Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR MANIPULATING COLOR IN A DISPLAY
Field
The field of the invention relates to microelectromechanical systems (MEMS).
Background
Microelectromechanical systems (MEMS) include micro mechanical elements,
actuators, and electronics. Micromechanical elements may be created using
deposition,
etching, and or other micromachining processes that etch away parts of
substrates and/or
deposited material layers or that add layers to form electrical and
electromechanical
devices. One type of MEMS device is called an interferometric modulator. As
used herein,
the term interferometric modulator or interferometric light modulator refers
to a device that
selectively absorbs and/or reflects light using the principles of optical
interference. In
certain embodiments, an interferometric modulator may comprise a pair of
conductive
plates, one or both of which may be transparent and/or reflective in whole or
part and
capable of relative motion upon application of an appropriate electrical
signal. In a
particular embodiment, one plate may comprise a stationary layer deposited on
a substrate
and the other plate may comprise a metallic membrane separated from the
stationary layer
by an air gap. As described herein in more detail, the position of one plate
in relation to
another can change the optical interference of light incident on the
interferometric
modulator. Such devices have a wide range of applications, and it would be
beneficial in
the art to utilize and/or modify the characteristics of these types of devices
so that their
features can be exploited in improving existing products and creating new
products that
have not yet been developed.
SUMMARY
The system, method, and devices of the invention each have several aspects, no
single one of which is solely responsible for its desirable attributes.
Without limiting the
scope of this invention, its more prominent features will now be discussed
briefly. After
considering this discussion, and particularly after reading the section
entitled "Detailed
Description of Preferred Embodiments" one will understand how the features of
this
invention provide advantages over other display devices.
One embodiment includes a display. The display includes a plurality of pixels.
Each of the pixels includes at least one red subpixel comprising at least one
interferometric
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modulator configured to output red light, at least one green subpixel
comprising at least one
interferometric modulator configured to output green light, at least one blue
subpixel
comprising at least one interferometric modulator configured to output blue
light, and at
least one white subpixel comprising at least one interferometric modulator
configured to
output colored light.
Another embodiment includes a display. The display includes a plurality of
interferometric modulators. The plurality of interferometric modulators
includes at least
one interferometric modulator configured to output red light, at least one
interferometric
modulator configured to output green light, at least one interferometric
modulator
configured to output blue light, and at least one interferometric modulator
configured to
output white light. The at least one interferometric modulator configured to
output white
light outputs white light having a standardized white point.
Another embodiment includes a display. The display includes a plurality of
display
elements. Each of the display elements includes a reflective surface
configured to be
positioned at a distance from a partially reflective surface. The plurality of
display
elements includes at least one of the plurality of display elements configured
to output
colored light and at least one of the plurality of display elements configured
to
interferometrically output white light.
Another embodiment includes a method of fabricating a display. The method
includes forming a plurality of display elements. Each of the plurality of
display elements
includes a reflective surface configured to be positioned at distance from a
partially
reflective surface. Each of the respective distances is selected so that at
least one of the
plurality of display elements is configured to output colored light and at
least one other of
the plurality of display elements is configured to interferometrically output
white light.
Another embodiment includes a display comprising means for displaying an
image.
The displaying means comprises means for reflecting light and means for
partially
reflecting light. The reflecting means is configured to be positioned at a
distance from the
partially reflecting means. The displaying means comprises first means for
outputting
colored light and second means for interferometrically outputting white light.
Another embodiment includes a display. The display includes a plurality of
pixels
each comprising red, green, and blue interferometric modulators that are
configured to
output red, green, and blue light, respectively. Each of the pixels are
configured to output a
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greater intensity of green light than red light and configured to output a
greater intensity of
green light than blue light when each of the interferometric modulators are
set to output red,
green, and blue light.
Another embodiment includes a method of fabricating a display. The method
includes forming a plurality of pixels. Forming the plurality of pixels
includes forming
interferometric modulators configured to output red light, forming
interferometric
modulators configured to output green light, and forming interferometric
modulators
configured to output blue light. Each of the pixels are configured to output a
greater
intensity of green light than red light and configured to output a greater
intensity of green
light than blue light when each of the interferometric modulators are set to
output red,
green, and blue light.
Another embodiment includes a display. The display includes a plurality of
pixels.
Each of the pixels comprises red, green, and blue interferometric modulators
that are
configured to output red, green, and blue light, respectively. Each of the
pixels are
configured to output a greater intensity of green light than red light and
configured to output
a greater intensity of green light than blue light. At least one of the
interferometric
modulators configured to output red light and the interferometric modulators
configured to
output blue light are configured to output light having a wavelength
to produce a more saturated color than said green light.
Another embodiment comprises a display comprising a plurality of means for
outputting red, a plurality of means for outputting green light, and a
plurality of means for
outputting blue light. The red, green, and blue outputting means form means
for displaying
= an image pixel. Each of the pixel displaying means are configured to
output a greater
intensity of green light than blue light when the red, green, and blue
outputting means are
set to output red, green and blue light.
Another embodiment comprises a display comprising a plurality of display
elements. The plurality of display elements comprise at least one color
display element
configured to output color light and at least one display element configured
to output white
light. The at least one display element configured to output white light
outputs white light
having a standardized white point.
Another embodiment comprises a display comprising means for displaying an
image. The displaying means comprising means for outputting colored light and
means for
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output white light. The white light outputting means outputs white light
having a
standardized white point.
Another embodiment comprises a method of fabricating a display comprising
forming a plurality of display elements comprising forming at least one color
display
element configured to output color light and at least one display element
configured to
output white light. The at least one display element configured to output
white light is
configured to output white light having a standardized white point.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view depicting a portion of one embodiment of an
interferometric modulator display in which a movable reflective layer of a
first
interferometric modulator is in a relaxed position and a movable reflective
layer of a second
interferometric modulator is in an actuated position.
FIG. 2 is a system block diagram illustrating one embodiment of an electronic
device incorporating a 3x3 interferometric modulator display.
FIG. 3 is a diagram of movable mirror position versus applied voltage for one
exemplary embodiment of an interferometric modulator of FIG. 1.
FIG. 4 is an illustration of a set of row and column voltages that may be used
to
drive an interferometric modulator display.
FIG. 5A illustrates one exemplary frame of display data in the 3x3
interferometric
modulator display of FIG. 2.
FIG. 5B illustrates one exemplary timing diagram for row and column signals
that
may be used to write the frame of FIG. 5A.
FIGS. 6A and 6B are system block diagrams illustrating an embodiment of a
visual
display device comprising a plurality of interferometric modulators.
FIG. 7A is a cross section of the device of FIG. 1.
FIG. 7B is a cross section of an alternative embodiment of an interferometric
modulator.
FIG. 7C is a cross section of another alternative embodiment of an
interferometric
modulator.
FIG 7D is a cross section of yet another alternative embodiment of an
interferometric modulator.
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FIG. 7E is a cross section of an additional alternative embodiment of an
interferometric modulator.
Figure 8 is a side cross-sectional view of an exemplary interferometric
modulator
that illustrates the spectral characteristics of output light by positioning
the movable mirror
in a range of positions.
