The present invention relates to a driving method for a liquid crystal device for use in flat-panel displays, projection displays, printers, etc.
As a type of a liquid crystal (liquid crystal device) for displaying various data (information) by using a liquid crystal, there have been known those using a nematic liquid crystal or a chiral smectic liquid crystal. A liquid crystal device using a chiral smectic liquid crystal has an advantage of, e.g., higher response speed than that using a nematic liquid crystal, thus being expected to be widely utilized.
More specifically, a twisted nematic (TN) liquid crystal has widely been used conventionally as a material for a liquid crystal device as described by M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol.18, No.4 (Feb. 15, 1971), pp. 127-128. The TN liquid crystal is used in an active matrix-type liquid crystal device (panel) in combination with switching elements such as thin film transistors (TFTs). The active matrix-type liquid crystal device is free from a problem of cross-talk and is produced with high productivity with respect to that having a size (diagonal length) of 10-17 in. with a progress of production technique.
However, the above-mentioned liquid crystal device using the TN liquid crystal has been accompanied with problems such as a slower response speed and a narrower viewing angle.
In order to solve the problems, various alignment modes including an optically compensated bend or birefringence (OCB) mode for improving a response speed, and In-Plain Switching mode and Vertical Alignment mode for improving a viewing angle have been proposed but are not said to be satisfactory for improvements in response seed and/or viewing angle.
In order to solve the problems of the conventional TN liquid crystal devices, a liquid crystal device using a chiral smectic liquid crystal exhibiting bistability has been proposed by Clark and Lagerwall (Japanese Laid-Open Application (JP-A) 56-107216, U.S. Pat. No 4367924). As the liquid crystal exhibiting bistability, a ferroelectric liquid crystal having chiral smectic C phase is generally used. Such a ferroelectric liquid crystal provides a very quick response speed because it causes inversion switching of liquid crystal molecules based on their spontaneous polarizations. In addition, the ferroelectric liquid crystal assumes bistable state showing a memory characteristic and further has an excellent viewing angle characteristic, thus being considered to be suitable for a display device or light-valve of high speed, high definition and larger area.
In recent years, an anti-ferroelectric liquid crystal exhibiting tristable state has been proposed by (chandani, Takezoe et al. (xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, vol. 27 (1988), pp. L729-). The anti-ferroelectric liquid crystal also provides a very quick response speed due to inversion switching based on spontaneous polarization similarly as in the ferroelectric liquid crystal.
As another type of the chiral smectic liquid crystal, there has been recently proposed a chiral smectic liquid crystal providing a V-character shaped response characteristic (voltage-transmittance characteristic) which is advantageous for gradational image display and is free from hysteresis (e.g., xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, Vol. 36 (1997), pp. 3586-).
Further, an active matrix-type liquid crystal device using such a chiral smectic liquid crystal providing the V-shaped voltage-transmittance characteristic has also been proposed (JP-A 9-50049).
In recent years, the above-mentioned liquid crystal devices are required to be used for displaying motion (picture) images.
In the case where motion (picture) image are displayed by a liquid crystal panel, images to be displayed (still (picture) images) are changed for each frame period. In this case, if such a change in image is always recognized by a viewer, a transitional state of the image change is also consequently recognized, thus lowering image qualities of motion images. In order to solve the problem, a backlight (unit) is turned on only in a period wherein the still image display is completed in the liquid crystal panel.
Such a liquid crystal panel for displaying motion images is, however, accompanied with a problem of a hysteresis with respect to an alignment state of a liquid crystal used.
Specifically, such a hysteresis is a phenomenon that even when a prescribed voltage is applied for displaying a gradational (display) state of 50% in a frame period, the gradational state (level) of 50% cannot be realized by the influence of a gradational state in its preceding frame period.
In the conventional liquid crystal devices, in order to solve the above hysteresis phenomenon, a reset voltage has been applied in each frame period.
More specifically, in the conventional liquid crystal device, as shown by VR at (e) in FIG. 14, a fixed voltage (0 V in the figure) has been applied as a reset voltage. For this purpose, the liquid crystal device is required to additionally providing a reset circuit including a switching element 30 (for forcedly providing buffer circuit with a uniform potential), a terminal (for providing the buffer circuit with a potential corresponding to a reset potential), and a gate terminal 32 of the switching element 30 (for controlling the timing for supplying the potential corresponding to a reset potential to the buffer circuit via the switching element) as shown in FIG. 15, thus resulting in a complicated pixel circuit. FIG. 15 shows an embodiment of an equivalent circuit of the conventional liquid crystal device. Referring to FIG. 15, in addition to the reset circuit 30 (encoding the reset line 31 and the reset switching element 32), the conventional liquid crystal device includes a liquid crystal 1, a pair of electrodes 2a and 2b, a first switching element 3, a gate line 4, a signal line 5, a first storage (holding) capacitor 6, a first buffer circuit 7, a second switching element 8, a second buffer circuit 9, a common control line 10, a counter electrode potential 11, a common potential 12 and a second storage (holding) capacitor 13.
When the conventional liquid crystal device as shown in FIG. 15 in driven for continuously displaying a black state in a certain pixel while setting a reset voltage of 0 V as shown at (b) in FIG. 16, a resultant voltage-transmittance characteristic (V-T characteristic) at the certain pixel as indicated by a curve connecting white squares (-xe2x96xa1-xe2x96xa1-) shown in FIG. 17 is different from a curve connecting black squares (-▪-▪-) shown in FIG. 17 indicating a V-T characteristic in the case of continuously displaying a white state at another pixel while setting a reset voltage of 0 V as shown at (a) in FIG. 16 (in this case, reset period =1 msec and writing period =8.33 msec are set), thus resulting in an occurrence of so-called image memory (burning or sticking) wherein a gradational image displayed on the liquid crystal panel as a whole is different from a gradational image to be displayed.
Further, in the case where the conventional liquid crystal device is driven for displaying full-color images according to a field-sequential driving scheme, a part of image data displayed in ia preceding frame period is displayed in a current frame period, thus resulting in an actually displayed color image which is different from a color image to be displayed originally, i.e., a poor color reproducibility.
An object of the present invention is to provide a driving method for a liquid crystal device capable of displaying appropriate gradational images while controlling a reset voltage for resetting a liquid crystal in a prescribed state without using an additional circuit (device) for exclusively applying a reset voltage.
Another object of the present invention is to provide a driving method for a liquid crystal device capable of suppressing an occurrence of image memory phenomenon.
According to the present invention, there is provided a driving method for a liquid crystal device comprising a pair of electrodes and a liquid crystal disposed between the electrodes, said driving method comprising:
a sequence of voltage application operations each comprising application of a reset voltage to the liquid crystal for placing the liquid crystal in a reset state in a reset period and application of a data voltage to the liquid crystal for placing the liquid crystal in a desired gradational display state in a writing period subsequent to the reset period, wherein
each reset voltage is set to provide a prescribed difference in voltage between said each reset voltage and a subsequent data voltage.