Note: Descriptions are shown in the official language in which they were submitted.
DRIVERS PROVIDING DC-BALANCED REFRESH SEQUENCES FOR
COLOR ELECTROPHORETIC DISPLAYS
[Para 11
BACKGROUND OF INVENTION
[Para 21 This invention relates to methods for driving electro-optic displays,
especially but not
exclusively electrophoretic displays capable of rendering more than two colors
using a single layer
of electrophoretic material comprising a plurality of colored particles, for
example white, cyan,
yellow, and magenta particles, wherein two particles are positively-charged
and two particles are
negatively-charged, and one positively-charged particle and one negatively-
charged particle has a
thick polymer shell.
[Para 3] The term color as used herein includes black and white. White
particles are often of the
light scattering type.
[Para 41 The term gray state is used herein in its conventional meaning in the
imaging art to refer
to a state intermediate two extreme optical states of a pixel, and does not
necessarily imply a black-
white transition between these two extreme states. For example, several of the
E Ink patents and
published applications referred to below describe electrophoretic displays in
which the extreme
states are white and deep blue, so that an intermediate gray state would
actually be pale blue. Indeed,
as already mentioned, the change in optical state may not be a color change at
all. The terms black
and white may be used hereinafter to refer to the two extreme optical states
of a display, and should
be understood as normally including extreme optical states which are not
strictly black and white,
for example the aforementioned white and dark blue states.
[Para 5] The terms bistable and bistability are used herein in their
conventional meaning in the
art to refer to displays comprising display elements having first and second
display states differing
in at least one optical property, and such that after any given element has
been driven, by means of
an addressing pulse of finite duration, to assume either its first or second
display state, after the
addressing pulse has terminated, that state will persist for at least several
times, for example at least
four times, the minimum duration of the addressing pulse required to change
the state of the display
element. It is shown in U.S. Patent No. 7,170,670 that some particle-based
electrophoretic displays
capable of gray scale are stable not only in their extreme
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Date Recue/Date Received 2022-06-22
black and white states but also in their intermediate gray states, and the
same is true of some
other types of electro-optic displays. This type of display is properly called
multi-stable rather
than bistable, although for convenience the term bistable may be used herein
to cover both
bistable and multi-stable displays.
[Para 6] The term impulse, when used to refer to driving an electrophoretic
display, is used
herein to refer to the integral of the applied voltage with respect to time
during the period in
which the display is driven.
[Para 71 A particle that absorbs, scatters, or reflects light, either in a
broad band or at selected
wavelengths, is referred to herein as a colored or pigment particle. Various
materials other than
pigments (in the strict sense of that term as meaning insoluble colored
materials) that absorb
or reflect light, such as dyes or photonic crystals, etc., may also be used in
the electrophoretic
media and displays of the present invention.
[Para 8] Particle-based electrophoretic displays have been the subject of
intense research and
development for a number of years. In such displays, a plurality of charged
particles
(sometimes referred to as pigment particles) move through a fluid under the
influence of an
electric field. Electrophoretic displays can have attributes of good
brightness and contrast, wide
viewing angles, state bistability, and low power consumption when compared
with liquid
crystal displays. Nevertheless, problems with the long-term image quality of
these displays
have prevented their widespread usage. For example, particles that make up
electrophoretic
displays tend to settle, resulting in inadequate service-life for these
displays.
[Para 91 As noted above, electrophoretic media require the presence of a
fluid. In most prior
art electrophoretic media, this fluid is a liquid, but electrophoretic media
can be produced using
gaseous fluids; see, for example, Kitamtua, T., et al., Electrical toner
movement for electronic
paper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al.,
Toner display
using insttlative particles charged triboelectrically, IDW Japan, 2001, Paper
AMD4-4). Sec
also U.S. Patents Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic
media appear
to be susceptible to the same types of problems due to particle settling as
liquid-based
electrophoretic media, when the media are used in an orientation which permits
such settling,
for example in a sign where the medium is disposed in a vertical plane.
Indeed, particle settling
appears to be a more serious problem in gas-based electrophoretic media than
in liquid-based
ones, since the lower viscosity of gaseous suspending fluids as compared with
liquid ones
allows more rapid settling of the electrophoretic particles.
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Date Recue/Date Received 2022-06-22
[Para 101 Numerous patents and applications assigned to or in the names of the
Massachusetts
Institute of Technology (MIT) and E Ink Corporation describe various
technologies used in
encapsulated electrophoretic and other electro-optic media. Such encapsulated
media comprise
numerous small capsules, each of which itself comprises an internal phase
containing
electrophoretically-mobile particles in a fluid medium, and a capsule wall
surrounding the
internal phase. Typically, the capsules are themselves held within a polymeric
binder to form
a coherent layer positioned between two electrodes. The technologies described
in these patents
and applications include:
(a) Electrophoretic particles, fluids and fluid additives; see for
example U.S. Patents Nos. 7,002,728 and 7,679,814;
(b) Capsules, binders and encapsulation processes; see for example
U.S. Patents Nos. 6,922,276 and 7,411,719;
(c) Microcell structures, wall materials, and methods of forming
microcells; see for example United States Patents Nos. 7,072,095 and
9,279,906;
(d) Methods for filling and sealing microcells; see for example
United States Patents Nos. 7,144,942 and 7,715,088;
(e) Films and sub-assemblies containing electro-optic materials; see
for example U.S. Patents Nos. 6,982,178 and 7,839,564;
(f) Backplanes, adhesive layers and other auxiliary layers and
methods used in displays; see for example U.S. Patents Nos. 7,116,318 and
7,535,624;
(g) Color formation color adjustment; see for example U.S. Patents
Nos. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875; 6,914,714;
6,972,893; 7,038,656; 7,038,670; 7,046,228; 7,052,571; 7,075,502***;
7,167,155; 7,385,751; 7,492,505; 7,667,684; 7,684,108; 7,791,789; 7,800,813;
7,821,702; 7,839,564***; 7,910,175; 7,952,790; 7,956,841; 7,982,941;
8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076; 8,363,299; 8,422,116;
8,441,714; 8,441,716; 8,466,852; 8,503,063; 8,576,470; 8,576,475; 8,593,721;
8,605,354; 8,649,084; 8,670,174; 8,704,756; 8,717,664; 8,786,935; 8,797,634;
8,810,899; 8,830,559; 8,873,129; 8,902,153; 8,902,491; 8,917,439; 8,964,282;
9,013,783; 9,116,412; 9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646;
9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511; 9,341,916; 9,360,733;
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Date Recue/Date Received 2022-06-22
9,361,836; 9,383,623; and 9,423,666; and U.S. Patent Applications Publication
Nos. 2008/0043318; 2008/0048970; 2009/0225398; 2010/0156780;
2011/0043543; 2012/0326957; 2013/0242378; 2013/0278995; 2014/0055840;
2014/0078576; 2014/0340430; 2014/0340736; 2014/0362213; 2015/0103394;
2015/0118390; 2015/0124345; 2015/0198858; 2015/0234250; 2015/0268531;
2015/0301246; 2016/0011484; 2016/0026062; 2016/0048054; 2016/0116816;
2016/0116818; and 2016/0140909;
(h)
Methods for driving displays; see for example U.S. Patents Nos.
5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970;
6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662;
7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,514; 7,259,744;
7,304,787; 7,312,794; 7,327,511; 7,408,699; 7,453,445; 7,492,339; 7,528,822;
7,545,358; 7,583,251; 7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606;
7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169; 7,859,742; 7,952,557;
7,956,841; 7,982,479; 7,999,787; 8,077,141; 8,125,501; 8,139,050; 8,174,490;
8,243,013; 8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784; 8,373,649;
8,384,658; 8,456,414; 8,462,102; 8,514,168; 8,537,105; 8,558,783; 8,558,785;
8,558,786; 8,558,855; 8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595;
8,665,206; 8,681,191; 8,730,153; 8,810,525; 8,928,562; 8,928,641; 8,976,444;
9,013,394; 9,019,197; 9,019,198; 9,019,318; 9,082,352; 9,171,508; 9,218,773;
9,224,338; 9,224,342; 9,224,344; 9,230,492; 9,251,736; 9,262,973; 9,269,311;
9,299,294; 9,373,289; 9,390,066; 9,390,661; and 9,412,314; and U.S. Patent
Applications Publication Nos. 2003/0102858; 2004/0246562; 2005/0253777;
2007/0091418; 2007/0103427; 2007/0176912; 2008/0024429; 2008/0024482;
2008/0136774; 2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568;
2009/0322721; 2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561;
2010/0283804; 2011/0063314; 2011/0175875; 2011/0193840; 2011/0193841;
2011/0199671; 2011/0221740; 2012/0001957; 2012/0098740; 2013/0063333;
2013/0194250; 2013/0249782; 2013/0321278; 2014/0009817; 2014/0085355;
2014/0204012; 2014/0218277; 2014/0240210; 2014/0240373; 2014/0253425;
2014/0292830; 2014/0293398; 2014/0333685; 2014/0340734; 2015/0070744;
2015/0097877; 2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257;
2015/0262255; 2015/0262551; 2016/0071465; 2016/0078820; 2016/0093253;
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Date Recue/Date Received 2022-06-22
2016/0140910; and 2016/0180777 (these patents and applications may
hereinafter be referred to as the MEDEOD (MEthods for Driving Electro-optic
Displays) applications);
(1) Applications of displays; see for example U.S.
Patents Nos.
7,312,784 and 8,009,348; and
(j) Non-electrophoretic displays, as described in U.S.
Patents Nos.
6,241,921; and U.S. Patent Applications Publication Nos. 2015/0277160; and
U.S. Patent Application Publications Nos. 2015/0005720 and 2016/0012710.
[Para 111 Many of the aforementioned patents and applications recognize that
the walls
surrounding the discrete microcapsules in an encapsulated electrophoretic
medium could be
replaced by a continuous phase, thus producing a so-called polymer-dispersed
electrophoretic
display, in which the electrophoretic medium comprises a plurality of discrete
droplets of an
electrophoretic fluid and a continuous phase of a polymeric material, and that
the discrete
droplets of electrophoretic fluid within such a polymer-dispersed
electrophoretic display may
be regarded as capsules or microcapsules even though no discrete capsule
membrane is
associated with each individual droplet; see for example, U.S. Patent No.
6,866,760.
Accordingly, for purposes of the present application, such polymer-dispersed
electrophoretic
media are regarded as sub-species of encapsulated electrophoretic media.
[Para 121 A related type of electrophoretic display is a so-called microcell
electrophoretic
display. In a microcell electrophoretic display, the charged particles and the
fluid are not
encapsulated within microcapsules but instead are retained within a plurality
of cavities formed
within a carrier medium, typically a polymeric film. See, for example, U.S.
