Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-11-12
2003-03-25
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C345S107000
Reexamination Certificate
active
06538801
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to electrophoretic displays using nanoparticles, that is particles having diameters substantially less than the wavelengths of visible light.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays use a display medium comprising a plurality of electrically charged particles suspended in a fluid. Electrodes are provided adjacent the display medium so that the charged particles can be moved through the fluid by applying an electric field to the medium. In one type of such electrophoretic display, the medium comprises a single type of particle having one optical characteristic in a fluid which has a different optical characteristic. In a second type of such electrophoretic display, the medium contains two different types of particles differing in at least one optical characteristic and in electrophoretic mobility; the particles may or may not bear charges of opposite polarity. The optical characteristic which is varied is typically color visible to the human eye, but may, alternatively or in addition, be any one of more of reflectivity, retroreflectivity, luminescence, fluorescence, phosphorescence, or (in the case of displays intended for machine reading) color in the broader sense of meaning a difference in absorption or reflectance at non-visible wavelengths.
Electrophoretic displays can be divided into two main types, namely unencapsulated and encapsulated displays. In an unencapsulated electrophoretic display, the electrophoretic medium is present as a bulk liquid, typically in the form of a flat film of the liquid present between two parallel, spaced electrodes. Such unencapsulated displays typically have problems with their long-term image quality which have prevented their widespread usage. For example, particles that make up such electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display differs from an unencapsulated display in that the particle-containing fluid is not present as a bulk liquid but instead is confined within the walls of a large number of small capsules. Encapsulated displays typically do 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.
For further details regarding encapsulated electrophoretic displays, the reader is referred to 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; 6,249,721; 6,252,564; 6,262,706; and 6,262,833, and International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10769; WO 99/10768; WO 99/10767; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/47970; WO 00/03349; WO 00/03291; WO 99/67678; WO 00/05704; WO 99/53371; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/36465; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/20922: WO 00/36666; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327; WO 01/02899; WO 01/07961; WO 01/08241; WO 01/08242; WO 01/17029; WO 01/17040; and WO 01/17041. The entire disclosures of all these patents and published applications, all of which are in the name of, or assigned to, the Massachusetts Institute of Technology or E Ink Corporation, are herein incorporated by reference.
Prior art electrophoretic displays use particles, which, while small (typically about 0.25 to 2 &mgr;m), are sufficiently large that they have essentially the bulk properties of the material from which they are formed. The particles keep the same optical properties during the time they are present in the electrophoretic display; the appearance of the display is changed by moving the particles within the suspending fluid using an appropriate electrical field. For example, consider the prior art electrophoretic display represented in a schematic manner in
FIG. 1
of the accompanying drawings. This display is provided on its front viewing surface (the top surface as illustrated in
FIG. 1
) with a common, transparent front electrode
100
, and on its rear surface with an opaque substrate
102
carrying a matrix of discrete electrodes; only two of these electrodes, designated
104
and
106
respectively, are shown in FIG.
1
. Each of the discrete electrodes
104
and
106
defines a pixel of the display. An encapsulated electrophoretic medium (generally designated
108
) is disposed between the common electrode
100
and the discrete electrodes
104
and
106
; for ease of illustration.
FIG. 1
shows only a s ingle capsule
110
of the medium
108
associated with each discrete electrode
104
and
106
, although in practice a plurality of capsules (typically at least 20) would be associated with each discrete electrode. Also for ease of illustration, the capsules are shown in
FIG. 1
as of circular cross-section, although in practice it is preferred that they have a flattened form.
Each of the capsules
110
comprises a capsule wall
112
, a dark colored fluid
114
(assumed for present purposes to be blue) contained within this capsule wall
112
and a plurality of light colored charged particles
116
(assumed for present purposes to be titania particles 250-500 nm in diameter) suspended in the fluid
114
. For purposes of illustration, it is assumed that the titania particles
116
are negatively charged so that they will be drawn to whichever of their associated discrete electrode and the common electrode is at the higher potential. However, the particles
116
could alternatively be positively charged. Also, the particles could be dark in color and the fluid
114
light in color so long as sufficient color contrast occurs as the particles move between the front and rear surfaces of the display medium, as shown in FIG.
1
.
In the display shown in
FIG. 1
, each of the discrete electrodes is held at either 0 or +V (where V is a drive voltage) while the common front electrode
100
is held at an intermediate voltage +V/2. Since the titania particles
116
are negatively charged, they will be attracted to whichever of the two adjacent electrodes is at the higher potential. Thus, in
FIG. 1
, discrete electrode
104
is shown as being held at 0, so that the particles
116
within the adjacent capsule move adjacent the common electrode
100
, and thus adjacent the top, viewing surface of the display. Accordingly, the pixel associated with discrete electrode
104
appears white, since light entering the viewing surface is strongly reflected from the titania particles adjacent this surface. On the other hand, discrete electrode
106
in
FIG. 1
is shown as being held at +V, so that the particles
116
within the adjacent capsule move adjacent the electrode
106
, and the color of the pixel associated with electrode
106
is that exhibited by light entering the viewing surface of the display, passing through the colored fluid
114
, being reflected from the titania particles adjacent electrode
116
, passing back through the colored fluid
114
, and finally re-emerging from the viewing surface of the display, i.e., the associated pixel appears blue.
It should be noted that the change in the appearance of a pixel of this electrophoretic display as the voltage on the associated discrete electrode changes is solely due to the change of the position of the titania particles within the fluid; the color and other optical characteristics of the titania particles themselves do not change during operation of the electrophoretic display. In both the pixels shown in
FIG. 1
, the function of the titania particles is to scatter light strongly.
Obviously, the type of display shown in
FIG. 1
can use particles of pigments other than titania, for example magenta pigments such as Hostaperm Pink E (Hoechst Celanese Corporation) and Lithol Scarlet (BASF), yellow pigments such as Diarylide Yellow (Domin
Drzaic Paul S.
Duthaler Gregg M.
Gray Caprice L.
Jacobson Joseph M.
McCreary Michael
Cole David J.
Dang Hung Xuan
E Ink Corporation
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