Liquid crystal display device with a hysteresis

Claim 1

A liquid crystal display device comprising two parallel supporting plates spaced at a distance d with facing surfaces; a pattern of row electrodes on a facing surface of one of said plates, and a pattern of column electrodes on an opposing facing surface of the other of said plates, said row electrodes crossing said column electrodes to form a matrix of picture cells at the crossings; a layer of nematic liquid crystal having a cholesteric addition disposed between said supporting plates, said liquid crystal having a positive dielectric anisotropy and a natural pitch p, said distance d and said pitch p forming a ratio d/p having a value between 0.6 and 0.9; and an orientation layer disposed on said facing surfaces and said electrodes, said orientation layer providing the molecules of said liquid crystal at one facing surface with an angle of tilt of between 0° and 70° and providing an angle of tilt of between 5° and 70° at said opposing facing surface, and said orientation layer providing a progressive molecular twist Φ across said distance d by the director of said liquid crystal molecules, said twist Φ having a value between 1.2π radians and 1.7π radians, wherein said value of d/p differs from the value of Φ/2π by 10%; wherein an electric field applied to said liquid crystal includes picture cells below an effective electric field strength E 1 being in a first stable state, picture cells above an effective field strength E 2 being in a second stable state differing optically from said first state, and picture cells at an effective field strength E H between E 1 and E 2 being either in said first stable state or said second stable state depending on the preceding stable state; wherein, starting from said first stable state and said effective field strength E H across said picture cells, said liquid crystal exhibits a small energy threshold to change a picture cell from said first stable state into said second stable state with a pulsatory field having an energy content above said energy threshold, and wherein with said pulsatory field having an energy content below said energy threshold, a picture cell returns to said first stable state by a metastable state having a given relaxation period if the recurrence frequency at which a picture cell is driven does not exceed the reciprocal value of said relaxation period.