A novel light modulator based on binary optics is proposed, which is consisted of two binary π/2 phase gratings, one for self-imaging and the other for phase compensation. An analytic expression for the modulation of any form of incident wave-front is obtained by using the Fresnel diffraction theory. Numerical simulations on the zero-order diffraction of Gaussian beams are given. It is concluded that tho modulability is directly proportional to the beam radius and reversely to the distance between the two gratings. Experiments results are given, too.