An optical system (10) includes an arrangement for splitting a source beam into a measurement beam and a reference beam. The reference beam is reflected off a reflective element (42) which mounted on a delay line (44). A target (35) scatters the radiation from the measurement beam. The scattered radiation and the reference beam are brought to interfere on a detector (40) by calibrating the delay line (44). The detected interference pattern is Fourier-transformed and filtered to select a region of interest around a side-band of the Fourier-transformed interference pattern in order to remove noise caused by stray radiation that hits the detector.