Liquid Crystal Reflection Cell with Improved Response Times

In recent papers the authors described a liquid crystal LC device, operated in the transmission mode, that had response times of the order of 0.2-0.4 msec and contrasts of 6. These response times are considerably faster than the 20-50 msec normally associated with cells using the twisted nematic configuration. The improvement in response time results from 1 the addition of a small amount of a cholesteric to a binary nematic, and 2 operation of the cell with the signal voltage superimposed on a square-wave pedestal voltage whose polarity is reversed at a 20-Hz rate. With an aim toward incorporating these features into a light valve of the type described by Beard et al., which uses a photoconductor-LC sandwich configuration, we have carried out experiments similar to those described earlier but on a cell in the reflection mode.