Threshold Characteristic and Operating Voltage

This equation was derived for an infinitely thick cholesteric layer where there is no torque exerted by the boundaries and cholesteric material has its natural pitch. However, it holds reasonably well for practical devices with large d/p ratios, and in particular for White-Taylor displays with homeotropic boundaries having no forced pitch. The threshold characteristic shows a noticeable hysteresis. The winding voltage Vnc is given by (see Fig. 16) 

With smaller d/p ratios d/p 2), the departure of the experimental curve from the theoretical values for the threshold characteristic shows the impact of increasing surface alignment with larger pitch [12, 16J. For a value of d/p below a critical value for homeotropically aligned sample, the effect of boundaries is so strong that the critical voltage is zero and a homeotropic nematic structure is obtained [120]. 

---