Abstract: A security inspection system is provided. The security inspection system includes a source configured to transmit a beam of radiation comprising a frequency of at least about 10 GHz. The system also includes an optical system configured to focus the beam of radiation on a sample. The system further includes at least one detector configured to detect one or more reflected beams from different locations of the sample in a focal plane of the optical system and generate a corresponding output signal. The system also includes a processor coupled to the at least one detector and configured to reconstruct a three dimensional image of the sample based upon the output signal.