A twisted nematic liquid-crystal television (LCTV) has been used together with Twyman-Green type interferometers to measure and compensate the phase distortion of a wave front. The twisted nematic LCTV has been operated as a phase-only modulator, and its phase retardation ability has been doubled in these experiments. The phase function of a phase object has been measured in different arrangements of the experimental setup. (C) 1996 Optical Society of America