U.S. patent application number 11/611324 was filed with the patent office on 2007-04-26 for methods for driving electro-optic displays, and apparatus for use therein.
This patent application is currently assigned to E INK CORPORATION. Invention is credited to Karl R. Amundson, Alexi C. Arango, Guy M. Danner, Jay Britton Ewing, Robert W. Zehner.
Application Number | 20070091418 11/611324 |
Document ID | / |
Family ID | 37985061 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070091418 |
Kind Code |
A1 |
Danner; Guy M. ; et
al. |
April 26, 2007 |
METHODS FOR DRIVING ELECTRO-OPTIC DISPLAYS, AND APPARATUS FOR USE
THEREIN
Abstract
A method for addressing a bistable electro-optic display having
at least one pixel comprises applying an addressing pulse to drive
the pixel to a first optical state; leaving the pixel undriven for
a period of time, thereby permitting the pixel to assume a second
optical state different from the first optical state; and applying
to the pixel a refresh pulse which substantially restores the pixel
to the first optical state, the refresh pulse being short relative
to the addressing pulse.
Inventors: |
Danner; Guy M.; (Somerville,
MA) ; Amundson; Karl R.; (Cambridge, MA) ;
Zehner; Robert W.; (Cambridge, MA) ; Arango; Alexi
C.; (Somerville, MA) ; Ewing; Jay Britton;
(Somerville, MA) |
Correspondence
Address: |
DAVID J COLE;E INK CORPORATION
733 CONCORD AVE
CAMBRIDGE
MA
02138-1002
US
|
Assignee: |
E INK CORPORATION
733 Concord Avenue
Cambridge
MA
02138-1002
|
Family ID: |
37985061 |
Appl. No.: |
11/611324 |
Filed: |
December 15, 2006 |
Related U.S. Patent Documents
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Application
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Patent Number |
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10249973 |
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11611324 |
Dec 15, 2006 |
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10065795 |
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7012600 |
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10249973 |
May 23, 2003 |
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09561424 |
Apr 28, 2000 |
6531997 |
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10065795 |
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09520743 |
Mar 8, 2000 |
6504524 |
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09561424 |
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60319321 |
Jun 18, 2002 |
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60319315 |
Jun 13, 2002 |
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60319007 |
Nov 20, 2001 |
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60319010 |
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60319034 |
Dec 18, 2001 |
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60319037 |
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60319040 |
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60131790 |
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Current U.S.
Class: |
359/296 |
Current CPC
Class: |
G09G 3/3453 20130101;
G09G 2310/063 20130101; G09G 2310/061 20130101; G09G 2320/041
20130101; G09G 3/2007 20130101; G09G 2310/0254 20130101; G09G
3/2018 20130101; G09G 2320/0285 20130101; G09G 3/2014 20130101;
G09G 2320/0247 20130101; G09G 3/344 20130101; G09G 2320/0252
20130101; G09G 2310/02 20130101; G09G 2340/16 20130101; G09G
2300/08 20130101; G02F 1/167 20130101; G09G 3/2011 20130101; G09G
2320/043 20130101; G09G 3/2077 20130101; G09G 2310/06 20130101;
G09G 2330/021 20130101; G09G 3/2074 20130101; G09G 2310/04
20130101; G09G 2310/068 20130101; G09G 2320/0204 20130101; G09G
2380/02 20130101; G09G 2320/0214 20130101; G09G 2310/027
20130101 |
Class at
Publication: |
359/296 |
International
Class: |
G02B 26/00 20060101
G02B026/00 |
Claims
1. A method for addressing a bistable electro-optic medium which
comprises applying to the medium an alternating current pulse
having a direct current offset.
2. A method according to claim 1 wherein the alternating current
pulse has substantially the form of an alternating square wave.
3. A method according to claim 1 wherein the application of the
alternating current pulse having a direct current offset to the
medium is continued for a period sufficient to cause the medium to
assume a substantially constant optical state.
4. A method according to claim 1 wherein there is first applied to
the medium an alternating current pulse having a first direct
current offset, thereby causing the medium to assume a first
optical state, and thereafter there is applied to the medium an
alternating current pulse having a second direct current offset
different from the first offset, thereby causing the medium to
assume a second optical state different from the first optical
state.
5. A method according to claim 1 wherein the electro-optic medium
is an electrophoretic medium in which a plurality of charged
particles move through a fluid under the influence of an electric
field.
6. A method according to claim 5 wherein the plurality of charged
particles and the fluid are confined within a plurality of
capsules.
7. A method according to claim 5 wherein the plurality of charged
particles and the fluid are retained within a plurality of cavities
formed within a carrier medium.
8. A method according to claim 5 wherein the plurality of charged
particles and the fluid are present as a plurality of droplets
surrounded by a continuous phase of a polymeric material.
9. A method according to claim 1 wherein the electro-optic medium
has a first display state which is substantially opaque and a
second display state which is light-transmissive.
10. A method for addressing a bistable electro-optic medium which
comprises applying to the medium an alternating current pulse, and
varying at least one of the duty cycle and the frequency of the
pulse to change the optical state of the electro-optic medium
following the alternating current pulse.
11. A method according to claim 10 wherein the alternating current
pulse has substantially the form of an alternating square wave.
12. A method according to claim 1 wherein the application of the
alternating current pulse to the medium is continued for a period
sufficient to cause the medium to assume a substantially constant
optical state.
13. A method according to claim 10 wherein the duty cycle but not
the frequency of the alternating current pulse is varied.
14. A method according to claim 13 wherein there is first applied
to the medium an alternating current pulse having a first duty
cycle offset, thereby causing the medium to assume a first optical
state, and thereafter there is applied to the medium an alternating
current pulse having a second duty cycle offset different from the
first duty cycle, thereby causing the medium to assume a second
optical state different from the first optical state.
15. A method according to claim 10 wherein the frequency but not
the duty cycle of the alternating current pulse is varied.
16. A method according to claim 10 wherein the electro-optic medium
is an electrophoretic medium in which a plurality of charged
particles move through a fluid under the influence of an electric
field.
17. A method according to claim 16 wherein the plurality of charged
particles and the fluid are confined within a plurality of
capsules.
18. A method according to claim 16 wherein the plurality of charged
particles and the fluid are retained within a plurality of cavities
formed within a carrier medium.
19. A method according to claim 16 wherein the plurality of charged
particles and the fluid are present as a plurality of droplets
surrounded by a continuous phase of a polymeric material.
20. A method according to claim 10 wherein the electro-optic medium
has a first display state which is substantially opaque and a
second display state which is light-transmissive.
21. A method for addressing a bistable electro-optic medium capable
of displaying at least three optical states including two extreme
optical states and at least one gray state intermediate the two
extreme optical states, which method comprises driving the medium
from one extreme optical state to a final gray state by first
applying to the medium a direct current pulse which drives the
medium from the one extreme optical state to an intermediate gray
state different from the final gray state, and thereafter applying
to the medium an alternating current pulse which drives the medium
from the intermediate gray state to the final gray state.
22. A method according to claim 21 which is carried out using drive
circuitry capable of applying only potential differences of +V, 0
and -V across the electro-optic medium, where V is an arbitrary
drive voltage.
23. A method for addressing a bistable electro-optic display having
a plurality of pixels arranged in a plurality of rows and a
plurality of columns, a plurality of row electrodes each associated
with one of the plurality of rows, a plurality of column electrodes
each associated with one of the plurality of columns, and drive
means arranged to select each of the row electrodes in turn and to
apply to the column electrodes during the selection of any given
row electrode voltages chosen so as to address the pixels in the
row associated with the selected row electrode and write one row of
a desired image on to the display, the method comprising: writing a
first image to the display; receiving data representing a second
image to be written to the display; comparing the first and second
images and dividing the rows of the display into a first set, in
which at least one pixel of the row differs between the first and
second images, and a second set, in which no pixel of the row
differs between the first and second images; and writing the second
image by sequentially selecting only the row electrodes associated
with the first set of rows, and applying voltages to the column
electrodes to write only the first set of rows, thereby forming the
second image to the display.
24. An electro-optic display having a plurality of pixels, at least
one of the pixels comprising a plurality of sub-pixels differing
from each other in area, the display comprising drive means
arranged to change the optical state of the sub-pixels
independently of one another.
25. An electro-optic display according to claim 24 wherein at least
two of the sub-pixels differ in area by substantially a factor of
two.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of copending application
Ser. No. 10/249,973, filed May 23, 2003 (Publication No.
2005/0270261), which is a continuation-in-part of application Ser.
No. 10/065,795, filed Nov. 20, 2002 (now U.S. Pat. No. 7,012,600),
which is itself a continuation-in-part of application Ser. No.
