Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Patent
1996-02-13
1998-11-24
Chow, Dennis-Doon
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
345 94, G09G 336
Patent
active
058414193
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
This invention broadly relates to liquid crystal matrix panels and more particularly it refers to a control method for matrix panels of a direct addressing, ferroelectric liquid crystal (FLC) type, to enable their improved operation.
As it is known, the panels to which this invention relates are used in devices for displaying images and for optical computation applications, both of the projection and of the direct vision types In this devices, each picture element (pixel) ideally corresponds to the intersection of an element of a first electrode set (for instance arranged as rows) and an element of a second electrode set (for instance arranged as columns) and materially it corresponds to an electro-optical cell comprising a ferroelectric liquid crystal in the room existing between two facing electrodes belonging to the above mentioned two electrode sets. In usual arrangements, a pair of crossed polarizers operatively completes the cell and makes the orientation changes visible of the director in the liquid crystal that can be of smectic C chiral type.
The device as a whole comprises the assembly of the described panel with the related electronic circuitry to generate the various voltage signals needed for its operation and with the interconnection elements to the panel electrodes. According to the expected application, in addition, polarizers, color filters, light sources and an optical system can be provided therein.
This invention additionally consists in the device comprising the above set forth assembly and operating according to the hereinafter described control method.
More precisely, this invention relates to a directly addressed FLC matrix panel wherein the ferroelectric liquid crystal cells operate according to a bistable or multistable behaviour in absence of voltage or in presence of a continuously applied, high frequency voltage having a sufficient and suitable rms amplitude, known as high frequency or alternated current stabilization voltage. The ferroelectric liquid crystal can be of smectic C chiral type and the cells can be of the chevron or partially straightened up chevron type. In both cases, the smectic layers are straightened up chevron type. In both cases, the smectic layers are approximately broken up into two halves, which are tilted in opposite directions with respect to a line normal to the cells, at an angle equal to (between 110% and 75% in the first case) or much smaller than (between 0% and 75% in the latter case) the characteristic angle of the SmC phase. Reference is made, for instance to P. Maltese, "Advances and problems in the development of ferroelectric liquid crystal displays", in Molecular Crystals and Liquid Crystals, Gordon and Breach, vol. 215, pages 57 and figures and to the references cited therein.
It is possible to obtain, as a result of each pulse, a cyclic transition of a cell from one extreme state to the other, by means of spaced apart rectangular pulses, of alternatively opposite polarities, possibly when a high frequency stabilization voltage Vhf having a predetermined rms amplitude is present between such pulses. This effect occurs when such pulses have a sufficient duration, which is a function of the amplitude of the pulses themselves (for a given rms stabilization voltage). Such sufficient duration has a minimum value, corresponding to a voltage Vtmin, below which the product of each sufficient duration by the corresponding pulse voltage varies to a small extent but at the same time it has a minimum value Amin in the voltage range between one and eight tenths of Vtmin. Often it is not possible to apply to the cells voltages sufficiently high to observe that the sufficient duration of the pulses increases as the voltage increases: in such case, the Vtmin should be evaluated by extrapolating the behaviours of the cells as observed at the applicable voltages.
Sufficiently small values of Vhf and Vtmin are achieved, as desired, when a large enough positive biaxiality of the dielectric constant tensor of the liquid crystal is avai
REFERENCES:
patent: 5162932 (1992-11-01), Kobayashi et al.
patent: 5379050 (1995-01-01), Annis et al.
patent: 5459481 (1995-10-01), Tanaka et al.
patent: 5519411 (1996-05-01), Okada et al.
Chow Dennis-Doon
Universita Degli Studi Di Roma "La Sapienza"
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