Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-06-08
2003-02-25
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C315S169300
Reexamination Certificate
active
06525704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display device for displaying an image, and more particularly to an image display device that displays an image by actively driving a multiplicity of two-dimensionally arranged organic EL (Electro-Luminescent) elements.
2. Description of the Related Art
EL displays for displaying a dot matrix image in which a multiplicity of organic EL elements are two-dimensionally arranged have currently been developed as image display devices for displaying various images in locations subject to radical changes in illumination, such as the interior of an automobile. Organic EL elements are light-emitting elements that spontaneously emit light and can be driven by a low-voltage direct current.
Methods of driving organic EL elements include passive matrix drive methods and active matrix drive methods. An active matrix drive method can achieve high luminance with high efficiency because the organic EL elements are lit continuously until updating of the display image.
As an example of an image display device of the prior art, explanation is presented with reference to FIG.
1
and
FIG. 2
regarding an EL display that actively drives organic EL elements.
As shown in
FIG. 1
, EL display
1
that is presented as an example of the prior art includes organic EL element
2
as well as power supply line
3
and ground line
4
as a pair of power supply electrodes. A predetermined drive voltage is constantly applied to power supply line
3
, and ground line
4
is constantly maintained at 0 V, which is the reference voltage.
Organic EL element
2
is directly connected to ground line
4
but is connected to power supply line
3
by way of drive TFT (Thin-Film Transistor)
5
. This drive TFT
5
includes a gate electrode, and the drive voltage that is applied to ground line
4
from power supply line
3
is supplied to organic EL element
2
according to a data voltage that is applied to this gate electrode.
One end of capacitor
6
is connected to the gate electrode of drive TFT
5
, and the other end of this capacitor
6
is connected to ground line
4
.
Data line
8
is connected to this capacitor
6
and the gate electrode of drive TFT
5
by way of switching TFT
7
, which is a switching element, and scan line
9
is connected to the gate electrode of this switching TFT
7
.
A data voltage for driving the light emission intensity of organic EL element
2
is supplied to data line
8
, and a scan voltage for controlling switching TFT
7
is applied to scan line
9
. Capacitor
6
holds the data voltage and applies it to the gate electrode of drive TFT
5
, and switching TFT
7
turns the connection between capacitor
6
and data line
8
ON and OFF.
In EL display
1
, (M×N, M and N are predetermined natural numbers) organic EL elements
2
are arranged two-dimensionally in M rows and N columns (not shown in the figures), and M rows of data lines
8
and N columns of scan lines
9
are connected in a matrix to these M rows and N columns of organic EL elements
2
. In the figures, the term “row” refers to the dimension parallel to the vertical direction and the term “column” refers to the dimension parallel to the horizontal direction, but this is merely a matter of definition, and the reverse case is also possible.
EL display
1
according to the above-described construction is capable of driving organic EL elements
2
with variable light emission intensity. In such a case, a scan voltage is applied to scan line
9
and switching TFT
7
is controlled to an ON state as shown in
FIG. 2
b
and
FIG. 2
c
, and a data voltage from the data line that corresponds to the light emission intensity of organic EL element
2
in this state is supplied to and held in capacitor
6
as shown in
FIG. 2
e.
The data voltage held by this capacitor
6
is applied to the gate electrode of drive TFT
5
as shown in
FIG. 2
d
, and as a result, as shown in
FIG. 2
f
, the drive voltage that is constantly generated at power supply line
3
and ground line
4
is supplied to organic EL element
2
by drive TFT
5
in accordance with the gate voltage. As a result, organic EL element
2
emits light at an intensity that accords with the data voltage that was supplied to data line
8
.
In EL display
1
, data voltage and scan voltage are applied in a matrix to M rows of data lines
8
and N columns of scan lines
9
, and each of M rows and N columns of organic EL elements
2
are therefore lit at different intensities, thereby displaying a dot-matrix image with the gray scale expressed in pixel units.
In such a case, the scan voltage is applied in order one column at a time to N columns of scan lines
9
in EL display
1
as shown in
FIG. 2
a
and
FIG. 2
b
, and when this scan voltage is being applied, one column of M data voltages is therefore applied in order to M rows of data lines
8
.
The state in which the drive voltage is applied to organic EL element
2
in accordance with the data voltage that is held by capacitor
6
as described in the foregoing explanation continues even when switching TFT
7
is placed in the OFF state by the scan voltage of scan line
9
. Organic EL element
2
thus continues emission that is controlled to a predetermined luminance until the next instance of control, and EL display
1
therefore is capable of displaying a bright and high-contrast image.
In EL display
1
in which organic EL elements
2
are actively driven as described above, however, organic EL elements
2
have a short life. Various explanations can be offered, but characteristically, it is clear that continuous application of the drive voltage of the same polarity to organic EL electrodes
2
results in a short life of the elements.
In an EL display (not shown) that passively drives organic EL elements
2
, for example, it has been confirmed that organic EL elements
2
have a longer life than in the case of active drive because the polarity of voltage applied to organic EL elements
2
reverses during the drive process. A passive-type EL display as described hereinabove, however, is incapable of driving organic EL elements
2
at both high luminance and high contrast, and such a display is therefore difficult to use in devices requiring high luminance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image display device capable of employing active drive to light organic EL elements at high luminance and high efficiency while enabling longer life of the elements.
According to one aspect of the present invention, (M×N) organic EL elements are arranged two-dimensionally in M rows and N columns, (M×N) data voltages that individually set the light-emission luminance of these (M×N) organic EL elements are applied in order N times for each of the M rows of data lines, and the scan voltage is applied in order to the N columns of scan lines in synchronization with the data voltages that are applied to these M rows of data lines. The scan voltage that is applied in order to these N columns of scan lines causes the M rows and N columns of switching elements to turn on one column at a time, and the (M×N) data voltages that are applied from the M rows of data lines in accordance with the ON state of these M rows and N columns of switching elements are individually held by M rows and N columns of data voltage holding means. The drive voltage that is constantly applied to the power supply electrode is applied to the (M×N) organic EL elements by the M rows and N columns of drive transistors in individual correspondence to the held voltage of the (M×N) voltage holding means. The M rows and N columns of organic EL elements are thus actively driven at individually differing luminances to display a multiple gray-scale dot matrix image.
Immediately before the application of the scan voltage to the scan line of the nth column, however, a conduction control element halts the application of the drive voltage to the M organic EL elements of the nth column. As a result, conduc
Kondo Yuji
Kota Atsushi
Piziali Jeff
Shalwala Bipin
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