An apparatus comprising means for: causing illumination of different areas of a sample with an optical frequency imaging beam at different positions at different times, wherein adjacent positions are configured to cause the corresponding areas to at least partially overlap; receiving signals indicative of back-scattering of the optical frequency imaging beam from the sample at the different times; and processing the received signals to obtain an image of the sample, wherein processing the received signals compensates for phase variations between the different positions at the different times using a matched filter derived from a scattering model of the sample.