Patents Nos.
6,672,921 and 6,788,449, both assigned to SiPix Imaging, Inc.
[Pam 131 Although electrophoretic media are often opaque (since, for example,
in many
electrophoretic media, the particles substantially block transmission of
visible light through the
display) and operate in a reflective mode, many electrophoretic displays can
be made to operate
in a so-called shutter mode in which one display state is substantially opaque
and one is light-
transmissive. See, for example, U.S. Patents Nos. 5,872,552; 6,130,774;
6,144,361; 6,172,798;
6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are
similar to
electrophoretic displays but rely upon variations in electric field strength,
can operate in a
similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optic
displays may also be
capable of operating in shutter mode. Electro-optic media operating in shutter
mode can be
used in multi-layer structures for full color displays; in such structures, at
least one layer
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Date Recue/Date Received 2022-06-22
adjacent the viewing surface of the display operates in shutter mode to expose
or conceal a
second layer more distant from the viewing surface.
[Para 141 An encapsulated electrophoretic display typically does not suffer
from the
clustering and settling failure mode of traditional electrophoretic devices
and provides further
advantages, such as the ability to print or coat the display on a wide variety
of flexible and rigid
substrates. (Use of the word printing is intended to include all forms of
printing and coating,
including, but without limitation: pre-metered coatings such as patch die
coating, slot or
extrusion coating, slide or cascade coating, curtain coating; roll coating
such as knife over roll
coating, forward and reverse roll coating; gravure coating; dip coating; spray
coating; meniscus
coating; spin coating; brush coating; air knife coating; silk screen printing
processes;
electrostatic printing processes; thermal printing processes; ink jet printing
processes;
electrophoretic deposition (See U.S. Patent No. 7,339,715); and other similar
techniques.)
Thus, the resulting display can be flexible. Further, because the display
medium can be printed
(using a variety of methods), the display itself can be made inexpensively.
[Para 151 As indicated above most simple prior art electrophoretic media
essentially display
only two colors. Such electrophoretic media either use a single type of
electrophoretic particle
having a first color in a colored fluid having a second, different color (in
which case, the first
color is displayed when the particles lie adjacent the viewing surface of the
display and the
second color is displayed when the particles are spaced from the viewing
surface), or first and
second types of electrophoretic particles having differing first and second
colors in an
uncolored fluid (in which case, the first color is displayed when the first
type of particles lie
adjacent the viewing surface of the display and the second color is displayed
when the second
type of particles lie adjacent the viewing surface). Typically the two colors
are black and white.
If a full color display is desired, a color filter array may be deposited over
the viewing surface
of the monochrome (black and white) display. Displays with color filter arrays
rely on area
sharing and color blending to create color stimuli. The available display area
is shared between
three or four primary colors such as red/green/blue (RGB) or
red/green/blue/white (RGBW),
and the filters can be arranged in one-dimensional (stripe) or two-dimensional
(2x2) repeat
patterns. Other choices of primary colors or more than three primaries are
also known in the
art. The three (in the case of RGB displays) or four (in the case of RGBW
displays) sub-pixels
are chosen small enough so that at the intended viewing distance they visually
blend together
to a single pixel with a uniform color stimulus ('color blending'). The
inherent disadvantage
of area sharing is that the colorants are always present, and colors can only
be modulated by
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Date Recue/Date Received 2022-06-22
switching the corresponding pixels of the underlying monochrome display to
white or black
(switching the corresponding primary colors on or off). For example, in an
ideal RGBW
display, each of the red, green, blue and white primaries occupy one fourth of
the display area
(one sub-pixel out of four), with the white sub-pixel being as bright as the
underlying
monochrome display white, and each of the colored sub-pixels being no lighter
than one third
of the monochrome display white. The brightness of the white color shown by
the display as a
whole cannot be more than one half of the brightness of the white sub-pixel
(white areas of the
display are produced by displaying the one white sub-pixel out of each four,
plus each colored
sub-pixel in its colored form being equivalent to one third of a white sub-
pixel, so the three
colored sub-pixels combined contribute no more than the one white sub-pixel).
The brightness
and saturation of colors is lowered by area-sharing with color pixels switched
to black. Area
sharing is especially problematic when mixing yellow because it is lighter
than any other color
of equal brightness, and saturated yellow is almost as bright as white.
Switching the blue pixels
(one fourth of the display area) to black makes the yellow too dark.
[Para 161 Multilayer, stacked electrophoretic displays are known in the art;
see, for example,
J. Heikenfeld, P. Drzaic, J-S Yeo and T. Koch, Journal of the SID, 19(2),
2011, pp. 129-156.
In such displays, ambient light passes through images in each of the three
subtractive primary
colors, in precise analogy with conventional color printing. U.S. Patent No.
6,727,873 describes
a stacked electrophoretic display in which three layers of switchable cells
are placed over a
reflective background. Similar displays are known in which colored particles
are moved
laterally (see International Application No. WO 2008/065605) or, using a
combination of
vertical and lateral motion, sequestered into microcells. In both cases, each
layer is provided
with electrodes that serve to concentrate or disperse the colored particles on
a pixel-by-pixel
basis, so that each of the three layers requires a layer of thin-film
transistors (Chi's) (two of
the three layers of TF'f's must be substantially transparent) and a light-
transmissive counter-
electrode. Such a complex arrangement of electrodes is costly to manufacture,
and in the
present state of the art it is difficult to provide an adequately transparent
plane of pixel
electrodes, especially as the white state of the display must be viewed
through several layers
of electrodes. Multi-layer displays also suffer from parallax problems as the
thickness of the
display stack approaches or exceeds the pixel size.
[Para 17] U.S. Applications Publication Nos. 2012/0008188 and 2012/0134009
describe
multicolor electrophoretic displays having a single back plane comprising
independently
addressable pixel electrodes and a common, light-transmissive front electrode.
Between the
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Date Recue/Date Received 2022-06-22
back plane and the front electrode is disposed a plurality of electrophoretic
layers. Displays
described in these applications are capable of rendering any of the primary
colors (red, green,
blue, cyan, magenta, yellow, white and black) at any pixel location. However,
there are
disadvantages to the use of multiple electrophoretic layers located between a
single set of
addressing electrodes. The electric field experienced by the particles in a
particular layer is
lower than would be the case for a single electrophoretic layer addressed with
the same voltage.
In addition, optical losses in an electrophoretic layer closest to the viewing
surface (for
example, caused by light scattering or unwanted absorption) may affect the
appearance of
images formed in underlying electrophoretic layers.
[Para 181 Attempts have been made to provide full-color electrophoretic
displays using a
single electrophoretic layer. For example, U.S. Patent Application Publication
No.
2013/0208338 describes a color display comprising an electrophoretic fluid
which comprises
one or two types of pigment particles dispersed in a clear and colorless or
colored solvent, the
electrophoretic fluid being disposed between a common electrode and a
plurality of pixel or
driving electrodes. The driving electrodes are arranged to expose a background
layer. U.S.
Patent Application Publication No. 2014/0177031 describes a method for driving
a display cell
filled with an electrophoretic fluid comprising two types of charged particles
carrying opposite
charge polarities and of two contrast colors. The two types of pigment
particles are dispersed
in a colored solvent or in a solvent with non-charged or slightly charged
colored particles
dispersed therein. The method comprises driving the display cell to display
the color of the
solvent or the color of the non-charged or slightly charged colored particles
by applying a
driving voltage which is about 1 to about 20% of the full driving voltage.
U.S. Patent
Application Publication No. 2014/0092465 and 2014/0092466 describe an
electrophoretic
fluid, and a method for driving an electrophoretic display. The fluid
comprises first, second
and third type of pigment particles, all of which are dispersed in a solvent
or solvent mixture.
The first and second types of pigment particles carry opposite charge
polarities, and the third
type of pigment particles has a charge level being less than about 50% of the
charge level of
the first or second type. The three types of pigment particles have different
levels of threshold
voltage, or different levels of mobility, or both. None of these patent
applications disclose full
color display in the sense in which that term is used below.
[Para 191 U.S. Patent Application Publication No. 2007/0031031 describes an
image
processing device for processing image data in order to display an image on a
display medium
in which each pixel is capable of displaying white, black and one other color.
U.S. Patent
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Date Recue/Date Received 2022-06-22
Applications Publication Nos. 2008/0151355; 2010/0188732; and 2011/0279885
describe a
color display in which mobile particles move through a porous structure. U.S.
Patent
Applications Publication Nos. 2008/0303779 and 2010/0020384 describe a display
medium
comprising first, second and third particles of differing colors. The first
and second particles
can form aggregates, and the smaller third particles can move through
apertures left between
the aggregated first and second particles. U.S. Patent Application Publication
No.
2011/0134506 describes a display device including an electrophoretic display
element
including plural types of particles enclosed between a pair of substrates, at
least one of the
substrates being translucent and each of the respective plural types of
particles being charged
with the same polarity, differing in optical properties, and differing in
either in migration speed
and/or electric field threshold value for moving, a translucent display-side
electrode provided
at the substrate side where the translucent substrate is disposed, a first
back-side electrode
provided at the side of the other substrate, facing the display-side
electrode, and a second back-
side electrode provided at the side of the other substrate, facing the display-
side electrode; and
a voltage control section that controls the voltages applied to the display-
side electrode, the
first back-side electrode, and the second back-side electrode, such that the
types of particles
having the fastest migration speed from the plural types of particles, or the
types of particles
having the lowest threshold value from the plural types of particles, are
moved, in sequence by
each of the different types of particles, to the first back-side electrode or
to the second back-
side electrode, and then the particles that moved to the first back-side
electrode are moved to
the display-side electrode. U.S. Patent Applications Publication Nos.
2011/0175939;
2011/0298835; 2012/0327504; and 2012/0139966 describe color displays which
rely upon
aggregation of multiple particles and threshold voltages. U.S. Patent
Application Publication
No. 2013/0222884 describes an electrophoretic particle, which contains a
colored particle
containing a charged group-containing polymer and a coloring agent, and a
branched silicone-
based polymer being attached to the colored particle and containing, as
copolymerization
components, a reactive monomer and at least one monomer selected from a
specific group of
monomers. U.S. Patent Application Publication No. 2013/0222885 describes a
dispersion
liquid for an electrophoretic display containing a dispersion medium, a
colored electrophoretic
particle group dispersed in the dispersion medium and migrates in an electric
field, a non-
electrophoretic particle group which does not migrate and has a color
different from that of the
electrophoretic particle group, and a compound having a neutral polar group
and a hydrophobic
group, which is contained in the dispersion medium in a ratio of about 0.01 to
about 1 mass %
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Date Recue/Date Received 2022-06-22
based on the entire dispersion liquid. U.S. Patent Application Publication No.
