode which has a lower voltage potential.

In this application, the term “driving voltage” is used to refer to the voltage potential difference experienced by the charged particles in the area of a pixel. The driving voltage is the potential difference between the voltage applied to the common electrode and the voltage applied to the pixel electrode. As an example, in a single particle system, positively charged white particles are dispersed in a black solvent. When zero voltage is applied to a common electrode and a voltage of +15V is applied to a pixel electrode, the “driving voltage” for the charged pigment particles in the area of the pixel would be +15V. In this case, the driving voltage would move the positively charged white particles to be near or at the common electrode and as a result, the white color is seen through the common electrode (i.e., the viewing side). Alternatively, when zero voltage is applied to a common electrode and a voltage of −15V is applied to a pixel electrode, the driving voltage in this case would be −15V and under such −15V driving voltage, the positively charged white particles would move to be at or near the pixel electrode, causing the color of the solvent (black) to be seen at the viewing side.

In another embodiment, the charged pigment particles 15 may be negatively charged.

In a further embodiment, the electrophoretic display fluid could also have a transparent or lightly colored solvent or solvent mixture and charged particles of two different colors carrying opposite charges, and/or having differing electro-kinetic properties. For example, there may be white pigment particles which are positively charged and black pigment particles which are negatively charged and the two types of pigment particles are dispersed in a clear solvent or solvent mixture.

The charged particles 15 may be white. Also, as would be apparent to a person having ordinary skill in the art, the charged particles may be dark in color and are dispersed in an electrophoretic fluid 13 that is light in color to provide sufficient contrast to be visually discernable.

The term “display cell” is intended to refer to a micro-container which is individually filled with a display fluid. Examples of “display cell” include, but are not limited to, microcups, microcapsules, micro-channels, other partition-typed display cells and equivalents thereof. In the microcup type, the electrophoretic display cells 10a, 10b, 10c may be sealed with a top sealing layer. There may also be an adhesive layer between the electrophoretic display cells 10a, 10b, 10c and the common electrode 11.

FIG. 2 is an example of a binary color system in which white particles are dispersed in a black-colored solvent.

In FIG. 2A, while the white particles are at the viewing side, the white color is seen.

In FIG. 2B, while the white particles are at the bottom of the display cell, the black color is seen.

In FIG. 2C, the white particles are scattered between the top and bottom of the display cell, an intermediate color is seen. In practice, the particles spread throughout the depth of the cell or are distributed with some at the top and some at the bottom. In this example, the color seen would be grey (i.e., an intermediate color).

While black and white colors are used in the application for illustration purpose, it is noted that the two colors can be any colors as long as they show sufficient visual contrast. Therefore the two colors in a binary color system may also be referred to as a first color and a second color.

The intermediate color is a color between the first and second colors. The intermediate color has different degrees of intensity, on a scale between two extremes, i.e., the first and second colors. Using the grey color as an example, it may have a grey scale of 8, 16, 64, 256 or more. In a grey scale of 8, grey level 0 may be a white color and grey level 7 may be a black color. Grey levels 1-6 are grey colors ranging from light to dark.

FIGS. 3 a and 3b show two driving waveforms WG and KG, respectively. As shown the waveforms have two driving phases (I and II). Each driving phase has a driving time of equal length, T, which is sufficiently long to drive a pixel to a full white or a full black state, regardless of the previous color state.

For brevity, in both FIGS. 3a and 3b, each driving phase is shown to have the same length of T. However, in practice, the time taken to drive to the full color state of one color may not be the same as the time taken to drive to the full color state of another color.

For illustration purpose, FIGS. 3a and 3b represent an electrophoretic fluid comprising positively charged white pigment particles dispersed in a black solvent.

In FIG. 3a, the common electrode is applied a voltage of −V and +V during Phase I and II, respectively. For the WG waveform, during Phase I, the common electrode is applied a voltage of −V and the pixel electrode is applied a voltage of +V, resulting a driving voltage of +2V and as a result, the positively charged white pigment particles move to be near or at the common electrode, causing the pixel to be seen in a white color. During Phase II, a voltage of +V is applied to the common electrode and a voltage of −V is applied to the pixel electrode for a driving time duration of t₁. If the time duration t₁ is 0, the pixel would remain in the white state. If the time duration t₁ is T, the pixel would be driven to the full black state. If the time duration t₁ is between 0 and T, the pixel would be in a grey state and the longer t₁ is, the darker the grey color. After t₁ in Phase II, the driving voltage for the pixel is shown to be 0V and as a result, the color of the pixel would remain in the same color state as that at the end of t₁ (i.e., white, black or grey). Therefore, the WG waveform is capable of driving a pixel from its initial color state to a full white (W) color state (in Phase I) and then to a black (K), white (W) or grey (G) state (in Phase II).

For the KG waveform in FIG. 3b, in Phase I, the common electrode is applied a voltage of +V while the pixel electrode is applied a voltage of −V, resulting in a −2V driving voltage, which drives the pixel to the black state. In Phase II, the common electrode is applied a voltage of −V and the pixel electrode is applied a voltage of +V for a driving time duration of t₂. If the time duration t₂ is 0, the pixel would remain in the black state. If the time duration t₂ is T, the pixel would be driven to the full white state. If the time duration t₂ is between 0 and T, the pixel would be in a grey state and the longer t₁ is, the lighter the grey color. After t₂ in Phase II, the driving voltage is 0V, thus allowing the pixel to remain in the same color state as that at the end of t₂. Therefore, the KG waveform is capable of driving a pixel from its initial color state, to a full black (K) state (in Phase I) and then to a black (K), white (W) or grey (G) state (in Phase II).