09/561,424, filed Apr. 28, 2000 (now U.S. Pat. No. 6,531,997),
which is itself a continuation-in-part of copending application
Ser. No. 09/520,743, filed Mar. 8, 2000 (now U.S. Pat. No.
6,504,524). Application Ser. No. 10/065,795 also claims priority
from the following Provisional Applications: (a) Ser. No.
60/319,007, filed Nov. 20, 2001; (b) Ser. No. 60/319,010, filed
Nov. 21, 2001; (c) Ser. No. 60/319,034, filed Dec. 18, 2001; (d)
Ser. No. 60/319,037, filed Dec. 20, 2001; and (e) Ser. No.
60/319,040, filed Dec. 21, 2001. Application Ser. No. 10/249,973
also claims priority from copending Application Ser. No.
60/319,315, filed Jun. 13, 2002 and copending Application Ser. No.
60/319,321, filed Jun. 18, 2002. Application Ser. No. 09/561,424
also claims priority from Application Ser. No. 60/131,790, filed
Apr. 30, 1999.
[0002] This application is also related to copending application
Ser. No. 10/063,236, filed Apr. 2, 2002 (Publication No.
2002/0180687). The entire contents of the aforementioned
applications, and of all U.S. patents and published applications
mentioned below, are herein incorporated by reference.
BACKGROUND OF INVENTION
[0003] This invention relates to methods and apparatus for driving
electro-optic displays, particularly bistable electro-optic
displays. The methods and apparatus of the present invention are
especially, though not exclusively, intended for use in driving
bistable electrophoretic displays.
[0004] The term "electro-optic" as applied to a material or a
display, is used herein in its conventional meaning in the imaging
art to refer to a material having first and second display states
differing in at least one optical property, the material being
changed from its first to its second display state by application
of an electric field to the material. Although the optical property
is typically color perceptible to the human eye, it may be another
optical property, such as optical transmission, reflectance,
luminescence or, in the case of displays intended for machine
reading, pseudo-color in the sense of a change in reflectance of
electromagnetic wavelengths outside the visible range.
[0005] 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 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 transition between the two extreme states may not be a color
change at all.
[0006] 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 the
aforementioned copending application Ser. No. 10/063,236 that some
particle-based electrophoretic displays capable of gray scale are
stable not only in their extreme 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.
[0007] The term "impulse" is used herein in its conventional
meaning of the integral of voltage with respect to time. However,
some bistable electro-optic media act as charge transducers, and
with such media an alternative definition of impulse, namely the
integral of current over time (which is equal to the total charge
applied) may be used. The appropriate definition of impulse should
be used, depending on whether the medium acts as a voltage-time
impulse transducer or a charge impulse transducer.
[0008] Several types of bistable electro-optic displays are known.
One type of electro-optic display is a rotating bichromal member
type as described, for example, in U.S. Pat. Nos. 5,808,783;
5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124;
6,137,467; and 6,147,791 (although this type of display is often
referred to as a "rotating bichromal ball" display, the term
"rotating bichromal member" is preferred as more accurate since in
some of the patents mentioned above the rotating members are not
spherical). Such a display uses a large number of small bodies
(typically spherical or cylindrical) which have two or more
sections with differing optical characteristics, and an internal
dipole. These bodies are suspended within liquid-filled vacuoles
within a matrix, the vacuoles being filled with liquid so that the
bodies are free to rotate. The appearance of the display is changed
to applying an electric field thereto, thus rotating the bodies to
various positions and varying which of the sections of the bodies
is seen through a viewing surface.
[0009] Another type of electro-optic medium uses an electrochromic
medium, for example an electrochromic medium in the form of a
nanochromic film comprising an electrode formed at least in part
from a semi-conducting metal oxide and a plurality of dye molecules
capable of reversible color change attached to the electrode; see,
for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood,
D., Information Display, 18(3), 24 (March 2002). See also Bach, U.,
et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this
type are also described, for example, in U.S. Pat. No. 6,301,038,
International Application Publication No. WO 01/27690, and in
copending application Ser. No. 10/249,128 filed Mar. 18, 2003 (now
U.S. Pat. No. 6,950,220).
[0010] Another type of electro-optic display, which has been the
subject of intense research and development for a number of years,
is the particle-based electrophoretic display, in which a plurality
of charged particles move through a suspending 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.
[0011] Numerous patents and applications assigned to or in the
names of the Massachusetts Institute of Technology (MIT) and E Ink
Corporation have recently been published describing encapsulated
electrophoretic media. Such encapsulated media comprise numerous
small capsules, each of which itself comprises an internal phase
containing electrophoretically-mobile particles suspended in a
liquid suspension 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. Encapsulated media of this type are described, for
example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;
6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;
6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;
6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;
6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;
6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;
6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;
6,521,489; 6,531,997; 6,535,197; 6,538,801; and 6,545,291; and U.S.
Patent Applications Publication Nos. 2002/0019081; 2002/0021270;
2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677;
2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832;
2002/0131147; 2002/0145792; 2002/0154382, 2002/0171910;
2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560;
2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855;
2003/0034949; 2003/0038755; and 2003/0053189; and International
Applications Publication Nos. WO 99/67678; WO 00/05704; WO
00/20922; WO 00/26761; WO 00/38000; WO 00/38001; WO 00/36560; WO
00/67110; WO 00/67327; WO 01/07961; and WO 01/08241.
[0012] 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, the
aforementioned 2002/0131147. Accordingly, for purposes of the
present application, such polymer-dispersed electrophoretic media
are regarded as sub-species of encapsulated electrophoretic
media.
[0013] 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; 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.
[0014] A related type of electrophoretic display is a so-called
"microcell electrophoretic display". In a microcell electrophoretic
display, the charged particles and the suspending 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, International Applications
Publication No. WO 02/01281, and published US Application No.
2002-0075556, both assigned to Sipix Imaging, Inc.
[0015] Although electrophoretic displays are often opaque (since
the particles substantially block transmission of visible light
through the display) and operate in a reflective mode,
electrophoretic displays can be made to operate in a so-called
"shutter mode" in which the particles are arranged to move
laterally within the display so that the display has one display
state which is substantially opaque and one which is
light-transmissive. See, for example, the aforementioned U.S. Pat.
Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552;
6,144,361; 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. Pat. No. 4,418,346. Other types of
electro-optic displays may also be capable of operating in shutter
mode.
[0016] The bistable or multi-stable behavior of particle-based
electrophoretic displays, and other electro-optic displays
displaying similar behavior, is in marked contrast to that of
conventional liquid crystal ("LC") displays. Twisted nematic liquid
crystals act are not bi- or multi-stable but act as voltage
transducers, so that applying a given electric field to a pixel of
such a display produces a specific gray level at the pixel,
regardless of the gray level previously present at the pixel.
Furthermore, LC displays are only driven in one direction (from
non-transmissive or "dark` to transmissive or "light"), the reverse
transition from a lighter state to a darker one being effected by
reducing or eliminating the electric field. Finally, the gray level
of a pixel of an LC display is not sensitive to the polarity of the
electric field, only to its magnitude, and indeed for technical
reasons commercial LC displays usually reverse the polarity of the
driving field at frequent intervals. In contrast, bistable
electro-optic displays act, to a first approximation, as impulse
transducers, so that the final state of a pixel depends not only
upon the electric field applied and the time for which this field
is applied, but also upon the state of the pixel prior to the
application of the electric field.
[0017] Although as already indicated, electrophoretic and some
other electro-optic displays exhibit bistability, this bistability
is not unlimited, and images on the display slowly fade with time,
so that if an image is to be maintained for extended periods, the
image may have to be refreshed periodically, so as to restore the
image to the optical state which it has when first written.
[0018] However, such refreshing of the image may give rise to its
own problems. As discussed in the aforementioned U.S. Pat. Nos.
6,531,997 and 6,504,524, problems may be encountered, and the
working lifetime of a display reduced, if the method used to drive
the display does not result in zero, or near zero, net
time-averaged applied electric field across the electro-optic
medium. A drive method which does result in zero net time-averaged
applied electric field across the electro-optic medium is
conveniently referred to a "direct current balanced" or "DC
balanced". If an image is to be maintained for extended periods by
applying refreshing pulses, these pulses need to be of the same
polarity as the addressing pulse originally used to drive the
relevant pixel of the display to the optical state being
maintained, which results in a DC imbalanced drive scheme.
[0019] According to one aspect of the present invention, it has
been found that images on displays can be refreshed, while reducing
the deleterious effects associated with DC imbalanced drive
schemes, if the refreshing is effected with short pulses.