2013/0222886
describes a dispersion liquid for a display including floating particles
containing: core particles
including a colorant and a hydrophilic resin; and a shell covering a surface
of each of the core
particles and containing a hydrophobic resin with a difference in a solubility
parameter of 7.95
(J/cm3)1/2 or more. U.S. Patent Applications Publication Nos. 2013/0222887 and
2013/0222888
describe an electrophoretic particle having specified chemical compositions.
Finally, U.S.
Patent Application Publication No. 2014/0104675 describes a particle
dispersion including first
and second colored particles that move in response to an electric field, and a
dispersion
medium, the second colored particles having a larger diameter than the first
colored particles
and the same charging characteristic as a charging characteristic of the first
color particles, and
in which the ratio (Cs/C1) of the charge amount Cs of the first colored
particles to the charge
amount Cl of the second colored particles per unit area of the display is less
than or equal to 5.
Some of the aforementioned displays do provide full color but at the cost of
requiring
addressing methods that are long and cumbersome.
[Para 201 U.S. Patent Applications Publication Nos. 2012/0314273 and
2014/0002889
describe an electrophoresis device including a plurality of first and second
electrophoretic
particles included in an insulating liquid, the first and second particles
having different
charging characteristics that are different from each other; the device
further comprising a
porous layer included in the insulating liquid and formed of a fibrous
structure. These patent
applications are not full color displays in the sense in which that term is
used below.
[Para 211 See also U.S. Patent Application Publication No. 2011/0134506 and
the
aforementioned Application Serial No. 14/277,107; the latter describes a full
color display
using three different types of particles in a colored fluid, but the presence
of the colored fluid
limits the quality of the white state which can be achieved by the display.
[Para 221 To obtain a high-resolution display, individual pixels of a display
must be
addressable without interference from adjacent pixels. One way to achieve this
objective is to
provide an array of non-linear elements, such as transistors or diodes, with
at least one non-
linear element associated with each pixel, to produce an "active matrix"
display. An addressing
or pixel electrode, which addresses one pixel, is connected to an appropriate
voltage source
through the associated non-linear element. Typically, when the non-linear
element is a
transistor, the pixel electrode is connected to the drain of the transistor,
and this arrangement
will be assumed in the following description, although it is essentially
arbitrary and the pixel
electrode could be connected to the source of the transistor. Conventionally,
in high resolution
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Date Recue/Date Received 2022-06-22
arrays, the pixels are arranged in a two-dimensional array of rows and
columns, such that any
specific pixel is uniquely defined by the intersection of one specified row
and one specified
column. The sources of all the transistors in each colimm are connected to a
single column
electrode, while the gates of all the transistors in each row are connected to
a single row
electrode; again the assignment of sources to rows and gates to columns is
conventional but
essentially arbitrary, and could be reversed if desired. The row electrodes
are connected to a
row driver, which essentially ensures that at any given moment only one row is
selected, i.e.,
that there is applied to the selected row electrode a select voltage such as
to ensure that all the
transistors in the selected row are conductive, while there is applied to all
other rows a non-
select voltage such as to ensure that all the transistors in these non-
selected rows remain non-
conductive. The column electrodes are connected to column drivers, which place
upon the
various column electrodes voltages selected to drive the pixels in the
selected row to their
desired optical states. (The aforementioned voltages are relative to a common
front electrode
which is conventionally provided on the opposed side of the electro-optic
medium from the
non-linear array and extends across the whole display.) After a pre-selected
interval known as
the "line address time" the selected row is deselected, the next row is
selected, and the voltages
on the column drivers are changed so that the next line of the display is
written. This process
is repeated so that the entire display is written in a row-by-row manner.
(Para 231 Conventionally, each pixel electrode has associated therewith a
capacitor electrode
such that the pixel electrode and the capacitor electrode form a capacitor;
see, for example,
International Patent Application WO 01/07961. In some embodiments, N-type
semiconductor
(e.g., amorphous silicon) may be used to from the transistors and the "select"
and "non-select"
voltages applied to the gate electrodes can be positive and negative,
respectively.
[Para 241
Figure 1 of the accompanying drawings depicts an exemplary equivalent circuit
of a single pixel of an electrophoretic display. As illustrated, the circuit
includes a capacitor
formed between a pixel electrode and a capacitor electrode. The
electrophoretic medium 20
is represented as a capacitor and a resistor in parallel. In some instances,
direct or indirect
coupling capacitance 30 between the gate electrode of the transistor
associated with the pixel
and the pixel electrode (usually referred to a as a "parasitic capacitance")
may create unwanted
noise to the display. Usually, the parasitic capacitance 30 is much smaller
than that of the
storage capacitor 10, and when the pixel rows of a display is being selected
or deselected, the
parasitic capacitance 30 may result in a small negative offset voltage to the
pixel electrode,
also known as a "kickback voltage", which is usually less than 2 volts. In
some embodiments,
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Date Recue/Date Received 2022-06-22
to compensate for the unwanted "kickback voltage", a common potential V., may
be supplied to
the top plane electrode and the capacitor electrode associated with each
pixel, such that, when V.
is set to a value equal to the kickback voltage (VKB), every voltage supplied
to the display may be
offset by the same amount, and no net DC-imbalance experienced.
[Para 251 Problems may arise, however, when Vcom is set to a voltage that is
not compensated for
the kickback voltage. This may occur when it is desired to apply a higher
voltage to the display
than is available from the backplane alone. It is well-known in the art that,
for example, the
maximum voltage applied to the display may be doubled if the backplane is
supplied with a choice
of a nominal +V, 0, or -V, for example, while V. is supplied with ¨V. The
maximum voltage
experienced in this case is +2V (i.e., at the backplane relative to the top
plane), while the minimum
is zero. If negative voltages are needed, the V. potential must be raised at
least to zero. Waveforms
used to address a display with positive and negative voltages using top plane
switching must
therefore have particular frames allocated to each of more than one V. voltage
setting.
[Para 26] A set of waveforms for driving a color electrophoretic display
having four particles
described in U.S. Application Serial No. 14/849,658. In U.S. Application
Serial No. 14/849,658,
seven different voltages are applied to the pixel electrodes: three positive,
three negative, and zero.
However, in some embodiments, the maximum voltages used in these waveforms are
higher than
that can be handled by amorphous silicon thin-film transistors. In such
instances, suitable high
voltages can be obtained by the use of top plane switching. When (as described
above) V. is
deliberately set to VKB, a separate power supply may be used. It is costly and
inconvenient, however,
to use as many separate power supplies as there are V. settings when top plane
switching is used.
Furthermore, top plane switching is known to increase kickback, thereby
degrading the stability of
the color states. Therefore, there is a need for methods to compensate for the
DC-offset caused by
the kickback voltage using the same power supply for the back plane and V.. Of
course, complete
DC-offset results in longer impulse sequences and therefore longer image
refreshes.
SUMMARY OF INVENTION
[Para 27] The invention involves drivers configured to deliver two-part reset
pulses to pixels in
color electrophoretic displays. The two-part reset pulses are effective in
removing last state
information, but do not require more energy or time than needed. As a result,
the described
controllers allow a three (or more)-particle electrophoretic display to update
faster while using less
energy. Surprisingly, the controllers also provide a larger color gamut when
the reset pulses
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are tuned for individual colors. The invention additionally provides a method
of driving an
electro-optic display which is DC balanced despite the existence of kickback
voltages and
changes in the voltages applied to the front electrode.
[Para 281 In an aspect the invention involves a method for driving an
electrophoretic display
having a front electrode, a backplane, and a display medium positioned between
the front
electrode and the backplane, the display medium comprising three sets of
differently-colored
particles. The method comprises applying a reset phase and a color transition
phase to the
display. The reset phase comprises applying a first signal having a first
polarity, a first
amplitude as a function of time, and a first duration on the front electrode,
applying a second
signal having a second polarity opposite the first polarity, a second
amplitude as a function of
time, during the first duration on the backplane, applying a third signal
having the second
polarity opposite the first polarity, a third amplitude as a function of time,
during the second
duration on the front electrode, applying a fourth signal equal to the sum of
the first and
second amplitudes, during the second duration on the backplane. The color
transition phase
comprises applying a fifth signal having the second polarity, a fourth
amplitude as a function
of time, and a third duration preceded by the first and second durations on
the front electrode,
applying a sixth signal having the first polarity, a fifth amplitude as a
function of time, and a
fourth duration preceded by the first and second durations on the backplane,
wherein the sum
of the first and second amplitudes as a function of time integrated over the
first duration, and
the sum of the first, second, and third amplitudes as a function of time
integrated over the
second duration, and the fourth amplitude as a function of time integrated
over the third
duration, and the fifth amplitude as a function of time integrated over the
fourth duration
produces an impulse offset designed to maintain a DC-balance on the display
medium over
the reset phase and the color transition phase. In some embodiments, the reset
phase erases
previous optical properties rendered on the display. In some embodiments, the
color
transition phase substantially changes the optical property displayed by the
display. In some
embodiments, the first polarity is a negative voltage. In some embodiments,
the first polarity
is a positive voltage. In some embodiments, the impulse offset is proportional
to a kickback
voltage experienced by the display medium. In some embodiments, the fourth
duration occurs
during the third duration. In some embodiments, the third duration and the
fourth duration
initiate at the same time.
[Para 291 In another aspect, the invention includes a method for driving an
electrophoretic
display having a front electrode, a backplane, and a display medium positioned
between the
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front electrode and the backplane, the display medium comprising three sets of
differently-
colored particles, the method comprises applying a reset phase and a color
transition phase to
the display. The reset phase comprises applying a first signal having a first
polarity, a first
amplitude as a function of time, and a first duration on the front electrode,
applying no signal
during the first duration on the backplane, applying a second signal having a
second polarity
opposite the first polarity, a second amplitude as a function of time, during
a second duration
on the front electrode, applying a third signal having the first polarity, and
a third amplitude
as a function of time, during the second duration on the backplane. The color
transition phase
comprises applying a fourth signal having the first polarity, a fourth
amplitude as a function
of time, and a third duration preceded by the first and second durations on
the front electrode,
applying a fifth signal having the second polarity, a fifth amplitude as a
function of time, and
a fourth duration preceded by the first and second durations on the backplane,
wherein the
sum of the first amplitude as a function of time integrated over the first
duration, and the sum
of the second and third amplitudes as a function of time integrated over the
second duration,
and the fourth amplitude as a function of time integrated over the third
duration, and the fifth
amplitude as a function of time integrated over the fourth duration produces
an impulse offset
designed to maintain a DC-balance on the display medium over the reset phase
and the color
transition phase. In some embodiments, the reset phase erases previous optical
properties
rendered on the display. In some embodiments, the color transition phase
substantially
changes the optical property displayed by the display. In some embodiments,
the first
polarity is a negative voltage. In some embodiments, the first polarity is a
positive voltage.
