An apparatus and method for imaging a sample, the apparatus comprising: a source for irradiating a sample with a beam of substantially continuous electromagnetic radiation having a frequency in the range 25GHz to 100THz; means for subdividing an area of the sample which is to be imaged into a two dimensional array of pixels; means for detecting radiation from each pixel wherein the detector is configured to detect a phase dependent quantity of the detected radiation which is measured relative to the radiation which irradiates the sample.