Diffuse reflectance spectroscopy apparatus for use in analysing a sample comprising a sample receiving location 2 for receiving a sample 3 for analysis; an illumination arrangement 4 for directing light towards a received sample; a detector 6 for detecting light reflected by a received sample; and collection optics 5 for directing light reflected by a received sample towards the detector. The illumination arrangement further comprises an interferometer 42 and a half beam block 45a, 45b which is disposed substantially at a focus in the optical path for blocking light which exits the interferometer, passes said focus, and is reflected from re-entering the interferometer. A half beam block 45a may be disposed in the optical path between the interferometer and the light source 41 for blocking light that exits the interferometer back towards the light source and is reflected by the light source from re-entering the interferometer and/or a half beam block 45b may be disposed in the optical path on the opposite side of the interferometer than the light source.