Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Patent
1998-04-02
2000-06-06
Sikes, William L.
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
349 88, 349 90, 349187, G02F 11333, G02F 113
Patent
active
060725524
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display, and more particularly relates to achieving a bright display without using a backlight.
Various liquid crystal display elements have been proposed and put to practical use as display elements for displaying information. Currently, a TN mode (Twisted Nematic; Japanese Patent Laid-open Publication No. Sho. 47-11737 (1972) and an STN mode (Super Twisted Nematic; Japanese Patent Laid-open Publication No. Sho. 60-107020 (1985)) are typical modes that are used in nematic liquid and these modes are widely employed.
In the TN mode, the direction of alignment of the liquid crystal molecules is twisted by 90 degrees when no voltage is applied. Light incident to this element is then polarized by the twisted structure of the liquid crystal and by birefringence, and polarized light is emitted. On the other hand, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules become aligned in the direction of the voltage, less double diffraction occurs and incident light is emitted without being polarized. By adopting a structure where this liquid crystal layer is sandwiched between two light polarizing plates, changes in the optical properties of the liquid crystal layer (changes in the polarized state) can be observed as changes in the strength of the emitted light.
In the STN mode the direction of orientation of the liquid crystal molecules is twisted by about 240 degrees, which is large compared with TN the mode. Changes in the optical properties of the liquid crystal layer (changes in the polarized state) can be observed as changes in the strength of the emitted light in this mode also by adopting a structure where this liquid crystal layer is sandwiched between two light polarizing plates.
Contrast between light and dark can then be obtained based on this operation theory in the TN and STN modes.
These display methods have the advantages of using remarkably little power, as compared with CRTs (Cathode Ray Tubes), and can be provided in the form of thin display panels. This display method is widely used in information processing equipment, such as personal computers and word processors.
However, because this method employs light polarizing plates as a matter of necessity, about half of the incident light is not transmitted by the liquid crystal display element. In reality therefore, a large number of liquid crystal displays are provided with light sources (backlights) at the rear of the liquid crystal display element to maintain brightness. The amount of light transmitted by the liquid crystal displays provided with color filters for information in color is even lower, and these displays therefore have to be provided with extremely powerful backlights. As the power consumed by the backlight is even greater than the power consumed by the driver circuit for driving the liquid crystal display element, this kind of display is not suitable as a display for portable information equipment that use batteries to provide power. There is then a trade-off in the related liquid crystal display method between brightness and power consumption, and a bright reflective liquid crystal display element that does not require a backlight is therefore preferred.
It is also preferable to obtain a reflective liquid crystal display element without a fluorescent backlight from the point of view of eye fatigue in the case of continued viewing of the display.
Liquid crystal display elements that do not use light polarizing plates have therefore been proposed in response to these problems, of which the "White-Taylor liquid crystal display" is typical (refer to J. Appl. Phys. 45, pp.4718-4723 (1974)). Here, a cholesteric liquid crystal to which dichroic dye is added is orientated substantially parallel to the faces of the plates. The dichroic dye then efficiently absorbs light because of spontaneous twisting of the cholesteric liquid crystal when there is no voltage applied. Absorption of transmitted light by the dichroic dye does not occu
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Ando Masahiko
Hirakata Jun-ichi
Hiyama Ikuo
Itou Osamu
Komura Shinichi
Chowdhury Tarifur R.
Hitachi , Ltd.
Sikes William L.
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