Schematic representation of a simple 3T non-redundant (left) and redundant (right) linear configuration for a dispersed fringe instrument. For each case, we show the pupil plane at the entrance of the spectrograph and the 2D Modulation Transfer Function (MTF). By adding an optical path difference (δ) on one of the pupil in the redundant case, one can separate the fringe signals in the MTF plane despite the redundancy.