Figure 9 is a graphical diagram illustrating the spectral response of one
embodiment
that includes cyan and yellow interferometric modulators to produce white
light.
Figure 10 is a side cross-sectional view of an interferometric modulator
illustrating
different optical paths through the modulator that result in different color
light being
reflected.
Figure 11 is a side cross-sectional view of the interferometric modulator
having a
layer of material for selectively transmitting light of a particular color.
Figure 12 is a graphical diagram illustrating the spectral response of one
embodiment that includes green interferometric modulators and a "magenta"
filter layer to
produce white light.
Figure 13 is a schematic diagram illustrating two pixels of an exemplary pixel
array
30. Rows 1-4 and columns 1-4 form one pixel 120a.
Figure 14A is a chromaticity diagram that illustrates the colors that can be
produced
by an exemplary color display that includes red, green, and blue display
elements.
Figure 14B is a chromaticity diagram that illustrates the colors that can be
produced
by an exemplary color display that includes red, green, blue, and white
display elements.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The following detailed description is directed to certain specific embodiments
of the
invention. However, the invention can be embodied in a multitude of different
ways. In
this description, reference is made to the drawings wherein like parts are
designated with
like numerals throughout. As will be apparent from the following description,
the
embodiments may be implemented in any device that is configured to display an
image,
whether in motion (e.g., video) or stationary (e.g., still image), and whether
textual or
pictorial. More particularly, it is contemplated that the embodiments may be
implemented
in or associated with a variety of electronic devices such as, but not limited
to, mobile
telephones, wireless devices, personal data assistants (PDAs), hand-held or
portable
computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game
consoles,
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wrist watches, clocks, calculators, television monitors, flat panel displays,
computer
monitors, auto displays (e.g., odometer display, etc.), cockpit controls
and/or displays,
display of camera views (e.g., display of a rear view camera in a vehicle),
electronic
photographs, electronic billboards or signs, projectors, architectural
structures, packaging,
and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS
devices of
similar structure to those described herein can also be used in non-display
applications such
as in electronic switching devices.
One embodiment is a display in which each of the pixels comprises a set of
display
elements, which may each comprise one or more interferometric modulators. The
set of
display elements includes display elements configured to output red, green,
blue, and white
light. In one embodiment, the "white light" display element outputs white
light having a
broader, higher intensity, spectral response than the combined spectral
response of the
"red," "green," and "blue" display elements. In one embodiment, the display
includes a
driver circuit configured to turn on the "white light" display element, when
the data for
driving the pixel. In addition, embodiments include color displays configured
to provide a
greater proportion of the intensity of output light in green portions of the
visible spectrum
in order to increase perceived brightness of the display.
One interferometric modulator display embodiment comprising an interferometric
MEMS display element is illustrated in Figure 1. In these devices, the pixels
are in either a
bright or dark state. In the bright ("on" or "open") state, the display
element reflects a large
portion of incident visible light to a user. When in the dark ("off' or
"closed") state, the
display element reflects little incident visible light to the user. Depending
on the
embodiment, the light reflectance properties of the "on" and "off' states may
be reversed.
MEMS pixels can be configured to reflect predominantly at selected colors,
allowing for a
color display in addition to black and white.
Figure 1 is an isometric view depicting two adjacent pixels in a series of
pixels of a
visual display, wherein each pixel comprises a MEMS interferometric modulator.
In some
embodiments, an interferometric modulator display comprises a row/column array
of these
interferometric modulators. Each interferometric modulator includes a pair of
reflective
layers positioned at a variable and controllable distance from each other to
form a resonant
optical cavity with at least one variable dimension. In one embodiment, one of
the
reflective layers may be moved between two positions. In the first position,
referred to
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herein as the relaxed position, the movable reflective layer is positioned at
a relatively large
distance from a fixed partially reflective layer. In the second position,
referred to herein as
the actuated position, the movable reflective layer is positioned more closely
adjacent to the
partially reflective layer. Incident light that reflects from the two layers
interferes
constructively or destructively depending on the position of the movable
reflective layer,
producing either an overall reflective or non-reflective state for each pixel.
The depicted portion of the pixel array in Figure 1 includes two adjacent
interferometric modulators 12a and 12b. In the interferometric modulator 12a
on the left, a
movable reflective layer 14a is illustrated in a relaxed position at a
predetermined distance
from an optical stack 16a, which includes a partially reflective layer. In the
interferometric
modulator 12b on the right, the movable reflective layer 14b is illustrated in
an actuated
position adjacent to the optical stack 16b.
The optical stacks 16a and 16b (collectively referred to as optical stack 16),
as
referenced herein, typically comprise of several fused layers, which can
include an
electrode layer, such as indium tin oxide (ITO), a partially reflective layer,
such as
chromium, and a transparent dielectric. The optical stack 16 is thus
electrically conductive,
partially transparent and partially reflective, and may be fabricated, for
example, by
depositing one or more of the above layers onto a transparent substrate 20. In
some
embodiments, the layers are patterned into parallel strips, and may form row
electrodes in a
display device as described further below. The movable reflective layers 14a,
14b may be
formed as a series of parallel strips of a deposited metal layer or layers
(orthogonal to the
row electrodes of 16a, 16b) deposited on top of posts 18 and an intervening
sacrificial
material deposited between the posts 18. When the sacrificial material is
etched away, the
movable reflective layers 14a, 14b are separated from the optical stacks 16a,
16b by a
defined gap 19. A highly conductive and reflective material such as aluminum
may be used
for the reflective layers 14, and these strips may form column electrodes in a
display device.
With no applied voltage, the cavity 19 remains between the movable reflective
layer
14a and optical stack 16a, with the movable reflective layer 14a in a
mechanically relaxed
state, as illustrated by the pixel 12a in Figure 1. However, when a potential
difference is
applied to a selected row and column, the capacitor formed at the intersection
of the row
and column electrodes at the corresponding pixel becomes charged, and
electrostatic forces
pull the electrodes together. If the voltage is high enough, the movable
reflective layer 14
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is deformed and is forced against the optical stack 16. A dielectric layer
(not illustrated in
this Figure) within the optical stack 16 may prevent shorting and control the
separation
distance between layers 14 and 16, as illustrated by pixel 12b on the right in
Figure 1. The
behavior is the same regardless of the polarity of the applied potential
difference. In this
Figures 2 through 5B illustrate one exemplary process and system for using an
array
of interferometric modulators in a display application.
Figure 2 is a system block diagram illustrating one embodiment of an
electronic
In one embodiment, the processor 21 is also configured to communicate with an
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row/column actuation protocol can be designed such that during row strobing,
pixels in the
strobed row that are to be actuated are exposed to a voltage difference of
about 10 volts,
and pixels that are to be relaxed are exposed to a voltage difference of close
to zero volts.
