Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1996-09-23
1999-11-16
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
349 99, 349121, 349 50, G02F 11335, G02F 11333
Patent
active
059867329
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a color liquid crystal display apparatus suitable for displaying an achromatic color by using a twisted nematic liquid crystal and capable of displaying each color of red, blue and green.
DISCUSSION OF THE BACKGROUND
There has been known a super-twisted element for a method of obtaining a high density dot matrix display by increasing a twist angle of liquid crystal molecules between a pair of electrodes to thereby cause a sharp change of voltage-transmittance characteristics (T. J. Scheffer and J. Nehring, Appl. Phycs. Lett. 45(10)1021-1023(1984).
In the conventional method, however, the product .DELTA.n.multidot.d of the refractive index .DELTA.n of liquid crystal in a liquid crystal display element used and the thickness d of a liquid crystal layer was substantially in a range of 0.8 .mu.m-1.2 .mu.m (Japanese Unexamined Patent Publication No. 10720/1985 which is referred to as conventional technique 1). According to the conventional technique, an excellent contrast could be obtained only by a specified combination of colors such as yellowish green and dark blue, bluish purple and pale yellow and so on.
Thus, in the conventional technique using such a liquid crystal display element, a monochrome display could not be effected. In order to improve the conventional technique, there was proposed a liquid crystal display apparatus capable of displaying a monochrome display and having a high contrast ratio wherein two liquid crystal cells of different helical structures are placed one on another; a voltage is applied to either cell and the other is merely used as an optically compensating plate (Report of Television Association 11 (27), p. 79 (1987) by Okumura et al.).
Also, there was proposed a method of providing a monochrome display by arranging a birefringent plate between the liquid crystal layer and a polarizing plate. Conventionally, a color liquid crystal display apparatus used for OA machines such as personal computers comprised the above-mentioned liquid crystal display element capable of effecting a monochrome display and color filters.
However, the color filters are expensive and have extremely low efficiency of utilization of light since a display is effected with three picture elements of red, blue and green. For instance, three picture elements of red, blue and green are used for displaying white, and even when the three picture elements are turned on, the brightness is 1/3 and accordingly, a bright display can not be obtained.
Several techniques have been proposed for color display apparatuses without having color filters. For instance, an electrically controlled birefringence (ECB) effect type liquid crystal display apparatus is known. In this apparatus, when gradation voltages (e.g., voltages for 8 gradations) are applied to a pixel, the orientation of liquid crystal molecules is changed depending on gradation voltages applied whereby .DELTA.n.multidot.d of the liquid crystal cells is changed. And various colors caused by the effect of the birefringence in the liquid crystal cell are used. In such an ECB effect type liquid crystal display apparatus, however, since liquid crystal is not have a twisted structure, a state of liquid crystal to be changed depending on an applied voltage was small, and a display by multiplexing driving could not be obtained.
Japanese Unexamined Patent Publication No. 118516/1990 (conventional technique 2) discloses that various colors can be displayed by changing a voltage applied to a cell including twisted liquid crystal molecules. In this conventional technique 2, however, there is a problem that colors which can be developed are yellow, red, purple, bluish purple, bluish green and green, and a display of achromatic color such as black or white is impossible. It has been known that the visibility in usually used displays is considerably reduced if a display of black or white is not used. For instance, in a display of graphs, achromatic color of black or white is often chosen a
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REC Mode for Reflective Color STN-LCD by Seiko Epson (6 pages), Iijma, pp. 171-176.
Mori Haruki
Ozeki Masao
Shidoji Eiji
Asahi Glass Company Ltd.
Sikes William L.
Ton Toan
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