Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-09-27
2004-08-10
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S082000, C315S169300
Reexamination Certificate
active
06774876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an EL panel in which EL elements formed on a substrate are enclosed between the substrate and a cover material. Further, the present invention relates to an EL module in which an IC is mounted in the EL panel. Note that EL panels and EL modules are referred to generically by the term “self light emitting device” in this specification. In addition, the present invention relates to electronic devices using the self light emitting device.
2. Description of the Related Art
EL elements have high visibility because light is self emitted, and are optimal for making a display thin because a backlight like used for an liquid crystal display (LCD) is not required. Along with this, their angle of view has no limits. Self light emitting devices using EL elements have thus come under the spotlight as substitute display devices for CRTs and LCDs.
EL elements have a layer containing an organic compound in which electro luminescence is generated by adding an electric field (hereafter referred to as an EL layer), an anode, and a cathode. There is emission of light in the organic compound in returning to a base state from a singlet excitation state (fluorescence), and in returning to a base state from a triplet excitation state (phosphorescence), and the self light emitting device of the present invention may use either type of light emission.
Note that all layers formed between the anode and the cathode are defined as EL layers in this specification. Specifically, layers such as a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer are included as EL layers. An EL element basically has a structure in which an anode, a light emitting layer, and a cathode are laminated in the stated order. In addition to this structure, the EL element may also have a structure in which an anode, a hole injecting layer, a light emitting layer, and a cathode are laminated in the stated order, or a structure in which layers such as an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, and a cathode are laminated in the stated order.
Furthermore, an EL element emitting light is referred to as the EL element being driven in this specification. Moreover, an element formed by an anode, an EL layer, and a cathode is referred to as an EL element within this specification.
There are mainly analog drive and digital drive as methods of driving a self light emitting display device which has EL elements. In particular, with respect to the digital drive, it is possible to display an image using a digital video signal with image information (digital video signal) without converting it to analog, corresponding to a digitalized broadcast signal, and therefore the digital drive is promising.
A surface area division driving method and a time division driving method can be given as driving methods for performing gray scale display in accordance with two voltage values of a digital video signal.
The surface area division driving method is a driving method for performing gray scale display by dividing one pixel into a plurality of sub-pixels and driving each sub-pixel independently based upon a digital video signal. One pixel must be divided into a plurality of sub-pixels with this surface area driving method. In addition, it is also necessary to form pixel electrodes corresponding to each of the sub-pixels in order to drive the divided sub-pixels independently. Thus, a difficulty that the pixel structure is complex develops.
On the other hand, the time division driving method is a driving method for performing gray scale display by controlling the length of time during which pixels are turned on. Specifically, one frame period is divided into a plurality of sub-frame periods. Each pixel is then placed in a turned on or turned off state in each sub-frame period in accordance with a digital video signal. The gray scale of a certain pixel is found by summing lengths of all the sub-frame periods that the pixel is turned on during, of the sub-frame periods within one frame period.
In general, the response speed of organic EL materials is fast compared to liquid crystals and the like, and therefore organic EL materials are suitable for time division driving.
A case of displaying mid-level gray scales by time division driving in accordance with a simple binary code method is explained in detail below using
FIGS. 27A and 27B
.
FIG. 27A
shows a pixel portion of a general self light emitting device, and the lengths of all sub-frame periods within one frame period in the pixel portion are shown in FIG.
27
B.
An image is displayed using a 6 bit digital video signal which is capable of displaying 1 to 64 gray scales in
FIGS. 27A and 27B
. The right half portion of the pixel portion performs displaying of 33rd (32+1) gray scale, and the left half of the pixel portion performs displaying of 32nd (31+1) gray scale.
Six sub-frame periods (sub-frame periods SF
1
to SF
6
) generally appear within one frame period in the case of using a 6 bit digital video signal. The first to the sixth bits of the digital video signal correspond to the sub-frame periods SF
1
to SF
6
, respectively.
The ratio of lengths of the sub-frame periods SF
1
to SF
6
become 2
0
::2
1
::2
2
::2
3
::2
4
::2
5
. The length of the sub-frame period SF
6
corresponding to the most significant bit (the sixth bit in this case) of the digital video signal is the longest, and the length of the sub-frame period corresponding to the least significant bit (the first bit) of the digital video signal is the shortest.
For a case of performing display of the 32nd gray scale, the pixels are placed in an on state in the sub-frame periods SF
1
to SF
5
, and the pixels are placed in an off state during the sub-frame period SF
6
. Further, the pixels are placed in a turned off state during the sub-frame periods SF
1
to SF
5
, and are turned on during the sub-frame period SF
6
, when performing display of the 33rd gray scale.
A pseudo contour may be visible at a boundary portion between the portion for performing display of the 32nd gray scale and the portion for performing display of the 33rd gray scale.
The term pseudo contour refers to an unnatural contour line which is repeatedly visible in performing time gray scale display in accordance with a binary code method, and it is considered that the main cause is fluctuations develop in the perceived brightness due to the characteristics of human sight. A mechanism for the generation of the pseudo contour is explained using
FIGS. 28A and 28B
.
FIG. 28A
shows a pixel portion of a self light emitting device in which a pseudo contour develops, and
FIG. 28B
shows the ratio of the lengths of sub-frame periods within one frame period.
An image is displayed using a 6 bit digital video signal which is capable of displaying 1 to 64 gray scales in
FIGS. 28A and 28B
. The right half portion of the pixel portion performs displaying 33rd gray scales, and the left half of the pixel portion performs displaying 32nd gray scales.
The pixels are placed in an on state during 31/63 of one frame period, and are placed in an off state during 32/63 of the one frame period, in the portion of the pixel portion for performing the 32nd gray scale. Periods during which the pixels are turned on appear alternately with periods in which the pixels are turned off.
Further, the pixels are placed in an on state during 32/63 of one frame period, and the pixels are placed in an off state during 31/63 of the one frame period, in portions of the pixel portion for performing the 33rd gray scale. Periods during which the pixels are turned on appear alternately with periods in which the pixels are turned off.
In a case of displaying a moving image, the boundary between portions for displaying the 32nd gray scale and portions for displaying the 33rd gray scale in
FIG. 28A
is taken, for example, as moving in the direction of the dotted line. Namely, the pixels swi
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Wu Xiao
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