After the strobe, the pixels are exposed to a steady state voltage difference
of about 5 volts
such that they remain in whatever state the row strobe put them in. After
being written,
each pixel sees a potential difference within the "stability window" of 3-7
volts in this
example. This feature makes the pixel design illustrated in Figure 1 stable
under the same
applied voltage conditions in either an actuated or relaxed pre-existing
state. Since each
pixel of the interferometric modulator, whether in the actuated or relaxed
state, is
essentially a capacitor formed by the fixed and moving reflective layers, this
stable state can
be held at a voltage within the hysteresis window with almost no power
dissipation.
Essentially no current flows into the pixel if the applied potential is fixed.
In typical applications, a display frame may be created by asserting the set
of
column electrodes in accordance with the desired set of actuated pixels in the
first row. A
row pulse is then applied to the row 1 electrode, actuating the pixels
corresponding to the
asserted column lines. The asserted set of column electrodes is then changed
to correspond
to the desired set of actuated pixels in the second row. A pulse is then
applied to the row 2
electrode, actuating the appropriate pixels in row 2 in accordance with the
asserted column
electrodes. The row 1 pixels are unaffected by the row 2 pulse, and remain in
the state they
were set to during the row 1 pulse. This may be repeated for the entire series
of rows in a
sequential fashion to produce the frame. Generally, the frames are refreshed
and/or updated
with new display data by continually repeating this process at some desired
number of
frames per second. A wide variety of protocols for driving row and column
electrodes of
pixel arrays to produce display frames are also well known and may be used in
conjunction
with the present invention.
Figures 4, 5A, and 5B illustrate one possible actuation protocol for creating
a
display frame on the 3x3 array of Figure 2. Figure 4 illustrates a possible
set of column and
row voltage levels that may be used for pixels exhibiting the hysteresis
curves of Figure 3.
In the Figure 4 embodiment, actuating a pixel involves setting the appropriate
column to ¨
Vbias, and the appropriate row to +AV, which may correspond to -5 volts and +5
volts
respectively Relaxing the pixel is accomplished by setting the appropriate
column to
+Vbias, and the appropriate row to the same +AV, producing a zero volt
potential difference
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across the pixel. In those rows where the row voltage is held at zero volts,
the pixels are
stable in whatever state they were originally in, regardless of whether the
column is at
Vbias, or -Vbias= As is also illustrated in Figure 4, it will be appreciated
that voltages of
opposite polarity than those described above can be used, e.g., actuating a
pixel can involve
setting the appropriate column to +Vbias, and the appropriate row to ¨AV. In
this
embodiment, releasing the pixel is accomplished by setting the appropriate
column to -
Vbias, and the appropriate row to the same -AV, producing a zero volt
potential difference
across the pixel.
Figure 58 is a timing diagram showing a series of row and column signals
applied
to the 3x3 array of Figure 2 which will result in the display arrangement
illustrated in
Figure 5A, where actuated pixels are non-reflective. Prior to writing the
frame illustrated in
Figure 5A, the pixels can be in any state, and in this example, all the rows
are at 0 volts,
and all the columns are at +5 volts. With these applied voltages, all pixels
are stable in
their existing actuated or relaxed states.
In the Figure 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are
actuated. To
accomplish this, during a "line time" for row 1, columns 1 and 2 are set to -5
volts, and
column 3 is set to +5 volts. This does not change the state of any pixels,
because all the
pixels remain in the 3-7 volt stability window. Row 1 is then strobed with a
pulse that goes
from 0, up to 5 volts, and back to zero. This actuates the (1,1) and (1,2)
pixels and relaxes
the (1,3) pixel. No other pixels in the array are affected. To set row 2 as
desired, column 2
is set to -5 volts, and columns 1 and 3 are set to +5 volts. The same strobe
applied to row 2
will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again, no
other pixels of the
array are affected. Row 3 is similarly set by setting columns 2 and 3 to -5
volts, and
column 1 to +5 volts. The row 3 strobe sets the row 3 pixels as shown in
Figure 5A. After
writing the frame, the row potentials are zero, and the column potentials can
remain at
either +5 or -5 volts, and the display is then stable in the arrangement of
Figure 5A. It will
be appreciated that the same procedure can be employed for arrays of dozens or
hundreds of
rows and columns. It will also be appreciated that the timing, sequence, and
levels of
voltages used to perform row and column actuation can be varied widely within
the general
principles outlined above, and the above example is exemplary only, and any
actuation
voltage method can be used with the systems and methods described herein.
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Figures 6A and 6B are system block diagrams illustrating an embodiment of a
display device 40. The display device 40 can be, for example, a cellular or
mobile
telephone. However, the same components of display device 40 or slight
variations thereof
are also illustrative of various types of display devices such as televisions
and portable
media players.
The display device 40 includes a housing 41, a display 30, an antenna 43, a
speaker
44, an input device 48, and a microphone 46. The housing 41 is generally
formed from any
of a variety of manufacturing processes as are well known to those of skill in
the art,
including injection molding, and vacuum forming. In addition, the housing 41
may be
made from any of a variety of materials, including but not limited to plastic,
metal, glass,
rubber, and ceramic, or a combination thereof. In one embodiment the housing
41 includes
removable portions (not shown) that may be interchanged with other removable
portions of
different color, or containing different logos, pictures, or symbols.
The display 30 of exemplary display device 40 may be any of a variety of
displays,
including a bi-stable display, as described herein. In other embodiments, the
display 30
includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD
as
described above, or a non-flat-panel display, such as a CRT or other tube
device, as is well
known to those of skill in the art. However, for purposes of describing the
present
embodiment, the display 30 includes an interferometric modulator display, as
described
herein.
The components of one embodiment of exemplary display device 40 are
schematically illustrated in Figure 6B. The illustrated exemplary display
device 40 includes
a housing 41 and can include additional components at least partially enclosed
therein. For
example, in one embodiment, the exemplary display device 40 includes a network
interface
27 that includes an antenna 43 which is coupled to a transceiver 47. The
transceiver 47 is
connected to a processor 21, which is connected to conditioning hardware 52.
The
conditioning hardware 52 may be configured to condition a signal (e.g. filter
a signal). The
conditioning hardware 52 is connected to a speaker 45 and a microphone 46. The
processor
21 is also connected to an input device 48 and a driver controller 29. The
driver controller
29 is coupled to a frame buffer 28, and to an array driver 22, which in turn
is coupled to a
display array 30. A power supply 50 provides power to all components as
required by the
particular exemplary display device 40 design.
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The network interface 27 includes the antenna 43 and the transceiver 47 so
that the
exemplary display device 40 can communicate with one ore more devices over a
network.
In one embodiment the network interface 27 may also have some processing
capabilities to
relieve requirements of the processor 21. The antenna 43 is any antenna known
to those of
skill in the art for transmitting and receiving signals. In one embodiment,
the antenna
transmits and receives RF signals according to the IEEE 802.11 standard,
including IEEE
802.11(a), (b), or (g). In another embodiment, the antenna transmits and
receives RF
signals according to the BLUETOOTH standard. In the case of a cellular
telephone, the
antenna is designed to receive CDMA, GSM, AMPS or other known signals that are
used
to communicate within a wireless cell phone network. The transceiver 47 pre-
processes the
signals received from the antenna 43 so that they may be received by and
further
manipulated by the processor 21. The transceiver 47 also processes signals
received from
the processor 21 so that they may be transmitted from the exemplary display
device 40 via
the antenna 43.
