100 volts over a period of 75 seconds while the fall curves 2b and 4b were obtained by ramping the voltage back down to zero over another 75 second interval as shown in fig1 each type of pdlc exhibits a threshold voltage below which it is non transmissive this threshold voltage is about 25 volts for curves 2 and about 6 volts for curves 4 above these thresholds levels the hysteresis fall curves 2b 4b are shifted to the left from the rise curves 2a 4a thus for any particular voltage above the threshold level and below the fully on level there is a higher degree of transmission through the pdlc on the fall curve than on the rise curve the invention makes beneficial use of this phenomenon by driving the pdlc cell with a shaped voltage waveform that forces the majority of the transmission period toward the fall curve and thus produces a substantial increase in the total optical transmission through the pdlc fig2 a 2c illustrate the problem of achieving good gray scale response with pdlc films using square wave pulse envelopes of 100 hz ac signals for 100 ms the fully on and fully off pdlc response times can be quite fast as shown in fig2 a the pdlc in this case was switched between zero volts and a fully on level of 70 volts rms with a 100 msec square wave pulse the turn off time along curve segment 6 was about 7 msec while the turn on time along curve 8 was about 1 msec however the dynamic response of the pdlc film was.