Abstract: A scanning optical interferometer (40) having a beamsplitter (42); a first input path (56) for an observation beam (52) and a second input path (78) for a reference beam (76) wherein both paths (56; 78) are directed towards the same beamsplitter (42) characterised in that the interferometer is configured with the first input path (56) and the second input path (78) intersecting at a location (L) before the beamsplitter (42) and in that the interferometer further comprises a dichroic filter (80) located at the intersection (L) of the two paths (56; 78) and acting to collocate both paths (56;78) in a direction towards the beamsplitter (42).