[0020] A further aspect of the present invention relates to dealing
with the problem that the aforementioned drive requirements of
bistable electro-optic displays render conventional driving methods
used for driving LCD's unsuitable for such bistable electro-optic
displays. Furthermore, as discussed in the aforementioned U.S. Pat.
Nos. 6,531,997 and 6,504,524, problems may be encountered, and the
working lifetime of a display reduced, if the method used to drive
the display does not result in zero, or near zero, net
time-averaged applied electric field across the electro-optic
medium. A drive method which does result in zero net time-averaged
applied electric field across the electro-optic medium is
conveniently referred to a "direct current balanced" or "DC
balanced". Similar problems could be encountered with LCD's, but
the insensitivity of such displays to the polarity of the applied
electric field, and the consequent ability to reverse polarity at
will, renders DC balance problems unimportant in LCD's. However,
the need for DC balance is an important consideration in devising
drive schemes for bistable electro-optic displays in which the
electro-optic medium is sensitive to the polarity of the applied
electric field.
[0021] Accordingly, a further aspect of the present invention
relates to methods and apparatus for driving electro-optic displays
which meet the particular requirements of bistable displays already
discussed. Certain methods and apparatus of the present invention
are especially intended for producing accurate gray scale rendition
in bistable displays.
SUMMARY OF THE INVENTION
[0022] Accordingly, in one aspect, this invention provides a method
for addressing a bistable electro-optic display having at least one
pixel, the method comprising:
[0023] applying an addressing pulse to drive the pixel to a first
optical state;
[0024] leaving the pixel undriven for a period of time, thereby
permitting the pixel to assume a second optical state different
from the first optical state; and
[0025] applying to the pixel a refresh pulse which substantially
restores the pixel to the first optical state, the refresh pulse
being short relative to the addressing pulse.
[0026] This aspect of the invention may hereinafter for convenience
be referred to as the "refresh pulse" method of the invention.
[0027] In this refresh pulse method, the refresh pulse will
typically have an impulse not greater than about 20 percent of the
impulse of the addressing pulse, desirably not greater than about
10 percent of this impulse, and preferably not greater than about 5
percent of this impulse. For reasons explained below, typically the
difference between the first and second optical states does not
exceed about 1 unit of L* (where L* has the usual CIE definition);
desirably this difference does not exceed about 0.5 unit of L*, and
preferably does not exceed about 0.2 unit of L*. A plurality of
refresh pulses may be applied to the pixel at regular
intervals.
[0028] In one form of the refresh pulse method, after application
of the refresh pulse, there is applied to the display a second
addressing pulse which drives the pixel to a third optical state
different from the first and second optical states, and wherein the
impulse applied by the second addressing pulse is the sum of (a)
the impulse needed to drive the pixel from the first to the third
optical state; and (b) an impulse equal in magnitude but opposite
in polarity to the algebraic sum of the refresh pulses applied to
the pixel between the first and second addressing pulses. The
second addressing pulse may be of constant voltage but variable
duration. In a display comprising a plurality of pixels, the second
addressing pulse may be a blanking pulse which drives all the
pixels of the display to one extreme optical state. In one
preferred form of such a "blanking pulse/refresh pulse" process,
the display comprises a plurality of pixels, the first addressing
pulse is applied to each pixel so as to drive a first group of
pixels white and a second group of pixels black, at least one
refresh pulse is applied to each pixel, and there are thereafter
applied to the display a first blanking pulse which turns all the
pixels black and a second blanking pulse which drives all the
pixels white, the two blanking pulses being applied in either
order. The impulse applied to each of the first group of pixels
during the first blanking pulse is the sum of (a) the impulse
needed to drive the pixel from white to black; and (b) an impulse
equal in magnitude but opposite in polarity to the algebraic sum of
the refresh pulses applied to the pixel between the first
addressing pulse and the first blanking pulse. Similarly, the
impulse applied to each of the second group of pixels during the
second blanking pulse is the sum of (c) the impulse needed to drive
the pixel from black to white; and (d) an impulse equal in
magnitude but opposite in polarity to the algebraic sum of the
refresh pulses applied to the pixel between the first addressing
pulse and the first blanking pulse.
[0029] The refresh pulse method of the invention may be used with
any of the types of electro-optic medium previously described.
Thus, in this method, the display may be a rotating bichromal
member or electrochromic display, or an electrophoretic display,
desirably an encapsulated electrophoretic display.
[0030] In another aspect, this invention provides a method for
addressing a bistable electro-optic medium which comprises applying
to the medium an alternating current pulse having a direct current
offset.
[0031] In another aspect, this invention provides a method for
addressing a bistable electro-optic medium which comprises applying
to the medium an alternating current pulse, and varying at least
one of the duty cycle and the frequency of the pulse to change the
optical state of the electro-optic medium following the alternating
current pulse.
[0032] In another aspect, this invention provides a method for
addressing a bistable electro-optic display having a plurality of
pixels arranged in a plurality of rows and a plurality of columns,
a plurality of row electrodes each associated with one of the
plurality of rows, a plurality of column electrodes each associated
with one of the plurality of columns, and drive means arranged to
select each of the row electrodes in turn and to apply to the
column electrodes during the selection of any given row electrode
voltages chosen so as to address the pixels in the row associated
with the selected row electrode and write one row of a desired
image on to the display. The method comprises:
[0033] writing a first image to the display;
[0034] receiving data representing a second image to be written to
the display;
[0035] comparing the first and second images and dividing the rows
of the display into a first set, in which at least one pixel of the
row differs between the first and second images, and a second set,
in which no pixel of the row differs between the first and second
images; and
[0036] writing the second image by sequentially selecting only the
row electrodes associated with the first set of rows, and applying
voltages to the column electrodes to write only the first set of
rows, thereby forming the second image on the display.
[0037] In another aspect, this invention provides an electro-optic
display having a plurality of pixels, at least one of the pixels
comprising a plurality of sub-pixels differing from each other in
area, the display comprising drive means arranged to change the
optical state of the sub-pixels independently of one another. In
such a display, desirably at least two of the sub-pixels differ in
area by substantially a factor of two.
BRIEF DESCRIPTION OF DRAWINGS
[0038] Preferred embodiments of the present invention will now be
described, though by way of illustration only, with reference to
the accompanying drawings, in which:
[0039] FIG. 1 is a graph showing variations of gray level with time
in a display addressed using a direct current pulse with pulse
length modulation;
[0040] FIG. 2 is a graph similar to FIG. 1 for a display addressed
using a direct current pulse with pulse height modulation;
[0041] FIG. 3 is a graph similar to FIG. 1 for a display addressed
using an alternating current pulse with a direct current offset in
accordance with the present invention;
[0042] FIG. 4 is a graph similar to FIG. 1 for a display addressed
using an alternating current pulse with duty cycle modulation in
accordance with the present invention;
[0043] FIG. 5 is a graph showing variations of gray level with time
in a display addressed using a double-prepulse slideshow
waveform;
[0044] FIG. 6 is a graph showing variations of gray level with time
in a display addressed using a single-prepulse slideshow waveform;
and
[0045] FIGS. 7A and 7B show possible arrangements of sub-pixels
within a single pixel of a display of the present invention.
DETAILED DESCRIPTION
[0046] As already indicated, the present invention provides a
number of improvements in methods for addressing electro-optic
media and displays, and in the construction of such displays. The
various aspects of the invention will now be described
sequentially, but it should be recognized that a single
electro-optic medium or display may make use of more than one
aspect of the invention. For example, a single electro-optic
display might use AC pulse with DC offset driving and also use
refresh pulses.
[0047] Refresh Pulse Method of the Invention
[0048] As already mentioned, in one aspect the present invention
provides a method for refreshing the image on an electro-optic
display by applying to the display a short refresh pulse. Thus, in
the method of the present invention, one first applies to a pixel
of a bistable display an addressing pulse which is sufficient to
change the optical state of that pixel. After leaving the display
undriven for an interval, one applies to the pixel a refresh pulse
which is short relative to the addressing pulse. Typically, the
impulse applied by the refresh pulse is not greater than about 20
(desirably not greater than 10, and preferably not greater than 5)
percent of the impulse applied by the addressing pulse. For
example, if a pixel requires an addressing pulse of 15 V for 500
msec, a refresh pulse could be 15 V for 10 msec, with an impulse of
2 percent of that of the addressing pulse.
[0049] The timing of the refresh pulses in this method should be
adjusted to take account of the sensitivity of the human eye to
abrupt small changes in optical state. The human eye is relatively
tolerant of gradual fading of an image so that, for example, the
bistability of an electro-optic medium of often measured as the
time necessary for the lightness L* (defined by the usual CIE
definition; see, for example, Hunt, R. W. G. Measuring Color, 3rd
edition, Fountain Press, Kingston-upon-Thames, England (1998).
