A new type of intensity spatial filter, based on the polarization interference figure from a crystal under conoscope is developed, and its application to the phase measurement in Fourier-transform quasi-interferometry is presented. This method does not require any Fourier-transforming lenses and masks for spatial filtering. Various configurations are introduced and their performances are analysed. Some contour-fringes systems may be produced and even with background fringes. Several examples of experiments are given.