In an alternative embodiment, the transceiver 47 can be replaced by a
receiver. In
yet another alternative embodiment, network interface 27 can be replaced by an
image
source, which can store or generate image data to be sent to the processor 21.
For example,
the image source can be a digital video disc (DVD) or a hard-disc drive that
contains image
data, or a software module that generates image data.
Processor 21 generally controls the overall operation of the exemplary display
device 40. The processor 21 receives data, such as compressed image data from
the
network interface 27 or an image source, and processes the data into raw image
data or into
a format that is readily processed into raw image data. The processor 21 then
sends the
processed data to the driver controller 29 or to frame buffer 28 for storage.
Raw data
typically refers to the information that identifies the image characteristics
at each location
within an image. For example, such image characteristics can include color,
saturation, and
gray-scale level.
In one embodiment, the processor 21 includes a microcontroller, CPU, or logic
unit
to control operation of the exemplary display device 40. Conditioning hardware
52
generally includes amplifiers and filters for transmitting signals to the
speaker 45, and for
receiving signals from the microphone 46. Conditioning hardware 52 may be
discrete
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components within the exemplary display device 40, or may be incorporated
within the
processor 21 or other components.
The driver controller 29 takes the raw image data generated by the processor
21
either directly from the processor 21 or from the frame buffer 28 and
reformats the raw
image data appropriately for high speed transmission to the array driver 22.
Specifically,
the driver controller 29 reformats the raw image data into a data flow having
a raster-like
format, such that it has a time order suitable for scanning across the display
array 30. Then
the driver controller 29 sends the formatted information to the array driver
22. Although a
driver controller 29, such as a LCD controller, is often associated with the
system processor
21 as a stand-alone Integrated Circuit (IC), such controllers may be
implemented in many
ways. They may be embedded in the processor 21 as hardware, embedded in the
processor
21 as software, or fully integrated in hardware with the array driver 22.
Typically, the array driver 22 receives the formatted information from the
driver
controller 29 and reformats the video data into a parallel set of waveforms
that are applied
many times per second to the hundreds and sometimes thousands of leads coming
from the
display's x-y matrix of pixels.
In one embodiment, the driver controller 29, array driver 22, and display
array 30
are appropriate for any of the types of displays described herein. For
example, in one
embodiment, driver controller 29 is a conventional display controller or a bi-
stable display
controller (e.g., an interferometric modulator controller). In another
embodiment, array
driver 22 is a conventional driver or a bi-stable display driver (e.g., an
interferometric
modulator display). In one embodiment, a driver controller 29 is integrated
with the array
driver 22. Such an embodiment is common in highly integrated systems such as
cellular
phones, watches, and other small area displays. In yet another embodiment,
display array
30 is a typical display array or a bi-stable display array (e.g., a display
including an array of
interferometric modulators).
The input device 48 allows a user to control the operation of the exemplary
display
device 40. In one embodiment, input device 48 includes a keypad, such as a
QWERTY
keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen,
a pressure- or
heat-sensitive membrane. In one embodiment, the microphone 46 is an input
device for the
exemplary display device 40. When the microphone 46 is used to input data to
the device,
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voice commands may be provided by a user for controlling operations of the
exemplary
display device 40.
Power supply 50 can include a variety of energy storage devices as are well
known
in the art. For example, in one embodiment, power supply 50 is a rechargeable
battery,
such as a nickel-cadmium battery or a lithium ion battery. In another
embodiment, power
supply 50 is a renewable energy source, a capacitor, or a solar cell,
including a plastic solar
cell, and solar-cell paint. In another embodiment, power supply 50 is
configured to receive
power from a wall outlet.
In some implementations control programmability resides, as described above,
in a
driver controller which can be located in several places in the electronic
display system. In
some cases control programmability resides in the array driver 22. Those of
skill in the art
will recognize that the above-described optimization may be implemented in any
number of
hardware and/or software components and in various configurations.
The details of the structure of interferometric modulators that operate in
accordance
with the principles set forth above may vary widely. For example, Figures 7A-
7E illustrate
five different embodiments of the movable reflective layer 14 and its
supporting structures.
Figure 7A is a cross section of the embodiment of Figure 1, where a strip of
metal material
14 is deposited on orthogonally extending supports 18. In Figure 7B, the
moveable
reflective layer 14 is attached to supports at the corners only, on tethers
32. In Figure 7C,
the moveable reflective layer 14 is suspended from a deformable layer 34,
which may
comprise a flexible metal. The deformable layer 34 connects, directly or
indirectly, to the
substrate 20 around the perimeter of the deformable layer 34. These
connections are herein
referred to as support posts. The embodiment illustrated in Figure 7D has
support post
plugs 42 upon which the deformable layer 34 rests. The movable reflective
layer 14
remains suspended over the cavity, as in Figures 7A-7C, but the deformable
layer 34 does
not form the support posts by filling holes between the deformable layer 34
and the optical
stack 16. Rather, the support posts are formed of a planarization material,
which is used to
form support post plugs 42. The embodiment illustrated in Figure 7E is based
on the
embodiment shown in Figure 7D, but may also be adapted to work with any of the
embodiments illustrated in Figures 7A-7C as well as additional embodiments not
shown.
In the embodiment shown in Figure 7E, an extra layer of metal or other
conductive material
has been used to form a bus structure 44. This allows signal routing along the
back of the
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interferometric modulators, eliminating a number of electrodes that may
otherwise have had
to be formed on the substrate 20.
In embodiments such as those shown in Figure 7, the interferometric modulators
function as direct-view devices, in which images are viewed from the front
side of the
transparent substrate 20, the side opposite to that upon which the modulator
is arranged. In
these embodiments, the reflective layer 14 optically shields the portions of
the
interferometric modulator on the side of the reflective layer opposite the
substrate 20,
including the deformable layer 34 and the bus structure 44. This allows the
shielded areas
to be configured and operated upon without negatively affecting the image
quality. This
separable modulator architecture allows the structural design and materials
used for the
electromechanical aspects and the optical aspects of the modulator to be
selected and to
function independently of each other. Moreover, the embodiments shown in
Figures 7C-7E
have additional benefits deriving from the decoupling of the optical
properties of the
reflective layer 14 from its mechanical properties, which are carried out by
the deformable
layer 34. This allows the structural design and materials used for the
reflective layer 14 to
be optimized with respect to the optical properties, and the structural design
and materials
used for the deformable layer 34 to be optimized with respect to desired
mechanical
properties.