(ISBN 0 86343 387 1)) to change by 2 units from the maximum for
white optical states (or minimum for black states) observed after
the conclusion of the addressing pulse. However, when one applies a
refresh pulse to a display, an abrupt change in the lightness of
the relevant pixel occurs, and abrupt changes substantially less
than 1 unit in L* are readily perceived by the human eye. Depending
upon the interval between refresh pulses, the changes in the image
caused by these pulses may appear as a "flicker" in the image, and
such flicker is highly objectionable to most observers. To avoid
such flicker, or other noticeable variation in the image caused by
the refresh pulses, it is desirable that the interval between the
addressing pulse and the first refresh pulse, or between successive
refresh pulses, be chosen so that each refresh pulse causes a
minimal change in the image. Thus, the change in L* caused by a
single refresh pulse should be less than about 1 unit of L*,
desirably less than about 0.5 unit, and preferably less than about
0.2 unit.
[0050] Although the use of refresh pulses in the present method
introduces some DC imbalance into the drive scheme during the
period in which the refresh pulses are being applied, it does not
preclude achieving long term DC balance in the drive scheme, and it
has been found that it is the long term rather than short term DC
balance which is of major importance in determining the operating
life of electro-optic displays. To achieve such long term DC
balance, after one or more refresh pulses have been applied, the
pixel which has received the refresh pulses may be driven to its
opposed optical state by a "switching" or second addressing pulse,
and the impulse applied in this switching addressing pulse may be
adjusted to provide DC balance (or at least minimal DC imbalance)
over the whole period since the first addressing pulse, by
adjusting the impulse of this second addressing pulse by an amount
substantially equal in magnitude, but opposite in polarity, to the
algebraic sum of the refresh pulses applied to the pixel between
the two addressing pulses. For example, consider a display which
can be switched between white and black optical states by applying
an impulse of .+-.15 V for 500 msec. Suppose a pixel of this
display is first switched from black to white by applying an
impulse of +15 V for 500 msec, and the white state of the pixel is
subsequently maintained by applying at intervals ten refresh pulses
each of +15 V for 10 msec. If after these ten refresh pulses, it is
desired to return the pixel to its black optical state, this may be
achieved by applying an addressing pulse of -15 V for 600 (rather
than 500) msec, thereby achieving overall DC balance over the whole
black-white-black transition of the pixel.
[0051] This type of adjustment of the switching addressing pulse
may be effected when a new image is to be written on the display
and it is thus necessary to change the optical states of certain
pixels. Alternatively, the adjustment may be carried out during the
application of "blanking pulses" to the display. As discussed in
the aforementioned application Ser. No. 10/065,795, it is often
necessary or desirable to apply at regular intervals to an
electro-optic display so-called "blanking pulses"; such blanking
pulses involve first driving all the pixels of the display to one
extreme optical state (for example, a white state), then driving
all the pixels to the opposite optical state (for example, black),
and then writing the desired image. Effecting the adjustment during
blanking pulses has the advantage that all of the pixels may be DC
balanced at substantially the same time; the pixels which were
black in the prior image (the image present immediately prior to
the blanking pulse) can be DC balanced during the blanking pulse
which drives all pixels white, while the pixels which were white in
the prior image can be DC balanced during the blanking pulse which
drives all pixels black, using the technique already described in
detail above. Also, effecting the adjustment during blanking pulses
can avoid the need to keep track of how many refresh pulses each
individual pixel has received since its previous addressing pulse;
assuming that black and white pixels are refreshed at the same
intervals (as will usually be the case), and that a blanking pulse
is inserted at each image transition, each pixel will need the same
adjustment (except for polarity) during the blanking pulse, this
adjustment being determined by the number of refresh pulses which
have been applied to the display since the previous blanking pulse.
Also, effecting DC balancing during blanking pulses provides a way
to apply the refresh pulse method to electro-optic displays having
more than two gray levels, since obviously adjusting the impulse
applied during a gray-to-gray transition in such a display may lead
to undesirable errors in gray levels.
[0052] The refresh pulse method of the present invention may be
used as an alternative to, or in combination with, additives for
increasing the bistability of an electro-optic medium. For example,
the present invention may be used with the electrophoretic media
described in the aforementioned 2002/0180687, which media have a
suspending fluid having dissolved or dispersed therein a polymer
which increases the bistability of the medium.
[0053] The following Example is now given, though by way of
illustration only, to show one embodiment of the refresh pulse
method of the present invention.
EXAMPLE 1
[0054] This Example uses displays containing an encapsulated dual
particle opposite charge type medium comprising a polymer-coated
titania white particle and a polymer-coated black particle with an
uncolored suspending fluid. The displays were prepared
substantially according to "Method B" described in Paragraphs
[0061]-[0068] of the aforementioned 2002/0180687.
[0055] The displays prepared as described above, which contained
multiple pixels, could be switched between their black and white
optical states using addressing pulses of .+-.15 V for 500 msec.
The displays had only limited bistability, the time necessary for
the white optical state to change by 2 L* units being only about 15
sec. at ambient temperature. However, it was determined empirically
that the white and black optical states could be maintained
indefinitely by applying short refresh pulses of .+-.15 V for 4
sec/min, a duty cycle of approximately 6.7 percent. To provide a
realistic test and avoid flicker in a standard image (containing
both black and white areas) used in these experiments, after an
initial 500 msec addressing pulse, .+-.15 V refresh pulses of
approximately 7 msec duration were applied to both the black and
white pixels of the display at intervals of approximately 100
msec.
[0056] To determine the effects of various periods of DC imbalanced
drive schemes on the displays, four different drive schemes were
tested:
[0057] Scheme 480:
[0058] The display was addressed with the standard image, and this
image was maintained using the aforementioned refresh pulses for
480 minutes. A series of blanking pulses were then applied, and the
cycle of addressing and refresh pulses repeated. No DC balancing
pulse was applied at any time. After 83 hours of operation, a
series of blanking pulses were applied and then separate areas of
the display, which had been white and black respectively in the
standard image were tested. The area of the display which had been
held white during the testing period is denoted by "480W" in the
Table below, while the area which has been held black is denoted by
"480D". Each tested area was driven to its white optical state by a
standard 500 msec addressing pulse, and its percentage reflectance
value measured; this value is denoted by "w %" in the Table. Each
tested area was then allowed to stand for 15 sec without any
refresh pulses being applied, and the change in L* measured after
this 15 second interval; the resultant change in L*, known as the
"bright holding difference", is denoted by "bhdl" in the Table.
After applying further blanking pulses, each tested area was driven
to its black optical state by a standard 500 msec addressing pulse,
and its percentage reflectance value measured; this value is
denoted by "d %" in the Table. Each tested area was then allowed to
stand for 15 sec without any refresh pulses being applied, and the
change in L* measured after this 15 second interval; the resultant
change in L*, known as the "dark holding difference", is denoted by
"dhdl" in the Table.
[0059] Scheme 60:
[0060] This scheme was identical to Scheme 480, except that the
image was maintained for only 60 minutes before blanking pulses
were applied. The area of the display which had been held white
during the testing period is denoted by "60W" in the Table below,
while the area which has been held black is denoted by "60D".
[0061] Scheme 10:
[0062] In this scheme, the image was written in the same way as in
Scheme 480, and maintained for 10 minutes using the same refresh
pulses as in Scheme 480. A 40 sec pulse of opposite polarity was
then applied to DC balance the display, and then the image was
rewritten and the cycle repeated. The area of the display which had
been held white during the testing period is denoted by "10W" in
the Table below, while the area which has been held black is
denoted by "10D".
[0063] Scheme 1:
[0064] This scheme was identical to Scheme 10, except that the
image was maintained for only 1 minute and then a 4 second DC
balancing pulse was applied and the cycle repeated. The area of the
display which had been held white during the testing period is
denoted by "1 W" in the Table below, while the area which has been
held black is denoted by "1D".
[0065] The results obtained in these experiments are shown in Table
1 below. TABLE-US-00001 TABLE 1 480 W 480 D 60 W 60 D w % 37.90
30.63 38.21 38.47 d % 2.89 2.69 3.03 2.45 dhdl 2.05 0.64 4.79 1.05
bhdl -1.34 -4.06 -0.47 -2.72 10 W 10 D 1 W 1 D w % 37.31 37.39
37.20 37.20 d % 2.75 2.75 3.14 3.13 dhdl 0.89 0.84 0.98 0.99 bhdl
-2.24 -2.30 -2.02 -1.98
[0066] From the data in Table 1, it will be seen that, in the
highly unbalanced Scheme 480, the white state reflectance differed
markedly between the areas of the display held white and black
during the testing period, and the bright and dark holding
differences also differed markedly. Thus, this highly unbalanced
drive scheme produced substantial change in the optical states of
the display, quite apart from any other effects, such as damage to
the electrodes, which may occur with such unbalanced drive schemes.
