A spatial light modulator (SLM) based on a deformable gel surface is presented. It has remarkable optical properties and its construction and operation are comparatively simple. It can be optically addressed through a photoconductor layer. The surface relief pattern is read out by total reflection and a schlieren optical system. The device provides good wavefront quality (X/10 over the whole aperture of 30 x 50 mm2) and has a spatial resolution of 10 line pairs/mm. Contrast ratios for modulation up to 40:1 were measured. The input sensitivity is typically 0.3 mW/cm2. The rise and decay times are both approximately 20 ms. Besides its primary application as a light valve in large screen TV projection, it can be used in optical information processing systems, e.g., as an incoherent-to-coherent transducer. Combined with a CRT, the SLM can be addressed electronically.