Twist angle determination in liquid crystal displays by location of local adiabatic points

In this work we present a method for the determination of the twist angle o f an arbitrary twisted nematic liquid crystal spatial light modulator. The method is based on the location of local adiabatic points, i.e., situations in which the liquid crystal SLM acts only as a rotation device. For these cases, the rotation induced on the polarization of the incident beam is equ al to the twist angle. Consequently, the twist angle can be determined with high precision. We show that local adiabatic regime may be achieved in two ways, either by changing the incident beam wavelength, or by applying a vo ltage to the electrodes of the display. However, the simple model that desc ribes the SLM in the off-state, may break down when a voltage is applied to the display, and it may affect the local adiabatic behaviour. We present t heoretical and experimental results. (C) 1998 Elsevier Science B.V. All rig hts reserved.