Comparison of the working principles between pulsed THz time-domain imaging and FMCW imaging. (a) Incident THz pulse and returned THz echo pulses, where the first peak corresponds to the sample surface reflection and the second peak is the interface reflection from within the sample. (b) Schematic diagram of a sample with a single layer with a thickness of d and a refractive index of n. (c) FMCW radar transmitted (TX) and returned (RX) signals. Note the different time delay (phase difference) corresponding to surface and interface reflections. (d) TX and RX frequency-varying signals with sawtooth frequency modulation. (e) The depth profile calculated from the FFT amplitude spectrum of the signal.