Patent ID: 7612341

Claim:
A method of investigating a sample, the sample comprising at least a first material in order to determine the presence of at least one further material, the method comprising the steps of: a) irradiating the sample with electromagnetic radiation in the range from 25 GHz to 100 THz; b) detecting radiation reflected from the sample; c) determining a parameter related to the amplitude of the radiation, which is reflected from the sample, in the time domain; d) analysing the parameter derived in step (c) in order to determine if there has been a 180° change in phase of the reflected radiation relative to the radiation irradiating the sample in order to determine if the sample further comprises the at least one further material; wherein the irradiating and reflected radiation passes through a window member having a refractive index that falls between the refractive indices of the at least first and at least one further materials such that the irradiating radiation undergoes no change in chase upon being reflected at an interface between the window member and the at least a first material and undergoes a 180° change in phase upon being reflected at an interface with between the window member and the at least one further material.