The term “full white” or “full black” state is intended to refer to a state where the white or black color has the highest intensity possible of that color for a particular display device. Likewise, a “full first color” or a “full second color” refers to a first or second color state at its highest color intensity possible.

Either one of the two waveforms (WG and KG) can be used to generate a grey level image as long as the lengths (t₁ or t₂) of the grey pulses are correctly chosen for the grey levels to be generated.

The present invention is directed to a driving method for a display device having a binary color system comprising a first color and a second color, which method comprises

a) applying waveform to drive each of pixels from its initial color state to the full first color state then to a color state of a desired level, or

b) applying waveform to drive each of pixels from its initial color state to the full second color state then to a color state of a desired level.

The term “initial color state”, throughout this application, is intended to refer to the color state before a waveform is applied, which can be the first color state, the second color state or an intermediate color state of any level.

In the WG waveform as shown in FIG. 3a, each of the pixels is driven from its initial color state to the full white color state and then to a color state of a desired level. In other words, some pixels are driven from their initial color states to the full white state and then to black, some from their initial color states to the full white state and remain white, some from their initial color states to the full white state and then to grey level 1, some from their initial color state to the full white state and then to grey level 2, and so on, depending on the images to be displayed.

In the KG waveform as shown in FIG. 3b, each of the pixels is driven from its initial color state to the full black color state and then to a color state of a desired level. In other words, some pixels are driven from their initial color states to the full black state and then to white, some from their initial color states to the full black state and remain black, some from their initial color states to the full black state and then to grey level 1, some from their initial color states to the full black state and then to grey level 2, and so on, depending on the images to be displayed.

The term “a color state of a desired level” is intended to refer to either the first color state, the second color state or an intermediate color state between the first and second color states.

FIGS. 4 a and 4b show alternative mono-polar driving waveforms. As shown, there are two driving waveforms, WKG waveform and KWG waveform.

The WKG waveform drive each of pixels from its initial color state, to the full white state, then to the full black state and finally to a color state of a desired level. The KWG waveform, on the other hand, drives each of pixels from its initial color state, to the full black state, then to the full white state and finally to a color state of a desired level.

The driving method as demonstrated in FIGS. 4a and 4b may be generalized as follows:

A driving method for a display device having a binary color system comprising a first color and a second color, which method comprises

a) applying waveform to drive each of pixels from its initial color state to the full first color state, then to the full second color state and finally to a color state of a desired level; or

b) applying waveform to drive each of pixels from its initial color state to the full second color state, then to the full first color state and finally to a color state of a desired level.

The bi-polar approach requires no modulation of the common electrode while the mono-polar approach requires modulation of the common electrode.

The present method may also be run on a bi-polar driving scheme. The two bi-polar waveforms WG and KG are shown in FIG. 5a and FIG. 5b, respectively. The bi-polar WG and KG waveforms can run independently without being restricted to the shared common electrode.

In practice, the common electrode and the pixel electrodes are separately connected to two individual circuits and the two circuits in turn are connected to a display controller. The display controller issues signals to the circuits to apply appropriate voltages to the common and pixel electrodes respectively. More specifically, the display controller, based on the images to be displayed, selects appropriate waveforms and then issues signals, frame by frame, to the circuits to execute the waveforms by applying appropriate voltages to the common and pixel electrodes. The term “frame” represents timing resolution of a waveform.

The pixel electrodes may be a TFT (thin film transistor) backplane.

EXAMPLES

FIG. 6 represents a driving method of the present invention which comprises four driving phases (T1, T2, T3 and T4) of the KWG waveform. In this example, the durations for T1, T2, T3 and T4 are 500 msec, 600 msec, 180 msec and 320 msec, respectively. The top waveform represents the voltages applied to the common electrode and the three waveforms below (I, II and III) represent how pixels may be driven to the black state, a grey state and the white state, respectively.

The voltage for the common electrode is set at +V in driving frame T1, −V in T2 and +V in T3 and T4.

In order to drive a pixel to the black state (waveform I), the voltage for the corresponding discrete electrode is set at −V in T1, +V in T2 and −V in T3 and T4.

In order to drive a pixel to a grey level (waveform II), the voltage for the corresponding discrete electrode is set at −V in T1, +V in T2, −V in T3 and +V in T4.

In order to drive a pixel to the white state (waveform III), the voltage for the corresponding discrete electrode is set at −V in T1 and +V in T2, T3 and T4.

FIG. 7 shows the consistency of reflectance levels achieved by the driving method of the example. The notations “W”, “B”, “G”, and “X” refers to the white state, black state, a grey level and any color state, respectively.

FIG. 8 demonstrates the bistability of the images achieved.

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

1. A driving method for a display device comprising a plurality of pixels wherein said display device has a binary color system comprising two contrasting colors of a first color and a second color, the method comprising: a) applying a waveform to drive each of said pixels from its initial color state to a full first color state for a length of time then directly from the full first color state to a full second color state for the same length of time, and finally directly to an intermediate color state between the full first color state and the full second color state;wherein (i) the length of time applied to drive the pixel from the initial color state to the full first color state is equal to the length of time applied to drive the pixel from the full first color state to the full second color state regardless of the initial color state, (ii) the length of time is sufficient to drive the pixel from the full first color state to the full second color state and from the full second color state to the full first color state, and (iii) the full first color state and the full second color state are the first color and the second color respectively at the highest color intensity possible.

2. The method of claim 1, wherein the two contrasting colors are black and white.

3. The method of claim 1, wherein the waveform is mono-polar driving waveform.

4. The method of claim 1, wherein the waveform is bi-polar driving waveform.