Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-02-14
2004-11-02
Eisen, Alexander (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
06812913
ABSTRACT:
This application is based on Japanese application Nos. 2000-39521, 2000-152504 and 2000-154634 filed in Japan, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display, and more specifically to a method of driving a liquid crystal display which comprises liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of the electric field applied thereto.
2. Description of Prior Art
In recent years, reflective type liquid crystal displays which use liquid crystal which exhibits a cholesteric phase at room temperature such as chiral nematic liquid crystal have been developed to be used as media for reproducing digital information as visual information because such liquid crystal displays consume little electric power and can be fabricated at low cost. However, such a liquid crystal display which uses liquid crystal with a memory effect has a demerit in that the driving speed is low.
A well-known prior art U.S. Pat. No. 5,748,277 discloses a method of driving bistable liquid crystal. The method comprises a preparation phase to cause the liquid crystal to come to a homeotropic state, a selection phase to select the liquid crystal to come to a focal-conic state or a planar state and an evolution phase to cause the liquid crystal to evolve to the desired final state. In the selection phase, by controlling the voltage applied to the liquid crystal to either one of two (high and low) levels, the state of the liquid crystal is selected.
However, this driving method has the following drawbacks: this driving method permits only a display with two tones and does not make a display with intermediate tones; this driving method requires a driving IC which has at least seven output levels to drive the scan electrodes and a driving IC which has at least two output levels to drive the data electrodes, that is, it requires costly drivers; and after the selection to determine the final states of pixels, pulse voltages are applied to the pixels from the data electrodes directly, and the image will be degraded because of crosstalk.
Incidentally, it is known that the response characteristic of chiral nematic liquid crystal to the voltage applied thereto changes depending on the temperature. Accordingly, when the temperature changes, the liquid crystal may make an incomplete display and may not be able to make a display.
When liquid crystal display layers for R, G and B are laminated to make a full-color display, the response characteristics of the respective liquid crystal materials in the liquid crystal display layers to the electric field applied thereto are different from each other. If mutually different optimal lengths are set as the respective selection steps of the display layers, the display layers will have different scanning times. The scanning time means the time from the start of selection step of a scan electrode to the start of selection step of the next scan electrode.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved method of driving a liquid crystal display and an improved liquid crystal display device which do not have the above problems.
In order to attain the object, a first driving method according to the present invention is a method for driving a liquid crystal display which comprises liquid crystal which exhibits a cholesteric phase and is capable of keeping an image displayed thereon after turn-off of an electric field applied thereto, and the method comprises: a reset step of applying a reset pulse to make the liquid crystal come to a homeotropic state; a selection step of, after the reset step, selecting a final state of the liquid crystal; and an evolution step of, after the selection step, applying an evolution pulse to stabilize the liquid crystal to the state selected in the selection step. The selection step comprises: a first step which comprises a time of applying substantially 0 volt to the liquid crystal; a second step of, after the first step, applying a selection pulse to select the final state of the liquid crystal; and a third step, after the second step, which comprises a time of applying substantially 0 volt to the liquid crystal.
In this first driving method, since the liquid crystal is driven in the reset step, in the selection step and in the evolution step, a desired image can be written on the liquid crystal at a relatively high speed. Moreover, in the first step and in the third step of the selection step, there are times in which the voltage applied to the liquid crystal is made substantially zero and consequently, the number of output levels of a driver can be reduced.
In the first driving method, the lengths of the reset step, the selection step and the evolution step may be multiples of the shortest one of these lengths. The selection pulse may be of a lower voltage than the reset pulse.
The state of the liquid crystal may be selected by changing the pulse width of the selection pulse. Variations of the selection pulse in pulse width result in display of various intermediate tones. In this case, in the second step, the pulse width of the selection pulse is adjusted in accordance with the desired final state of the liquid crystal.
In the first driving method, the liquid crystal display further comprises a plurality of scan electrodes and a plurality of data electrodes which are arranged to face and cross each other with the liquid crystal in-between, and these scan electrodes and data electrodes define a plurality of display units of the liquid crystal display. The scan electrodes may be selected serially to be subjected to at least the selection step and the evolution step. All the scan electrodes may be selected at one time to be subjected to the reset step.
In the case of selecting all the scan electrodes simultaneously for the reset step, while the display units on one of the scan electrodes are in the selection step, an evolution pulse of a voltage with an absolute value larger than 0 is applied to the scan electrode which is to come to the selection step next. With the application of the evolution pulse, the display units on the scan electrode which is to come to the selection step next can be kept in the reset state.
In the case of selecting the scan electrodes serially for the reset step, the scan electrodes are selected at lags of a time which corresponds to the length of the selection step. In this case, at least one of the first step and the third step may include a time of applying a compensation pulse to avoid necessity of changing the length of the selection step. Thereby, the length of the selection step can be fixed in any case. If compensation pulses are applied both in the first step and in the third step, the application times of the compensation pulses in the first step and in the third step are preferably equal to each other.
At least one of the first step and the third step may include a time of applying a compensation pulse to compensate for a change in responsiveness of the liquid crystal with a change in temperature. In this case, the form of the compensation pulse depends on the temperature. Thereby, regardless of the temperature, display of a quality image can be realized at all times. Also, it is possible to change the length of the selection step depending on in what range the temperature is.
At least one of the first step and the third step may include a time of applying a compensation pulse to avoid necessity of changing the length of the selection step. In a liquid crystal display which comprises a plurality of liquid crystal layers, by applying appropriate compensation pulses to the respective liquid crystal layers, the differences among the liquid crystal layers in responsiveness can be corrected, and it becomes possible that the liquid crystal layers can have the same scanning speed. Further, the application times of the compensation pulses applied to the respective liquid crystal layers may
Komatsu Mikihiro
Mandai Makiko
Masazumi Naoki
Yagi Tsukasa
Eisen Alexander
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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