THz-TDS measures the electric field of a pulse and not just the power. Thus, THz-TDS measures both the amplitude and phase information of the frequency components it contains. In contrast, measuring only the power at each frequency is essentially a photon counting technique; information regarding the phase of the light is not obtained. Thus, the waveform is not uniquely determined by such a power measurement.
Even when measuring only the power reflected from a sample, the complex optical response constant of the material can be obtained. This is so because the complex nature of an optical constant is not arbitrary. The real and imaginary parts of an optical constant are related by the Kramers–Kronig relations. There is a difficulty in applying the Kramers-Kronig relations as written, because information about the sample (reflected power, for example) must be obtained at all frequencies. In practice, far separated frequency regions do not have significant influence on each other, and reasonable limiting conditions can be applied at high and low frequency, outside of the measured range.
THz-TDS, in contrast, does not require use of Kramers-Kronig relations. By measuring the electric field of a THz pulse in the time-domain, the amplitude and phase of each frequency component of the THz pulse are known (in contrast to the single piece of information known by a power measurement). Thus the real and imaginary parts of an optical constant can be known at every frequency within the usable bandwidth of a THz pulse, without need of frequencies outside the usable bandwidth or Kramers-Kronig relations.