Also, as shown by the differences in bright and dark holding
differences, the unbalanced drive scheme introduced a "bias" to the
display, in the sense that areas held white for long periods tended
to remain white thereafter whereas areas held black for long
periods tended to remain black thereafter. The results obtained
from the unbalanced Scheme 60 were similar but, as would be
expected, less marked. In contrast, both the DC balanced Schemes 10
and 1 showed essentially no differences between the areas held
white and black.
[0067] Thus, these experiments showed that the temporary DC
imbalance produced by the use of short refresh pulses did not
adversely affect the properties of the displays provided that long
term DC balance was produced by spaced blanking pulses.
[0068] Electrophoretic media used in the refresh pulse method of
the present invention may employ the same components and
manufacturing techniques as in the aforementioned E Ink and MIT
patents and applications, to which the reader is referred for
further information.
[0069] Fundamental Elements of Grayscale Drive Waveforms (Including
use of AC Pulses)
[0070] Currently, as described in the aforementioned U.S. Pat. Nos.
6,531,997 and 6,504,524, many displays are switched from one
extreme optical state to the other (for example, from black to
white or vice versa) by applying a voltage pulse of sufficient
duration to saturate the electro-optic medium; for example, in a
particle-based electrophoretic medium, to move charged particles
all the way to a front or back electrode. The conventional
requirement that the electro-optic medium be addressed until the
optical state becomes saturated does not allow for intermediate
gray states. An electro-optic display that achieves grayscale
offers significant advantages in graphics capability and image
quality.
[0071] For convenience, a voltage waveform or drive scheme capable
of achieving grayscale in a bistable electro-optic display may
hereinafter be called a "grayscale waveform" or "grayscale drive
scheme" respectively. There are five fundamental grayscale waveform
elements which may be used in such a grayscale waveform or drive
scheme; the term "grayscale waveform element" is used to mean a
voltage pulse, or series of voltage pulses, that is capable of
producing a change in optical state of an electro-optic display. A
grayscale waveform element is itself capable of generating
grayscale, and one or more grayscale waveform elements arranged in
a particular sequence together form a grayscale drive waveform. A
grayscale drive waveform is capable of switching a pixel of a
display from one gray state to another. A sequence of one or more
drive waveforms makes up a drive scheme, which is capable of
displaying any series of grayscale images on a display.
[0072] Drive waveform elements fall into two basic categories,
namely direct current (DC) voltage pulses and alternating current
(AC) voltage pulses. In both cases, the parameters of the pulse
that can be varied are the pulse height and the pulse length.
[0073] Although the generation of a grayscale optical state in an
electro-optic medium is critically dependent on the manner in which
voltage is applied to the medium, the ability of the medium to hold
that grayscale optical state once the voltage is no longer applied
is equally important in grayscale addressing schemes and this
ability will depend upon the nature of the medium, as indeed will
all grayscale switching properties. In this application, grayscale
addressing schemes will primarily be discussed with reference to
encapsulated particle-based electrophoretic media, but it is
considered that the necessary modifications of such schemes to
allow for the properties of other types of bistable electro-optic
media will readily be apparent to those skilled in the technology
of such media.
[0074] The fundamental elements of grayscale drive waveforms are as
follows:
[0075] DC Pulse with Pulse Length Modulation
[0076] One of the least complex methods of realizing a desired gray
state is to stop addressing the pixel in the middle of a transition
from one extreme optical state to the other. In FIG. 1 of the
accompanying drawings, the inset illustrates the DC pulse length
modulated waveform elements used to produce the grayscale
transitions in an encapsulated electrophoretic medium shown in the
main part of the Figure. (The displays used in this and subsequent
experiments described below were prepared substantially according
to "Method B" described in Paragraphs [0061]-[0068] of the
aforementioned 2002/0180687.) The three pulses used were 15 V for
200, 400 and 600 msec respectively, and the three curves produced
are labelled accordingly; note that the time scale in the inset is
not the same as that in the main Figure. Thus, the pulse height was
fixed, while the duration of the pulse was varied for different
changes in reflectivity. In FIG. 1, the reflectivity of a pixel,
whose reflective state was changed from black to different levels
of gray upon application of these voltage pulses, is plotted
against time; it will be seen that longer pulse lengths yielded
greater changes in reflectivity.
[0077] The display tested responded immediately to the end of the
applied voltage pulse, and its optical state ceased to evolve. On
the microscopic level, it may be presumed that the electrophoretic
particles instantly stop in transit from one electrode to the other
and remain suspended at an intermediate position within the
capsule.
[0078] An advantage of a DC grayscale drive pulse with pulse length
modulation is the speed with which the desired gray state is
achieved.
[0079] DC Pulse with Pulse Height Modulation
[0080] Another approach to achieving a desired gray state is to
address a pixel with a lower voltage than is required to fully
switch from one extreme optical state of the pixel to the other. In
FIG. 2 of the accompanying drawings, the inset illustrates the DC
pulse height modulated waveform elements used to produce the
grayscale transitions in an encapsulated electrophoretic medium
shown in the main part of the Figure. The voltage pulse length is
fixed at the length of time required to completely switch the
medium at the maximum voltage level. The three pulses used were 5,
10 and 15 V for 500 msec respectively, and the three curves
produced are labelled accordingly; note that the time scale in the
inset is not the same as that in the main Figure. Thus, the pulse
length was fixed, while the height of the pulse was varied for
different changes in reflectivity. In FIG. 2, the reflectivity of a
pixel, whose reflective state was changed from black to different
levels of gray upon application of these voltage pulses, is plotted
against time; it will be seen that greater pulse heights yielded
greater changes in reflectivity.
[0081] It may be presumed that the electrophoretic particles travel
through the suspending fluid at slower speeds at lower voltages and
remain suspended when the driving voltage ceases to be applied.
[0082] An advantage of a DC grayscale drive pulse with pulse height
modulation is accurate control of the gray state achieved.
[0083] AC Pulse with Dc Offset Modulation
[0084] Grayscale driving of the aforementioned encapsulated
electrophoretic medium has been effected with oscillating (AC)
electric fields; the switching mechanism with such AC fields is
presumed to be entirely different from that effected in the DC
driving of the same medium discussed above. In FIG. 3 of the
accompanying drawings, the inset illustrates the AC pulse with DC
offset modulation waveform elements used to produce the grayscale
transitions in an encapsulated electrophoretic medium shown in the
main part of the Figure. In all cases, the frequency of the AC
component (approximately 10 Hz) was set at a value that allowed the
particles to respond to the oscillating field, while the magnitude
and direction of the DC offset (which was 0, -1 or -2.5 V, as
indicated for the three curves in FIG. 3) determined the gray state
that the pixel eventually attained. As in previous Figures, the
time scale used in the inset is not the same as that in the main
Figure. In FIG. 3, the reflectivity of a pixel, whose reflective
state was changed from black to different levels of gray upon
application of these voltage pulses, is plotted against time; it
will be seen that greater DC offsets yielded greater changes in
reflectivity.
[0085] Upon application of an AC field, the electrophoretic
particles oscillate in the suspending fluid and this oscillation is
observed motion as a cyclic variation in reflectivity, superimposed
upon the overall change in reflectivity, as is readily seen on the
left hand side of FIG. 3. There was no net effect on reflectivity,
however, until the DC offset was applied. Under the influence of a
DC offset, the reflectivity approaches a constant value after the
waveform has been applied for some time. It appears that there must
be a restoring force that opposes the force on the particles due to
the DC offset voltage, otherwise, the particles would continue to
flow to the cell wall. This restoring force may be due to the
motion of fluid in between the capsule wall and the particles
and/or to the interaction of the particles directly with the cell
wall. The stability of the optical state after the voltage is
released appears consistent with that of other waveform
elements.
[0086] An advantage of AC waveform elements is the ability to reach
a particular reflectivity state by specifying the parameters of the
waveform element, while DC waveform elements enable only a change
in reflectivity. An advantage of an AC waveform element with DC
offset over other AC waveform elements is that precise timing of
the addressing pulse is not required.
