A method for imaging a sample, the method comprising the steps of: (a) irradiating the surface of a sample with a source of coherent substantially continuous radiation with a frequency in the range 25 GHz to 100 THz; (b) detecting said radiation reflected from or transmitted by the sample; (c) providing and detecting a probe beam having a phase related to that of the radiation leaving the source; (d) analysing the detected radiation for at least one characteristic feature within the radiation and monitoring any change in phase of such at least one feature in order to derive structural information about the sample as a function of depth from the surface of the sample.