Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-10-06
2004-10-12
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000
Reexamination Certificate
active
06803901
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to display devices with a display panel including pixels which are arranged in two dimensions, each pixel being constituted by an element capable of controlling transmittance and reflection of light, and light sources for use with the display devices.
BACKGROUND OF THE INVENTION
The moving-image-display quality (moving-image quality) of a typical LCD (Liquid Crystal Display) is inferior to that of a CRT (Cathode Ray Tube). This is regarded as a result of slow response speed of the liquid crystal in used.
For the purpose of solving this problem, Journal of the Japanese Liquid Crystal Society (Vol.3, No.2, 1999, pp., 99-106) describes an attempt to improve moving-image quality through an increased response speed of liquid crystal, by adopting a Pi-cell structure whereby a Pi-cell is flanked by optical compensators as shown in FIG.
17
.
The paper mentions that a Pi-cell shows an improvement in response speed of liquid crystal over a TN liquid crystal cell: namely, a turn-on time of 1 ms and a turn-off time of 5 ms.
The Pi-cell structure successfully yields a response speed that is fast enough to draw an image in a single frame period. However, the moving-image quality of an LCD with a Pi-cell structure is still inferior to that of the CRT. See
FIGS. 18
a
and
19
a
illustrating the moving image display on a CRT and a LCD with a Pi-cell structure respectively. The moving images are supposed to be moving in the directions denoted by the arrows.
The paper attributes the quality differences to illuminating characteristics of the CRT and the LCD.
FIG. 18
b
shows the “impulse-type” illuminating characteristics of the CRT whereby pixels emit an impulse of light lasting for a short period of time. In contrast,
FIG. 19
b
shows the “hold-type” illuminating characteristics of the LCD whereby pixels are hold alight continuously. According to the paper, the degradation of moving-image quality occurs in the LCD, because images in successive fields appear overlapping as a result of the motion of viewpoint.
The paper mentions that the problem is solved by the use of a backlight with impulse-type illuminating characteristics similar to those of the CRT. SID (Society for Information Display), 1997, pp., 203-206, “Improving the Moving-Image Quality of TFT-LCDs”, describes a technique to impart impulse-type illuminating characteristics to the LCD (first technique).
According to the first technique, a fluorescent lamp is adopted for use as a backlight of an LCD originally having a hold-type transmittance as shown in
FIG. 20
b
. The fluorescent lamp is flashed as shown in
FIG. 20
c
, using a switching circuit for use with a fluorescent lamp configured as shown in
FIG. 20
a
. The result is impulse-type illuminating characteristics as shown in
FIG. 20
d
(hereinafter, such an LCD will be referred to as an “entire surface flash type”). The fluorescent lamp in
FIG. 20
a
exhibits illuminating characteristics as show in
FIG. 21
a
when a voltage in
FIG. 21
b
is applied.
The paper describes, as detailed above, a further improvement of moving-image quality of an OCB (Optically Compensated Bend) cell by means of the first technique. A Pi-cell is a type of OCB cell.
The paper further discusses a second technique, whereby the pixels per se of the liquid crystal panel are used as a shutter to impart impulse-type illuminating characteristics to the LCD.
Specifically, a TFT panel
116
is used in which the display section is divided horizontally into an upper screen and a lower screen which are driven by various signals supplied from source drivers
117
and
118
provided to the respective upper and lower screens as shown in FIG.
22
d.
The upper and lower source drivers
117
and
118
supplies a black signal and a video signal alternately as shown in
FIGS. 22
a
and
FIG. 22
c
to each pixel of the TFT panel
116
. In synchronism with the supply, a gate driver
119
supplies a gate signal shown in
FIG. 22
b
to the TFTs each constituting a pixel of the TFT panel
116
. The result is a blanking signal and a video signal being applied within a field period as shown in
FIGS. 23
b
to
23
d (hereinafter, such an LCD will be referred to as an “black blanking type”).
According to the second technique, a black display period (interval between RS periods) appears on the hold-type video image in
FIG. 23
a
, moving from the top to the bottom of the panel as shown in
FIGS. 23
b
to
23
d
. This explains a successful improvement of moving-image quality.
From a viewpoint of flashing a backlight in an LCD module as above, the concept of field sequential color, whereby a color image display is effected by displaying red, green, and blue images in a time series, is similar to the concept of improving moving-image quality.
SID (Society for Information Display), 1999, DIGEST, pp., 1098-1101, “Field-Sequential-Color LCD Using Switched Organic EL Backlighting” describes a conventional driving method for a field sequential color display. According to the driving method, the device is driven in the time sequence shown in FIG.
24
.
Referring to
FIG. 24
, voltage is applied to a TFT pixel in period (1), response of liquid crystal is awaited in period (2), and an EL (electro-luminescence) backlight is flashed across the screen in period (3). The backlight of this kind of LCD is flashed across the screen similarly to that of the entire-surface-flash-type LCD.
According to the new driving method introduced in the paper, voltage is applied to TFT pixels starting in the top line of the panel and moving down to the bottom line of the panel as shown in FIG.
25
. In synchronism with the voltage application to a particular line (however, after a response time of liquid crystal is elapsed), an EL backlight corresponding to that line is flashed.
In prior art example described in the paper, an EL is used as a backlight for use with a field sequential color display; however, a fluorescent lamp may be used instead. In the event, the flashing of the fluorescent lamp should be controlled using the circuit for controlling the flashing of a fluorescent lamp disclosed in Japanese Laid-Open Patent Application No. 11 160675/1999 (Tokukaihei 11 160675; published on Jun. 18, 1999).
FIG. 26
shows the arrangement of a circuit for controlling the flashing of a fluorescent lamp described as a conventional example in the Laid-Open Patent Application.
The circuit for controlling the flashing of a fluorescent lamp, as shown in
FIG. 26
, includes: high voltage generating means
115
constituted by a DC power source
105
and an inverter
107
; and three cold cathode tubes
108
,
109
, and
110
emitting red, green, and blue light respectively. The cold cathode tubes
108
,
109
, and
110
are connected in series to switches
111
,
112
, and
113
respectively. The switches
111
to
113
are each constituted by a high-voltage-resistant bidirectional thyristor which is readily available on the market at a cheap price. By closing one of the switches
111
to
113
, a path is established for the high voltage generating means
115
to apply voltage only to the corresponding one of the cold cathode tubes
108
to
110
.
This field sequential color technique corresponds to the conventional driving method mentioned above in reference to the SID '99 paper.
However, in a circuit in
FIG. 26
disclosed in the Laid-Open Patent Application, the switches
111
to
113
each constituted by a bidirectional thyristor are not resistant enough to high voltage when they are all open; if the high voltage generating means
115
applies voltage, breakdown takes place in one or more of the open cold cathode tubes
108
to
110
, disrupting a complete dark state.
To solve this problem, the Laid-Open Patent Application suggests the use of a novel circuit for controlling the flashing a fluorescent lamp which includes high voltage generating means
114
with an additional switch
106
interposed between the DC power source
105
and the inverter
107
as shown in FIG.
27
. When no breakdown is desired in
Hjerpe Richard
Nguyen Jennifer T.
Sharp Kabushiki Kaisha
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