In some embodiments, the impulse offset is proportional to a kickback voltage
experienced
by the display medium. In some embodiments, the fourth duration occurs during
the third
duration. In some embodiments, the third duration and the fourth duration
initiate at the same
time.
[Para 301 In another aspect, the invention includes a controller for an
electrophoretic display
comprising a front electrode, a backplane, and a display medium positioned
between the front
electrode and the backplane, the display medium comprising three sets of
differently-colored
particles, the controller being operatively coupled to the front electrode and
the backplane,
and configured to apply a reset phase and a color transition phase to the
display. The reset
phase comprises applying a first signal having a first polarity, a first
amplitude as a function
of time, and a first duration on the front electrode, applying a second signal
having a second
polarity opposite the first polarity, a second amplitude as a function of
time, during the first
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Date Recue/Date Received 2022-06-22
duration on the backplane, applying a third signal having the second polarity
opposite the
first polarity, a third amplitude as a function of time, during the second
duration on the front
electrode, applying a fourth signal equal to the sum of the first and second
amplitudes, during
the second duration on the backplane. The color transition phase comprises
applying a fifth
signal having the second polarity, a fourth amplitude as a function of time,
and a third
duration preceded by the first and second durations on the front electrode,
applying a sixth
signal having the first polarity, a fifth amplitude as a function of time, and
a fourth duration
preceded by the first and second durations on the backplane, wherein the sum
of the first and
second amplitudes as a function of time integrated over the first duration,
and the sum of the
first, second, and third amplitudes as a function of time integrated over the
second duration,
and the fourth amplitude as a function of time integrated over the third
duration, and the fifth
amplitude as a function of time integrated over the fourth duration produces
an impulse offset
designed to maintain a DC-balance on the display medium over the reset phase
and the color
transition phase. In some embodiments, the controller applies a different
reset phase
depending upon the color to be displayed by the electrophoretic display. In
some
embodiments, the display medium comprises white, cyan, yellow, and magenta
particles. In
some embodiments, the display medium comprises white, red, blue, and green
particles.
[Pam 31] In another aspect, the invention includes a controller for an
electrophoretic display
comprising a front electrode, a backplane, and a display medium positioned
between the front
electrode and the backplane, the display medium comprising three sets of
differently-colored
particles, the controller being operatively coupled to the front electrode and
the backplane,
and configured to apply a reset phase and a color transition phase to the
display. The reset
phase comprises applying a first signal having a first polarity, a first
amplitude as a function
of time, and a first duration on the front electrode, applying no signal
during the first duration
on the backplane, applying a second signal having a second polarity opposite
the first
polarity, a second amplitude as a function of time, during a second duration
on the front
electrode, applying a third signal having the first polarity, and a third
amplitude as a function
of time, during the second duration on the backplane. The color transition
phase comprises
applying a fourth signal having the first polarity, a fourth amplitude as a
function of time, and
a third duration preceded by the first and second durations on the front
electrode, applying a
fifth signal having the second polarity, a fifth amplitude as a function of
time, and a fourth
duration preceded by the first and second durations on the backplane, wherein
the sum of the
first amplitude as a function of time integrated over the first duration, and
the sum of the
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second and third amplitudes as a function of time integrated over the second
duration, and the
fourth amplitude as a function of time integrated over the third duration, and
the fifth
amplitude as a function of time integrated over the fourth duration produces
an impulse offset
designed to maintain a DC-balance on the display medium over the reset phase
and the color
transition phase. In some embodiments, the controller applies a different
reset phase
depending upon the color to be displayed by the electrophoretic display. In
some
embodiments, the display medium comprises white, cyan, yellow, and magenta
particles. In
some embodiments, the display medium comprises white, red, blue, and green
particles.
[Para 32] The electrophoretic media used in the display of the present
invention may be any
of those described in the aforementioned Application Serial No. 14/849,658.
Such media
comprise a light-scattering particle, typically white, and three substantially
non-light-scattering
particles. The electrophoretic medium of the present invention may be in any
of the forms
discussed above. Thus, the electrophoretic medium may be unencapsulated,
encapsulated in
discrete capsules surrounded by capsule walls, or in the form of a polymer-
dispersed or
microcell medium.
BRIEF DESCRIPTION OF DRAWINGS
[Para 331 FIG. 1 illustrates an exemplary equivalent circuit of a single pixel
of an
electrophoretic display.
(Para 34] FIG. 2 is a schematic cross-section showing the positions of the
various colored
particles in an electrophoretic medium of the present invention when
displaying black, white,
the three subtractive primary and the three additive primary colors.
(Para 351 FIG. 3 shows in schematic form the four types of different pigment
particles used
in a multi-particle electrophoretic medium;
(Para 361 FIG. 4 shows in schematic form the relative strengths of
interactions between pairs
of particles in a multi-particle electrophoretic medium;
[Para 371 FIG. 5 shows behavior of multiple different particles in an
electrophoretic medium
when subjected to electric fields of varying strength and duration;
[Para 38] FIG. 6 is an exemplary waveform including a two-part reset phase (A)
and a color
transition phase (B);
[Para 39] FIG. 7 is a schematic voltage against time diagram showing the
variation with time
of the front and pixel electrodes, and the resultant voltage across the
electrophoretic medium,
of a waveform used to generate one color in a drive scheme of the present
invention;
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[Para 401 FIG. 8A shows experimental data of color gamuts produced with
various voltage
combinations of two-part reset phases;
Mara 411 FIG. 8B shows the total experimental color gamut available by
implementing a
controller that changes the two-part reset phase depending upon the desired
color;
[Para 421 FIG. 9 shows an embodiment of a DC-balanced reset pulse;
[Para 43] FIG. 10 shows the DC-balanced reset pulse of FIG. 9 as experienced
by the
electrophoretic particles.
DETAILED DESCRIPTION
1Para 441 As indicated above, the present invention may be used with an
electrophoretic
medium which comprises one light-scattering particle (typically white) and
three other particles
providing the three subtractive primary colors. Such as system is shown
schematically in FIG.
2, and it can provide white, yellow, red, magenta, blue, cyan, green, and
black at every pixel.
[Para 451 The three particles providing the three subtractive primary colors
may be
substantially non-light-scattering ("SNLS"). The use of SNLS particles allows
mixing of colors
and provides for more color outcomes than can be achieved with the same number
of scattering
particles. The aforementioned US 8,587,859 uses particles having subtractive
primary colors,
but requires two different voltage thresholds for independent addressing of
the non-white
particles (i.e., the display is addressed with three positive and three
negative voltages). These
thresholds must be sufficiently separated for avoidance of cross-talk, and
this separation
necessitates the use of high addressing voltages for some colors. In addition,
addressing the
colored particle with the highest threshold also moves all the other colored
par
[Para 461 Particles, and these other particles must subsequently be switched
to their desired
positions at lower voltages. Such a step-wise color-addressing scheme produces
flashing of
unwanted colors and a long transition time. The present invention does not
require the use of a
such a stepwise waveform and addressing to all colors can, as described below,
be achieved
with only two positive and two negative voltages (i.e., only five different
voltages, two positive,
two negative and zero are required in a display, although as described below
in certain
embodiments it may be preferred to use more different voltages to address the
display).
[Para 471 As already mentioned, FIG. 2 of the accompanying drawings is a
schematic cross-
section showing the positions of the various particles in an electrophoretic
medium of the
present invention when displaying black, white, the three subtractive primary
and the three
additive primary colors. In FIG. 2, it is assumed that the viewing surface of
the display is at the
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Date Recue/Date Received 2022-06-22
top (as illustrated), i.e., a user views the display from this direction, and
light is incident from
this direction. As already noted, in preferred embodiments only one of the
four particles used
in the electrophoretic medium of the present invention substantially scatters
light, and in FIG.
2 this particle is assumed to be the white pigment. Basically, this light-
scattering white particle
forms a white reflector against which any particles above the white particles
(as illustrated in
FIG. 2) are viewed. Light entering the viewing surface of the display passes
through these
particles, is reflected from the white particles, passes back through these
particles and emerges
from the display. Thus, the particles above the white particles may absorb
various colors and
the color appearing to the user is that resulting from the combination of
particles above the
white particles. Any particles disposed below (behind from the user's point of
view) the white
particles are masked by the white particles and do not affect the color
displayed. Because the
second, third and fourth particles are substantially non-light-scattering,
their order or
arrangement relative to each other is unimportant, but for reasons already
stated, their order or
arrangement with respect to the white (light-scattering) particles is
critical.
[Para 481 More specifically, when the cyan, magenta and yellow particles lie
below the white
particles (Situation [A] in FIG. 2), there are no particles above the white
particles and the pixel
simply displays a white color. When a single particle is above the white
particles, the color of
that single particle is displayed, yellow, magenta and cyan in Situations [B],
[D] and [F]
respectively in FIG. 2. When two particles lie above the white particles, the
color displayed is
a combination of those of these two particles; in FIG. 2, in Situation [C],
magenta and yellow
particles display a red color, in Situation [E], cyan and magenta particles
display a blue color,
and in Situation [G], yellow and cyan particles display a green color.
Finally, when all three
colored particles lie above the white particles (Situation [H] in FIG. 2), all
the incoming light
is absorbed by the three subtractive primary colored particles and the pixel
displays a black
color.
[Para 491 It is possible that one subtractive primary color could be rendered
by a particle that
scatters light, so that the display would comprise two types of light-
scattering particle, one of
which would be white and another colored. In this case, however, the position
of the light-
scattering colored particle with respect to the other colored particles
overlying the white
particle would be important For example, in rendering the color black (when
all three colored
particles lie over the white particles) the scattering colored particle cannot
lie over the non-
scattering colored particles (otherwise they will be partially or completely
hidden behind the
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Date Recue/Date Received 2022-06-22
scattering particle and the color rendered will be that of the scattering
colored particle, not
black).
[Para 501 It would not be easy to render the color black if more than one type
of colored
particle scattered light.
[Para 511 FIG. 2 shows an idealized situation in which the colors are
uncontaminated (i.e., the
light-scattering white particles completely mask any particles lying behind
the white particles).
In practice, the masking by the white particles may be imperfect so that there
may be some
small absorption of light by a particle that ideally would be completely
masked. Such
contamination typically reduces both the lightness and the chroma of the color
being rendered.
