Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2000-01-27
2003-03-04
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S695000
Reexamination Certificate
active
06529213
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a display device, especially to an improved technology of gradation display.
2. Description of the Related Art
TFT-ELDs, namely thin-film-transistor (TFT) driven electoroluminescent displays, which include electoroluminescent elements (EL elements) driven and controlled by thin film transistors, are considered as future potential displays due to their light weight, small size, high resolution, wide visual field, low electric consumption, etc.
FIG. 4
is a circuit diagram of a conventional TFT-ELD, and
FIG. 5
is a cross section of such TFT-ELD.
FIG. 4
shows a unit pixel
11
of the TFT-ELD, a scanning line
12
, a signal line
13
, current supplying line
14
, a retention capacitor
15
, a selective transistor
16
, a driving transistor
17
, and an EL element
15
. As shown in
FIG. 5
, the driving transistor
17
for adjusting light emission intensity (gradation) of the EL element
18
is formed on a glass substrate
10
. A drain electrode of the driving transistor
17
is connected to a cathode (transparent electrode)
21
of the EL element
18
, and a source electrode is connected to the analog signal supply line
14
. The EL element
18
is formed of the anode
21
, a luminescent layer
22
, and an cathode
23
. The EL element
18
may be a inorganic electroluminescent element, a low-molecular organic electroluminescent element, or a high-molecular organic electroluminescent element.
The selective transistor
16
includes a gate electrode connected to the scanning line
12
, a source electrode connected to a signal line
13
, and a drain electrode connected to a gate electrode of the driving transistor
17
. The retention capacitor
15
is provided between the analog signal supplying line
14
and the source electrode of the selective transistor
16
.
In order to cause the EL element
18
to emit light in the aforementioned structure, the scanning line
12
and the signal line
13
are set at level “H”, and current is conducted between the drain and the source of the selective transistor
16
, whereby the driving transistor
17
is on state. An analog signal supplied from the analog supplying line
14
in this condition is delivered to the retention capacitor
15
and alters the conductance of the driving transistor
17
. As a result, the EL element
18
emits light with light emission intensity pursuant to the analog signal, thereby accomplishing gradations of light emission intensity.
However, as a problem of the above-described. structure, resolution of the picture lowers due to the EL element
18
included in each pixel emitting light with unequal light emission intensity, especially in the middle gradation, because of the difference in the transistor properties of the driving transistor
17
.
In order to solve this problem, the applicant of the present invention suggested in Japanese Patent Laid-Open Publication No. HEI 11-73158 a technology of displaying respective gradations by controlling on/off states of light emission of EL elements and changing the luminous area for each gradation.
FIG. 6
is a circuit diagram of the TFT-ELD disclosed in said Laid-Open Publication.
FIG. 6
shows an EL element included in each pixel, which is formed of EL elements
18
-
1
and
18
-
2
. Such structure allows display of four gradations by controlling on/off states of EL elements
18
-
1
and
18
-
2
respectively via a 2-bit signal line formed of signal lines
13
-
1
and
13
-
2
. More specifically, there are: gradation “0”, where neither EL element
18
-
1
nor
18
-
2
emits light; gradation “1”, where only EL element
18
-
1
emits light; gradation “2”, where only EL element
19
-
2
emits light; and gradation “3”, where both EL elements
18
-
1
and
18
-
2
emit light. Luminous areas of EL element
18
-
1
and EL element
18
-
2
are in a ratio of 1:2.
As shown in
FIG. 7
, in the structure above, signals S, D
1
, and D
2
are respectively supplied to the scanning line
12
, signal line
13
-
1
and signal line
13
-
2
. When signal S is set at level “H”, current is conducted between the drain and the sources of selective transistors
16
-
1
and
16
-
2
. In
FIG. 7
, gradation “1” is obtained when signal S is set at level “H”, signal D
1
at level RHO, and signal D
2
at level “L”. As a consequence, driving transistor
17
-
1
is turned on, and transistor
17
-
2
is turned off, whereby only EL element
18
-
1
emits light. Furthermore, in order to realize gradation “2”, signal S should be set at level “H”, signal D
1
at level “L”, and signal D
2
at level “H”. By doing so, driving transistor
17
-
2
is turned on and transistor
17
-
1
is turned off, and consequently, only EL element
18
-
2
emits light.
In this method, driving transistors
17
-
1
and
17
-
2
are to be regarded as either almost completely on state or almost completely off state. When driving transistors
17
-
1
and
17
-
2
are on state,resistance is negligibly small compared to the resistance of driving transistors
18
-
1
and
18
-
2
, such that the amount of current conducted through driving transistors
17
-
1
,
17
-
2
,
18
-
1
and
18
-
2
depends substantially on the resistance of driving transistors
18
-
1
and
18
-
2
alone. Accordingly, light emission intensity is never uneven due to the difference in the transistor properties of driving transistors
18
-
1
and
18
-
2
. Furthermore, when driving transistors
17
-
1
and
17
-
2
are off state, the voltage applied to EL elements
18
-
1
and
18
-
2
will be smaller than the threshold voltage, and driving transistors
18
-
1
and
19
-
2
, will not emit light at all. Therefore, also in this case, the light emission intensity of EL elements
18
-
1
and
18
-
2
is never uneven by the difference in the transistor properties of driving transistors
18
-
1
and
18
-
2
.
However, as a disadvantage of the aforementioned structure, the luminous center (the average position of the luminescent portion) shifts for each gradation and visibility is thereby decreased. Characteristics of such disadvantage will be explained with reference to
FIGS. 8A-D
.
FIG. 8C
, for example, shows a luminous center
40
of the unit pixel element
11
. The EL element
18
-
1
shown with oblique lines means that no light is emitted, and the EL element
18
-
2
shown in white means that light is emitted. In
FIG. 8A
, the EL elements
18
-
1
,
18
-
2
do not emit light. In
FIG. 8B
, only EL element
18
-
1
emits light. In
FIG. 8C
, only EL element
18
-
2
emits light. Finally, in
FIG. 8D
, both EL elements
18
-
1
,
18
-
2
emit light. It is clear from these drawings that the position of the luminous center
40
changes for each gradation. As a consequence, when the brightness of a displayed image is changed, the position of the image shifts unfavorably. Furthermore, if the displayed image is actually observed here, the displayed image will be seen to flicker, causing an impression of unnatural display or fatigue to the viewer.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to overcome such disadvantage and to provide a display device wherein a luminous center does not shift for each light emission gradation.
In the present invention, in order to achieve said object, a unit pixel is formed of multiple EL elements whose luminescent portions corresponding to each gradation are arranged point-symmetrically with one another with respect to a prescribed point. Such structure allows provision of a display device wherein the position of a luminous center does not change for each gradation. “Prescribed position” here means, for example, a luminous center of the EL element upon realizing the gradation of minimum luminance.
Furthermore, each electroluminescent element is preferably configured to have a state of “emission” or “non-emission”. By controlling on/off of the multiple Aluminescent elements, it is possible to prevent uneven aluminance caused by difference in the properties of luminescent elements. In order to achieve the str
Chang Kent
Oliff & Berridg,e PLC
Seiko Epson Corporation
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