Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1997-04-25
2001-05-15
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S089000
Reexamination Certificate
active
06232942
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal display device using an antiferroelectric liquid crystal display panel that has a plurality of columns electrodes and a plurality of row electrodes.
BACKGROUND ART
An antiferroelectric liquid crystal is stable in an antiferroelectric state when left in a condition that no voltage (zero) is applied to the liquid crystal. Hereinafter, this stable state will be referred to as a neutral state. An antiferroelectric liquid crystal panel may be configured in such a manner as to effect either a dark display or a bright display in this neutral state. Although antiferroelectric liquid crystal panels of the present invention be applied to both a dark display and a bright display, an antiferroelectric liquid crystal panel which is adapted to effect a dark display in the neutral state will be described hereinbelow.
FIG. 7
is an example of a graph illustrating the optical transmittance of an antiferroelectric liquid crystal relative to a voltage applied thereto. In this graph, the axis of abscissa represents the applied voltages; and the axis of ordinates represents the optical transmittances.
When applying a positive voltage to the crystal, which has been in the neutral state at a point O, and increasing the positive voltage, the transmittance abruptly increases at a voltage Ft. Then, the transmittance reaches nearly the maximum value at a voltage Fs and the crystal is put into a saturated ferroelectric state. Thence, the optical transmittance does not change much even when a higher voltage is applied thereto. Next, when the applied voltage is gradually decreased, the optical transmittance abruptly drops at a voltage At. Further, the transmittance nearly reaches zero at the voltage As, and the crystal returns to an antiferroelectric state. Similarly, if a negative voltage is applied to the crystal from the voltage 0, the transmittance abruptly rises at a voltage (−Ft). Then, the transmittance nearly reaches the maximum value at a voltage (−Fs), and the crystal is put into a saturated ferrorelectric state. Thence, when the applied negative voltage is gradually reduced to 0 V, the transmittance abruptly drops at a voltage (−At). Further, the transmittance becomes almost zero at a voltage (−As), and the crystal returns to the antiferroelectric state. As above described, there are two ferroelectric states of the liquid crystal. Namely, one is the application of the positive voltage, and the other is the application of the negative voltage. Hereunder, the ferroelectric state due to the former case will be referred to as (+) ferroelectric state, while the ferroelectric state due to the latter case will be referred to as (−) ferroelectric state. Further, |Ft| designates a ferroelectric threshold voltage; |Fs| a ferroelectric saturation voltage; |At| designates an antiferroelectric threshold voltage; and |As| an antiferroelectric saturation voltage.
Generally, it is the case that the curves (namely, hysteresis curves) of
FIG. 7
representing the optical transmittance characteristics of a liquid crystal relative to the voltage applied thereto are obtained by applying thereto a triangular-wave-like voltage in which the absolute value of the ratio of a change in this voltage relative to time, namely, the value of |dV/dt| is constant. However, in this case, if the value of |dV/dt| is changed, the shapes of the hysteresis curves also change. Moreover, the values of the aforementioned values As, Ft, Fs and At also vary. It is, accordingly, necessary to specify these values to specify the aforesaid value of |dV/dt|. However, the inventor obtained
FIG. 7
by the following method (hereunder referred to as a time fixation method 1) so as to obtain values more corresponding to actual driving conditions.
It is assumed that the duration of one frame (to be described later) of a display device to be used in a working temperature, is Pt and that the length of a time period, in which a selection voltage (to be described later) is applied, is Wt.
(1) A pulse voltage, whose duration is Wt and voltage level is Vx, is applied to the liquid crystal that is in a stable antiferroelectric state (namely, in the neutral state). Further, the relationship between the optical transmittance and the pulse voltage Vx at the time of completion of the application of this pulse voltage is plotted. Moreover, this operation is repeated by changing the value of the voltage Vx. Thereby, the curve drawn from the point O to Fs through Ft of
FIG. 7
, as well as the curve drawn from the point O to (−Fs) through the (−Ft), is obtained.
(2) Next, the liquid crystal is first put into the saturated ferroelectric state by applying thereto a voltage which is not lower than the aforementioned voltage |Fs|. Then, at a moment 0, the applied voltage is reduced to |Vz|. Thence, after the elapse of the assumed relaxation period (to be described later), the relationship between the value of the optical transmittance and the applied voltage Vz is plotted. This operation is repeated by changing the value of the voltage |vz|. Thereby, the curve drawn from Fs to the point O through At and As of
FIG. 7
, as well as the curve drawn from (−Fs) to the point O through (−At) and (−As), is obtained.
When some liquid crystal panels are used, the curve (namely, the curve drawn from Fs or (−Fs) to the point O in
FIG. 7
) obtained in the aforementioned case (2) sometimes intersects the ordinate axis. The main cause of this is the responsivity of the liquid crystal. Namely, in the case that the liquid crystal is maintained in the ferroelectric state by applying thereto a voltage, which is not lower than the aforementioned voltage |Fs|, and that at the moment 0, the applied voltage Vz is changed into 0, the liquid crystal finally becomes stable in the antiferroelectric state after the elapse of a certain time period (hereunder referred to as a relaxation time tn). However, if this relaxation time tn is longer than the relaxation period (to be described later), the curve obtained in the aforementioned case (2) intersects the ordinate axis.
When actually driven, it is difficult to bring such a liquid crystal panel into a complete antiferroelectric state, and a dark display cannot be effected and that the contrast is extremely degraded.
Generally, a liquid crystal panel is driven by performing the following process. Namely, first, N row electrodes and M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. Further, a scanning signal is applied to each of the row electrodes through a row-electrode drive circuit, while a display signal depending on display data of each pixel (incidentally, a part of the display signal is sometimes not dependent on the display data) is applied to each of the column electrodes through a column-electrode drive circuit. Moreover, a voltage (hereunder referred to simply as a synthesis voltage), which corresponds to the difference between the scanning signal and the display signal, is applied to a liquid crystal layer. The time period required to scan all of the row electrodes (namely, 1 vertical scanning interval) is usually referred to as 1 frame (or 1 field). In the case of driving the liquid crystal panel, the polarity of a driving voltage is reversed or inverted each frame (or every frames) in order to prevent the liquid crystal from being adversely affected (for example, the degradation due to non-uniform distribution of ions).
FIG. 9
illustrates the waveforms of signals flowing through the row electrodes, the column electrodes and the pixel synthesis electrodes of a liquid crystal panel in which the N row electrodes and the M column electrodes are formed in such a manner as to be arranged as a matrix of N rows and M columns. The display conditions or states of pixels are assumed as follows. Namely, i
Ebihara Heihachiro
Imoto Satoshi
Awad Amr
Chow Dennis-Doon
Citizen Watch Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
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