Abstract: Selective modulation interferometric detector, intended for specifically detecting and/or identifying at least one gas in a gaseous mixture, especially with the aim of carrying out analysis and/or proportioning operations, the assembly E consists of a first strip L.sub.1 of interferometric quality having plane and parallel faces, of thickness e and index n introducing a displacement .DELTA.=2ne adapted to the gas to be analyzed, this strip L.sub.1 supplying on the detector a brilliant fringe, and a second strip L.sub.2 made of the same material as the first strip L.sub.1 which is slightly prismatic and supplying on the detector average illumination.