In the electrophoretic medium of the present invention, such color
contamination should be
minimized to the point that the colors formed are commensurate with an
industry standard for
color rendition. A particularly favored standard is SNAP (the standard for
newspaper
advertising production), which specifies L*, a* and b* values for each of the
eight primary
colors referred to above. (Hereinafter, "primary colors" will be used to refer
to the eight colors,
black, white, the three subtractive primaries and the three additive primaries
as shown in FIG.
2.)
[Para 521 Methods for electrophoretically arranging a plurality of different
colored particles
in "layers" as shown in FIG. 2 have been described in the prior art. The
simplest of such
methods involves "racing" pigments having different electrophoretic
mobilities; see for
example U.S. Patent No. 8,040,594. Such a race is more complex than might at
first be
appreciated, since the motion of charged pigments itself changes the electric
fields experienced
locally within the electrophoretic fluid. For example, as positively-charged
particles move
towards the cathode and negatively-charged particles towards the anode, their
charges screen
the electric field experienced by charged particles midway between the two
electrodes. It is
thought that, while pigment racing is involved in the clectrophoretic of the
present invention,
it is not the sole phenomenon responsible for the arrangements of particles
illustrated in FIG.
2.
[Para 531 A second phenomenon that may be employed to control the motion of a
plurality of
particles is hetero-aggregation between different pigment types; see, for
example, the
aforementioned US 2014/0092465. Such aggregation may be charge-mediated
(Coulombic) or
may arise as a result of, for example, hydrogen bonding or Van der Waals
interactions. The
strength of the interaction may be influenced by choice of surface treatment
of the pigment
particles. For example, Coulombic interactions may be weakened when the
closest distance of
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Date Recue/Date Received 2022-06-22
approach of oppositely-charged particles is maximized by a steric barrier
(typically a polymer
grafted or adsorbed to the surface of one or both particles). In the present
invention, as
mentioned above, such polymeric barriers are used on the first, and second
types of particles
and may or may not be used on the third and fourth types of particles.
[Para 54] A third phenomenon that may be exploited to control the motion of a
plurality of
particles is voltage- or current-dependent mobility, as described in detail in
the aforementioned
Application Serial No. 14/277,107.
[Para 551 FIG. 3 shows schematic cross-sectional representations of the four
pigment types
(1-4) used in preferred embodiments of the invention. The polymer shell
adsorbed to the core
pigment is indicated by the dark shading, while the core pigment itself is
shown as unshaded.
A wide variety of forms may be used for the core pigment: spherical, acicular
or otherwise
anisometric, aggregates of smaller particles (i.e., "grape clusters"),
composite particles
comprising small pigment particles or dyes dispersed in a binder, and so on as
is well known
in the art. The polymer shell may be a covalently-bonded polymer made by
grafting processes
or chemisorption as is well known in the art, or may be physisorbed onto the
particle surface.
For example, the polymer may be a block copolymer comprising insoluble and
soluble
segments. Some methods for affixing the polymer shell to the core pigments are
described in
the Examples below.
(Para 56] First and second particle types in one embodiment of the invention
preferably have
a more substantial polymer shell than third and fourth particle types. The
light-scattering white
particle is of the first or second type (either negatively or positively
charged). In the discussion
that follows it is assumed that the white particle bears a negative charge
(i.e., is of Type 1), but
it will be clear to those skilled in the art that the general principles
described will apply to a set
of particles in which the white particles are positively charged.
!Para 571 In the present invention the electric field required to separate an
aggregate formed
from mixtures of particles of types 3 and 4 in the suspending solvent
containing a charge control
agent is greater than that required to separate aggregates formed from any
other combination
of two types of particle. The electric field required to separate aggregates
formed between the
first and second types of particle is, on the other hand, less than that
required to separate
aggregates formed between the first and fourth particles or the second and
third particles (and
of course less than that required to separate the third and fourth particles).
[Para 581 In FIG. 3 the core pigments comprising the particles are shown as
having
approximately the same size, and the zeta potential of each particle, although
not shown, is
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Date Recue/Date Received 2022-06-22
assumed to be approximately the same. What varies is the thickness of the
polymer shell
surrounding each core pigment. As shown in FIG. 3, this polymer shell is
thicker for particles
of types 1 and 2 than for particles of types 3 and 4¨ and this is in fact a
preferred situation for
certain embodiments of the invention.
[Para 591 In order to understand how the thickness of the polymer shell
affects the electric
field required to separate aggregates of oppositely-charged particles, it may
be helpful to
consider the force balance between particle pairs. In practice, aggregates may
be composed of
a great number of particles and the situation will be far more complex than is
the case for simple
pairwise interactions. Nevertheless, the particle pair analysis does provide
some guidance for
understanding of the present invention.
[Para 601 The force acting on one of the particles of a pair in an electric
field is given by:
F ¨F +F +F +F
Total ¨ App VW (1)
Where FApp is the force exerted on the particle by the applied electric field,
Fc is the Coulombic
force exerted on the particle by the second particle of opposite charge, Fvw
is the attractive Van
der Waals force exerted on one particle by the second particle, and FD is the
attractive force
exerted by depletion flocculation on the particle pair as a result of
(optional) inclusion of a
stabilizing polymer into the suspending solvent.
[Para 611 The force FApp exerted on a particle by the applied electric field
is given by:
frApp= qE = 47rvre0(a+s)c E (2)
where q is the charge of the particle, which is related to the zeta potential
(C) as shown in
equation (2) (approximately, in the Huckel limit), where a is the core pigment
radius, s is the
thickness of the solvent-swollen polymer shell, and the other symbols have
their conventional
meanings as known in the art.
[Para 621 The magnitude of the force exerted on one particle by another as a
result of
Coulombic interactions is given approximately by:
F ¨ 47cer8o (al + s1)(a2+ s2)ci.c2
c¨ (3)
(a1 + s1+ a2 + s2
for particles 1 and 2.
[Para. 631 Note that the FApp forces applied to each particle act to separate
the particles, while
the other three forces are attractive between the particles. If the FApp force
acting on one particle
is higher than that acting on the other (because the charge on one particle is
higher than that on
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Date Recue/Date Received 2022-06-22
the other) according to Newton's third law, the force acting to separate the
pair is given by the
weaker of the two FApp forces.
[Para 641 It can be seen from (2) and (3) that the magnitude of the difference
between the
attracting and separating Coulombic terms is given by:
F ¨ pp Fc = 4nErgo ((a + s); 1 E 1 ¨c2)
A (4)
if the particles are of equal radius and zeta potential, so making (a + s)
smaller or larger will
make the particles more difficult to separate. Thus, in one embodiment of the
invention it is
preferred that particles of types 1 and 2 be large, and have a relatively low
zeta potential, while
particles 3 and 4 be small, and have a relatively large zeta potential.
[Para 651 However, the Van der Waals forces between the particles may also
change
substantially if the thickness of the polymer shell increases. The polymer
shell on the particles
is swollen by the solvent and moves the surfaces of the core pigments that
interact through Van
der Waals forces further apart. For spherical core pigments with radii (al,
a2) much larger than
the distance between them (si-Fs2),
Aa,a2
F ¨
vw ¨ 6(a1+ a2)(s1+ s2)2 (5)
where A is the Hamaker constant. As the distance between the core pigments
increases the
expression becomes more complex, but the effect remains the same: increasing
si or 52 has a
significant effect on reducing the attractive Van der Waals interaction
between the particles.
[Para 66] With this background it becomes possible to understand the rationale
behind the
particle types illustrated in FIG. 3. Particles of types 1 and 2 have
substantial polymeric shells
that are swollen by the solvent, moving the core pigments further apart and
reducing the Van
der Waals interactions between them more than is possible for particles of
types 3 and 4, which
have smaller or no polymer shells. Even if the particles have approximately
the same size and
magnitude of zeta potential, according to the invention it will be possible to
arrange the
strengths of the interactions between pairwise aggregates to accord with the
requirements set
out above.
[Para 67] For fuller details of preferred particles for use in the display of
FIG. 3, the reader is
referred to the aforementioned Application Serial No. 14/849,658.
[Para 681 FIG. 4 shows in schematic form the strengths of the electric fields
required to
separate pairwise aggregates of the particle types of the invention. The
interaction between
particles of types 3 and 4 is stronger than that between particles of types 2
and 3. The interaction
-22-
Date Recue/Date Received 2022-06-22
between particles of types 2 and 3 is about equal to that between particles of
types 1 and 4 and
stronger than that between particles of types 1 and 2. All interactions
between pairs of particles
of the same sign of charge as weak as or weaker than the interaction between
particles of types
1 and 2.
[Pam 691 FIG. 5 shows how these interactions may be exploited to make all the
primary colors
(subtractive, additive, black and white), as was discussed generally with
reference to FIG. 2.
[Para 701 When addressed with a low electric field (FIG. 5(A)), particles 3
and 4 are
aggregated and not separated. Particles 1 and 2 are free to move in the field.
If particle 1 is the
white particle, the color seen viewing from the left is white, and from the
right is black.
Reversing the polarity of the field switches between black and white states.
The transient colors
between black and white states, however, are colored. The aggregate of
particles 3 and 4 will
move very slowly in the field relative to particles 1 and 2. Conditions may be
found where
particle 2 has moved past particle 1 (to the left) while the aggregate of
particles 3 and 4 has not
moved appreciably. In this case particle 2 will be seen viewing from the left
while the aggregate
of particles 3 and 4 will be seen viewing from the right. In certain
embodiments of the
invention the aggregate of particles 3 and 4 is weakly positively charged, and
is therefore
positioned in the vicinity of particle 2 at the beginning of such a
transition.
[Para 711 When addressed with a high electric field (FIG. 5(B)), particles 3
and 4 are
separated. Which of particles 1 and 3 (each of which has a negative charge) is
visible when
viewed from the left will depend upon the waveform (see below). As
illustrated, particle 3 is
visible from the left and the combination of particles 2 and 4 is visible from
the right.
[Para 72] Starting from the state shown in FIG. 5(B), a low voltage of
opposite polarity will
move positively charged particles to the left and negatively charged particles
to the right.
However, the positively charged particle 4 will encounter the negatively
charged particle 1, and
the negatively charged particle 3 will encounter the positively charged
particle 2. The result is
that the combination of particles 2 and 3 will be seen viewing from the left
and particle 4
viewing from the right.
[Para 731 As described above, preferably particle 1 is white, particle 2 is
cyan, particle 3 is
yellow and particle 4 is magenta.
[Para 741 The core pigment used in the white particle is typically a metal
oxide of high
refractive index as is well known in the art of electrophoretic displays.
Examples of white
pigments are described in the Examples below.