As discussed above with reference to Figure 1, the modulator 12 (i.e., both
modulators 12a and 12b) includes an optical cavity formed between the mirrors
14 (i.e.,
mirrors 14a and 14b) and 16 (mirrors 16a and 16b, respectively). The
characteristic
distance, or effective optical path length, d, of the optical cavity
determines the resonant
wavelengths, k, of the optical cavity and thus of the interferometric
modulator 12. A peak
resonant visible wavelength, X, of the interferometric modulator 12 generally
corresponds
to the perceived color of light reflected by the modulator 12. Mathematically,
the optical
path length d is equal to 1/2 N X, where N is an integer. A given resonant
wavelength, X, is
thus reflected by interferometric modulators 12 having optical path lengths d
of Y2 X, (N=1),
X (N=2), 3/2 X (N=3), etc. The integer N may be referred to as the order of
interference of
the reflected light. As used herein, the order of a modulator 12 also refers
to the order N of
light reflected by the modulator 12 when the mirror 14 is in at least one
position. For
example, a first order red interferometric modulator 12 may have an optical
path length d of
about 325 nm, corresponding to a wavelength k of about 650 nm. Accordingly, a
second
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order red interferometric modulator 12 may have an optical path length d of
about 650 nm.
Generally, higher order modulators 12 reflect light over a narrower range of
wavelengths,
e.g., have a higher "Q" value, and thus produce colored light that is more
saturated. The
saturation of the modulators 12 that comprise a color pixel affects properties
of a display
such as the color gamut and white point of the display. For example, in order
for a display
using a second order modulator 12 to have the same white point or color
balance as a
display that includes a first order modulator reflecting the same general
color of light, the
second order modulator 12 may be selected to have a different central peak
optical
wavelength.
Figure 8 is a side cross-sectional view of an exemplary interferometric
modulator 12
that illustrates the spectral characteristics of light that would be output by
positioning the
movable mirror 14 at a range of positions 61-65. The exemplary modulator
includes a
conductive layer 52 of indium-tin-oxide (ITO) acting as a column electrode. In
the
exemplary modulator, the movable mirror 14 includes the row conductor.
Each of a particular group of positions 61-65 of the movable mirror 14 is
shown by
an arrow extending from the fixed mirror 16. The point of each arrow indicates
a particular
one of the positions 61-65 of the movable mirror. The color of light reflected
from the
interferometric modulator is determined by the optical path length, d, between
the movable
and fixed mirrors 14 and 16. The distances 61-65 are selected so as to account
for the
thickness and index of refraction of the dielectric layer 54 in the optical
path length, d.
Accordingly, a movable mirror 14 positioned at a different one of the
positions 61-65, each
corresponding to a different distance, d, would result in a modulator 12 that
outputs light to
a viewing position 51 with a different spectral response, which corresponds to
different
colors of incident light being reflected by the modulator 12. Moreover, at
position 61, the
movable mirror 14 is sufficiently close to the fixed mirror 16, that the
effects of
interference are negligible and modulator 12 acts as a mirror that reflects
substantially all
colors of incident visible light substantially equally, e.g., as white light.
The broadband
mirror effect is caused because the small distance d is too small for optical
resonance in the
visible band. The mirror 14 thus merely acts as a reflective surface with
respect to visible
light.
With the mirror 14 positioned at the position 62, the modulator 12 exhibits a
shade
of gray as the increased gap distance between the mirrors 14 and 16 reduces
the reflectivity
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of the mirror 14. At the position 63, the distance d is such that the cavity
operates
interferometrically but reflects substantially no visible wavelengths of light
because the
resonant wavelength is outside the visible range.
As the distance d is increased further, a peak spectral response of the
modulator 12
moves into visible wavelengths. Thus, when the movable mirror 14 is at
position 64, the
modulator 12 reflects blue light. When the movable mirror 14 is at the
position 65, the
modulator 12 reflects green light. When the movable mirror 14 is at the non-
deflected
position 66, the modulator 12 reflects red light.
In designing a display using interferometric modulators 12, the modulators 12
may
be formed so as to increase the color saturation of reflected light.
Saturation refers to the
intensity of the hue of color light. A highly saturated hue has a vivid,
intense color, while a
less saturated hue appears more muted and grey. For example, a laser, which
produces a
very narrow range of wavelengths, produces highly saturated light. Conversely,
a typical
incandescent light bulb produces white light that may have a desaturated red
or blue color.
In one embodiment, the modulator 12 is formed with a distance d corresponding
to higher
order of interference, e.g., 2nd or 3rd order, to increase the saturation of
reflected color
light.
An exemplary color display includes red, green, and blue display elements.
Other
colors are produced in such a display by varying the relative intensity of
light produced by
the red, green, and blue elements. Such mixtures of primary colors such as
red, green, and
blue are perceived by the human eye as other colors. The relative values of
red, green, and
blue in such a color system may be referred to as tristimulus values in
reference to the
stimulation of red, green, and blue light sensitive portions of the human eye.
In general, the
more saturated the primary colors, the greater the range of colors that can be
produced by
the display. In other embodiments, the display may include modulators 12
having sets of
colors that define other color systems in terms of sets of primary colors
other than red,
green, and blue.
Another consideration in the design of displays incorporating interferometric
modulators 12 is the generation of white light. "White" light generally refers
to light that is
perceived by the human eye to include no particular color, i.e., white light
is not associated
with a hue. While black refers to an absence of color (or light), white refers
to light that
includes such a broad spectral range that no particular color is perceived.
White light may
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refer to light having a broad spectral range of visible light at approximately
uniform
intensity. However, because the human eye is sensitive to certain wavelengths
of red,
green, and blue light, white can be created by mixing intensities of colored
light to produce
light that has one or more spectral peaks which is perceived by the eye as
"white." The
color gamut of a display is the range of colors that the device is able to
reproduce, e.g., by
mixing red, green, and blue light.
In a reflective display, white light produced using saturated interferometric
modulators tends to have a relatively low intensity to a viewer because only a
small range
of incident wavelengths is reflected with relatively high intensities to form
the white light.
In contrast, a mirror reflecting broadband white light, e.g., substantially
all incident
wavelengths, has a greater intensity because a greater range of incident
wavelengths is
reflected. Thus, designing reflective displays using combinations of primary
colors to
produce white light generally results in a tradeoff between color saturation
and color gamut
and the brightness of white light output by the display.
In one embodiment, the movable mirror 14 is positioned so that in a first
position
the modulator 12 is non-reflective of visible light (e.g., position 63 of
Figure 8) and in a
second position the distance between the movable mirror 14 and the fixed
mirror 16 is too
small for interferometric modulation of the incident visible light so that the
mirror 14
reflects a broadband white (e.g., position 61 of Figure 8). In such an
embodiment, the
movable mirror 14 reflects incident light with a broad, relatively uniform
spectral response
across the visible spectrum. If the incident light comprises white light, the
light reflected
by the modulator 12 in the second position may be a substantially similar
white light. The
spectral response of such a "white" reflective state of the modulator 12 may
be generally
uniform across the visible spectrum. In one embodiment, the spectral response
is tuned by
selection of the materials of the modulator. For example, different materials,
e.g.,
aluminum or copper, may be used for the reflective surface of the movable
mirror 14 so as
to tune the spectral response of the modulator 12 when in the white reflective
state. In
another embodiment, a filter may be used to selectively absorb certain
wavelengths of
reflected or incident light to affect the output of such a broadband white
modulator.
