which is formed the image of the slot f 0 through the objective o 1 the cell c the interferometry unit i and the objective o 2 this slot f 1 constitutes the input slot of a holographic concave grating having a small radius of curvature and is associated with an output slot f 2 disposed in front of the sensitive element of a detector pm which delivers a signal s once it is illuminated the interferometry unit i comprises two polarizing prisms w 1 and w 2 having a double image and of the same type and between them a birefringent plate l and a photoelastic modulator m the two prisms w 1 and w 2 are two double image polarizing prisms of the same type for example two wollaston prisms or two rochon prisms or analog they can for example be made of magnesium fluorine their elements being adherent and not stuck w 1 and w 2 are wollaston prisms as represented in fig1 the birefringent plate l is temperature controlled for adjusting the path difference that it introduces the photoelectric modulator m is a silica or fluorine plate excited by a piezoelectric ceramic thereby giving this plate a birefringency which is variable by compression the optical axis of the modulator m is parallel to that.