Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
2000-03-21
2004-06-29
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S134000, C349S184000, C349S191000
Reexamination Certificate
active
06757045
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal device and a liquid crystal apparatus, such as a liquid crystal display apparatus, including the liquid crystal device for use in light valves for flat-panel displays, projection displays, printers, etc., and particularly relates to those using a chiral smectic liquid crystal.
Active matrix-type liquid crystal devices wherein each pixel is provided with an active element (e.g., a thin film transistor (TFT)) are known as a type of a nematic liquid crystal display device.
Twisted nematic (TN) liquid crystal, as disclosed by M. Schadt and W. Helfrich, Appl. Phys. Lett., vol. 18, no. 4, pp. 127-128 (1971), has been used as a nematic liquid crystal material used for such an active matrix-type liquid crystal device using a TFT.
In recent years, liquid crystal devices of both In-Plain Switching mode, utilizing an electric field applied in a longitudinal direction of the device, and Vertical Alignment mode have been proposed to improve a viewing angle characteristic which is poor in conventional liquid crystal displays.
Accordingly, there are various liquid crystal modes suitable for the TFT-type liquid crystal device using the nematic liquid crystal material. In any mode, however, the resultant nematic liquid crystal display device has encounters a problematic slow response speed of several ten milliseconds or more.
In order to improve the response characteristic of the conventional types of nematic liquid crystal devices, several liquid crystal devices using a specific chiral smectic liquid crystal, such as a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized ferroelectric liquid crystal or an anti-ferroelectric liquid crystal showing no threshold (voltage) value, have been proposed. Although these devices have not been put into practical use sufficiently, it has been reported that a high speed responsiveness on the order of below millisecond is realized.
With respect to the chiral smectic liquid crystal device, our research group has proposed a liquid crystal device as in U.S. patent application Ser. No. 09/338426 (filed Jun. 23, 1999), wherein a chiral smectic liquid crystal has a phase transition series on temperature decrease of isotropic liquid phase (Iso)—cholesteric phase (Ch)—chiral smectic C phase (SmC*) or Iso—SmC*, and liquid crystal molecules are monostabilized at a position inside an edge of a virtual cone. During the phase transition of Ch—SmC* or Iso—SmC*, liquid crystal molecular layers are uniformly oriented or aligned in one direction, e.g., by applying a DC voltage of one polarity (+ or −) between a pair of substrates to improve high speed responsiveness and gradation control performance and realize a high luminance liquid crystal device excellent in motion picture image qualities with a high mass productivity.
Further, our search group has also proposed a chiral smectic liquid crystal device as in U.S. patent application Ser. No. 09/257032 (filed Feb. 25, 1999; corresponding to Japanese Laid-Open Patent Application (JP-A) 11-311812), wherein a chiral smectic liquid crystal is placed in an alignment state in chiral smectic C phase such that the liquid crystal comprises smectic molecular layers forming a chevron structure, under no electric field application, the liquid crystal has an average molecular axis substantially in alignment with the uniaxial alignment axis and/or a bisector of a maximum angle formed between two extreme molecular axes established under electric field application, and under electric field application, the liquid crystal provides an effective tilt angle and a transmittance that continuously changed depending on an electric field applied thereto. The smectic molecular layers provide a layer inclination angle &dgr; with respect to a normal to the substrates, and the liquid crystal provides a tilt angle H in chiral smectic C phase satisfying the following relationship at at least an operating temperature: &dgr;≧H.
The above-mentioned ferroelectric or anti-ferroelectric liquid crystals effecting inversion switching based on spontaneous polarization are liquid crystal materials showing chiral smectic phase.
Accordingly, in a sense of solving the problem of the conventional nematic liquid crystal devices, i.e., improvement in response speed, the realization of a practical liquid crystal device using a chiral smectic liquid crystal material is expected for use in advanced displays with high speed responsiveness and high gradation display performance.
In this regard, however, optimum conditions for designing such a liquid crystal device, particularly a cell gap or thickness, have not yet been sufficiently proposed.
In the case of, e.g., a liquid crystal device using a surface-stabilized ferroelectric liquid crystal (SSFLC) assuming only two (bright and dark) states, when liquid crystal molecules are aligned in parallel with a pair of substrates, it is possible to obtain a transmitted light quantity in a light-transmission state (white display state) based on a product (&Dgr;nd) of a refractive index anisotropy (birefringence) (&Dgr;n=n
//
−n
⊥
) inherent to the liquid crystal material used and a cell gap (d).
Specifically, under the cross-nicol condition, a transmitted light quantity T
1
may be calculated according to the following equation (A):
T
1
=sin
2
(&pgr;&Dgr;nd/&lgr;) (A),
wherein &lgr; denotes a wavelength of incident light.
By using the equation (A), the cell gap (d) may appropriately be determined so as to provide a desired waveform-transmittance characteristic, thus optimizing the liquid crystal device.
In the case of using a SSFLC providing a large (high) pretilt angle, it is possible to calculate an effective refractive index anisotropy (&Dgr;n
effect
) within a liquid crystal device by taking an angle of liquid crystal molecules rising from a planar (horizontal) surface of the substrate into consideration. By using the effective refractive index anisotropy (&Dgr;n
effect
) in place of &Dgr;n in the above equation (A), it is similarly possible to readily obtain an optimum cell gap for the liquid crystal device using the SSFLC providing the large tilt angle.
However, as a result of our detailed study, when a liquid crystal device using a monostabilized FLC (ferroelectric liquid crystal) according to the above-mentioned earlier-filed U.S. patent application Ser. Nos. 09/338426 and 09/257032 is prepared based on the above-mentioned cell gap designing, a transmittance at the time of actual drive of the liquid crystal device results in a value different from the calculated value based on the equation (A), although the FLC material is used. In other words, it has been experimentally confirmed that the light-transmission characteristic of the SSFLC (showing bistability) and that of the monostabilized FLC as used in the earlier-filed U.S. patent applications are different in behavior from each other.
More specifically, when a liquid crystal device providing no pretilt angle is prepared according to the earlier-filed U.S. patent applications and its cell gap is set to optimize a value &Dgr;nd based on the above-described equation (A), the resultant transmitted light quantity is lower in intensity than the calculated (expected) transmitted light quantity (obtained according to the equation (A)). Further, a color temperature for white is shifted to a higher temperature side, thus resulting in a peak wavelength shifted toward a shorter wavelength side.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems, an object of the present invention is to provide a liquid crystal device capable of providing a high transmittance state even in the case of using a liquid crystal material assuming a chiral smectic phase.
Another object of the present invention is to provide a liquid crystal apparatus including the liquid crystal device.
According to the present invention, there is provided a liquid crystal device comprising a layer of a chiral smectic liquid crystal, a pair of substrates oppositely
Asao Yasufumi
Terada Masahiro
Togano Takeshi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ngo Julie
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