A method is described to determine the Fourier analysis of the image of a slit of finite width using polarization interferometers. Sinusoidal fringes of variable spatial frequency and of contrast one are projected on the slit image and the output flux is recorded. The modulation of the flux when the fringe system is displaced with respect to the slit image yields the spatial spectrum of the slit image. The spatial spectrum of the slit object being known, the transfer function (both amplitude and phase) of the optical system can be determined.