In linear optics, the visualization of phase object and the contrast improvement of the image of amplitude object are obtained using a small absorbing and dephasing screen at the Fourier spectrum plane of the objet (Zernike method). In this communication it is shown that the screen can be replaced by a non linear medium displaying either non linear absorption or depletion of the exciting beam by non linear scatterings, and non linear change in the refractive index at convenient levels. The conditions leading to the visualization of phase object or to contrast inversion in amplitude objects are studied for a tenth of liquids. The advantage of the proposed method is that it needs neither the construction of a screen nor precise alignments.