[0087] AC Pulse with Duty Cycle Modulation
[0088] Another way to induce a DC bias with an oscillating field is
to modulate the duty cycle. In FIG. 4 of the accompanying drawings,
the inset illustrates the AC pulses with duty cycle modulation used
to produce the grayscale transitions shown in the main part of the
Figure. In each of the pulses, the voltage is set to a maximum
value, and the duty cycle (the percentage of time that the voltage
is in the positive or negative direction) determines the
reflectivity. The three duty cycles used were 50, 47 and 40
percent, as indicated in FIG. 4. As in previous Figures, the time
scale used in the inset is not the same as that in the main Figure.
In this Figure, the reflectivity of a pixel, whose reflective state
was changed from black to different levels of gray upon application
of the voltage pulses, is plotted against time.
[0089] It will be seen from FIG. 4 that, as with the AC/DC offset
pulses used to generate the curves shown in FIG. 3, the curves
shown in FIG. 4 approach a constant value after the pulses have
been applied for some time. Thus, as with AC/DC offset, with duty
cycle modulation there appears to exist a restoring force that
forces the particles away from the cell wall, maintaining a
constant gray state. The physical mechanism for the restoring force
appears likely the same as discussed above. Again, the gray state
ceases to change immediately the pulses cease to be applied.
[0090] An advantage of an AC waveform with duty cycle modulation is
that voltage modulation is not required.
[0091] AC Pulse with Frequency Modulation
[0092] Another approach to AC grayscale switching is to apply to an
electro-optic medium an AC field which causes the optical state of
the medium to oscillate and then to terminate the AC field in
mid-cycle at the point having the desired reflectivity. The voltage
may be set at a maximum value and the AC frequency varied in order
to achieve a greater or lesser reflectivity range. The frequency
determines the amplitude of the oscillation in reflectivity.
[0093] When such an approach is applied to an encapsulated
particle-based electrophoretic medium, the electrophoretic
particles respond to the AC field by oscillating around their
initial positions. Since the reflectivity typically does not reach
either the extreme black or white optical state, interactions with
the cell wall are minimized and the response of the reflectivity is
relatively linear with the applied voltage.
[0094] An advantage of AC pulses with frequency modulation is that
voltage modulation is not required.
[0095] By combining the types of pulses discussed above, a
multitude of waveform elements can be developed, each involving
unique switching mechanisms, thus providing versatile methods for
driving differing electro-optic media with differing switching
characteristics.
[0096] In one specific application of the drive scheme principles
discussed above, pulse width modulation and AC pulses are used to
achieve an intermediate gray state in an electro-optic display
which otherwise would be capable of achieving only black and white
states.
[0097] For reasons already discussed, the ability to achieve gray
scale is highly desirable in electro-optic displays. However,
providing a large number of gray levels requires either pulse width
modulation with a high frame rate driver (the high frame rate being
needed to "slice" the pulse width into a large number of intervals,
thereby enabling pulse width and hence gray scale to be controlled
very accurately) or a driver capable of voltage modulation. Either
type of driver is substantially more costly than the simple
tri-level drivers, which enable individual pixels of a display to
be set only to +V, -V and 0 potential (where V is an arbitrary
operating potential) relative to the potential of a common front
plane electrode, and which are commonly used to drive displays
capable of only black and white states.
[0098] This invention provides a drive scheme which enables a
tri-level driver to produce a gray level intermediate the black and
white levels of a bistable electro-optic display. The drive scheme
is most easily appreciated from the following Table 2, which shows
the voltage applied during the successive frames of various types
of transitions in such a display of the present invention:
TABLE-US-00002 TABLE 2 0 1 2 3 4 5 6 . . . N - 1 N white to black
+V +V +V +V +V +V +V . . . +V +V black to white -V -V -V -V -V -V
-V . . . -V -V white to gray +V +V +V +V -V +V -V . . . +V -V black
to gray -V -V -V -V +V -V +V . . . -V +V gray to black +V +V +V 0 0
0 0 0 0 0 gray to white -V -V -V 0 0 0 0 0 0 0
[0099] As may be seen from Table 2 above, the transitions from
black to white or vice versa are the same as in a binary
(black/white only) display. The transitions to gray, on the other
hand, have two parts. The first part is a square wave pulse (i.e.,
a plurality of frames at the same potential) of the proper polarity
and length to bring the reflectivity of the electro-optic medium as
close as possible to the desired middle gray lightness. The
accuracy possible with this step will be limited by the frame rate
of the display. The second part of the addressing pulse consists of
an equal number of positive and negative voltage pulses, each one
frame in width. As discussed above with reference to FIGS. 3 and 4,
it has previously been demonstrated that the application of an AC
square wave to an encapsulated particle-based electrophoretic
medium causes the medium to "relax" to some "middle gray" state.
Therefore, this second part of the pulse will bring all of the
pixels to the same uniform middle gray state, regardless of
previous pulse history. Addressing out from the gray state to black
or white is accomplished with a short pulse of the proper
polarity.
[0100] More generally, the AC portion of the pulse need not switch
polarity every frame, but may switch at a lower frequency, with the
voltage alternating every other frame (frequency=frame rate/4), or
more generally every nth frame (frequency=frame rate/2n).
[0101] Thus, this invention provides a method to produce a single
gray level in an otherwise binary electro-optic display using only
a simple tri-level driver rather than without the use of a complex
and costly voltage modulated driver.
[0102] In a second specific application of the drive scheme
principles discussed above, this invention provides a collection of
two-dimensional transition matrices, wherein each element in the
matrix specifies how to get from an initial optical state (denoted
"the row index" herein, although obviously the allocation of the
initial optical states to rows is arbitrary) to a final optical
state (denoted "the column index" herein). Each element of this
matrix is constructed from a series of waveform elements (as
defined above), and in general, for an n-bit grayscale display,
this matrix will contain 2.sup.(2N) elements. The matrices of the
present invention take account of such considerations as the need
for DC balancing of the drive scheme (as discussed above),
minimizing "memory" effects in certain electro-optic media (i.e.,
effects whereby the result of applying a particular pulse to a
pixel depends not only upon the current state of the pixel but also
upon certain prior states), thereby producing uniform optical
states and maximizing the switching speed of the display, while
working within the constraints of an active matrix drive scheme.
The present invention also provides a method for determining the
optimal values of each term of the elements in such a matrix for
any specific electro-optic medium. For further discussion of such
matrices and their use in driving electro-optic displays, the
reader is referred to the aforementioned copending application Ser.
No. 10/065,795.
[0103] The presently preferred waveforms of the invention are
described below in terms of pulse width modulation (PWM) as
discussed above. However, the same or similar results may be
achieved using pulse height modulation, or the various hybrid types
of AC modulation discussed above, and various different types of
modulation may be employed within a single waveform, for example,
pulse width modulation for all but the last section of the pulse,
followed by voltage modulation on the last section of the
pulse.
[0104] The first two waveforms of the present invention described
below are "slide show" waveforms, which return from one gray state
to the black state before addressing out to the next gray state.
Such waveforms are most compatible with a display update scheme
where the entire screen blanks at once, as in a slide
projector.
[0105] Double-Prepulse Slideshow Waveform
[0106] In this waveform, a preferred form of which is illustrated
in FIG. 5 of the accompanying drawings, a pixel of the
electro-optic medium is initially driven (as indicated at 100) from
black to an initial (first) gray state using a partial pulse. To
change the pixel from this initial gray state to a different
desired (second) gray state, the pixel is first driven (at 102)
from the first gray state to white, and then (at 104) from white to
black. Finally, the proper pulse to reach the second gray state is
applied at 106. To ensure that this type of waveform preserves
overall DC balance, it is necessary that the sum of the lengths of
the addressing pulse at 106 and the white pulse at 102 equal the
length of the white-black pulse at 104. This waveform requires a
maximum of three times the switching time of the medium (i.e., the
time necessary for a single pixel to switch from its black optical
state to its white optical state or vice versa) to effect a
transition between any two arbitrary gray states, and is therefore
referred to as a 3.times. waveform.
[0107] Single-Prepulse Slideshow Waveform
[0108] In this waveform, a preferred form of which is illustrated
in FIG. 6 of the accompanying drawings, a pixel of the
electro-optic medium is initially driven (as indicated at 110) from
black to an initial (first) gray state using a partial pulse, in
the same way as in the double-prepulse waveform discussed in
Section 6 above. To change the pixel from this initial gray state
to a different desired (second) gray state, the pixel is first
driven (at 112) from the first gray state to black, then the proper
pulse to reach the second gray state is applied at 114. Obviously,
before a second transition, the pixel will again be returned to
black at 116. This type of waveform preserves DC balance of the
overall waveform, since the impulses applied at 112 and 116 are
equal (except for polarity) to the impulses applied at 110 and 114
respectively. This waveform requires a maximum of twice the
switching time of the medium to effect a transition between any two
arbitrary gray states, and is therefore referred to as a 2.times.
waveform.