In one embodiment of the pixel array 30, each pixel includes one or more color
modulators 12, e.g., modulators configured to reflect red, green, and blue
light, and one or
more "white" modulators 12 configured to reflect white light. In such an
embodiment, light
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from the red, green, and/or blue modulators 12 in their reflective states
combines to output
colored light. Light from the white modulators 12 can be used to output white
or gray light.
Use of white in combination with color may increase the brightness or
intensity of the
pixels.
The white point of a display is the hue that is considered to be generally
neutral
(gray or achromatic). The white point of a display device may be characterized
based on a
comparison of white light produced by the device with the spectral content of
light emitted
by a black body at a particular temperature ("black body radiation"). A black
body radiator
is an idealized object that absorbs all light incident upon the object and
which reemits the
light with a spectrum dependent on the temperature of the black body. For
example, the
black body spectrum at 6,500 K may be referred to as white light having a
color
temperature of 6,500 K. Such color temperatures, or white points of
approximately
5,000 40,000 K are generally identified with daylight.
The International Commission on Illumination (OE) promulgates standardized
white points of light sources. For example, light source designations of "d"
refer to
daylight. In particular, standard white points D55, D65, and D75, which
correlate with color
temperatures of 5,500 K, 6,500 K, and 7,500 K, are standard daylight white
points.
A display device may be characterized by the white point of the white light
produced by a display. As with light from other light sources, human
perception of a
display is at least partially determined by the perception of white light from
the display.
For example, a display or light source having a lower white point, e.g., D55,
may be
perceived as having a yellow tone by a viewer. A display having a higher
temperature
white point, e.g., D75 may be perceived as having a "cooler" or bluer tone to
a user. Users
generally respond more favorably to displays having higher temperature white
points.
Thus, controlling the white point of a display desirably provides some control
over a
viewer's response to a display. Embodiments of the interferometric modulator
array 30
may be configured to produce white light in which the white point is selected
to conform to
a standardized white point under one or more anticipated lighting conditions.
White light can be produced by the pixel array 30 by including one or more
interferometric modulators 12 for each pixel. For example, in one embodiment,
the pixel
array 30 includes pixels of groups of red, green, and blue interferometric
modulators 12. As
discussed above, the colors of the interferometric modulators 12 may be
selected by
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selecting the optical path length d using the relation of d = V2 N X. In
addition, the balance,
or relative proportions, of the colors produced by each pixel in the pixel
array 30 may be
further affected by the relative reflective areas of each of the
interferometric modulators 12,
e.g., of the red, green, and blue interferometric modulators 12. Further,
because the
modulators 12 selectively reflect incident light, the white point of reflected
light from the
pixel array 30 of interferometric modulators 12 is generally dependent on the
spectral
characteristics of incident light. In one embodiment, the white point of
reflected light may
be configured to be different than the white point of incident light. For
example, in one
embodiment, the pixel array 30 may be configured to reflect D75 light when
used in D65
sunlight.
In one embodiment, the distances d and areas of the interferometric modulators
12
in the pixel array 30 are selected so that white light produced by the pixel
array 30
corresponds to a particular standardized white point in an anticipated
lighting condition,
e.g., in sunlight, under fluorescent light, or from a front light positioned
to illuminate the
pixel array 30. For example, the white point of the pixel array 30 may be
selected to be
D55, D65, or D75 in particular lighting conditions. Moreover, the light
reflected by the pixel
array 30 may have a different white point than the light of an anticipated or
configured light
source. For example, a particular pixel array 30 may be configured to reflect
D75 light
when viewed under D65 sunlight. More generally, the white point of a display
may be
selected with reference to a source of illumination configured with the
display, e.g., a front
light, or with reference to a particular viewing condition. For example, a
display may be
configured to have a selected white point, e.g., D55, D65, or D75, when viewed
under
anticipated or typical sources of illumination such as incandescent,
fluorescent, or natural
light sources. More particularly, a display for use in a handheld device, for
example, may
be configured to have a selected white point when viewed under sunlight
conditions.
Alternatively, a display for use in an office environment may be configured to
have a
selected white point, e.g., D75, when illuminated by typical office
fluorescent lights. In
various embodiments, different distances d and areas of modulators 12 may be
selected to
produce other standardized white point settings for different viewing
environments.
Further, the red, green, and blue modulators 12 may also be controlled so as
to be in
reflective or non-reflective states for different amounts of time so as to
further vary the
relative balance of reflected red, green, and blue light, and thus the white
point of reflected
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light. In one embodiment, the ratio of reflective areas of each of the color
modulators 12
may be selected so as to control the white point in different viewing
environments. In one
embodiment, the optical path length d may be selected so as to correspond to a
common
multiple of more than one visible resonant wavelength, e.g., first, second, or
third order
peaks of red, green, and blue, so that the interferometric modulator 12
reflects white light
characterized by three visible peaks in its spectral response. In such an
embodiment, the
optical path length d may be selected so that the white light produced
corresponds to a
standardized white point.
In addition to groups of red, green, and blue interferometric modulators 12 in
the
pixel array 30, other embodiments include other ways of generating white
light. For
example, one embodiment of the pixel array 30 includes cyan and yellow
interferometric
modulators 12, e.g., interferometric modulators 12 that have respective
separation distances
d so as to produce cyan and yellow light. The combined spectral response of
the cyan and
yellow interferometric modulators 12 produces light with a broad spectral
response that is
perceived as "white." The cyan and yellow modulators are positioned
proximately so that a
viewer perceives such a combined response. For example, in one embodiment, the
cyan
modulators and yellow modulators are arranged in adjacent rows of the pixel
array 30. In
another embodiment, the cyan modulators and yellow modulators are arranged in
adjacent
columns of the pixel array 30.
Figure 9 is a graphical diagram illustrating the spectral response of one
embodiment
that includes cyan and yellow interferometric modulators 12 to produce white
light. The
horizontal axis represents the wavelength of reflected light. The vertical
axis represents the
relative reflectance of light incident on the modulators 12. A trace 80
illustrates the
response of the cyan modulator, which is a single peak centered in the cyan
portion of the
spectrum, e.g., between blue and green. A trace 82 illustrates the response of
the yellow
modulator, which is a single peak centered in the yellow portion of the
spectrum, e.g.,
between red and green. A trace 84 illustrates the combined spectral response
of a pair of
cyan and yellow modulators 12. The trace 84 has two peaks at cyan and yellow
wavelengths but is sufficiently uniform across the visible spectrum so that
reflected light
from such modulators 12 is perceived as white.