-23-
Date Recue/Date Received 2022-06-22
[Para 751 The core pigments used to make particles of types 2-4, as described
above, provide
the three subtractive primary colors: cyan, magenta and yellow.
[Para 761 A display device may be constructed using an electrophoretic fluid
of the invention
in several ways that are known in the prior art. The electrophoretic fluid may
be encapsulated
in microcapsules or incorporated into microcell structures that are thereafter
sealed with a
polymeric layer. The microcapsule or microcell layers may be coated or
embossed onto a
plastic substrate or film bearing a transparent coating of an electrically
conductive material.
This assembly may be laminated to a backplane bearing pixel electrodes using
an electrically
conductive adhesive.
[Para 771 A first embodiment of waveforms used to achieve each of the particle
arrangements
shown in FIG. 2 will now be described. In this discussion it is assumed that
the first particles
are white and negatively charged, the second particles cyan and positively
charged, the third
particles yellow and negatively charged, and the fourth particles magenta and
positively
charged. Those skilled in the art will understand how the color transitions
will change if these
assignments of particle colors are changed, as they can be provided that one
of the first and
second particles is white. Similarly, the polarities of the charges on all the
particles can be
inverted and the electrophoretic medium will still function in the same manner
provided that
the polarity of the waveforms (see next paragraph) used to drive the medium is
similarly
inverted.
[Para 781 In the discussion that follows, the waveform (voltage against time
curve) applied to
the pixel electrode of the backplane of a display of the invention is
described and plotted, while
the front electrode is assumed to be grounded (i.e., at zero potential). The
electric field
experienced by the electrophoretic medium is of course determined by the
difference in
potential between the backplane and the front electrode and the distance
separating them. The
display is typically viewed through its front electrode, so that it is the
particles adjacent the
front electrode which control the color displayed by the pixel, and if it is
sometimes easier to
understand the optical transitions involved if the potential of the front
electrode relative to the
backplane is considered; this can be done simply by inverting the waveforms
discussed below.
[Para 791 These waveforms require that each pixel of the display can be driven
at five
different addressing voltages, designated +Vino, +V10,, 0, -Viow and -Vhigh,
illustrated as 30V,
15V, 0, -15V and -30V. In practice it may be preferred to use a larger number
of addressing
voltages. If only three voltages are available (i.e., +Vhigh, 0, and -Vhigh)
it may be possible to
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Date Recue/Date Received 2022-06-22
achieve the same result as addressing at a lower voltage (say, Vhigh/n where n
is a positive
integer > 1) by addressing with pulses of voltage Vhigh but with a duty cycle
of 1/n.
[Para 801 Waveforms used in the present invention may comprise three phases: a
DC-
balancing phase, in which a DC imbalance due to previous waveforms applied to
the pixel is
corrected, or in which the DC imbalance to be incurred in the subsequent color
rendering
transition is corrected (as is known in the art), a "reset" phase, in which
the pixel is returned to
a starting configuration that is at least approximately the same regardless of
the previous optical
state of the pixel, and a "color rendering" phase as described below. The DC-
balancing and
reset phases are optional and may be omitted, depending upon the demands of
the particular
application. The "reset" phase, if employed, may be the same as the magenta
color rendering
waveform described below, or may involve driving the maximum possible positive
and
negative voltages in succession, or may be some other pulse pattern, provided
that it returns
the display to a state from which the subsequent colors may reproducibly be
obtained.
[Para 811 The general principles used in production of the eight primary
colors (white, black,
cyan, magenta, yellow, red, green and blue) using this second drive scheme
applied to a display
of the present invention (such as that shown in FIG. 2) will now be described.
It will be assumed
that the first pigment is white, the second cyan, the third yellow and the
fourth magenta. It will
be clear to one of ordinary skill in the art that the colors exhibited by the
display will change if
the assignment of pigment colors is changed.
[Para 821 The greatest positive and negative voltages (designated Vmax in
FIG. 6) applied
to the pixel electrodes produce respectively the color formed by a mixture of
the second and
fourth particles (cyan and magenta, to produce a blue color ¨ cf. FIG. 2[E]),
or the third
particles alone (yellow ¨ cf. FIG. 2[B] ¨ the white pigment scatters light and
lies in between
the colored pigments). These blue and yellow colors are not necessarily the
best blue and
yellow attainable by the display. The mid-level positive and negative voltages
(designated
\Inn(' in FIG. 6) applied to the pixel electrodes produce colors that are
black and white,
respectively (although not necessarily the best black and white colors
attainable by the display
¨ cf. FIG. 5(A)).
[Para 83] From these blue, yellow, black or white optical states, the other
four primary colors
may be obtained by moving only the second particles (in this case the cyan
particles) relative
to the first particles (in this case the white particles), which is achieved
using the lowest applied
voltages (designated \Tina, in FIG. 6). Thus, moving cyan out of blue (by
applying -Vmin to
the pixel electrodes) produces magenta (cf. FIG. 2[E] and [D] for blue and
magenta
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Date Recue/Date Received 2022-06-22
respectively); moving cyan into yellow (by applying +Vmin to the pixel
electrodes) provides
green (cf. FIG. 2[B] and [G] for yellow and green respectively); moving cyan
out of black (by
applying -Vmin to the pixel electrodes) provides red (cf. FIG. 2[H] and [C]
for black and red
respectively), and moving cyan into white (by applying +Vmin to the pixel
electrodes) provides
cyan (cf. FIG. 2[A] and [F] for white and cyan respectively).
[Para 84] While these general principles are useful in the construction of
waveforms to
produce particular colors in displays of the present invention, in practice
the ideal behavior
described above may not be observed, and modifications to the basic scheme are
desirably
employed.
[Para 851 A generic waveform for addressing a color electrophoretic display of
the invention
is illustrated in FIG. 6, in which the abscissa represents time (in arbitrary
units) and the ordinate
represents the voltage difference between a pixel electrode and the common
front electrode.
The magnitudes of the three positive voltages used in the drive scheme
illustrated in FIG. 6
may lie between about +3V and +30V, and of the three negative voltages between
about -3V
and -30V. In one preferred embodiment, the highest positive voltage, +Vmax, is
+30V, the
medium positive voltage, +Valid, is 1 5V, and the lowest positive voltage,
+Vmm, is 9V. In a
similar manner, negative voltages ¨Vim., -Vaaa and ¨Vmm are; in a preferred
embodiment -30V,
-15V and -9V. It is not necessary that the magnitudes of the voltages 1+VI = I-
VI for any of the
three voltage levels, although it may be preferable in some cases that this be
so.
(Para 861 There are two distinct phases in the generic waveform
illustrated in FIG. 6. In
the first phase, there are supplied pulses (wherein "pulse" signifies a
monopole square wave,
i.e., the application of a constant voltage for a predetermined time) at +Vmax
and -Vmax that
serve to erase the previous image rendered on the display (i.e., to "reset"
the display). The
lengths of these pulses (ti and t3) and of the rests (i.e., periods of zero
voltage between them (t2
and t4) may be chosen so that the entire waveform (i.e., the integral of
voltage with respect to
time over the whole waveform as illustrated in FIG. 6) is DC balanced (i.e.,
the integral of
voltage over time is substantially zero). DC balance can be achieved by
adjusting the lengths
of the pulses and rests in phase A so that the net impulse supplied in this
phase is equal in
magnitude and opposite in sign to the net impulse supplied in phase B, during
which phase the
display is switched to a particular desired color.
[Para 871 Herein the term "frame" refers to a single update of all the
rows in the display.
It will be clear to one of ordinary skill in the art that in a display of the
invention driven using
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Date Recue/Date Received 2022-06-22
a thin-film transistor (TFT) array the available time increments on the
abscissa of FIG. 6 will
typically be quantized by the frame rate of the display. Likewise, it will be
clear that the display
is addressed by changing the potential of the pixel electrodes relative to the
front electrode and
that this may be accomplished by changing the potential of either the pixel
electrodes or the
front electrode, or both. In the present state of the art, typically a matrix
of pixel electrodes is
present on the backplane, whereas the front electrode is common to all pixels.
Therefore, when
the potential of the front electrode is changed, the addressing of all pixels
is affected. The basic
structure of the waveform described above with reference to FIG. 6 is the same
whether or not
varying voltages are applied to the front electrode.
[Para 881 The generic waveform illustrated in FIG. 6 requires that the
driving electronics
provide as many as seven different voltages to the data lines during the
update of a selected
row of the display. While multi-level source drivers capable of delivering
seven different
voltages are available, many commercially-available source drivers for
electrophoretic displays
permit only three different voltages to be delivered during a single frame
(typically a positive
voltage, zero, and a negative voltage). It is possible to modify the generic
waveform of FIG. 6
to accommodate a three level source driver architecture provided that the
three voltages
supplied to the panel (typically +V, 0 and ¨V) can be changed from one frame
to the next. (i.e.,
such that, for example, in frame n voltages (+Vmax, 0, ¨Vmm) could be supplied
while in frame
n+1 voltages (+Vmid, 0, -Vmax) could be supplied).
[Para 89] Sometimes it may be desirable to use a so-called "top plane
switching" driving
scheme to control an electrophoretic display. In a top plane switching driving
scheme, the top
plane common electrode can be switched between ¨V, 0 and +V, while the
voltages applied to
the pixel electrodes can also vary from ¨V, 0 to +V with pixel transitions in
one direction being
handled when the common electrode is at 0 and transitions in the other
direction being handled
when the common electrode is at +V.
[Para 901 When top plane switching is used in combination with a three-
level source
driver, the same general principles apply as described above with reference to
FIG. 6. Top plane
switching may be preferred when the source drivers cannot supply a voltage as
high as the
preferred V.. Methods for driving electrophoretic displays using top plane
switching are well
known in the art.
-27-
Date Recue/Date Received 2022-06-22
[Para 911 A typical waveform of the (E Ink) prior art is shown below in
Table 1, where
the numbers in parentheses correspond to the number of frames driven with the
indicated
backplane voltage (relative to a top plane assumed to be at zero potential).