[0109] Gray-to-Gray Waveforms
[0110] Instead of using the slideshow waveforms described above, a
display may be updated by addressing it directly from one gray
state to another without passing through a black or white state.
Since there are no obvious artifacts (i.e., black and/or white
"flashes") associated with such a transition, it may be referred to
as "gray-to-gray" addressing. There are two main forms of
gray-to-gray waveform, namely DC-balanced and DC-imbalanced.
[0111] In a DC-balanced gray-to-gray waveform, the transition
between any two gray states is effected by applying a modulated
pulse of the precise length necessary to shift between the two
states. The electro-optic medium does not pass through any
intermediate black or white states. Since the maximum pulse length
is equal to the addressing time of the ink, such a waveform may be
referred to as a 1.times. waveform. To maintain DC balance, for a
display with n gray states, there are n-1 free parameters available
in the optimization of the transition matrix associated with any
specific waveform. This results in a highly over-constrained
system. For example, all transitions are required to be equal and
opposite in impulse to the reverse transition (i.e. 2-3 must be the
same as 3-2, except for polarity).
[0112] A DC-imbalanced gray-to-gray waveform is fundamentally the
same as a DC-balanced one, except that the pulse lengths are no
longer constrained by the restriction of DC balancing. Thus, each
of the 2.sup.(2N) entries in the transition matrix can vary
independently of all the others.
[0113] The various waveforms discussed above enable grayscale
addressing in active matrix displays, which is crucial for the use
of electro-optic media in personal digital assistant (PDA) and
electronic book applications. These waveforms minimize the effects
of memory in the electro-optic medium, and such memory can lead to
image ghosting. By choosing optimal pulse lengths and sequences,
desired gray optical states can be achieved in a minimum number of
pulses.
[0114] Selective Row Driving
[0115] Another aspect of the present invention relates to improving
the performance of an active matrix bistable electro-optic display
by selective driving of the rows of the display.
[0116] As already mentioned, and as discussed in more detail in
some of the aforementioned patents and applications, to maintain a
desired image on a conventional LCD, the whole image area must be
continuously refreshed, since typically liquid crystals are not
bistable and an image on an LCD will fade within a very short time
unless refreshed. As is well known to those skilled in the art of
active matrix LCD's, in such displays the continuous refreshment is
effected by using a row driver to turn on the gates of the
transistors associated with one row of pixels of the display,
placing on column drivers (connected to the source electrodes of
the transistors in each column of the display) the potentials
needed to write to the pixels in the selected row the relevant
portion of the desired image on the display, and thus writing the
selected row of the display. The row driver then selects the next
row of the display and the process is repeated, with the rows thus
being refreshed cyclically. (The assignment of the row drivers to
gate electrodes and the column drivers to source electrodes is
conventional but essentially arbitrary, and could of course be
reversed if desired.)
[0117] Because an LCD requires continuous refreshing of an image, a
change of only part of a displayed image is handled as part of the
overall refreshing procedure. In a continuously-refreshed display
there need be no provision for updating part of an image; since in
effect a new image is being written to the display several times
per second (in the case of an LCD), any change of part of an image
fed to the display automatically appears effect on the display
within a short interval. Consequently, the conventional circuitry
developed for use with LCD's makes no provision for updating of
only part of an image.
[0118] In contrast, bistable electro-optic displays do not need
continuous refreshing, and indeed such continuous refreshing is
undesirable, since it unnecessarily increases the energy
consumption of the display. Furthermore, during such refreshing,
the gate (row) lines may deliver capacitative voltage spikes to
pixel electrodes, and any driver voltage errors or uncompensated
gate feedthrough bias errors can accumulate; all these factors can
result in undesirable shifts in the optical states of the pixels of
the display. Accordingly, in bistable electro-optic displays it is
desirable to provide some means for updating a portion of an image
without the need to rewrite the whole image on the display, and one
aspect of this invention relates to a bistable electro-optic
display provided with such "partial updating" means. According to
the present invention, this is done by comparing successive images
to be written to the display, identifying the rows which differ in
the two images, and addressing only the rows thus identified.
[0119] In the present method, to effect a partial update of a
display only the rows of the display containing pixels which are to
change optical state are identified. In a preferred form of this
method, for every line of the display, a display controller (cf.
the aforementioned copending application Ser. No. 10/065,795)
examines all of the desired pixel electrode output voltages. If all
of the output voltages for that line are equal to the potential
V.sub.com of the common front electrode of the display (i.e., if no
pixel in that row needs to be rewritten), then the controller
outputs a synchronizing (V.sub.sync) pulse without loading data
values into the column drivers, and without issuing a corresponding
output enable (OE) command. The net effect of this is that the
token bit for the row drivers is passed to the next row of the
display without activating the current row. Data is only loaded
into the column drivers, and output enable is only asserted, for
rows where at least one pixel needs to be rewritten.
[0120] This invention gives two distinct types of advantages.
Firstly, many sources of spurious voltage are eliminated for pixels
that are not rewritten. There is no capacitative gate spike for
these pixels, and errors in the column driver voltage will not be
passed on to a pixel in frames where it is not addressed. Because
of the relatively lower resistivity of many electro-optic media,
especially electrophoretic media, as compared with liquid crystals,
the pixel electrode will tend to relax to the actual front plane
voltage, thus maintaining the hold state of the electro-optic
medium. Secondly, power consumption of the display is minimized.
For every row that is not rewritten, the corresponding gate line
does not have to be charged. In addition, when data is not loaded
into the column drivers of the display, the additional power
consumption of moving that data across the display interface is
also eliminated.
[0121] Spatial Area Dithering
[0122] The aspects of the present invention previously discussed
relate to the waveforms used to drive electro-optic displays.
However, the behavior of such displays can also be changed by
varying the structure of the backplane, and this aspect of the
invention relates to dividing one or more pixel, and preferably
each pixel, of a display into a plurality of sub-pixels having
differing areas.
[0123] As already noted, it is highly desirable to provide
grayscale in an electro-optic display. This gray scale may be
achieved either by driving a pixel of the display to a gray state
intermediate its two extreme states. However, if the medium is not
capable of achieving the desired number of intermediate states, or
if the display is being driven by drivers which are not capable of
providing the desired number of intermediate states, other methods
must be used to achieve the desired number of states, and this
aspect of the invention relates to the use of spatial dithering for
this purpose.
[0124] A display may be divided into a plurality of "logical"
pixels, each of which is capable of displaying the desired number
of gray or other optical states. However, obviously more than one
physically separate area can be present at each logical pixel, and
indeed it is common in color displays to make use of "full color"
logical pixels each of which comprises three sub-pixels of primary
colors, for example red, green and blue; see, for example, the
aforementioned 2002/0180688. Similarly, one could achieve gray
scale by using as a logical pixel an assembly of sub-pixels, each
of which was capable of only binary switching. For example, a
logical pixel comprising four independently controllable sub-pixels
of equal area could be used to provide two-bit gray scale. However,
for anything more than one- or two-bit gray scale, the number of
sub-pixels becomes inconveniently great, since the required number
of sub-pixels doubles for each one-bit increase in gray scale.
[0125] The present invention provides an electro-optic display
having at least one pixel which comprises a plurality of
sub-pixels, these sub-pixels being of differing areas. In a
preferred embodiment of this invention, at least two sub-pixels
differ in area by substantially a factor of two. Thus, for example,
a logical pixel might have sub-pixels with areas of 1X, 2X and 4X,
where X is an arbitrary area. A logical pixel of this type is
illustrated schematically in FIG. 7A of the accompanying drawings.
This logical pixel achieves three-bit grayscale using only three
electrodes, whereas achieving the same three-bit grayscale with
sub-pixels of equal area would require eight sub-pixels.
[0126] When driven, each sub-pixel reflects or transmits a portion
of the incoming light, and the fractional amount is dictated by the
area of the sub-pixel. If the reflectance/transmission is averaged
over the area of the logical pixel, then a binary weighting of
driven area is achieved, and hence spatially dithered
grayscale.
[0127] The areas of the sub-pixels are arbitrary. The ones shown in
FIG. 7A are weighed by reflectance. If one were to use a non-linear
weighting (as would be appropriate for equal steps in L* or a gamma
corrected grayscale spacing), the areas would be changed
accordingly.
[0128] Careful consideration should be given to the shape of the
sub-pixels, in addition to consideration of their relative areas.