Generally, the color of light reflected by an interferometric modulator 12
shifts
when the modulator 12 is viewed from different angles. Figure 10 is a side
cross-sectional
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view of an interferometric modulator 12 illustrating different optical paths
through the
modulator 12. The color of light reflected from the interferometric modulator
12 may vary
for different angles of incidence (and reflection) with respect to an axis AA
as illustrated in
Figure 10. For example, for the interferometric modulator 12 shown in Figure
10, as light
travels along the off-axis path A1, the light is incident on the
interferometric modulator at a
first angle, reflects from the interferometric modulator, and travels to a
viewer. The viewer
perceives a first color when the light reaches the viewer as a result of
optical interference
between a pair of mirrors in the interferometric modulator 12. When the viewer
moves or
changes his/her location and thus view angle, the light received by the viewer
travels along
a different off-axis path A2 corresponding to a second different angle of
incidence (and
reflection). Optical interference in the interferometric modulator 12 depends
on optical
path length of light propagated within the modulator, d. Different optical
path lengths for
the different optical paths A1 and A2 therefore yield different outputs from
the
interferometric modulator 12. With increasing view angle, the effective
optical path of the
interferometric modulator is decreased according to the relationship 2d cos PI
= NX, where
13 is the view angle (the angle between the normal to the display and the
incident light).
With increasing view angle, the peak resonant wavelength of the reflected
light is
decreased. The user therefore perceives different colors depending on his or
her angle of
view. As described above, this phenomenon is referred to as a "color shift."
This color
shift is typically identified with reference to a color produced by an
interferometric
modulator 12 when viewed along the axis AA.
In one embodiment, the pixel array 30 includes a first order yellow
interferometric
modulator and a second order cyan interferometric modulator. When such a pixel
array 30
is viewed from increasingly larger off-axis angles, light reflected by the
first order yellow
modulator is shifted toward the blue end of the spectrum, e.g., the modulator
at a certain
angle has an effective d equal to that of a first order cyan. Concurrently,
light reflected by
the second order cyan modulator shifts to correspond to light from the first
order yellow
modulator. Thus, the overall combined spectral response is broad and
relatively uniform
across the visible spectrum even as the relative peaks of the spectrum shift.
Such pixel
array 30 thus produces white light over a relatively large range of viewing
angles.
In one embodiment, a display having a cyan and yellow modulators may be
configured to produce white light having a selected standardized white point
under one or
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more viewing conditions. For example, the spectral response of the cyan
modulator and of
the yellow modulator may be selected so that reflected light has a white point
of D55, D65,
D75, or any other suitable white point under selected illumination conditions
that include
D55, D65, or D75 light such as sunlight for a display suited for outdoor use.
In one
embodiment, the modulators may be configured to reflect light that has a
different white
point than incident light from an expected or selected viewing condition.
Figure 11 is a side cross-sectional view of the interferometric modulator 12
having a
layer 102 of material for selectively transmitting light of a particular
color. In an exemplary
embodiment, the layer 102 is on the opposite side of the substrate 20 from
modulator 12. In
one embodiment, the layer 102 of material' comprises a magenta filter through
which green
interferometric modulators 12 are viewed. In one embodiment, the layer 102 of
material is
a dyed material. In one such embodiment, the material is a dyed photoresist
material. In
one embodiment, the green interferometric modulators 12 are first order green
interferometric modulators. The filter layer 102 is configured to transmit
magenta light
when illuminated with a broadly uniform white light. In the exemplary
embodiment, light
is incident on the layer 20 from which filtered light is transmitted to the
modulator 12. The
modulator 12 reflects the filtered light back through the layer 102. In such
an embodiment,
the light passes through the layer 102 twice. In such an embodiment, the
thickness of the
layer 102 of material may be selected to compensate for, and utilize, this
double filtering.
In another embodiment, a front light structure may be positioned between the
layer 102 and
the modulator 12. In such an embodiment, the layer 102 of material acts only
on light
reflected by the modulator 12. In such embodiments, the layer 102 is selected
accordingly.
Figure 12 is a graphical diagram illustrating the spectral response of one
embodiment that includes the green interferometric modulators 12 and the
"magenta" filter
layer 102. The horizontal axis represents the wavelength of reflected light.
The vertical
axis represents the relative spectral response of light incident on the green
modulator 12
and filter layer 102 over the visible spectrum. A trace 110 illustrates the
response of the
green modulator 12, which is a single peak centered in the green portion of
the spectrum,
e.g., near the center of the visible spectrum. A trace 112 illustrates the
response of the
magenta filter formed by the layer of material 102. The trace 112 has two
relatively flat
portions on either side of a central u-shaped minimum. The trace 112 thus
represents the
response of a magenta filter that selectively transmits substantially all red
and blue light
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while filtering light in the green portion of the spectrum. A trace 114
illustrates the
combined spectral response of the pairing of the green modulator 12 and the
filter layer
102. The trace 114 illustrates that the spectral response of the combination
is at a lower
reflectance level than the green modulator 12 due to the filtering of light by
the filter layer
102. However, the spectral response is relatively uniform across the visible
spectrum so
that the filtered, reflected light from the green modulator 12 and the magenta
filter layer
102 is perceived as white.
In one embodiment, a display having a green modulator 12 with the magenta
filter
layer 102 may be configured to produce white light having a selected
standardized white
point under one or more viewing conditions. For example, the spectral response
of the
green modulator 12 and of the magenta filter layer 102 may be selected so that
reflected
light has a white point of D55, D65, D75, or any other suitable white point
under selected
illumination conditions that include D55, D65, or D75 light such as sunlight
for a display
suited for outdoor use. In one embodiment, the modulator 12 and filter layer
102 may be
= configured to reflect light that has a different white point than incident
light from an
expected or selected viewing condition.
Figure 13 is a schematic diagram illustrating two pixels of an exemplary pixel
array
30. Rows 1-4 and columns 1-4 form one pixel 120a. Rows 5-8 and columns 1-4
form a
second pixel 120b. Each pixel 120a and 120b includes at least one modulator 12
configured to reflect red (column 1), green, (column 2), blue (column 3), and
white
(column 4) light. Each pixel of the exemplary pixel array 30 includes 4
display elements of
each of red, green, blue, and white to form a "4 bit" per color display which
can output
24=16 shades of each of red, green, blue, or white/gray for a total of 216
shades of color.
Figure 14A is a chromaticity diagram that illustrates the colors that can be
produced
by an exemplary color display that includes red, green, and blue display
elements. A wide
range of colors are produced in such a display by varying the relative
intensity of light
produced by the red, green, and blue elements. A chromaticity diagram
illustrates how a
display may be controlled to generate the mixtures of primary colors such as
red, green, and
blue that are perceived by the human eye as other colors. The horizontal and
vertical axes
of Figure 14 define a chromaticity coordinate system on which color values may
be
depicted. In particular, points 130 illustrate the color of light reflected by
exemplary red,
green, and blue interferometric modulators. The triangular trace 133 encloses
a region 134
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that corresponds to the range of colors that can be produced by mixing the
light produced at
points 120. This range of colors may be referred to as the color gamut of the
display. In
operation, each of the red, green and blue display elements in a pixel can be
controlled to
produce different mixtures of the red, green, and blue light that combine to
form each color
within the color gamut.