TABLE 1
Reset Phase High/Mid V Phase (N repetitions Low/Mid V
phase
of frame sequence below)
K -Vmax(60+AR) Vmax(60- AK) Vmid(5) Zero(9) Zero(50)
B -Vmax(60H-AR) Vmax(60- AB) Vmax(2) Zero(5) -Vmid(7) Vmid(40) Zero(10)
R -Vmax(60+AR) Vmax(60- AR) Vmax(7) Zero(3) -Vmax(4) Zero(50)
M -Vmax(60+Am) Vmax(60- Am) Vmax(4) Zero(3) -Vmid(7) Zero(50)
G -Vmax(60+AG) Vmax(60- AG) Vmid(7) Zero(3) -Vmax(4) Vmin(40) Zero(1O)
C -Vmax(60+Ac) Vmax(60- Vmax(2) Zero(5) -Vmid(7) Vmin(40) Zero( 1 0)
Y -Vmax(60+Ay) Vmax(60- Ay) Vmid(7) Zero(3) -Vmax(4) Zero(50)
W -Vmax(60+Aw) Vmax(60- Aw) Vmax(2) Zero(5) -Vmid(7) Zero(50)
[Para 921 In the reset phase of this waveform, pulses of the maximum
negative and
positive voltages are provided to erase the previous state of the display. The
number of frames
at each voltage are offset by an amount (shows as Ax for color x) that
compensates for the net
impulse in the High/Mid voltage and Low/Mid voltage phases, where the color is
rendered. To
achieve DC balance, Ax is chosen to be half that net impulse. It is not
necessary that the reset
phase be implemented in precisely the manner illustrated in the Table; for
example, when top
plane switching is used it is necessary to allocate a particular number of
frames to the negative
and positive drives. In such a case, it is preferred to provide the maximum
number of high
voltage pulses consistent with achieving DC balance (i.e., to subtract 2Ax
from the negative or
positive frames as appropriate).
[Para 931 In the High/Mid voltage phase, as described above, a sequence of
N repetitions
of a pulse sequence appropriate to each color is provided, where N can be 1-
20. As shown, this
sequence comprises 14 frames that are allocated positive or negative voltages
of magnitude
Vmax or Vmid, or zero. The pulse sequences shown are in accord with the
discussion given
above. It can be seen that in this phase of the waveform the pulse sequences
to render the colors
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Date Recue/Date Received 2022-06-22
white, blue and cyan are the same. Likewise, in this phase the pulse sequences
to render yellow
and green are the same (since green is achieved starting from a yellow state).
l'ara 94J In
the Low/Mid voltage phase the colors blue and cyan are obtained from white,
and the color green from yellow.
!Para 951 The foregoing discussion of the waveforms, and specifically the
discussion of DC
balance, ignores the question of kickback voltage. In practice, as previously,
every backplane
voltage is offset from the voltage supplied by the power supply by an amounts
equal to the
kickback voltage VKB. Thus, if the power supply used provides the three
voltages +V, 0, and -
V, the bacicplane would actually receive voltages V+Vicn, VKB, and ¨V+ VKB
(note that VKB, in
the case of amorphous silicon TFTs, is usually a negative number). The same
power supply
would, however, supply +V, 0, and ¨V to the front electrode without any
kickback voltage
offset. Therefore, for example, when the front electrode is supplied with ¨V
the display would
experience a maximum voltage of 2V+ VKB and a minimum of VKB. Instead of using
a separate
power supply to supply VKB to the front electrode, which can be costly and
inconvenient, a
waveform may be divided into sections where the front electrode is supplied
with a positive
voltage, a negative voltage, and VKB.
[Para 961 As discussed above, in some of the waveforms described in the
aforementioned
Application Serial No. 14/849,658, seven different voltages can be applied to
the pixel
electrodes: three positive, three negative, and zero. Preferably, the maximum
voltages used in
these waveforms are higher than that can be handled by amorphous silicon thin-
film transistors
in the current state of the art. In such cases, high voltages can be obtained
by the use of top
plane switching, and the driving waveforms can be configured to compensate for
the kickback
voltage and can be intrinsically DC-balanced by the methods of the present
invention. Figure
7 depicts schematically one such waveform used to display a single color. As
shown in FIG.
7, the waveforms for every color have the same basic form: i.e., the waveform
is intrinsically
DC-balanced and can comprise two sections or phases: (1) a preliminary series
of frames that
is used to provide a "reset" of the display to a state from which any color
may reproducibly be
obtained and during which a DC imbalance equal and opposite to the DC
imbalance of the
remainder of the waveform is provided, and (2) a series of frames that is
particular to the color
that is to be rendered; cf. Sections A and B of the waveform shown in FIG. 6.
[Para 971 During the first "reset" phase, the reset of the display ideally
erases any memory of
a previous state, including remnant voltages and pigment configurations
specific to previously-
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Date Recue/Date Received 2022-06-22
displayed colors. Such an erasure is most effective when the display is
addressed at the
maximum possible voltage in the "reset/DC balancing" phase. In addition,
sufficient frames
may be allocated in this phase to allow for balancing of the most imbalanced
color transitions.
Since some colors require a positive DC-balance in the second section of the
waveform and
others a negative balance, in approximately half of the frames of the
"reset/DC balancing"
phase, the front electrode voltage V. is set to Vol (allowing for the maximum
possible
negative voltage between the backplane and the front electrode), and in the
remainder, V. is
set to Vim (allowing for the maximum possible positive voltage between the
backplane and the
front electrode). Empirically it has been found preferable to precede the
\icon, = VnH frames by
the V. = VpH frames.
[Para 98] The "desired" waveform (i.e., the actual voltage against time curve
which is desired
to apply across the electrophoretic medium) is illustrated at the bottom of
FIG. 7, and its
implementation with top plane switching is shown above, where the potentials
applied to the
front electrode rn, tv and
to the backplane (BP) are illustrated. It is assumed that the column
, co
driver is used connected to a power supply capable of supplying the following
voltages: Vol,
Vnii (the highest positive and negative voltages, typically in the range of
10-15 V), Vpi,
(lower positive and negative voltages, typically in the range of 1-10 V),
and zero. In addition
to these voltages, a kickback voltage VKB (a small value that is specific to
the particular
backplane used, measured as described, for example, in U.S. Patent No.
7,034,783) can be
supplied to the front electrode by an additional power supply.
[Para 991 As shown in FIG. 7, every backplane voltage is offset by VKH (shown
as a negative
number) from the voltage supplied by the power supply while the front
electrode voltages are
not so offset, except when the front electrode is explicitly set to VKH, as
described above.
[Para 100] Although the display of the invention has been described as
producing the eight
primary colors, in practice, it is preferred that as many colors as possible
be produced at the
pixel level. A full color gray scale image may then be rendered by dithering
between these
colors, using techniques well known to those skilled in imaging technology.
For example, in
addition to the eight primary colors produced as described above, the display
may be
configured to render an additional eight colors. In one embodiment, these
additional colors are:
light red, light green, light blue, dark cyan, dark magenta, dark yellow, and
two levels of gray
between black and white. The terms "light" and "dark" as used in this context
refer to colors
having substantially the same hue angle in a color space such as CIE L*a*b* as
the reference
color but a higher or lower L*, respectively.
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Date Recue/Date Received 2022-06-22
[Para 101] In general, light colors are obtained in the same manner as dark
colors, but using
waveforms having slightly different net impulse in phases B and C. Thus, for
example, light
red, light green and light blue waveforms have a more negative net impulse in
phases B and C
than the corresponding red, green and blue waveforms, whereas dark cyan, dark
magenta, and
dark yellow have a more positive net impulse in phases B and C than the
corresponding cyan,
magenta and yellow waveforms. The change in net impulse may be achieved by
altering the
lengths of pulses, the number of pulses, or the magnitudes of pulses in phases
B and C.
[Para 102] Gray colors are typically achieved by a sequence of pulses
oscillating between low
or mid voltages.
[Para 10311t will be clear to one of ordinary skill in the art that in a
display of the invention
driven using a thin-film transistor (TFT) array the available time increments
on the abscissa of
FIG. 7 will typically be quantized by the frame rate of the display. Likewise,
it will be clear
that the display is addressed by changing the potential of the pixel
electrodes relative to the
front electrode and that this may be accomplished by changing the potential of
either the pixel
electrodes or the front electrode, or both. In the present state of the art,
typically a matrix of
pixel electrodes is present on the backplane, whereas the front electrode is
common to all
pixels. Therefore, when the potential of the front electrode is changed, the
addressing of all
pixels is affected. The basic structure of the waveform described above with
reference to FIG.
7 is the same whether or not varying voltages are applied to the front
electrode.
I Para 1041 The generic waveform illustrated in FIG. 7 requires that the
driving electronics
provide as many as seven different voltages to the data lines during the
update of a selected
row of the display. While multi-level source drivers capable of delivering
seven different
voltages are available, many commercially-available source drivers for
electrophoretic displays
permit only three different voltages to be delivered during a single frame
(typically a positive
voltage, zero, and a negative voltage). Herein the term "frame" refers to a
single update of all
the rows in the display. It is possible to modify the generic waveform of FIG.
7 to accommodate
a three level source driver architecture provided that the three voltages
supplied to the panel
(typically +V, 0 and ¨V) can be changed from one frame to the next. (i.e.,
such that, for
example, in frame n voltages (+Vmax, 0, ¨Vmin) could be supplied while in
frame n+1 voltages
(+Vmid, 0, -Vma) could be supplied).
[Para 105] Referring now to FIG. 6, phase A (the reset phase) it is seen that
this phase is divided
into two sections of equal duration (illustrated by the dotted lines). When
top plane switching
is used, the top plane will be held at one potential in the first of these
sections, and at a potential
-3 1 -
Date Recue/Date Received 2022-06-22
of the opposite polarity in the second section. In the particular case of FIG.
6, during the first
such section the top plane would have been held at VpH, and the backplane at
V11, to achieve
a potential drop across the electrophoretic fluid of VH - VpH (where the
convention is used
of referencing the backplane potential relative to that of the top plane).
During the second
section, the top plane would have been held at VH, and the backplane at VpH.
As shown,
during the second section the electrophoretic fluid would have been subjected
to a potential of
VpH - VH, the highest potential available. For rendition of certain colors,
however, exposure
to this high voltage might result in an initial pigment arrangement from which
an ideal final
configuration would be difficult to achieve. For example, as noted in the
prior art, in order to
render the color cyan, it is necessary for the magenta pigment (which has the
same charge
polarity as the cyan pigment) to be tied up in an aggregate with the yellow
pigment. Such an
aggregate would be split by a high applied potential, and thus the magenta
would not be
controlled and would contaminate the cyan.
[Para 106] It is not necessary, however, to use the maximum possible voltages
in both sections
of Phase A of the waveform. All that is required in Phase A is that the prior
color state be erased
such that the newly rendered color is the same no matter which color preceded
it, and that the
net impulse provided in Phase A balance the net impulse in Phase B.
[Para 107]Therefore, an experiment was conducted in which Phase B of a
waveform of the
type illustrated in Table 1 was held constant, while the voltages applied in
each of two sections
of phase A was varied (although the same number of frames was allocated to
Phase A in each
case: 120 frames in total, 60 frames for the first and 60 frames for the
second sections). After
addressing the display, the CIELab L*, a* and b* values of each primary color
were measured.