Simple large blocks, as in FIG. 7A, allow simple patterning of the
sub-pixel array, but under certain conditions the sub-pixels may be
resolved by a viewer. Also, if a large area (covering many logical
pixels) is displayed with a mid-level gray (so that (say) only the
area 4 in FIG. 7A is driven in each logical pixel), a viewer still
see a line or grating pattern arising from the pattern of
sub-pixels.
[0129] Increasing the resolution of the logical pixels reduces
these problems, but requires a large number of additional pixels,
however, as the number of pixels increases as the square of the
resolution. Instead, the problems of sub-pixel visibility and/or
visible patterning can be reduced by interdigitating the sub-pixels
as shown, for example, in FIG. 7B; note that this Figure is only
intended to illustrate interdigitation, and not to accurately
represent the relative areas of the sub-pixels. Many interdigitated
patterns similar to that of FIG. 7B can be used to improve image
quality.
[0130] Another approach to dealing with the problems of sub-pixel
visibility and/or visible patterning is to randomly orient the
sub-pixels. For example, in an array of pixels each of which has
the sub-pixel arrangement shown in FIG. 7A, individual pixels could
have, at random, each of the four possible orientations of the
arrangement shown in FIG. 7A. Such "randomization" of the
sub-pixels helps to break up patterns and render them less
noticeable to an observer.
[0131] Although the embodiments of the present invention shown in
FIGS. 7A and 7B produce three-bit grayscale, it will be appreciated
that the present invention can produce any number of bits of gray
scale simply by adding additional sub-pixels.
[0132] This aspect of the present invention has the advantages
that:
[0133] (a) The electro-optic medium itself does not need to be
capable of gray scale; essentially the display can be a black/white
display, and sub-pixels turned on and off to produce gray scale. In
a scanned array, the necessary control of the sub-pixels can be
achieved by providing additional column drivers for the same number
of rows, as in color sub-pixel arrays. This reduces demands upon
the electro-optic medium used; for example, one does not need to
worry about possible drift of gray levels of the medium over its
operating lifetime.
[0134] (b) There is no need for complicated column drivers; the
present invention is compatible with simple use binary level
drivers used in many conventional displays. thus facilitating the
use of a variety of electro-optic media with readily available,
inexpensive "off-the-shelf" components. Some methods of generating
grayscale require voltage modulated drivers for the column
electrodes, and such drivers are not widely available and are more
expensive/harder to build than binary level drivers.
[0135] (c) The thin film transistor (TFT) design for an active
matrix array using the present invention need be no harder than
that required for full color, where there are three sub-pixels (for
example, RGB) per pixel, and the amount of data which needs to be
supplied to the various components is no greater. Thus no new
technology development is required in an active matrix backplane to
implement the present invention.
[0136] Miscellaneous Techniques
[0137] In most conventional active matrix drive schemes for
electro-optic displays, the voltages of the pixel electrodes on the
display backplane are varied in order to impose desired voltages
across pixels. The top plane is typically held at a particular
voltage deemed advantageous for addressing the pixels. For example,
if the data line voltage supplied to the pixel electrodes varies
between zero volts and a voltage V.sub.0, the top plane may be held
at V.sub.0/2 in order to permit voltage drops across the pixel to
be as large as V.sub.0/2 in both directions.
[0138] According to one aspect of this invention, the voltage of
the top plane may be varied to enhance the addressing of the
electro-optic medium. For example, the top plane voltage could be
held at zero volts in order to permit the total pixel voltage drop
(top plane minus pixel voltage) to be as low at -V.sub.0. Raising
the top plane up to V.sub.0 permits a pixel voltage drop as large
as V.sub.0. These larger voltage drops permit faster addressing of
the electro-optic medium.
[0139] More generally, it may be advantageous to be able to set the
top plane voltage not only at voltages zero and V.sub.0, but to
other voltages as well. For example, it may be advantageous to
apply a global time-varying voltage across the electro-optic medium
in concert with pixel-to-pixel voltages imposed by the
backplane.
[0140] It is known to provide an electro-optic display with a
capacitor formed between a pixel electrode and an electrode formed
by an extension of a select line so as to charged with the same
voltage as the select line; as described in the aforementioned WO
01/07961, the provision of such a capacitor reduces the rate of
decay of the electric field across the relevant pixel after the
driving voltage is removed. In another aspect, this invention
provides an electro-optic display having a storage capacitor formed
between a pixel electrode and a (second) electrode that has a
voltage that can be varied independently from the select lines of
the display. In a preferred embodiment, the second electrode
follows the top plane voltage, that is, its voltage differs from
the top plane only by a time-independent constant. The provision of
this type of capacitor greatly reduces the capacitative voltage
spikes experienced by the pixel, as compared with a storage
capacitor is created by an overlap between a pixel electrode and a
select line that controls the adjacent (previous) row of the
display.
[0141] Another aspect of the present invention relates to reducing
or eliminating unwanted switching of the electro-optic medium by
select and data lines.
[0142] As discussed above, select and data lines are essential
elements of an active matrix panel in that they provide the
voltages required for charging pixel electrodes to desired values.
However, the select and data lines can have the unwanted effect of
switching the electro-optic medium adjacent the data lines. The
undesirable optical artifacts caused by such switching can be
eliminated by using a black mask to hide the regions switched by
the data and/or select lines from a viewer. However, providing such
a black mask requires registration of the front plane of the
display with its back plane and a reduction in the fraction of the
electro-optic medium that is exposed to the viewer. The result is a
display darker and lower in contrast than one could achieve without
the black mask.
[0143] In another aspect of the present invention, the need for a
black mask is avoided by making the data lines to have a small
lateral extent in one direction so that they do not appreciably
address the adjacent electro-optic medium during normal display
operation. This obviates the need for a black mask.
[0144] A related aspect of the present invention relates to the use
of passivated electrodes and modification of the drive scheme used
to drive the electro-optic medium. An impulse-driven electro-optic
medium can be electronically addressed when it is in a thin film
form between two electrodes. Generally, the electrodes make contact
with the electro-optic medium. However, it is also possible to
address the medium even when a dielectric material with a long
electronic relaxation time exists between one or both electrodes
and the medium. Passivation of one or both electrodes may be
desirable to avoid adverse chemical or electrochemical interactions
at the backplane or front plane of a display device; see the
aforementioned WO 00/38001. Although the ability to sustain a
voltage across the electro-optic medium is greatly reduced by the
presence of a dielectric layer, a voltage impulse can still be
applied to the medium and the medium can be addressed through these
voltage impulses if the dielectric layer is properly
engineered.
[0145] The optical state of an electro-optic medium is of course
achieved by changing the voltage on a pixel electrode. This voltage
change results in a voltage across the electro-optic medium that
decays as charge leaks through the medium. If an external
dielectric layer (i.e., a dielectric layer between the medium and
one electrode) is sufficiently thin and the electro-optic medium is
sufficiently resistive, the voltage impulse across the medium will
be sufficient to cause a desirable shift in the optical state of
the medium. Electronic addressing of an electro-optic medium
through a dielectric layer is therefore possible. The addressing
scheme is different, however, from addressing an electro-optic
medium with electrodes in direct contact with the medium since, in
the latter case, the medium is addressed by applying voltages
across the pixel, whereas, in the former case, addressing is
achieved by causing a change in the pixel voltage. At every change,
a voltage impulse is experienced by the electro-optic medium.
[0146] Finally, this invention provides drive schemes for reducing
cross-talk in active matrix electro-optic displays.
[0147] Inter-pixel cross-talk, where addressing one pixel affects
the optical state of other pixels, is undesirable but has many
causes. One cause is the finite current flow through transistors in
the off state. Bringing a data line to a voltage intended for
charging one pixel can charge up transistors in unselected rows
because of off state current leakage. A solution is to use
transistors with a low off-state current.
[0148] Another source of cross talk is current leakage between
neighboring pixels. Current can leak through elements of the
backplane or through the electro-optic medium in contact with the
backplane. A solution to such cross talk is to design the backplane
with a large insulating gap between the pixel electrodes. A larger
gap will result in smaller leakage currents.
[0149] As already indicated, a preferred type of electro-optic
medium for use in the present invention is an encapsulated
particle-based electrophoretic medium. Such electrophoretic media
used in the methods and apparatus of the present invention may
employ the same components and manufacturing techniques as in the
aforementioned E Ink and MIT patents and applications, to which the
reader is referred for further information.
[0150] Numerous changes and modifications can be made in the
preferred embodiments of the present invention already described
without departing from the spirit of the invention. Accordingly,
the foregoing description is to be construed in an illustrative and
not in a limitative sense.
* * * * *