Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
1998-05-01
2001-01-16
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S173000, C349S174000
Reexamination Certificate
active
06175401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an LCD (Liquid Crystal Display) device using an AFLC (AntiFerroelectric Liquid Crystal), and more particularly to a display device which is capable of performing gradation display and a method for driving the display device.
2. Description of the Related Art
An LCD device, which uses an LC material whose molecules have spontaneous polarizations, and which drives the LC molecules through utilization of interactions between an electric field and the spontaneous polarizations of the LC molecules in order to perform a display operation, has received attention due to its quicker response and wider angle of visibility than those of an TN liquid crystal display device using a nematic liquid crystal.
Known FLC display devices include an FLC display device which uses an FLC and an AFLC display device which uses an AFLC.
An AFLC display device displays images by utilization of three stable alignment states of an AFLC.
This matter will now be discussed in more detail. An AFLC has three stable alignment states in which; (1) when a voltage equal to or higher than a first threshold value is applied to the LC, the LC exhibits a first ferroelectric phase wherein the LC molecules are aligned in a first direction or a second ferroelectric phase wherein the LC molecules are aligned in a second direction, in accordance with the polarity of the applied voltage; and (2) when a voltage equal to or lower than a second threshold value less than the first threshold value is applied to the AFLC, the liquid crystal exhibits an antiferroelectric phase which differs in the alignment of the LC molecules from the first and second ferroelectric phases. Determining the directions of the transmission axes of a pair of polarization plates arranged one on either side of an LCD device, on the basis of the optical axis of a liquid crystal layer while in the antiferroelectric phase, enables a display device to display images, with control of the light transmittance in accordance with the applied voltage.
Even when a variation occurs in the applied voltage, an AFLC stays in the first/second ferroelectric phase or the antiferroelectric phase, as far as the value of the applied voltage lies in a range between the first and second threshold values. This property is known as a memory property. A conventional AFLC display device is driven in a simple matrix mode through utilization of this memory property.
The memory property of an AFLC is determined by a difference in magnitude between a voltage, which causes a phase transition from the first/second ferroelectric phase to the antiferroelectric phase in the liquid crystal, and a voltage which causes a phase transition from the antiferroelectric phase to the first/second ferroelectric phase in the liquid crystal. The greater the difference between those voltages, the greater will be the memory property. In other words, the greater the hysteresis of the optical characteristic of the liquid crystal, the greater the memory property.
Due to this, a conventional AFLC display device which is driven in a simple matrix mode uses, as an AFLC, such a liquid crystal that the difference between the magnitudes of the above-described voltages is large.
However, the light transmittance of the conventional AFLC display device, which uses an AFLC having an excellent memory property, can not be controlled. That is, the control of a display gradation is almost impossible and multi-gradation display cannot be realized.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an LCD device which can achieve high-contrast gradation display.
According to the first aspect of the present invention having the above-described object, there is provided a liquid crystal display device comprising:
a first substrate on which first electrodes are formed;
a second substrate on which a second electrode is formed facing the first electrodes;
a liquid crystal layer formed of a smectic liquid crystal which exhibits a mixed phase when the liquid crystal is sealed between the first and second substrates, the liquid crystal in the mixed phase containing liquid crystal molecules aligned in a plurality of alignment states which differ from each other in an alignment order of liquid crystal molecules forming adjoining smectic layers; and
director changing means for changing a director of the liquid crystal layer by controlling the alignment of liquid crystal molecules exhibiting the mixed phase.
According to the above-described structure, the liquid crystal layer exhibits a mixed phase in which the liquid crystal layer contains liquid crystal molecules aligned in a plurality of alignment states which differ from each other in the alignment order of the liquid crystal molecules forming adjoining smectic layers. In the mixed phase, the direction of the director of the liquid crystal layer varies continuously in accordance with the applied voltage and its polarity. By providing the liquid crystal display device with polarization plates, an electro-optical characteristic exhibiting no hysteresis and which changes linearly over a wide range can be attained. According to this characteristic, the applied voltage determines the display gradation. Therefore, by applying a voltage corresponding to the desired gradation to the liquid crystal layer, an image can be displayed at the desired gradation.
The plurality of alignment states differ from each other in the alignment of liquid crystal molecules which are in a plane substantially parallel with the first and second substrates.
An electric field applied between the first and second substrates induces the mixed phase. In this case, it is preferred that the interactions between smectic layers contained in the liquid crystal layer be stronger than the aligning forces of alignment films formed on the inner surfaces of the substrates in order to align the liquid crystal molecules in a predetermined direction.
In the mixed phase, the liquid crystal contains liquid crystal molecules which exhibit one of ferroelectric, antiferroelectric and ferrielectric phases.
The liquid crystal layer is formed of a liquid crystal material which exhibits an antiferroelectric phase while the liquid crystal material is in the bulk state. In the states wherein the liquid crystal material is sealed between the substrates and wherein no voltage is applied between the first and second electrodes, the liquid crystal material exhibits the antiferroelectric phase. When a voltage is applied between the first and second electrodes, the liquid crystal material exhibits the aforementioned mixed phase.
The director changing means includes means for applying, between the first and second electrodes, a voltage for changing the antiferroelectric phase to the mixed phase and controlling the alignment of the liquid crystal molecules exhibiting the mixed phase in order to change the director of the liquid crystal layer.
The surface effect which takes place at the interface between the liquid crystal layer and the alignment films induces the mixed phase.
In this case, alignment means and liquid crystal material are selected so that the aligning forces are substantially equal to or stronger than the interactions acting between the smectic layers.
When alignment means and liquid crystal material are selected so that the aligning forces are substantially equal to the interactions acting between the smectic layers, the liquid crystal layer while in the bulk state exhibits the antiferroelectric phase, and exhibits the ferrielectric phase under the condition wherein no voltage is applied to the first and second electrodes, and exhibits the mixed phase under the condition wherein a voltage is applied between the first and second electrodes. In the mixed phase, the director is controlled by an applied electric field.
When alignment means and liquid crystal material are selected so that the interactions between the smectic layers are weaker than the aligning forces, the liquid crystal layer while in the bulk
Ogura Jun
Takei Manabu
Tanaka Tomio
Yoshida Tetsushi
Casio Computer Co. Ltd.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Parker Kenneth
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