An optical wavefront sensor based on a non-linear Zernike filter is described. The sensor's key element is a light valve (optically addressed spatial light modulator), which is composed of ferroelectric liquid crystal (FLC) and semi-transparent photoconducting film of amorphous silicon carbide. The circularly polarized input light beam is focused onto the light valve by a Fourier lens in standard phase contrast filtering geometry. For the high intensity (zero order) spectral component the deviation of the FLC director by @ /4 (45°) results in a ~ /2 phase shift. This provides a wavefront phase distortion visualization similar to a conventional Zernike filter. The response time of the sensor is estimated on the order of 200 w s. The sensitivity of the sensor is on the level of units of nW. The fast FLC switchable quarter wave plate allows one to design a high contrast differential Zernike filter.