Broadband light is directed along sample and reference paths such that light reflected by a sample surface and light reflected by a reference surface interfere. A mover effects relative movement between the sample and reference surfaces along a scan path. A detector senses a series of light intensity values representing interference fringes produced by a sample surface region during the movement. A data processor processes the intensity values as they are received to produce coherence peak position data and, after completion of a measurement operation, uses this coherence peak position data to obtain data indicative of the height of the surface region. The data processor includes a correlator for correlating intensity values with correlation function data to provide correlation data to enable identification of a position of a coherence peak. A surface topography determiner determines a height of a sample surface region from coherence peak position data.