Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-09-28
2003-12-02
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S060000, C348S761000
Reexamination Certificate
active
06657609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal displays. More specifically, it relates to reducing flicker in reflective liquid crystal display devices.
2. Discussion of the Related Art
Producing a color image using a Liquid Crystal Display (LCD) is well known. Such displays are particularly useful for producing images that are updated by frames, such as in color televisions. Typically, each image frame is composed of color sub-frames, usually red, green and blue sub-frames.
Such LCD systems employ a light crystal light panel that is comprised of a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprised of pixel rows and pixel columns. To produce a desired image, the individual pixel elements are modulated in accordance with image information. Typically, the image information is applied to the individual pixel elements by rows, with each pixel row being addressed in each frame period.
Pixel element matrix arrays are preferably “active” in that each pixel element is connected to an active switching element of a matrix of such switching elements. One particularly useful active matrix liquid crystal display is the reflective active-matrix liquid crystal display (RLCD). An RLCD display is typically produced on a silicon substrate and is often based on the twisted nematic (TN) effect. Thin film transistors (TFTs) are usually used as the active switching elements. Such RLCD displays can support a high pixel density because the TFTs and their interconnections can be integrated onto the silicon substrate.
FIG. 1
schematically illustrates a single pixel element
10
of a typical RLCD. The pixel element
10
is comprised of a twisted nematic liquid crystal layer
12
that is disposed between a transparent electrode
14
and a pixel electrode
16
. For convenience,
FIG. 1
shows the transparent electrode applied to a common ground. However, in practice the transparent electrode is usually biased, say at +7 volts. Additionally, a storage element
18
is connected to complementary data terminals
20
and
22
. The storage element receives control signals on a control terminal
24
. In responsive to a “write” control signal the storage element
18
selectively latches the voltage on one of the data terminals
20
and
22
, and applies that latched voltage to the pixel electrode
16
via a signal line
26
. The voltages on the data terminals
20
and
22
are complementary. That is, if the transparent electrode is at ground, when one line is at +2 volts, the other is at −2 volts. Still referring to
FIG. 1
, and as explained in more detail subsequently, the liquid crystal layer
12
rotates the polarization of the light
30
, with the amount of polarization rotation dependent on the voltage across the liquid crystal layer
12
. Ideally, the pixel element
10
is symmetrical in that the polarization rotation depends only on the magnitude of the latched signal on the signal line
26
. By alternating complementary signals in consecutive frames, unwanted charges across the liquid crystal layer
12
are prevented. If only one polarity was used, ions would build up across the capacitance formed by the transparent electrode
14
, the liquid crystal layer
12
, and the pixel electrode
16
. Such charges would bias the pixel element
10
. Thus, the pixel elements are driven by complementary signals in consecutive frame periods. Those frame periods are thus grouped into even frames and odd frames, with the even and odd frames being interlaced.
The light
30
is derived from incident non-polarized light
32
from an external light source (which is not shown). The non-polarized light is polarized by a first polarizer
34
to form the light
30
. The light
30
passes through the transparent electrode
14
, through the liquid crystal layer
12
, reflects off the pixel electrode
16
, passes back through the liquid crystal layer
12
, passes out of the transparent electrode
14
, and then is directed onto a second polarizer
36
. During the double pass through the liquid crystal layer
12
the polarization of the light beam is rotated in accord with the magnitude of the voltage on the signal line
26
. Only the portion of the light
30
that is parallel with the polarization direction of the second polarizer
36
passes through that polarizer. Since the passed portion depends on the amount of polarization rotation, which in turn depends on the voltage on the signal line
26
, the voltage on the signal line controls the intensity of the light that leaves the pixel element.
The storage element
18
is typically a capacitor connected to a thin film transistor switch. When a control signal is applied to the gate electrode of the thin film transistor that transistor turns on. Then, the voltage applied to the source of the thin film transistor passes through the thin film transistor and charges the capacitor. When the control signal is removed, the thin film transistor opens and the capacitor potential is stored on the pixel electrode
16
.
FIG. 2
schematically illustrates a pixel element matrix. As shown, a plurality of pixel elements
10
, each having an associated switching thin film transistor and a storage capacitor, are arranged in a matrix of rows (horizontal) and columns (vertical). For simplicity, only a small portion of a matrix array is shown. In practice there are numerous rows, say 1290, and numerous columns, say 1024. Referring to
FIG. 2
, the pixel elements of a row are selected together by applying a gate (switch) control signal on a gate line, specifically the gate lines
40
a
,
40
b
, and
40
c
. A constant voltage (which is shared by all of the pixel elements) is applied to the transparent electrode
14
from a ramp source
41
via a line
42
. Furthermore, the ramp source
41
applies complementary ramp signals on lines
20
and
22
(which are also shared by all of the pixel elements
10
). Furthermore, column select lines
46
a
,
46
b
, and
46
c
, control the operation of the pixel elements
10
.
A row of pixel elements is selected by the application of a signal on an appropriate one of the gate lines
40
a
-
40
c
. This turns on all of the pixel elements in that row. Then, the ramp source
41
applies a ramp to either line
20
or line
22
(which line is used is varied in each frame). The ramp begins charging all of the storage capacitors in the selected row. As the other rows are not energized, the ramp source only charges the OFF-state capacitance of the other pixels. When the ramp voltage reaches the desired state for a particular pixel, the column select line (
46
a
-
46
c
) voltage for that particular pixel element
10
turns the pixel switch OFF. Then, the ramp voltage that existed when the particular pixel element
10
was turned OFF is stored on that element's storage capacitor. Meanwhile, the ramp voltage continues to increase until all of the column select lines (
46
a
-
46
c
) cause a ramp voltage to be HELD on an associated pixel element. After that, a new row of pixel elements is selected and the process starts over. After all rows have been selected, the process starts over again in a new frame period, this time using the complement of the previous ramp.
The foregoing process is generally well known and is typically performed using digital shift registers, microcontrollers, and voltages sources that are beneficially fabricated on a common substrate using semiconductor processing technology on polysilicon and/or amorphous silicon.
While RLCD displays are generally successful, they have their problems. For example, in practice the pixel elements are not ideal in that the polarization rotation of the light
30
is not symmetrical. That is a +1 volt signal does not necessarily produce the same rotation as a −1 volt signal. While the physical principles behind this asymmetry are not fully understood, it appears that one explanation for this phenomenon is that the liquid crystal layer
12
, the transparent electrode
Chang Kent
Koninklijke Philips Electronics , N.V.
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