Optical instruments having, inter alia, optics to process wavelengths of electromagnetic radiation to produce an interferogram. The instruments include an optical path and optical elements positioned along this path for splitting and recombining the wavelengths which interfere with each other to produce a plurality of different fringes of different wavelengths. The optics include matched gratings which are positioned along the optical path outside of the interferometer optics to produce first and second sets of spectrally dispersed beams. The interferometer optics includes a beam splitter and first and second mirrors. In two embodiments the beam splitter has an internal surface including three zones. The instruments can all include a detector for detecting the interferogram and means for processing the detected interferogram to produce spectral information that is spatially distributed.