Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase
Reexamination Certificate
2000-04-24
2001-03-27
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within nematic phase
C349S097000, C349S100000, C349S172000, C349S184000
Reexamination Certificate
active
06208403
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device which uses an AFLC (AntiFerroelectric Liquid Crystal), and, more particularly, to an AFLC display device which can ensure gradation display.
2. Description of the Related Art
A ferroelectric liquid crystal (FLC) display device which uses a ferroelectric liquid crystal has a faster response and wider angle of visibility as compared with a TN liquid crystal display (LCD) device which uses 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.
The AFLC display device displays images using the stability of the alignment states an AFLC has.
This will be discussed more specifically. An AFLC has three stable alignment states of liquid crystal (LC) molecules. (1) When a voltage equal to or greater than a first threshold value is applied to the AFLC, the liquid crystal is set to a first ferroelectric phase where the LC molecules are aligned in a first direction or to a second ferroelectric phase where the LC molecules are aligned in a second direction according to the polarity of the applied voltage. (2) When a voltage equal to or smaller than a second threshold value lower than the first threshold value is applied to the AFLC, the liquid crystal is set to an antiferroelectric phase as an intermediate aligning state between the first and second ferroelectric phases. By setting the directions of the transmission axes of a pair of polarization plates located on both sides of the LCD device based on the optical axis in the antiferroelectric phase as a reference, it is possible to display images by controlling the transmittance of light in accordance with the applied voltage, as shown in FIG.
39
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The AFLC stays set to the first or second ferroelectric phase or the antiferroelectric phase even if the applied voltage changes as long as the variation lies between the first and second threshold values. That is, the AFLC has a memory property. The conventional AFLC display device is driven in a simple matrix mode using this memory property.
The memory property of the AFLC is determined by the difference between the voltage which causes the liquid crystal to change its phase to the antiferroelectric phase from the first or second ferroelectric phase and the voltage which causes the liquid crystal to change its phase to the first or second ferroelectric phase from the antiferroelectric phase. The greater this voltage difference becomes, the higher the memory property of the alignment state gets. That is, the memory property becomes greater as the hysteresis of the optical characteristic increases.
The conventional AFLC display device which is driven in a simple matrix mode therefore uses an AFLC whose voltage difference is large. But, it is not possible to arbitrarily control the light transmittance of the conventional AFLC display device which uses an AFLC having a high memory property. That is, it is extremely difficult to control the display gradation, so that multi-gradation display cannot be accomplished.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an AFLC display device capable of accomplishing clear gradation display. To achieve this object, an AFLC display device according to the first aspect of this invention comprises:
a first substrate on which a plurality of pixel electrodes and a plurality of active elements connected to the pixel electrodes are arranged;
a second substrate on which a common electrode opposing the pixel electrodes is formed; and
an antiferroelectric liquid crystal, sealed between the first and second substrates and having a first ferroelectric phase where LC molecules are substantially aligned in a first aligning direction, a second ferroelectric phase where LC molecules are substantially aligned in a second aligning direction and an antiferroelectric phase where there are LC molecules aligned in the first aligning direction and LC molecules aligned in the second aligning direction with an average direction of directors being substantially coincident with a direction of a normal line of a layer formed by a chiral smectic phase, whereby those LC molecules aligned to one of the first aligning direction and the second aligning direction are aligned in the other aligning direction in accordance with an applied voltage, thus forming a plurality of regions of different alignment states in a range shorter than a wavelength of light in a visible light band, and directions of the directors are changed in accordance with a ratio of regions in a first alignment state to regions in a second alignment state.
To achieve this object, an AFLC display device according to the second aspect of this invention comprises:
a first substrate;
a second substrate facing the first substrate;
an antiferroelectric liquid crystal, sealed between the first and second substrates and having a first ferroelectric phase where LC molecules are substantially aligned in a first aligning direction, a second ferroelectric phase where LC molecules are substantially aligned in a second aligning direction and an antiferroelectric phase where there are LC molecules aligned in the first aligning direction and LC molecules aligned in the second aligning direction with an average direction of directors being substantially coincident with a direction of a normal line of a layer formed by a chiral smectic phase, whereby those LC molecules aligned to one of the first aligning direction and the second aligning direction are aligned in the other aligning direction in accordance with an applied voltage, thus forming a plurality of regions of different alignment states in a range shorter than a wavelength of light in a visible light band, and directions of the directors are changed in accordance with a ratio of regions in a first alignment state to regions in a second alignment state; and
control means for applying a voltage to the liquid crystal and changing the applied voltage to control the ratio of regions in the first and second alignment states, thereby setting the directions of the directors of the antiferroelectric liquid crystal to an arbitrary direction between the first and second aligning directions.
In the AFLC display devices according to the first and second aspects of this invention, the LC molecules of the antiferroelectric liquid crystal change their state from one of the first and second alignment states (stable states) to the other in accordance with the applied voltage and the ratio of the change is associated with the applied voltage. It is therefore possible to control the ratio of minute regions where the LC molecules are in the first stable state to minute regions where the LC molecules are in the second stable state by adjusting the applied voltage. The directions of the directors of the antiferroelectric liquid crystal vary in accordance with this ratio. Therefore, the directions of the directors can be controlled to an arbitrary direction between the first and second aligning directions.
An AFLC display device according to the third aspect of this invention comprises:
a first substrate on which a plurality of pixel electrodes and a plurality of active elements connected to the pixel electrodes are arranged;
a second substrate on which a common electrode opposing the pixel electrodes is formed; and
an antiferroelectric liquid crystal, sealed between the first and second substrates and having first and second ferroelectric phases where alignment states of LC molecules differ from each other, and an antiferroelectric phase, the antiferroelectric liquid crystal having an intermediate alignment state which lies between the ferroelectric phases and the antiferroelectric phase and to which directors are changed by movement of the LC molecules caused by a phase transition precursor in accordance with an applied voltage.
An AFLC display device according to the fourth aspect of this invention comprises:
a first substrate;
a second substrate facing
Ogura Jun
Shimoda Satoru
Tanaka Tomio
Yoshida Tetsushi
Casio Computer Co. Ltd.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Ngo Julie
Sikes William L.
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