A spectrometric instrument comprising: a scanning interferometer having a beamsplitter 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 for launching a reference beam into the interferometer along a first propagation path to be initially incident on a first face of the beamsplitter; an observation optical radiation source for launching a divergent observation beam into the interferometer along a second propagation path to be initially incident on the first face of beamsplitter and overlap the reference beam at the first face; wherein the radiation sources cooperate to generate a first angle between the directions of propagation of the two beams along respective first and second propagation paths when initially and simultaneously incident at the first face which is larger than a divergence half-angle of the observation beam 64.