1. A device for the two-dimensional reading of an optical image using elastic surface waves, comprising a first piezoelectric medium at the surface of which elastic waves are capable of being propagated, and a second medium which is both semiconductive and photoconductive and in which the electrical fields associated with said elastic waves are capable of creating nonlinear interactions and where said image is projected, producing therein a spatial variation in conductivity, said first medium carring at least two electromechanical transducers which transmit elastic surface waves having the same frequency, in two separate directions of which the intersection defines a zone of interaction where said nonlinear interactions occur, the intensity of said interactions being modulated by said spatial variation in conductivity, said waves being transmitted respectively by said transducers with a period at least equal to the scanning time of the zone of interaction by an elastic wave and with variable relative delays, said device further comprising means for extracting the time independent signals produced by said nonlinear interaction, said extracting means having a spatial periodicity which corresponds to the one of said time independent signals.