A spectrometric instrument (38) comprising: a scanning interferometer (40,42,44) having a beamsplitter (40) for dividing incident optical radiation into a reflected beam, following a reflected beam path and a transmitted beam following a transmitted beam path; a monochromatic optical radiation source (52) for launching a reference beam into the interferometer (40,42,44) along a first propagation path (62) to be initially incident on a first face (40') of the beamsplitter (40); an observation optical radiation source (46) for launching a divergent observation beam (64) into the interferometer (4,6,8) along a second propagation path (66) to be initially incident on the first face (40') of beamsplitter (40) and overlap the reference beam at the first face (40'); wherein the radiation sources (52; 46) cooperate to generate a first angle (Theta) between the directions of propagation of the two beams along respective first (62) and second (66) propagation paths when initially and simultaneously incident at the first face (40') which is larger than a divergence half-angle (alpha) of the observation beam (64).