An optical spatial phase modulator consisting of a thin polymer membrane mirror applied over a compartmented sub-structure has been devised for use as an input transducer for coherent optical computers. This device is called the ‘membrane light modulator’ or MLM. Membrane elements are 10 to 50 microns in diameter, have a sub-microsecond response time, and require only 10 picojoules per element to produce a full 360° phase reversal in reflection. An array of approximately 2000 elements is described having an active area of 4 by 4 mm. This MLM has been used as an input transducer for a coherent optical computer which takes one-dimensional Fourier transforms.