[Para 108]Table 2 shows the default case in which the maximum possible
negative and
positive voltages are applied in the first and second sections of Phase A.
This is done using top
plane switching, in which the first listed voltage is applied to the backplane
while the second
listed voltage is applied to the top plane. The color gamut, measured as the
volume of the
convex hull containing the eight points listed in Table 2, is 21,336 AE3.
[Para 1091 Table 3 shows the case where the backplane is held at zero during
the first section
of Phase A. The voltage applied is in this case less than in the case of Table
2. The voltage
applied in the second section of Phase A is the same as the case for Table 2.
In order to maintain
DC balance, the time of application of the lower voltage must of course be
correspondingly
longer. The color gamut, measured as the volume of the convex hull containing
the eight points
listed in Table 2, is 20,987 AE3.
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Date Recue/Date Received 2022-06-22
[Para 110] Table 4 shows the case where the bacicplane is held at zero during
the second section
of Phase A. The voltage applied in the first section of Phase A is the same as
the case for Table
2. The color gamut, measured as the volume of the convex hull containing the
eight points
listed in Table 2, is 20,339 AE3.
Table 2
First reset V Second reset V Color L* a* b*
Val - VpH VpH - VnH K 24.67 2.68 -12.53
VnH - VpH VpH - VnH B 37.26 0.97 -14.51
VnH - VpH VpH - Via R 43.2 16.16 11.34
VnH - VpH VpH - VnH M 43.56 21.93 -10.65
Vnil - VpH VpH - VnH G 36.29 -19.89 13.13
VnH - VpH VpH - VnH C 48.34 -9.82 -6.73
VnH - VpH VpH - VnH Y 67.99 -10.29 56.06
Vail - VpH VpH - Voll IN 70.29 -1.24 7.83
Table 3
First reset V Second reset V Color L* a* ' b*
0- Vpil VpH - V.H K 27.82 2.2 -15.78
0- VpH VpH - Vnli B 37.99 0.41 -14.78
0- VpH VpH - VnH R 43.7 17 11.4
0- Vpil VpH - Vnil M 44.02 22.03 -10.39
0- VpH VpH - VnH G 37.37 -21.57 13.38
0- VpH VpH - VnH C 49.06 -9.96 -7.78
0- VpH VpH - VnH Y 67.73 -10.25 53.71
0- VpH VpH - VnH W 70.02 -0.99 6.7
Table 4
First reset V Second reset V . Color L* a* b*
Val - VpH 0 - VnH , K 27.42 -4.03 -10.77
VnH - VpH 0 - VnH . B 31.99 -7.38 -11.16
VnH - VpH 0 - VnH R 46.19 8.49 21.11
VnH - VpH 0 - VnH , M 47.46 12.8 -3.05
VnH - VpH 0 - VnH G 33.33 -24.63 11.2
VnH - VpH 0 - VnH . C 43.03 -19.38 -9.32
Val - VpH 0 - VnH . Y 67.21 -9.44 59.36
VnH - VpH 0 - VnH W 70.12 -3.49 14.26
[Para 111ifigure 8A shows the results of these experiments as a projection
onto the a*/b*
plane: the abscissa represents a* and the ordinate b*. It can be seen that
certain colors (for
example, red, magenta, and blue) are rendered better by the Phase A settings
corresponding to
Tables 2 or 3, while other colors (cyan, green and yellow) are rendered better
by Phase A
settings corresponding to Table 4.
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Date Recue/Date Received 2022-06-22
[Para 112] Interestingly, the alternative experiment in which the order of the
first and second
sections of Phase A was reversed gave very poor results, with all colors being
contaminated
with yellow.
[Para 113] Table 5 shows the combination of best colors from this experiment.
The color
gamut, measured as the volume of the convex hull containing the eight points
listed in Table 2,
is 28,092 tlE3. Thus, by appropriate choice of the voltages applied in the
reset phase (Phase A)
of the waveform, the color gamut was increased by a factor of about 50%. The
results of Table
are depicted in FIG. 8B.
[Para 114]The method of this invention is particularly important when it is
desired to make
the waveform as short as possible. With fixed voltages in Phase A, Phase B
needs to be made
longer in order to compensate for the bias introduced in Phase A for certain
colors.
[Para 115]Although the invention was described with only two sections in Phase
A, those of
skill in the art will understand that any reasonable number of sections may be
used. However,
when top plane switching is employed, the same structure of top plane
potentials is fixed no
matter which color is to be rendered. According to the invention, the
backplane settings
corresponding to each top plane potential are varied in Phase A of the
waveform according to
which color is being rendered, but without violating the condition that the
overall waveform,
comprising Phases A and B, be DC-balanced.
Table 5
First reset V Second reset V Color L* a* b*
V.H - VpH VpH - VH K 24.67 2.68 -12.53
0 - VpH VpH - VnH B 37.99 0.41 -14.78
0 - VpH VpH - Val R 43.7 17 11.4
0 - VpH VpH - VnH M 44.02 22.03 -10.39
VnH - VpH 0- VnH G 33.33 -24.63 11.2
VnH - VpH 0- VnH C 43.03 -19.38 -9.32
VnH - VpH 0 - VnH V 67.21 -9.44 59.36
0 - VpH VpH - VnH W 70.02 -0.99 6.7
[Para 116] DC-balancing the reset pulse can be achieved in the following way:
[Para 117] For a DC-balancing reset process, one set of voltages must be
chosen for all
transitions in the waveform. Choosing a set of voltages can be problematic
because certain
palette colors require high voltage, while others require low voltage. For a
device with a large
amount of simultaneous bacicplane voltages available, this is not a problem,
as each transition
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Date Recue/Date Received 2022-06-22
can be balanced individually, but in the case of top-plane switching, each
transition is coupled
together by the top-plane, which forces transitions to be aligned with each
other. An additional
constraint is enforced by source-driver standards, which currently limit the
number of
simultaneous backplane voltages to three.
[Para 1181A transition is a sequence of voltages applied to the backplane and
top plane, Tj =
(VI ,117), where is
the backplane voltage for transition j at frame i, andig is the top plane
voltage at frame L. Let tui = E(4/ ¨ +
nfiKB be the total impulse of T1 prior to
applying the DC-balancing reset, where nj is the update length (in frames) of
711, and VKB is the
kickback voltage of the display.
[Para 1191Let aj be the desired DC-balance impulse offset (time*V), dr be the
desired total
duration of the DC-balancing reset. The DC-balancing reset has two pulses in
it, so top-plane
voltages will need to be chosen for each pulse, and backplane voltages will
need to be chosen
ic
for each pulse and each transition. Let Vkip = vrj _ + VKB
be the voltage of the kth pulse
of transition TI, where VBrki is the backplane voltage for the kth reset pulse
of transition T1, and
VP is the top-plane voltage for the kth reset pulse. It is important that the
voltages for the two
pulses be chosen so that Kip and Vip are of opposite signs for each
transition.
(Para 1201A "zero" voltage needs to be selected, which would ideally be OV,
although that is
not always possible
vizil
= vzkf VP 1VKB
Where
Vicii I
¨Bzkiiirk v = argmin I VB ¨
[Para 1211Next, compute the global maximum duration for each of the two pulses
4174 ¨ aj
= m ax . __ .
-
UJ
2p ip
d2 dr ¨
[Para 122] Then compute the "ideal" duration of each pulse for each
transition, which is the
duration in the case that I = 0. Define the notation [x] la' = min(b, max(a,
x)). Then
dl
_ Viz ¨ c12 Vip )
2p 1p 0
c--2
(Cri µ12V2iz ¨ (Kip ¨ Vilz)) 1
di
2 VIP
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Date Recue/Date Received 2022-06-22
[Para 123] We then break each pulse into an "active" portion and a "zero"
portion in order to
balance the transition:
= aj c-12V2jz
= r_4(v2ip_ V2jz. ldi
1p ¨
ip lz
= di
=ri Cl1P(Vlip Vljz)1 2
2p Vi ¨ Vj
2p 2z
= ¨
= ¨
2z 2 2p
[Para 1241Now we are ready to construct the DC-balancing reset phase of the
waveform. The
top-plane is driven at VP for duration di, followed by Iv for duration d2. For
each transition
T1, we drive at V:" for duration dL, followed by V:" for duration dlp,
followed by VBz2i for
duration 4z, followed by VBr" for duration di as shown in FIG. 9. The
resulting waveform
2p,
that is experienced by the ink is shown in FIG. 10.
[Para 125] At first glance it might appear that the sequential scanning of the
various rows of
an active matrix display might upset the above calculations designed to ensure
accurate DC
balancing of waveforms and drive schemes, because when the voltage of the
front electrode is
changed (typically between successive scans of the active matrix), each pixel
of the display
will experience an "incorrect" voltage until the scan reaches the relevant
pixel and the voltage
on its pixel electrode is adjusted to compensate for the change in the front
electrode voltage,
and the period between the change in front plane voltage and the time when the
scan reaches
the relevant pixel varies depending upon the row in which the relevant is
located. However,
further investigation will show that the actual "error" in the impulse applied
to the pixel is
proportional to the change in front plane voltage times the period between the
front plane
voltage change and the time the scan reaches the relevant pixel. The latter
period is fixed,
assuming no change in scan rate, so that for any series of changes in front
plane voltage which
leaves the final front plane voltage equal to the initial one, the sum total
of the "errors" in
impulse will be zero, and the overall DC balance of the drive scheme will not
be affected.
[Para 126]Thus, the invention provides for DC-balanced waveforms for multi-
particle
electrophoretic displays. Having thus described several aspects and
embodiments of the
technology of this application, it is to be appreciated that various
alterations, modifications,
and improvements will readily occur to those of ordinary skill in the art.
Such alterations,
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Date Recue/Date Received 2022-06-22
modifications, and improvements are intended to be within the spirit and scope
of the
technology described in the application. For example, those of ordinary skill
in the art will
readily envision a variety of other means and/or structures for performing the
function and/or
obtaining the results and/or one or more of the advantages described herein,
and each of such
variations and/or modifications is deemed to be within the scope of the
embodiments described
herein. Those skilled in the art will recognize, or be able to ascertain using
no more than routine
experimentation, many equivalents to the specific embodiments described
herein. It is,
therefore, to be understood that the foregoing embodiments are presented by
way of example
only and that, within the scope of the appended claims and equivalents
thereto, inventive
embodiments may be practiced otherwise than as specifically described. In
addition, any
combination of two or more features, systems, articles, materials, kits,
and/or methods
described herein, if such features, systems, articles, materials, kits, and/or
methods are not
mutually inconsistent, is included within the scope of the present disclosure.
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Date Recue/Date Received 2022-06-22
