1. A method of investigating an object, comprising the steps of: (a) irradiating the object with an optically-generated pulse of electromagnetic radiation, said pulse having a plurality of frequencies in the range from 100 GHz to 100 THz; (b) detecting radiation transmitted and/or reflected from the object to obtain a time domain waveform; (c) repeating steps (a) and (b) for a plurality of points on the object; (d) combining data from step (c) to produce a time domain waveform for the object which has been averaged over the plurality of points; and (e) removing the directly reflected or directly transmitted portion of the signal obtained in step (d) and applying a Fourier transform to the remaining portion of the waveform.