An interferometer is disclosed, such as may be incorporated into a hand-held spectrometer. The interferometer comprises enclosed path optics and a detector, the enclosed path optics comprise at least two reflecting elements and a beamsplitter, the beamsplitter is arranged to divide an input beam into first and second beams. The enclosed path optics are arranged to direct the first and second beams in opposite directions around paths enclosing an area and to output the first and second beams towards the detector. The enclosed path optics also focus the first and second beams onto the detector. The detector is arranged to detect a pattern produced by interference of the first and second beams. In a preferred embodiment the two reflecting elements are a pair of concave mirrors, and the enclosed path optics enclose a triangular area. The use of concave mirrors for both reflection and focussing provides the interferometer with compactness.