Phase control of the voltages applied to opposite electrodes for a cholesteric to nematic phase transition display

Claim 1

An electro-optical device comprising: 1. a liquid crystal cell having a first electrode and a second electrode for appling a voltage ot a liquid crystal layer between said first and said second electrodes and having a threshold voltage Vth above which its optical transmission changes, at least one of said first and second electrodes being light-transparent, said liquid crystal being cholesteric liquid crystal having positive dielectric anisotropy and consisting of nematic liquid crystal of 30 to 99 weight % having positive dielectric anisotropy and a compound of 70 to 1 weight % selected from steroidal compounds and optically active compounds; and 2. a means for changing optical transmission of said liquid crystal cell, comprising; a. a first circuit connected to said first electrode which generates a.c. voltage pulse having amplitude v, frequency f, phase phi 1 time duration T1 and d.c. bias voltage of the same amplitude as that of said a.c. voltage pulse, said amplitude V being higher than a half of said threshold voltage Vth and equal to or lower than said threshold voltage Vth, said frequency f being within a range from 10 Hz to 1 KHz, and b. a second circuit connected to said second electrode which generates an a.c. voltage having the same amplitude V, the same frequency f, the same bias voltage as those of said a.c. voltage pulse, respectively and phase phi 1 switchable to another phase phi 2 for time interval T2, said time interval T2 being varied from O to T1 for changing the optical transmission, whereby, an a.c. bias resulted from the difference between said d.c. bias voltage generated by said first circuit and said a.c. voltage generated by said second circuit keeps an excited light-transparent state of said liquid crystal cell and reduces fliker.