Properties and new possible applications of the slit phase contrast technique are presented. They are tested by an advanced computer simulation program. A digital 2-D spatial filtering process is performed. A composition of phase objects (e.g. , circles, rectangles, gratings, fibers) is transformed into its Fourier form, multiplied by the transmittance of the phase strip, and finally retransformed. According to the simulation results, several soot filters are prepared and applied to a modified commercial microscope. The experiments mainly involve biological specimens. Special attention is given to measurements of the refractive index profile of the optical fibers. A detailed theoretical background is presented and the results of additional computer simulation based on the operation of convolution are discussed.