  We have built a Terahertz Dispersive Fourier Transform Spectrometer
\cite{Birch1987} to study frequency properties of superconducting films used
for fabrication of THz detectors. The signal reflected from the tested film is
measured in time domain, which allows to separate it from the other
reflections. The complex conductivity of the film depends on frequency and
determines the reflection coefficient. By comparing the film reflection in the
superconducting state (temperature is below $T_c$) with the reflection of the
normal state, we characterise the film quality at terahertz frequencies. The
method was applied to 70 and 200nm thick Nb films on a silicon wafer and to
360nm thick NbTiN films on silicon and quartz wafers. The strong-coupling
coefficient, $\alpha$, was found to be 3.52 for Nb, and 3.71-4.02 for the NbTiN
films. The experimental results were fitted using extended Mattis-Bardeen
theory \cite{Noguchi2012} and show a good agreement.