As illustrated in Figure 13, in one embodiment, an exemplary display 30
includes
pixels having subpixels of red, green, blue, and white. One embodiment of a
scheme for
driving such a display defines each color to be displayed by a pixel in terms
of
combinations of (i) red, green, and white, (ii) red, blue, and white, and
(iii) blue, green, and
white chromaticity values that define three different color gamuts. In
operation of such an
embodiment, when the display controller determines that a particular pixel is
to be set to a
color value expressed in terms of red, green, and blue, the display controller
translates the
color value into a value expressed in terms of one of (i) red, green, and
white; (ii) red, blue,
and white, and (iii) blue, green, and white.
Figure 14B is a chromaticity diagram that illustrates the colors that can be
produced
by such a color display. The overall color gamut of the display is defined by
the area
defined by a trace 140 that connects each of the points 130 corresponding to
the
chromaticity of the display primary colors red, green, and blue. In addition,
a point 130a
corresponds to the chromaticity of light emitted by the white subpixel. This
point 130a may
be in other locations depending on the white produced by the white subpixel.
Traces 144a,
144b, and 144c connect the point 130a corresponding to the white subpixel to
each of the
points 130 corresponding to red, blue, and green, respectively. Along with the
trace 140,
the traces 144a, 144b, and 144c define three regions 146a, 146b, and 146c
within the color
gamut of the display corresponding to the colors that may be produced by the
(i) red, green,
and white, (ii) red, blue, and white, and (iii) blue, green, and white display
elements,
respectively. Thus, conceptually, one embodiment of a drive scheme for such a
display
includes identifying within which of the three regions 146a, 146b, or 146c a
desired color to
be displayed falls. The input color, represented as values of red, green, and
blue may then
be converted to a new chromaticity. This chromaticity coordinate will fall
within one of the
three identified regions 146a, 146b, or 146c. New output values may then be
used to drive
each of the three identified display elements bounding the region within which
the desired
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chromaticity coordinate falls, (i) red, green, and white, (ii) red, blue, and
white, or (iii) blue,
green, and white display elements, of the pixel to output the desired color of
light.
In one embodiment, when a chromaticity value is within a selected distance
(e.g., on
the chromaticity diagram) of the point 130a of the white display element, both
the color and
the white display elements are activated so as to produce a brighter output
from the pixel
for such colors.
In another embodiment, in order to drive such a pixel array, when the total
hue of
the pixel data is below a threshold value, e.g., the pixel data is a gray or
substantially gray
color, a driver circuit sets the white modulators in column 4 to a
corresponding reflective
state. In one embodiment, the red, green, and blue modulators may also be in
their
reflective states. When the total hue for the pixel data is above a threshold
value, e.g., the
pixel data is not substantially gray, the driver circuit sets the white
modulators in column 4
to their non-reflective state and the color modulators in columns 1-3 are set
to reflective
states.
In certain embodiments, white display elements may be activated in combination
with color display elements to add additional brightness. For example, if a
pixel is to
output red light, all of the red display elements in the pixel may be
activated. Additionally,
one or more of the white display elements may also be activated to produce
other color
combinations.
In certain embodiments, a driver circuit can adjust input data to compensate
for the
additional white surface area so that such a display produces images with
color balances
that are substantially unchanged by the white reflective areas (although the
display is
enhanced in its relative brightness).
In one embodiment, the white interferometric modulators are grouped with the
other
white interferometric modulators, such as in an extra column as illustrated in
Figure 13. In
another embodiment, the white interferometric modulators are distributed
evenly
throughout the pixel, e.g., interlaced between red, green, and blue display
elements.
Moreover, in some embodiments, the number of white display elements in each
pixel is
different from the numbers of, for example, red, green, or blue display
elements.
In addition to using additional interferometric modulators configured to
reflect
white light to increase the intensity of reflected white light, embodiments of
the pixel array
30 may be formed that increase the overall apparent brightness of the system
by other
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means. For example, the human eye is more sensitive to green light than to
other hues.
Thus, in one embodiment, the apparent brightness of the interferometric
modulator system
is increased by using an additional green interferometric modulator in every
pixel. For
example, in some embodiments, there are an equal number of green, red, and
blue
interferometric modulators per pixel. In one embodiment, similar to that
illustrated in
Figure 13, a second column of green interferometric modulators can be also
included. In
another embodiment, the pixel array 30 may include a 4th column, such as
illustrated in
Figure 13, in which some of the display elements reflect white light and some
reflect green
light.
In one embodiment, the additional green interferometric modulators can be
grouped
with the other green interferometric modulators, such as in an extra column as
illustrated in
Figure 13. In other embodiments, the additional green interferometric
modulators can be
distributed evenly throughout the pixel, e.g., interlaced between red, green,
and blue display
elements. Moreover, in some embodiments, the number of extra green display
elements in
each pixel can be different from the numbers of, for example, red, green, or
blue display
elements. In one embodiment, the display elements are interferometric
modulators in
which the optical path lengths, d, of the red and blue modulators are selected
to compensate
for the additional green pixels in the color balance of the display. Moreover,
in one
embodiment, the optical path lengths, d, of one or both of the red and blue
display elements
may be selected to produce a more saturated color. In one such embodiment, the
optical
path lengths, d, of the red or blue display elements may be selected to
produce a higher
order (2nd order or greater) reflected light. Second order corresponds to an
optical path
length, d, equal to 1 x k. As interferometric modulators having a more
saturated response
reflect a smaller portion of incoming light, such modulators tend have less
intense (darker)
outputs. However, by increasing the relative intensity of reflected green
light, such a
display may be configured to have a brighter appearance to a viewer. In one
embodiment,
the ratio of area of red to blue is one to one while the area of green to red
(or blue is greater
than one to one. For example, in one embodiment, expressed as a percentage of
total
reflective area of each pixel, 33-50% of the pixel is green. In one
embodiment, 38-44% of
the pixel is green.
In one embodiment, the ratio of the surface area of the green interferometric
modulators to the total reflective surface area of the pixel can be larger
than the ratio of the
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surface area of the red and blue interferometric modulators in order to
increase the
perceived brightness. In another embodiment, the duration that the green
interferometric
modulators are in a reflecting state is increased to increase the green color
relative to the
duration of the other color generating interferometric modulators. In one
embodiment, the
blue and red interferometric modulators are tuned towards the green spectra to
increase the
appearance of green and thus increase the perceived brightness in the system.
As will be
recognized by one of skill in the art, a driver circuit can adjust input data
to compensate for
the additional green surface areas so that such a display produces images with
color
balances that are substantially unchanged by the additional green reflective
areas (although
the display is enhanced in its relative brightness). In one embodiment, the
extra green
display elements are used in display modes where brightness is more important
than color
accuracy, e.g., text display.
While the above detailed description has shown, described, and pointed out
novel
features of the invention as applied to various embodiments, it will be
understood that
various omissions, substitutions, and changes in the form and details of the
device or
process illustrated may be made by those skilled in the art without departing
from the scope
of the claims. As will be recognized, the present invention may be embodied
within a
form that does not provide all of the features and benefits set forth herein,
as some features
may be used or practiced separately from others. The scope of the invention is
indicated by
the appended claims rather than by the foregoing description. All changes
which come
within the meaning and range of equivalency of the claims are to be embraced
within their
scope.
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